The Differential Drive - A New Breed of Actuator
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- Опубликовано: 27 апр 2021
- I designed, built, and tested a unique actuator I'm calling the differential drive. It utilizes two identical motors, and the output rotation speed is the sum of the motors individual rotation speeds (with an additional overall reduction). It's something I've never seen before!
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They are used in tracked vehicles and are called adder/subtractor gear boxes. Essentially two differentials linked together. They allow one high torque motor to provide the main motive force to two outputs and a second lower torque motor to add and subtract its speed to those outputs. It allows precise control over the relative speeds of the outputs even with motors of similar torque.
and with 3 differentials and you can use the "steering motor" to move torque from one diff to the other via the central, allowing for a constatant total output, but varying difference between the two outputs. Thats how you get fine control on racing tanks like the Ripsaw. this way you dont lose speed and dont put too much backlash on the steering motor at high speed while maintaining steering reaction time.
That's a very well known application, and I didn't even think of it. Nice.
Can you mention some vehicles that use this? I had looked up this exact thing and found nothing
@@BrokeWrench cat differential steering
cant like cause 69
At the end of the day, every engineer's true desire: " It does look good and it is shiny".
You just described the “transmission” of the Prius, except that the Prius uses three input motors, mg1, mg2, and ICE.
Exactly. Ford/Toyota eCVT. I made one based off the linear toyota variant to dynamically share output from a windmill with two generators and one motor, one gen for low winds and the other geared for high wind. The motor/braking takes some power off the gens but it speed sets the distribution between the gens. Cost effective, no; Fun yes.
Check out the Fendt Vario or AGCO CVT transmissions
I knew I had seen this before but couldn't remember where. Hybrid cars. That's where.
Also some CVT were there's an electric motor who's sole purpose is to vary the gear ratio. I think modern tanks use something like this.
Not precisely... A combiner gearbox, which is what the Prius has, is a little different. Similar, yes, but not _quite_ the same. It's nice to see him make something to test his theories though.
Another gearing to study the history of the Subaru CVT transmission,, they were one of the first to try.. www.subaruoutback.org/attachments/cvt-diagram-jpg.26499/
Little fun fact:
The Toyota Prius is using a planetary differential to couple an electric generator, motor (and ice engine) and the driveshaft.
This system is widely belived to be the most cost effective and efficient way to build a hybrid drive train. In german, we call this system a "Leistungsverzweigter Hybridantrieb". 😅😁
toyota calls it eCVT
@@Eugensson in Toyota’s R&D lab it’s nicknamed the “power splitter”
Also the Chevy Volt's drivetrain www.greencarreports.com/news/1096942_2016-chevrolet-volt-powertrain-how-it-works-in-electric-hybrid-modes
German: “CONCATENATE ALL THE THINGS”
@@liesdamnlies3372 Luxembourgish: Do the same as germans, but in a most unexpected way, eg ZWEEEEËG
The best way to learn is to fail :)
Congrats on not giving up!
Agreed. Also, documenting expectations/hypotheses and how they relate to test results is fun to watch and learn about. One of the best aspects of MythBusters has been executed well here! Right on Levi.
Thank you, and I absolutely agree. The world needs more failures!
To be mentioned in the same thought as Mythbusters is an honor, thank you
Well, failure is how we learn and I really admire your full honest approach and not omitting the failures. I found this very fascinating, keep up the great work and we'll all learn something worth while. Awesome!!
The "reverse" differential drive you describe was used for a century (I believe) to actuate rudders on ships. Two large DC traction motors ran in opposite directions at high speed, with the rudder linked to the output shaft. The rudder could then be moved by varying the electrical power fed to the motors, one more and one less, resulting in a high torque at the rudder very quickly. This was before the days of sophisticated hydraulics, and avoided the need to spin up a single traction motor from rest every time a rudder movement was required.
This is beautiful. And your style is flawless.
I love that you turned a "failure" into a video about what you learned. Quality video!
I'm learning now, how to turn any quality video into an unmitigated disaster, or irreversible failure. But, thanks for watching.
@@witoldgrabowski9263 n. m m
.mmm
Awesome video, I like that you show where you went wrong, really showing people that engineering is about analyzing failures and not only about making perfect systems all the time.
15:42 "you live, you learn." - Great dude!
Beautiful build quality. Great presentation and example of the value of lab tests in conjunction with thought experiments. The first computer I worked on professionally, used physical differentials to perform vector math based on heading to integrate acceleration, and speed into distance traveled in latitude and longitude for the F4 fighter jets. It worked well - Back in my day. The entire navigation and weapons delivery was done in analog computing and was build before Apollo.
