User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--14
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
The DL450 loop controller operates in one of two modes, either Manual or
Automatic.
Manual
In manual mode, the control output is determined by the operator, not the loop
controller. While in manual mode, the loop controller will still monitor all of the alarms
including High--High, High, Low, Low--Low, Yellow deviation, Orange deviation and
Rate--of--Change.
Automatic
In automatic mode, the loop controller computes the c ontrol output based on the
programmed parameters stored in V--memory. All alarms are monitored while in
automatic.
Cascade
Cascade mode is an option with the DL450 PLC and is used in special control
applications. If the cascade feature is used, the loop will operate as it would if in
automatic mode except for the fact that a cascaded loop has a setpoint which is the
control output from another loop.
Reverse Acting Loop
Although the PID algorithmis used in a direct, orforward, acting loop controller, there
are times when a reverse acting control output is needed. The DL450 loop controller
allows a loop to operate as reverse acting. With a reverse acting loop, the output is
driven in the opposite direction of the error. For example, if SP > PV, then a reverse
acting controller will decrease the output to increase the PV.
Mx = --Ki * e
n
+Mx
n--1
M=--Kc*e
n
+ Kr(PV
n
-- P V
n--1
)+Mx
n
Square Root of the Process Variable
Square root is selected whenever the PV is from a device such as an orifice meter
which requires this calculation.
Error Squared Control
Whenever error squared control is selected, the error is calculated as:
e
n
= (SP -- PV
n
) * ABS(SP -- PV
n
)
A loop using the error squared is less responsive than a l oop using just the error,
however, it will respond faster with a large error. The smaller the error, the less
responsive the loop. Error squared control would typically be used in a pH control
application.
Loop Operating
Modes
Special Loop
Calculations