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TRANSDUCERS
• In a measurement system, the measurand (quantity under measurement) makes its first contact with system
through a detector.
• The measurand is converted into an analogous form by the detector.
• The measurand or the input signal is called information for the measurement system.
• The information may be in the form of a physical phenomenon or it may be an electrical signal.
• The function of the detector is to sense the information and convert it into a convenient form for acceptance by the
later stages of the system.
• The process of detection and conversion of input signal or information from one form to another requires energy.
• This energy may be extracted from the signal thereby causing loading effects.
• Thus if energy is extracted from the signal, it will not be faithfully reproduced after conversion which leads to
errors.
• In fact, efforts should be made to supply energy required by the detector from an external source for conversion so
that the input signal is converted into a usable output without drawing an undue amount of energy from it.
• The ideal conversion is when absolutely no energy is extracted from the signal so that it is not distorted and the
analogous output of the detector is a faithful representation of the input.
TRANSDUCERS
TYPCIAL PRIMARY TRANSDUCER ELEMENTS
ELECTRIC TRANSDUCERS
• In order to measure non-electrical quantities a detector is used which usually converts the physical quantity into a
displacement.
• This displacement actuates an electric transducer, which acting as a secondary transducer, gives an output that is electrical
in nature.
• The electrical quantity so produced is measured by standard methods used for electrical measurements.
• The result (electrical output) gives the magnitude of the physical quantity or condition being measured.
• The electrical signal may be a current or a voltage or a frequency and production of these signals is based upon, resistive,
capacitive, inductive effects etc.
• The first stage of a measurement system may simply be called a transducer stage instead of detector transducer
stage by redefining a transducer.
• A transducer, in general form, may be defined as a device which converts energy from one form to another.
• However, this definition has to be restricted, many a time especially in the field of electrical instrumentation.
• Keeping this restriction in view, transducer may be defined as a device which converts a physical quantity or a
physical condition into an electrical signal.
• Another name for a transducer is pick up.
TRANSDUCERS
• There are a number of transducers which transform a variety of physical quantities and phenomena into electrical signals.
• The reasons for transforming a physical phenomenon into electrical form are numerous.
1. electrical amplification and attenuation can be done easily and that too with static devices.
2. The mass-inertia effects are minimized. In fact, when dealing with electrical or electronic signals, the inertia effects are due to electrons
which have negligible mass. In many situations, we do not come across mass or inertia problems at all.
3. The effects of friction are minimized.
4. The electrical or electronic systems can be controlled with a very small power level.
5. The electrical output can be easily used, transmitted and processed for the purpose of measurement.
6. Telemetry is used in almost all sophisticated measurement systems. The entire aerospace research and development is based upon telemetry
and remote control.
7. The ever enlarging field of radio monitoring in space research has left us with no alternative but to resort to electronic means. This
completely eliminates the data transmission through mechanical means and hence electrical and electronic principles have to be employed
for these conditions. The remote indication or recording is an essential part of modern day technology.
8. There has been an explosive development in the field of electronic components and devices. This development is on account of the fact that
electronic devices are very amenable to miniaturization. Components which are compact, have always an, advantage. Miniaturization on
account of use of ics (integrated circuits) has completely revolutionized the field of instrumentation.
TRANSDUCERS
A.Based on principle of transduction
The transducers can be classified on the basis of principle of transduction as resistive, inductive, capacitive etc. Depending upon
how they convert the input quantity into resistance, inductance or capacitance respectively.
They can be classified as
1. Piezo electric,
2. Thermoelectric,
3. Magnetorestrictive,
4. Electro-kinetic and
5. Optical.
TRANSDUCERS
B. Primary and Secondary Transducers
TRANSDUCERS
C. Active and Passive Transducers
• Passive transducers derive the power required for transduction from an auxiliary power source.
• They also derive part of the power required for conversion from the physical quantity under measurement.
• They are also known as "externally powered transducers".
• Typical examples of passive transducers are resistive, inductive and capacitive transducers.
• A typical example of a passive transducer is a ‘POT' which is used for measurement of displacement.
• A 'POT' is a resistive transducer powered by a source voltage ei as shown in fig. .
• This 'POT' is used for measurement of linear displacement xi,
Suppose L is the total length of potentiometer whose total resistance Ri.
The input displacement is xi.
∴ 𝑂𝑢𝑡𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
𝑒𝑜 =
𝑥𝑖
𝐿
𝑒𝑖 𝑜𝑟 𝑥𝑖 =
𝑒𝑜
𝑒𝑖
𝐿
In the absence of external power, the transducer cannot work and it hence is called a
passive transducer.
TRANSDUCERS
C. Active and Passive Transducers
• Active transducers are those which do not require an auxiliary power source to produce their output.
• They are also known as self generating type since they develop their own voltage or current output.
• The energy required for production of output signal is obtained from the physical quantity being measured.
• Velocity, temperature, light intensity and force can be transduced with the help of active transducers.
• These transducers include tachogenerators, thermocouples, photovoltaic cells and piezoelectric crystals.
• Consider the case of a piezoelectric crystal used for measurement of acceleration as shown in fig. the crystal is sandwiched
between two metallic electrodes, and the entire sandwich is fastened to a base which may be the floor of a rocket. A fixed
mass is placed on the top of the sandwich.
TRANSDUCERS
C. Active and Passive Transducers
• The property of the piezo-electric crystals is that when a force is applied to them, they produce an output voltage.
• The mass exerts a certain force on account of acceleration on the crystal due to which a voltage is generated.
• The acceleration is applied to the base, due to which the mass produces a force.
• The mass being fixed, the force is proportional to acceleration.
• The voltage output is proportional force and hence is proportional to acceleration (the mass being fixed).
