First thing: This doesn't look like it was designed - more like it has "happened". It may have evolved over some time and no one bothered to clean the house.
U1 is an inverting amplifier with gain -0.5. It turns the transducer input (0-5 V) into a 0- -2.5 V signal. R4 and R5 are there to make U1 see same resistance on its positive and negative inputs. Very unusual to do like this. Normal way is to use the equivalent value (3.3 k) instead. Things like these makes me think that this "controller" wasn't designed at all.
U2 is a voltage follower with gain +1.0, which buffers the setpoint from R36. So, setpoint moves 0- +5 V and actual value moves 0- -2.5 V. It seems that only half of the setpoint potentiometer can be utilized before saturating the controller.
U3 takes the difference between the two signals and amplifies it. It also reduces noise (C3 makes it behave like a first order low-pass). Gain is not so obvious - the voltage divider r7/r6 says that it is +7.9. The voltage follower configuration adds 1, so total gain is +8.9 - I would use +9 for gain.
Then, you have some gain adjustment/range switching around U4. Lowest gain is when S1 is open and S2 in opposite position. It is then (18//2.4)/150=0.0141 - the rest can be calculated correspondingly.
The controller as such is made up of U5 and its feed-back. P is set with R37 and ranges from 0 to 1.83. Integration time (input-output, not FB integral) is a fixed 1200 milliseconds. FB integral ranges from 0 to 2200 milliseconds.
U6/Q1 is the output stage with negative feed-back from Q1 collector to U6 positive input. Looks like a difficult thing to make stable. Perhaps the extremely high base resistor R34 - 330 k) helps. But what about gain spread in Q1?
This whole circuit stinks. Very unprofessionally made. There are so many unnecessary components and don'ts (Like the 2.2 k resistor at the opamp outputs - not necessary at all. And the "unusual" output stage). There is one thing the designer has learned, though - the use of balanced input resistors. I certainly hope that this isn't a commercial controller.
Equation: the PI part is obvious, use numbers above. The setpoint and actual value channels as said above. Output stage (if at all stable) has a gain ranging from 7.5 - 27 when R38 goes from 0 to 1 Mohm.
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
