I need to slow the rise & fall time or 'average' the output from a process controller, the output is currently 4-20mA but can be 0-10V if desired.
It's for a temperature control on a very unstable process, the items respond very quickly to temp changes and the controller (PID) cannot find a steady state, it tends to swing the output too far either way when it really needs a slow and steady rise or fall.
Many people have tried PID tuning but the process is too unstable, it has (and needs) a very large heat input and the products have little or no heat mass (sheets of paper), the heat input is large as the process can handle thick card requiring massive heat input or thin paper requiring less input, it is also speed / exposure time dependant.
Is there a cost effective way of averaging or smoothing the output in a control loop setup like this? The signal is isolated from the process by an LDR/Led optocoupler. I have had some luck in slowing the fall time by placing a cap across the Led/resistor but this has no effect on the rise time.
I have not modelled or measured as I am doing this real-time, yes the LDR is a CDS cell and I'm not sure how to check if it is linear or not. I will have a look tomorrow if I get a chance. The system is very unstable and a P only controller was no better.