Hi folks,
1756-L72, Studio 5000 V31. Jacket temperature loop on a mixing vessel, steam valve, the standard PID instruction, not PIDE. The vessels landed on me in the spring so treat me as a beginner on loops.
Last week I moved the PID rung out of a 500 ms periodic task into MainRoutine, gains untouched, Kp 2.5, Ki 0.3, Kd 0. Since then the temperature swings about 2 degrees either side of setpoint with a period of around 40 seconds and it won't settle. Before the move it held within half a degree for two years.
I tried dropping Ki to 0.1 to see what it did and the swing got slower, but it's still sitting there. Not sure that's the right end of it.
What's different about a PID in a continuous task? I'm mostly a TIA person and PID_Compact never cared where I called it from, or so I thought. Does the instruction maybe need telling how often it's being called?
Was the valve proved before you blamed the software? Stroke it 0 to 100 from the HMI in manual and watch the position feedback follow. A positioner with a dead spot gives you exactly this kind of slow cycle, and it can start the same week as a program change by pure coincidence.