User manual

PID Loop Operation
(DL450 Only)
Maintenance
8--11
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
If the “Freeze Bias” option is selected when setting up the PID loop (discussed later)
then the CPU simply stops changing the bias (Mx) whenever the computed
normalized output (M) goes outside the interval 0.0 to 1.0.
Mx = Ki * e
n
+Mx
n--1
M=Kc*e
n
-- Kr(PV
n
-- P V
n--1
)+Mx
M
n
=0 “ifM<0”
M
n
=M “if0<M>1”
M
n
=1 “ifM>1”
Mx
n
=Mx “if0<M>1”
Mx
n
=Mx
n--1
“otherwise”
Thus in this example, the bias will probably not go all the way to zero so that, when
the PV does begin to c ome down, the loop will begin to open the valve sooner than it
would have if the bias had been allowed to go all the way to zero. This action has the
effect of reducing the amount of overshoot.
The normal action of the CPU is to adjust the bias term when the output goes out of
range as shown below.
Mx = Ki * e
n
+Mx
n--1
M=Kc*e
n
-- Kr(PV
n
-- P V
n--1
)+Mx
M
n
=0 “ifM<0”
M
n
=M “if0<M>1”
M
n
=1 “ifM>1”
Mx
n
=Mx “if0<M>1”
Mx
n
=M
n
-- K c * e
n
-- Kr(PV
n
-- P V
n--1
) “otherwise”
By adjusting the bias, the valve will begin to open as soon as the PV begins to come
down. If the loop is properly tuned, overshoot can be eliminated entirely. If the output
went out of range due to a setpoint change, then the loop probably will oscillate
because we must wait for the bias term to stabilize again.
The choice of whether to use the default loop action or to freeze the bias is
dependent on the application. If large step changes to the setpoint are
anticipated, then it is probably better to select the freeze bias option (see page
8--34).
Freeze Bias
Adjusting the Bias