Red Lion TCU - Manual (Page 78)

The following additional parameters are calculated and set as a result of
Auto-Tuning of the secondary:
Secondary Proportional Band (Pb-2)
Secondary Integral Time (It-2)
Secondary Derivative Time (dt-2)
Secondary Output Power Dampening (OPd2)
Auto-Tuning of the secondary presents two different control points at
which the controller cycles power. In Automatic mode of operation, the
secondary control point is the setpoint directed by the primary at the instant
Auto-Tune is started. In Manual mode of operation, the secondary control
point is the secondary process value reading at the instant Auto-Tune is
started. The secondary is normally Auto-Tuned with the controller in the
Manual mode of operation unless the process is reasonably under control.
Prior to tuning the Secondary, it is essential that it is scaled to match the actual
secondary process range. This is important for proper Auto-Tuning of the
primary. Subsequent changes made to scaling values may require re-tuning.
The following procedure may be used to initially tune an Internal Cascade
controller:
1) Place the controller into Manual (USEr) mode of operation.
2) Adjust output power level until primary variable is close to primary
setpoint. ( ± 10% of range)
3) Auto-Tune the secondary.
4) Auto-Tune the primary.
5) Place controller into Automatic (Auto) mode of operation.
6) Initial tuning is complete.
After the process has stabilized, the primary and secondary may be
re-tuned in Automatic mode of operation. Normally, the primary requires
re-tuning whenever the secondary PID constants are changed.
Auto-Tune Of External Cascade Systems (Remote Setpoint)
External Cascade systems involve the use of two controllers, the Primary
and the Secondary, that have a Remote Setpoint Input. In such a system, the
Secondary controller is normally tuned first followed by tuning of the
primary controller. Prior to tuning the Secondary controller, it is essential
that the Remote Setpoint is scaled to match the actual secondary process
range. This is important for proper Auto-Tuning of the primary controller.
Subsequent changes made to scaling values may require re-tuning. The
following procedure may be used to initially tune an External Cascade
controller:
1) Place the Secondary controller into Local Setpoint mode and Manual
(USEr) mode of operation.
2) Adjust output power level of the secondary until primary variable is close
to primary setpoint. ( ± 10% of range)
3) Key-in secondary setpoint value equal to secondary process value.
4) Auto-Tune the secondary controller while in Local Setpoint mode.
5) Place the secondary controller into Remote Setpoint mode and Automatic
(Auto) mode of operation.
6) Auto-tune the primary controller while the primary is in Automatic mode
of operation.
7) Initial tuning of system is complete.
After the process has stabilized, the primary and secondary may be
re-tuned in Automatic mode of operation. Normally, the primary requires
re-tuning whenever the secondary PID constants are changed.
Note: For Remote Setpoint controllers, the Auto-tune control point is derived
from the Remote Setpoint when in Remote Setpoint mode and it is derived from
the Local Setpoint when in Local Setpoint mode.
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