VVP10 HART
HART
®
VALVE POSITIONER
INSTALLATION, OPERATION, CONFIGURATION AND MAINTENANCE MANUAL
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40
Example 1: Assuming Mileage Deadband = 1% and PV = 50%. When the valve goes to 45%, the sum will be
incremented by 5%. If the valve returns to 50%, the sum will be increased again by 5%.
Example 2: Considering Mileage Deadband = 1% and PV = 50%. When the valve goes to 49.1%, the sum will not be
incremented as the displacement will not have exceeded the deadband value. If, in the sequence, the valve goes to
48%, the sum will be increased by 2%.
The diagnostics also has an Operation Time count, informing how long it has been active since the last reset of the
diagnostics. On restarting, mileage value, operating time and alarm are reset.
The default values for the Mileage parameters are
Mileage Deadband = 1%
and
Mileage Limit = 1000
.
Deviation
Diagnostics to check for excessive deviation in the PID control. Deviation is characterized by an error greater than
Deviation Deadband
for a time greater than
Deviation Time
. With each new deviation, a counter is incremented. After
the deviation count, if the error in the PID control becomes less than the value configured by the user in
Deviation
Deadband
, the deviation state is turned off, the time reference is reset and a new verification is started.
The user can configure the maximum value for the counter (
Deviation Counter Limit
), in order to generate an alarm
(
Deviation Limit Alert
) when it is exceeded. This alarm is persistent, forcing the user to manually deactivate it, after
verifying the cause of the deviation alert.
With the proper knowledge of the process, the user will be able to configure the counter limit to an expected value of
deviations during a certain period (with a certain tolerance, in order to avoid false alarms).
In the chart of figure 3.23, the deviation starts at time T
1
, as soon as the error (
SP – PV
) becomes greater than the
Deviation Deadband
parameter. The error remains above the value configured for the deviation for a time longer than
the
Deviation Time
, counting the deviation in the counter at time T
2
and activating the non-persistent alarm
Control
Deviation Alert
.
Example 1: Considering Deviation Deadband = 5% and Deviation Time = 10s. With SP = 50% and PV = 50% there is
no control deviation. Assuming that the Setpoint is changed to SP = 60% and that the PV remains at PV = 50%, the
error becomes 10% and the deviation verification time count starts. Note that at this time the deviation is not yet
accounted for. If the error is not less than 5% for 10 seconds, the deviation is characterized, and the counter is
incremented. After the deviation occurs, a new deviation will only be counted if the error is less than 5% at some point
and the process is repeated.
Example 2: Considering the same initial conditions as in
Example 1, without control deviation (SP = PV = 50%).
Assuming that the Setpoint is changed in order to create an
error greater than the Deviation Deadband, the deviation time
count starts. At T = 9s of the deviation time count, the Setpoint
returns to SP = 50%. At this moment, the deviation count is
reset and, if the error rises again, the count will be restarted
from T = 0s, ignoring the previous nine seconds.
The diagnostics also has an Operation Time count, informing
how long it has been active since the last reset of the
diagnostics. On restarting, counter, operation time and alarms
are reset.
Figure 3.23 – Example of PID control deviation.
The default values for the Deviation parameters are
Deviation Deadband = 5%, Deviation Time = 10s
and
Deviation
Counter Limit = 1000
.
HINT
The Deviation diagnostic is used to identify problems directly related to valve position control. It may indicate
an error in the PID control tuning or friction problems that disturb the correct modulation of the system.
