Micromod MOD: 30ML Functions Data Base Reference - Manual (Page 132)

MOD 30ML Functions
CONFIGURATION EXAMPLES
3.3.9 CAS&DSP (CASCADE WITH DISPLAY) COMPOUND
The CAS&DSP compound is a fully functional cascade control and display configuration for
the MOD 30ML Multiloop Controller. This compound provides for cascade control of a
multiloop system where the primary variable is controlled by adjusting the set-point of a PID
algorithm which is controlling a related secondary variable. The secondary variable then
affects the primary variable through the process. In this compound the PID algorithm
receiving the primary process variable is called the master, and the PID algorithm receiving
the secondary process variable is called the slave. All auto/manual switching for the system
is done in the slave control loop.
Operation of the cascade arrangement provided by this compound is as follows:
• The slave set-point tracks (automatically remains equal to) the slave process variable while in
manual. This feature eliminates the need for the slave PID algorithm to respond immediately
to any deviation which exists when the mode is changed from manual to cascade. Without
set-point tracking the algorithm responds to any deviation which exists by ramping the slave
output at the reset rate established when the slave loop was tuned.
• The output of the master PID algorithm tracks the set-point of the slave while the slave is in
manual. This ensures that the local (slave) set-point and remote set-point (master output)
are in balance prior to placing the slave loop in cascade (automatic) mode.
• The set-point of the master tracks the master process variable while the slave is in manual.
This causes the set-point of the master to vary indirectly in response to manual output
changes in the slave loop. When the system is switched to cascade, the master set-point
switches from track to local, and there is no process upset because the track function
eliminates any set/process deviation in the master loop.
• Since the control element (valve, air damper, etc.) is directly manipulated by the slave, the
cascade master is configured to permit only the automatic mode which simplifies operations
by ensuring that all mode transfers and manual intervention can be accomplished exclusively
at the slave.
3.3.10 Configuration Overview
The blocks which make up the CAS&DSP compound are shown in Figure 3-38. This
compound consists of an input stage, a control stage, an output stage, and a display stage.
The input stage contains analog input blocks and input function blocks which process the
master and slave process variable inputs. The control stage contains two PID algorithms
which provide the cascade control function. The output stage contains a single analog output
block which outputs an analog control signal to the built-in current output circuit. The display
stage contains display and tune list blocks which determine how the system inputs, outputs,
status indications, and tunable parameters are displayed on the front panel.
Each block in the compound has been preconfigured and all required connections between
the blocks have been made. Each block can be queried to view its edit display, and block
attributes can be edited to meet specific application requirements. The master and slave
process inputs are configured as current signals connected to the built-in universal analog
input circuits. The configuration can be modified to use any process input signal which can
be accepted by the MOD 30ML instrument
3-38