• It should be noted from above that this transducer called "accelerometer" which converts acceleration into
electrical voltage does not need any auxiliary power source to convert a physical phenomenon (acceleration in
this case) to an electrical output (voltage in this case) and therefore is an active transducer.
TRANSDUCERS
D. Analog and digital transducers
1. Analog transducers
• These transducers convert the input quantity into an analog output which is a continuous function of time.
• Thus a strain gauge, an L.V.D.T., A thermocouple or a thermistor may be called as "analog transducers" as they give an output
which is a continuous function of time.
2. Digital transducers
• These transducers convert the input quantity into an electrical output which is in the form of pulses.
• As the binary system uses only two symbols 0 and 1 it can be easily represented by opaque and transparent areas on a glass
scale or non-conducting and conducting areas on a metal scale.
• A scale constructed to show the linear position on a movable object and having five digits is shown in fig..
• The complete binary number denoting position is obtained by scanning the pattern across the scale at a stationary index mark.
• Glass scales can be read optically by means of a light source, an optical system and photocells.
• Metal scales are scanned by brushes making electrical contact with individual tracks.
• The resolution depends upon the digits comprising the binary number and is 1/2n of full scale where n is the number of digits.
TRANSDUCERS
E. Transducers and Inverse Transducers
1. Transducers:
A transducer can be broadly defined as a device which converts a non-electrical quantity into an electrical quantity.
2. Inverse transducers:
• An inverse transducer is defined as a device which converts an electrical quantity into a non-electrical quantity.
• It is a precision actuator which has an electrical input and a low power non-electrical output.
• A piezoelectric crystal acts as an inverse transducer because when a voltage is applied across its surfaces, it changes its
dimensions causing a mechanical displacement.
• A current carrying coil moving in a magnetic field is also an inverse transducer because current carried by it is converted into
a force which causes translational or rotational displacement.
• Many data indicating and recording devices are inverse transducers.
• An analog ammeter or voltmeter converts current into mechanical displacement. However, such devices which include
instruments like indicating instruments, pen recorders, oscilloscopes that convert the electrical signals to a mechanical
movement are placed at the output stage (data presentation stage) are called output transducers.
• The most useful application of inverse transducers is in feedback measuring systems. The development of transducers and
inverse transducers, and the advantages gained through use of feedback has increased their applications manifold and there is
reason to believe that they will continue to grow in importance.
SELECTION OF TRANSDUCERS
1. Range
The range of a sensor indicates the limits between which the input can vary. For example, a thermocouple for the measurement of
temperature might have a range of 25-225 °C.
2. Span
The span is difference between the maximum and minimum values of the input. Thus, the above-mentioned thermocouple will have a span
of 200 °C.
3. Error
Error is the difference between the result of the measurement and the true value of the quantity being measured. A sensor might give a
displacement reading of 29.8 mm, when the actual displacement had been 30 mm, then the error is –0.2 mm.
4. Accuracy
The accuracy defines the closeness of the agreement between the actual measurement result and a true value of the measurand. It is often
expressed as a percentage of the full range output or full–scale deflection. A piezoelectric transducer used to evaluate dynamic pressure
phenomena associated with explosions, pulsations, or dynamic pressure conditions in motors, rocket engines, compressors, and other
pressurized devices is capable to detect pressures between 0.1 and 10,000 psig (0.7 KPa to 70 MPa). If it is specified with the accuracy of
about ±1% full scale, then the reading given can be expected to be within ± 0.7 MPa.
5. Sensitivity
Sensitivity of a sensor is defined as the ratio of change in output value of a sensor to the per unit change in input value that causes the
output change. For example, a general purpose thermocouple may have a sensitivity of 41 µV/°C.
SELECTION OF TRANSDUCERS
6. Nonlinearity
• The nonlinearity indicates the maximum deviation of the actual measured curve of a sensor
from the ideal curve.
• Figure shows a somewhat exaggerated relationship between the ideal, or least squares fit, line
and the actual measured or calibration line.
• Linearity is often specified in terms of percentage of nonlinearity, which is defined as:
Nonlinearity (%) = Maximum deviation in input ⁄ Maximum full scale input - - - - - (1)
• The static nonlinearity defined by Equation 1 is dependent upon
environmental factors, including temperature, vibration, acoustic
noise level, and humidity. Therefore it is important to know under
what conditions the specification is valid.
SELECTION OF TRANSDUCERS
7. Hysteresis
• The hysteresis is an error of a sensor, which is defined as the maximum difference in output at any measurement
value within the sensor’s specified range when approaching the point first with increasing and then with decreasing
the input parameter.
• Figure shows the hysteresis error might have occurred during measurement of temperature using a thermocouple.
The hysteresis error value is normally specified as a positive or negative percentage of the specified input range.
SELECTION OF TRANSDUCERS
8. Resolution
Resolution is the smallest detectable incremental change of input parameter that can be detected in the output signal. Resolution can be
expressed either as a proportion of the full-scale reading or in absolute terms. For example, if a LVDT sensor measures a displacement up
to 20 mm and it provides an output as a number between 1 and 100 then the resolution of the sensor device is 0.2 mm.
9. Stability
Stability is the ability of a sensor device to give same output when used to measure a constant input over a period of time. The term ‘drift’
is used to indicate the change in output that occurs over a period of time. It is expressed as the percentage of full range output.
10.Dead band/time
The dead band or dead space of a transducer is the range of input values for which there is no output. The dead time of a sensor device is
the time duration from the application of an input until the output begins to respond or change. 11.Repeatability
It specifies the ability of a sensor to give same output for repeated applications of same input value. It is usually expressed as a percentage
of the full range output:
Repeatability = (maximum – minimum values given) X 100 ⁄ full range
12.Response time
Response time describes the speed of change in the output on a step-wise change of the measurand. It is always specified with an
indication of input step and the output range for which the response time is defined.
