🚀 Hey Jugglebot Enthusiasts! 🤖 I'm thrilled to finally share this update of the actuator journey with all of you! Your insights, curiosity, and passion have been pivotal in this progression. If you were designing this actuator, what would YOU do differently? Or better yet, if you could see one feature or capability added to these actuators, what would it be? 🧐 Don't forget, if you want to learn more about the project, or to lend a helping hand, check out our Zulip site, here: pdj.zulipchat.com/
@@MichaelMantion Hmm interesting. I'm in two minds about this. Do you think just making the music a bit quieter would be better? Or would you prefer just none at all?
Love the series and the focus on the process of basing the design in proper design requirements. Inspiring thank you, clearly you are on a great path with the project 👊👊👍
For the bearings, I would suggest a delrin or Teflon roller. You can get a pack of these easily, marketed as 3d printer rollers. They would stay on the bearings better, resist abrasion, and have a self-lubricating effect with the tubes. For the synchromesh cable, you may be able to design a clamping plate similar to those used for bike cables.
Very good suggestions, cheers! I think the bike clamping plate will probably be the final solution for this problem; super simple, small and light. Great thinking!
The wheels used by 3D printers to roll on 2020 could also be used. The wheels are really cheap. There are also different shapes. Maybe a concave shape is better for rolling on the carbon fiber. You can buy them with a groove. Or if you can control the spinning modify them with a static spinning drill bit and slowly turning the wheel.
Some of these have a Delrin "v" groove profile that would give you 2 points of contact per bearing on your carbon tube. This is the first thing that jumped to mind when I saw your printed sleeves....I would imagine that this would keep things located a little more positively (at least, short of a concave profile to match the tube radius) . You might be able to use 3 of the same on the captured end of the moving tube in place of the 2 guide rollers you currently have riding one of the outside tubes...basically the same thing you have holding that moving tube inside your printed end, but flipped inside out. That would both keep it centered AND prevent axial rotation given the 2 contact points inside the V. Anyway super interesting project, and very promising.
Super cool project! Immediately got a couple ideas as well. Sorry if these are already named, but here we go: - Route the extend and retract spring via the same wire, so kind of get a capstan drive. - Make the sleeves for the inner bearings slightly concave. to make them self align to the rod? Maybe also make a groove in the middle of them to make a channel for the wire, to turn that issue into a feature. It might remove an extra bearing for routing the wire. - Make the moving bearings three sets at a 90deg angle, to get a kind of rollercoaster setup. - Add a fan to the output shaft of the motor, that spins a cooling fan over the brake resistor. Well now that that's out in the world so I don't think about it again, good luck!
It's been a treat watching this be developed! Some suggestions for future testing: - Conduct endurance testing while the actuator is (reasonably) loaded, to better match end-use conditions - Implement multiple load cases into the motion accuracy test (and record deviations in both absolute and relative positions with each move). Under a single constant load (or no load) the stiffness largely "disappears" since the parts' deflections under stress are not really changing. In the final application, the load will be constantly varying and this may have a large effect.
Cheers! I did consider that but I couldn't think of a good way to mount the actuator to support a load when upright... I imagine it'd be fairly similarly strong in both directions, though?
@@harrisonlowyou could use string and pulley so that it pushes the string downwards and the string pulls some load. But youll have to have at least two strings so that side forces cancel out and dont deflect towards the string
@@TrueHolarctic That'd definitely work and I think I considered something like that but quickly abandoned it after thinking of the direct lifting test that I showed in the video. I'm not super concerned about strength and I would expect the two directions to be similarly strong so I didn't bother with testing the other direction 😛
Hello stumbled into the video and the topic pulled me in. Retired Engineer in Medical, Automation and Robotics Systems. You do a great job of scientific thinking and setting goals. We used a Small Stewart Platform to shake test medical Data collectors in Scanners. It was amazing as we rolled through the frequency scale how bits would actually start to vibrate so much we could only see a blur where the part was. We added braces and stiffeners and in the end it improved our end result significantly. The advantage of having access to Aerospace folks to help. We also used Carbon Fiber for patient support to keep them nice and still. Later i used Carbon Fiber to build robotic arms, Large ones for Defense manufacturing, we added active and passive vibration dampening to get long arms with heavy grippers to move fast and stop without rebound. I appreciate what you doing and how you are doing it but still unclear what gizmo is doing. oh and this work i mention was done in the late 70s and early 80s. Maybe you were born after that time. Keep it going troop you are educating as well as fixing your machine. We had to know position as well so we added a rotary counter on the motor to feed a location calculator. Have fun and keep up the good work. Dennis another thought on the center Round Shaft. I used a square Shaft on one device, machined two corners opposite each other and a three V shaped rollers to capture the shaft and maintain alignment with lowest drag. Those were all metal. I was surprised to see you running metal bearings on six sides of resin cured Carbon Fiber tube. The resin will require some compliance as you have learned. We found Delrin worked well it is machinable and robust with some compliance to prevent damage to the resin surface of the composite tubes. D
Regarding terminating synchro mesh cables: You could probably create a part with an internal "thread" that matches the cable. Perhaps create a two/three piece part that can be pushed onto the cable and match its shape. As the pieces are forced together with the cable between, they'd pinch the cable and hold it tight. Then you pinch that part in a collared holder that prevents it from slipping out and provide pressure to keep the cable captured. Very hard to explain, but perhaps you get the idea. Not sure if it'd stand up to the tension, but it probably could. Let me know if you want me to elaborate and produce diagrams!
I was just about to post a comment like this. My Prusa 3D printer does a similar thing for terminating the axis belts. You push the belt sideways into a cutout that matches the profile of the belt, and it engages several of the teeth to keep it in place.
Interesting! I think I get what you mean and I wonder if I could use this for the spool end of the cable? Something like: a conical part with a cutout in the side that the cable slips into. The conical part then fits into a similar conical cutout in the spool. As the cable is pulled tight, the first part (the one the cable is actually touching) is pulled further into the conical cutout, compressing it and strengthening its grasp on the cable. Is that along the lines of what you were thinking? I like the idea, and I imagine it'd be fairly straightforward to turn this into a mechanism that would allow for quite fine adjustments to be made to the tension. Cheers for the idea!
@@harrisonlow I'm sure there are many ways - I'm thinking of a cylindrical "nut" where the thread of the nut fits the cable. A typical nut probably wouldn't hold the cable because the clearance needed to rotate the nut would make it sloppy. But two half nuts with the proper thread could be clamped onto the cable with a set screw. Perhaps you could even tighten the cable by rotating the two half nuts in their clamping recess - though I believe you'd have to both pull the cable and rotate the nut to overcome friction while fine tuning. I'd design the half nuts to have some kind of flange to allow turning it.
@@mattiasfagerlund Hmm, I'm not sure I follow exactly what you're describing. Are you suggesting having the synchromesh cable be fed through a cylindrical tube that has a sort of internal helical cutout? The synchromesh cable would need to be twisted to feed through, and shouldn't pull back through if exposed to a purely axial load (no torsion)-assuming the friction between the cable and the tube is high enough. If you want to share images etc. may I suggest posting to the Zulip Topic on the matter? I'll post a link as a reply to this comment (in case YT removes it). If that doesn't work, you can find it in "#General > How to terminate synchromesh cable" on Zulip.
Link to Zulip Topic for Terminating Synchromesh cable: pdj.zulipchat.com/#narrow/stream/399279-General/topic/How.20to.20Terminate.20Synchromesh.20Cable.3F
Indeed, if there's no strong reason to keep the motor in-line (or not use of a 90° gearbox), it could make things much simpler. It certainly looks cooler like this though
For the cables pulleys You can absolutely do the compression with 2 pulleys : Use the first pulley to first route the cable throught any point of the desired destination line, and then put a pulley at that point to get the destination line First step is always possible due to a point and a line always being in the same plane, and same for 2 intersecting lines in the second step Good luck with your project
You should look at 1-1.5mm Dyneema rope for your cable, very light, very flexible, very strong, low friction, low elongation/elasticity and easy to splice to create loop or termination
Kevlar would be better as dyneema does exhibit some degree of creep when maintained under tension for long periods. Kevlar is absolutely stable. Also is it possible to arrange it as a continuous loop running back to the capstan (motor) where it would be anchored by a tensioning screw widget. Would you ease the termination clamp problem.
One thing you can do rather than use brake resistors is have a common rail for your motor drivers, so when one motor is generating it can power the other motors on the rails, and this will tie in with regenerative breaking too
In order to utilize regenerative breaking efficiently, you really need multiple servo drives sharing a common power bus. This is a gross over simplification but large automation companies use this strategy regularly to help drastically lower power consumption, heat dissipation, and panel space.
Super glue probably doesn't stick well to TPU. Scotch Weld PR40 adheres very well to TPU. You could also print the TPU sleeves to be concave on the outside so they are self centering
For your linear bearings, a lot of newer industrial bearings use metalized ceramics. Systems that used to be rows of ball bearings on hardened steel rails have become ceramic sleeves which fit snug to the rail. I don't know if that applies to carbon fiber tubing, or much for your project - but a good lool at linear bearings might give you some ideas for how to stabilize your tube. I would even recommend going with some kind of graphite coating on the rods that can act as a dry lubricant. An oil or grease probably won't fit the application well, but can make a world of difference in longevity. Gears that run for decades with oil in them won't last a day without it. A little can go a very long way. You could look at just trying a teflon sleeve or set of pads that form to the wall of the tube. Play around with contact area and pressure to get what you need. The resin binder for the carbon fiber is probably going to be easy to wear. You may want to consider adding or trying to identify tubing made with a harder material. A silicon dioxide coating may improve wear characteristics (or perhaps an alumina spinel - quartz/saphire coat). If you want to go aerospace, you could try a sputtered titanium nitride coating. Not sure how well (or horrible) it would fuse - but if you're looking to go off the wall with it - that would be a direction. For the string, you might actually look into amarid/kevlar. The nomenclature to use is "tow" when looking for strings of fabric material (same with carbon fiber) - I spent days losing my mind looking for a carbon fiber string because I didn't know that was called a "tow". Well - a tow is unbraided, so I suppose not technically a string/line. You might look at fishing lines for off the shelf solutions and even talk to those communities about line routing.
It’s also worth mentioning that some strings are stretchy. Using string with low elasticity may improve precision too. FWIW, strength of ropes (atleast in climbing scenarios) are usually measured in newtons/kilonewtons. The fact that the string was given in Kg (assuming they didn’t do the conversion themselves) is a bit of a red flag. I don’t think the material was mentioned in the video, (fishing line?) but dyneema, if they can find a suitably thin version, seems like it would work well here.
@@techheck3358 The color size and rating make me think kevlar. Spectra, dyneema, dacron and kevlar might all be good choices. You can put them in a woven dacron sleeve and tie a knot in that to get a strong loop on the end without loosing as much strength... Look up trick kite lines and bridals for that info.
@bkuker I agree, though this does change based on what knot you tie. For the string strength testing, I tied figure of eights, which (AFAIK) are fairly good with not severely diminishing rope/string strength. Interestingly the string actually broke in the middle, far away from both knots 🤷♂️ As for the string in the actuators themselves, this is a big reason why I wanted to avoid knots altogether. I believe the system is fairly good with having no "stress concentrations" on the string, and when it does brake (eg. from the motor not slowing down fast enough and the actuator ramming into one of its ends) it seems to be braking in different places each time; some of these breakages being in sections where the string is perfectly straight at all times!
@techhech3358 Interesting point re. stretchyness and precision! I've just been treating these strings as perfectly (axially) stiff, though you do raise a good point that this, of course, isn't true. Perhaps I should test how stretchy these strings are compared to synchromesh cable. I should've mentioned in the video, but these strings are made of kevlar and are usually used for kites. The MBS/WLL is actually quoted in lbs and I see where you're coming from re. a "red flag" though I'm not sure I'd expect kite-hobbyists to care too much about the differences between weight and mass 🤷♂️
@@harrisonlow The strength loss of knots is a factor of the bend radius of the knot, knots like the figure-8 have a pretty big radius; but the way you're mounting the string also has a radius. I do think in this case it's best to avoid knots, so maximizing the radius of the mounting points will help. I can't tell where it failed in your lift test so this might not be the issue now but you could run into it later on. A good strategy is often to coil the rope around a perpendicular rod a few times before securing it. This coil forms a friction knot and will take up a lot of the force before it gets to the mounting point. Maybe add a large radius to the 3D print and then coil around axial to the carbon fiber rod?
