Sensing plastic gearmotor stall

I’m going to try to use your 180:1 plastic gearmotor to turn the shaft that controls the vent on my RV. I’ve selected your MAX14870 driver.

I’ll operate the vent either all the way open or closed. It takes 2.5 turns on the shaft to go from one to the other. I do not wish to add limit switches, so I’d like to just command the motor in the open or close direction then stop when the gearmotor clutch is slipping.

Finally my questions:

  1. will the gearmotor suffer early death if driven to clutch slippage every time?
  2. If 1) is ok, how should I sense this condition?

And thank you so much for shipping my order during this crazy Coronovirus time.
Stay Healthy,

Hello, Kent.

Even though that gearmotor has a built in safety-clutch, I would not recommend using it the way you are proposing. I would anticipate a reduction in your motor’s operating lifetime since the clutch typically slips just before the gear teeth begin shearing or the motor stalls, which means there is still a lot of stress on the system.

Can you explain why you want to avoid using limit switches?

- Patrick

well, I think that installing limit switches is going to increase the mechanical complexity because I’d have to find a way to mount them. So then I’m mounting two switches in addition to the motor, in a vent built 30 years ago (it’s an old RV). Plus, on vent closing I want to pull it tight to avoid loosing heat in the winter.

I was thinking of sensing the current and stopping the open/close action as soon as the current reaches some threshold, say one-half of the stall current. Kinda worried that the current while start high on the open action so have to ignore the current at start.

Do you think that sensing the current and stopping quickly would be some protection, or maybe it’ll stall anyway before the MCU can react and stop the motor?

Maybe I should be looking at a stepper motor or a multi-turn servo. But those are both more expensive, for the comparable torque right?

Thanks for your help,

You could try to use a current sensor for that, but I am not sure if it will work well in your application. The presence of the clutch might make it difficult to determine a current threshold that works reliably. However, if you decide to try that out, you might consider using one of our current sensors, and I would be interested to hear how it goes.

Alternatively, if you use one of our Mini Plastic Gearmotors with an extended motor shaft and our Magnetic Encoder Pair Kit for Mini Plastic Gearmotors to provide incremental position feedback, then you would only need one fixed reference point, like a limit switch.

Any stepper motor system would probably be at least a little more expensive, and you would still need at least one absolute position reference. This does seem like a good application for a multi-turn servo, and I suspect you could find something reasonable, though I do not have any specific suggestions about that.

- Patrick

Thank you for your help. I finally get that that using current flow to sense a stall means the motor is going to be stalled for some amount of time on every action. The motor has to stall first and then the current increases. Duh. Still lots of alternatives to consider. Thanks.

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