DC motor for high torque, low RPM options

Using Arduino Motors, Mechanics, Power and CNC
1

Hi - i'm looking to turn a knob via an arduino. The knob won't be turned much - perhaps 5-10 rotations over the course of an hour's use. Typical rotation would be less than a quarter turn on the knob.

Requrements are:

  • The knob still needs to be turned manually, so whatever motor that is attached should be relatively easy to spin when not energized.
  • Movement doesn't need to be exact - perhaps within 20% accuracy
  • Rotation speed of the knob should be perhaps a rotation every 2 sec - speed not critical

I currently have it working driven with a NEMA17 stepper motor (59Ncm of torque), belt and geared sleeve that I 3d printed. A full turn on the motor is about a half turn on the knob. I seem to have enough torque, and I can fairly easily turn the knob when the stepper isn't energized.

I'd like to eventually battery-power this. I think the stepper is overkill. Is there any option with a standard DC motor? Is there a motor i can drive slowly, have decent torque, and run with non-rechargeable batteries? I tried a geared motor, but they were hard to turn when off.

Thanks in advance - Joe

2

You might try a gearmotor with a lower ratio (10:1?) that would be easy to backturn but with enough torque for the pot (shouldn't be much) and if it spun too fast, roll your own low frequency PWM, say, 20 Hz or so to "chop" it around.

3

With small hobby motors, low speed and high torque are mutually exclusive. This why they have gear boxes to increase torque.

The previous poster mentioned low frequency PWM. Don't bother, it will not work. A lower ratio means lower torque and at some point, probably much higher than you would expect, the motor will not start due the higher torque required to break free and begin to turn. You might find you can start the motor at 50% speed (or more) then quickly reduce speed but it's going to make it more difficult to know what the resultant pot setting is.

Ultimately, the best way to do this might be with a digital pot. Replace the analog pot with an encoder which writes to the digital pot and since there is no absolute value to an encoder, you can change pot value without changing encoder position. More suited to battery operation as well, as it significantly reduces battery capacity needs.