I'm currently developing a motor controller with FOC (Field Oriented Control) and I have some trouble with the offset calibration of a motor.

What I actually do to calculate the offset: I inject a voltage directly in the d-axis, such that the motor alignes. The offset is then chosen as this position. When I then calculate the electrical angle, I add PI/2, such that I get an alignement with the q-axis.

What I expect: When the offset is chosen the right way with the method above, the motor produces the maximum amount of torque/ampere for the motor and it should run the same in both directions.

What I get: The motor produces the maximum amount of torque, but the motor turns way better in one direction than the other. I see also high id spikes when this happens as shown below. When I manually adjust offset a little, then it turns the same in both directions and the spikes depicted below are gone, but it has no more the maximum torque.

What I further see is, that when I exchange the Phase U and W, it works better in the other direction, so the motor should not be the problem here. Has anyone an idea, what could cause this problems?

Speed Current Peaks

  • 1
    \$\begingroup\$ Commutation spikes are from your lack of dead-time and snubber design not shown. Asymmetry in direction is also not shown due to motor + Load friction differences but mainly your full bridge RdsOn and timing differences. Show all design calculations , details and layout for reactance RdsOn (Vgs-Vt)/gm, motor DCR and kV/RPM no load in both directions . \$\endgroup\$ – Tony Stewart EE75 Jul 14 '19 at 14:36

I experimented with a number of different offset calibration ideas while building out my FOC system. The method I started with is the method you described, locking the motor electrical angle with an inverse park transform and comparing it against my encoder angle. I quickly moved away from this technique because it is wildly inconsistent. It's easy to show why; request a fixed electrical angle via inverse park, plot out your encoder angle, and introduce mechanical disturbances to your rotor. It will settle to a slightly different angle every time, due to real-world effects (friction, cogging torque).

Based on experience, your observation that the motor 'spins better' in one direction vs. the other almost certainly comes from a bad estimate of the offset angle. You should be able to tweak your offset value and observe consistent torque in both motor directions.


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