Smart Materials and New Technologies
photodiode measures the amount of reflected light. (See also Semiconductors, and Photovoltaics).
Sound sensors The most common type of sound sensor is based on the use of piezoelectric materials. In a piezoelectric material, a mechanical force produces a measurable electrical current. In a sound sensor, acoustical sound wave pressures produce a force in a piezoelectric material in a microphone, and a detectable current is thus generated. Thermal sensors Specific technologies for detecting changes in the thermal environment include various classic thermometers, thermocouples, thermistors and others. Several are mechanically based. A room thermostat, for example, works by the exchange of thermal energy to induce the bending of a bimetallic strip, in which the two metals have different thermal expansion coefficients. The bending, in turn, generates some sort of output signal. How this is done varies, but can range from triggering a simple switch to generating an electrical signal via strain gages (see below) or other electrically based devices. Thermistors, by contrast, are resistors that change their electrical resistances in a predictable way with a change in environmental temperature. Any change in resistance can in turn be detected by any of a variety of electrical circuits, which in turn can be converted into a digital display output. Humidity sensors Measurement of absolute and relative humidity levels is a common environmental need. Measurements can be difficult, however, because of the way air pressure, temperature and moisture content in the air interrelate. The classic psychrometer evaluates relative humidity by measuring the temperature difference between a ‘wet bulb’ and a ‘dry bulb’ thermometer. Other technologies include different capacitive or impedance devices. In impedance devices, the resistivity of a moisture-absorbent material changes with the amount of moisture present. Its impedance or resistivity is then measured. A capacitance device is used that has a moisture absorbent material whose dielectric properties change with the absorption of moisture. The resulting change in the dielectric properties corresponds to the capacitance of the material, which in turn can be easily measured. In an Elements and control systems
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