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Low-speed control with STEVAL-SPIN3201 and FOC SDK

Question asked by Max Schachtschabel on Nov 30, 2017
For the past few weeks, I have been working with the SPIN3201 Eval-Board to control a small BLDC motor which is usually used for electrical-powered Longboards. I have chosen a purposely slow model with only 60KV (rpm/V), which I want to run on 12V DC Voltage (max. speed of 720rpm).
I was able to set up the board, the internal Hall-Sensors, the Firmware and I measured/estimated the neccessary motor parameters; now I can control the rotation Speed of the motor quite well using an external Servo tester (basically a voltage divider) or the Motor Monitor within the ST Motor Workbench.
Now, I have some issues when I want to set the motor to a particulary low speed compared to the maximum, e.g. 100rpm or lower. Once I set the speed ramp to such a value, the motor either comes to a stop or does some fidging around, clearly showing that the FOC is not aligned with the current rotor position. Since I am quite sure that the
motor parameters are correct at the moment, I tried adjusting the PID constants of the Torque-, Flux and Speed-control, which brought nice results with higher Speeds (good adjusting time, no overshoot, no offset), but the lower speeds still show the same behaviour. When I apply small loads during these moments, the motor is not able to bring the neccessary force to rotate and makes noticable jumps between the phases.
Since my application has a focus on low rotation/high torque scenarios, which should fit the motor quite well (1800W, up to 55A peak current and 8.5Nm of tourque), I am searching for more ways to adapt the FOC SDK for smoother and more robust control schemes.
Does anyone had similar experiences with another motor and can give me a few hints where to look?
For those wo are interested, this is the motor I currently use:
And here is the list of parameters that are not stated and I needed to measure manually:
Pole pairs = 7
R_s = 0.55 Ohm
L_s = 0.5 mH
B_Emf constant = 12.5 Vrms/krpm

Many thanks in advance,
Max Schachtschabel