GPS-X Technical Reference

Tools Object 352
FEEDFORWARD-FEEDBACK CONTROLLERS
In feedforward control, an input disturbance is measured and the controller adjusts the
manipulated variable to compensate for the disturbance before the controlled variable deviates
from its setpoint. Feedforward control is usually combined with feedback control to retain the
beneficial properties of feedback.
The pidforward model of the Tools object combines a PID controller for feedback and a lead/lag
algorithm for feedforward control. Lead/lag feedforward control is a simple feedforward strategy
that provides enough flexibility for most applications. The PID controller algorithm is presented
in the section entitled PID Controllers above. In this section we present the lead/lag algorithm.
A feedforward term may be added to the feedback control action of the PID controller when
measurement of a process disturbance is available. The lead/lag feedforward controller is
represented in transfer function form as:
Equation 12.22
Its time-domain representation is given by:
Equation 12.23
where:
MV FF
D
K FF
ld
lg
I FF
= additional control action provided by feedforward control
= the measured disturbance
= the feedforward gain, a controller tuning parameter
= the feedforward dead time, a controller tuning parameter
= the feedforward lead time, a controller tuning parameter
= the feedforward lag time, a controller tuning parameter
= the initialization constant of the feedforward controller
GPS-X Technical Reference