BoilingHeat said:
With the OpAmp, RC represents the time it takes the integrator to reach the input value when it's constant (
I don't quite grasp that statement. You may have tried to convey the right concept I'm not sure.
We calibrate controllers on the bench
open loop ie not in a system.
It's down in the dirt simple - we apply step changes and watch output usually on a stripchart recorder so there's a calibration record
To check integral time here's how i might do it
Set proportional to 1.0
set integral to 1 repeat per minute
apply input to create zero error
set output to some convenient value , industrial controllers have a provision for manual control that allows this
transfer from manual control to auto, allowing integral to become active
controller output should hold constant since integrator sees zero error
start a stopwtach and
change input from zero to (?)some convenient amount of error say 10%
output should change immediately by same amount because of proportional term
output should continue changing at uniform rate by 10%(the amount of error) every minute
after a few minutes return error to zero
output should drop by 10%due to proportional , and hold at new value because integrator once again sees 0 error at input.
i have thereby verified that the proportional term is correct, output step was same size as inpuit step
and the integral term is correct, output repeats error once per reset time AND integration is linear
nonlinearity in integration says there's a hardware problem maybe an air leak in pneumatic or an unbalanced opamp in electronic
BoilingHeat said:
I thought it was the time it takes the integrator to correct a unity error,
That sound to me like something you could only measure in a closed loop and the remainder of the loop's dynamics would get into the act. We test them open loop because it's so much easier to interpret the result. Any adjustment when the thing is plugged into the plant is for purposes of tuning the process.
For example
<<<< boring anecdote alert>>>
our feedwater controllers ran for twenty years with the same settings.
Engineering decided to increase by 10% the amount of feedwater valve travel
when they did we observed that at low power we got sinewave oscillation in feedwater flow, large enough to make operators uneasy.
AHA! They'd raised loop gain by making that valve move farther
It only caused trouble at low power where valve is barely open and a small movement is a big % of the small flow
a small tweak downward on the proportional dial fixed it immediately.
Plant manager said :" That dog'll hunt, Thanks."
I
honestly replied "Thanks for the kind words, but really It was nothing."
Hope that too long dissertation helps
you have to understand controllers open loop, then integrate them into closed loop controlling a process. It's two ways of thinking - just train yourself to separate them
old jim