Desired response of PI controller in motor vector controller

control systemmotorpid controller

In vector control (field-oriented control) of motors (specifically induction motors), a rotating reference frame is attached to the rotor flux to give "DC" values in the current controller. This enables PI control to be used (which is not possible, or at least not ideal, with continuously varying signals, e.g. sinusoidal waveforms in a static reference frame).

The gains of the PI controller can be tuned to give a good response to changes in current demand. My question is: how does the response of the current controller (level of damping) affect the torque response of the drive?

This is probably application-specific, but are there any general rules, like "always under-damp since the mechanical system has a slower response" or something like that (this is just a hypothetical example).

Best Answer

In control system theory, "is a critically damped response ideal" is relative. Critically damped systems ARE ideal, only if you are looking for the fastest possible rise time with zero overshoot and fastest possible settling time (due to there being no ripple).

The Clark/Park transforms in Field Oriented Control are what provide DC style signals you refer to.

If the application of the control system favours very fast reaction speed (for example, a load is added to a spinning device which slows it down, the ability to return to the required/set speed is what the control system would do) and cares little about overshoot would have very little damping, and high proportional gain in order to get in this case, the "ideal" response. That is, the designers do not care if the speed of the spinning device goes over the set point, and oscillates a little bit before settling back to the correct speed with the load now on it.

If your device is a medical robot arm that does surgery on people, you would very much like a damped response, perhaps "critically damped" is too difficult to achieve, so the designers would allow for a nice, slow, safe control of the robotic arm movement - perhaps in reaction to the movement of a haptic feedback device operated by a doctor. Overshoot in this case might cut people in strange places, so is undesirable! Control systems are very arbitrary in exactly what they are "controlling" and what the inputs