外文翻译传感器的基础知识.docx

1、外文翻译传感器的基础知识Basic 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 tran

2、sduction 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 ElementsAlthou

3、gh 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 i

4、nto 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 othe

5、r 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

6、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 Tran

7、sducerThe 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 tra

8、nsducer 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

9、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 characteristic

10、s. 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 typesThes

11、e 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 revers

12、ible. 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

13、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, m

14、easured 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 ba

15、rwires. 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, provid

16、ed 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

17、 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 sens

18、ing 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 referr

19、ed 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 displacemen

20、t, 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 achie

21、ved 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 i

22、n 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 w

23、ire 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

24、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 cha

25、nges 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,

26、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

27、 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

28、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

29、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,

30、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 ar

31、e 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 c

32、admium 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 pho