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...5 CONTROL OPERATION
5.1 Operating Page Introduction
5.1.1 Set Point Tracking
With set point tracking enabled (
) the local set point value tracks the
CONFIGURATION LEVEL
process variable when the controller is in Manual control mode.
In this mode of operation the set point limits do not apply. If the
set point value is outside its limits when Automatic control mode
is selected, the local set point remains outside its limits and can
only be adjusted in one direction, towards its limits. Once inside
the limits they apply as normal.
With remote set point tracking enabled, the local set point tracks
the remote set point value when in the remote set point mode. In
this mode of operation the local set point limits do not apply. If
the set point value is outside its limits when the local set point
value is selected, the local set point remains outside its limits and
can only be adjusted in one direction, towards its limits. Once
inside the limits they apply as normal.
5.1.2 Auto/Manual Transfer
All auto-to-manual transfers are bumpless. If the local set point
is used and set point tracking is enabled, all manual-to-auto
transfers are bumpless, since the set point is always at the same
value as the process variable. Without set point tracking
enabled, the response following a manual-to-auto transfer
depends on the control settings. With an integral action setting
the output is ramped up or down to remove any process variable
offset from the set point (providing the process variable is within
the proportional band). If the integral action is off, the output may
step to a new value when the controller is transferred back to
automatic control mode.
With remote set point tracking enabled, the control set point
switches automatically from remote to local when manual mode
is selected.
5.1.3 Profile Control – Fig. 5.3
100%
Cool
Output
0%
Controller Output
0%
Fig. 5.3 Overall Profile Example
12
Set Points Page, CONTROL
100%
Output Off
Hysteresis
Heat
Ouput
0%
Crossover
Value
100%
5.1.4 Cascade Control
The master in a cascade set-up is always channel 1 and the
slave is always channel 2. If the slave is switched to manual
control with cascade set point selected, the slave's set point
reverts automatically to local set point.
Ratio and bias are applied to the master output value so that the
slave's cascade set point value = Ratio x Master Output + Bias.
With
Output Tracking enabled
mode or local set point, the master is switched automatically to
manual. The manual output of the master tracks the local set
point value of the slave. The value fed back to the master takes
into account any ratio and bias settings.
With
Output Tracking disabled
mode or local set point does not affect the operation of the
master.
To return to full cascade control carry out the following
procedure:
a) Switch the Slave controller into automatic control mode.
b) Switch the Slave Controller set point to 'Cascade'.
c) Switch the Master controller to automatic control mode (if
currently in Manual)
5.1.5
Heat/Cool Control – Fig. 5.4
When in automatic control mode both the heat and cool outputs
are turned off when in the Output Off Hysteresis Band. In manual
control mode the Output Off Hysteresis Band has no effect. If the
PID output is within the Off Hysteresis Band when the controller
is returned to auto control mode, the Off Hysteresis Band has no
effect until either the PID output goes outside the band or
becomes equal to the Crossover Value.
Overall Profile
Segment
Numbers
Eng.
Units
S1
S2
T1
T2
Program 1
Start
at SS
Start at S2
Fig. 5.4 Heat/Cool Control – Principle of Operation
– if the slave is switched to manual
– switching the slave to manual
S3
S4
S5
Time
T3
T4
T5
End
at S3
Program 2
End at S5
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