Page 115 - Electric Drives and Electromechanical Systems

108 Electric Drives and Electromechanical Systems

The collections of large amounts of measurement data in real time is one aspect of the
Industry 4.0 concept that will allow the adaptive control of any machining process to
improve quality.
In this chapter we will restrict our considerations to those sensors and transducers
that are normally used in a closed-loop motion-control system, where feedback is used
to minimise the difference between the demanded and actual output, this is further
discussed in Chapter 10, Controllers for Automation. In a motion-control system, the
controlled variable is normally either velocity or position. The overall performance of a
motion-control system will depend, to a large extent, on the type and quality of the
transducer which is used to generate the feedback signal. It should be noted that
velocity- or position-measuring transducers need not be used; other process variables
(for example, temperature and chemical composition) can be used to determine the
speed or position of a drive within a manufacturing process. However, as this book is
concerned with robotic and machine-tool applications, the primary concentration will
be on velocity and position transducers, however there are a large number of mea-
surement techniques that are used in both the calibration of systems and the inspection
of the final component, it is estimated around 20% of the machining time is taken up
with measurement as part of the inspection process (Harding, 2013).
In order to consider the key features of available systems, the performance of mea-
surement systems in general must be considered, before individual types are reviewed.
In addition to transducers, this chapter also discussed encoders, where a transducer
generates a coded reading of a measurement.

4.1 The performance of measurement systems

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 which is not caused by a change in
the input; typically, it is caused by thermal effects on the transducer or on its con-
ditioning system.