Rockwell Automation T8019 Trusted Process Control Algorithm Software Package - Manual (Page 26)

Trusted
TM
Process Control Algorithms T8019
Issue 8 Sep 07 PD-T8019 26
4. Sample Application Program
A sample application program using selected Process Control Algorithms to implement a simple PID
loop is shown in Figure 3. Note that many of the input parameters to the ipid function block will usually
be program constants, and the extra outputs are not necessary for a single loop.
The mantrk blocks allow read/write control of setpoint and output. For the setpoint, the mantrk block
copies the measured variable to the setpoint input whilst the loop is in manual, to allow a bumpless
transfer to automatic mode. On entering automatic mode the setpoint will remain at this value until the
setpoint is changed. Note that since the ipid function block uses incremental action, this function is only
necessary if the output should not move on entering automatic mode until the setpoint is entered.
Without this function, the output will move to control to the setpoint but without a bump to the output.
The second mantrk block allows manual control of the output in manual mode, using the ‘operator
manual output’ tag. In automatic mode, the mantrk block follows the automatic output and writes to the
ipid’s XO input. On entering manual mode, the ipid block output follows XO, but the operator can now
change the output using ‘operator manual output’.
The slew function block limits the rate of change of the output, and is optional. Note that although this
will smooth the action to the control valve, it will also change the control response in a non-linear
fashion.
For cascade (master-slave) pairs of ipid blocks, wire the LIMIT outputs of the slave back to the
INCst/DECst inputs of the master. This will stop the master’s action if the slave output has hit its limit.
Wire the output of the master to the setpoint of the slave. If the slave is not in auto and reading its
setpoint from the master’s output (‘remote’ mode) then the master should be forced into manual mode
because its output is not being used. For any loop, if the measured variable is suspect, force the loop
into manual mode. If the remote setpoint is suspect, force the loop into local automatic mode with a
manually adjustable setpoint. Always wire intermediate signals like remote setpoints and master-slave
interactions through variables, so that their state can be monitored.
Figure 3 Sample Application Program