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Chapter 4 Velocity and position transducers 129

FIG. 4.19 Encoder output frequencies as a function of speed for a range of incremental encoders, (ppr, pulses per
revolution).

FIG. 4.20 Optical linear encoders. Photograph reproduced by permission of Heidenhain (GB) Ltd, Burgess Hill,
West Sussex. (A) Examples of sealed optical linear encoders which are protected from dust, chips and fluids. (B)
An exposed linear encoders consisting of a scale and a scanning head. The scanning hear is supplied separately to
allow matching the application to the position controller.
reflective metallic tape can be provided in lengths in excess of 30 m, in both cases an
accuracy of 2 mm is readily available.
In operation, the sensing head moves over the stationary grating, which is either a
glass scale or a reflective steel strip, a typical system is shown in Fig. 4.20, with the key