Stepper motor control limits

One of the fundamental questions that arose during the control and testing of the SCARA arm was what is the maximum speed achievable and the torque available at that speed.

Some digging around in the world-wide-web revealed the following:

  • The speed is limited by the torque availability from the stepper to rotate. For steppers, as the torque drops exponentially with speed, that places an upper limit on the speed we can run it at for a given load torque.
  • Typical stepper pullout torque curve:

Stepper_torque pullout curve

  • Stepper motors are used for precise, open-loop positioning. The maximum speed is limited by the time it takes for the stepper coils to energize to its maximum holding current, and then de-energize as the polarity flips. (Reference from http://www.daycounter.com/Calculators/Stepper-Motor-Calculator.phtml)
    • Current through the coil (I) is proportional to the time (T), the voltage has been applied (V) and inversely proportional to the inductance (L):I= \frac{V*T}{L}
    • For one step the current must go from 0 to latex Imax and back to 0, or alternatively from -Imax to +Imax:

I= 2*I_{max}

T= \frac{L*I_{max}*2}{V}

where, T is the number of seconds for a single step.

 

 

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