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Homework Help: Incremental rotary encoder motor help

  1. Jan 14, 2016 #1
    1. The problem statement, all variables and given/known data
    Hi please see the image that I have attached. The text colored in red is the data I need to calculate.

    2. Relevant equations

    3. The attempt at a solution
    The encoder has 128 counts per revolution
    So to calculate corresponding angle change this I what I did
    I divided encoder count in decimal number by 128 which gave the number of revolutions, I then multiplied it by 360 to find the angle change

    So for example I did
    6350/128 = 49.61 revolutions
    49.61 * 360 = 17859.34 degrees

    Is this method right?

    To calculate no load speed RPM, I divided the revolutions by (time*60) to get it to RPM

    So for the 6350 encoder count which gave 49.61 revolutions

    I did

    49.64/(5.25*60) = 0.1575 RPM

    Is this RPM calculation right?

    For me it looks as if the RPM is well below and so I must have made a mistake somewhere but I don't know specifically where

    Attached Files:

  2. jcsd
  3. Jan 14, 2016 #2


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    Gold Member

    That's correct.

    To calculate the speed, you must know the sample frequency. Say it is 1Hz. Then you will have 6350 counts per second.
    Convert to RPM. ( 128 counts per rev, 60 sec/min ).

    The result is close to 3000 RPM.

    If the sample frequency were 2Hz, the motor speed would be the double ( ≈ 6000 RPM ).
    Last edited: Jan 14, 2016
  4. Jan 15, 2016 #3
    Hi can you please tell me where RPM came from?
    I know that it took 5.25 sec for the angle change and so frequency would be 1/T = 1/5.25 = 0.1905Hz

    But I don't really know how to find RPM
  5. Jan 15, 2016 #4


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    Gold Member

    So 49.61 rev. within 5.25 sec, which gives 9.449 rev./sec ≈ 567 rev/min. = 567 RPM. ( 1 min = 60 sec ).

    The RPM's in #2 is just an example, not knowing your mentioned 5.25 sec.
  6. Jan 16, 2016 #5
    Ok thanks I understand it now
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