Page 367 - The Mechatronics Handbook

18.5 Repeatability

Repeatability (or reproducibility) refers to a sensor’s ability to give identical outputs for the same input.
Precision (or random) errors cause a lack of repeatability. Fortunately, precision errors can be accounted
for by averaging several measurements or other operations such as low-pass filtering. Electrical noise and
hysteresis (described later) both contribute to a loss of repeatability.

18.6 Linearity and Accuracy

The accuracy of a sensor is inversely proportional to error, i.e., a highly accurate sensor produces low
errors. Many manufacturers specify accuracy in terms of the sensor’s linearity. A least-squares straight-
line fit between all output measurements and their corresponding inputs determines the nominal output
of the sensor. Linearity (or accuracy) is specified as a percentage of full scale (maximum valid input), as
shown in Fig. 18.3, or as a percentage of the sensor reading, as shown in Fig. 18.4. Figures 18.3 and 18.4
show both of these specifications for 10% linearity, which is much larger than most actual sensors.
Accuracy and precision are two terms that are frequently confused. Figure 18.5 shows four sets of
histograms for ten measurements of angular velocity of an actuator turning at a constant 100 rad/s. The
first set of data shows a high degree of precision (low standard deviation) and repeatability, but the

FIGURE 18.3 Linearity specified at full scale.

FIGURE 18.4 Linearity specified at reading.

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