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【正文】 rings, bourdon tubes, and cantilevers are sensing elements which respond to changes in pressure or force and convert these physical quantities into a displacement. This displacement may then be used to change an electrical parameter such as voltage, resistance, capacitance, or inductance. Such bination of mechanical and electrical elements form electromechanical transducing devices or transducers. Similar bination can be made for other energy input such as thermal. Photo, magic and chemical,giving thermoelectric, photoelectric,electromaaic, and electrochemical transducers respectively. Transducer Sensitivity The relationship between the measured and the transducer output signal is usually obtained by calibration tests and is referred to as the transducer sensitivity K1= outputsignal increment / measured increment . In practice, the transducer sensitivity is usually known, and, by measuring the output signal, the input quantity is determined from input= outputsignal increment / K1. Characteristics of an Ideal Transducer The high transducer should exhibit the following characteristics a) high fidelitythe transducer output waveform shape be a faithful reproduction of the measured。 however, if an alternating voltage is applied across the material, the transducer exhibits the reversible effect by deforming or vibrating at the frequency of the alternating voltage. Resistance Transducers Resistance transducers may be divided into two groups, as follows: i) Those which experience a large resistance change, measured by using potentialdivider methods. Potentiometers are in this group. ii) Those which experience a small resistance change, measured by bridgecircuit methods. Examples of this group include strain gauges and resistance thermometers. Potentiometers A linear wirewound potentiometer consists of a number of turns resistance wire wound around a nonconducting former, together with a wiping contact which travels over the barwires. The construction principles are shown in figure which indicate that the wiper displacement can be rotary, translational, or a bination of both to give a helicaltype motion. The excitation voltage may be either . or . and the output voltage is 3 3 proportional to the input motion, provided the measuring device has a resistance which is much greater than the potentiometer resistance. Such potentiometers suffer from the linked problem of resolution and electrical noise. Resolution is defined as the smallest detectable change in input and is dependent on the crosssectional area of the windings and the area of the sliding contact. The output voltage is thus a serials of steps as the contact moves from one wire to next. Electrical noise may be generated by variation in contact resistance, by mechanical wear due to contact friction, and by contact vibration transmitted from the sensing element. In addition, the motion being measured may experience significant mechanical loading by the inertia and friction of the moving parts of the potentiometer. The wear on the contacting surface limits the life of a potentiometer to a finite
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