The performance of a measurement system is dependent on both the static and dynamic characteristics of the transducers selected. In the case of motion-control systems where the measured quantities are rapidly changing, the dynamic relationships between the input and the output of the measurement system have to be considered, particularly when discrete sampling is involved. In contrast, the measured parameter may change only slowly in some applications; hence the static performance only needs to be considered during the selection process.
The key characteristics of a transducer include the following:
Accuracy is a measure of how the output of the transducer relates to the true value at the input. In any speciﬁcation of accuracy, the value needs to be qualiﬁed by a statement of which errors are being considered and the conditions under which they occur.
Dead band is the largest change in input to which the transducer will fail to respond; this is normally caused by mechanical effects such as friction, backlash, or hysteresis.
Drift is the variation in the transducer’s output that is not caused by a change in the input; typically, it is caused by thermal effects on the transducer or on its conditioning system.
Linearity is a measure of the consistency of the input/output ratio over the useful range of the transducer. Its input-output characteristics should be linear and it should produce these characteristics in a symmetrical way.
Repeatability is a measure of the closeness with which a group of output values agrees for constant input, under a given set of environmental conditions. It should reproduce the same output signal when the same input signal is applied again and again under fixed environmental conditions e.g. temperature, pressure, humidity, etc.
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 the figure.
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