Figure 12-1. Typical Cs21 Two Loop Cascade Controller Application - ABB ControlIT 53MC5000 Instruction Manual

12.0 CS21 - TWO LOOP CASCADE
12.1 CS21 - TWO LOOP CASCADE CONTROLLER
The Two Loop Cascade Controller provides two standard PID controllers that function as a primary
and secondary controller in one unit. The output of the primary controller, based on its setpoint
and process variable, becomes the setpoint input to the secondary controller. This controller setup
is suited to process applications where the primary loop is usually slow, having changes that occur
over a long time duration, and the secondary loop is much faster, with rapid changes (distur-
bances) that affect the primary loop. The Two Loop Cascade Controller overcomes the slow proc-
ess time lags of the primary loop by providing immediate response to changes in the secondary
loop, which minimizes their affect on the primary loop. Transfer between local and cascade control
in the Two Loop Cascade Controller is bumpless and balanceless because the primary controller's
output is forced to match the secondary controller's setpoint when the secondary controller is in lo-
cal mode. A typical low-resolution display Two Loop Cascade Controller application is illustrated in
Figure 12-1 where the temperature (primary variable) of a liquid in a tank is maintained by regulat-
ing cold water flow to adjust tank jacket temperature (secondary variable).
Ensure the controller is capable of supporting more than one control loop by comparing the model
number of the controller (found on the controller tag) with the model number breakdown provided
in Section 1.
AI1 - REMOTE SETPOINT PRIMARY
AI3 - ADDITIVE FEED FORWARD
SECONDARY
DI0 - CASCADE ENABLE
(CLOSED CONTACT)
DI1 - REMOTE ENABLE PRIMARY
(CLOSED CONTACT)
AI2 - PROCESS
VARIABLE (SECONDARY)
AI0 - PROCESS
VARIABLE (PRIMARY)
A
2
TT
PUMP

Figure 12-1. Typical CS21 Two Loop Cascade Controller Application

Section 12. CS21 - Two Loop Cascade Controller
CONTROLLER
A
1
TT
A , B, C = PRIMARY LOOP
1
A , B, C = SECONDARY LOOP
2
DO0 - PROCESS ALARMS
DO1 - PROCESS ALARMS
AN01 - SECONDARY SETPOINT
AO0 - CONTROL
OUTPUT (SECONDARY)
B
VALVE
C
SECONDARY
PRIMARY
TRANSMIT
COLD
WATER
12-1