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I have successfully used PID control for heating in the past and just started to try and implement one for Cryo control of a platen that uses a LN2 solenoid valve to digitally control flow. The issue I ran into was that the PID only seems to accept positive set point values which makes the coding especially complicated. Has anyone used PID for Cryo control which demanded negative set point values?
I have not, but perhaps you could try passing absolute values and using a negative gain? Or negating the output (if symmetric, if not, then flipping it relative to your 0 output level) of the PID setup before controlling the valve?
I started to experiment with this type of scheme but ran into complications that can be worked out with the proper logic but I was hoping that I was missing something since the PID does not state it will only accept positive values. The complications that I run into it that both the set point and the process values (TC) can both be either positive or negative making a simple sign flip of either not that easy.
I have not, but perhaps you could try passing absolute values and using a negative gain? Or negating the output (if symmetric, if not, then flipping it relative to your 0 output level) of the PID setup before controlling the valve?
