Speed control of a BLDCM at low speed

Question

I have here a BLDC motor that is controllable with FOC and I already implemented a position controller. The next step is to implement a velocity controller, but I actually don't see how to control the velocity at very low speeds. The speed is proportional to the reference torque producing current i_d for FOC and so for low speeds, there is only a very low current and so nearly no power. This immediately can be seen, that the motor becomes bumpy for low speeds. The implemented speed controller looks like this and works fine for speeds around 50rpm or higher.

i_q_ref = i_q_ref_old + K_Perr_speed + K_Isum_err_speed

The speed measurement is pretty accurate.

What kind of other control structures can be used to achieve a velocity controller for low speeds with FOC?

Answer 1

If you already have a functional position controller, you can just integrate the target speed to get a target position (which is moving). This should work relatively well for low speeds, but probably not when the motor starts moving a significant fraction of a revolution per control loop iteration.

But I think the underlying problem is probably either latency or inaccuracy in the speed measurement. I assume it comes from some kind of encoder? At low speeds, it can take a long time for the motor to move to the next encoder tick, thus the velocity change is not immediately visible to the controller. And if you don't have any smoothing on the velocity reading, it can render PID D-term quite ineffective; and if you add smoothing, it will increase the latency.

One way to increase the speed of the feedback loop is to form some kind of a feed-forward system. Consider having a Kalman filter with state variables (velocity, inertia, external_torque). Then you can update that Kalman filter by both your velocity measurements when you get them, and more often by torque values that you drive to the motor.