Drawing too much current: is torque control the answer?

Hi everyone,

I’m doing an application where I’m doing essentially a “pick and place” robot. I have a ungeared maxon motor RE30 moving the robot in about a 20cm range, driven by a one of the MC33926 Motor Drivers.

What happens right now is that when the motor is trying to do PID control, its drawing over 5A. It isn’t feeling too much resistance since its on a linear rail and friction should be minimal.

First, what is the problem? Why am I still seeing 5A, when the machine is still running and moving to the right place? Second, How do I solve this problem? I heard torque control is the answer from a friend, but I waons wondering if anyone could give me a more detailed explanation of torque control (e.g. is it just controlling the current?), and how it can be used?

Thank you very much!


I suspect either your motor is wants to draw 5A when running at the voltage you are giving it, or your PID control is causing the motor to change directions very quickly.

In brushed DC motors, torque is proportional to current, so controlling current would control torque. The MC33926 has current sense, so you could sense the current and control the PWM duty cycle to reach a particular current. I am not sure how effective it would be. An easier thing to do would be to just limit the max duty cycle to whatever the motor driver can reasonably handle or reduce the supply voltage.

- Ryan