Mentor DC drive loosing signal?

[riiska]

Im using an old Mentor Dc drive to control a "take up" that is spooling wire on it. I have a potentiometer controlling it by torque but if the motor is at "stall tension" it will release tension and then grab again, then release and grab just going back and forth until I turn the tension down. If I slowly increase the tension using the pot it seems to hold but will randomly start the jerking motion when I stop spooling or increase tension too quick. Can any one offer any Ideas why, and maybe a solution.

 

[itsmoked]

I suspect it's because the drive is controlling to torque and because you have no moderating aspect in your mechanical system the drive system begins to oscillate. Usually with a take-up system like you describe there's a tensioning arm somewhere (dancer) that the material runs around and as tension changes the dancer changes angle. The angle is what is used to control the take-up giving the control system a better variable to work with. This gives the control loop something to look at that isn't pretty much just 'too-tight/too-loose'. The dancer is also spring loaded which provides tension over a length of the wire giving the servo system time to work.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

Torque control is really current control to the motor. If the motor is not turning then it must overcome static friction before it starts. Since dynamic friction is less than static friction, the tension will overshoot. If you have a gearbox on the motor then torque control is almost useless for good tension control, although I have seen it done many times, and operators have to deal with the consequences.

This is really a pretty complex problem with voltage trying to control the speed while current is trying to control tension. Each controller will behave differently. Dancer arms solve this problem.