Emerson Rosemount 644h, Справочное руководство

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Reference Manual
00809-0100-4728, Rev HA
December 2002
3-7
Models 644H and 644R
Process Variable (PV) Damping
The PV Damp command changes the response time of the transmitter to
smooth variations in output readings caused by rapid changes in input.
Determine the appropriate damping setting based on the necessary response
time, signal stability, and other requirements of the loop dynamics of the
system. The default damping value is 5.0 seconds and can be reset to any
value between 0 and 32 seconds.
The value chosen for damping affects the response time of the transmitter.
When set to zero (or disabled), the damping function is off and the transmitter
output reacts to changes in input as quickly as the intermittent sensor
algorithm allows (refer to “Intermittent Threshold” on page 3-12 for a
description of the intermittent sensor algorithm). Increasing the damping value
increases the transmitter response time.
With damping enabled, if the temperature change is within 0.2% of the sensor
limits, the transmitter measures the change in input every 500 milliseconds
and outputs values according to the following relationship:
At the value to which the damping time constant is set, the transmitter output
is at 63% of the input change and it continues to approach the input according
to the damping equation above.
For example, as illustrated in Figure 3-2, if the temperature undergoes a step
change—within 0.2% of the sensor limits—from 100 degrees to 110 degrees,
and the damping is set to 5.0 seconds, the transmitter calculates and reports
a new reading every 500 milliseconds using the damping equation. At 5.0
seconds, the transmitter outputs 106.3 degrees, or 63% of the input change,
and the output continues to approach the input curve according to the
equation above.
For information regarding the damping function when the input change is
greater than 0.2% of the sensor limits, refer to “Intermittent Threshold” on
page 3-12.
Figure 3-2. Change in Input vs.
Change in Output with Damping
Set to Five Seconds
Fast Key Sequence 1, 3, 3, 1, 3
Damped Value
N P
–
( )
2
T U –
2
T U
+
------------------
 
 
× P +
=
P =previous damped value
N =new sensor value
T = damping time constant
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4_01A