calculate RPM of a stepper motor

Discussion in 'General Electronics Chat' started by bug13, May 5, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi guys,

    Can I calculate the RPM of a stepper motor without feedback from the numbers of steps and time?

    take this one for example
     
    Metalmann likes this.
  2. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    Do you know the frequency of the steps?
     
  3. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    no,

    I am hoping I can get a variable RPM output, without feedback. (that's what I have given, MCU + stepper motor driver + stepper motor)

    here is how I drive the motor:
    Code ( (Unknown Language)):
    1.  
    2. motor_multiple_step( int x_step )
    3. {
    4.      do( x_step )
    5.      {
    6.           send_signal_for_one_step();
    7.           delay_one_ms();
    8.      }  
    9. }      
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    What stepper motor driver do you have ?
     
  5. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38

    I am using Pololu A4983
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    The example motor you show has 400 steps per revolution. By controlling the step rate, you control the speed. 400 steps/second would give 60 RPM, and 200 sps would give 30 rpm. That assumes that the load does not exceed the torque limit and result in missed steps.
     
    bug13 and Metalmann like this.
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Here's some info on typical max speed for stepper motors:
    http://www.daycounter.com/Calculators/Stepper-Motor-Calculator.phtml

    You have a good stepper driver which provides current limiting and micro-stepping.
    Current limiting allows for somewhat higher speeds by using a higher voltage supply than the stepper motors rated voltage while limiting the current to a safe value. Check it out using the calculator above.
    Just be aware of heat sink/cooling requirement for the chip.
    You might want to consider a motor with a bit less current draw so you are not so close to the max for the chip.

    The micro-stepping feature controls the motor in finer increments than the full-step mode. With the motor posted: 800 steps/rev, 1600 steps/rev etc..
     
    Last edited: May 6, 2013
    bug13 likes this.
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,782
    941
    The problem with a simple calculation for RPM will be a reference timebase.

    Somewhere in this process of software 'counting' you will have to provide a stable ref freq as a timebase which your count can be refered to. Due to differences in processing power and clock speeds your software will produce different motor speeds when used with different computers. If your controller provides a crystal oscillator then you are golden. :)

    Otherwise you are at the mercy of your processor clock cycle which will be variable from platform to platform.
     
  9. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Thanks BillB3858, why haven't I though of that? I think this will be good enough for my application.
     
  10. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi tubeguy, thanks for your link, that is going to be a very good reference for me, it's going to be very useful.
     
    tubeguy likes this.
  11. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    guess I am golden, thanks:)
     
Loading...