EAF
Process Control Instructions
Chapter 16
16-28
Feedforward or Bias (Word 07)
This input is added to the calculated control output of the PID equation. It
is used when a bias is needed or if feedforward control is desired. This
value can be a storage constant or a calculated variable of
process measurement.
Proportional only control will give a steady-state offset of the process
variable from setpoint. A bias value can be added into the control output
using this word to bring the process variable to the set point.
The feedforward value (Figure 16.9) represents a measured feed system
disturbance fed into the PID equation. Its effect is to adjust the output
before the disturbance has a chance to change the process variable, thereby
reducing the impact of the disturbance on the system.
Figure 16.9
This Is an Example of Feedforward Control
PID
Equation
Flow
PV
Level
Detector
CO
Σ
SP
Σ
FT
Transmitter
10374-I
Feedforward/Bias Sign Bit (Word 02, Bit 02)
Set/reset this bit to change the sign of the feedforward value.
Reset (0)
value is positive
Set (1)
value is negative
