Appendix A: Autotuning - Fuji Electric PXZ Series Operation Manual

APPENDIX A
Autotuning
By autotuning, the controller selects what it calculates to be the opti-
mal 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 PXZ
uses the same autotuned PID parameters until the Autotune function
is again initiated. The Autotune parameters are only good for the
process the Autotune function was used on. If the setpoint is signifi-
cantly changed, the input sensor is changed, the load or output
device is changed or relocated, or any other disturbances occur
which might change the dynamics of the system, the Autotune func-
tion 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 PXZ's autotuning algorithm 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 PXZ would never be able to
autotune such an unstable process.
2. The system is very dynamic. The process variable changes very
quickly. Certain pressure and flow applications would be charac-
terized as very dynamic. Because of how the Autotune function is
performed, a very dynamic system would create very large over-
shoots which could damage the process.
3. The system is very insulated and cannot cool down in a timely
manner. With such heating systems the autotuning function would
take a very long time to complete with questionable results.
In Autotune, the PXZ sends test signals to the process. The test sig-
nals are 100% output and 0% output at the Autotune point. The
Autotune point can either be at setpoint or 10% of full scale below
setpoint..
The controller performs as an On/Off controller. See diagram below.
Autotune
Point
Process
Variable
ON OFF
100%
Output
0%
Autotune Period
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Time
ON OFF ON
Time
The PXZ then reads the reaction of these test signals on the process.
Keep in mind that every process is different and therefore every reac-
tion to the test signals is different. This is why PID parameters are not
the same for different processes. The amplitude (L) or lag time which
is the overshoot and undershoot of the system when autotuning and
the time constant (T) which is the time the process takes to go
through one On/Off cycle is measured. See diagram below.
Time Constant
Time
OFF
The measurements are then used with the Autotune algorithm for cal-
culation of the proper PID parameters for the system. See the PXZ
Autotune algorithm below, where K is the proportionality constant and
S is the Laplace operator.
K
(1+TS)
16
Overshoot
Amplitude
Undershoot
Time
ON
e -LS