i'm trying to control a two position valve (110vAC) with a simple stand alone process controller (no plc, no pc, etc.). set point is % time open. full cycle time can be from 20 to 100 seconds. anybody have simple off the shelf solution?

 

If the setpoint (SP) is percentage output, then there's nothing for a PID controller to calculate. There's no error to deal with, because the SP = % output. In effect, its a manual contoller.

Your commercial solution is a mid level, stand alone, PID controller
- with an on-off output suitable for driving your AC load, either electromechanical relay or solid state relay, your preference,
- that can be 'locked' into manual mode.
- preferably one that you can setup to wake up on power up in manual mode.

You set the controller up for what is known as time proportioning PID with a 100 second cycle time.

That allows an output to modulate ON & OFF over 100 seconds. Since you picked 100 seconds for a full on-time, that makes a 1:1 correlation between the 100% output of the controller and 100 second cycle time.

You lock the controller into manual mode and you don't bother with the setpoint (SP has no bearing in manual mode), rather, you use the output setting (since it's in manual mode, you have manual control over the output) to establish which value between 20 and 100 that you want. You have a 1:1 correlation between % output and % time on.

If you want 30 seconds on, enter 30. The output will be on for 30 seconds, off for 70 seconds.

I tried it on a Honeywell UDC 2500 with dual displays (% output in lower display) and got the performaance described above. The upper display would display whatever PV you're looking at.

The only thing it couldn't do was to limit the output in manual mode to 20% or higher. In manual mode, the one I used would span 0-100% output.

BE warned that many of the 1/16 DIN controllers don't have space for a manual key on the faceplate. Most bury some form of manual mode in the setup. To change the output in manual mode on these requires that one go into setup or config mode. You've dug own grave if you put a controller into a process with the setup or configuration mode "open" and exposed. Disaster awaits. Get something that can be locked into manual mode outside of the config mode.

Dan