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Stepper motors vs DC gear motors

main article: Motor FAQ

Stepper motors are wonderful things. They can set positions with extreme accuracy. They do have some big disadvantages, though. They...

  • require expensive control boards
  • are power pigs
  • are extremely heavy for the amount of torque that they deliver

Gear motors, on the other hand, are...

  • incredibly cheap on a torque to torque basis
  • very light with respect to the torque they deliver
  • very efficient

They aren't accurate, however.

Turning a DC geared motor into a cheap stepping motor

You can make an inexpensive GM3 gearmotor behave like an expensive stepper motor.

They aren't accurate, however. You can slap a cheap shaft encoder chip on one and achieve very good speed and final position control. The downside to this is that if you want to drive an xy positioning table with two of them you are hampered by the fact that you can not operate them using voltage control down to zero rpm as you can with a stepper motors.

There is an alternative. You can force a DC gearmotor to behave like a stepper motor. Here's how you do it.

  • set up to count the encoder pulses via an interrupt
  • start the motor at full voltage
  • when you get an interrupt stop the motor
  • wait a calculated number of milliseconds
  • start the motor again
  • repeat

So far I've run one down to 0.5 rpm with a considerable amount of torque available. You can see the individual steps occurring at that rate. I suspect that I can make it go as slow as you need.

There is enough torque at those speed that I can not stall the gearmotor shaft by squeezing it with my fingers. I have been able to stall the gearmotor operating at low voltages using PWM with little problem.

The smallest step that I've achieved so far appears to be about 7.5 degrees. For a threaded rod with a 1 mm pitch that gives us a resolution of 0.021 mm.

I suspect that if I were to use the L298N chip and apply braking between steps I could get even more torque and sharper braking out of it.

Here is a short File:GearMotorStepper-Gearmotorbehavinglikeasteppermotor02.MPG video clip of a Solarbotics GM3 gearmotor being operated in stepper motor mode at very low rotational speeds. Please turn down your speakers a bit.