Preferred FlexFit 110 - General Steps for Tuning the FlexFit PID Loop

Commissioning – Tuning 3-55
FlexFit – Linkageless Control – Revision 1.0
PREFERRED
UTILITIES MFG CORPORATION
CC
OO
MM
MM
II
SS
SS
II
OO
NN
II
NN
GG
Tuning
The PID provides a Proportional, Integral, and Derivative control algorithm. The PID equation used in the FlexFit is called the
"Parallel" form by the Instrument Society of America (ISA).
The Proportional Function
The Proportional constant is expressed as Proportional Band (PB) as opposed to Gain (G). PB = (100/G), i.e. a PB of 5% equals
a Gain of 20.
In the FlexFit, the PB is expressed as the setpoint to process variable deviation (error) that results in a change in the controlled
output.
The Proportional Band is expressed in the same engineering units as the process being controlled. The amount of error required
to produce full-movement servo degrees or loop output is noted in the PB Parameter.
The PB output values for the two PID loops in the FlexFit are expressed as such:
• Firing Rate – the actual temperature or pressure change that will result in a 100% ring rate change.
• O2 Trim – the ue gas oxygen change that results in a change from minimum trim to maximum trim.
The Integral Function
The Integral constant in the PID function can be expressed either as a reset rate (repeats per minute) or as a time constant
(minutes per repeat). The FlexFit uses minutes per repeat. The values set in the Minutes Per Repeat is the time it takes before the
PID ramps up or down 1 additional PB move. A smaller value causes more integral control action.
The Derivative Function
The derivative is a rate function that is not used in the FlexFit.
Recommended Procedure for Tuning a PID Loop
The default values in the PID related parameters are approximate and only a suggested starting point for tuning. Tuning a
controller requires subjective judgments. The person tuning the PID loop must be fully aware of the systems operational constraints
and safety considerations before proceeding. Always be ready to put the controller in manual should uncontrollable cycling occur.
Never tune a PID loop unless you have sufcient time to monitor the operation. During the monitoring period, try to simulate
every probable load swing condition that might upset the loop.
BE VERY PATIENT! Observe the process variable, the controller output, and the entire plant operation during various load
conditions to ensure smooth controller performance.
General Steps for Tuning the FlexFit PID Loop
Step 1 – Identify the control loop about to be tuned and determine the relative speed of the loop. Examples are as follows:
• A fast loop has a response time from less than one second to about 10 seconds, such as a ow loop.
• A medium speed loop has a response time of several seconds up to about 30 seconds, such as ow, temperature, and
pressure.
• A slower loop has a response time of more than 30 seconds, such as most temperature, steam pressure, and level loops.
Step 2 – Identify the units of the PID controller.
• Proportional Band as related to the magnitude of output change for a given SP vs. PB error.
• Consider the time constant as it relates to the loops speed. How often should the PB ramp be repeated?
Step 3 – Adjust the Proportional Band: With the loop in automatic, make a small change in the setpoint or wait for a disturbance
in the process. Then watch for process variable (PV) and control output responses. Keep the minutes per repeat at a
higher value at this point so that it imposes very little inuence on the loop.
• If no visible change in the output occurs upon a change in the setpoint, or there is no overreaction, decrease the PB by 50%.
• If the PV is unstable or has sustained oscillation, with overshoot greater than 25%, increase the PB by 50%.
• A smaller PB value will cause repeated loop oscillation, too large a PB value will result in a very sluggish loop.
• Continue to create loop upsets by changing the setpoint or wait for an actual change in the process. Adjust the PB value
until the loop responds with a tolerable overshoot followed by a settling effect without continued oscillation.
Step 4 – Add Integral: When the PB tuning is reasonable, the observed action of the loop will show that the process will
never seem to be able to return to the set point in a reasonable amount of time. This is where the integral will help. By