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This column is moderated by Béla Lipták (http://belaliptakpe.com/), automation and safety consultant, former chief instrument engineer at C&R and former Yale professor of process control and editor of the Instrument Engineer's Handbook. If you have automation related questions for this column, write to firstname.lastname@example.org
Q: My question is this: Can we replace a P controller with a linear function generator in controlling the level on a deaerator?
General information about the control loop(s):
The operational issues:
Do you agree with the proposed change? What control issues could that change generate? Any suggestions for any changes in the loop would be very welcome.
A: Gap level control can be provided by using two controllers or by splitting the output of a single controller among two valves with a gap in the center of the control signal where both valves are closed.In your deaerator level control system, the make-up valve must have an open-failure position, which means that the controller has to be reverse-acting, and therefore its gain has to be negative (Figure 1). If it's not, that explains your problems. When a proportional-only (P) controller is started up, the bias is normally adjusted to 50%, so that when the level is on the setpoint, the valve will be 50% open, and this opening will provide the "normal" flow. If the load (the required water flow) moves away from the 50% setpoint, the proportional controller is incapable of changing the flow to return the level to setpoint because it needs an error (e) in order to change its output signal (m) to the required new opening. This output is m = (controller gain)(e) + b, so that the output "m" equals the 50% bias (b), when the error is zero.
Therefore, whenever the required valve opening (the load) changes, an error must develop, and this error is called the "offset." For example, if the load "m" has changed to 60%, while the normal load (b) was 50%, the offset error is Offset Error = (60% - 50%)/(controller gain). Consequently, if the controller gain is high, (the slope of the operating curve in Figure 1 is steep), and proportional control is acceptable because the level will not change much while the valve is throttled (the offset is small).
Therefore, as shown in Figure 1, the two-controller system will "bump" the valve by the amount of the offset when the loop is switched between manual and automatic, but this does no harm to the level control, and the operator should simply be educated to accept it.
Another option is to provide logic that guarantees bumpless transfer, but it will not improve the quality of level control, only eliminate the "offset bump."
The correct system parameters are listed in Table 1.
A: I'm guessing that the current implementation does not use true P controllers. Digital systems based on incremental (velocity) algorithms have a floating output bias that changes whenever the controller is placed in manual or hits a limit. True proportional control is what you need, and you can get it with simple linear functions. The only caveat is that you can't transfer bumplessly from manual to auto. But it will control level. Teach the operators to accept the variable offset.