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handbook of modern sensors

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274 7 Position, Displacement, and Level<br />

(A)<br />

(B)<br />

Fig. 7.23. Output signal from the amplifiers for direction 1 (A) and 2 (B).<br />

Fig. 7.24. A magnetostrictive detector uses ultrasonic waves to detect position <strong>of</strong> a permanent<br />

magnet.<br />

7.4.6 Magnetostrictive Detector<br />

A transducer which can measure displacement with high resolution across long distances<br />

can be built by using magnetostrictive and ultrasonic technologies [8]. The<br />

transducer is comprised <strong>of</strong> two major parts: a long waveguide (up to 7 m long) and a<br />

permanent ring magnet (Fig. 7.24). The magnet can move freely along the waveguide<br />

without touching it. A position <strong>of</strong> that magnet is the stimulus which is converted by<br />

the sensor into an electrical output signal. A waveguide contains a conductor which,<br />

upon applying an electrical pulse, sets up a magnetic field over its entire length. Another<br />

magnetic field produced by the permanent magnet exists only in its vicinity.<br />

Thus, two magnetic fields may be setup at the point where the permanent magnet is<br />

located. A superposition <strong>of</strong> two fields results in the net magnetic field, which can be<br />

found from the vector summation. This net field, although helically formed around the<br />

waveguide, causes it to experience a minute torsional strain, or twist at the location<br />

<strong>of</strong> the magnet. This twist is known as the Wiedemann effect.

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