User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--34
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Enable Deadband y When selected, the enable deadband function takes a range
of small error values near zero, and simply substitutes zero as the value of the error.
If the error is larger than the deadband range, then the error value is used normally.
Freeze Bias
The term reset windup refers to an undesirable characteristic of integrator behavior
which occurs naturally under certain conditions. Refer to the figure below. Suppose
the PV signal becomes disconnected, and the PV value goes to zero. While this is a
serious loop fault, it is made worse by reset windup. Notice the bias (reset) term
keeps integrating normally during the PV disconnect, until its upper limit is reached.
When the PV signal returns, the bias value is saturated (windup) and takes a long
time to return to normal. The loop output consequently has an extended recovery
time. Until recovery, the output level is wrong and causes further problems.
PV
Output
0
Bias
Reset windup
Freeze bias enabled
Recovery time Recovery time
PV loss PV loss
In the second PV signal loss episode in the figure, the freeze bias feature is enabled.
It causes the bias value to freeze when the control output goes to its range limits.
Much of the reset windup is thus avoided, and the output recovery time is much less.
For most applications, the freeze bias feature will work with the loop as described
above. It is suggested to enable this feature by selecting it in the dialog. Bit 10 of PID
Mode 1 Setting (V+00) word can also be set in RLL.
NOTE: The freeze bias feature stops the bias term from changing when the control
output reaches the end of the data range. If you have set limits on the control output
other than the range (i.e, 0--4095 for a unipolar/12 bit loop), the bias term still uses
the end of range for the stopping point and bias freeze will not work.