Red Lion PCU - Manual (Page 64)

SECOND ANALOG INPUT OPTION
The Second Analog Input option is an additional analog input used for
Remote Setpoint or Internal Cascade operation. The mode of operation is
selected by programming. The Second Analog Input reading can be viewed in
the secondary display. Front panel annunciator SEC illuminates to indicate
this display mode.
REMOTE SETPOINT
The PCU with Second Analog Input can be configured as a Remote
Setpoint. This mode of operation enables Cascade control (external), Ratio
control and Process Setpoint Slave control, among others.
The Remote Setpoint value used internally by the controller is:
Remote Setpoint = (Scaled Second Analog Input * rtio) + bIAS
where rtio = 0.000 to 9.999
bIAS = -999 to 9999
The rtio and bIAS parameters offer on-line scaling of the Remote Setpoint
to adjust control ratios or biases among related processes.
In Remote Setpoint mode, the front panel annunciator REM is illuminated.
When in Local Setpoint mode, this annunciator is off. In either Local or
Remote Setpoint mode, the Manual (USEr) mode is indicated by the REM
annunciator flashing.
The Remote Setpoint is restricted to the setpoint limit values SPLO and
SPHI. These parameters may be used to limit the range of the Remote Setpoint
to a safe or more stable control range. For Remote Setpoint signal sources that
change wildly or are too sensitive to process upsets, the Setpoint Ramp
parameter (SPrP) can be used to ramp (rate limit) the Remote Setpoint reading.
This can subsequently reduce the fluctuations of the Secondary control loop.
Flow Ratio Control
Example: For processing purposes, it is necessary to control the flow of one
process with respect to the flow rate of another. The PCU is to control the
flow at a ratio of 1.5x that of the uncontrolled flow. The PCU with Remote
Setpoint programmed for a ratio value of 1.500 (rtio) and a bias value of 0
(bias) suits this application.
Process Remote Setpoint Slave Control
Example: Multiple PCUs are used to regulate the temperature zones of a
continuous drying oven. To reduce thermal shock to the product, the
setpoint levels of incoming zone controllers are low, while the other
controllers have setpoints that are increasingly ramped up to the ideal
drying temperature. The PCUs are slave controllers that have Remote
Setpoint with unique bias values to implement the ramp in setpoint values
of the drying oven. One PCU is the master controller. The master controller
re-transmits the setpoint value via the linear DC output (4 to 20 mA) to the
slave zone controllers. The slave zone controllers receive the 4 to 20 mA
signal as a Remote Setpoint.
CASCADE CONTROL
Cascade control involves the separation of a process into two control
loops: the Primary and the Secondary.
The Secondary control loop is designed to regulate the manipulated
variable which is normally the faster responding variable. The Primary loop
controller establishes this setpoint to the Secondary to maintain Primary loop
regulation. Disturbances occurring to the Secondary control loop are quickly
compensated for, before the effect appears in the Primary loop output. This
early loop compensation or “feed forward” action of Cascade control can
improve control quality compared with standard single loop control. Since
the Primary and Secondary “see” different processes, they normally have
different tuning values, with the Secondary normally faster responding.
The PCU is capable of two modes of Cascade control: External Cascade and
Internal Cascade. External cascade involves the use of two controller units, one
of which is a Remote Setpoint controller. Internal Cascade has both the Primary
and Secondary control loops implemented within one controller unit.
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