Instruction Manual

Instruction Manual
D103434X012
Blocks
November 2014
88
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:
D Shed logic never results in a non‐permitted target mode.
D 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 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 B‐6 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.
Figure B‐6. PID Function Block Set Point Selection
B2722
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
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.