Page 389 - The Mechatronics Handbook

Commercially available devices of this nature are called “Hall potentiometers” and have a variety of outputs
(e.g., sine and cosine, or a linear ramp repeating with each revolution). In contrast to potentiometers with
resistive strips and sliders, Hall pots allow continuous 360∞ rotation and experience no wear. All Hall
effect devices are susceptible to external magnetic fields, however.
Hall sensors are also excellent transducers of short linear or arc-like motions. The motion of a bar
magnet past a Hall sensor exposes the sensor to a magnetic field—which can be arranged to vary linearly
with displacement—over a range of several millimeters up to several centimeters. (The bar magnet travels
less than its own length.) Commercial implementations are known as throttle position sensors.
Tape-Based Sensors
There are a number of linear and rotational sensors, both incremental and absolute, which are similar
to optical encoders but use magnetic patterns rather than optical ones. Linear applications are likely to
require an exposed strip. In exposed applications, magnetic sensors have advantages in resistance to dirt,
although the magnetic stripes must be protected from damage.

Ultrasonic

Ultrasonic (US) sensors use the time-of-flight of a pulse of ultrasonic sound through air or liquid to
measure distance. Sensors are available with ranges from a few centimeters to 10 m. A great advantage of
US sensors is that all of the sensor’s electronic and transducer components are in one location, out of
harm’s way. The corresponding disadvantages are that US sensors tend to be indiscriminate: they may
detect spurious targets, even very small ones, especially if these are near the transducer. Sensors are available
with carefully shaped beams (down to 7∞) to minimize detection of spurious targets. Some include
compensation for variation in air temperature, which affects sound velocity. US sensors can be used in
surprising geometries. For instance, they can be used to detect the liquid level in a vertical pipe; back-
reflection of sound pulses from the walls of a smooth pipe are minimal.
There is also an inexpensive and easily interfaced US sensor from Polaroid, derived from a ranging
device for an instant camera, which is popular with experimenters.
Ultrasonic sensors typically have an analog output proportional to distance to target. Accuracies of
the 1% level can be expected in a well-controlled environment.

Magnetostrictive Time-of-Flight
A more accurate technique for using time-of-flight to infer distance is the magnetostrictive wire transducer
(MTS). A moving magnet forms the “target” corresponding to the acoustic target in US sensors, and need
not touch the magnetrostrictive wire which is the heart of the device. The magnet’s field acting on the
magnetrostrictive wire creates an ultrasonic pulse in the wire when a current pulse is passed through
the wire. The time interval from the current pulse to the detection of the ultrasonic pulse at the end of the
wire is used to determine the position of the magnet along the wire (Fig. 19.16).

FIGURE 19.16 Principle of operation of a magnetostrictive linear position sensor, courtesy of Temposonics, Inc.

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