Red Lion TCU - Manual (Page 69)

External Cascade Control
External Cascade Control involves the use of two controllers, one of which
has a Remote Setpoint Input.
The outer loop controller (Primary controller) directs the setpoint of the
inner loop controller (Secondary controller) through the linear DC 4-20 mA
output. The Secondary controller has a Second Analog Input option
configured as a Remote Setpoint (RSP) in order to receive the directed
setpoint. See Figure 29, for more details.
The Remote Setpoint of the Secondary controller must be scaled prior to
tuning the controllers. Normally, the Remote Setpoint is scaled to equal the
actual process range of the secondary. Scaled in this way, the Primary
controller can direct the setpoint of the Secondary controller over its
operating range. The internal Remote Setpoint value is as follows:
Remote Setpoint = (Scaled Second Analog Input * Ratio Parameter) + Bias
Parameter
Example : A TCU (temperature input) and PCU (process input) are used in an
External Cascade arrangement to regulate the temperature of a large dye vat.
The PCU is the Secondary controller with Remote Setpoint to regulate steam
pressure. The TCU is the primary controller with linear DC output which
directs the setpoint of the Secondary controller in order to maintain vat
temperature. The range of the secondary steam process is 0.0 to 200.0 PSI.
The following data configures the Remote Setpoint of the PCU:
OPEr - rSP Select Remote Setpoint mode
root - NO No square root linearization necessary
dPt2 - 0.0 Select decimal point position to match that of main input
dSP1 - 0.0 Scale Remote Setpoint to match main input range of PCU
INP1 - 4.00 Scale input range to match 4-20 mA output of TCU
dSP2 - 200.0 Scale Remote Setpoint to match main input range of PCU
INP2 - 20.00 Scale input range to match 4-20 mA output of TCU
SPtr - Auto Select bumpless Local/Remote setpoint transfer
In some cases the Remote Setpoint signal may change too rapidly or have
excessive process noise. This may lead to instability or even oscillation of the
Secondary controller. The Setpoint Ramp parameter (SPrP) is effective in
limiting the amount of change of the Remote Setpoint. The Setpoint Ramp
parameter should be set to a minimum ramp value consistent with the
response time of the Primary process. Additionally, Setpoint Limit Low and
Setpoint Limit High parameters (SPLO, SPHI) may be used to constrain the
Remote Setpoint value to safe limits or narrow the operating range for
stability purposes.
See Auto-tune , page 68, for tuning procedure of External Cascade
controllers.
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Figure 29, External Cascade