*Old Design Issues* - Bearings wore out quickly. - String clamping led to wear. - Assembly required hammering, causing parts to crack. - Not modular; any issue with one actuator affects the entire system. - Bowden tubes kinked and had slop. *New Design Improvements* - Strings internally routed, making it sleek and modular. - Motor contained within the actuator for easy replacement. - Clamping mechanisms simplify assembly. - TPU sleeves minimize wear on bearings. *Performance Metrics* - Endurance: Over 5 hours of continuous runtime. - Speed: Max 3.4 m/s. - Precision: Less than 0.06 mm. - Lifting Force: Almost 7 kg. *Future Improvements* - Use motor body as the spool for the string. - Consider using synchromesh cable. - Replace TPU sleeves with heat shrink. - Modify bottom cap to reduce torsion. *Community Engagement* - Set up a Zulip site for better communication. *Positive Points (Sorted by Importance)* 1. *Modularity:* The new design allows for easy replacement of individual actuators, reducing downtime. 2. *Endurance:* Over 5 hours of continuous runtime, passing 50,000 cycles with minimal wear. 3. *Speed:* Achieved a maximum speed of 3.4 m/s, meeting the design requirements. 4. *Precision:* Less than 0.06 mm, close to the theoretical limit of 8.4 micrometers. 5. *Lifting Force:* Capable of lifting almost 7 kg, exceeding expectations. 6. *Ease of Assembly:* New clamping mechanisms eliminate the need for hammering parts together. 7. *Reduced Wear:* TPU sleeves and internal routing of strings minimize wear and tear. 8. *Community Engagement:* Set up a Zulip site for better communication and sharing of ideas. *Negative Points (Sorted by Importance)* 1. *String Wear:* The string broke during precision testing, indicating potential durability issues. 2. *Torsion Issue:* When fully compressed, the actuator has little resistance against torsion. 3. *TPU Sleeve Problems:* TPU sleeves can slip off and are not perfectly smooth or symmetric. 4. *Complex String Routing:* The design requires complex string routing, which could be simplified. 5. *Braking Limitations:* Difficulty in slowing down the actuators, indicating a need for better braking mechanisms. 6. *Assembly Cracks:* The old design required hammering, leading to cracked parts. 7. *Old Design Wear:* Bearings and strings in the old design wore out quickly, affecting longevity.
@dine9093 tbh even if it were ChatGPT-made, I reckon it's still pretty good! I wouldn't think it'd be so effective just from (presumably) the transcript?
For terminating the synchromesh cable, you could use a metal ring attached to the end of the the rod/the rod stop, and simply thread a small hole for a screw that then clamps on the cable. Im having trouble explaining it, but if you google crossfit jump rope, it's the little screw at the end that I'm talking about. It should be strong enough with simply the clamping pressure, but you could always allign the screw up against the black coiled wire. I hope this helps. Fantastic work btw!
When adding heat inserts, as soon as you pull the iron away, flip the part over and press the face with the insert (and still hot plastic) directly against your workbench for 3-5 seconds. The flat surface of the workbench will keep the small raised ring from forming around the insert as it cools, so you won't need to shave it off manually. It will also help make sure the insert is aligned perpendicular to the face. Alternatively, you can press a small block of copper of aluminum (like a spare heatsink) against the fresh insert to accomplish the same task.
I see a lot of comments about using concave TPU profiles but I don’t think this is a good idea. It sounds intuitive but you’d be over-constraining the system. Also, don’t use heat-shrink! And don’t use Teflon. The key to preventing wear with rolling surfaces is matching the hardnesses as closely as possible. The reason the TPU works well is that you’re better able to control the preload, but the gold standard here is going back to the plain bearings but with a more precise way of adjusting how tight they are. Also this is where the twist in the central carbon fiber rod comes from: slight misalignment in the guiding bearings, in the vertical direction. I was glad to see you ditch the pulley system for measurement. Very cool to see you got such good repeatability. By the way this is most commonly expressed in terms of 3-sigma repeatability, so if your standard deviation was 0.057mm you’d say you have about 180um repeatability. This is extremely good for something like this, well done!
Cheers for the input! Some thoughts: 1. I actually originally had the TPU sleeves being concave instead of convex and they had an insane amount of friction (as I suspect wouldn't come as a surprise for you 😛) 2. There has been a ton of feedback on this bearing block and I'm starting to think the best solution would be to buy some higher-tolerance tubing and just use linear bearings. Would be sleeker, lighter, simpler, etc. (just more expensive 😅) 3. Well that's nice to hear about repeatability! I'll take it 😁 Honestly I would've been happy with < 1mm precision/repeatability, so I'm stoked by the actual performance! Cheers 😊
I once took apart an old photocopier and it had a steel cable moving the photocopier head around, so i think you might have some success with steel cables rather than nylon/other. And it really did surprise me how supple the steel cable was, so there might be some specific type of cable for applications like this.
Interesting application! After the last video I actually picked up a bunch of steel cable and the sizes that are flexible enough weren't that much stronger than the kevlar line (and a lot more expensive) so I ultimately decided to stick with the kevlar. Cheers for the suggestion 😊
The TPU sleeve over the bearing will work way better if you design them to fully engage with the surface area of the carbon fiber tubes. Make them like a circular indent like doing a sphere revolve cut in your cad program where the sphere is the same size as your carbon fiber rods. It will also help hold them centered if you add a bit either side of the bearing on the flat side that helps lock them in, like a U shape that comes down about a millimeter from the outside diameter of the bearing towards the center of the diameter. Hope this helps. 👍👍
There are v groove pulleys which will increase your contact surface area for the bearings. Doing the heat shrink around the surface will increase the grip while allowing some compliance. I would use spectra cord if you can since they are about as strong as you can find and very stiff. For torsional stiffness you would be far better off using a square profile tube rather than round. If you use v groove bearings you could run with 4 bearings, 2 on each corner which should give you plenty of stiffness.
Late to the party, I know, but as this video got started I couldn't help but think he needed either spectra/dyneema or vectran, along w/ delrin bearings...as the thing reminds me very much of an internal cascading outhaul on a sailboat boom I built years ago. Vectran and dyneema, while feeling very different, are both quite 'slippery', but also they both will allow each end to be brummel spliced so that the 'strings' are terminated with eyes on each end.
Two options for cable tensioning: Use a simple turnbuckle so it doesn't twist the cable as you tension. To secure it to the turnbuckle you can loop and crimp it or use a grub screw or a throttle cable style connector. Alternatively look up the Cable Rail Tensioner by Cable Bullet. It seems to be a compact tensioning system that uses a screw to tention and does not twist the cable. The latter option basically uses a crimp on the end of the cable which passes through a hole that has a ring on it to push on the crimp and it is adjusted with a grub screw. You could certainly develop your own system similar.
I'm glad you mentioned this! I couldn't think of a succinct way to include this in the video but that was actually my first approach to this problem! Unfortunately there was an ungodly amount of rolling resistance between the 6 TPU sleeves and the tube. For reference, the 6 TPU sleeves had a concavity that perfectly matched (within 0.3 mm tolerance) the profile of the CF tube
@@harrisonlow If you make a rigid sleeve around the bearing with a semicircular groove, I'm sure you can find an off-the-shelf rubber o-ring that will fit in the groove snugly and provide friction.
@@bobbylox Fantastic idea! Very simple and would be easy to replace parts for. I'll get some O-rings next time I'm at Bunnings and give it a test. Cheers for the idea!
High tensile threads need real care in end termination; if you create a good load sync with a gentle bend radius you'll be able to get the MBS. Try using a boss at each end with a number of "dead" wraps which will distribute the load into the thread. If you use the thread as one continuous wrap you'll only need two ends as well. You can eliminate one bearing by aligning the bearing to the tangent of the drive drum as well. If you put a counter rotation feature in the fixed end you won't have to pay the weight in the moving end as well ;)
Just lying here, thinking that there are a couple of things that i might be able to usefully comment on... a bit off the wall, but i wonder if using two motors, although more complex, might give you control over things like stretch in a string, and providing some kind of dynamic control over the string action and loading... with encoders on the motors, and optical "zeroing" sensors, you get calibration on every cycle... i wonder if some kind of silicone 'oil' impregnating the string might improve lifetime ... indeed, steel cable with a 'dry' lubricant might work well, even on the smallish radius pulleys? I guess that a lubricant could be 'wet', if the actuator 'tube' was enclosed....anyway, sick in bed, just the ramblings of an amateur....
Interesting idea with two motors! I'm not sure it'd be worth the added complexity in my case though 🤔 Silicone lube on the string is a neat idea! Since this video I've done a heap of testing with different strings (video to come eventually...) and have found dyneema to be really good; high strength, low wear over repeated cycles around small pulleys, low stretch etc. Cheers for the input, and I hope you get better soon!
I think you should consider using Dyneema/Spectra for your ropes. It is made from high strength polyethylene fibers, and it's very resistant to wear (and thus quite hard to cut, even with a proper knife). It's used a lot in sailing and fishing, and I've seen some only 1mm in diameter, which I guess should be thin enough for your application, claiming several hundred kilos of working load.
Interesting project from engineering perspective. Endurance during 10x thousands of movement cycles seems the relevant criteria … and the weakest part might be these "strings" :-) Indeed we use dyneema/spectra on performance racing yachts, easily with very dynamic loads on main sheets and standing rigging between 5 tons and 20 tons (on boats of 16-18 meters), and much higher loads on bigger superyachts ... the rope producers also offer "pre-stretched" materials, so there is no need to worry of losing tensioning. :-) good luck for all projects coming along !!
Terminating the synchormesh cable can be done with a guitar string winder. (The part of the guitar that turns the string to adjust the tuning). This also gives a nice way to adjust the tension. I'm wondering why the motor isn't just turned 90 degrees instead of having the strings go through pulleys. I think regenerative breaking would be ideal and would allow your juggler to run on batteries for shows where power isn't available.
Or make a long tin hole to thread the synchromesh cable through and then have a 2nd perpendicular hole through which a screw is tightened down to impinge upon and lock the cable in place in the hole.
@chrisBruner Addressing your points in order: 1 - Good idea! I don't play guitar and this idea hadn't come to mind. I'll do some research into how these work and I'll see if I can make a similar mechanism at the small scale/weight that I'd need. Cheers! 2 - The main reason I didn't set the motor perpendicular to the extension axis is because I don't want the motor to be sticking out the side of the actuator. I have a gut sense that this would likely cause issues down the line once I put all 6 actuators together into a Stewart platform so I want to keep everything as "neat" and in-line as possible. I think using the motor body as the spool would still work, though, because I'd be able to keep the centre of mass of the motor approximately coincident with the extension axis. 3 - I agree. I love the idea of Jugglebot running off batteries and being fully portable, though I have precisely 0 experience with these and don't want to find myself in the position of having to pause work during troubleshooting in order to charge batteries (not to mention any physical risks/dangers of high-power batteries)
@Superkuh2 I agree with techheck here - the synchromesh cable needs a fairly clear path to be pulled through and I think a convoluted tensioning path would be tricky to make fine adjustments to. FWIW one of my patrons had a few great suggestions that I've written up on Zulip. I'll post the link as a reply to this comment in case RUclips takes a disliking to the external link. (The thread can be found in General > How to Terminate Synchromesh Cable) Cheers for the suggestion!
Awesome design, I am working on a project that needs a linear actuator and nothing I have found online will suit my needs. I cannot believe it never occurred to me that I could make my own, thank you for inspiring me.
Two things you didn't consider when you thought of using rotor as spool: it'll have way worse precision and way less torque, by a factor of 3 it would seem like.
I think the precision would be if anticogging were set up, but after doing that for one motor + ODrive I'm not too keen on to do it for all of the motors. It's a very finnicky process that occasionally just doesn't work 🤷♂ Good point re. torque! Cheers for the input 😊
To improve strength, you could have 3 pairs of strings. You also have to be careful when considering cord strength. When you're pulling the bag up and down by hand, the jerk you're applying is definitely high enough that it could reasonably be producing tensile loading in excess of the rating. The literal weight/mass rating doesn't matter, you need to be thinking in terms of force, which will make it more obvious how jerk and acceleration can create very high peak forces. If you improve the motion controller to enforce jerk limits, it should improve the performance of any cord or cable, and also reduce shock forces on the machine overall. This kind of thing is being used in 3D printers to improve print quality, reduce ringing, and go faster with fewer consequences.
Haha I was wondering if anyone would call me out on my sketchy testing of the string strength 😂 I agree that my "jiggling" of the bag would increase the force in the line, but it felt like my jiggles were fairly small and (I would think) shouldn't increase the force on the line by a factor of 4 (MBS/bag weight). As for the motion controller, I actually don't know if it's possible to set jerk limits with the ODrives. I'm currently planning to use trapezoidal trajectory planning (with velocity being the trapezoidally-controlled parameter) which does a pretty good job of smoothly moving between setpoints. It's yet to be seen how well this does when the setpoint is being updated at eg. 100 Hz... Cheers for the input!
@@harrisonlowAt work we have a garage door, and you pull it shut with a cord. When I started this was generic 90lb nylon diamond weave. It's not hard to close but the way the guys pull on it, not even really a yank, was causing the rope to fail every month or two, it would just rip in half mostly at mid-span. I'm the knot person, boss asked me to fix it, I just did mostly the same thing but using 550lb rated paracord, and a knot that's better at distributing the load. It's been maybe 8 months, no problem even with spending hours a day in direct sun. It's just that easy to generate high shock loads by accident. I assume your cord, due to being smaller and not diamond weave with a useless fluff core, has a shorter elastic range. I'm not sure what cord you've got, but in the vein of paracord there's "1.18mm Micro Cord", a lot of paracord dealers have it. Supposedly it's "rated" for 45kg, but they're not clear about what kind of rating that is. It's some kind of weave, which may widen the elastic range and make it a little more forgiving, but whose to say how the overall stretch will differ. Honestly the other commentors seem to have your synchromesh cord solved, I've never seen that stuff before, it's cool. Edit: one more thing: all paracord melts, any cord you can fuse the end on can melt, they can melt from rubbing and from loading. Once they start melting the mechanical properties are out the window. You won't be able to feel this heating, the cord is just too insubstantial to heat your finger. The paracord family could easily just be useless to you.