RESISTIVE TRANSDUCERS
• Resistance is changed by a physical phenomena.
• The translational and rotational potentiometers which work on the basis of change in resistance value
with change in length of the conductor can be used for measurement of translational or rotary
displacements.
• Strain gauges work on the principle that resistance of a conductor or semi-conductor changes when
strained.
• This property can be used for measurement of displacement, force, and pressure.
• The resistive of material changes with changes in temperature thus causing a change of resistance. This
property may be used for measurement of temperature.
• Thus electrical resistance transducers have a wide field of application.
• Examples are Linear and Rotary Potentiometers, Strain Gauges, Resistance Thermometers (RTDs),
Thermistor, Thermocouples
INDUCTIVE TRANSDUCERS
The variable inductance transducers work, generally, upon one of the following three principles.
1. Change of self inductance
2. Change of mutual inductance
3. Production of eddy currents
By change of self inductance
• The self inductance of a coil 𝐿 =
𝑁2
𝑅
where N = number of turns, and
R = reluctance of the magnetic circuit.
• The reluctance of the magnetic circuit 𝑅 =
𝑙
𝜇𝐴
• Inductance 𝐿 = 𝑁2
𝜇
𝐴
𝑙
= N² µG...(1)
where µ = effective permeability of the medium in and around the coil; H/m
G=A/1 = geometric form factor
A = area of cross-section of coil; m²
1 = length of coil; m
• It is clear from eqn. 1 that the variation in inductance may be caused by:
(I) change in number of turns, N,
(II) Change in geometric configurations, G, and (III) Change in permeability, p.
• Inductive transducers are mainly used for measurement of displacement. The displacement to be measured is arranged to cause,
variation of any of three variables in eqn. 1 and thus alter the self inductance L by ΔL
INDUCTIVE TRANSDUCERS
By change of mutual inductance
• An inductive transducer working on the principle of variation of mutual inductance uses multiple coils.
• The mutual inductance between two coils is
𝑀 = 𝐾 𝐿1𝐿2
• Where L1 and L2 = self-inductances of two coils, and K = co-efficient of coupling.
• Thus mutual inductance between the coils can be varied by variation of self-inductances or the co-efficient of
coupling.
• However, the mutual inductance can be converted into a self-inductance by connecting the coils in series.
• The self-inductance of such an arrangement varies between L + L-2M to L + L +2M with one of the coils being
stationary while the other is movable.
• The self-inductance of each coil is constant but the mutual inductance changes depending upon the displacement of
the movable coil.
INDUCTIVE TRANSDUCERS
Inductive transducers working on principle of production of Eddy Currents
• These inductive transducers work on the principle that if a conducting plate is placed near a coil carrying alternating
current, eddy currents are, produced in the conducting plate.
• The conducting plate acts as a short-circuited secondary winding of a transformer.
• The eddy currents flowing in the plate produce a magnetic field of their own which acts against the magnetic field
produced by the coil.
• This results in reduction of flux and thus the inductance of the coil is reduced.
• The nearer is the plate to the coil, the higher are the eddy currents and thus higher is the reduction in the inductance
of the coil.
• Thus the inductance of the coil alters with variation of distance between the plat and the coil.
EXAMPLES OF INDUCTIVE TRANSDUCERS
• Linear Variable Differential Transformer (LVDT)
• Rotary variable Differential Transformer (RVDT)
• Synchros
• Resolvers
CAPACITIVE TRANSDUCERS
• The principle of operation of capacitive Transducers is based upon the familiar equation for capacitance of parallel
plate capacitor
𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒 𝐶 = 𝜀
𝐴
𝑑
= 𝜀𝑜𝜀𝑟
𝐴
𝑑
−−−−− − 1
Where
A = overlapping area of plates; m^2,
D=distance between two plates;m
𝜀 = 𝜀𝑜𝜀𝑟=permittivity of medium;F/m
𝜀𝑟 = 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑝𝑒𝑟𝑚𝑖𝑡𝑡𝑖𝑣𝑖𝑡𝑦
𝜀𝑜 = 𝑝𝑒𝑟𝑚𝑖𝑡𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑓𝑟𝑒𝑒 𝑠𝑝𝑎𝑐𝑒; 8.85 𝑋 10−12
𝐹/𝑚
• The capacitive transducer works on the principle of change in
Capacitance which may be caused by
(i) Change in overlapping area A,
(ii) Change in distance d between the plates and
(iii)Change in dielectric constant
• These changes are caused by physical variables like displacement, force, and pressure in most of the cases. The
change in capacitance may be caused by change in dielectric constant as in the case of measurement of liquid or
gas levels.
• Applications are variable capacitance pressure gauges, capacitor microphone, dielectric guage
PIEZOELECTRIC TRANSDUCERS
• A piezo-electric material is one in which an electric potential appears across certain surfaces of a crystal if the dimensions
of the crystal are changed by the application of a mechanical force.
• This potential is produced by the displacement of charges.
• The effect is reversible, i.e., conversely, if a varying potential is applied to the proper axis of the crystal, it will change the
dimensions of the crystal thereby deforming it.
• This effect is known as piezo-electric effect.
• Elements exhibiting piezo-electric qualities are called as electro resistive elements.
• Common piezo-electric materials include Rochelle salts, ammonium dihydrogen phosphate,
lithium sulphate, dipotassium tartrate, potassium dihydrogen phosphate, quartz and ceramics A and B.
