Watlow Electric Revision 5 - Manual (Page 59)

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range of the process control span. If the process elements are not correctly
sized then it will be difficult and even impossible to tune the controller.
8.2.2 TUNING PID LOOPS
1. First set PB to 2% of the desired SP, Reset to .2 R/M, Rate to 0, Filter to
0. Set control in AUTO. Set the plot function for the proper range to record
the PV over an appropriate time base such as one hour.
2. The SP is set to the desired control point. If overshoot cannot be
tolerated, set the SP to a value below the final SP for tuning. Most heat
processes are slow reacting compared to many other types of control
systems. Usually 20 minutes are required between adjustments to see any
effective change. After a suitable wait, look at the Plot Function. If cycling
is not occurring, the PB is set correctly. If cycling is occurring, double the
PB. After a suitable interval [normally at least 20 minutes] if cycling is still
occurring, double the PB again. Repeat this process until the cycling stops.
The time between cycling peaks will usually be about 5 to 15 minutes. A
small amount of cycling may be removed by using the digital output filter
allowing a narrow PB without cycling.
3. With the PB at the Setting from Step 2, increase the reset in steps of .3
R/M. Keep increasing the reset until, by watching the Plot Function,
cycling is occurring at a slow rate. The time period between peaks will
probably be 20-40 minutes. Reduce the reset in .1 R/M steps until the
cycling stops. Remember to take the time between adjustments to allow the
process to settle out.
4. With the PB and RESET set according to Step #2 and #3, move the SP
upscale from the present SP by at least 20%. If the present SP is 450, the
new SP should be at least 540. If this cannot be done due to process
considerations, then allow the process to cool off. No matter the method, a
step change in SP is required. A ramp of the PV is needed to check the
overshoot of the PV, in order to adjust the RATE. Starting the system up in
the morning may be the only way to get a ramp of the PV. After seeing the
overshoot on the Plot Function, turn on RATE to a small number such as
.08, for a small overshoot and a larger number like .2, for a larger
overshoot. Keep increasing the number until there is no overshoot. If, the
RATE gets too large, the PV will undershoot. If undershooting occurs,
reduce the RATE setting in small steps until it is eliminated. This is the
RATE setting when little or no over or undershooting occurs.
The above steps may be time consuming, especially at first. With
experience the tuning process can be quick and straight forward.
Many processes have been controlled with the following values:
PB=40 RESET=.4 R/M [150 SEC.] RATE=0 [OFF] FILTER=4