5-DRV8825 Questions

[Falon]

For my application I need to drive a stepper motor at 60RPM with 2.5Nm of torque. I have chosen a stepper motor with a maximum allowable torque of 3Nm and a current rating of 0.6A per phase. The stepper motor is a NEMA 16 stepper motor with a 14:1 planetary gearbox so the step angle is 0.131 degrees.

My question is how do I calculate at what RPM the 5-DRV8825 stepper motor driver can drive the stepper motor?

I have read the TI datasheet for the 5-DRV8825 and it says the maximum step frequency is 250kHz, which I am assuming is for the highest microstepping resolution of 1/32 and a maximum driver output current of 2.5A.

Is there a reference table or simple formula that I can use to determine the maximum step frequency at lower microstepping resolutions and lower driver output currents?

Is the relationship between microstepping and torque linear? From what I understand, microstepping reduces torque?

 

[MarkzP]

Apart from full stepping & the special case of half-step overdrive mode that some stepper drivers will offer, microstepping count should have a minimal impact on final torque.
For a spinning stepper motor (i.e. not stalled!), the biggest factor will be supply voltage - as the motor spins faster, the generated EMF will increase, and ultimately torque depends on the difference between supply voltage & this internally generated voltage. Unlike DC motors, a stepper will eventually start missing steps & stall completely if torque is insufficient.

The driver will also have a measurable impact. I have no experience with this specific driver, but some modules will provide more torque at higher speed by using various techniques such as hybrid open loop/closed loop vector control. Some are better at high speed, some are more efficient at very low speed.

If all you need is raw torque & don't care about jitter nor efficiency, go full step. If you need a smooth turning motor, go as high as you can - even at 1/32, you are still well below 250k steps/s

Marc

 

[MarkT]

Microstepping reduces the tendancy to miss-step, so it usually increases the _usable_ torque. 60 rpm is within most stepper's power band (torque can drop off very rapidly with speed at a few hundred rpm). Dynamic torque is a lot less than static torque typically though, 3Nm holding torque does not imply 3Nm dynamic torque. Good motors have performance curves for a variety of supply voltages in their datasheets.
Often the limiting factor with a stepper is mechanical damping - without this resonances can lead to miss-stepping and stalling at resonant frequencies, especially with full steps.