【正文】 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 structure 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 only, 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。 按照轉(zhuǎn)換原理和測量形式對傳感器進(jìn)行分類。如振動膜、波紋管、應(yīng)力管和應(yīng) 力環(huán)、低音管和懸臂都是敏感元件，它們對壓力和力作出響應(yīng)把物理量轉(zhuǎn)變成位移。熱的，光的，磁的和化學(xué)的相互結(jié)合產(chǎn)生的熱電式、光電式、電磁式和電化學(xué)式傳感器。 b）可測量最小的干擾，任何時候傳感器的出現(xiàn)不能改變被測量。 f）傳感器的固有頻率應(yīng)該避開被測量的頻率和諧波。 a） 變參 數(shù)型，包括： i） 電阻式； ii） 電容式； iii） 自感應(yīng)式； iv） 互感應(yīng)式； 這些傳感器的工作依靠外部電壓。 電阻式傳感器 電阻式傳感器可以分為兩大類： i）那些表現(xiàn)為大電阻變化的物理量可通過分壓方式進(jìn)行測量，電位器就屬于此類。 如果測量設(shè)備的電阻比電位器的電阻大，那么電壓既可以是交流也可以是直流，且輸出電壓與輸入運(yùn)動成正比。 電子噪聲可以通過接觸電阻的振動、觸頭摩擦形成的機(jī)械磨損以及從敏感元件傳出的觸頭振動產(chǎn)生。 空載電位器電路的輸出電壓 V0 由下式 決定：設(shè)電阻 R1= xi/xt *Rt，其中 xi 為輸入位移， xt 為最大可能位移， Rt為電位器的電阻。 電阻應(yīng)變儀 電阻應(yīng)變儀是由機(jī)械應(yīng)變產(chǎn)生電阻變化的傳感器。 b） 非耦合應(yīng)變儀 一典型應(yīng)變儀表明細(xì)電阻絲在懸臂彈簧偏差作用下改變電阻絲張力進(jìn)而改變電阻絲的阻值。 a)金屬電阻溫度傳感器 在窄溫度變化范圍內(nèi)，此類傳感器取決于以下關(guān)系： R1=R0[1+a(b1b0)] 式中， a 阻抗系數(shù)， R0 為 b0=0176。 電熱調(diào)節(jié)器通常被制成附有玻璃質(zhì)釉的半導(dǎo)體圓盤形狀。光敏元件常用的半導(dǎo)體材料有硫化鎘、硫化鉛和銅鍺化合物。陰極就會吸收這些電子產(chǎn)生一個通過電阻 R 的電流，從而形成一輸出電壓 V。不象電位器，變極距型電容傳感器有無限的分辨率，這最適合測量微小的位移增