Rockwell Automation Publication 1444-UM001D-EN-P - June 2018
133
Measurement Definition
Chapter 4
Overall Time Constant
Regardless of the signal detection method, the basic function of the overall
time constant is to make the measurement more or less responsive to sudden
changes in magnitude. Often “very responsive” is the natural inclination.
However, the general feeling that “faster is better” isn’t always the case when
the consequence of too fast a response could be an unexpected and unnecessary
machine trip.
This section provides guidance for how the time constant affects the
measurement. However, the provided data is specific to the test cases applied,
and includes any inaccuracies that are associated with the signal source and
other factors (see
). Actual response behavior varies,
depending on the specifics of the measurement, the starting signal level and
behavior, and the magnitude and duration of the spike or step change.
Consider the expected measurement behavior during steady state, signal spike,
and step change events when determining Overall measurement-based alarm
and trip settings, including alarm limits, time delays, and voting logic.
True Peak and True Peak to Peak Measurements
True Peak behavior for steady state data is illustrated in
and
. As illustrated, to apply the minimum time constant (0.1) can
significantly reduce measurement accuracy. While there are applications that
require this level of responsiveness, to apply a time constant of less than about
0.3 to True Peak measurements is not recommended.
Overall (0/1) Signal Detection
Select from:
• True pk
• True pk-pk
• RMS
• Scaled pk
• Scaled pk-pk
Select the signal detection method for the Overall magnitude measurement.
• True measurements are measurements that are based on the actual peak or peak-to-peak
values in the signal. These values are recommended when the measurement must consider
the actual maximum of the measurement (such as maximum displacement) or when non-
sinusoidal signals, such as impacts, must be detected. Note though that this method is also
more sensitive to noise.
• Scaled measurements are calculated as the Square Root of 2 x the RMS value (2x if pk-pk), or
approximately 1.707 (or 2.414) x the RMS value. These values are recommended when the
measurement must consider the total energy in the signal
Overall (0/1) Units
Displays the Engineering Units for the
measurement
The Units for Overall (0) are the units after any integration is applied and are the same as
“Measurement Units” shown on the Hardware Configuration page.
The Units for Overall (1) are the same as the “Xdcr Units” specified in the Hardware
Configuration page as this measurement is always taken from the signal before any required
integration is applied.
Overall (0/1) Time Constant
0.100…60.000
Enter the time constant for the overall measurement.
• The time constant is written to the RMS or the PEAK tag value depending on the selected
Signal Detection method.
• The detection time constant defines the output smoothing filter for RMS-based detection
methods, or the decay rate of the peak detection methods. Set longer time constants to
reduce the responsiveness of the measurement to rapid changes (spikes / noise), or shorter
to increase the responsiveness.
See
Overall Time Constant on page 133
for more information about the effect of time constant
measurements.
Table 28 - Overall (continued)
Parameter
Values
Comment