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 transduc
2、tion 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 Althoug
3、h 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 in
4、to 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
5、 energy input such as 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 an
6、d 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 T
7、ransducer 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 t
8、he 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,
9、pressure 2 2 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 Transducers Electrical transducers exhibit many of the ide
10、al 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) resistance ii) capacitance iii) inductance iv) mutu
11、al-inductance types These transducers all rely on external excitation 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 inpu
12、t 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
13、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 expe
14、rience 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 wip
15、ing 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. or d.c. and the output voltage is 3
16、3 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 d
17、ependent 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
18、 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
19、 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 d
20、isplacement, 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 w
21、ould 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 Gauges Resistance strain
22、 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 bein
23、g 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. 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.
