Torque limitation in Field Oriented Control of PMSM
Torque limitation in Field Oriented Control of PMSM
(OP)
Hi,
I'm controlling my PMSM at zero speed. While I'm trying to move its shaft I don't want my motor to generate its maximum torque to resist my movement but the limited one.
Everything went fine with classic control, but as soon as I tried to limit it's torque -- by limiting outputs of speed and Q-current PI controllers on some smaller values -- the motor started spinning (?!).
I really have no idea how to limit the torque generated by motor, exept by limiting outputs of PI controlers in FOC code.
Any suggestions?
(I'm working with Microchip's dsPIC 5015 in MPLAB X IDE.)
Thanks in advance!
I'm controlling my PMSM at zero speed. While I'm trying to move its shaft I don't want my motor to generate its maximum torque to resist my movement but the limited one.
Everything went fine with classic control, but as soon as I tried to limit it's torque -- by limiting outputs of speed and Q-current PI controllers on some smaller values -- the motor started spinning (?!).
I really have no idea how to limit the torque generated by motor, exept by limiting outputs of PI controlers in FOC code.
Any suggestions?
(I'm working with Microchip's dsPIC 5015 in MPLAB X IDE.)
Thanks in advance!





RE: Torque limitation in Field Oriented Control of PMSM
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Torque limitation in Field Oriented Control of PMSM
And could you please tell why can't it be done simply by limiting outputs of speed and/or Q-current PI controllers?
RE: Torque limitation in Field Oriented Control of PMSM
What size is this motor?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Torque limitation in Field Oriented Control of PMSM
I didn't implement any braking functions in my code since I do not have braking resistor.
RE: Torque limitation in Field Oriented Control of PMSM
Would it be safe for motor, because D component of stator current is usually set to zero in speed range from 0 to nominal speed?
RE: Torque limitation in Field Oriented Control of PMSM
However, one thing you must do is to make sure your integrator is not "winding up" when your torque output is saturated on the limit. This "anti-windup" protection is simple (just don't add to the integrator if saturated), but very important to maintain stability.
I think what others are suggesting is basically to operate the motor like a stepper motor instead of a servo motor when stopped. Under closed-loop servo control, your Iq command output to try to hold position is proportional (P) to your angle error and the time integral (I) of that error, and your Id command is usually 0.
Under "stepper" control, you just have a fixed Id command allotted to the phases based on commanded (not actual) angle. The restoring torque is proportional to the (sine of the) physical angle deviation from this commanded angle. (All this happens inside the motor, not in the code.) While this is commonly used in applications without an angle sensor, I do not recommend it if you have a sensor on the shaft. Figure out what is wrong with your code instead.
Curt Wilson
Delta Tau Data Systems
RE: Torque limitation in Field Oriented Control of PMSM
Do I have to limit exits of both speed and q-current controllers or just one of them?
RE: Torque limitation in Field Oriented Control of PMSM
RE: Torque limitation in Field Oriented Control of PMSM
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Torque limitation in Field Oriented Control of PMSM
RE: Torque limitation in Field Oriented Control of PMSM
I've got 4096 PPR encoder on my surface mounted PMSM.
RE: Torque limitation in Field Oriented Control of PMSM
RE: Torque limitation in Field Oriented Control of PMSM
I am not sure about your final goal. Do you expect to make a smooth movement of shaft (example - 0 deg to 90 deg - Robot arm to show the movement-->exercise)?
If that is the intention, please consider using a servo motor (or use the motor with position feedback). Movement of shaft by a small degree without feedback will be difficult because of the non linear characteristics due static friction etc. Hence a closed loop control with a smooth ramp to the desired position control scheme will do a good job. Hope this helps.
Kind Regards
Srikanthan Krishnan
RE: Torque limitation in Field Oriented Control of PMSM
RE: Torque limitation in Field Oriented Control of PMSM
If his set up is a direct drive unit, like a bar attached to the motor output shaft and someone pushes against it, then with proper torque limit via Iq current limit it will not 'spin' as he says but just let the outside force move the bar. Here there will be no 'spinning' if torque limit exceeded unless his control algorithms are flat wrong.
If his set up is some gadget like weight machine where output moves a wire around a pulley as someone pushes against something, with proper torque limit via Iq current limit it will 'spin' until the motion is finished. This is as it should be of course.
I would suggest you consider simply buying a quality servo motor drive to run your motor instead of trying to invent what has been invented and proven well for years. It will be much cheaper than the time you put into trying to invent what has been invented and refined before. A good 2kw 400v drive will only cost about $ 800 and be available from stock from many sources.
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