PI control of PMDC motor at low speeds

[Umit45]

Hi everybody,

I have implemented PI control of a DC motor and it works properly at high speeds (50rad/s) but if I lower the reference speed to 5 rad/s, there are fluctuations in its motion. Initially, I thought it could be due to the limitation of my current sensor (LTS 6 np) but I added some load, so that current is at a higher value (around 30mA) but the fluctuations are still present. I tried to increase the frequency of the PWM but due to limitations in my board, could not test beyond 20KHz.

Will increasing the PWM frequency have any positive effect at low frequencies?

Your comments and suggestions are welcome.

Thanks,

 

[chuckey]

At very low speeds the motor while "cog", i.e. jump positions from pole to pole. try a flywheel.
Frank

 

[Umit45]

Do you mean a freewheeling diode (1N5822)? I have them across the MOSFETS (IRF530).
I could not find the max input frequency for the MOSFET or the diode from their datasheets, hence I was not able to test beyond 20KHz. Do you think at high frequencies the performance will improve?

 

[FvM]

You post is very unclear. You tell about PI controller, but which quantity is actually measured as process value and controlled? If it's a speed controller, how are you measuring speed? What's the role of measured motor current?

 

[Umit45]

I am measuring both the speed and the current of the motor. I am using current (inner) and speed (outer) PI control loops for controlling the speed. Current is being measured by LTS 6NP and speed by the encoder, WDG 50B

 

[FvM]

You didn't yet tell that it's a cascaded controller topology, that's essential information, I think.

Using an inner current control loop won't be my first idea for PMDC, at least I would add voltage (e.m.f.) feed forward, bypassing the current controller. But it should work somehow as is.

What's the encoder pulse frequency in the operation point with "fluctuations"?
By first guess would be a problem of discrete speed measurement.

In any case you should out which controller primarly brings up the fluctuations. It should be possible to operate both separately.