Micromod MOD: 30ML Operation and Template Setup - Manual (Page 45)

MOD 30ML Multiloop Controller
OPERATION
3-13
3.9.4 Single Loop Automatic Operation with Feedforward
Feedforward control is a strategy used to compensate for disturbances in a system before
they affect the controlled process variable. A feedforward control system measures a
disturbance variable, predicts its effect on the process, and applies corrective action to cancel
the effect of the disturbance. A block diagram of this type of system is shown below.
This system provides a combination of feedforward and feedback control. Feedback provides
its normal function of holding the process at the set-point. Feedforward helps the feedback
function by modifying the control output as required to cancel the effect of variations in the
disturbance variable. The feedforward input (set up as a percent of output) is multiplied by a
constant (gain), and a bias value (%) is added to or subtracted from the signal. The gain and
bias values are tunable. The feedforward function can be configured to either add the signal
to the control output or multiply the output by the signal.
The front panel display and control key operations for feedforward control resemble the
example below.
[AUT] indicates that the controller is in automatic with
feedforward input.
[LOC] indicates local set-point.
[FF] indicates that the current feedforward input value is
displayed on Line 6 (bottom line).