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136 Rockwell Automation Publication 1444-UM001D-EN-P - June 2018
Chapter 4 Measurement Definition
Figure 58 - Time Constant Effect: RMS Signal Detection, Steady State – Absolute
Figure 59 - Time Constant Effect: RMS Signal Detection, Steady State – Percent Deviation
A signal spike can be the result of an actual process or machine-related event.
However, in most cases a spike is more likely the result of an electrical or
mechanical anomaly.
When a spike occurs, a measurement that uses RMS signal detection reacts to it
quickly, regardless of the setting of the time constant. However, a measurement
that uses a large TC may not have time to resolve the actual magnitude of the
spike before the measurement begins to decay.
Figure 60 illustrates how quickly measurements with various TC values react to
a spike, and how quickly the signal decays back to the actual measured value
following the spike.
As the TC increases, the decay time does as well. While an additional few
seconds of decay time are not a problem in how operators and engineers use the
measurement, it could be problematic if alarm delays are short, so could be
triggered by a spike.