@@Ziraya0 Haha I love that your work has a "knot person" 😂 What do you do for work? Cheers for that writeup! Useful thought experiments re. woven vs braided thread and elastic range. I haven't really paid too much attention to the structure of the thread before! (FWIW, the kevlar string I'm using is made from two twisted threads) As for melting: I mentioned in the video that I accidentally routed the prototype actuator incorrectly. This routing had the string going straight over a printed part at a rather aggressive angle. Cut straight through it! For better or worse, the string is far more robust than the printed parts so they tend to fail first.
@@harrisonlow You probably have enough space in this design to go up one or two sizes in kevlar line literally without changing anything else. The line I'm seeing isn't two threads, but it is 0.8mm, 45kg, safe working load limit 9kg. 90kg/18kg line is only 1.1mm, and 136kg/27kg is 1.5mm. If the synchromesh gets too complicated, this seller goes up to 476kg 2.6mm cord; at that point it's mainly a question of how big you may need to make the spools and how that'll impact the torque across the range of motion.
@@Ziraya0drop Kevlar for Dyneama thread. Size AA/30 thread is 0.28mm, holds 40+ pounds easily. It also stretches less than Kevlar. Thread is easier to track down, and that's all kite fancy kite string is.
Regarding the Pulliies.. you could turn the green "middle - direction" pullie such that its output guides the string on the tangent of the main pullie.. this would result in an really odd angle but would reduce the number of pullies. But I think three is much simpler, as you don't have to "think" about this odd rotation.. :)
@@harrisonlowyes any two lines in 3d space can be connected with two pulleys. the way to think about this is choose any two random points, one on each line, and then connect a third line between those points. then you place pulleys at each of the points, tangent to one of the original lines and the newly constructed third line. hope that makes sense
you should be able to hold the synchromesh cable by having two small metal plates that clamp together with two bolts, when you tighten the bolts it will crush the cable in between the plates and hold onto it real tight, I am taking inspiration from how electric cables are held into some fixtures, its often molded into the plastic in consumer electronics so that when the chassis of whatever is screwed together the two mating surfaces pinch the cable to act as strain relief on where the cable is attached inside.
If you need to dump break power for a very short time with significantly longer time between breaks, you could use supercapacitors, as they will essentially be shorts when they're empty
In terms of the slowing down, you may want to look into powered steering, a technique called drive by wire. It's essentually adds pressure from one motor that is generated from another to create a force. One motor controls the other. You also have to remember moving mass is heaver mass in terms of the string. Terminating the line you can drill a hole in a screw and feed it though and tighten, or use a couple of washers.
This drilling-a-hole-through-a-screw idea keeps coming up! I'm very surprised as I'd never heard of it before. Have you used this termination method before? What for?
I'm liking your design! Here's a couple ideas for ya. Change from the string to the cable as you mentioned. Set the cable tension with a adjustable pully. Put the first pully from your motor to be on a slot, and a screw on either side to adjust it to set the tension. Oh and the tension should be just tight enough, so when you pluck it you get a thump and not a strum like a guitar. Then the ends of the cable could be securely mounted, perhaps looped over a rod you have ran thru your carbon tube, then crimped, just like the common method for larger cables.
For the slowing down of the actuator, you can put a coil on the ends of the main frame and have magnets attached at the ends of sliding rod. By using hall sensors you can determine the proximity of the magnets and switch on the coils accordingly and hence decreasing the speed effectively. This can moreover be used in a way that woks like a coil gun and help accelerate the moving arm after a full stop.
Here's an idea for terminating the syncromesh cables: route the cable around an eyelet and crimp the end onto the cable. That is a cable terminal that will not break (the cable itself will break before the terminal does). Then you can use a hook screw and a hex but to tighten the cable as needed.
Regarding all the bearings, but more specifically the end cap alignment thing, I once took apart an HP inkjet and found that they use felt bearings. That's right! It was a piece of felt with a hole a bit smaller than the rod so it held firmly. there were metal rings on both side of the felt centering it around the polished linear rod. This helped keep the mass down, cost down and noise down. What I'm suggesting is to create triangular end cap with all 3 holes to prevent twist. It would be 3d printed and hold 3 pieces of felt-washers. You would need thick felt (1mm or more) and could mass produce them if you created 2 hand made punches. Take a metal tube that is slightly smaller than the carbon fiber tube and sharpen the inside edge of it. Then hammer down on the tube against the felt to create an inside punch. Do the same for the outside punch. Make tons of them and redesign the end cap to encapsulate 3 felt washers and all 3 rods. You can get rid of the wheels and bearings this way and solve for that odd twist. Make the end cap organically shaped to reduce weight (like a semi-collapsed triangle, don't know how to describe it). I'm unsure if this suggestion would also work for the primary alignment bearing block but try it if the end cap proves useful. You would essentially create a new block with many felt washers to center and hold firmly the center rod. It would be cool if you could drop most of the bearings to reduce cost and rotational inertia and wear on the shaft! Good luck. BTW, I couldn't create an account nor use the Google signup... Anyway... On that previous video I was the one to recommend the synchromesh wire. Thank you for covering that in your video I feel honored and excited to continue to help!
To terminate the syncromesh cable, use a set-screw inside a metal screw insert. They're used a lot in woodworking. Drill a hole into the insert near the bottom, then feed the wire through, screw I to the insert. For the wrapping around the motor housing, that would decrease the speed, but would massively increase the strength I think. This is amazing progress. Not sure what you're planning on using this for, but it's really cool!
Interesting! I imagine you'd have to use fairly large inserts to do what you're describing? To check that I'm understanding you correctly, the wire would be "pressed into" by the bolt, but would it have anything touching it on the other side? Or does the strength of this idea come from the wire being bent by the bolt, disallowing it from being pulled back through? Cheers!
You can eliminate one pulley on each line, rotating the first one 90° and placing it on the right place. I'd also swap the strings on the spools for better clearance, probably. For the strings, I don't know what you are using, but 1.5mm or 1.7mm dynemma could work, or something similar in Kevlar if temperature or wear is a problem. Friction is quite low with dynemma, have an eye out for creep, there are some variants with lower creep that could help to facilitate the tensioning. If you add 4 more bearings to the moving block you could spare 3 from the stationary block. Using the motor as a spool seem problematic to route the line, unless you spread the bars, and you need them parallel so seems like a packaging hassle. Also, you remove flexibility as changing the spool size gives freedom in the velocity/force compromise. Lastly, adding regenerative breaking should be really good, you should be able to find a motor driver that could handle that. More advanced drivers can also help with the different drivers working together, I don't know how the premature disassembly got to be as 6 degrees of freedom with 6 drivers should allow for small errors in actuator position to translate to small errors in platform position, not unscheduled disassembly, but start slower next time and maybe incorporate some compliance in the connections until you dial the control setup.
I think you can have fewer than 3 pulleys per side if you offset the spool insted of constraining it to be coaxial with the moving rod. The minimum might be with the spool tilted to be perpendicular to the rod. Rock west prefabs hex section carbon rod which might be useful for improving the torsion issue.
If mounting flex leading to the string generating derailing forces can reliably be ignored, then absolutely he can use just 2 pulleys per string with the design shown. If derailing forces can't be ignored, then using pulleys with a "taller lip" (to retain the string better) may be enough to compensate. It's probably not worth focussing on right now, but definitely something worth considering on the maybe-last version. I can't give any formulas for how to do the calculations off the top of my head, but the concept is fairly straightforward: 1) Each pulley corresponds to a plane, which it's embedded in (and parallel with); one of the sections of string (e.g. "from the spool" or "to the attachment point") must also be embedded and parallel with this plane, while the other section won't be; 2) The task is to intersect these planes with each other in such a way that: 2a) the intersection is tangent to both pulleys, AND 2b) the section of the intersection between the two pulleys doesn't pass through anything else (e.g. the spool), AND 2c) the plane with the string section that passes to the spool should be as close to perfectly tangential to the spool as possible (to minimize the risk of derailings).
Great work, especially with iterations. One thing that can't go un-noticed is the cyclic testing. With no load, almost every actuator outperforms their intended application. It's the testing of cycles under different loads and disturbances that can really speak volumes for a design. Loved the video. I wonder if you shouldn't consider extruded aluminum hexagonal cross section, as the part in motion. Would make for better load distributions. You could also add steel wear surfaces if you figured a good way to bond thin shim stock to aluminum.
I agree, cycle testing under load is going to define how well the design will hold up. The forces are going increase dramatically depending on how fast you accelerate and decelerate.
If you still dont know how to terminate the syncro mesh cable you can crimp electral wire terminals onto the end. Youd want to do some strength testing for your needs, but in my experience, it works great for a mounting point.
On the TPU sleeves, you should make them concave instead of convex. This will allow the bearings to have more positive contact with the carbon fiber sleeves. This will make the system more rigid.
I actually originally had the TPU sleeves being concave instead of convex and they had an insane amount of friction (though they were a very good fit for the CF tube). Someone also recently had a great thought about this on Zulip: "The problem with sleeves that follow the CF tube is that you will have contact points with the tube that are at different radii from the axis of the bearing." Very clever way to think about it!
Definately use the dual wheel design on all 3 outter shafts to stop torsional forces. And for braking you can use another mini actuator to press a cylindrical bearing down against the wheels in the bearing block and the harder it presses the more it brakes the bearing and an electrical actuator is very percise and fast so with the right algorithms it can precisely brake the movement.
for the sincro-mesh cable put a helical cut into the 3dprint to match the synchromesh cable and a tab to hold the cable into the groves (tab could be grooved too but not required)
For the precision/accuracy: not sure about the details of your design, but when your motor spins in either direction, the string is looping around it, effectively changing the diameter of the Motor's pulley. Making it bigger or smaller will dynamically change your "degrees per mm" conversion! If, initially you calculate 1 rotation = 1 mm, at the end stroke it would probably be more than that. For the central fiber twist: My main guess would be the alignment of your pullies. If they are not aligned properly, the efforts in your strings mechanism will not be symmetrical enough (with this system it cannot be symmetrical) and I suppose that after numerous passes, combined with vibration and dynamic load, some screws started to losen themselves ?
Loving seeing this develop,thanks for sharing. Couple of quick thoughts... 1. Could you use concentric tubes, so one large and one small instead of four small? Also, can you get carbon fibre tubes in square section, which will help with the twist?
Cheers! I briefly toyed with the idea of using one large CF tube for the support structure (rather than three small ones) but I wasn't a huge fan of it for two main reasons: - Large CF tubes are super expensive - I don't like how opaque they are; I like being able to see the "inner workings" of the actuator. I've come to find that having an "open" design is also super helpful for fixing issues as they arise The square-section tubing is a good idea! I wonder how that would affect the geometry of the upper "bearing block"? I'll have a think about this. Thanks for the thoughts!
I don't know if you are having differential tension problems between the two stings, but if you are there is a way to use a single loop to accomplish the same thing. I don't know what it is, but airplane ailerons are set up this way. Should be easy to find.
You could reduce the number of pulleys by putting the motor axis at a right angle to the axis of the actuator. If you make the drive spool the right diameter and offset it a little (so one side is in the center and the other side is in line with the side tube) you would only need the one pulley at the far end of the 'blue string.'
Lock the synchromesh cable through three screws running in a line (more if needed), where each screw is tightened down in sequence with the cable at an angle. The cable is mostly straight, running through the screws. Spacing of the screws can accomodate the bend radius of the cable.
1) Why are the TPU "tires" rounded (positive curvature) when they are contacting a round shaft? They could just as easily be flat or even negative curvature. 2) How are you controlling tension of the cables? Could one of the redirecting pulleys be adjustable to allow tightening the loop after fixing the ends? This would also allow adjusting if there is any creep in the line, allowing tension tuning like a guitar 3) As an actuator it would also be interesting to see the stiffness in addition to the failure strength. How much would a small load on the end effector change it's position?
1 - I actually originally thought the same thing and had the TPU sleeves being concave instead of convex and they had an insane amount of friction (though they were a very good fit for the CF tube). I figured reversing that would flip the problem and minimize friction 2 - I'm doing this by keeping the string tight while I screw in the clamping bolt. I really like your idea of an adjustable pulley. The one at the end of the actuator is in the perfect spot for this and that mechanism probably wouldn't be too complicated to design. I'll have a think about this! 3 - I agree! I reckon there are a ton of aspects to this design that'd be interesting to explore (another being max power output), though unfortunately I have nothing close to a good lab setup and I think (for now, at least) my time is better spent moving forward Cheers for the (neatly formatted) suggestions!!