Wow, that's fascinating!
I never thought of a rear differential as being a planetary gear pulled into 3D, or a planetary being like a rear differtial folded inside out and flattened. Thanks for that!
I know it has been a while but timestamps would be useful
I designed this and applied for a patent on it about 12 years ago, it is a great system never put in market
May i get your patent number?
"I'm not saying I'm the first to ever invent this, but I am saying I developed it independently"
Also known as the "patent loophole" :-D
@@TheNefastor or: Why Patents Are Bullshit
I mean seriously what is sane about "I had that idea so no one else is ever allowed to think of it"?
@@kingmasterlord well that's not an accurate portrayal. It's more "I had an idea so everyone else must pay me to use it".
@@TheNefastor it's not a race to have unique ideas. you don't deserve my money just because you had an idea earlier than I did. I'm not using your idea, I'm using mine.
@@kingmasterlord yeah except it's hard to prove your idea is original if someone else has published it first. You can't really prove your ignorance (short of being a brexit voter 😅)
all you have to do is invert one of the inputs and you've just created the mechanical equivalent to an electronic operational amplifier where the gear ratios determine the "gain" and you can even feed the output back to one of the inputs with a variable gear ratio and you can change the "gain" or input to output ratio of the drive unit.
Go on ...
@@fire17102 "What is an op amp" ruclips.net/video/7FYHt5XviKc/видео.html just substitute "rpms" instead of "volts". That's a pretty sick idea btw.
@@DFPercush also torque equals current. And if you multiply torque*rpm you get horsepower which then can be directly converted to watts. The same energy you see in electrical systems
@@megadjc192 so what sort of applications or purpose would this serve? Sorry, not as knowledgeable about this stuff, but its really cool!
The primary usage of this is in automatic transmissions as they allow the syncronized smooth change of gears where one of the inputs is connected to the engine then the gears are set at a specific ratio then the other input is forced to zero rpm thus allowing you to selectively change the rpm and torque without have to mesh gears together. A planetary gearset is a mechanical operational amplifier. Also, one can use this at a simple analog computer. For example, you can compare two rpms of two different rotational shafts then compare the output rpm and that output rpm will be the addition or subration of the two input rpms depending on the configuration. You can use this effect to accurately measure a high rpm source by converting it down to a lower rpm and then allowing an extremely precise measurement. Basically anything you can do with an electronic op amp you can do with a mechanical one. You can also integrate and differentiate the same way and this was commonly used in old mechanical computers. Also the mechanical op amp is completely reversibale unlike its electronic cousin so as long as the two outputs add up to the same input you can have any configuration of rpm and torque on the two outputs. This is how a car differential works. The limited slip differential just has clamps on both of the outputs so that no one output will equal the input in terms of rpm or torque. Otherwise the other output will be zero which is usually undesireable with a differential.
This is exactly why I love the RUclips community. 5 minutes in my brain is all fireworks and i’m sketching and writing out ideas to compare with your findings at the end. You’ve earned a follower! Keep up the great work.
I just returned to this video after having seen it back when you first posted it. It’s clever and very well-explained!
I realized that this is essentially the same principle used in the ancient “north-pointing chariot”. There, the inputs were the rotation of the two chariot wheels, and the output was a pointer that s always pointed in the same direction, regardless of how you rotated the chariot.
Anyway, a great project and explanation of the project!
Not only is this a really interesting video, it is incredibly generous and presented with admirable humility. Thanks so much for taking us on your journey of exploration. Coming up with novel ideas, such as this, and testing them out is how we ultimately advance technology. Really, really enjoyed this. Fantastic project and presentation!
I really appreciate you documenting and sharing your experiments. Great work.
This is pretty similar to a differential swerve drivetrain that some FIRST robotics teams have experimented with before. It is also similar to some types of robotic gripper/wrist mechanisms, since you can actually pull two outputs out of the differential.
Brilliant and quick explanation of the differential gear - not an easy task. Well done!
This was great, thanks for you work. Your inclusion of your failed hypotheses and your thought processes prior and after testing made for an authentic watch.
It is the first time for me as well to see this idea. Very neat. You might be able to write a publishable paper on it.
Core XY mechanisms technically also double the speed achievable, although not in all directions thus strictly speaking don't double the velocity.
Even a 'failure' is a success, because something new is learned.
Fascinating - great video - seeing the process and your knowledge, is impressive
What a humble learner!
I like that you posted a video about a “failure”. You live and learn is something it seems many people do not subscribe to these days. It is nice to actually see the failures that contribute to the overall successes. We can all learn from your learning experience. Thank you.
