Red Lion TCU - Manual (Page 70)

Internal Cascade Control
The Internal Cascade control mode of the TCU embodies the function of
two Cascade controllers into a single unit. In all other respects, Internal
Cascade yields the same control flexibility and control quality as External
Cascade.
In Internal Cascade, the Primary loop provides an internal setpoint for the
Secondary loop. The Primary loop output power (0-100%) is scaled
internally by the “DSP1” and “DSP2” scaling parameters to yield the
Secondary (directed) setpoint. This setpoint is used by the secondary loop to
calculate the actual output (physical output). The setpoint can be viewed
during operation by the SP-2 parameter. See Figure 30, for more details.
For proper Auto-tuning of the Primary loop, it is necessary that “DSP1”
and “DSP2” represent the actual process low and process high values,
respectively, of the Secondary process. The tuning parameters (Prop, Intt,
dErt, OPdP) pertain to the primary loop and the tuning parameters (Pb-2, It-2,
dt-2, OPd2) pertain to the secondary loop.
Example : A TCU with Second Analog Input is used in an Internal Cascade
arrangement to regulate the temperature of a large dye vat. The Second
Analog Input is the input to the Secondary loop. This loop regulates steam
pressure. The Primary loop (temperature) directs the setpoint of the
Secondary to maintain vat temperature. The range of the Secondary steam
process is 0.0 to 200.0 PSI transmitted by a 4-20 mA transducer. The
following data configures the Internal Cascade controller:
OPEr - CSCd Select Internal Cascade mode
root - NO No square root linearization necessary
dPt2 - 0.0 Select decimal point position for 0.0 PSI
dSP1 - 0.0 Secondary process display low value (0.0 PSI)
INP1 - 4.00 Secondary process transmitter low value (4 mA)
dSP2 - 200.0 Secondary process display high value 200.0 PSI)
INP2 - 20.00 Secondary process transmitter high value (20 mA)
OPd2 - 2 Secondary output power (physical output) dampening
The Secondary process value can be monitored during operation in the
secondary display.
See Auto-tune, page 68, for tuning procedure of Internal Cascade
controllers.
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Figure 30, Internal Cascade