Feedback and Feedforward Control
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- Опубликовано: 26 сен 2017
- Four exercises are designed to classify feedback and feedfoward controllers and develop control systems with sensors, actuators, and controllers. See apmonitor.com/pdc/index.php/Ma...
Наука
Feed-forward design is a type of voltage regulator circuit that doesn't require negative feedback. It's simpler than a negative feedback design and is less likely to oscillate, making it more stable. However, it can be less accurate than a negative feedback design and is more sensitive to component variations.
Hey John, I'm very appreciative of everything on apmonitor as I try to relearn process control. ( I have a Ph.D. in chemical engineering and this was my worst class as an undergrad). One of the issues I have with Exercise 2 is that I think the valve should be placed downstream of the overflow tank. I do this for two reasons, the first is that overflow tanks are used to allow uninterrupted operation of the upstream side; so a valve on the upsteam side defeats this purpose. The second reason for placing the valve on the downstream is to prevent de-priming the pump. The LC & FC should connect to both the valve and the pump, the FT should be upstream the tank. The tank then needs to be sufficiently sized to handle any operational variability. Ideally, you might want to use multiple pumps in order to maintain continuous downstream operations. Thanks for the videos.
Thanks for the great suggestions.
Great audio and voice
could cascade be used in the example at 13:40? where concentration controller feeds a setpoint for a temperature controller. the temperature controller receives the measured temperature and adjusts the valve. could this be possible or beneficial?
Yes, that is certainly a possibility. Cascade and feedforward are often two options.
Please what is the textbook name that exercise took from? Thank you
It is an online course: apmonitor.com/pdc/index.php/Main/ControllerDesign
I love the duck! :D
I don't remember a duck in that video. Could you remind me?
Thank you for the good explanation. However, in exercise 4, shouldn't the TT be connected to an FC, because you are actually controlling the flow of the fuel, not the temperature.
Exercise 4 solution is correct but I see why you have the question. It is mostly a terminology issue. We are controlling the temperature by manipulating the inlet flow. The fuel flow is the Controller Output (OP), the setpoint (SP) is a temperature, and the Process Variable (PV) is also a temperature. You write the symbol (TC) for the process value that is associated with the SP.
@APMonitor.com I'm having a hard time understanding the feedforward part of the Exercise 3. Why is the temperature measured in the tank a measurement of an input disturbance? I would have expected to have a measurement of the disturbance maybe in the input pipe that feeds the polluted water. Measuring the temperature in the tank feels like measuring the output (like when dealing with a feedback controller). May you explain this a little more please?
Good question. A disturbance that we reject with a feedforward trim isn't necessarily a thing that is positionally located. It is something like temperature that then affects the concentration. We can either create a cascade control to maintain a temperature or else a feedforward controller that compensates for fluctuation in temperature.
@@apm Thank you very much for your answer!
It is a nice video. I have not understood why controller 2 is feedback controller. It seems to me that it is feed forward.
Good question. It is a feedback controller if you are directly measuring the thing your are trying to control. Feedforward is if you measure a disturbance and try to proactively adjust the actuator to anticipate the effect of the disturbance before it creates a deviation from the set point. Feedforward is rarely used without a feedback controller so example #3 on problem 1 is not very realistic. Controller #2 of problem 1 is feedback control even though the measurement is upstream of the actuator. The mass flow upstream of the valve is equal to the mass flow at the valve or downstream of the valve, unless you had a very long pipeline and a compressible fluid. There is another video on this topic: ruclips.net/video/QUfNGwBQJ-s/видео.html
Let's consider gas flow: * Feedback is the system recognizing High flow rates and controller signal adjusts flow i.e. the system is going 'flow is too high, therefore I am going to control flow' ? * *Feed forward is the system picking up an increase in pressure and going ' uh oh this is not good, this is going to affect the flow rate of the gas. Let's correct the high flow rates by adjusting the temperature to compensate for the pressure' ? * *If I am correct, is adjusting the pressure still feed forward or does that make it feedback? Essentially is 'fixing' the pressure by adjusting the pressure (e.g. controlling a compressor when flow rate is the focus downstream) feed forward? *
Feedforward control in your case would be to adjust the flow loop valve based on a pressure disturbance upstream. If you are correcting for pressure changes by adjusting temperature then you have a separate control loop that is actively rejecting the disturbance. The two seem like they are independent loops. There is also cascade control if the flow loop (master control) were to give a setpoint to the upstream pressure controller (slave control). More info on cascade is here: apmonitor.com/pdc/index.php/Main/CascadeControl