PID Control of Motor...

[Bluenoser337]

I have a motor/gearbox unit that chain drives a machine assembly, at 6 inches per second, along a shaft over a total travel distance of one foot. The motor FWD/REV is PLC controlled and positioning is done manually by an operator. I want to add an analog positioning rod to allow controlling positioning automatically. The assembly must stop at precise (1/16" tolerance) positions along the way based on operator input. My question is regarding PID. I know PID does a lot of pulsing to achieve accurate positioning. Has anyone used PID on a motor driven application like this? Too much pulsing would be tough on the motor, etc. The brake reaction time must be considered. How can I program this to achieve repeatable accuracy? Thanks!!!!

 

[Skogsgurra]

It is being done all the time. In saw mills, steel works, machine-tools. Lots of info out there for anyone that cares to google for it. But you do not need PID. PI does it.

 

[vonbad]

I just programmed a PID controller from Watlow. I am using a 4..20 mA signal from a sensor. I adjusted the scale from -2 to 102 for my purpose. You could adjust the scale as need just depends on the accuracy of the sensor output.

As for the pulsing issue; this can also be adjusted as needed.

 

[jraef]

What type of motor and controller do you have? Servo or stepper motors will of course have a controller so will be no problem at all. If it is 3 phase AC I hope you have a drive on that motor. The banging on and off of DOL starters will be a) very inacurate and b) very hard on the motor. The same will be true if you have 1 phase AC motors as well, but there will be fewer drive options. DC motors will be less of a problem but should have a drive as well.

"Venditori de oleum-vipera non vigere excordis populi"

 

[leonar40]

If you can use a stepper motor than you might not need any feedback control. You can just move the motor exactly the number of steps it takes to achieve your positions, and assuming that you do not stall or otherwise overload the motor , then your position will be accurate.

 

[Bluenoser337]

Thanks for all the replies. Unfortunately, I have a 3 phase motor (3/4 HP) controlled by an ordinary fwd/rev starter...and that likely won't be changing. I'm still wondering if PID (or PI) will work with this arrangement. Maybe a small VFD would be the answer for fine tuning position?!

 

[Skogsgurra]

OOPS! A DOL fwd/rev motor!? And positioning? The "PID pulsing" is not your problem. A frequency inverter will definitely help.