• Except for quartz and ceramics A and B, the rest are man-made crystals grown from
aqueous solutions under carefully controlled conditions.
• The ceramic materials are poly crystalline in nature.
• They are, basically, made of barium titanate. They do not have
piezo-electric properties in their original state but these properties are produced by special polarizing treatment.
HALL EFFECT TRANSDUCERS
• The principle of working of a Hall Effect Transducer is that if a strip of conducting material carries a current in the
presence of a transverse magnetic field as shown in Fig., a difference of potential is produced between the opposite edges
of the conductor.
• The magnitude of the voltage depends upon the current, the strength of magnetic field and the property of the conductor
called Hall Effect.
• The Hall effect is present in metals and semiconductors in varying amounts, depending upon the densities and mobilities
of carriers.
𝑂𝑢𝑡𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑖𝑠, 𝐸𝐻 =
𝐾𝐻𝐼𝐵
𝑡
−−−−− −(1)
Where
𝐾𝐻 = 𝐻𝑎𝑙𝑙 𝑒𝑓𝑓𝑒𝑐𝑡 𝑐𝑜𝑒𝑓𝑓𝑖𝑖𝑐𝑖𝑒𝑛𝑡
𝑡 = 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑡𝑟𝑖𝑝
𝐼 𝑎𝑛𝑑 𝐵 𝑎𝑟𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑖𝑛 𝑎𝑚𝑝𝑒𝑟𝑒 𝑎𝑛𝑑 𝑓𝑙𝑢𝑥 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑖𝑛
𝑊𝑏
𝑚2
• Thus the voltage produced may be used for measurement of either the current I or field strength B.
• Applications include Magnetic to Electric Transducer, Measurement of Displacement, Measurement of Current.
OPTO ELECTRONIC TRANSDUCERS
• The opto-electronic transducers include photo voltaic cell, semiconductor photodiode and photo transistors.
1 Photo-voltalic cell
• The "photo-voltaic cell", or "solar cell" as it is sometimes called, produces an electrical current when connected to a load.
Both silicon (Si) and selenium types are used.
• Photo-voltaic cells may be used in a number of applications.
• Multiple-unit silicon voltaic devices may be used for sensing light as a means of reading punched cards in the data
processing industry.
• Gold-doped germanium cells with controlled spectral responses act as photovoltaic devices in the infra-red region of the
spectrum and may be used as infra-red detectors.
• Applications are in photographic exposure meters, in space crafts to
supply electrical energy.
OPTO ELECTRONIC TRANSDUCERS
• The opto-electronic transducers include photo voltaic cell, semiconductor photodiode and photo transistors.
2. Photoconductive Cell
• Another Photo-electric effect that has proved very useful is the photo-conductive effect, which is used in photo-
conductive cells or photo-cells.
• In this type of device, the electrical resistance of the material varies with the amount of light energy striking it.
• A typical control circuit utilizing a photo conductive cell is illustrated in Fig.
• The potentiometer is used to make adjustments to compensate for manufacturing
tolerances in photocell sensitivity and relay operating sensitivity.
• When the photo-cell has the appropriate light incident upon it,
its resistance is low and the current through the relay is consequently high to operate the relay.
• When the light is interrupted or shut off partially or completely, the resistance of the photocell increases thereby reducing
the current through the relay.
• The current may drop down to a level where it will not be able to operate the relay thereby de-energizing the relay.
• Applications are for counting packages moving on a conveyer belt, in a burglar alarm circuit.
OPTO ELECTRONIC TRANSDUCERS
• The opto-electronic transducers include photo voltaic cell, semiconductor photodiode and photo transistors.
3. Photo diode
• A reverse biased semi-conductor diode passes only a small leakage current (a fraction of 1 µA in typical silicon diodes) if
the junction is not exposed to light.
• Under illumination, however, the current rises almost in direct proportion to the light intensity.
• Thus, the photodiode can be used in applications similar to those in a photo-conductive cell.
• When the device operates with a reverse voltage applied, it functions as a photo-conductive device.
• When operating without the reverse voltage, it functions as a photo-voltaic device.
• It is also possible to arrange for a photodiode to change from photo-conductive mode to photo-voltaic mode.
• However, the photodiode has very important advantages over the photo-conductive cell.
• These advantages are its response, time is much faster, so that it may be used in applications in which light fluctuations
occur at high frequencies.
• Applications are similar to first two.
OPTO ELECTRONIC TRANSDUCERS
• The opto-electronic transducers include photo voltaic cell, semiconductor photodiode and photo transistors.
3. Photo transistor
• A Phototransistor is an electronic switching and current amplification component which relies on exposure to light to
operate.
• When light falls on the junction, reverse current flows which are proportional to the luminance.
• Phototransistors are used extensively to detect light pulses and convert them into digital electrical signals.
• These are operated by light rather than electric current.
• Providing a large amount of gain, low cost and these phototransistors might be used in
numerous applications.
• Applications are Punch-card readers, Security systems, Encoders – measure speed and
Direction, IR detectors photo, electric controls, Computer logic circuitry., Relays, Lighting
control (highways etc), Level indication, Counting systems
DIGITAL TRANSDUCERS
• Digital transducers are called Encoders.
• They are available but they are normally in the form of linear or rotary displacement transducers.
• Digital encoding transducers, or Digitizers, enable a linear or rotary displacement to be directly converted into digital
form without intermediate forms of analog to digital (A/D) conversion.
• Such digitizers may be known as digital encoders or linear digitizers, or for rotary applications, shaft digitizers or shaft
encoders.
• A digitizer is perhaps the most elementary form of analog to digital (A/D) converter because it converts a continuous an
analog quantity to be defined incrementally in some binary or decimal code.
