proportional value control question


Thread Starter

Joe Howell

I have an application where there are two hydraulic cylinders that push product into station in a machine. The problem is that how the machine is designed now it at times will push uneven and calls jams or racking. We are planning on using linear transducers on the hydraulic cylinders to sense there true position and we are
planning on using proportional value on each cylinder to controll them. This machine is controlled by a Allen Bradley PLC-5 and my question is this. Is there any easy plc-5 command that can read these two linear inputs and control each output to allow the machine to push even until the cylinders are fully extended. If anybody has any ideas of how to program this with a PLC-5 I sure would like to hear from you.

Thank You.
You might consider, just as you described, but run signals into diff. amp and look at error signal. the error signal limit would be just below mis-alignment tolerance. Unlinked (or loosely linked) cylinders are a bear to contol.

Use a rotory flow divider on cylinders to match input oil flow, if you arent already. Not sure if you can adapt the analog inputs directly on the PLC to do same. Hope that helps

James Johnson

This is a typical scenario for many industrial and machine applications.
I would try using 2 independent PID closed loops with a common set point input.

Good luck
James Johnson
You didn't mention performance requirements (speed, tolerable position error, etc) but you have everything you need. I assume you have analog inputs and outputs for feedback and proportional control. As mentioned in the previous post simply close both loops with a PID function (assume PLC-5 model you have supports this... thought all 5 series PLCs did).

The only thing I would add is that you would have a more robust system if you designated one actuator as primary and the other as a follower... feed your profile setpoint to the primary and have the follower use the primary actuator's position feedback as the follower's setpoint.

Good Luck-Webb

Peter Nachtwey

There are two solutions:
1. Buy a 1771QB card that can control two cylinders.
2. Buy a RMC100-M1-ENET or Serial. This is a remote motion controller that communicates with the PLC5 via Ethernet or DF1. Actually this controller can move the axes without the PLC5.
Don't be fooled into thinking this is a simple PID application. Depending on your performance requirements, you might get away with putting together a cascade configuration - inner loop for velocity with output to the proportional valve; outer loop for position with input from the linear transducer. The setpoint for the position loop has to ramp from starting point to finishing point. You will need to provide velocity feedback to the inner loop - perhaps you can differentiate the position feedback. Do you need to control motion in both directions? Hydraulic cylinders have different flow vs velocity characteristics in each direction due to the volume occupied by the rod. If all this was easy, A-B wouldn't sell so many 1771-QB modules. Check it out.
Hi, Sorry if I repeated my reply. I suggest using a hydraulic flow divider. One input / two outputs. This is the traditional solution to get off on the right footing. Secondly, once you get position sensors installed, usually the magneto-restrictive type (MTS), consider looking at the error signal at the output of a dif. amp as the indication that the cylinders are out of alignment and correct the flow accordingly. -flyboy4250