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Video Recorder – User Manual 238
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.
Table 7-10 Stages Of Pretune
Order Status Meaning
1 STOPPED Pretuning not operating, waiting to be started by operator. See STOPPED menu.
2 IDENTIFYING 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
3 CALCULATING Identification is complete and calculation of new tuning parameters is in process.
See Table 7-12
4 COMPLETE Calculations are complete and new parameters are ready to replace the loop's
tuning parameters, if desired. See Table 7-13
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.
Table 7-11 Pretune STOPPED Prompts
Prompt Range/Selections Definition
STATUS This is a read-only prompt. Status stopped.
PV This is a read-only prompt. Current process variable value of the loop being tuned.
OUTPUT This is a read-only prompt. Current output value of the loop being tuned.
SETPOINT This is a read-only prompt. Current working set point value of the loop being tuned
TUNE SET Select which set of tuning
parameters (#1 or #2) will
be pretuned and installed.
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.
OPTIMIZE Select SET PT or LOAD This optimizes the new tuning parameters according to their
intended use (that is, controlling changes in setpoint or
process load).
OVERSHOOT Select YES or NO. This determines whether or not some overshoot is
acceptable in the pretune specified tuning.
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