Cheap motor with position/force control

[aj7897]

Hey guys,

I'm interested in creating a low-cost solution for force/position/speed controlled motor for a prototype. Basically I need to be able to extend a cable 3 ft with 0-250 lbf of dynamic resistance (from the motor), then retract it with a similar dynamic force. I'm a mechanical engineer, but my knowledge of electric motors is somewhat limited - so hopefully you folks can help me out.

I found that I can buy a 5-hp series-wound bi-directional DC winch motor for less than $100. I'm planning on using an Atmel ATMega168 microcontroller with a PID program and a digital potentiometer to control it. A Wheatstone bridge from a digital scale (and an op amp) attached to a pulley will measure cable tension. A Hall effect sensor (maybe harvested from a car's ABS system) will measure rotary displacement. So I'm basically making a low-cost (& relatively low accuracy) servo motor.

Does this sound reasonable/possible? What kind of problems might I run into? Also, I couldn't find much info about "overloading" a brushed DC motor, but which I mean extending a cable with an external force as the motor works to try to retract it. This won't damage it, right? (I think most winches can spool out under load). Also, if this type of motor creates 100% of torque at 0 RPM, why do winches run their motors at high speed and use a planetary gearset to reduce? I'm guessing it's because of better power efficiency at that speed?

Any feedback is appreciated!

Thanks,
Andrew

 

[itsmoked]

What you propose will generally work.

Stalling motors tends to zorch them, with a few exceptions. That one won't like it. In that app you'd want a current limited drive to protect the motor when it's stalled.

Back to physics. hp = torque x rpm. Torque is one thing, but you need hp to actually move something. That's why they run the motors really fast.

You should probably consider a VFD running a 3ph induction motor. It would save you a huge amount of time and expensive learning curves. VFDs could also provide a lot of feedback info for your research.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jcaden]

You will need a compatible motor drive that works for 4 quadrant operation. That's because you will be forcing the motor to rotate the opposite direction from which it is powering rotation. I have built tensioners using drives from KB Electronics that can be run in torque mode. DC permanent magnet motors can be stalled if current is limited.