GPS-X Technical Reference

351 Tools Object
The controller tuning parameters for the PID controller are accessed by selecting
Parameters > Control Variable > More... in the Process Data menu of the tools object.
Setting tuning to ON starts the tuning mode. The controller sampling time must be set
to an appropriate value. The process should have been brought to steady-state, or
reasonably close to steady-state, prior to the activation of the tuning mode. This may be
done with or without a PID controller currently ON. While this mode is active, a step
input is applied to the manipulated variable, forcing the controlled variable to be
perturbed.
This mode may be deactivated by manually setting Tuning to OFF, at which point
suggested PID tuning parameters will be displayed in the simulation Log window, based
on an analysis of the time series data collected during the tuning operation. It is not
necessary to wait for a new steady-state before turning off the tuning operation, but it
may be desirable to do so. Tuning parameters will also be suggested if the simulation
terminates while the tuning mode is ON.
The estimates of the process parameters (i.e. process gain, dead time, and time
constant) may be tagged for display in the Display Variables menu of the object or
stream associated with the controller.
The other settings which affect the operation of the tuning mode are:
Fractional step size: The size of the step to be applied in the manipulated
variable, specified as a fraction of its initial value.
Time of step: The interval of time that will elapse between the activation of the
tuning mode and the application of a step in the manipulated variable.
Maximum possible dead time: Upper bound on admissible values of dead time
in the first-order plus dead time model. This may be used to reduce the time
required to compute tuning constants, or may be used to force a dead time of zero
in the model used to derive the tuning constants.
While operating in tuning mode, time series data of the manipulated and controlled
variables are sampled (at each controller sampling time) and stored in arrays of fixed
maximum size. Data for 3000 sampling times can be stored. If that capacity is to be
exceeded, a warning will be displayed, and a scrolling time window that will retain
data for the last 3000 sampling times will be initialized. This should not be a limitation
in most cases, as this storage capacity is enough for data sampled every 10 minutes
over 20 days. The storage capacity is a global setting labeled controller tuning array
size, found in General Data > System > Parameters > miscellaneous. If this value is
increased, the layout must be recompiled for the change to take effect, i.e., this value
may not be changed inside a scenario. For more information on tuning PID
controllers, consult Marlin (1995) or Perry and Chilton (1973). The Ciancone
correlations are presented and discussed in Marlin (1995).
GPS-X Technical Reference