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Survival of the Fittest
The process time constants and process gain are highly nonlinear in batch operation because the reaction rate is a moving target. If you throw a transition between heating and cooling into the mix, you are headed for less than the best quality and efficiency. The preemptive adaptation of the controller to changing batch conditions is as important as the preemptive adaptation of a company to changing business conditions. The primary controller tuning settings should be scheduled based on totalized feeds and the secondary controller tuning settings scheduled based on the position of the split ranged valves4. The adaptive controller should trim the settings as the reactor moves into the scheduled regions and provide a closed loop response faster than the open loop response.
Time Delay – Dead Time
Derivative Action – Rate Action
Derivative Time – Rate Time
Digital Valve Controller – Digital Valve Positioner
Integral Action – Reset Action
Integral Time – Reset Time
Time Lag – Time Constant*
Open Loop Gain – Process Gain
Proportional Action – Gain Action
* A closed loop time constant defines the exponential response for a step change in the controller set point with the controller in automatic. An open loop time constant defines the exponential response for a step change in the controller output with the controller in manual. An open loop time constant for a self-regulating process is a negative feedback time constant. The positive feedback time constant for a runaway process is not common but is useful for describing the acceleration. There is no steady state for a runaway except as forced by the gain action of a controller in automatic.
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