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2025-03-24 07:22本頁面
【正文】 ucture which is being examined. Examples of bonded gauges are i) fine wire gauges cemented to paper backing ii) photoetched grids of conducting foil on an epoxyresin backing iii) a single semiconductor filament mounted on an epoxyresin backing with copper or nickel leads. Resistance gauges can be made up as single elements to measuring strain in one direction onl y, or a bination of elements such as rosettes will permit simultaneous measurements in more than one direction. 4 4 b) unbonded strain gauges A typical unbondedstraingauge arrangement shows fine resistance wires stretched around supports in such a way that the deflection of the cantilever spring system changes the tension in the wires and thus alters the resistance of wire. Such an arrangement may be found in mercially available force, load, or pressure transducers. Resistance Temperature Transducers The materials for these can be divided into two main groups: a) metals such as platinum, copper, tungsten, and nickel which exhibit and increase in resistance as the temperature rises。 but, if large temperature changes are experienced, special linearizing techniques are used in the measuring circuits to produce a linear relationship of resistance against temperature. Thermistors are normally made in the form of semiconductor discs enclosed in glass vitreous enamel. Since they can be made as small as 1mm,quite rapid response times are possible. Photoconductive Cells The photoconductive cell , uses a lightsensitive semiconductor material. The resistance between the metal electrodes decrease as the intensity of the light striking the semiconductor increases. Common semiconductor materials used for photoconductive cells are cadmium sulphide, lead sulphide, and copperdoped germanium. The useful range of frequencies is determined by material used. Cadmium sulphide is mainly suitable for visible light, whereas lead sulphide has its peak response in the infrared region and is, therefore , most suitable for flamefailure detection and temperature measurement. Photoemissive Cells 5 5 When light strikes the cathode of the photoemissive cell are given sufficient energy to arrive the cathode. The positive anode attracts these electrons, producing a current which flows through resistor R and resulting in an output voltage V. Photoelectrically generated voltage V= Where Ip=photoelectric current(A),and photoelectric current Ip= Where Kt=sensitivity (A/im),and B=illumination input (lumen) Although the output voltage does give a good indication of the magnitude of illumination, the cells are more often used for counting or control purpose, where the light striking the cathode can be interrupted. Capacitive Transducers The capacitance can thus made to vary by changing either the relative permittivity, the effective area, or the distance separating the plates. The characteristic curves indicate that variations of area and relative permittivity give a linear relationship only over a small range of spacings. Thus the sensitivity is high for small values of d. Unlike the potentionmeter, the
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