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eQep question

Sung Han1
Prodigy 150 points

Hello, quick question

I have encoder with 1024 lines and trying to calculate rpm off that.

Equation 1: w = delta_cnt  / (4 * 1024 lines)] * fsampling.

Equation 2: rpm= w*60

Is this correct equation so far?

  • 0
    TI__Guru 61235 points

    Sung,

    Looks correct.  You're just calculating speed = delta_x/T.  In your equations, fsampling is in samples/s and w is in revolutions/s.

    Note that at low speed, the delta_X/T method of computing speed will fail because you will get delta_x = 0 for many sample instances (i.e., the rotation is slow so the encoder does not 'tick' between samples).  In this case, you can use the eQEP in time capture mode and compute speed = X/delta_T.  This is discussed in the various technical reference manuals, for example F2806x TRM, SPRUH18D, p.465.  It depends on your application as to whether you are concerned with low speed estimation or not.

    Regards,

    David

  • 0 in reply to David M. Alter
    Prodigy 150 points

    David, 

    Trying to understand better, 

    Is there way I could figure out fsampling without oscilloscope? 

    Thank you very much

  • 0 in reply to Sung Han1
    TI__Guru 61235 points

    Sung,

    fsampling is whatever you configure to read the eQEP reading.  You would know what you are using in your code.  For example, you could use a timer interrupt to take eQEP readings.  Then, fsampling would be whatever your timer period was.

    - David

  • 0 in reply to David M. Alter
    Prodigy 150 points

    David,

    Is ECAPCTL[CCPS] the one I am configuring for fsampling ?

    I have Sysclock is 100MHz and CCPS=7 thus, that gives me 781kHz?

    Thank You

  • 0 in reply to Sung Han1
    TI__Guru 61235 points

    Sung,

    No.  QCAPCTL[CCPS] controls the prescalar to a clock used as the timebase for capture mode results.  This would be used in the low-speed velocity estimation I mentioned earlier, but has nothing to do with the regular v = delta_x/T speed estimation.  You need to setup a timer (e.g., ePWM timer, or CPU Timer) that triggers an interrupt.  In the interrupt ISR, you read the eQEP results and perform your speed calculations.

    - David

  • 0 in reply to David M. Alter
    Prodigy 150 points

    David,

    Sorry . My mistake.

    Thank You very much. It seems working fine now.

  • 0 in reply to Sung Han1
    Intellectual 260 points

    Hi song,

    I have encoder with 1024 lines and trying to calculate rpm off that.  but I can not do it.

    Can you send all of your program? please. 

    I do not understand where I went wrong

    regards...

  • 0 in reply to David M. Alter
    Intellectual 260 points

    Hi david,

    You said "You need to setup a timer (e.g., ePWM timer, or CPU Timer) that triggers an interrupt.  In the interrupt ISR, you read the eQEP results and perform your speed calculations."

    How can I  do this?

    regards...