I think you could take the first bearing out of the blue string set if you align the central rotational plane of the second bearing tangential to the spool
Hey, Cool project, just discovered your channel. Couple of thoughts, the line you use is quite rough which could potentially contribute to the precision issues you're facing. There's a product using a dimensionally controlled kevlar cable, it's a bit thick for your application perhaps (2mm), but a lot more round. For the TPU and bearing sleeves, UHMWPE could also be an option, also thin silicone sleeves with a high shore value could. Last thing regarding the precision, I haven't seen any tensioners for the strings and how do you control the stack up of the string in the spool ?
Hello. Harrison. I'm writing because I think I can help you a little, with an idea. You talked about being able to tighten and change your strings on your Linear Actuators. when you tune the strings on a guitar, you turn a small handle. very simple mechanical structure. What might not be so good about it is the weight. But it can certainly be made very light in weight. Thanks for a great channel. Finn
Great project! What about dynema as the string? Very low stretch, very high strength. Standard braid so making sliced loops and terminations is easy. Also, compression cones for attaching a possibility.
I've actually recently been doing a TON of testing of different string materials and dyneema is looking quite good! What do you mean by compression cones? I'm not familiar with that term
You should be able to get the pulley count down to 2 per string: * for the extension string, just rotate the existing bottom-of-tube pulley around the tube's axis until the input string is tangent to the drive pulley, and remove the other motor-side pulley. * for the retraction string, chose a random point "A" in the plane of the drive pulley ~3cm away, draw a line from "A" tangent to the drive pulley in "B"; then set a point "C" on the central axis ~1cm above the pulley; the string should follow the path "BA" "AC" then up the central axis from "C". The 2 pulleys are semicircles at "A" and "C", tangent to "BA" and "AC", and "AC" "central axis" (in CAD, create a plane from each pair of lines and just add the circle) [for clearance, you might need to swap the extension and retraction drums]
A couple ideas come to mind. To terminate the twisted cable try a double plate clamp, 2 screws with a bump in between should do. The pulley "shoes" as I would call them, having them rounded may decrease their friction coefficient however making them concaved instead of rounded would reduced their odds of slipping off and also serve as an self-aligning guide sort of speak. I wouldn't make it exactly the same as your center shaft, maybe 1/2 a mm wider then the shaft on each side. effectively giving you a maximum of 1mm tolerance and ever so slightly reducing the friction vs a matched curve to shaft. really like what you did so far. hope my advice helps. feel free to message me if I wasn't clear and you want clarification. I can fab something up easily if needed
I have in Thingiverse a design for Customizable wheel for 608 bearing, that locks in place and won't slip off. I've been playing with printing the middle part of the wheel in spiral vase mode to get rid of any small bumps (printing artifacts), but I have problems with the round wheel profile getting distorted. Your application does not need a round profile and thus no variable extrusion width, so I think it would be more straightforward to get printed in vase spiral more. Some manual G-code editing would be needed to switch between spiral vase mode and normal printing..
You could consider using a very large super capacitor as a "breaking load" for the ESC. If you dump into the capacitor and then pull power back out, you should end up with a net loss in power usage, thus the batteries will be at net discharge without worrying about chemistry issues.
Hi @harrisonlow - great work! I wanted to point your attention to a type of string that has much higher strength called "spectra" or "dyneema". used most commonly in sailing, paragliding and kitesurfing, such strings can support 100s of kilograms in rather thin gauges. The fibers are also cut-resistant but you might have a heat problem with them and might consider lubrication to control that.
Thanks for the suggestion! I've actually since swapped to dyneema (0.8 mm) and I love it! 😍 Super strong, really thin and flexible and barely any creep. I actually have a video in the works on my testing of a bunch of strings 😁
Regenerative breaking can be done with a 3 phase rectifier and some way to either store the energy or dissipate the energy. A good place to start is looking up "delta alternator diode layout". Delta alternators have nearly identical wiring to most brush-less motors and you can use the same type of rectifier layout because of this. The trick however is using PNP transistors instead of diodes. By using PNP transistors, you can monitor the motor driver's voltages at the transistor's base and limit the current of the rectifier when the motor driver is engaged. Keep going on the project, very interested to see where this goes!
Thankfully the ODrive is capable of handling the electrical stuff for regen braking, but the problem is that my electrical knowledge is subpar right now so I need to learn/figure out what batteries or brake resistors to use. I've tried the naive "increase brake resistor power, keeping nominal resistance the same" but that didn't work for some reason 🤷♂
Very good improvements! I am looking to make some actuators for a very different application, but endurance is important to me, keeping cost in mind. I'd very very keen on seeing you implement your planned improvements.
brake resistors are really job specific. Got to do the calculation for the brake power. This is the weight of the load, time to stop and a few other things. I have always went with what the manufacturer recommends. Industrial Allen Bradley (Rockwell) will have a chart/calculator to tell you what size drive and brake you need. This also keeps the parameter settings straight forward by using all the same system. Also, if you start getting into higher end drive systems some of them are modular. Allowing them to be all hooked together and then controlled through tcp or other standard protocol.
@110:30. For the bearing surfaces, can you use an o-ring of something like teflon or viton? The viton would even give some resistance to tube rotation.
Can it be done with less than 3 pulleys? Referring to the green string, my thought is to have the first pulley be slightly tilted and the correct diameter to stretch the distance between the motor pulley edge and the upper central pulley edge, so that the intermediate pulley essentially does the job of both pulleys you have right now, but it keeps the string in line coming off of the motor pulley and the upper pulley.
Just a quick idea on terminating that funky syncro cable, how about a copper and/or brass compression lug used in wiring electrical service, but with a plastic sleeve such that you're not directly compressing on the syncro-cable
If you get the angle correct on the spool pulleys, you only need 1 coming off the pulley instead of two. The strings will wrap more than 180* around the pulley though
I’d be curious what the accuracy of this (or the next iteration) would be with a bunch of consecutive movements that are all in the same direction, like 5 random points chosen to be consecutively descending or ascending, and measuring primarily the relative deltas that are then achieved _between_ the points, and then comparing that to the requested point deltas. I would not be surprised if your consecutive deltas end up showing far higher precision when moving repeatedly in the same direction without the driving string having to change. If this is the case it could be a good indication that some undesirable inaccuracy is coming from the slack transferring between the two strings, and you might be able to design movement paths around that (ie. preferring multiple consecutive movements in the same direction if at all possible)
For the synchromesh cable look at how bike brake and derailleur cables are clamped, its still a bolt, but with a grooved washer that holds it against the part.
You can do 2 pulleys per string. Remove the middle one, rotate the closest one to the motor 45° clockwise with the axis of the rope going to he motor. Rotate the other pulley 45° conterclockwise about the vertical axis. For BLDC motors, when the H bridged are configured where the coils are energyzed similarly to a pwm where 0 speed is 50% +V and 50% -V, it gives a good speed control because the shortest pulse polarity acts like a regenerative brake by itself. Apparently its called 6 step commutation and it required a bipolar wirering.
Yep, I think you're right about the two pulley idea! Good thinking! Re. BLDC motors: I'm not sure I want to deviate from using the ODrive controllers that I'm currently using; I trust the makers of the ODrive to be *far* more competent as designing these circuits than I'd be. It's interesting to know that this sort of configuration is possible, though! Cheers 😊
Why not flip the motor perpendicular to the direction of the linear actuator and have the body be the spool as you mentioned and then have much more simple routing of the string direction to the rod?
Yeah I've realised that I didn't do the motor-as-spool idea justice in the video. The viewer's suggestion was to have the motor sideways so that most of the routing would be a non-issue anymore. As for why I didn't do this: I didn't think of it! (the motor-as-spool part, at least.) I did consider the perpendicular motor idea, but decided against it because I don't want the motor to be sticking out the side of the actuator. I have a gut sense that this would likely cause issues down the line once I put all 6 actuators together into a Stewart platform so I want to keep everything as "neat" and in-line as possible. I think using the motor body as the spool would still work, though, because I'd be able to keep the centre of mass of the motor approximately coincident with the extension axis.
To hold the synchromesh, take a bolt and drill hole in it large enough for the synchromesh to fit through, thread on one jam-nut so it goes below the hole, insert synchromesh, thread on 2nd jam-nut and then use the 2 to clamp the synchromesh in place. Could be changed in seconds and wouldn't require any design changes and parts could be made in minutes.
I've had to sand flexible filament before it will bond with other materials. Also, you could try a concave profile for the bearing sleeves, that might give it more surface contact with the rods vs the current round profile. Def see the rolling brace being more stable when against all 3 rods. Look at how roller coasters work, you should be locked in place with just 2 bearings per rod if all are angled for opposing forces. Congrats on the progress! Looking great
Very interesting point re. sanding. I'll try that out! I actually originally had the TPU sleeves being concave instead of convex and they had an insane amount of friction (though they were a very good fit for the CF tube) I agree that four bearings are all that should be necessary, but I can't think of a good way to position them so that they're robust against forces in all directions! Cheers 😊
@@harrisonlow you'll also want to sand the metal surface of the bearings where the TPU bonds - I use rubbing alcohol to clean both sides before gluing. I'd start w/ equal angles on all rods - 2 bearings per rod, on opposite sides of the rod (angled more than 90 degrees to each other) so the the tension is pushing out, away from the center equally. Provided the sleeves don't make contact w/ each other, they could be all aligned - so a ring of 6 bearings in a sort of star pattern. If you test the TPU sleeves w/ dif infills, you might find the right density to keep tension w/ out too much friction
Consider nylon for instead of TPU for the bearing sleeves, it will make it more rigid / stiff / precise. You might also consider engaging more of the small linear rods with bearings.
For your cable clamp, you'll need 2 metal plates with a hole at each end (10x5mm, with M3 holes should suffice) - fix the plates with screws at each end and clamp the cable between the plates. Might work better if you set a shallow channel in it to keep the cable from moving about. If you can't make the plates, plates from a bicycle chain would work.
To get rid of the torsion along the central CF tube, instead of using six bearings and increasing the cost. Maybe consider using bushings around all three outer tubes. My thinking was PTFE but, another material could perform better
What about attaching a small spring to a small rubber o-ring then attach string to the o-ring available in those plumbing kits to fix a leaky tap. Might be smoother and not require that spiral string. Would be interesting to see which lasts longer. Just a thought...
I suggest you to put the bearings in different orientations, 3 to 3 with some angle (I sopose 60 degrees in between) to get more points of contact with the bars and to prevent them of moving around its axes. Great design, thank you!
It may cover the rolling bearing in a concave shape instead convex shape. That will make it more surface to contact. For the motor braking, you may use a big (small super) capacitor. (store the energy instead of dissipating)
8:42 For measuring precision, I bet you could mount the actuator to a pivoting mirror and reflect a laser off the mirror. Then, depending on how the (measurable) laser point moves with your (incredibly small) movement of the actuator, you can use trigonometry to determine your actuator's precision.
No clue if this will work or not but have you tried to do a compression clamping mechanism like you did with the carbon rods? You could make it a passthrough one so you could adjust the tension as needed.
🚀 Hey Jugglebot Enthusiasts! 🤖
I'm thrilled to finally share this update of the actuator journey with all of you! Your insights, curiosity, and passion have been pivotal in this progression.
If you were designing this actuator, what would YOU do differently? Or better yet, if you could see one feature or capability added to these actuators, what would it be? 🧐
Don't forget, if you want to learn more about the project, or to lend a helping hand, check out our Zulip site, here: pdj.zulipchat.com/
Please don't add music. I really wanted to see how this was made but just couldn't with the music.
@@MichaelMantion Hmm interesting. I'm in two minds about this. Do you think just making the music a bit quieter would be better? Or would you prefer just none at all?
Would love to make a design for you but i just cant do things for free anymore
Love the series and the focus on the process of basing the design in proper design requirements. Inspiring thank you, clearly you are on a great path with the project 👊👊👍
Cheers Martin!
For the bearings, I would suggest a delrin or Teflon roller. You can get a pack of these easily, marketed as 3d printer rollers. They would stay on the bearings better, resist abrasion, and have a self-lubricating effect with the tubes. For the synchromesh cable, you may be able to design a clamping plate similar to those used for bike cables.
Very good suggestions, cheers!
I think the bike clamping plate will probably be the final solution for this problem; super simple, small and light. Great thinking!
The wheels used by 3D printers to roll on 2020 could also be used. The wheels are really cheap. There are also different shapes. Maybe a concave shape is better for rolling on the carbon fiber. You can buy them with a groove. Or if you can control the spinning modify them with a static spinning drill bit and slowly turning the wheel.
Why not linear bearings? Recirculating ball type used by many machine tool makers for linear movement. Robust, accurate, hard wearing and cheapish.
@@seeker1015 Weight to stiffness ratio is worse (if you include the rails) than carbon fiber. More weight means: requires bigger motors.
Some of these have a Delrin "v" groove profile that would give you 2 points of contact per bearing on your carbon tube. This is the first thing that jumped to mind when I saw your printed sleeves....I would imagine that this would keep things located a little more positively (at least, short of a concave profile to match the tube radius) . You might be able to use 3 of the same on the captured end of the moving tube in place of the 2 guide rollers you currently have riding one of the outside tubes...basically the same thing you have holding that moving tube inside your printed end, but flipped inside out. That would both keep it centered AND prevent axial rotation given the 2 contact points inside the V. Anyway super interesting project, and very promising.
