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Chapter 4 Velocity and position transducers 119

4.2.4 Electromechanical pulse encoders

Using the counting techniques discussed above, it is possible to replace an optical
encoder with a system based a non-contact sensor. In these applications a steel or a soft-
iron toothed wheel will be fitted to the shaft, and a magnetic, inductive, or capacitive
proximity sensor is used to detect the presence of the teeth. While such a system is not
normally capable of producing highly accurate speed measurements it can provide a
rugged system which can be used in high-reliability applications in particular over/
under speed detectors for motors or generators.

4.3 Position transducers

Position transducers are available in three main types: incremental, semi-absolute, and
absolute. A typical incremental encoder is an encoder that produces a set number of
pulses per revolution, which are counted to produce the positional information. If the
power is lost, or the data is corrupted, re-zeroing is required to obtain the true infor-
mation. An incremental encoder can be improved by the addition of a once-
per-revolution marker; this will correct against noise in the system, but complete
re-zeroing will still be required after a power loss, because the counting of the number of
revolutions is also lost. An absolute transducer will maintain the zero and thus it will
provide true information despite a loss of power for any length of time.

4.3.1 Brushed potentiometers
The principle of a potentiometer can be used in either a linear or a rotary absolute-
position transducer in which the output voltage is a function of displacement.
Excellent performance and reliability can be obtained if the drawbacks of the non-
uniform track resistance and of the brush contact are considered to be acceptable. The
overall accuracy of the potentiometer as a measurement device ultimately dependents
on regulation of the excitation voltage. A typical servo grade device will have a resolution
of 0.05% of the full scale, with an accuracy of 0.1%. The maximum operating speed of a
rotational version is typically limited to 500 rev min 1 by the brushes.

4.3.2 Linear variable differential transformers - LVDT

One of the most common methods of directly measuring a linear displacement to a high
degree of accuracy uses a linear variable differential transformer (LVDT); the principal
features of LVDTs are shown in Fig. 4.12. The operation is based on a transformer in
which the coupling between the primary and secondary coils (see Fig. 4.12A and B), is
determined by the position of a movable ferromagnetic core. The core is assembled
using precision linear bearings to give low friction and wear. The most widely used
design has a secondary winding which is split into two, on either side of the primary. The
secondary coils can be wound in opposite directions and they are half the length of the