• There are several techniques used for achieving this conversion, each with its own advantages and limitations, which
include cost, simplicity of associated circuitry and reliability etc.
• The are classified an contacting and non-contacting type and also as tachometer transducers, incremental transducers,
absolute transducers.

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Transducers.pptx

  • 1. TRANSDUCERS • In a measurement system, the measurand (quantity under measurement) makes its first contact with system through a detector. • The measurand is converted into an analogous form by the detector. • The measurand or the input signal is called information for the measurement system. • The information may be in the form of a physical phenomenon or it may be an electrical signal. • The function of the detector is to sense the information and convert it into a convenient form for acceptance by the later stages of the system. • The process of detection and conversion of input signal or information from one form to another requires energy. • This energy may be extracted from the signal thereby causing loading effects. • Thus if energy is extracted from the signal, it will not be faithfully reproduced after conversion which leads to errors. • In fact, efforts should be made to supply energy required by the detector from an external source for conversion so that the input signal is converted into a usable output without drawing an undue amount of energy from it. • The ideal conversion is when absolutely no energy is extracted from the signal so that it is not distorted and the analogous output of the detector is a faithful representation of the input.
  • 4. ELECTRIC TRANSDUCERS • In order to measure non-electrical quantities a detector is used which usually converts the physical quantity into a displacement. • This displacement actuates an electric transducer, which acting as a secondary transducer, gives an output that is electrical in nature. • The electrical quantity so produced is measured by standard methods used for electrical measurements. • The result (electrical output) gives the magnitude of the physical quantity or condition being measured. • The electrical signal may be a current or a voltage or a frequency and production of these signals is based upon, resistive, capacitive, inductive effects etc. • The first stage of a measurement system may simply be called a transducer stage instead of detector transducer stage by redefining a transducer. • A transducer, in general form, may be defined as a device which converts energy from one form to another. • However, this definition has to be restricted, many a time especially in the field of electrical instrumentation. • Keeping this restriction in view, transducer may be defined as a device which converts a physical quantity or a physical condition into an electrical signal. • Another name for a transducer is pick up.
  • 5. TRANSDUCERS • There are a number of transducers which transform a variety of physical quantities and phenomena into electrical signals. • The reasons for transforming a physical phenomenon into electrical form are numerous. 1. electrical amplification and attenuation can be done easily and that too with static devices. 2. The mass-inertia effects are minimized. In fact, when dealing with electrical or electronic signals, the inertia effects are due to electrons which have negligible mass. In many situations, we do not come across mass or inertia problems at all. 3. The effects of friction are minimized. 4. The electrical or electronic systems can be controlled with a very small power level. 5. The electrical output can be easily used, transmitted and processed for the purpose of measurement. 6. Telemetry is used in almost all sophisticated measurement systems. The entire aerospace research and development is based upon telemetry and remote control. 7. The ever enlarging field of radio monitoring in space research has left us with no alternative but to resort to electronic means. This completely eliminates the data transmission through mechanical means and hence electrical and electronic principles have to be employed for these conditions. The remote indication or recording is an essential part of modern day technology. 8. There has been an explosive development in the field of electronic components and devices. This development is on account of the fact that electronic devices are very amenable to miniaturization. Components which are compact, have always an, advantage. Miniaturization on account of use of ics (integrated circuits) has completely revolutionized the field of instrumentation.
  • 6. TRANSDUCERS A.Based on principle of transduction The transducers can be classified on the basis of principle of transduction as resistive, inductive, capacitive etc. Depending upon how they convert the input quantity into resistance, inductance or capacitance respectively. They can be classified as 1. Piezo electric, 2. Thermoelectric, 3. Magnetorestrictive, 4. Electro-kinetic and 5. Optical.
  • 7. TRANSDUCERS B. Primary and Secondary Transducers
  • 8. TRANSDUCERS C. Active and Passive Transducers • Passive transducers derive the power required for transduction from an auxiliary power source. • They also derive part of the power required for conversion from the physical quantity under measurement. • They are also known as "externally powered transducers". • Typical examples of passive transducers are resistive, inductive and capacitive transducers. • A typical example of a passive transducer is a ‘POT' which is used for measurement of displacement. • A 'POT' is a resistive transducer powered by a source voltage ei as shown in fig. . • This 'POT' is used for measurement of linear displacement xi, Suppose L is the total length of potentiometer whose total resistance Ri. The input displacement is xi. ∴ 𝑂𝑢𝑡𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑒𝑜 = 𝑥𝑖 𝐿 𝑒𝑖 𝑜𝑟 𝑥𝑖 = 𝑒𝑜 𝑒𝑖 𝐿 In the absence of external power, the transducer cannot work and it hence is called a passive transducer.
  • 9. TRANSDUCERS C. Active and Passive Transducers • Active transducers are those which do not require an auxiliary power source to produce their output. • They are also known as self generating type since they develop their own voltage or current output. • The energy required for production of output signal is obtained from the physical quantity being measured. • Velocity, temperature, light intensity and force can be transduced with the help of active transducers. • These transducers include tachogenerators, thermocouples, photovoltaic cells and piezoelectric crystals. • Consider the case of a piezoelectric crystal used for measurement of acceleration as shown in fig. the crystal is sandwiched between two metallic electrodes, and the entire sandwich is fastened to a base which may be the floor of a rocket. A fixed mass is placed on the top of the sandwich.
