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Online Operation Using Menus
Pretune Loop
Pretune calculates optimum values for a loop's Proportional Band/Gain, Reset and Rate by analyzing
the reaction of the loop to a "step change" in setpoint or output. After these new tuning values have
been calculated you have the option of applying (installing) or not applying them to a preselected tune
set of the loop. You can pretune a loop while another loop is pretuning.
To use pretune, your process time constant must be at least 60 seconds. The process time constant is
defined as the time needed for the PV to change by 63% of a step change in the setpoint. For example,
if the setpoint changes instantaneously by 100 degrees, the time constant is the amount of time needed
for the PV to change 63 degrees.
To pretune a loop, select PRETUNE from the TUNE LOOP online menu. Select a loop to tune. Pretune
occurs in 4 stages as indicated by the status. Each status has its own menu. Table 7-10 shows the
stages of pretune.
Order
Status
1
STOPPED
2
IDENTIFYING
3
CALCULATING
4
COMPLETE
NOTE: While using Pretune, it is recommended that Overshoot Suppression is turned off. With
overshoot suppression on, Pretune may not work or may give inaccurate tunings.
Pretune STOPPED menu
Table 7-11 describes the Pretune STOPPED prompts.
Prompt
STATUS
This is a read-only prompt.
PV
This is a read-only prompt.
OUTPUT
This is a read-only prompt.
SETPOINT
This is a read-only prompt.
TUNE SET
Select which set of tuning
parameters (#1 or #2) will
be pretuned and installed.
OPTIMIZE
Select SET PT or LOAD
OVERSHOOT
Select YES or NO.
238
Table 7-10 Stages Of Pretune
Pretuning not operating, waiting to be started by operator. See STOPPED menu.
Pretune is identifying process dynamics as a result of a setpoint or output change.
This process may take up to 4 to 6 process time constants. See Table 7-12
Identification is complete and calculation of new tuning parameters is in process.
See Table 7-12
Calculations are complete and new parameters are ready to replace the loop's
tuning parameters, if desired. See Table 7-13
Table 7-11 Pretune STOPPED Prompts
Range/Selections
Video Recorder – User Manual
Meaning
Status stopped.
Current process variable value of the loop being tuned.
Current output value of the loop being tuned.
Current working set point value of the loop being tuned
If tuning a split loop, set#1 applies to loop output between 0
and +100; set#2 applies to loop output between 0 and -100.
This optimizes the new tuning parameters according to their
intended use (that is, controlling changes in setpoint or
process load).
This determines whether or not some overshoot is
acceptable in the pretune specified tuning.
Definition
Continued on next page
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