equation (PID) for controller 1

[john111smith]

Hi All,
I wish you a Merry Christmas and a Happy New Year.

I try to use a PID controller instead of an old hardware vacuum controller. While, a turbo pump decrease pressure; a valve connected to the vacuum chamber, increase pressure.
What is the equation that represents the function of the proposed controller?
Also, how can I determine the PID controller constants and gains?

Regards,
J. Smith

 

[ScottyUK]

Hi John,

Try posting in Circuit Engineering forum240. In fact it is probably worth transferring the discussion there straight away and RFing this one so it gets deleted.

Try to re-arrange the sketch into the conventional forms for, say, an invetrting or NI amplifier. There are a number of fairly standard 'building blocks' in there which are not immediately apparent because of their configuration. Something like 'The Art of Electronics' would be a great reference for you to start with - a touch pricey perhaps but it's Christmas so why not! The individual building blocks should be fairly straightforward to analyse.

It might be worth getting hold of a simple circuit analysis package. I think Microcap 8 (MC8) was available as a free download with restricted capacity a little while ago - might be worth checking out. At first glance I don't see any sign of a full PID controller - this looks more like PI without the 'D'. Unless you've got an awkward process to control this shouldn't pose a serious practical disadvantage.

----------------------------------

Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

I think it is OK to stay in this forum - we oldies are used to circuit diagrams.

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.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[john111smith]

the above circuit extracted from a mono layer old PCB.
because of simplicity, I didn't place 10K potentiometers (offset null) of all 741 opamps circuits. and also, RC [1k, 2n4] between B and C of Q1.
R37 is 67K7
the ouput is enough to drive piezoelectric voltage controlled valve.

if its possible, please write final equation with constants and gains.