Appendix A: Autotuning - Omega CN4321 Manual

APPENDIX A
Autotuning
By autotuning, the controller selects what it calculates to be the optimal
PID control parameters for a particular process and then stores them in
EEPROM memory for future use. The PID parameters are stored so that
when the controller is powered up after being shut down, the controller
does not need to be autotuned again. The controller uses the same auto-
tuned PID parameters until the Autotune function is again initiated. The
Autotune parameters are only good for the process the Autotune func-
tion was used on. If the setpoint is significantly changed, the input sen-
sor is changed, the load or output device is changed or relocated, or any
other disturbances occur which might change the dynamics of the sys-
tem, the Autotune function should be performed again. The autotuned
control parameters are not always perfect for every application but
almost always give the operator a good starting point from which further
refinement of the control parameters can be performed manually.
The autotuning algorithm used here is particularly suited for temperature
control applications and may not always autotune effectively for other
processes. Here are cases where the Autotune function does not per-
form well or does not perform at all:
1. The system is affected by process disturbances external to the control
loop. Adjacent heater zones, changing material levels, exothermic
reactions are examples of process disturbances which are external
to the control loop. The controller would never be able to autotune
such an unstable process.
2. The system is very dynamic. The process variable changes very quick-
ly. Certain pressure and flow applications would be characterized as
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