Is it just me.... or every time I watch video of this guy I say: "I really like this guy". Awesome project! very good idea and straight to the point!
Cheers for the feedback 😊
Every time I watch this guy I say, "I hate your perfect jawline and chin."
Super cool project! Immediately got a couple ideas as well.
Sorry if these are already named, but here we go:
- Route the extend and retract spring via the same wire, so kind of get a capstan drive.
- Make the sleeves for the inner bearings slightly concave. to make them self align to the rod? Maybe also make a groove in the middle of them to make a channel for the wire, to turn that issue into a feature. It might remove an extra bearing for routing the wire.
- Make the moving bearings three sets at a 90deg angle, to get a kind of rollercoaster setup.
- Add a fan to the output shaft of the motor, that spins a cooling fan over the brake resistor.
Well now that that's out in the world so I don't think about it again, good luck!
It's been a treat watching this be developed! Some suggestions for future testing:
- Conduct endurance testing while the actuator is (reasonably) loaded, to better match end-use conditions
- Implement multiple load cases into the motion accuracy test (and record deviations in both absolute and relative positions with each move). Under a single constant load (or no load) the stiffness largely "disappears" since the parts' deflections under stress are not really changing. In the final application, the load will be constantly varying and this may have a large effect.
It's a great design. You should also measure the force you can apply in extension - the opposite of your weight lifting test.
Cheers! I did consider that but I couldn't think of a good way to mount the actuator to support a load when upright... I imagine it'd be fairly similarly strong in both directions, though?
@@harrisonlowyou could use string and pulley so that it pushes the string downwards and the string pulls some load. But youll have to have at least two strings so that side forces cancel out and dont deflect towards the string
@@TrueHolarctic That'd definitely work and I think I considered something like that but quickly abandoned it after thinking of the direct lifting test that I showed in the video. I'm not super concerned about strength and I would expect the two directions to be similarly strong so I didn't bother with testing the other direction 😛
@@harrisonlow an extension strength test would test buckling characteristics.
@@harrisonlowput it on a scale, Like weight scale?
Hello stumbled into the video and the topic pulled me in. Retired Engineer in Medical, Automation and Robotics Systems. You do a great job of scientific thinking and setting goals. We used a Small Stewart Platform to shake test medical Data collectors in Scanners. It was amazing as we rolled through the frequency scale how bits would actually start to vibrate so much we could only see a blur where the part was. We added braces and stiffeners and in the end it improved our end result significantly. The advantage of having access to Aerospace folks to help. We also used Carbon Fiber for patient support to keep them nice and still. Later i used Carbon Fiber to build robotic arms, Large ones for Defense manufacturing, we added active and passive vibration dampening to get long arms with heavy grippers to move fast and stop without rebound. I appreciate what you doing and how you are doing it but still unclear what gizmo is doing. oh and this work i mention was done in the late 70s and early 80s. Maybe you were born after that time. Keep it going troop you are educating as well as fixing your machine. We had to know position as well so we added a rotary counter on the motor to feed a location calculator. Have fun and keep up the good work. Dennis
another thought on the center Round Shaft. I used a square Shaft on one device, machined two corners opposite each other and a three V shaped rollers to capture the shaft and maintain alignment with lowest drag. Those were all metal. I was surprised to see you running metal bearings on six sides of resin cured Carbon Fiber tube. The resin will require some compliance as you have learned. We found Delrin worked well it is machinable and robust with some compliance to prevent damage to the resin surface of the composite tubes. D
Regarding terminating synchro mesh cables: You could probably create a part with an internal "thread" that matches the cable. Perhaps create a two/three piece part that can be pushed onto the cable and match its shape. As the pieces are forced together with the cable between, they'd pinch the cable and hold it tight. Then you pinch that part in a collared holder that prevents it from slipping out and provide pressure to keep the cable captured. Very hard to explain, but perhaps you get the idea. Not sure if it'd stand up to the tension, but it probably could. Let me know if you want me to elaborate and produce diagrams!
I was just about to post a comment like this. My Prusa 3D printer does a similar thing for terminating the axis belts. You push the belt sideways into a cutout that matches the profile of the belt, and it engages several of the teeth to keep it in place.
Interesting! I think I get what you mean and I wonder if I could use this for the spool end of the cable? Something like: a conical part with a cutout in the side that the cable slips into. The conical part then fits into a similar conical cutout in the spool. As the cable is pulled tight, the first part (the one the cable is actually touching) is pulled further into the conical cutout, compressing it and strengthening its grasp on the cable.
Is that along the lines of what you were thinking? I like the idea, and I imagine it'd be fairly straightforward to turn this into a mechanism that would allow for quite fine adjustments to be made to the tension.
Cheers for the idea!
@@harrisonlow I'm sure there are many ways - I'm thinking of a cylindrical "nut" where the thread of the nut fits the cable. A typical nut probably wouldn't hold the cable because the clearance needed to rotate the nut would make it sloppy. But two half nuts with the proper thread could be clamped onto the cable with a set screw.
Perhaps you could even tighten the cable by rotating the two half nuts in their clamping recess - though I believe you'd have to both pull the cable and rotate the nut to overcome friction while fine tuning. I'd design the half nuts to have some kind of flange to allow turning it.
@@mattiasfagerlund Hmm, I'm not sure I follow exactly what you're describing. Are you suggesting having the synchromesh cable be fed through a cylindrical tube that has a sort of internal helical cutout? The synchromesh cable would need to be twisted to feed through, and shouldn't pull back through if exposed to a purely axial load (no torsion)-assuming the friction between the cable and the tube is high enough.
If you want to share images etc. may I suggest posting to the Zulip Topic on the matter? I'll post a link as a reply to this comment (in case YT removes it). If that doesn't work, you can find it in "#General > How to terminate synchromesh cable" on Zulip.
Link to Zulip Topic for Terminating Synchromesh cable:
pdj.zulipchat.com/#narrow/stream/399279-General/topic/How.20to.20Terminate.20Synchromesh.20Cable.3F
I love the cable routing. Flipping the motor 90 deg may be better and less complicated, but I love complicated mechanisms. It just looks awesome
Sort of an aesthetically pleasing Heath Robinson affair.
Indeed, if there's no strong reason to keep the motor in-line (or not use of a 90° gearbox), it could make things much simpler.
It certainly looks cooler like this though
For the cables pulleys
You can absolutely do the compression with 2 pulleys :
Use the first pulley to first route the cable throught any point of the desired destination line, and then put a pulley at that point to get the destination line
First step is always possible due to a point and a line always being in the same plane, and same for 2 intersecting lines in the second step
Good luck with your project
You should look at 1-1.5mm Dyneema rope for your cable, very light, very flexible, very strong, low friction, low elongation/elasticity and easy to splice to create loop or termination
Kevlar would be better as dyneema does exhibit some degree of creep when maintained under tension for long periods. Kevlar is absolutely stable. Also is it possible to arrange it as a continuous loop running back to the capstan (motor) where it would be anchored by a tensioning screw widget. Would you ease the termination clamp problem.
One thing you can do rather than use brake resistors is have a common rail for your motor drivers, so when one motor is generating it can power the other motors on the rails, and this will tie in with regenerative breaking too
In order to utilize regenerative breaking efficiently, you really need multiple servo drives sharing a common power bus. This is a gross over simplification but large automation companies use this strategy regularly to help drastically lower power consumption, heat dissipation, and panel space.
Super glue probably doesn't stick well to TPU. Scotch Weld PR40 adheres very well to TPU. You could also print the TPU sleeves to be concave on the outside so they are self centering
For your linear bearings, a lot of newer industrial bearings use metalized ceramics. Systems that used to be rows of ball bearings on hardened steel rails have become ceramic sleeves which fit snug to the rail. I don't know if that applies to carbon fiber tubing, or much for your project - but a good lool at linear bearings might give you some ideas for how to stabilize your tube.
I would even recommend going with some kind of graphite coating on the rods that can act as a dry lubricant. An oil or grease probably won't fit the application well, but can make a world of difference in longevity. Gears that run for decades with oil in them won't last a day without it. A little can go a very long way.
You could look at just trying a teflon sleeve or set of pads that form to the wall of the tube. Play around with contact area and pressure to get what you need. The resin binder for the carbon fiber is probably going to be easy to wear. You may want to consider adding or trying to identify tubing made with a harder material. A silicon dioxide coating may improve wear characteristics (or perhaps an alumina spinel - quartz/saphire coat).
If you want to go aerospace, you could try a sputtered titanium nitride coating. Not sure how well (or horrible) it would fuse - but if you're looking to go off the wall with it - that would be a direction.
For the string, you might actually look into amarid/kevlar. The nomenclature to use is "tow" when looking for strings of fabric material (same with carbon fiber) - I spent days losing my mind looking for a carbon fiber string because I didn't know that was called a "tow". Well - a tow is unbraided, so I suppose not technically a string/line.
You might look at fishing lines for off the shelf solutions and even talk to those communities about line routing.
Regarding the MBS of that string, you're going to get something closer to half of that once you tie a knot in it.
It’s also worth mentioning that some strings are stretchy. Using string with low elasticity may improve precision too.
FWIW, strength of ropes (atleast in climbing scenarios) are usually measured in newtons/kilonewtons. The fact that the string was given in Kg (assuming they didn’t do the conversion themselves) is a bit of a red flag.
I don’t think the material was mentioned in the video, (fishing line?) but dyneema, if they can find a suitably thin version, seems like it would work well here.
@@techheck3358 The color size and rating make me think kevlar. Spectra, dyneema, dacron and kevlar might all be good choices. You can put them in a woven dacron sleeve and tie a knot in that to get a strong loop on the end without loosing as much strength... Look up trick kite lines and bridals for that info.
@bkuker I agree, though this does change based on what knot you tie. For the string strength testing, I tied figure of eights, which (AFAIK) are fairly good with not severely diminishing rope/string strength. Interestingly the string actually broke in the middle, far away from both knots 🤷♂️
As for the string in the actuators themselves, this is a big reason why I wanted to avoid knots altogether. I believe the system is fairly good with having no "stress concentrations" on the string, and when it does brake (eg. from the motor not slowing down fast enough and the actuator ramming into one of its ends) it seems to be braking in different places each time; some of these breakages being in sections where the string is perfectly straight at all times!
@techhech3358 Interesting point re. stretchyness and precision! I've just been treating these strings as perfectly (axially) stiff, though you do raise a good point that this, of course, isn't true. Perhaps I should test how stretchy these strings are compared to synchromesh cable.
I should've mentioned in the video, but these strings are made of kevlar and are usually used for kites. The MBS/WLL is actually quoted in lbs and I see where you're coming from re. a "red flag" though I'm not sure I'd expect kite-hobbyists to care too much about the differences between weight and mass 🤷♂️
@@harrisonlow The strength loss of knots is a factor of the bend radius of the knot, knots like the figure-8 have a pretty big radius; but the way you're mounting the string also has a radius. I do think in this case it's best to avoid knots, so maximizing the radius of the mounting points will help. I can't tell where it failed in your lift test so this might not be the issue now but you could run into it later on. A good strategy is often to coil the rope around a perpendicular rod a few times before securing it. This coil forms a friction knot and will take up a lot of the force before it gets to the mounting point. Maybe add a large radius to the 3D print and then coil around axial to the carbon fiber rod?
*Old Design Issues*
- Bearings wore out quickly.
- String clamping led to wear.
- Assembly required hammering, causing parts to crack.
- Not modular; any issue with one actuator affects the entire system.
- Bowden tubes kinked and had slop.
*New Design Improvements*
- Strings internally routed, making it sleek and modular.
- Motor contained within the actuator for easy replacement.
- Clamping mechanisms simplify assembly.
- TPU sleeves minimize wear on bearings.
*Performance Metrics*
- Endurance: Over 5 hours of continuous runtime.
- Speed: Max 3.4 m/s.
- Precision: Less than 0.06 mm.
- Lifting Force: Almost 7 kg.
*Future Improvements*
- Use motor body as the spool for the string.
- Consider using synchromesh cable.
- Replace TPU sleeves with heat shrink.
- Modify bottom cap to reduce torsion.
*Community Engagement*
- Set up a Zulip site for better communication.
*Positive Points (Sorted by Importance)*
1. *Modularity:* The new design allows for easy replacement of individual actuators, reducing downtime.
2. *Endurance:* Over 5 hours of continuous runtime, passing 50,000 cycles with minimal wear.
3. *Speed:* Achieved a maximum speed of 3.4 m/s, meeting the design requirements.
4. *Precision:* Less than 0.06 mm, close to the theoretical limit of 8.4 micrometers.
5. *Lifting Force:* Capable of lifting almost 7 kg, exceeding expectations.
6. *Ease of Assembly:* New clamping mechanisms eliminate the need for hammering parts together.
7. *Reduced Wear:* TPU sleeves and internal routing of strings minimize wear and tear.
8. *Community Engagement:* Set up a Zulip site for better communication and sharing of ideas.
*Negative Points (Sorted by Importance)*
1. *String Wear:* The string broke during precision testing, indicating potential durability issues.
2. *Torsion Issue:* When fully compressed, the actuator has little resistance against torsion.
