GPS-X Technical Reference

Tools Object 348
Derivative Kick Protection
Derivative kick protection is a commonly used modification to the standard PID
algorithm. The modification, which affects the derivative mode only, prevents large
`jumps' from occurring in the manipulated variable as the result of setpoint changes.
These jumps occur because a discrete setpoint change introduces a discontinuity into the
error signal, resulting in an "infinite" derivative (in the continuous case), or a large value
in the difference error signal (in the discrete case). Therefore, it is recommended that
derivative kick protection be used for all PID controllers, unless the behaviour described
above is specifically desired.
Derivative Filtering
Derivative filtering is another strategy by which large jumps in the manipulated variable
can be prevented. Filtering reduces the effect of sudden setpoint changes and high
frequency noise present in the error signal, which may produce large values of the error
derivative. When derivative filtering is selected, the signal fed to the derivative mode of
the PID (error or controlled variable) is passed through a discrete first-order filter before
being used in the calculation of the derivative mode. The filter is tuned by a single
parameter, the cutoff frequency, which corresponds to the filter's `corner frequency'.
(i.e.-the reciprocal of its time constant)
PID Controller Parameters
The parameters available for PID controllers are listed below. These parameters are
divided into two groups: control variable parameters, and manipulated variable
parameters. The control variable parameters are accessed by selecting
Parameters > Control Variable from the Tools Process Data menu. The manipulated
variable parameters are accessed by selecting Parameters > Manipulated Variable from
the Tools Process Data menu.
In addition to the PID controller in the tools object, PID controllers for common variable
pairings are built into many of GPS-X's process objects. For example, a PID controller
for controlling the dissolved oxygen concentration is available in the plug flow tank
object. These controllers have essentially the same parameters as the PID controller in the
tools object except that the manipulated variable and possibly the controlled variable
have already been defined.
Control Variable Parameters
Controller (ON - OFF): This parameter determines whether or not the PID controller is
active.
Setpoint for control variable: This is the value of the setpoint for the controlled
variable. The controller acts to keep the controlled variable at the setpoint.
GPS-X Technical Reference