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handbook of modern sensors

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284 7 Position, Displacement, and Level<br />

Fig. 7.35. The PSD sensor measures distance by applying a triangular principle.<br />

Fig. 7.36. Design <strong>of</strong> a one-dimensional<br />

PSD.<br />

reflective properties <strong>of</strong> an object. Diffusive reflectivity in the near-infrared spectral<br />

range is close to that in the visible range; hence, the intensity <strong>of</strong> the light incident<br />

on the PSD has a great deal <strong>of</strong> variation. Nevertheless, the accuracy <strong>of</strong> measurement<br />

depends very little on the intensity <strong>of</strong> the received light.<br />

APSD operates on the principle <strong>of</strong> photoeffect. It makes use <strong>of</strong> a surface resistance<br />

<strong>of</strong> a silicon photodiode. Unlike MOS and CCD <strong>sensors</strong> integrating multielement photodiode<br />

arrays, the PSD has a nondiscrete sensitive area. It provides one-dimensional<br />

or two-dimensional [11] position signals on a light spot traveling over its sensitive<br />

surface. A sensor is fabricated <strong>of</strong> a piece <strong>of</strong> high-resistance silicon with two layers (p<br />

and n + types) built on its opposite sides (Fig. 7.36).Aone-dimensional sensor has two<br />

electrodes (A and B) formed on the upper layer to provide electrical contacts to the<br />

p-type resistance. There is a common electrode (C) at the center <strong>of</strong> the bottom layer.<br />

Photoelectric effect occurs in the upper p-n junction. The distance between two upper<br />

electrodes is D, and the corresponding resistance between these two electrodes is R D .<br />

Let us assume that the beam incidence strikes the surface at distance x from the<br />

A electrode. Then, the corresponding resistance between that electrode and the point<br />

<strong>of</strong> incidence is respectively, R x . The photoelectric current I 0 produced by the beam

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