3. *TPU Sleeve Problems:* TPU sleeves can slip off and are not perfectly smooth or symmetric.
4. *Complex String Routing:* The design requires complex string routing, which could be simplified.
5. *Braking Limitations:* Difficulty in slowing down the actuators, indicating a need for better braking mechanisms.
6. *Assembly Cracks:* The old design required hammering, leading to cracked parts.
7. *Old Design Wear:* Bearings and strings in the old design wore out quickly, affecting longevity.
Wow! What a thorough summary!
definitely not openAI output.
Good boy. You paid attention. Have a chocolate fish.
@dine9093 tbh even if it were ChatGPT-made, I reckon it's still pretty good! I wouldn't think it'd be so effective just from (presumably) the transcript?
@@harrisonlow It would if it also used your previous videos transcripts. keep up the cool work though!
For terminating the synchromesh cable, you could use a metal ring attached to the end of the the rod/the rod stop, and simply thread a small hole for a screw that then clamps on the cable. Im having trouble explaining it, but if you google crossfit jump rope, it's the little screw at the end that I'm talking about. It should be strong enough with simply the clamping pressure, but you could always allign the screw up against the black coiled wire. I hope this helps. Fantastic work btw!
When adding heat inserts, as soon as you pull the iron away, flip the part over and press the face with the insert (and still hot plastic) directly against your workbench for 3-5 seconds. The flat surface of the workbench will keep the small raised ring from forming around the insert as it cools, so you won't need to shave it off manually. It will also help make sure the insert is aligned perpendicular to the face. Alternatively, you can press a small block of copper of aluminum (like a spare heatsink) against the fresh insert to accomplish the same task.
That's a *really* good idea! Thanks heaps for sharing it!
I see a lot of comments about using concave TPU profiles but I don’t think this is a good idea. It sounds intuitive but you’d be over-constraining the system.
Also, don’t use heat-shrink! And don’t use Teflon. The key to preventing wear with rolling surfaces is matching the hardnesses as closely as possible. The reason the TPU works well is that you’re better able to control the preload, but the gold standard here is going back to the plain bearings but with a more precise way of adjusting how tight they are.
Also this is where the twist in the central carbon fiber rod comes from: slight misalignment in the guiding bearings, in the vertical direction.
I was glad to see you ditch the pulley system for measurement. Very cool to see you got such good repeatability. By the way this is most commonly expressed in terms of 3-sigma repeatability, so if your standard deviation was 0.057mm you’d say you have about 180um repeatability. This is extremely good for something like this, well done!
Cheers for the input! Some thoughts:
1. I actually originally had the TPU sleeves being concave instead of convex and they had an insane amount of friction (as I suspect wouldn't come as a surprise for you 😛)
2. There has been a ton of feedback on this bearing block and I'm starting to think the best solution would be to buy some higher-tolerance tubing and just use linear bearings. Would be sleeker, lighter, simpler, etc. (just more expensive 😅)
3. Well that's nice to hear about repeatability! I'll take it 😁 Honestly I would've been happy with < 1mm precision/repeatability, so I'm stoked by the actual performance!
Cheers 😊
I once took apart an old photocopier and it had a steel cable moving the photocopier head around, so i think you might have some success with steel cables rather than nylon/other.
And it really did surprise me how supple the steel cable was, so there might be some specific type of cable for applications like this.
Interesting application! After the last video I actually picked up a bunch of steel cable and the sizes that are flexible enough weren't that much stronger than the kevlar line (and a lot more expensive) so I ultimately decided to stick with the kevlar.
Cheers for the suggestion 😊
The TPU sleeve over the bearing will work way better if you design them to fully engage with the surface area of the carbon fiber tubes. Make them like a circular indent like doing a sphere revolve cut in your cad program where the sphere is the same size as your carbon fiber rods. It will also help hold them centered if you add a bit either side of the bearing on the flat side that helps lock them in, like a U shape that comes down about a millimeter from the outside diameter of the bearing towards the center of the diameter. Hope this helps. 👍👍
There are v groove pulleys which will increase your contact surface area for the bearings. Doing the heat shrink around the surface will increase the grip while allowing some compliance. I would use spectra cord if you can since they are about as strong as you can find and very stiff. For torsional stiffness you would be far better off using a square profile tube rather than round. If you use v groove bearings you could run with 4 bearings, 2 on each corner which should give you plenty of stiffness.
hi dexterm, have you become a flat earther yet?
Late to the party, I know, but as this video got started I couldn't help but think he needed either spectra/dyneema or vectran, along w/ delrin bearings...as the thing reminds me very much of an internal cascading outhaul on a sailboat boom I built years ago. Vectran and dyneema, while feeling very different, are both quite 'slippery', but also they both will allow each end to be brummel spliced so that the 'strings' are terminated with eyes on each end.
Two options for cable tensioning:
Use a simple turnbuckle so it doesn't twist the cable as you tension. To secure it to the turnbuckle you can loop and crimp it or use a grub screw or a throttle cable style connector.
Alternatively look up the Cable Rail Tensioner by Cable Bullet. It seems to be a compact tensioning system that uses a screw to tention and does not twist the cable.
The latter option basically uses a crimp on the end of the cable which passes through a hole that has a ring on it to push on the crimp and it is adjusted with a grub screw. You could certainly develop your own system similar.
For the TPU on bearings, you can use a concave instead of a convex shape.
I'm glad you mentioned this! I couldn't think of a succinct way to include this in the video but that was actually my first approach to this problem! Unfortunately there was an ungodly amount of rolling resistance between the 6 TPU sleeves and the tube. For reference, the 6 TPU sleeves had a concavity that perfectly matched (within 0.3 mm tolerance) the profile of the CF tube
@@harrisonlow If you make a rigid sleeve around the bearing with a semicircular groove, I'm sure you can find an off-the-shelf rubber o-ring that will fit in the groove snugly and provide friction.
@@bobbylox Fantastic idea! Very simple and would be easy to replace parts for. I'll get some O-rings next time I'm at Bunnings and give it a test. Cheers for the idea!
To ensure symmetry you could print your bearing sleeves in two halves.
Would it be better two have two o rings per bearing or just one centered o ring?
High tensile threads need real care in end termination; if you create a good load sync with a gentle bend radius you'll be able to get the MBS. Try using a boss at each end with a number of "dead" wraps which will distribute the load into the thread. If you use the thread as one continuous wrap you'll only need two ends as well. You can eliminate one bearing by aligning the bearing to the tangent of the drive drum as well. If you put a counter rotation feature in the fixed end you won't have to pay the weight in the moving end as well ;)
Just lying here, thinking that there are a couple of things that i might be able to usefully comment on... a bit off the wall, but i wonder if using two motors, although more complex, might give you control over things like stretch in a string, and providing some kind of dynamic control over the string action and loading... with encoders on the motors, and optical "zeroing" sensors, you get calibration on every cycle... i wonder if some kind of silicone 'oil' impregnating the string might improve lifetime ... indeed, steel cable with a 'dry' lubricant might work well, even on the smallish radius pulleys? I guess that a lubricant could be 'wet', if the actuator 'tube' was enclosed....anyway, sick in bed, just the ramblings of an amateur....
Interesting idea with two motors! I'm not sure it'd be worth the added complexity in my case though 🤔 Silicone lube on the string is a neat idea! Since this video I've done a heap of testing with different strings (video to come eventually...) and have found dyneema to be really good; high strength, low wear over repeated cycles around small pulleys, low stretch etc.
Cheers for the input, and I hope you get better soon!
I think you should consider using Dyneema/Spectra for your ropes. It is made from high strength polyethylene fibers, and it's very resistant to wear (and thus quite hard to cut, even with a proper knife). It's used a lot in sailing and fishing, and I've seen some only 1mm in diameter, which I guess should be thin enough for your application, claiming several hundred kilos of working load.
Interesting project from engineering perspective. Endurance during 10x thousands of movement cycles seems the relevant criteria … and the weakest part might be these "strings" :-) Indeed we use dyneema/spectra on performance racing yachts, easily with very dynamic loads on main sheets and standing rigging between 5 tons and 20 tons (on boats of 16-18 meters), and much higher loads on bigger superyachts ... the rope producers also offer "pre-stretched" materials, so there is no need to worry of losing tensioning. :-) good luck for all projects coming along !!
Terminating the synchormesh cable can be done with a guitar string winder. (The part of the guitar that turns the string to adjust the tuning). This also gives a nice way to adjust the tension. I'm wondering why the motor isn't just turned 90 degrees instead of having the strings go through pulleys. I think regenerative breaking would be ideal and would allow your juggler to run on batteries for shows where power isn't available.
Making the motor wider would mean a more complex/flimsy swivel mechanism between the actuator and the base
Or make a long tin hole to thread the synchromesh cable through and then have a 2nd perpendicular hole through which a screw is tightened down to impinge upon and lock the cable in place in the hole.
@@Superkuh2 that makes it hard to adjust the cable
@chrisBruner Addressing your points in order:
1 - Good idea! I don't play guitar and this idea hadn't come to mind. I'll do some research into how these work and I'll see if I can make a similar mechanism at the small scale/weight that I'd need. Cheers!
2 - The main reason I didn't set the motor perpendicular to the extension axis is because I don't want the motor to be sticking out the side of the actuator. I have a gut sense that this would likely cause issues down the line once I put all 6 actuators together into a Stewart platform so I want to keep everything as "neat" and in-line as possible. I think using the motor body as the spool would still work, though, because I'd be able to keep the centre of mass of the motor approximately coincident with the extension axis.
3 - I agree. I love the idea of Jugglebot running off batteries and being fully portable, though I have precisely 0 experience with these and don't want to find myself in the position of having to pause work during troubleshooting in order to charge batteries (not to mention any physical risks/dangers of high-power batteries)
@Superkuh2 I agree with techheck here - the synchromesh cable needs a fairly clear path to be pulled through and I think a convoluted tensioning path would be tricky to make fine adjustments to. FWIW one of my patrons had a few great suggestions that I've written up on Zulip. I'll post the link as a reply to this comment in case RUclips takes a disliking to the external link. (The thread can be found in General > How to Terminate Synchromesh Cable)
Cheers for the suggestion!
Awesome design, I am working on a project that needs a linear actuator and nothing I have found online will suit my needs. I cannot believe it never occurred to me that I could make my own, thank you for inspiring me.
Two things you didn't consider when you thought of using rotor as spool: it'll have way worse precision and way less torque, by a factor of 3 it would seem like.
I think the precision would be if anticogging were set up, but after doing that for one motor + ODrive I'm not too keen on to do it for all of the motors. It's a very finnicky process that occasionally just doesn't work 🤷♂
Good point re. torque! Cheers for the input 😊
For the brakes, use magnetic damper less work on the brake, plus it will give it a shove back the other way.
To improve strength, you could have 3 pairs of strings. You also have to be careful when considering cord strength. When you're pulling the bag up and down by hand, the jerk you're applying is definitely high enough that it could reasonably be producing tensile loading in excess of the rating. The literal weight/mass rating doesn't matter, you need to be thinking in terms of force, which will make it more obvious how jerk and acceleration can create very high peak forces. If you improve the motion controller to enforce jerk limits, it should improve the performance of any cord or cable, and also reduce shock forces on the machine overall. This kind of thing is being used in 3D printers to improve print quality, reduce ringing, and go faster with fewer consequences.
Haha I was wondering if anyone would call me out on my sketchy testing of the string strength 😂 I agree that my "jiggling" of the bag would increase the force in the line, but it felt like my jiggles were fairly small and (I would think) shouldn't increase the force on the line by a factor of 4 (MBS/bag weight).
As for the motion controller, I actually don't know if it's possible to set jerk limits with the ODrives. I'm currently planning to use trapezoidal trajectory planning (with velocity being the trapezoidally-controlled parameter) which does a pretty good job of smoothly moving between setpoints. It's yet to be seen how well this does when the setpoint is being updated at eg. 100 Hz...
Cheers for the input!
@@harrisonlowAt work we have a garage door, and you pull it shut with a cord. When I started this was generic 90lb nylon diamond weave. It's not hard to close but the way the guys pull on it, not even really a yank, was causing the rope to fail every month or two, it would just rip in half mostly at mid-span. I'm the knot person, boss asked me to fix it, I just did mostly the same thing but using 550lb rated paracord, and a knot that's better at distributing the load. It's been maybe 8 months, no problem even with spending hours a day in direct sun.
It's just that easy to generate high shock loads by accident. I assume your cord, due to being smaller and not diamond weave with a useless fluff core, has a shorter elastic range.
I'm not sure what cord you've got, but in the vein of paracord there's "1.18mm Micro Cord", a lot of paracord dealers have it. Supposedly it's "rated" for 45kg, but they're not clear about what kind of rating that is. It's some kind of weave, which may widen the elastic range and make it a little more forgiving, but whose to say how the overall stretch will differ.
Honestly the other commentors seem to have your synchromesh cord solved, I've never seen that stuff before, it's cool.
Edit: one more thing: all paracord melts, any cord you can fuse the end on can melt, they can melt from rubbing and from loading. Once they start melting the mechanical properties are out the window. You won't be able to feel this heating, the cord is just too insubstantial to heat your finger. The paracord family could easily just be useless to you.
