傳感器外文翻譯.doc

10Basic knowledge of transducersA transducer is a device which converts the quantity being measured into an optical, mechanical, or-more commonly-electrical signal.The energy-conversion process that takes place is referred to as transduction.Transducers are classified according to the transduction principle involved and the form of the measured. Thus a resistance transducer for measuring displacement is classified as a resistance displacement transducer. Other classification examples are pressure bellows, force diaphragm, pressure flapper-nozzle, and so on.1、Transducer ElementsAlthough there are exception ,most transducers consist of a sensing element and a conversion or control element. For example, diaphragms,bellows,strain tubes and 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 combination of mechanical and electrical elements form electromechanical transducing devices or transducers. Similar combination can be made for other energy input such as thermal. Photo, magnetic and chemical,giving thermoelectric, photoelectric,electromaanetic, and electrochemical transducers respectively.2、Transducer SensitivityThe relationship between the measured and the transducer output signal is usually obtained by calibration tests and is referred to as the transducer sensitivity K1= output-signal increment / measured increment . In practice, the transducer sensitivity is usually known, and, by measuring the output signal, the input quantity is determined from input= output-signal increment / K1. 3、Characteristics of an Ideal TransducerThe high transducer should exhibit the following characteristicsa) high fidelity-the transducer output waveform shape be a faithful reproduction of the measured; there should be minimum distortion.b) There should be minimum interference with the quantity being measured; the presence of the transducer should not alter the measured in any way.c) Size. The transducer must be capable of being placed exactly where it is needed.d) There should be a linear relationship between the measured and the transducer signal.e) The transducer should have minimum sensitivity to external effects, pressure transducers,for example,are often subjected to external effects such vibration and temperature.f) The natural frequency of the transducer should be well separated from the frequency and harmonics of the measurand.4、Electrical TransducersElectrical transducers exhibit many of the ideal characteristics. In addition they offer high sensitivity as well as promoting the possible of remote indication or mesdurement.Electrical transducers can be divided into two distinct groups:a) variable-control-parameter types,which include:i) resistanceii) capacitanceiii) inductanceiv) mutual-inductance typesThese transducers all rely on external excitation voltage for their operation.b) self-generating types,which includei) electromagneticii) thermoelectriciii) photoemissiveiv) piezo-electric typesThese all themselves produce an output voltage in response to the measurand input and their effects are reversible. For example, a piezo-electric transducer normally produces an output voltage in response to the deformation of a crystalline material; 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.5、Resistance TransducersResistance transducers may be divided into two groups, as follows:i) Those which experience a large resistance change, measured by using potential-divider methods. Potentiometers are in this group.ii) Those which experience a small resistance change, measured by bridge-circuit methods. Examples of this group include strain gauges and resistance thermometers.5.1 PotentiometersA linear wire-wound potentiometer consists of a number of turns resistance wire wound around a non-conducting 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 combination of both to give a helical-type motion. The excitation voltage may be either a.c. ord.c. and the output voltage is 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 cross-sectional 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 manufactures 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+( Rt-R1)=V*R1/Rt=V*xi/xt*Rt/Rt=V*xi/xt. This shows that there is a straight-line 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 .5.2 Resistance Strain GaugesResistance 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 arei) fine wire gauges cemented to paper backingii) photo-etched grids of conducting foil on an epoxy-resin backingiii) a single semiconductor filament mounted on an epoxy-resin backing with copper or nickel leads.Resistance gauges can be made up as single elements to measuring strain in one direction only, or a combination of elements such as rosettes will permit simultaneous measurements in more than one direction.b) unbonded strain gaugesA typical unbonded-strain-gauge 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 commercially available force, load, or pressure transducers.5.3 Resistance Temperature TransducersThe 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; they