BDC Motor Control - feedforward

BDC Motor Control

I have a BDC motor in witch I have to maintain a really constant speed. The rotor has considerable inertia. They happen random rapid changes in the load. In the rotor attached is a simple optical encoder from witch i read the speed. In general this is a feedback system, so I make corrections based on the difference between velocity SP and velocity PV.

I have good results, but by definition in one only-feedback control system I could not avoid small changes in the PV because this same changes feed the control loop. So i thing about feedforward. But there isn't other values except velocity and current that I could use as input.

So my question is: in this system when a rapid increase in the load happens, if instead of the decrease in the speed i expect for input an increase in the current, I would benefit something in response speed? Or in other words, the motor current increases before the speed falls ?

Thanks in advance

Re: BDC Motor Control

Hello t_maggot,

I think that you'll gain nothing, because the increase of current is the consequence of the control loop action: The speed decreses, so (SP-PV) increases commanding more tension (or more current if your power driver uses current feedback).
I think that you must think in the delay introduced by the speed calculation from the encoder input (maybe the sample interval to do that is excesive for your expected dynamic response).
If you can reengineer your system, you could think in a PLL-type control loop.

Hope this helps and best regards.

BDC Motor Control

Hi jorgito,

I thing also that probably I will gain nothing, as for the delay from encoder, this is a fact, and the speed read is much slower than the current measurement, but my question is if we take this delays as zero.

I dont know much about PLL motor control. Can you give me any references? What are the dis/advantages comparing to PID ?

Thanks

Re: BDC Motor Control

A higher PPR encoder could be a solution.

Nandhu

(But as per my idea there will be no zero delay)