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。 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 word文档 可自由复制编辑

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 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 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 as combination of mechanical and electrical elements form electromechanical transduction devices or transducers. Similar combination can be made for other energy input such as thermal. Photo, magnetic and chemical giving thermoelectric, photoelectric electromagnetic 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 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 Transducer The high transducer should exhibit 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 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 word文档 可自由复制编辑

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 measured. 4、Electrical Transducers Electrical transducers exhibit many of the ideal characteristics. In addition they offer high sensitivity as well as promoting the possible of remote indication or measurements. 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 excitation voltage for their operation. b) self-generating types, which include I) electromagnetic ii) thermoelectric iii) photo emissive IV)piano-electric types these all themselves produce an output voltage in response to the measured input and their effects are reversible. For example, a piano-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 Transducers 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 group 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 barbwires. The construction principles are shown in figure which indicate that the wiper displacement can be word文档 可自由复制编辑

rotary, translational or a combination of both to give a helical-type motion. The excitation voltage may be either act. Or deco. 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 serial 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/ox *Rat where xi = input displacement, at= maximum possible displacement, Rat total resistance of the potentiometer. Then output voltage V0= V* R1/ (R1+ (Rt-R1)) =V*R1/Rt=V*xi/at*Retort=V*xi/it. 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=(Part)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 bonded or unbounded 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 epoxy-resin backing iii) a single semiconductor filament mounted on an epoxy-resin backing with copper or nickel leads. word文档 可自由复制编辑