handbook of modern sensors

258 7 Position, Displacement, and Level
(A) (B) (C)
Fig. 7.4. Optoelectronic inclination sensor: (A) design; (B) a shadow at a horizontal position;
(C) a shadow at the inclined position.
and road shapes and the flatness of an iron plate, which cannot be done by conventional
methods. The sensor (Fig. 7.4A) consists of a light-emitting diode (LED) and a
hemispherical spirit level mounted on a p-n-junction photodiode array. A shadow of
the bubble in the liquid is projected onto the surface of the photodiode array. When the
sensor is kept horizontal, the shadow on the sensor is circular, as shown in Fig. 7.4B,
and the area of the shadow on each photodiode of the array is the same. However,
when the sensor is inclined, the shadow becomes slightly elliptic, as shown in Fig.
7.4C, implying that the output currents from the diodes are no longer equal. In a practical
sensor, the diameter of the LED is 10 mm and the distance between the LED and
the level is 50 mm, and the diameters of the hemispherical glass and the bubble are
17 and 9 mm, respectively. The outputs of the diodes are converted into digital form
and calibrated at various tilt angles. The calibration data are compiled into look-up
tables which are processed by a computing device. By positioning the sensor at the
cross point of the lines drawn longitudinally and latitudinally at an interval on the
slanting surface of an object, x and y components of the tilt angle can be obtained
and the shape of the object is reconstructed by a computer.
7.3 Capacitive Sensors
The capacitive displacement sensors have very broad applications, they are employed
directly to gauge displacement and position and also as building blocks in other sensors
where displacements are produced by force, pressure, temperature, and so forth. The
ability of capacitive detectors to sense virtually all materials makes them an attractive
choice for many applications. Equation (3.20) of Chapter 3 states that the capacitance