Hello everyone!

I need to be able to control the tightening or losing of a screw knob on my gym. It is about 10 to 15 turns full tight to full loose and requires about 1.5 kg/cm torque to turn. The resolution could be a half of turn. I can use a 12v motor with reduction gears and coupled a 10 turn pot for position info and controlled all with relays and an Arduino. This works ok but the pot every once in awhile will get destroyed when the project goes haywire and the motor goes past the pot's 10 turns. Is there a better way to do this? Maybe an absolute multiturn encoder instead of a pot? They are so expensive. A multiturn servo is much cleaner but doesn't have enough turns or torque. Sorry this is so long but hopefully it's helpful.

 

You can get a quadrature opto and 100 slot disk on ebay for about $2.50.
Another alternative is a open loop stepper and gear box.
Max.

 

Use a linear pot with travel longer than the total possible travel of the screw.
You will need a bearing that floats with the screw, to transmit the linear motion to the pot - without backlash.

Add mechanical limit switches to prevent haywire, assume the motor will over-travel the limit switches due to the stored inertia in the motor.
(don't let the limit switches get crushed)

When designing something like this, you have to assume it's going to go haywire, it must not self-destruct when this happens.

The incremental encoder idea would work, but you have to "home" it when the power comes up- every time.
Absolute encoders will be absolute overkill, economically speaking.

 

I wonder how they manage to stop the motor on auto-up / auto-down window controls in automobiles. It has to be simple, cheap, and extremely reliable.

 

AFAIK the modern version is an electronic breaker, when a certain current is reached it switches off.
Max.

 

AKA, an automobile window up/down switch. I was thinking that would a simple and direct way to control this motor without a lot of hullabaloo! I replaced the one in my DeVille for $25 new last year. BUT the currents / voltages involved might be an issue. Something to check out.