VVP10 HART
HART
®
VALVE POSITIONER
INSTALLATION, OPERATION, CONFIGURATION AND MAINTENANCE MANUAL
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38
PREDICTIVE DIAGNOSTICS OF POSITION
Reversal
Diagnostic for verification of course transitions from the control system. With each reversal of sense of the movement,
a counter is increased. The reversal is accounted for when the valve reverses its direction with a moving above
Reversal Deadband
, configured by the user between 0% and 20%.
In addition, the user can configure the maximum value to the counter (
Reversal Counter Limit
) in order to generate an
alarm (
Reversal Limit Alert
) when it is exceeded. This alarm is persistent, forcing the user to deactivate it manually
after verifying the cause of the reversal alert.
With proper knowledge of the process, the user can configure the counter limit to an expected value of motion reversals
for a certain period (with a certain tolerance to avoid false alarms).
In the chart of Figure 3.20, considering variations d1 and d2, where d1 <
Reversal Deadband
and d2 >
Reversal
Deadband
, the reversal counter will be increased only in reversal of direction in d2, ignoring the small d1 reversal,
because it is less than minimum value of dead zone configured.
Example 1: Considering Reversal Deadband = 1% and PV = 0%. If the valve goes to 25% and then to 23.9%, the
counter will be increased as it will have changed direction beyond 1%. If the valve went to 25% and then to 24.1%, the
counter would not be increased as it would not have left the deadband of 1% of the reversal.
Example 2: Considering Reversal Deadband = 1% and PV = 100%. If the valve travels the sequence 50%, 55%, 45%
and 45.5%, the counter will be increased twice (50% to 55% and 55% to 45%). Note that the movement from 45% to
45.5%, despite being in reverse to the previous one, will not be considered a reversal, as it does not exceed the
deadband configured.
The diagnosis also has Operation Time count, informing how long it has been active since the last reset of diagnosis.
On restarting, counter, operation time and alarm are reset.
Default values for Reversal parameters are
Reversal Deadband = 1%
and
Reversal Counter Limit = 1000
.
Figure 3.20 – Example of reversal occurrence.
Figure 3.21 – Example of strokes occurrence.
Stroke
Diagnosis to verify position at the end of the valve course. With each entrance to the end of the course, a counter is
increased. The end of the course regions are the ends of the system, configured by the user via
Stroke Edge
parameter
(between 0% and 20%), and is therefore considered by the positioner as
Stroke Edge
, to the lower region, and (100%
-
Stroke Edge
), to the upper region.
HINT
The Reversal diagnosis is used to identify unwanted fluctuations in the system and may indicate error in
PID control tune or friction problems that disturb the correct modulation of the system.
