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1、 1 1 Basic knowledge of transducers A 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
2、 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 Elements Although there are
3、 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 displacemen
4、t.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
5、thermal.Photo,magnetic and chemical,giving thermoelectric,photoelectric,electromaanetic,and electrochemical transducers respectively.2、Transducer Sensitivity The relationship between the measured and the transducer output signal is usually obtained by calibration tests and is referred to as the tran
6、sducer 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 Transducer The high transducer should exhib
7、it the following characteristics a)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
8、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 2 2 transducers,for example,are often subjecte
9、d 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 Transducers Electrical transducers exhibit many of the ideal characteristics.In addition they offer high sensitivity
10、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)resistance ii)capacitance iii)inductance iv)mutual-inductance types These transducers all rely on external excitatio
11、n voltage for their operation.b)self-generating types,which include i)electromagnetic ii)thermoelectric iii)photoemissive iv)piezo-electric types These all themselves produce an output voltage in response to the measurand input and their effects are reversible.For example,a piezo-electric transducer
12、 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 Transducers
13、Resistance 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 gro
14、up include strain gauges and resistance thermometers.5.1 Potentiometers A 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
15、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.or d.c.and the output voltage is 3 3 proportional to the input motion,provided the measuring device has a resistance which is much gr
16、eater 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 v
17、oltage 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 experie
18、nce 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
19、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/(
20、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
21、 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 Gauges Resistance strain gauges are transducers which exhibit a change in electrical resistance in response to mechanical strain.They may be of the bond
22、ed 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)photo-etched grids of conducting foil on an ep
23、oxy-resin backing iii)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 mor
24、e than one direction.4 4 b)unbonded strain gauges A 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 arrang
25、ement may be found in commercially available force,load,or pressure transducers.5.3 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;th
26、ey 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)meta
27、l resistance temperature transducers These 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 internat
28、ional practical temperature scale is based on the platinum resistance thermometer,which covers the temperature range-259.35 to 630.5.b)thermistor resistance temperature transducers Thermistors are temperature-sensitive resistors which exhibit large non-liner resistance changes with temperature varia
29、tion.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 resist
30、ance 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.5.4 Photoconductive Cells The photoconductive cell,uses a light-sensitive semiconductor material.
31、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,lead sulphide,and copper-doped germanium.The useful range of frequencies is determined by materi
