Reset Windup Protection - Automation Direct DL05 User Manual

Chapter 8: PID Loop Operation
The DL05 also combines the integral sum and the initial output into a single term called the
1
bias (Mx). This results in the following set of equations:
2
3
The DL05 by default will keep the normalized output M in the range of 0.0 to 1.0. This is
done by clamping M to the nearer of 0.0 or 1.0 whenever the calculated output falls outside
4
this range. The DL05 also allows you to specify the minimum and maximum output limit
values (within the range 0 to 4095 in binary if using 12 bit unipolar).
5
NOTE: The equations and algorithms , or parts of, in this chapter are only for references. Analysis of these
equations can be found in most good text books about process control.
6

Reset Windup Protection

Reset windup can occur if reset action (integral term) is specified and the computation of the
7
bias term Mx is:
8
For example, assume the output is controlling a valve and the PV remains at some value
greater than the setpoint. The negative error (e
9
decrease until the output M goes to 0 closing the valve. However, since the error term is still
negative, the bias will continue to decrease becoming ever more negative. When the PV
10
finally does come back down below the SP, the valve will stay closed until the error is positive
for long enough to cause the bias to become positive again. This will cause the process
variable to undershoot.
11
One way to solve the problem is to simply clamp the normalized bias between 0.0 and 1.0.
The DL05 CPU does this. However, if this is the only thing that is done, then the output will
12
not move off 0.0 (thus opening the valve) until the PV has become less than the SP. This will
also cause the process variable to undershoot.
13
The DL05 CPU is programmed to solve the overshoot problem by either freezing the bias
term, or by adjusting the bias term.
14
A
B
C
D
8–10
DL05 Micro PLC User Manual, 6th Edition, Rev. A
Mx = M
o o
Mx = Ki * e + Mx
n n-1
M = Kc * e - Kr(PV -PV
n n n
Mx = Ki * e + Mx
n n-1
) + Mx
n-1 n
) will cause the bias term (Mx) to constantly
n