Glossary: Position Sensor Accuracy

Cylinder Sensors , Know-how , Linear Sensors
Wednesday 5 May 2021

Accuracy can be defined as the closeness of the measured value in comparison with the true value. In a theoretically perfect world, the output signal of a proportional position sensor should be the exact representation of the position. The output signal should go in a straight line between 0-100% in proportion to the position change. However, in reality, different types of accuracy errors occur. Below is a short glossary of some of the most common terms:

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Linearity Error

Linearity, or non-linearity, is the deviation between the theoretically ideal straight line and the actual output signal. Linearity can be expressed in absolute terms, e.g., millimeters [mm] or milliamps [mA], or relative to the entire full scale [FS], e.g., 0.05%.

Hysteresis Error

Hysteresis is the difference in the output signal for increasing versus decreasing values. Hysteresis can be seen as an accuracy error in proportional position sensors but is used as a feature in many switching applications. For instance, proximity sensors and thermostats use hysteresis to avoid noise (rapid, repeated switching on/off) at the switching point.

Repeatability

Repeatability is a measure of how consistently the characteristics of the output match between repeated strokes. A position sensor should present the same output characteristics for repeated strokes. If the output signal varies between movements over the stroke length, there is a deviation in repeatability. Unlike linearity and hysteresis errors, which are systematic and do not change between strokes and can therefore be calibrated out, errors in repeatability vary with every movement across the stroke and are thus difficult to calibrate away.

Resolution

Resolution is the smallest change a sensor can detect. Whether resolution should be classified as an accuracy error or a feature is a subject of endless debate, as the measurement is not necessarily incorrect. However, too low a resolution may result in a rough representation of the true value by the output signal, similar to a low-resolution photo, which is not wrong but not detailed enough.

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