Increase current to motor (Maximum current & Torque desired)

Robotics and electronics discussion Electronics
1

Hi all,
I’m using a Pololu 2998 - TB67S128FTG to drive a Lin Engineering 4418x-0111RO stepper motor. This is being used to drive a conveyor belt, for which maximum torque is desired (to be able to move larger, heavier objects). The current limit is being set using a DAC on a microcontroller - not the potentiometer. Using an oscilloscope with a current probe that takes RMS current measurements, I found the following data (RMS Current is the Y axis, DAC Vref value is the X axis.):

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With AGC disabled, and microstepping set to full steps only, I was able to achieve the highest current of 1.1A. However this is still well below the rating of the motor, the Pololu stepper driver, and the power supply being used. Torque pins are all set to their highest values, and the gain selector is set to the higher range. I’m not sure what else is causing the cut-off of current at 1.1A. If anyone has any ideas on things I could try, that would be really beneficial. Thank you!

2

Hello.

What stepping speed and operating voltage were you using when those measurements were taken? At higher stepping speeds, there is increased back EMF from the motor, which limits how quickly the current ramps up. In that case, using a higher operating voltage could help, as mentioned in the first answer of the FAQs tab of the driver’s product page. You could also try running the same tests at slower stepping speeds.

Did you modify the TB67S128FTG carrier by removing the potentiometer or is it still on the board? If it is still functional, could you try using the potentiometer to set the VREF voltage to 1.92V and remove the connection to your DAC and seeing if you get any different results, particularly in your full-step test with AGC off ? (A VREF of 1.92V should set the current limit to 1.5A per phase when the GAIN_SEL pin is high.)

Brandon