  • 10. TRANSDUCERS C. Active and Passive Transducers • The property of the piezo-electric crystals is that when a force is applied to them, they produce an output voltage. • The mass exerts a certain force on account of acceleration on the crystal due to which a voltage is generated. • The acceleration is applied to the base, due to which the mass produces a force. • The mass being fixed, the force is proportional to acceleration. • The voltage output is proportional force and hence is proportional to acceleration (the mass being fixed). • It should be noted from above that this transducer called "accelerometer" which converts acceleration into electrical voltage does not need any auxiliary power source to convert a physical phenomenon (acceleration in this case) to an electrical output (voltage in this case) and therefore is an active transducer.
  • 11. TRANSDUCERS D. Analog and digital transducers 1. Analog transducers • These transducers convert the input quantity into an analog output which is a continuous function of time. • Thus a strain gauge, an L.V.D.T., A thermocouple or a thermistor may be called as "analog transducers" as they give an output which is a continuous function of time. 2. Digital transducers • These transducers convert the input quantity into an electrical output which is in the form of pulses. • As the binary system uses only two symbols 0 and 1 it can be easily represented by opaque and transparent areas on a glass scale or non-conducting and conducting areas on a metal scale. • A scale constructed to show the linear position on a movable object and having five digits is shown in fig.. • The complete binary number denoting position is obtained by scanning the pattern across the scale at a stationary index mark. • Glass scales can be read optically by means of a light source, an optical system and photocells. • Metal scales are scanned by brushes making electrical contact with individual tracks. • The resolution depends upon the digits comprising the binary number and is 1/2n of full scale where n is the number of digits.
  • 12. TRANSDUCERS E. Transducers and Inverse Transducers 1. Transducers: A transducer can be broadly defined as a device which converts a non-electrical quantity into an electrical quantity. 2. Inverse transducers: • An inverse transducer is defined as a device which converts an electrical quantity into a non-electrical quantity. • It is a precision actuator which has an electrical input and a low power non-electrical output. • A piezoelectric crystal acts as an inverse transducer because when a voltage is applied across its surfaces, it changes its dimensions causing a mechanical displacement. • A current carrying coil moving in a magnetic field is also an inverse transducer because current carried by it is converted into a force which causes translational or rotational displacement. • Many data indicating and recording devices are inverse transducers. • An analog ammeter or voltmeter converts current into mechanical displacement. However, such devices which include instruments like indicating instruments, pen recorders, oscilloscopes that convert the electrical signals to a mechanical movement are placed at the output stage (data presentation stage) are called output transducers. • The most useful application of inverse transducers is in feedback measuring systems. The development of transducers and inverse transducers, and the advantages gained through use of feedback has increased their applications manifold and there is reason to believe that they will continue to grow in importance.
  • 13. SELECTION OF TRANSDUCERS 1. Range The range of a sensor indicates the limits between which the input can vary. For example, a thermocouple for the measurement of temperature might have a range of 25-225 °C. 2. Span The span is difference between the maximum and minimum values of the input. Thus, the above-mentioned thermocouple will have a span of 200 °C. 3. Error Error is the difference between the result of the measurement and the true value of the quantity being measured. A sensor might give a displacement reading of 29.8 mm, when the actual displacement had been 30 mm, then the error is –0.2 mm. 4. Accuracy The accuracy defines the closeness of the agreement between the actual measurement result and a true value of the measurand. It is often expressed as a percentage of the full range output or full–scale deflection. A piezoelectric transducer used to evaluate dynamic pressure phenomena associated with explosions, pulsations, or dynamic pressure conditions in motors, rocket engines, compressors, and other pressurized devices is capable to detect pressures between 0.1 and 10,000 psig (0.7 KPa to 70 MPa). If it is specified with the accuracy of about ±1% full scale, then the reading given can be expected to be within ± 0.7 MPa. 5. Sensitivity Sensitivity of a sensor is defined as the ratio of change in output value of a sensor to the per unit change in input value that causes the output change. For example, a general purpose thermocouple may have a sensitivity of 41 µV/°C.
  • 14. SELECTION OF TRANSDUCERS 6. Nonlinearity • The nonlinearity indicates the maximum deviation of the actual measured curve of a sensor from the ideal curve. • Figure shows a somewhat exaggerated relationship between the ideal, or least squares fit, line and the actual measured or calibration line. • Linearity is often specified in terms of percentage of nonlinearity, which is defined as: Nonlinearity (%) = Maximum deviation in input ⁄ Maximum full scale input - - - - - (1) • The static nonlinearity defined by Equation 1 is dependent upon environmental factors, including temperature, vibration, acoustic noise level, and humidity. Therefore it is important to know under what conditions the specification is valid.
  • 15. SELECTION OF TRANSDUCERS 7. Hysteresis • The hysteresis is an error of a sensor, which is defined as the maximum difference in output at any measurement value within the sensor’s specified range when approaching the point first with increasing and then with decreasing the input parameter. • Figure shows the hysteresis error might have occurred during measurement of temperature using a thermocouple. The hysteresis error value is normally specified as a positive or negative percentage of the specified input range.
  • 16. SELECTION OF TRANSDUCERS 8. Resolution Resolution is the smallest detectable incremental change of input parameter that can be detected in the output signal. Resolution can be expressed either as a proportion of the full-scale reading or in absolute terms. For example, if a LVDT sensor measures a displacement up to 20 mm and it provides an output as a number between 1 and 100 then the resolution of the sensor device is 0.2 mm. 9. Stability Stability is the ability of a sensor device to give same output when used to measure a constant input over a period of time. The term ‘drift’ is used to indicate the change in output that occurs over a period of time. It is expressed as the percentage of full range output. 10.Dead band/time The dead band or dead space of a transducer is the range of input values for which there is no output. The dead time of a sensor device is the time duration from the application of an input until the output begins to respond or change. 11.Repeatability It specifies the ability of a sensor to give same output for repeated applications of same input value. It is usually expressed as a percentage of the full range output: Repeatability = (maximum – minimum values given) X 100 ⁄ full range 12.Response time Response time describes the speed of change in the output on a step-wise change of the measurand. It is always specified with an indication of input step and the output range for which the response time is defined.
