Cylinder Control

[tbrun 0]

I need to make a cylinder extend and close a limit switch every three seconds then retract ; I am able to vary the pressure to the cylinder by a portional vavle which receives a voltage input from a compactlogix PLC. I need to make this a automatic procedure

[Ken Roach 194]

Please ask a more specific question. Is this a theoretical system for a class project, a real system for a class project, or a real system in a real application ? Is this a pneumatic cylinder, or a hydraulic cylinder ? Does it have discrete (retract / extend) valve as well as a pressure controller ?

[tbrun 0]

sorry that I didn't really specifiy my intentions; just trying to look for another approach to the this project

[paulengr 44]

You don't have any way to measure position so trying to use the prop valve for any kind of acceleration/deceleration is a waste of time (you don't have enough info). Thus the timing information is also totally useless because you can't tell how fast you are going. With what you have, create two variables. When you need to go up, set the prop valve to value 1. When you need to go down, set the prop valve to value 2. Play with those variables until you get the timing right. Put in a pressure regulator. Ideally temperature regulation on your working fluid, and some way to compensate for valve wear is also needed, but again, you can't measure velocity or acceleration so you can't compensate for it in the PLC. All it can do is issue open loop commands.

[Ken Roach 194]

This system confused me a lot the first few times I read the description. But I think I get it now: the idea is that the system will repeatedly extend and retract the cylinder and you will use PID to control the air flow so that the cylinder cycle takes 3 seconds. 3 seconds between the UP valve being turned on and the limit switch closing is your "Setpoint". The actual measured time between the UP valve being turned on an the limit switch closing is your "Process Variable" The difference between the measured time and 3 seconds (PV - SP) is the "Error". If the Process Variable is greater than the Setpoint (let's say it takes 4 seconds) then you want the air flow to increase a small amount to increase the speed of the extension, so that the Error will be smaller. This is called a "forward-acting" loop because you will increase the output (the Controlled Variable) to increase the measured value (the Process Variable). The Controlled Variable is the signal you give to the proportional valve. Does this description of the system match your understanding of the assignment ?

[tbrun 0]

thank you for that clearification i have another question, is there a way to make this PV,time, a varaiable which I can program into the PID function into RSlogix5000. this is where I've been having the real challenge