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【正文】 uantity being 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 number of full strokes or rotations usually referred to in the manufacture?s specification as the ?number of cycles of life expectancy?, a typical value being 20*1000000 cycles. The output voltage V0 of the unload potentiometer circuit is determined as follows. Let resistance R1= xi/xt *Rt where xi = input displacement, xt= maximum possible displacement, Rt total resistance of the potentiometer. Then output voltage V0= V* R1/(R1+( RtR1))=V*R1/Rt=V*xi/xt*Rt/Rt=V*xi/xt. This shows that there is a straightline relationship between output voltage and input displacement for the unloaded potentiometer. It would seen that high sensitivity could be achieved simply by increasing the excitation voltage V. however, the maximum value of V is determined by the maximum power dissipation P of the fine wires of the potentiometer winding and is given by V=(PRt)1/2 . Resistance Strain Gauges Resistance strain gauges are transducers which exhibit a change in electrical resistance in response to mechanical strain. They may be of the bonded or unbonded variety . a) bonded strain gauges Using an adhesive, these gauges are bonded, or cemented, directly on to the surface of the body or str
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