@@Ziraya0 Haha I love that your work has a "knot person" 😂 What do you do for work?
Cheers for that writeup! Useful thought experiments re. woven vs braided thread and elastic range. I haven't really paid too much attention to the structure of the thread before! (FWIW, the kevlar string I'm using is made from two twisted threads)
As for melting: I mentioned in the video that I accidentally routed the prototype actuator incorrectly. This routing had the string going straight over a printed part at a rather aggressive angle. Cut straight through it! For better or worse, the string is far more robust than the printed parts so they tend to fail first.
@@harrisonlow You probably have enough space in this design to go up one or two sizes in kevlar line literally without changing anything else. The line I'm seeing isn't two threads, but it is 0.8mm, 45kg, safe working load limit 9kg. 90kg/18kg line is only 1.1mm, and 136kg/27kg is 1.5mm. If the synchromesh gets too complicated, this seller goes up to 476kg 2.6mm cord; at that point it's mainly a question of how big you may need to make the spools and how that'll impact the torque across the range of motion.
@@Ziraya0drop Kevlar for Dyneama thread. Size AA/30 thread is 0.28mm, holds 40+ pounds easily. It also stretches less than Kevlar. Thread is easier to track down, and that's all kite fancy kite string is.
It’s a shame I’ve only just today discovered your channel, your communication of your development process is very inspirational to me!
Rest assured, there's still a lot of development to go! 😁
Regarding the Pulliies..
you could turn the green "middle - direction" pullie such that its output guides the string on the tangent of the main pullie.. this would result in an really odd angle but would reduce the number of pullies. But I think three is much simpler, as you don't have to "think" about this odd rotation.. :)
Very good thinking! I wonder if it's true then that any two "skew" lines can be joined with just two pulleys?
I thought exactly the same.
@@harrisonlowyes any two lines in 3d space can be connected with two pulleys.
the way to think about this is choose any two random points, one on each line, and then connect a third line between those points. then you place pulleys at each of the points, tangent to one of the original lines and the newly constructed third line.
hope that makes sense
@@waxt0n Ah of course! Great way to think about it. Cheers for the explanation 😊
you should be able to hold the synchromesh cable by having two small metal plates that clamp together with two bolts, when you tighten the bolts it will crush the cable in between the plates and hold onto it real tight, I am taking inspiration from how electric cables are held into some fixtures, its often molded into the plastic in consumer electronics so that when the chassis of whatever is screwed together the two mating surfaces pinch the cable to act as strain relief on where the cable is attached inside.
If you need to dump break power for a very short time with significantly longer time between breaks, you could use supercapacitors, as they will essentially be shorts when they're empty
Just what I thought too. As a bonus it reduces energy consumption.
In terms of the slowing down, you may want to look into powered steering, a technique called drive by wire. It's essentually adds pressure from one motor that is generated from another to create a force. One motor controls the other. You also have to remember moving mass is heaver mass in terms of the string. Terminating the line you can drill a hole in a screw and feed it though and tighten, or use a couple of washers.
This drilling-a-hole-through-a-screw idea keeps coming up! I'm very surprised as I'd never heard of it before. Have you used this termination method before? What for?
I'm liking your design! Here's a couple ideas for ya. Change from the string to the cable as you mentioned. Set the cable tension with a adjustable pully. Put the first pully from your motor to be on a slot, and a screw on either side to adjust it to set the tension. Oh and the tension should be just tight enough, so when you pluck it you get a thump and not a strum like a guitar. Then the ends of the cable could be securely mounted, perhaps looped over a rod you have ran thru your carbon tube, then crimped, just like the common method for larger cables.
For the slowing down of the actuator, you can put a coil on the ends of the main frame and have magnets attached at the ends of sliding rod. By using hall sensors you can determine the proximity of the magnets and switch on the coils accordingly and hence decreasing the speed effectively. This can moreover be used in a way that woks like a coil gun and help accelerate the moving arm after a full stop.
Here's an idea for terminating the syncromesh cables: route the cable around an eyelet and crimp the end onto the cable. That is a cable terminal that will not break (the cable itself will break before the terminal does). Then you can use a hook screw and a hex but to tighten the cable as needed.
Regarding all the bearings, but more specifically the end cap alignment thing, I once took apart an HP inkjet and found that they use felt bearings. That's right! It was a piece of felt with a hole a bit smaller than the rod so it held firmly. there were metal rings on both side of the felt centering it around the polished linear rod. This helped keep the mass down, cost down and noise down. What I'm suggesting is to create triangular end cap with all 3 holes to prevent twist. It would be 3d printed and hold 3 pieces of felt-washers.
You would need thick felt (1mm or more) and could mass produce them if you created 2 hand made punches. Take a metal tube that is slightly smaller than the carbon fiber tube and sharpen the inside edge of it. Then hammer down on the tube against the felt to create an inside punch. Do the same for the outside punch. Make tons of them and redesign the end cap to encapsulate 3 felt washers and all 3 rods. You can get rid of the wheels and bearings this way and solve for that odd twist. Make the end cap organically shaped to reduce weight (like a semi-collapsed triangle, don't know how to describe it).
I'm unsure if this suggestion would also work for the primary alignment bearing block but try it if the end cap proves useful. You would essentially create a new block with many felt washers to center and hold firmly the center rod. It would be cool if you could drop most of the bearings to reduce cost and rotational inertia and wear on the shaft! Good luck.
BTW, I couldn't create an account nor use the Google signup... Anyway... On that previous video I was the one to recommend the synchromesh wire. Thank you for covering that in your video I feel honored and excited to continue to help!
To terminate the syncromesh cable, use a set-screw inside a metal screw insert. They're used a lot in woodworking. Drill a hole into the insert near the bottom, then feed the wire through, screw I to the insert.
For the wrapping around the motor housing, that would decrease the speed, but would massively increase the strength I think.
This is amazing progress. Not sure what you're planning on using this for, but it's really cool!
Interesting! I imagine you'd have to use fairly large inserts to do what you're describing? To check that I'm understanding you correctly, the wire would be "pressed into" by the bolt, but would it have anything touching it on the other side? Or does the strength of this idea come from the wire being bent by the bolt, disallowing it from being pulled back through?
Cheers!
You can eliminate one pulley on each line, rotating the first one 90° and placing it on the right place. I'd also swap the strings on the spools for better clearance, probably.
For the strings, I don't know what you are using, but 1.5mm or 1.7mm dynemma could work, or something similar in Kevlar if temperature or wear is a problem. Friction is quite low with dynemma, have an eye out for creep, there are some variants with lower creep that could help to facilitate the tensioning.
If you add 4 more bearings to the moving block you could spare 3 from the stationary block.
Using the motor as a spool seem problematic to route the line, unless you spread the bars, and you need them parallel so seems like a packaging hassle. Also, you remove flexibility as changing the spool size gives freedom in the velocity/force compromise.
Lastly, adding regenerative breaking should be really good, you should be able to find a motor driver that could handle that. More advanced drivers can also help with the different drivers working together, I don't know how the premature disassembly got to be as 6 degrees of freedom with 6 drivers should allow for small errors in actuator position to translate to small errors in platform position, not unscheduled disassembly, but start slower next time and maybe incorporate some compliance in the connections until you dial the control setup.
I think you can have fewer than 3 pulleys per side if you offset the spool insted of constraining it to be coaxial with the moving rod. The minimum might be with the spool tilted to be perpendicular to the rod.
Rock west prefabs hex section carbon rod which might be useful for improving the torsion issue.
If mounting flex leading to the string generating derailing forces can reliably be ignored, then absolutely he can use just 2 pulleys per string with the design shown. If derailing forces can't be ignored, then using pulleys with a "taller lip" (to retain the string better) may be enough to compensate. It's probably not worth focussing on right now, but definitely something worth considering on the maybe-last version.
I can't give any formulas for how to do the calculations off the top of my head, but the concept is fairly straightforward:
1) Each pulley corresponds to a plane, which it's embedded in (and parallel with); one of the sections of string (e.g. "from the spool" or "to the attachment point") must also be embedded and parallel with this plane, while the other section won't be;
2) The task is to intersect these planes with each other in such a way that:
2a) the intersection is tangent to both pulleys, AND
2b) the section of the intersection between the two pulleys doesn't pass through anything else (e.g. the spool), AND
2c) the plane with the string section that passes to the spool should be as close to perfectly tangential to the spool as possible (to minimize the risk of derailings).
Harrison as a Chartered Engineer I am blown away by you work, well done!
Great work, especially with iterations. One thing that can't go un-noticed is the cyclic testing. With no load, almost every actuator outperforms their intended application. It's the testing of cycles under different loads and disturbances that can really speak volumes for a design. Loved the video. I wonder if you shouldn't consider extruded aluminum hexagonal cross section, as the part in motion. Would make for better load distributions. You could also add steel wear surfaces if you figured a good way to bond thin shim stock to aluminum.
I agree, cycle testing under load is going to define how well the design will hold up. The forces are going increase dramatically depending on how fast you accelerate and decelerate.
This is awesome!! I've been needing some scalable linear actuators for a project. These look Great!
Awesome! Be sure to check out the Printables link in the description for all the files. Let me know if you make one - I'd love to see it!
If you still dont know how to terminate the syncro mesh cable you can crimp electral wire terminals onto the end. Youd want to do some strength testing for your needs, but in my experience, it works great for a mounting point.
On the TPU sleeves, you should make them concave instead of convex. This will allow the bearings to have more positive contact with the carbon fiber sleeves. This will make the system more rigid.
I actually originally had the TPU sleeves being concave instead of convex and they had an insane amount of friction (though they were a very good fit for the CF tube). Someone also recently had a great thought about this on Zulip:
"The problem with sleeves that follow the CF tube is that you will have contact points with the tube that are at different radii from the axis of the bearing."
Very clever way to think about it!
Definately use the dual wheel design on all 3 outter shafts to stop torsional forces. And for braking you can use another mini actuator to press a cylindrical bearing down against the wheels in the bearing block and the harder it presses the more it brakes the bearing and an electrical actuator is very percise and fast so with the right algorithms it can precisely brake the movement.
for the sincro-mesh cable put a helical cut into the 3dprint to match the synchromesh cable and a tab to hold the cable into the groves (tab could be grooved too but not required)
Wow, what an elegant design. Kind of reminds me of something you'd find in an IBM Selectric typewriter.
What an amazing mechanism! Cheers for bringing that to my attention!
Who did you make bldc motor to run in both directions at a time , as I know that they need esc that allow them to run in one direction only🤔
how far you've come on this project! Thanks for being so detailed and sharing it with us.
Haha yeah it's pretty crazy to think back to the earlier designs! Still quite a ways to go though 😁 I'm glad you're enjoying following the process 😊
What an excellent project update. I love the collaboration happening, thanks for sharing.
For the precision/accuracy: not sure about the details of your design, but when your motor spins in either direction, the string is looping around it, effectively changing the diameter of the Motor's pulley. Making it bigger or smaller will dynamically change your "degrees per mm" conversion! If, initially you calculate 1 rotation = 1 mm, at the end stroke it would probably be more than that.
For the central fiber twist: My main guess would be the alignment of your pullies. If they are not aligned properly, the efforts in your strings mechanism will not be symmetrical enough (with this system it cannot be symmetrical) and I suppose that after numerous passes, combined with vibration and dynamic load, some screws started to losen themselves ?
Loving seeing this develop,thanks for sharing. Couple of quick thoughts... 1. Could you use concentric tubes, so one large and one small instead of four small? Also, can you get carbon fibre tubes in square section, which will help with the twist?
Cheers!
I briefly toyed with the idea of using one large CF tube for the support structure (rather than three small ones) but I wasn't a huge fan of it for two main reasons:
- Large CF tubes are super expensive
- I don't like how opaque they are; I like being able to see the "inner workings" of the actuator. I've come to find that having an "open" design is also super helpful for fixing issues as they arise
The square-section tubing is a good idea! I wonder how that would affect the geometry of the upper "bearing block"? I'll have a think about this. Thanks for the thoughts!
I don't know if you are having differential tension problems between the two stings, but if you are there is a way to use a single loop to accomplish the same thing. I don't know what it is, but airplane ailerons are set up this way. Should be easy to find.
You could reduce the number of pulleys by putting the motor axis at a right angle to the axis of the actuator. If you make the drive spool the right diameter and offset it a little (so one side is in the center and the other side is in line with the side tube) you would only need the one pulley at the far end of the 'blue string.'
This is a lot better than the geometrical fiddling that I suggested.
Lock the synchromesh cable through three screws running in a line (more if needed), where each screw is tightened down in sequence with the cable at an angle. The cable is mostly straight, running through the screws. Spacing of the screws can accomodate the bend radius of the cable.
Use a collet block to terminate the synchromesh cable. The bonus is that a collet will draw it tighter as your cinch down on it.
1) Why are the TPU "tires" rounded (positive curvature) when they are contacting a round shaft? They could just as easily be flat or even negative curvature.
