Starting output value pid control

[Limarq]

In a process which will be periodically started and stopped we would like to store the output control value when pid is stopped and load this as a starting value again when the process starts again. Is this possible?

[Dave]

Haven't tried this before. Sounds tricky. There is an instruction to pause the derivative and integral control values in the PID struct, as well as an instruction to force the integral error to a particular value. I don't know if  this alone will "freeze" the output value. You probably would have to manually control the process value as well. 

[Ausman]

Later Edit:  I thought this was in Visilogic's area.   Tired...sorry.    But in some ways the fundamental idea might work for Uni.

I don't use PID, but perhaps simply running the output through an intermediary MI that is stored, I would think that you could get a very close thing to what is wanted.  Perhaps even holding the previous MI output value for a little while on restart until things stabilise a bit at around the stored output.  I know this is my "simplified non-user" impression, but it might be worth a try.  On the other hand, I might be talking absolute nonsense!

cheers, Aus

 

 

[Joe Tauser]

You can't "force" the PID algorithm.  You can intercept the CV output and copy it to your control device, and when the PID block is off you can continue writing that same value, but when the PID block is activated again it will do its thing and come up with whatever value based on the tuning parameters.

When you remove power from the PID block, the Integral value is retained unless you reset it using the Reset Integral function.  The control value consists of P, I, and D components.  If the PV -SP error is the same as when you turned the block off (so the P component will be the same) the block should pick right up where it left off.

Joe T.

[David_2019]

You can use the "FORCE ERROR INTEGRAL" function.