Torque Motor Calibration 1

[RedCoat999]

We have setup a dynamic calibration of a torque motor and system. One end of the torque motor has a constant 3 rpm input from an air motor through a gearbox. The other end has a copper wire attached to a 12 lb weight. We record the torque as we raise and lower the weight at a constant speed (altering the direction of rotation of the motor too). We consistently have ~11.4 in-lb torque when pulling the weight up, and ~10.3 in-lb when lowering the weight (at the same speed controlled by the 3 rpm input).

I am having a brain-freeze as to why such a 'big' difference. I've drawn many Free Body Diagrams and there's not much too them... Wire Tension Force, mass, gravity. There is no acceleration to speak of since the input end of the torque motor is controlled to 3 rpm.

Any help matching theory to actual would be appreciated.

 

[Philrock]

I expect the hystersis you are seeing is due to the copper wire. 2 things are happening here:

1. If the ratio of the drum diameter to the wire diameter is low enough, the wire is yielding every time it enters or leaves the drum.

2. As the wire enters and leaves the drum, there is a small amount of sliding of the surface of the wire against the surface of the drum. The wire may also be sliding against wire that is already on the drum.

Try using as thin a wire as possible. Thinnest possible music wire, lightly lubricated, would probably be best. Nylon fishing line would probably be better than copper.

 

[RedCoat999]

Thanks for the suggestions Philrock. Am I right in saying there should technically be almost zero difference between the torque pulling the weight going up and down at a constant speed?
Thanks

 

[Philrock]

Yes, provided that the torque measurement is accurate and there is no friction in the drum/wire system.

Some torque measuring setups have bearings that can introduce friction/hysteresis.