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Marathon Monitors Inc.
AACC 2000 Carbon Nov. 1, 1997
57
Setting the cutback values
The above procedure sets up the parameters for optimum steady state control. If
unacceptable levels of overshoot or undershoot occur during start-up, or for large step
changes in process variable, then manually set the cutback parameters ‘Lcb’ and ‘Hcb’.
Proceed as follows:
1. Set the low and high cutback values to three proportional bandwidths (that is to say,
Lcb = Hcb = 3 x Pb).
2. Note the level of overshoot, or undershoot, that occurs for large atmosphere changes
(see the diagrams below).
In example (a) increase ‘Lcb’ by the overshoot value. In example (b) reduce ‘Lcb’ by
the undershoot value.
Example (a)
Example (b)
Where the atmosphere approaches setpoint from above, you can set ‘Hcb’ in a similar
manner.
Atmosphere
Time
Setpoint
Atmosphere
Undershoot
Setpoint
Overshoot