32、al 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 5 5 When light strikes the cathode of the photoemissive cel
33、l 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.Rl Where Ip=photoelectric current(A),and photoelectric current Ip=Kt.B Wher
34、e 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 Transducers The capacitanc
35、e 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 hi
36、gh 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 thu
37、s 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
38、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 infinite ii)accuracy+-0.1%of full scale is quoted iii)displacement ranges 25*10-
39、6 m to 10-3m iv)rise time less than 50us possible Typical measurands are displacement,pressure,vibration,sound,and liquid level.8、Linear Variable-differential Ttransformer 9、Piezo-electric Transducers 10、Electromagnetic Transducers 11、Thermoelectric Transducers 6 6 12、Photoelectric Cells 13、Mechanic
40、al Transducers and Sensing Elements 7 7 传感器的基础知识 传感器是一种把被测量转换为光的、机械的或者更平常的电信号的装置。能量转换的过程称之为换能。按照转换原理和测量形式对传感器进行分类。用来测量位移的电阻式传感器被归为电阻式位移传感器。*的分类如压力波纹管、压力膜和压力阀等。1、传感器元件 除特例外，大多数的传感器都由敏感元件、转换元件或控制元件组成。如振动膜、波纹管、应力管和应力环、低音管和悬臂都是敏感元件，它们对压力和力作出响应把物理量转变成位移。然后位移可以改变电参数，如电压、电阻、电容或者感应系数。机械式和电子式元件合并形成机电式传感设备或传感
41、器。这样的组合可用来输入能量信号。热的，光的，磁的和化学的相互结合产生的热电式、光电式、电磁式和电化学式传感器。2、传感器灵敏度 通过校正测量系统获得的被测物理量和传感器输出信号的关系叫做传感器灵敏度 K1，也就是 K1=输出信号增量/测量增量。实际中，传感器的灵敏度是已知的，并且通过测量输出信号，输入量由下式决定，输入量=输出信号增量/K1。3、理想传感器的特性 a）高保真性：传感器输出波形应该真实可靠地再现被测量，并且失真很小。b）可测量最小的干扰，任何时候传感器的出现不能改变被测量。c）尺寸：传感器必须能正确地放在所需的地方。d）被测量和传感器信号之间应该有一个线性关系。e）传感器对外部
42、影响的灵敏度应该小，例如压力传感器经常受到外部振动和温度的影响。f）传感器的固有频率应该避开被测量的频率和谐波。4、电传感器 电传感器具有许多理想特性。它们不仅实现远程测量和显示，还能提供高灵敏度。电传感器可分为两大类。a）变参数型，包括：i）电阻式；ii）电容式；iii）自感应式；iv）互感应式；这些传感器的工作依靠外部电压。b）自激型，包括：i）电磁式；8 8 ii）热电式；iii）光栅式；iv）压电式。这些传感器根据测量输入值产生输出电压，而且这一过程是可逆的。比如，在一般情况下，压电式传感器可根据晶体材料的变形产生一个输出电压；但是，如果在材料上施加一个可变电压，传感器可以通过变形或与
43、变电压同频率的振动来体现可逆效应。5、电阻式传感器 电阻式传感器可以分为两大类：i）那些表现为大电阻变化的物理量可通过分压方式进行测量，电位器就属于此类。ii）那些表现为小电阻变化的物理量可通过桥电路方式进行测量，这一类包括应变仪和电阻温度计。5.1 电位器 绕线式电位器由许多绕在非导体骨架的电阻丝以及滑行在线圈上的触头组成。结构原理如图，触头能够转动、直线式运动或者两运动合成的螺旋式运动。如果测量设备的电阻比电位器的电阻大，那么电压既可以是交流也可以是直流，且输出电压与输入运动成正比。这样的电位器存在着分辨率和电子噪声的问题。分辨率是指传感器能检测到的最小的输入增量，分辨率大小取决于线圈与滑
44、动触头围成的面积。因此，输出电压为触头从一端移到另一端时一系列阶跃。电子噪声可以通过接触电阻的振动、触头摩擦形成的机械磨损以及从敏感元件传出的触头振动产生。另外，测得的运动量可以通过惯性和电位器中移动元件的摩擦获得较大的机械载荷。触头表面的磨损将电位器的寿命限制为多少转。通常指的是生产商在说明书中提及的“寿命转数”，一个典型值为 20*1000000 转。空载电位器电路的输出电压 V0 由下式决定：设电阻 R1=xi/xt*Rt，其中 xi 为输入位移，xt 为最大可能位移，Rt 为电位器的电阻。那么输入电压 V0=V*R1/(R1+(Rt-R1)=V*R1/Rt=V*xi/xt*Rt/Rt=
45、V*xi/xt 上式表明，对于空载电位器输出电压和输入位移呈直线关系。通过提高激励电压 V 可以获得高的灵敏度。但是，V 的最大值由电位器线圈金属丝的功率损耗 P 决定，即 V=(PRt)1/2。5.2 电阻应变仪 电阻应变仪是由机械应变产生电阻变化的传感器。它们可以是耦合的或者非耦合的。a）耦合应变仪 运用黏合剂可将应变仪与被检测的结构或部件的表面粘合或粘牢。耦合应变仪分为：i)粘合在绝缘纸背后的金属细丝仪 9 9 ii)在环氧树脂上粘贴导电箔片的光栅 iii）在环氧树脂上粘贴铜或镍的半导体丝 电阻应变仪可作为单个元件仅在一个方向测量应力，或者几个元件的组合体可在几个方向同时进行测量。b）非
46、耦合应变仪 一典型应变仪表明细电阻丝在悬臂弹簧偏差作用下改变电阻丝张力进而改变电阻丝的阻值。商业上通常在力、负载、压力传感器上运用此方法。5.3 电阻温度传感器 此传感器的材料有以下两大类 a)金属（如铂、铜、钨、镍）的阻值会随着温度的升高而增大，即有一个正温度电阻系数。b)半导体，如用锰、钴、铬或镍的氧化物制成的电热调节器，其阻值变化与温度变化存在一个非线性关系，即通常有一个负温度电阻系数。a)金属电阻温度传感器 在窄温度变化范围内，此类传感器取决于以下关系：R1=R01+a(b1-b0)式中，a 阻抗系数，R0 为 b0=0时 C 的电阻 b)电热调节器（半导体）电阻温度传感器 电热调节器
47、为感温电阻器，其阻值变化与温度变化呈非线性关系。通常此类传感器有一负温度系数。对于小的温度增量，阻值的变化大体呈线性，但是如果存在大的温差，测量电路需运用特定线性化技术生成电阻随温度变化的线性关系。电热调节器通常被制成附有玻璃质釉的半导体圆盘形状。由于电热调节器可以小到1mn，所以响应的时间非常快。5.4 光敏元件 光敏元件采用光敏半导体材料做成。当照射在半导体上的光强度增大，金属电极间的阻抗就会降低。光敏元件常用的半导体材料有硫化镉、硫化铅和铜锗化合物。频率的有效范围由所用材料决定。硫化镉主要适用于可见光，硫化铅在红外线区有峰值响应，所以最适合于光故障检测以及温度测量。5.5 放射性光元件
48、当光照射到放射性光元件的阴极时，电子就会获取足够能量到达阴极。阴极就会吸收这些电子产生一个通过电阻R 的电流，从而形成一输出电压V。产生的光电压 V=I.R 式中，I 为光发射电流，I=K.B 且为灵敏度，B 输入照度（lm）尽管输出电压能够表示照明的强度，这类元件却更多的应用于计算或调节，这里照射到阴极的光可被中断。6、电容式传感器 电容量随着相对介电常数、截面面积、或者极板间的距离的变化而变化。电容的特征曲线表明，在空间的一段范围内，截面面积和相对介电常数的变化与电容量变化成线性关系。不象电位器，变极距型电容传感器有无限的分辨率，这最适合测量微小的位移增量的位移。7、电感式传感器 电感可以通过改变电感电路的阻抗来调节。电容式和电感式传感器的测量技术：a）用差分式电容或电感作为交流电桥 b）用交流电位计电路做动态测量 10 10 c）用直流电路为电容器提供正比于容值变化的电压 d）采用调频法，C 或者 L 随着振荡电路频率的变化而改变 电容式和电感式传感器的一些重要特性如下：i)分辨率无限 ii）精确到满量程的+-0.1%iii）位移范围从 25*10-6m 到 10-3m iv）上升时间小于 50us 典型的被测量是位移、压力、振动量、声音和液位。8、线性调压器 9、压电式传感器 10、电磁式传感器 11、热电式传感器 12、光电管 13、机械式传感器及敏感元件