In the world of sensors, there are also 3 R’s: Repeatability, Resolution, and Response. As important as these sensor parameters are, there is often confusion in the mind of users about exactly what they mean and in what ways they tend to interact with each other. This article attempts to explain these 3 R’s for position sensors and to dispel any confusion that exists.
Repeatability is a measure of the variation between outputs of a sensor-based measuring system for repeated trials of an identical mechanical input in a constant environment. Common practice is to use at least 3 repeated inputs, but 5 or more identical inputs are considered to be an even better sample for determining this parameter. Repeatability is usually evaluated by applying an averaging process to the variations in output values observed for the multiple trials. It is typically specified as a percentage of Full Scale Output or Full Span Output (FSO), but sometimes it is specified in absolute terms such as parts per million or fractions of the mechanical units applicable to the actual sensor-based measurement.
A constraint on repeatability measurement is that the trial inputs have to be applied in the same way, usually from a lower value to higher value, to eliminate any effects from hysteresis. Hysteresis error is a measure of the difference in system output when the mechanical input is rising up to the desired input value from a lower value compared to an identical input coming down from a higher input value to the desired value. For most contactless position sensors, hysteresis error is smaller than repeatability error.