PLC5 Bumpless Transfer

[alphite 0]

I didn't see an exact answer to what I was looking for, so here it goes. 

Right now a chilled water control valve is in PID auto the majority of the time (using a PLC5). When a button on the HMI is pressed, the valve should close until a heating cycle completes. At this point, it should revert back to PID control.

As it is, the valve closes correctly for the duration of the cycle, but when it switches back to auto, it's going to full open instantly (the current PID CV). What is the best way to implement a bumpless transfer, without having to receive the value from an HMI? I just want to hard-code the 100% closed value (4095). The main issue is that if the valve snaps back, it causes a rapid drain on the system and faults a chiller. I considered a second PID that's less reactive for the transfer, but bumpless seems like the correct, more elegant way?

Something else that was strange to me - with the current code, the CV of the PID is set to 4095 during the heating cycle (closed). However, our historian values show a moving CV, which gradually goes from 4095 to 0. How exactly is this happening, as the valve remains closed?

[Izzybe 13]

Alphite, 

It sounds you might have some other issues. For heating applications there are a few options to explore (bang bang control, PID, fuzzy). PID setup can be a bit tricky - what is the PV when you are checking this out?  check out document 1785-rm001_-en-p  - page 198 and look at tieback if you need it. Is your bias set right? Hope I can point you out in the right direction.

[alphite 0]

Thanks Izzybe.

Normally, hot water is fed to the heat exchanger, and it cools to a setpoint without issue. Once a day, there's a heat sanitization cycle, where the cooling needs to be disabled. Switching from PID control to manual and then back seems to be where the issue is, as theres no bumpless transfer. It looks like the PID is left in auto mode, and the program is constantly writing 4095 (closed) to the CV. This means the PID is trying to reach a CV of 0 (full open), and the valve suddenly goes from full closed to full open when 4095 stops getting written to the CV location. I think I need to put the PID in manual, and then I can hard code 4095 into the tie back value? Does tie back have to link to a N-address?