Hysteresis is the inability of a device to return to a previously established position when the input to the device is repeated. In control valves, hysteresis is distinguished from deadband by expecting that small reversals of input may not produce reversals of valve travel. Controller integral action drives the output in the other direction, setting up a distinctive continuous oscillation. Control Engineering Controller influences Controllers exist to maintain a measured variable equal to a setpoint. The affects filtering have on controller performance is a seldom addressed topic.
Some transmitters provide electronic filtering of the output signal.
Many digital control systems allow users to apply one or more filters on input signals. Regardless of the form, filters add lag to the signal and, when inappropriately used, can mask measurement variability and create unsafe conditions. If examination of the raw measured-variable input indicates frequent, random spikes which cannot be eliminated at their source, then application of a filter as near the source of the noise is appropriate. The amount of filter should be the minimum necessary to filter excessive, but not all the noise from the signal. Use extra care when applying filters to integrating processes.