User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--68
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Feedforward Control
Feedforward control is an enhancement to standard closed-loop control. It is most
useful for diminishing the effects of a quantifiable and predictable loop disturbance
or sudden change in setpoint. Use of this feature is an option available to you on the
DL450. However, it’s best to implement and tune a loop without feedforward, and
adding it only if better loop performance is still needed. The term “feed-forward”
refers to the control technique involved, shown in the diagram below. The incoming
setpoint value is fed forward around the PID equation, and summed with the output.
Process Variable
Loop
Calculation
Σ
+
--
Control OutputSetpoint
Σ
+
kf
Feedforward path
+
In the previous section on the bias term, wesaid that “the bias term value establishes
a “working region” or operating point for the control output. When the error fluctuates
around its zero point, the output fluctuates around the bias value.” Now, when there
is a change in setpoint, an error is generated and the output must change to a new
operating point. This also happens if a disturbance introduces a new offset in the
loop. The loop does not really “know its way” to the new operating point... the
integrator (bias) must increment/decrement until the error disappears, and then the
bias has found the new operating point.
Suppose that we are able to know a sudden setpoint change is about to occur
(common in s ome applications). We can avoid much of the resulting error in the first
place, if we can quickly change the output to the new operating point. If we know
(from previous testing) what the operating point (bias value) will be after the setpoint
change, we can artificially change the output directly (which is feedforward). The
benefits from using feedforward are:
S The SP--PV error is reduced during predictable setpoint changes or loop
offset disturbances.
S Proper use of feedforward will allow us to reduce the integrator gain.
Reducing integrator gain gives us an even more stable control system.
Feedforward is very easy to use in the DL450 loop controller, as shown below. The
bias term has been made available to the user in a special read/write location, at PID
Parameter Table location V+04.
Process Variable
Σ
Error Term
+
--
Control OutputSetpoint
Σ
+
P
I
D
Loop Calculation
+
+
kp
ki
kd
V+04
Bias TermXXXX