have a positive temperature coefficient of resistance.b) semiconductors, such as thermistors which use oxides of manganese, cobalt, chromium, or nickel. These exhibit large non-linear resistance changes with temperature variation and normally have a negative temperature coefficient of resistance.a) metal resistance temperature transducersThese depend, for many practical purpose and within a narrow temperature range, upon the relationship R1=R0*1+a*(b1-b2) where a coefficient of resistance in -1,and R0 resistance in ohms at the reference temperature b0=0 at the reference temperature range .The international practical temperature scale is based on the platinum resistance thermometer, which covers the temperature range -259.35 to 630.5.b) thermistor resistance temperature transducersThermistors are temperature-sensitive resistors which exhibit large non-liner resistance changes with temperature variation. In general, they have a negative temperature coefficient.For small temperature increments the variation in resistance is reasonably linear; but, if large temperature changes are experienced, special linearizing techniques are used in the measuring circuits to produce a linear relationship of resistanceagainst 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.5.4 Photoconductive Cells The photoconductive cell , uses a light-sensitive semiconductor material. The resistance between the metal electrodes decrease as the intensity of the light striking the semiconductor increases. Common semiconductor materials used for photo-conductive cells are cadmium sulphide, leadsulphide, and copper-doped 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 infra-red region and is, therefore , most suitable for flame-failure detection and temperature measurement.5.5 Photoemissive Cells 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=Ip.RlWhere Ip=photoelectric current(A),and photoelectric current Ip=Kt.BWhere 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.6、Capacitive TransducersThe 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 variable-distance capacitive transducer has an infinite resolution making it most suitable for measuring small increments of displacement or quantities which may be changed to produce a displacement.7、Inductive Transducers The inductance can thus be made to vary by changing the reluctance of the inductive circuit. Measuring techniques used with capacitive and inductive transducers:a) A.C. excited bridges using differential capacitors inductors.b) A.C. potentiometer circuits for dynamic measurements.c) D.C. circuits to give a voltage proportional to velocity for a capacitor.d) Frequency-modulation methods, where the change of C or L varies the frequency of an oscillation circuit.Important features of capacitive and inductive transducers are as follows:i) resolution infiniteii) accuracy+- 0.1% of full scale is quotediii) displacement ranges 25*10-6 m to 10-3miv) rise time less than 50us possibleTypical measurands are displacement, pressure, vibration, sound, and liquid level.8、 Linear Variable-differential Ttransformer9、 Piezo-electric Transducers10、Electromagnetic Transducers11、Thermoelectric Transducers12、Photoelectric Cells13、Mechanical Transducers and Sensing Elements傳感器的基礎(chǔ)知識傳感器是一種把被測量轉(zhuǎn)換為光的、機(jī)械的或者更平常的電信號的裝置。能量轉(zhuǎn)換的過程稱之為換能。按照轉(zhuǎn)換原理和測量形式對傳感器進(jìn)行分類。用來測量位移的電阻式傳感器被歸為電阻式位移傳感器。*的分類如壓力波紋管、壓力膜和壓力閥等。1、傳感器元件除特例外，大多數(shù)的傳感器都由敏感元件、轉(zhuǎn)換元件或控制元件組成。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。然后位移可以改變電參數(shù)，如電壓、電阻、電容或者感應(yīng)系數(shù)。機(jī)械式和電子式元件合并形成機(jī)電式傳感設(shè)備或傳感器。這樣的組合可用來輸入能量信號。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。2、傳感器靈敏度通過校正測量系統(tǒng)獲得的被測物理量和傳感器輸出信號的關(guān)系叫做傳感器靈敏度K1，也就是K1=輸出信號增量/測量增量。實際中，傳感器的靈敏度是已知的，并且通過測量輸出信號，輸入量由下式?jīng)Q定，輸入量=輸出信號增量/K1。3、理想傳感器的特性a）高保真性：傳感器輸出波形應(yīng)該真實可靠地再現(xiàn)被測量，并且失真很小。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。c）尺寸：傳感器必須能正確地放在所需的地方。d）被測量和傳感器信號之間應(yīng)該有一個線性關(guān)系。e）傳感器對外部影響的靈敏度應(yīng)該小，例如壓力傳感器經(jīng)常受到外部振動和溫度的影響。f）傳感器的固有頻率應(yīng)該避開被測量的頻率和諧波。4、電傳感器電傳感器具有許多理想特性。它們不僅實現(xiàn)遠(yuǎn)程測量和顯示，還能提供高靈敏度。電傳感器可分為兩大類。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。b） 自激型，包括：i） 電磁式；ii） 熱電式；iii） 光柵式；iv） 壓電式。這些傳感器根據(jù)測量輸入值產(chǎn)生輸出電壓，而且這一過程是可逆的。比如，在一般情況下，壓電式傳感器可根據(jù)晶體材料的變形產(chǎn)生一個輸出電壓；但是，如果在材料上施加一個可變電壓，傳感器可以通過變形或與變電壓同頻率的振動來體現(xiàn)可逆效應(yīng)。5、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。ii）那些表現(xiàn)為小電阻變化的物理量可通過橋電路方式進(jìn)行測量，這一類包括應(yīng)變儀和電阻溫度計。5.1 電位器 繞線式電位器由許多繞在非導(dǎo)體骨架的電阻絲以及滑行在線圈上的觸頭組成。結(jié)構(gòu)原理如圖，觸頭能夠轉(zhuǎn)動、直線式運(yùn)動或者兩運(yùn)動合成的螺旋式運(yùn)動。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。 這樣的電位器存在著分辨率和電子噪聲的問題。分辨率是指傳感器能檢測到的最小的輸入增量，分辨率大小取決于線圈與滑動觸頭圍成的面積。因此，輸出電壓為觸頭從一端移到另一端時一系列階躍。 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。另外，測得的運(yùn)動量可以通過慣性和電位器中移動元件的摩擦獲得較大的機(jī)械載荷。觸頭表面的磨損將電位器的壽命限制為多少轉(zhuǎn)。通常指的是生產(chǎn)商在說明書中提及的“壽命轉(zhuǎn)數(shù)”，一個典型值為20*1000000轉(zhuǎn)。空載電位器電路的輸出電壓V0由下式?jīng)Q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。那么輸入電壓V0= V* R1/(R1+( R