DVC6000f Series
March 2006
5-82
the next highest permitted non-remote mode. If Cas or
Auto are not available, the block will shed by default to
Man.
Retained Target—The retained target mode is the
mode the block was in before changing to one of the
remote target modes. On failure of a remote
connection, the block attempts to attain the retained
target mode.
Auto—On failure of a remote connection, the block
attempts to attain Auto, if permitted, until the remote
connection is restored.
Man—On failure of a remote connection, the block
sheds to Man until a remote connection is restored.
Shed With No Return Options
For any shed with no return option, the target mode
changes as determined by the option. Therefore, there
is no attempt to restore the connection following
failure. The behavior on change to a remote target
mode is identical to that for Shed With Return Options.
Normal—On failure of a remote connection, the block
sets the target mode to the highest permitted
non-remote mode. Cas is the highest permitted
non-remote mode and Auto is is the next permitted
non-remote mode. If Cas or Auto are not available, the
block will shed by default to Man.
Retained Target—The retained target mode is the
mode the block was in before changing to one of the
remote target modes. On failure of a remote
connection, the block sets the target mode to the
retained target mode.
Auto—On failure of a remote connection, the block
sets the target mode to Auto, if permitted.
Man—On failure of a remote connection, the block
sets the target mode to Man, if permitted.
The user may configure SHED_OPTS [34] so that it
calls for a target mode that is not permitted. When
doing this, the mode logic uses the following rules as
applied by the remote logic:
Shed logic never results in a non-permitted target
mode.
Shed logic never attempts to attain an actual
mode of Auto or Cas if that mode is not permitted.
Status Handling
If the input status on the PID block is Bad, the mode of
the block reverts to Manual. In addition, you can select
the Target to Manual if Bad IN status option to direct
Figure 5-13. PID Function Block Set Point Selection
B2722 / IL
Setpoint
Limiting
SP
HI LIM
SP LO LIM
Auto
Man
CAS
SP
RATE UP
SP RATE DN
Operator
Set point
Rate
Limiting
Auto
Man
CAS
the target mode to revert to manual. You can set the
status option in Manual or Out of Service mode only.
Set Point Selection and Limiting
The set point of the PID block is only valid when the
block is in Auto, Cas, or RCas. Figure 5-13 illustrates
the method for set point selection. You can configure
the SP_HI_LIM [21] and SP_LO_LIM [22] parameters
to limit the set point. In Cascade or Remote Cascade
mode, the set point is adjusted by another function
block or by a host computer, and the output is
computed based on the set point.
In Automatic mode, the set point is entered manually
by the operator, and the output is computed based on
the set point. In Auto mode, you can also adjust the
set point limit and the set point rate of change using
the SP_RATE_UP [20] and SP_RATE_DN [19]
parameters.
In Manual mode the output is entered manually by the
operator. In Remote Output mode, the output is
entered by a host computer.
Output Selection and Limiting
Output selection is determined by the mode and the
set point. In Automatic, Cascade, or Remote Cascade
mode, the output is computed by the PID control
equation. In Manual and Remote Output mode, the
output may be entered manually (see also Set Point
Selection and Limiting). You can limit the output by
configuring the OUT_HI_LIM [28] and OUT_LO_LIM
[29] parameters.
Filtering
The filtering feature changes the response time of the
device to smooth variations in output readings caused
by rapid changes in input. You can adjust the filter
time constant (in seconds) using the PV_FTIME [16]
or SP_FTIME [69] parameters. Set the filter time
constant to zero to disable the filter feature.
5
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