  • 17. RESISTIVE TRANSDUCERS • Resistance is changed by a physical phenomena. • The translational and rotational potentiometers which work on the basis of change in resistance value with change in length of the conductor can be used for measurement of translational or rotary displacements. • Strain gauges work on the principle that resistance of a conductor or semi-conductor changes when strained. • This property can be used for measurement of displacement, force, and pressure. • The resistive of material changes with changes in temperature thus causing a change of resistance. This property may be used for measurement of temperature. • Thus electrical resistance transducers have a wide field of application. • Examples are Linear and Rotary Potentiometers, Strain Gauges, Resistance Thermometers (RTDs), Thermistor, Thermocouples
  • 18. INDUCTIVE TRANSDUCERS The variable inductance transducers work, generally, upon one of the following three principles. 1. Change of self inductance 2. Change of mutual inductance 3. Production of eddy currents By change of self inductance • The self inductance of a coil 𝐿 = 𝑁2 𝑅 where N = number of turns, and R = reluctance of the magnetic circuit. • The reluctance of the magnetic circuit 𝑅 = 𝑙 𝜇𝐴 • Inductance 𝐿 = 𝑁2 𝜇 𝐴 𝑙 = N² µG...(1) where µ = effective permeability of the medium in and around the coil; H/m G=A/1 = geometric form factor A = area of cross-section of coil; m² 1 = length of coil; m • It is clear from eqn. 1 that the variation in inductance may be caused by: (I) change in number of turns, N, (II) Change in geometric configurations, G, and (III) Change in permeability, p. • Inductive transducers are mainly used for measurement of displacement. The displacement to be measured is arranged to cause, variation of any of three variables in eqn. 1 and thus alter the self inductance L by ΔL
  • 19. INDUCTIVE TRANSDUCERS By change of mutual inductance • An inductive transducer working on the principle of variation of mutual inductance uses multiple coils. • The mutual inductance between two coils is 𝑀 = 𝐾 𝐿1𝐿2 • Where L1 and L2 = self-inductances of two coils, and K = co-efficient of coupling. • Thus mutual inductance between the coils can be varied by variation of self-inductances or the co-efficient of coupling. • However, the mutual inductance can be converted into a self-inductance by connecting the coils in series. • The self-inductance of such an arrangement varies between L + L-2M to L + L +2M with one of the coils being stationary while the other is movable. • The self-inductance of each coil is constant but the mutual inductance changes depending upon the displacement of the movable coil.
  • 20. INDUCTIVE TRANSDUCERS Inductive transducers working on principle of production of Eddy Currents • These inductive transducers work on the principle that if a conducting plate is placed near a coil carrying alternating current, eddy currents are, produced in the conducting plate. • The conducting plate acts as a short-circuited secondary winding of a transformer. • The eddy currents flowing in the plate produce a magnetic field of their own which acts against the magnetic field produced by the coil. • This results in reduction of flux and thus the inductance of the coil is reduced. • The nearer is the plate to the coil, the higher are the eddy currents and thus higher is the reduction in the inductance of the coil. • Thus the inductance of the coil alters with variation of distance between the plat and the coil.
  • 21. EXAMPLES OF INDUCTIVE TRANSDUCERS • Linear Variable Differential Transformer (LVDT) • Rotary variable Differential Transformer (RVDT) • Synchros • Resolvers
  • 22. CAPACITIVE TRANSDUCERS • The principle of operation of capacitive Transducers is based upon the familiar equation for capacitance of parallel plate capacitor 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒 𝐶 = 𝜀 𝐴 𝑑 = 𝜀𝑜𝜀𝑟 𝐴 𝑑 −−−−− − 1 Where A = overlapping area of plates; m^2, D=distance between two plates;m 𝜀 = 𝜀𝑜𝜀𝑟=permittivity of medium;F/m 𝜀𝑟 = 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑝𝑒𝑟𝑚𝑖𝑡𝑡𝑖𝑣𝑖𝑡𝑦 𝜀𝑜 = 𝑝𝑒𝑟𝑚𝑖𝑡𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑓𝑟𝑒𝑒 𝑠𝑝𝑎𝑐𝑒; 8.85 𝑋 10−12 𝐹/𝑚 • The capacitive transducer works on the principle of change in Capacitance which may be caused by (i) Change in overlapping area A, (ii) Change in distance d between the plates and (iii)Change in dielectric constant • These changes are caused by physical variables like displacement, force, and pressure in most of the cases. The change in capacitance may be caused by change in dielectric constant as in the case of measurement of liquid or gas levels. • Applications are variable capacitance pressure gauges, capacitor microphone, dielectric guage
  • 23. PIEZOELECTRIC TRANSDUCERS • A piezo-electric material is one in which an electric potential appears across certain surfaces of a crystal if the dimensions of the crystal are changed by the application of a mechanical force. • This potential is produced by the displacement of charges. • The effect is reversible, i.e., conversely, if a varying potential is applied to the proper axis of the crystal, it will change the dimensions of the crystal thereby deforming it. • This effect is known as piezo-electric effect. • Elements exhibiting piezo-electric qualities are called as electro resistive elements. • Common piezo-electric materials include Rochelle salts, ammonium dihydrogen phosphate, lithium sulphate, dipotassium tartrate, potassium dihydrogen phosphate, quartz and ceramics A and B. • Except for quartz and ceramics A and B, the rest are man-made crystals grown from aqueous solutions under carefully controlled conditions. • The ceramic materials are poly crystalline in nature. • They are, basically, made of barium titanate. They do not have piezo-electric properties in their original state but these properties are produced by special polarizing treatment.