I love this old black/white video explaining a differential, it´s genius and way way ahead of it´s time.
Really cool! Looking forward for the speed and precision test
Good video man. I appreciate your transparency.
Dude! This is cool! I love being along for the ride of discovery!
Thank you for showing us your whole development process and not just the successful parts.
Most interesting video I've watched in a while - comments are cool too! Subscribed
That was very educational, Mr. Janssen!
Very cool! Congrats on surpassing one million total views!
I'd really like to see a demonstration of the variable speed feature you described. Put encoders on the motors and output and test your theory.
It might be my bias in favor of encoders, but I have a hard time believing this mechanism will allow precise speed control. As with standard gearboxes, I think changing loads will require adjusting the power input to maintain a desired speed.
Excellent video. Thanks for sharing your projects with us.
Agreed, he "confirms" his two theories but didn't even demonstrate those two theories at all.
Look up Nissan electric variable valve timing.
That is a really good educational video on systematic testing of ideas and building knowledge. So thanks!
I used something like this in a lego truck a while back so that when stationary the model engine would run at an idle speed and then rev up when it started moving. Cool to see it implemented on this scale.
Keep up the good work. You're getting somewhere!
I used this same technique to win an electric tractor pull competition in 2011. This is a cool investigation and explanation :)
Very Awesome my man! well done!
Geez, someone hire this guy! This is very impressive.
Great vid!
You sir have my attention and earned a sub.
2:20 this method of planetary gears is used in many hybrid cars, the Chevy Volt does this where the outer ring gear can be held and use the reduced speed of the planetary or the planetary gears can be held to overdrive the ring, which it uses to recharge the battery. Great video!
No way, I was literally just working on one of these. Great job as always!
Fantastic experimenting, kudos!
Very cool. Sure it's been done before but it's the first time I'm seeing this concept. 👍
You should look at the drive system for the Sherman tanks, they use the difference of 2 motors to do forward backward and speed control. It is a very old idea
It's also used in mechanically controlled gear hobbing machines.
They had double and triple differential steering in certain tanks.
Paused the video to come say how much I appreciate that you laid out your hypotheses up front, admitted two were wrong (rather than cutting them), then moved on. Thumbed up and subscribed right away.
Very good idea; also i personally like very much this scientific method approach!! Well done
Before smart electronics, this was a common way to drive an equatorial telescope to both track the natural apparent motion of the sky and also position the telescope automatically to a new object. Because the "sky would move" some distance during the time of the reposition operation, the known destination (number of motor steps or encoder counts), would always be incorrect. By using a differential with one motor tracking and the other positioning, at the end of the move (in either direction), the desired object would be in the center of the field. In the early 1980s, while working on a telescope for the Navy's sky transparency survey, I found that making a numerical model of such a differential simplified the design equations significantly. Good memories.
Cheers on your clever design.
Quality content Levi! I'm so glad your videos are getting traction!
- Kaleb Rush
I really appreciate that you show when things go wrong, really true to life haha
With regards to the arbitrarily precise positioning; my understanding is that under applied load, in order to energise the windings; a motor controller requires either a sensor or encoder to know the exact position of the permanent magnets.
I don't quite understand how this requirement could be removed - as soon as you add load to the mechanism, the rotation speed of the motors will reduce. So without an encoder or sensor; you end up losing most of your torque as the wrong coils are energised in relation to the poles on the rotor. Perhaps you're implying the use of back EMF to determine the rotor position?
I should have read more comments before adding one myself. You just gave a better explanation of what I was trying to say in my comment. I really don't understand how this mechanism can produce precise speeds without encoder feedback.
Sensorless motors do have positional feedback, but it's achieved via measuring the back EMF across the coils, and sufficient back EMF is only generated at speed. So, a sensorless driver can maintain a specified velocity, compensating for external load based off the timing of the back EMF. Arbitrarily precise movement can then be achieved because the actuator can stall when the motors are at or above that threshold speed.
@@LeviJanssen Thanks. Thought that might be the case
So, to summarise;
back EMF can be used over a certain speed threshold and accurately maintain that speed
Run both motors above this rpm threshold.
run in opposing directions to "stall" the output (motors are still running at an rpm above back EMF threshold)
output entirely depends on ratio between the two motor speed+direction.
The most important is to have an idea and try to develop something new... Thanks for your story !
This is cool shit..... glad to have found the channel
Thumbs up, and Subscribed. Great job!
Absolutely invertable , nice
Great job, including hypothesis was great
What a great video! Thanks for sharing your ideas. This line of thinking could be relevant to a hybrid drive system. The idea reminds me of a Toyota Prius drive system where a motor/generator and an engine share inputs to a differential and torque is routed to where is is needed.
