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丑陋传感器的基础知识中英文对照外文翻译文献

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2021-01-20 20:54
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2021年1月20日发(作者:marcher)
英文文献翻译

中英文对照外翻译



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
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 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.

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英文文献翻译

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 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 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 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 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 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
barwires. The construction principles are shown in figure which indicate that the wiper

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英文文献翻译

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
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 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/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 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 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 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,

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英文文献翻译

or a combination of elements such as rosettes will permit simultaneous measurements in more
than one direction.
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 arrangement 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; 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 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 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 transducers
Thermistors 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 resistance 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. 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 material used. Cadmium sulphide is mainly
suitable for visible light, whereas lead sulphide has its peak response in the infra-red region

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英文文献翻译

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=
Where Ip=photoelectric current(A),and photoelectric current Ip=Kt.B
Where 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 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 infinite
ii) accuracy+- 0.1% of full scale is quoted
iii)displacement ranges 25*10-6 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

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