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CCS/F28M35H52C: LOW SPEED ACCURACY USING QEP?

user4328968
Prodigy 60 points
Part Number: F28M35H52C
Other Parts Discussed in Thread: CONTROLSUITE, C2000WARE

Tool/software: Code Composer Studio

The precondition is : encoder is 2500 lines and MCU is F28M35H52C1.

We use qep to test a pmsm motor,we find the speed accuracy when low  is very bad : +/-20rpm for 50rpm.

So we want to know what is the low speed accuracy for this IC using qep? do you have example code or inner block for qep?

Thanks for your help

Longfei

  • 0
    TI__Mastermind 27200 points

    Longfei,

    Speed accuracy is not solely a function of the device.  It depends on the resolution of your encoder, as well as your measurement rate.  In your case, I'm guessing you are measuring speed at a rate of about 1kHz, in which case at 50 rpm there are only ~2 encoder lines per measurement.  Possibly, sometimes the QEP sees only one line, sometimes three.  This is why you're seeing such a large variation in measurement.

    At high speeds, the encoder pulse count method works well, but at low speeds the edge count per measurement period falls and resolution is lost.  Typically, to maintain resolution over a very wide speed range, a designer would use an edge timer method at low speeds, and an edge counting method at high speeds.  The attached document describes this trade-off, and should help you to assess the accuracy part of your question.

    Regarding example code, I can only direct you towards the motor control code in C2000Ware and controlSUITE.  I do not know if any of the examples actually change measurement mode depending on speed, but if not this may be something you'll need to implement yourself.

    I hope this helps.

    Regards,

    Richard

    (QEP resolution).pdf