2) How are you controlling tension of the cables? Could one of the redirecting pulleys be adjustable to allow tightening the loop after fixing the ends? This would also allow adjusting if there is any creep in the line, allowing tension tuning like a guitar
3) As an actuator it would also be interesting to see the stiffness in addition to the failure strength. How much would a small load on the end effector change it's position?
1 - I actually originally thought the same thing and had the TPU sleeves being concave instead of convex and they had an insane amount of friction (though they were a very good fit for the CF tube). I figured reversing that would flip the problem and minimize friction
2 - I'm doing this by keeping the string tight while I screw in the clamping bolt. I really like your idea of an adjustable pulley. The one at the end of the actuator is in the perfect spot for this and that mechanism probably wouldn't be too complicated to design. I'll have a think about this!
3 - I agree! I reckon there are a ton of aspects to this design that'd be interesting to explore (another being max power output), though unfortunately I have nothing close to a good lab setup and I think (for now, at least) my time is better spent moving forward
Cheers for the (neatly formatted) suggestions!!
I think you could take the first bearing out of the blue string set if you align the central rotational plane of the second bearing tangential to the spool
Hey,
Cool project, just discovered your channel.
Couple of thoughts, the line you use is quite rough which could potentially contribute to the precision issues you're facing. There's a product using a dimensionally controlled kevlar cable, it's a bit thick for your application perhaps (2mm), but a lot more round.
For the TPU and bearing sleeves, UHMWPE could also be an option, also thin silicone sleeves with a high shore value could.
Last thing regarding the precision, I haven't seen any tensioners for the strings and how do you control the stack up of the string in the spool ?
Hello. Harrison. I'm writing because I think I can help you a little, with an idea.
You talked about being able to tighten and change your strings on your Linear Actuators. when you tune the strings on a guitar, you turn a small handle. very simple mechanical structure. What might not be so good about it is the weight. But it can certainly be made very light in weight. Thanks for a great channel. Finn
Great project!
What about dynema as the string? Very low stretch, very high strength. Standard braid so making sliced loops and terminations is easy. Also, compression cones for attaching a possibility.
I've actually recently been doing a TON of testing of different string materials and dyneema is looking quite good!
What do you mean by compression cones? I'm not familiar with that term
You should be able to get the pulley count down to 2 per string:
* for the extension string, just rotate the existing bottom-of-tube pulley around the tube's axis until the input string is tangent to the drive pulley, and remove the other motor-side pulley.
* for the retraction string, chose a random point "A" in the plane of the drive pulley ~3cm away, draw a line from "A" tangent to the drive pulley in "B"; then set a point "C" on the central axis ~1cm above the pulley; the string should follow the path "BA" "AC" then up the central axis from "C". The 2 pulleys are semicircles at "A" and "C", tangent to "BA" and "AC", and "AC" "central axis" (in CAD, create a plane from each pair of lines and just add the circle) [for clearance, you might need to swap the extension and retraction drums]
A couple ideas come to mind. To terminate the twisted cable try a double plate clamp, 2 screws with a bump in between should do. The pulley "shoes" as I would call them, having them rounded may decrease their friction coefficient however making them concaved instead of rounded would reduced their odds of slipping off and also serve as an self-aligning guide sort of speak. I wouldn't make it exactly the same as your center shaft, maybe 1/2 a mm wider then the shaft on each side. effectively giving you a maximum of 1mm tolerance and ever so slightly reducing the friction vs a matched curve to shaft. really like what you did so far. hope my advice helps. feel free to message me if I wasn't clear and you want clarification. I can fab something up easily if needed
I have in Thingiverse a design for Customizable wheel for 608 bearing, that locks in place and won't slip off. I've been playing with printing the middle part of the wheel in spiral vase mode to get rid of any small bumps (printing artifacts), but I have problems with the round wheel profile getting distorted. Your application does not need a round profile and thus no variable extrusion width, so I think it would be more straightforward to get printed in vase spiral more. Some manual G-code editing would be needed to switch between spiral vase mode and normal printing..
This is such a nice design! I love cables in actuators, they feel like an underated technique!
You could consider using a very large super capacitor as a "breaking load" for the ESC. If you dump into the capacitor and then pull power back out, you should end up with a net loss in power usage, thus the batteries will be at net discharge without worrying about chemistry issues.
Hi @harrisonlow - great work! I wanted to point your attention to a type of string that has much higher strength called "spectra" or "dyneema". used most commonly in sailing, paragliding and kitesurfing, such strings can support 100s of kilograms in rather thin gauges. The fibers are also cut-resistant but you might have a heat problem with them and might consider lubrication to control that.
Thanks for the suggestion! I've actually since swapped to dyneema (0.8 mm) and I love it! 😍 Super strong, really thin and flexible and barely any creep. I actually have a video in the works on my testing of a bunch of strings 😁
Great video by great creator!
Really happy to hear and follow your project
Cheers for the support 😊
Regenerative breaking can be done with a 3 phase rectifier and some way to either store the energy or dissipate the energy. A good place to start is looking up "delta alternator diode layout". Delta alternators have nearly identical wiring to most brush-less motors and you can use the same type of rectifier layout because of this. The trick however is using PNP transistors instead of diodes. By using PNP transistors, you can monitor the motor driver's voltages at the transistor's base and limit the current of the rectifier when the motor driver is engaged. Keep going on the project, very interested to see where this goes!
Thankfully the ODrive is capable of handling the electrical stuff for regen braking, but the problem is that my electrical knowledge is subpar right now so I need to learn/figure out what batteries or brake resistors to use. I've tried the naive "increase brake resistor power, keeping nominal resistance the same" but that didn't work for some reason 🤷♂
Very good improvements! I am looking to make some actuators for a very different application, but endurance is important to me, keeping cost in mind. I'd very very keen on seeing you implement your planned improvements.
brake resistors are really job specific. Got to do the calculation for the brake power. This is the weight of the load, time to stop and a few other things. I have always went with what the manufacturer recommends. Industrial Allen Bradley (Rockwell) will have a chart/calculator to tell you what size drive and brake you need. This also keeps the parameter settings straight forward by using all the same system. Also, if you start getting into higher end drive systems some of them are modular. Allowing them to be all hooked together and then controlled through tcp or other standard protocol.
@110:30. For the bearing surfaces, can you use an o-ring of something like teflon or viton? The viton would even give some resistance to tube rotation.
This is awesome! I'm so glad to see your progress!. This is awesome! I'm so glad to see your progress!.
Can it be done with less than 3 pulleys? Referring to the green string, my thought is to have the first pulley be slightly tilted and the correct diameter to stretch the distance between the motor pulley edge and the upper central pulley edge, so that the intermediate pulley essentially does the job of both pulleys you have right now, but it keeps the string in line coming off of the motor pulley and the upper pulley.
Yep, I now realise that this is very doable with only two pulleys per string. Good thinking!
@@harrisonlow Awesome. Thanks for replying to my comments. Can't wait for the next update!
Just a quick idea on terminating that funky syncro cable, how about a copper and/or brass compression lug used in wiring electrical service, but with a plastic sleeve such that you're not directly compressing on the syncro-cable
If you get the angle correct on the spool pulleys, you only need 1 coming off the pulley instead of two. The strings will wrap more than 180* around the pulley though
I’d be curious what the accuracy of this (or the next iteration) would be with a bunch of consecutive movements that are all in the same direction, like 5 random points chosen to be consecutively descending or ascending, and measuring primarily the relative deltas that are then achieved _between_ the points, and then comparing that to the requested point deltas.
I would not be surprised if your consecutive deltas end up showing far higher precision when moving repeatedly in the same direction without the driving string having to change.
If this is the case it could be a good indication that some undesirable inaccuracy is coming from the slack transferring between the two strings, and you might be able to design movement paths around that (ie. preferring multiple consecutive movements in the same direction if at all possible)
That's a very good point! Either way, I'm super happy with sub-millimetre precision (let alone < 0.06 mm!)
For the synchromesh cable look at how bike brake and derailleur cables are clamped, its still a bolt, but with a grooved washer that holds it against the part.
Commenting so that I can come back to this later.... this is a great paradigm to work with.
These actuators are awesome! I think they'd probably be useful for a really wide range of applications
Cheers! I'm keen to see if anyone ever uses them for anything 😁
You can do 2 pulleys per string. Remove the middle one, rotate the closest one to the motor 45° clockwise with the axis of the rope going to he motor.
Rotate the other pulley 45° conterclockwise about the vertical axis.
For BLDC motors, when the H bridged are configured where the coils are energyzed similarly to a pwm where 0 speed is 50% +V and 50% -V, it gives a good speed control because the shortest pulse polarity acts like a regenerative brake by itself.
Apparently its called 6 step commutation and it required a bipolar wirering.
Yep, I think you're right about the two pulley idea! Good thinking!
Re. BLDC motors: I'm not sure I want to deviate from using the ODrive controllers that I'm currently using; I trust the makers of the ODrive to be *far* more competent as designing these circuits than I'd be. It's interesting to know that this sort of configuration is possible, though!
Cheers 😊
I thought the same. As long as the pulley is in the plane of the contact points between the top pulley and the motor, it should work?
Yep, I think you're right. I'm not sure how I didn't see this when CAD'ing!
Why not flip the motor perpendicular to the direction of the linear actuator and have the body be the spool as you mentioned and then have much more simple routing of the string direction to the rod?
Ah I see this may be what you were talking about. I thought you meant keep it in line with the rod, but just use the body as the wheels.
Yeah I've realised that I didn't do the motor-as-spool idea justice in the video. The viewer's suggestion was to have the motor sideways so that most of the routing would be a non-issue anymore. As for why I didn't do this: I didn't think of it! (the motor-as-spool part, at least.) I did consider the perpendicular motor idea, but decided against it because I don't want the motor to be sticking out the side of the actuator. I have a gut sense that this would likely cause issues down the line once I put all 6 actuators together into a Stewart platform so I want to keep everything as "neat" and in-line as possible.
I think using the motor body as the spool would still work, though, because I'd be able to keep the centre of mass of the motor approximately coincident with the extension axis.
I would recommend a capstan pulley if you are trying to increase precision
To hold the synchromesh, take a bolt and drill hole in it large enough for the synchromesh to fit through, thread on one jam-nut so it goes below the hole, insert synchromesh, thread on 2nd jam-nut and then use the 2 to clamp the synchromesh in place. Could be changed in seconds and wouldn't require any design changes and parts could be made in minutes.
I've had to sand flexible filament before it will bond with other materials. Also, you could try a concave profile for the bearing sleeves, that might give it more surface contact with the rods vs the current round profile. Def see the rolling brace being more stable when against all 3 rods. Look at how roller coasters work, you should be locked in place with just 2 bearings per rod if all are angled for opposing forces. Congrats on the progress! Looking great
Very interesting point re. sanding. I'll try that out!
I actually originally had the TPU sleeves being concave instead of convex and they had an insane amount of friction (though they were a very good fit for the CF tube)
I agree that four bearings are all that should be necessary, but I can't think of a good way to position them so that they're robust against forces in all directions!
Cheers 😊
@@harrisonlow you'll also want to sand the metal surface of the bearings where the TPU bonds - I use rubbing alcohol to clean both sides before gluing. I'd start w/ equal angles on all rods - 2 bearings per rod, on opposite sides of the rod (angled more than 90 degrees to each other) so the the tension is pushing out, away from the center equally. Provided the sleeves don't make contact w/ each other, they could be all aligned - so a ring of 6 bearings in a sort of star pattern. If you test the TPU sleeves w/ dif infills, you might find the right density to keep tension w/ out too much friction
Consider nylon for instead of TPU for the bearing sleeves, it will make it more rigid / stiff / precise. You might also consider engaging more of the small linear rods with bearings.
For your cable clamp, you'll need 2 metal plates with a hole at each end (10x5mm, with M3 holes should suffice) - fix the plates with screws at each end and clamp the cable between the plates. Might work better if you set a shallow channel in it to keep the cable from moving about.
If you can't make the plates, plates from a bicycle chain would work.
To get rid of the torsion along the central CF tube, instead of using six bearings and increasing the cost. Maybe consider using bushings around all three outer tubes. My thinking was PTFE but, another material could perform better
I know I’m really late in the game, but to terminate that synchromesh cable you could use soldered tubular cable lugs. Just a thought.
What about attaching a small spring to a small rubber o-ring then attach string to the o-ring available in those plumbing kits to fix a leaky tap. Might be smoother and not require that spiral string. Would be interesting to see which lasts longer. Just a thought...
I suggest you to put the bearings in different orientations, 3 to 3 with some angle (I sopose 60 degrees in between) to get more points of contact with the bars and to prevent them of moving around its axes.
Great design, thank you!
It may cover the rolling bearing in a concave shape instead convex shape. That will make it more surface to contact.
For the motor braking, you may use a big (small super) capacitor. (store the energy instead of dissipating)
8:42 For measuring precision, I bet you could mount the actuator to a pivoting mirror and reflect a laser off the mirror. Then, depending on how the (measurable) laser point moves with your (incredibly small) movement of the actuator, you can use trigonometry to determine your actuator's precision.
No clue if this will work or not but have you tried to do a compression clamping mechanism like you did with the carbon rods? You could make it a passthrough one so you could adjust the tension as needed.