Posted by Adam Krajewski at 2/20/2001 2:00 PM EST
Engineer on team #221, MI Roboworks, from CCISD and Michigan Tech..
In Reply to: Backdrive disable for the van door motors?
Posted by Brandon Heller on 2/18/2001 7:46 PM EST:
I have not taken apart one of the van door motors identical to the ones used this year, but the ones used in previous years (allen head screw instead of torx screw on the top of the motor if looking directly down the shaft) a mechanical brake is present. The brake can be used effectively to decrease backdrive. The brake works well when a small amount of force is needed to stop backdriving (using it to stop the cable spool on the lift of the '99 T3 robot), but not so well when a large amount of force is needed to stop backdriving (using it to hold the arm in position on the '00 T3 robot). The brake consists of a setscrew and a spring mounted "plunger" that makes contact with the large plastic gear inside of the van door motor. Overtightening the screw causes the "plunger" to wear until you reach the setscrew. The shape edge of the setscrew then tends to bind on the gear. I do not believe this has caused any of our motors to fail, but it does change the motion of the arm and may cause failure down the road. In short, don't try to use the van door brake to hold a heavy arm for long periods of time.
Your best option would be to try to incorporate some type of closed-loop proportional control system. For more info on that type of control, check out the "Industrial Control" book at
www.parallaxinc.com
Adam