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Originally Posted by Tom Bottiglieri
I've always thought it was a rule of thumb to use PI for position control and PD for velocity control, especially with the dead band on the victors.
In position control, you want to accurately stick to one position. (yeah, I know... duh... ) With just P control, the error can get so small that you are sending a output of 0 (-10 - 10 - victor dead band) to the motor control. You want the integral term to look at that error building up and over power it.
In velocity control, you want to change your output on the fly, and compensate for where you think you need to be. PD works fine.
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Maybie i am mistaken, but it thought the opposite was true and you were suppsoed to use PD for position and PI for velocity/temperature. It seems to me that when controlling velocity, I would be very important because it ends up canceling out friction and other forces that would prevent P alone from reaching the setpoint. It seems like D is very important in a position control system because it makes the system stable. If you model a perfect frictionless system with a position P controller applying a force to a mass, the system will oscillate in a sinusiodal fashion. If you add D, it becomes stable. You make a good point though about I eliminating the steady state error.
In my PD code i have a minimum output signal that the controller is allowed to output so long as the controller is outside of its allowable tolerance. This minimum is just enough to overcome friction, the victor deadband etc.
Perhaps i should code in an I to my control loop.
Obviously PID is the ideal solution.