[PowerElectronix]

The closed loop experiences a phase margin loss that is exponential with the frequency. At lower frecuencies it is negligible, but if you get close to the frequency of the delay the phase margin reduction becomes dramatic and the control goes from stable to unstable very fast.

If the sensor has a limited bandwidth, you add the conversion delay and then the computation delay on top of that you end up with a max workable loop bandwidth in the low tens of kHz and anything higher will have overshoots, oscillations, etc.

[topspin]

You see this in low cost products like MKS SERVO42x, where they're doing FoC with a GD32 MCU. It works; the motor runs cool, smooth and quiet, but the system is limited to 3000 RPM, and struggles with rapid acceleration because the control loop is too slow.

[mianos]

I have tried one. It has no torque. For what looks like an awesome product, it does not have the power to drive a peristaltic pump. I used the same motor on a TMC stepper controller and it's completely silent and works. It's open loop, so comparing apples to oranges but I am not sure what the MKS servo driver on a motor could actually do, aside from spin unloaded.

[topspin]

These can deliver 2.5-3A/phase, which should ample for a pump. Respectfully, I wonder what motor was involved and whether the current was configured: they come out of the box with conservative configuration so people don't burn up motors.

[mianos]

Nema 17

[GeorgeTirebiter]

This is exactly correct. Low 10s of kHz is quite functional for machines moving in human space / speed.

If one is trying to do some assembly line (max # of operations per second), the power requirements alone get hard to manage. And then you're managing back EMF, eddy currents, heck, air resistance!

My rule: have dedicated low-level hw provide smooth PID response, mostly on the P term; and have a higher-level control produce the setpoint. Faster response means less need to rely on I or D terms as much (just because delta-T is so relatively small).