I like how it look like, very nice aluminum + 3D printed, nice video for people who like mechanics and we like to try different types of drivers.
Levi, you're an awesome man. One day, you'll save us all. (hopefully, you'll long be done by then, with these dark and awkward DARPA projects) !
Love this content!
I was wondering how to do this. Amazing youtube algorythm. Kings to you my friend. Brilliant
This is already done for agriculture machinery, high torque hydraulic motor (s) coupled to Diesel engine crankshaft to adjust for loads.
This reminds me of a constant-speed drive system from an aircraft alternator. It uses a differential kind of mechanism to limit the output shaft to be a constant speed regardless of whether the aircraft engine was spooling at idle or at max thrust. It uses a motor to spin the other side of the differential to change what the speed of the output is. It's not exactly what you've done here but it is similar and I'd recommend looking into that mechanism
Wow this is really well applied! In 2018 I actually did my high school senior year engineering project on this concept using a 3D printed herringbone planetary gear set. It was designed as a way to include two motors on an RC car. In my prototype I opted for biasing the gear ratio to take advantage of torque and power over a wide range of rpm. (The concept was designed around putting an electric assist engine on a gas powered RC by replacing the transmission and clutch, creating a unique RC hybrid) it was pretty cool Bc theoretically the electric can spin in reverse in order to let the gas engine idle without a clutch, and also combine the power of both engines for max power during runtime. Back driving the electric motor was an issue, just like it was in your test, so I think any future version of the coupler I made would need some sort of servo actuated or electric brake on the electric motor. 3D printed transmission burned out in only a few minutes but it was a fun project!
Have the electric motor drive the outside of ring gear via worm screw.
As far as I know they use such gearsets to drive the shaft of big ships. Seamless way to add a second motor to the output. really nice video & work!!
I have cnc mill, lathe and wire edm but my favorite is my 3d printer. Best tool to bring simple to complex ideas to life quickly and go through the revision process with minimal pain.
This is why we do experiments. You didn't get the result you hoped for, but we all learned something. Great job, keep experimenting, who knows what you will develop.
This will work great for my rc cars. I've been dabbling with a similar concept just with 3 planetary gears and 2 motors
you gave this a lot of thought. Keep those gears turning, congrats
The outboard spoilers (used for additional roll control at lower air speeds) on A B-1b aircraft use triple redundant motors driving through two differentials. I was the product engineer for the electronics that control them for most of the production run.
I am wondering if the the torque issue could be because when you run the motors in opposite directions and then apply force to the output shaft there is always a motor spinning in the same direction as you're applying torque to.
Great design!
Is it posiible for this designe to use stpper mottors, or BLDC motor are reuired? I was thinikng about it, and it seems that with stepper motors if primary motor would be power up, but wouldnt turn at the moemnt, and the primary housing was to rotate becouse of the secondery motor, the cycloid dysk would stuck in one position and resist the rotation of the primary houseing. If there is missunderstanding on my part, would anybody like to explain it to me? I would love to designe something like this using stpper motors becoise bldc are to expensive for me.
What type are the two cycloidal gear units? Are they compound cycloidal gears each?
New viewer here: cool video! Well done, really enjoyed the editing and pace. Small tip: I got a bit distracted by the reflections om your glasses though. Have a good one!
The easiest way for me to think about this is to just mount next motor to the output shaft of the previous motor. If you imagine a longer chain of motors, then I think how the stall torque distributes begins to make more sense.
And it also explains why Hypothesis 4 doesn't hold water: If you add up two values with independent errors, the error add up quadratically. So the precision of speed or position control of two coupled motors will always be worse than a single motor. Of course you can get below the minimum RPM of a single motor, but with a horrendous quality of control.
Thanks for sharing!
This is an extremely impressive video. Fantastic work mate and well done on being so honest about your hypothesizes.
How is the backlash? I'd be guessing that you could code the motors to reduce it to an acceptable level.
Ooh, I built something like this a while back to make a two wheel drive robot go straight. I used a differential style input to each wheel with a drive motor and a steering motor. The steering motor added to one side and subtracted from the other. The motors were set up with different ratios because you never want to steer as fast as you drive, the steering motor was smaller and I was trying to avoid back-driving the steering so it had a higher ratio. Very interesting to see the more general application... I thought it was interesting at the time, but never played with it further.
good video. I like the original ideas.
Really cool video. I have actually used this method several times when building with Legos. The lego motor system is usually full power on or off. So when building a tank for example the turning is not smooth and can be hard to control. Using this method of adders and subtracters by means of a differential I was able to add speed to one side while subtracting from the other side. This would create smooth turns and allow for more functionality and tank like movements.
