Encoder requirements for a motion application (low speed/holding positions)

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Thread Starter

rpicatoste

Dear all,

I am trying to figure out the resolution that I need for an application where I plan to use Field Oriented Control and motors that will stop and hold in fixed positions.

I have found somewhere that Siemens has a rule of thumb like this:
PPR > 275/RPM_min

The thing is that I do not have an specification for RPM_min, but I do need to avoid strange effects at very low speeds/holding positions like the motor shaking and so on.

Am I approaching this the wrong way? Probably I can use a different control scheme to avoid the speed loop in stopped positions? Now I am using a PI closing the loop on speed by setting the quadrature current that is needed, and a P controller for the position loop.

Any help or guidance will be very welcome! And regarding the project if more info would be helpful let me know which.

Thanks!
Ricardo
 
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Peter Nachtwey

How fast you want to go is usually not a problem. How slow you want to go is. If the controller has a loop time of 1 millisecond then how many counts will you get in 1 millisecond or loop time? If you get 1000 counts on the average there will be times when there are 1001 or 999 but the jitter is only 0.1%. However the encoder is getting 1M counts per second and usually there is a limit of around 8-12 million counts per second on the input due to screwing phase by the opto isolators.

If you buy a 1000 PPR or 4000 CPR encoder then at 1 rev per second the encoder input will see about 4 counts +/- 1 or a 20 percent jitter. This make using the derivative gain a problem.

I tell my customers to bet the most resolution they can find providing it won't exceed the high count rate limit.

Usually there is no reason not to get and encoder with about 50,000 PPR as a minimum that should result in 200,000 counts per rev. 1800 RPM or 30 rev per second push the count rate to 6M counts per second.

You really need to know the RPM or RPS of the encoder. I have customer using much higher resolution encoders because the roller it is attached to will never exceed 15 RPS.
 
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