can be measured Displacement is also commonly measured by the movement of a ferromagnetic core inside of an inductor coil. The displacement produces a change in inductance which can be measured by placing the inductor in an oscillator circuit and measuring the change in frequency of oscillation The most commonly used force sensor is the strain gage. It consists of metal wires which are stretched in response to a force. The resistance of the wire changes as it undergoes strain, i. e, a change in length, since the the metal,F a wire is R= resistivity x length/cross-sectional area. The wire's resistivity is a bulk property of the metal which is a constant for constant temperature. For example, a strain gage can be used to measure acceleration by attaching both ends of the wire to a cantilever beam, with one end of the wire at the attached am end and the other at the free end. The cantilever beam free end moves in response to an applied force, such as the force due to acceleration which produces strain in the wire and a subsequent change in resistance. The sensitivity of a strain gage is described by the unitless gage factor, G=(AR/R)/(AL/L). For metal wires, gage factors typically range from 2 to 3. Semiconductors are known to exhibit piezoresistivity, which is a change resistance in response to strain which involves a large change in resistivity in addition to the change in linear dimension. Piezoresistors have gage factors as high as 130. Piezoresistive strain gages are frequently used in microsensors, described in Section 56.5 Optical Radiation The intensity and frequency of optical radiation are parameters of growing interest and utility in consumer products such as the video camera and home security systems and in optical communications systems. The conversion of optical energy to electronic signals can be accomplished by several mechanisms(see radiant to electronic transduction in Table 56.1 ); however, the most commonly used is the photogeneration of carriers in semiconductors. The most often-used device is the p-n junction photodiode(Section III). The construction of this device is very similar to the diodes used in electronic circuits as rectifiers. The diode is operated in reverse bias,where very little current normally flows. When light is incident on the structure and is absorbed in the semiconductor, energetic electrons are produced. These electrons flow in response to the electric field sustained internally across the junction, producing an externally measurable current. The current magnitude is propo tional to the light intensity and also depends on the frequency of the light. Figure 56.2 shows the effects of varying incident optical intensity on the terminal current versus voltage behavior of a p-n junction. Note that for zero applied voltage, a net negative current flows when the junction is illuminated. This device can therefore also be a source of power(a solar cell) 56.3 Chemical sensors Chemical measurands include ion concentration, chemical composition, rate of reactions, reduction-oxidation potentials, and gas concentration. The last column of Table 56.1 lists some of the transduction mechanisms that have been, or could be, employed in chemical sensing. Two examples of chemical sensors are described FIGURE 56.2 Sketch of the variation of current versus voltage characteristics of a p-n photodiode with incident light c 2000 by CRC Press LLC© 2000 by CRC Press LLC can be measured. Displacement is also commonly measured by the movement of a ferromagnetic core inside of an inductor coil. The displacement produces a change in inductance which can be measured by placing the inductor in an oscillator circuit and measuring the change in frequency of oscillation. The most commonly used force sensor is the strain gage. It consists of metal wires which are stretched in response to a force. The resistance of the wire changes as it undergoes strain, i.e., a change in length, since the resistance of a wire is R = resistivity ¥ length/cross-sectional area. The wire’s resistivity is a bulk property of the metal which is a constant for constant temperature. For example, a strain gage can be used to measure acceleration by attaching both ends of the wire to a cantilever beam, with one end of the wire at the attached beam end and the other at the free end. The cantilever beam free end moves in response to an applied force, such as the force due to acceleration which produces strain in the wire and a subsequent change in resistance. The sensitivity of a strain gage is described by the unitless gage factor, G = (DR/R)/(DL/L). For metal wires, gage factors typically range from 2 to 3. Semiconductors are known to exhibit piezoresistivity, which is a change in resistance in response to strain which involves a large change in resistivity in addition to the change in linear dimension. Piezoresistors have gage factors as high as 130. Piezoresistive strain gages are frequently used in microsensors, described in Section 56.5. Optical Radiation The intensity and frequency of optical radiation are parameters of growing interest and utility in consumer products such as the video camera and home security systems and in optical communications systems. The conversion of optical energy to electronic signals can be accomplished by several mechanisms (see radiant to electronic transduction in Table 56.1); however, the most commonly used is the photogeneration of carriers in semiconductors. The most often-used device is the p-n junction photodiode (Section III). The construction of this device is very similar to the diodes used in electronic circuits as rectifiers. The diode is operated in reverse bias, where very little current normally flows. When light is incident on the structure and is absorbed in the semiconductor, energetic electrons are produced. These electrons flow in response to the electric field sustained internally across the junction, producing an externally measurable current. The current magnitude is propor￾tional to the light intensity and also depends on the frequency of the light. Figure 56.2 shows the effects of varying incident optical intensity on the terminal current versus voltage behavior of a p-n junction. Note that for zero applied voltage, a net negative current flows when the junction is illuminated. This device can therefore also be a source of power (a solar cell). 56.3 Chemical Sensors Chemical measurands include ion concentration, chemical composition, rate of reactions, reduction-oxidation potentials, and gas concentration. The last column of Table 56.1 lists some of the transduction mechanisms that have been, or could be, employed in chemical sensing. Two examples of chemical sensors are described FIGURE 56.2 Sketch of the variation of current versus voltage characteristics of a p-n photodiode with incident light intensity