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Chapter 4 Velocity and position transducers 109

Linearity is a measure of the consistency of the input/output ratio over the useful
range of the transducer.
Repeatability is a measure of the closeness with which a group of output values
agree for a constant input, under a given set of environmental conditions.
Resolution is the smallest change in the input that can be detected with certainty
by the transducer.
Sensitivity is the ratio of the change in the output to a given change in the input.
This is sometimes referred to as the gain or the scale factor.
A clear understanding is required of the interaction between accuracy, repeatability
and resolution as applied to a measurement system. It is possible to have measurement
systems with either high or low accuracy and repeatability; the measurements compared
to the target position are shown in Fig. 4.1. A motor drive system needs to incorporate a

FIG. 4.1 Effect of accuracy and repeatability on the performance of a measurement system. The dots represent
the individual measurements, with the real value at the centre of the grid. (A) Low repeatability and low
accuracy. (B) High repeatability and high accuracy. (C) High repeatability and low accuracy. Only when the system
has both high accuracy and repeatability is the measurement error minimised.