I'm working to design a motor to control a mechanical shutter. After considering some other possibilities I have decided to try using a stepper-type micro motor. We're using a shutter from Edmond optical. I'd like to pair it with one of the Micromo stepper motors.
The critical datasheet specification seems to be the holding torque which is rated as nMn. I take this to be Millinewton/meter. I rigged up the shutter with a simple pulley and found that a weight of 12.8 grams was sufficient to turn the shutter's lever (which opens and closes the aperture).
This explanation might benefit from a diagram, so I drew up one to illustrate the experiment:
If w=mg then the weight exerts a force of 126 mN on the lever…ignoring the geometry of the pulley and that the lever turns on a circular orbit. My naive self imagines that I need a motor which provides a torque of more than 126 nMn. Is this a correct estimate?
If so it seems all the Micromo stepper motors are too weak to turn the shutter.
Best Answer
You have calculated the force required to operate the shutter but not the torque. Assuming that the lever is perfectly horizontal and the pulley cable is perfectly vertical, you must multiply the force of 126 mN by the distance between the shutter axis and the point on the lever to which the cable is attached. For example, assuming your diagram shows the shutter displayed 1:1 on my monitor (which I'm sure it doesn't!), I measure the distance as 30mm giving a torque of 30E-3 * 12.8E-3 * 9.81 = 3.8 mNm.