  • 24. HALL EFFECT TRANSDUCERS • The principle of working of a Hall Effect Transducer is that if a strip of conducting material carries a current in the presence of a transverse magnetic field as shown in Fig., a difference of potential is produced between the opposite edges of the conductor. • The magnitude of the voltage depends upon the current, the strength of magnetic field and the property of the conductor called Hall Effect. • The Hall effect is present in metals and semiconductors in varying amounts, depending upon the densities and mobilities of carriers. 𝑂𝑢𝑡𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑖𝑠, 𝐸𝐻 = 𝐾𝐻𝐼𝐵 𝑡 −−−−− −(1) Where 𝐾𝐻 = 𝐻𝑎𝑙𝑙 𝑒𝑓𝑓𝑒𝑐𝑡 𝑐𝑜𝑒𝑓𝑓𝑖𝑖𝑐𝑖𝑒𝑛𝑡 𝑡 = 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑡𝑟𝑖𝑝 𝐼 𝑎𝑛𝑑 𝐵 𝑎𝑟𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑖𝑛 𝑎𝑚𝑝𝑒𝑟𝑒 𝑎𝑛𝑑 𝑓𝑙𝑢𝑥 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑖𝑛 𝑊𝑏 𝑚2 • Thus the voltage produced may be used for measurement of either the current I or field strength B. • Applications include Magnetic to Electric Transducer, Measurement of Displacement, Measurement of Current.
  • 25. OPTO ELECTRONIC TRANSDUCERS • The opto-electronic transducers include photo voltaic cell, semiconductor photodiode and photo transistors. 1 Photo-voltalic cell • The "photo-voltaic cell", or "solar cell" as it is sometimes called, produces an electrical current when connected to a load. Both silicon (Si) and selenium types are used. • Photo-voltaic cells may be used in a number of applications. • Multiple-unit silicon voltaic devices may be used for sensing light as a means of reading punched cards in the data processing industry. • Gold-doped germanium cells with controlled spectral responses act as photovoltaic devices in the infra-red region of the spectrum and may be used as infra-red detectors. • Applications are in photographic exposure meters, in space crafts to supply electrical energy.
  • 26. OPTO ELECTRONIC TRANSDUCERS • The opto-electronic transducers include photo voltaic cell, semiconductor photodiode and photo transistors. 2. Photoconductive Cell • Another Photo-electric effect that has proved very useful is the photo-conductive effect, which is used in photo- conductive cells or photo-cells. • In this type of device, the electrical resistance of the material varies with the amount of light energy striking it. • A typical control circuit utilizing a photo conductive cell is illustrated in Fig. • The potentiometer is used to make adjustments to compensate for manufacturing tolerances in photocell sensitivity and relay operating sensitivity. • When the photo-cell has the appropriate light incident upon it, its resistance is low and the current through the relay is consequently high to operate the relay. • When the light is interrupted or shut off partially or completely, the resistance of the photocell increases thereby reducing the current through the relay. • The current may drop down to a level where it will not be able to operate the relay thereby de-energizing the relay. • Applications are for counting packages moving on a conveyer belt, in a burglar alarm circuit.
  • 27. OPTO ELECTRONIC TRANSDUCERS • The opto-electronic transducers include photo voltaic cell, semiconductor photodiode and photo transistors. 3. Photo diode • A reverse biased semi-conductor diode passes only a small leakage current (a fraction of 1 µA in typical silicon diodes) if the junction is not exposed to light. • Under illumination, however, the current rises almost in direct proportion to the light intensity. • Thus, the photodiode can be used in applications similar to those in a photo-conductive cell. • When the device operates with a reverse voltage applied, it functions as a photo-conductive device. • When operating without the reverse voltage, it functions as a photo-voltaic device. • It is also possible to arrange for a photodiode to change from photo-conductive mode to photo-voltaic mode. • However, the photodiode has very important advantages over the photo-conductive cell. • These advantages are its response, time is much faster, so that it may be used in applications in which light fluctuations occur at high frequencies. • Applications are similar to first two.
  • 28. OPTO ELECTRONIC TRANSDUCERS • The opto-electronic transducers include photo voltaic cell, semiconductor photodiode and photo transistors. 3. Photo transistor • A Phototransistor is an electronic switching and current amplification component which relies on exposure to light to operate. • When light falls on the junction, reverse current flows which are proportional to the luminance. • Phototransistors are used extensively to detect light pulses and convert them into digital electrical signals. • These are operated by light rather than electric current. • Providing a large amount of gain, low cost and these phototransistors might be used in numerous applications. • Applications are Punch-card readers, Security systems, Encoders – measure speed and Direction, IR detectors photo, electric controls, Computer logic circuitry., Relays, Lighting control (highways etc), Level indication, Counting systems
  • 29. DIGITAL TRANSDUCERS • Digital transducers are called Encoders. • They are available but they are normally in the form of linear or rotary displacement transducers. • Digital encoding transducers, or Digitizers, enable a linear or rotary displacement to be directly converted into digital form without intermediate forms of analog to digital (A/D) conversion. • Such digitizers may be known as digital encoders or linear digitizers, or for rotary applications, shaft digitizers or shaft encoders. • A digitizer is perhaps the most elementary form of analog to digital (A/D) converter because it converts a continuous an analog quantity to be defined incrementally in some binary or decimal code. • There are several techniques used for achieving this conversion, each with its own advantages and limitations, which include cost, simplicity of associated circuitry and reliability etc. • The are classified an contacting and non-contacting type and also as tachometer transducers, incremental transducers, absolute transducers.