RSL500 setting initial PID CV output?

Hello all,

On occasion, I need to manually set a PID control variable's initial output when the process is first started- say a bump to 100% to get a process going quickly. I've tried doing this with a oneshot to energize the .AM bit to set the PID in manual mode, while also using the oneshot to write the initial value to the CV output word. For whatever reason, I can't get this to work- the PID starts out exactly as it would normally. I'm assuming it's because the PID doesn't update every scan, and the PID doesn't "see" the oneshot.

My workaround is to trigger a 1 second timer to "hold" the .AM bit energized while also "holding" the MOV instruction to the CV output word.

That works, but it seems clunky to use a timer- is there a better way to do this? Do I need to break the PID instruction's rung to "reset" it when I manually change the output? Or is it better to just write a value to the feed forward word and leave it?

Curious what you guys do in this situation.

I am not sure of your process, but you should not have to bump the output to 100%. The PID loop should do this automatically when starting out.
Tuning the loop may be required.
http://control.com/thread/943391977
http://www.plctalk.net/qanda/showthread.php?t=2290
The above are a couple of ways of tuning the loop.

Hope this helps,

Thanks for the response. I agree, normally it is a non-issue as most PIDs can ramp up in a reasonable amount of time, however occasionally I will have a process slow enough to require very conservative gains, which would end up taking a number of minutes for the PID to ramp up on its own. Usually the first startup of a large heater or cooler has this sort of issue for me. So I do indeed need to set the initial PID output or startup is unacceptably slow.

You can use the bias
Use open loop or manual control
Change gains on the fly.
I would prefer to change the bias. Use the bias to estimate the control output so the PID has to do as little as possible. The trick is to keep the integrator from winding up.