Micromod Micro-DCI: 53MC5000 Multi-Loop Process Controller FLEXIBLE CONTROL STRATEGIES - Manual (Page 152)

A.1.8 CS22 - TWO-LOOP OVERRIDE CONTROLLER
The two-loop override controller has two standard PID control loops. Override control is used
when two interdependent variables are being controlled with a single final element and neither vari-
able may exceed a safe limit. Whichever variable is changing in the undesirable direction must be
the loop maintaining control of the final element. To accomplish this, the two control loops are ar-
ranged so that both control outputs are routed through a high/low selector. The output of the selec-
tor is fed back to both loops, holding the unselected loop near the active control output range.
Loading CS22 sets the controller for high limit (low selector) override control by initializing ANI2
(C258) engineering span to 100.0, C078 to 100.0, C089 to 1.0, and B202 to 138. As a high limit
override controller, the lowest output value is selected to drive the final element via ANO0. (After
CS22 is started, to set the controller for low limit [high selector] override control, configure data-
points C088 to 1.0, C089 to 2.0, and B202 to 102.) Also, for high or low limit override control, the
display list is expanded to add the second display group (CON1).
The control scheme is shown in block diagram form in Figure A-8. Also provided is Table A-8,
which is a worksheet that shows the CS22 wirelist connections; wirelist datapoints not connected
are left at default. Table A-8 is partitioned by FCS module type applicable to CS22 (those modules
illustrated in Figures 2-1 and 2-2). The datapoint parameters for each module are shown with their
default values.
The following I/O assignments result from loading CS22:
ANI0 Process Variable Loop 1
ANI1 Remote Setpoint Loop 1
ANI2 Process Variable Loop 2
ANI3 Remote Setpoint Loop 2
ANO0 Control Output (Secondary)
ANO1 Override Status
CCI0 Remote Enable Loop 1
CCI1 Remote Enable Loop 2
CCO0 Process Alarms Loop 1
CCO1 Process Alarms Loop 2
The description that follows is for Loop 1 with Loop 2 referenced parenthetically.
A setpoint value can be derived from a remote signal through ANI1 (ANI3). This remote setpoint
signal is selected as the setpoint when a closed contact is present on CCI0 (CCI1) and the R/L
push button is toggled to select remote. The control signal selected from one of the two loops is
provided for the final control element at ANO0. ANO1 is used to indicate which loop is providing
the active control. When control is from Loop 2, 100 percent of output is present at ANO1; when
control is from Loop 1, 0 percent of output is present at ANO1. Alarm conditions for both control
loops are indicated by a closed contact on the CCOs. Loop 1 alarms appear at CCO0, while Loop
2 alarms appear at CCO1.
FCS 53MC5000 Flexible Control Strategies
A–62
418-TXT