What if you have reducers before coupling both together. I think this actually can work with some modification
Hey I did this back in highschool with a vex robot, I used a planetary gear model instead of a cycoidal drive, but it still had to use a chain to reach in the gear and drive the planet gears directly, it was one of my favorite designs to work with, I wasn't useful for any of the competitions but was very fun anyway
Very cool!
An IVT from a Toyota hybrid uses the same operating principle. The benefit is that you effectively "blend" the two ratios, allowing for a combination of generation and drive, as well as the fact that you can run the electric motors in reverse, or idle them, allowing the petrol motor to run without stalling even when the output isn't operating.
I know of at least one radio telescope that uses two motors driving a gear. By having one pushing against the opposite gear faces, there is zero backlash, and driving in either direction is simply a matter of increasing torque on one and decreasing torque on the other. It’s good for arc second stability.
This sounds a lot like an IVT or infinitely variable transmission, versions are used in some hybrid cars and john deer tractors. Usually 3 motor inputs (john deer uses 3 cvt inputs) are used to control the sun, ring, and carrier of a planetary creating "infinite" gearing.
I initially thought this would be an IVT/CVT system, hence the double-torque hypothesis. But, considering my final thought experiment and the experiment, this can't be the case. As far as understand.
I used the same idea for a hybrid transmission, but after some research found it was just a simplified version of what is used for the CVT part of most hybrids
This was done in 2006 in the agriculture tractors. It was a CVT transmission. On a planetary gear set. The sun gear was driven by the engine RPM. The planetary carrier is splined to the rear differential. The ring gear was driven my a hydraulic variable displacement piston motor. The pump that supplied said motor was driven by the flywheel of the engine. It was also variable displacement. This gave the tractor great control over torque and speed. To me. It's the way CVT should be done
Your brain works a lot like mine, so I instantly thumbs up, and subbed. Digging the channel already.
Same idea in some mechanism that have to be able to move fast and slow using different reductions in each motor you can achieve A+B/2 A-B/2 in both directions.
Some aircraft system that require redundancy in the motor but not in the driveshaft has similar setups.
This idea is used in the Koenigsegg Regera to make a high performance transmission with no gear shifting.
There is a electric motor joined with a internal combustion engine. So running the motor is reverse slows down the engines output speed for low gears while running it forwards increases the output speed for high gears. This also removes the need for a clutch because the output can have 0 RPM while the internal combustion engine is running. To assist the lack of torque at low speeds extra motors are used in parallel with the wheels to provide additional torque (Electrical motors are very good at making high torque at low rpm so this nicely uses there advantage)
FYI, Toyota uses these in their hybrid transaxles. This mechanism is used to couple the gas motor to the generator and starter motor, so that the starter motor and Generator can spin freely, while the engine is not turning, but also can be stopped or turned start the engine, as well as act as a Cvt.
Would it be more likely to work with planetary gears like in the first graphic?
so, im not too certain about the answer to your question, but i think the answer would be no. the reason being the motors are going to be exerting different torque on the output because the outer motor will have a different gear reduction compared to the inner motor. this falls back on the weakest link of the motor, what ever motor has the weakest torque rating that that said speed will fail first. the only way to remedy this issue will be to directly link the motors.
It should work the same for all reduction systems.
@Levi Janssen Hey man, totally off-topic, but today I was binging your coilgun videos and concepts, and was wondering, if the projectile was modified in some way, would that open up possibilities towards a more efficient build? One example I though of off the cuff was a steel slug with a taper hollowed out with the wide end towards the rear, and filled with lead. Would that possibly mitigate the pull that interferes with the speed as the projectile transfers to the next coil in the sequence?
Lots of load on the OutPut! Hmm, how could get a Trans-Stall on that backwards Diff?
What if you did this with a non back drivable gear set, like a strain wave gear for each motor. Shouldn’t the new stall torque be double the stall torque of each harmonic gear individually? I may be thinking about this wrong Ive only been thinking for the last minute or two, but it seems that since they cant be back driven, it would be the outer shell that would have to fail in order to back drive. Since the 2 outer shells are attached with a toothed belt, you would have to cause both gears to fail together in order to break it?
A similar system is use on Aircraft to drive generators at the correct frequency, it's called a Constant speed drive unit (CSDU). At low engine speed the drive speeds the alternator up with a motor, at high engine rpm the motor is driven in reverse to lower the alternator frequency. Avionics on Aircraft are frequency sensitive unlike cars that have frequency wild alternators.