5 ms·

Using only PD (no I) when tracking also works well. Might be worth adding a short section about that to compare against the full PID.

by cgg1 3y ago

azeemba 3y ago
Interestingly, there was another PID post earlier today and someone specifically commented on using a PID for camera tracking: https://news.ycombinator.com/item?id=39011630#39016836 https://news.ycombinator.com/item?id=39011630#39016836 I am surprised to hear about a PD controller though. In my testing, a PI controller seemed to behave much better than a PD controller. In my research, it seemed a common strategy to drop the D-component completely but I did not see the suggestion to drop the I-component.
- cgg1 3y ago
  YMMV I guess :)
- ok_dad 3y ago
  I used a PD controller for a specific application where we were trying to maintain a specific temperature in the future, and cared more about the trajectory of the temperature over time to reach the goal. We didn’t much care for the integration of that temperature since that wasn’t important in this particular case. An integration factor would actually wind up the rate of temperature change too much!
  - dwattttt 3y ago
    Integral windup is the term for this issue, there's a few ways to deal with the windup if you need to keep the integral component (e.g. for tracking a moving set point, or to overcome a stable error). https://en.wikipedia.org/wiki/Integral_windup https://en.wikipedia.org/wiki/Integral_windup
- claytonwramsey 3y ago
  In terms of the theoretical guarantees, a PD controller is (nearly) always stable and pretty easy to critically damp. However, if the set point is moving with a nonzero velocity or there is some load, then the PD controller will not converge. A PI and a PID controller can converge even if the target is moving. Practically speaking, I would recommend encoding as much model information as possible before pulling out the I term: for instance, if you know the velocity of your tracked object and assume an intertidal model, you can just include some of that data in your feed forward rather than the controller, which simplifies things greatly.
  - fho 3y ago
    Do you have some resources on that?
    - YZF 3y ago
      You can use your intuition in the sense that if what you measure just has a constant error without an integrator in the loop that error will never be closed. Think about a motion control system with a proportional gain where you are pushing the actuator with your hand away from the current position. The proportional gain will apply a force against your push but generally that will leave some residual error in steady state. This is where the integrator comes in. And I agree with the parent that I've mostly seen the D term dropped from controllers, definitely with motion control, though sometimes there are other weird components or multiple loops which may end up having a similar effect. One thing that I think is often confused the matter is that P, I, and D all relate to what you measure and what you're controlling. E.g. you might measure velocity or position in a motion control system, and generally your output is going to be current. In a temperature control system your measurement would be temperature and your output is likely going to be current (which influences acceleration, not position or velocity). Ofcourse velocity being the derivative of position means that the meaning of "P" is different. I haven't done PID in a long while but I've always used to ground myself in motion control systems to get a sense what I want to put the loop over. EDIT: E.g. IIRC it's common in motion control to have a PI loop over the position error and an additional proportional controller over the velocity (and then filters, feed-forward and a bunch of other components ;) ).

Using only PD (no I) when tracking also works well. Might be worth adding a short section about that to compare against the full PID.

by cgg1 3y ago

Using only PD (no I) when tracking also works well. Might be worth adding a short section about that to compare against the full PID.

azeemba 3y ago

Interestingly, there was another PID post earlier today and someone specifically commented on using a PID for camera tracking: https://news.ycombinator.com/item?id=39011630#39016836 https://news.ycombinator.com/item?id=39011630#39016836 I am surprised to hear about a PD controller though. In my testing, a PI controller seemed to behave much better than a PD controller. In my research, it seemed a common strategy to drop the D-component completely but I did not see the suggestion to drop the I-component.

cgg1 3y ago
YMMV I guess :)
ok_dad 3y ago
I used a PD controller for a specific application where we were trying to maintain a specific temperature in the future, and cared more about the trajectory of the temperature over time to reach the goal. We didn’t much care for the integration of that temperature since that wasn’t important in this particular case. An integration factor would actually wind up the rate of temperature change too much!
- dwattttt 3y ago
  Integral windup is the term for this issue, there's a few ways to deal with the windup if you need to keep the integral component (e.g. for tracking a moving set point, or to overcome a stable error). https://en.wikipedia.org/wiki/Integral_windup https://en.wikipedia.org/wiki/Integral_windup
claytonwramsey 3y ago
In terms of the theoretical guarantees, a PD controller is (nearly) always stable and pretty easy to critically damp. However, if the set point is moving with a nonzero velocity or there is some load, then the PD controller will not converge. A PI and a PID controller can converge even if the target is moving. Practically speaking, I would recommend encoding as much model information as possible before pulling out the I term: for instance, if you know the velocity of your tracked object and assume an intertidal model, you can just include some of that data in your feed forward rather than the controller, which simplifies things greatly.
- fho 3y ago
  Do you have some resources on that?
  - YZF 3y ago
    You can use your intuition in the sense that if what you measure just has a constant error without an integrator in the loop that error will never be closed. Think about a motion control system with a proportional gain where you are pushing the actuator with your hand away from the current position. The proportional gain will apply a force against your push but generally that will leave some residual error in steady state. This is where the integrator comes in. And I agree with the parent that I've mostly seen the D term dropped from controllers, definitely with motion control, though sometimes there are other weird components or multiple loops which may end up having a similar effect. One thing that I think is often confused the matter is that P, I, and D all relate to what you measure and what you're controlling. E.g. you might measure velocity or position in a motion control system, and generally your output is going to be current. In a temperature control system your measurement would be temperature and your output is likely going to be current (which influences acceleration, not position or velocity). Ofcourse velocity being the derivative of position means that the meaning of "P" is different. I haven't done PID in a long while but I've always used to ground myself in motion control systems to get a sense what I want to put the loop over. EDIT: E.g. IIRC it's common in motion control to have a PI loop over the position error and an additional proportional controller over the velocity (and then filters, feed-forward and a bunch of other components ;) ).