Red-lion TCU User Manual download pdf (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 d ye 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 steamprocess 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.

Figure 29, External Cascade

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Red-lion TCU User Manual Page 69