Analog filtering to reduce aliasing
To reduce the effects of aliasing when sampling analog signals, analog filtering must first be used to reduce the higher frequencies. Data sampled for process control use will typically have a first order analog RC (resistor-capacitor) filter. For a first order analog filter, a filter time constant at least 3 times the sample interval is often appropriate for reducing aliasing in a diagnostic system. The required analog filtering is called an anti-aliasing filter.
Supervisory systems like diagnostics are prone to aliasing problems
Diagnostic systems typically work from data sampled at a lower rate than that used for process control. As a result, aliasing is a concern. This is especially an issue in monitoring systems for the process industries, because of the prevalence of control loops that cause cycling. Be sure that adequate filtering is present before sampling for diagnostic use to significantly reduce the effects of these frequencies, except for cases where high frequency noise is a symptom of a fault of interest. For an exponential filter in the more rapidly scanned process control system, to minimize aliasing in the more slowly scanned diagnostic system, a filter time constant equal to several times the diagnostic sampling time could be appropriate. The diagnostic system should have a sample time less than 1/3 or 1/4 the period of noise frequencies related to controller cycling.
Copyright 2010 - 2013, Greg Stanley
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