Red-lion TCU User Manual download pdf (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 p arameter. See Figure 30, for more details.

For proper Auto-tuning of the Primary loop, it is necessary that “DSP1”

and “ DSP2” represent the actual pro cess low and process h igh 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.

Figure 30, Internal Cascade

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Red-lion TCU User Manual Page 70