Transducers and Data Aquisition Systems.pdf

SRI RAMAKRISHNA INSTITUTE OF TECHNOLOGY, COIMBATORE - 641010
(An Autonomous Institution)
(Approved by AICTE, New Delhi and Affiliated to Anna University, Chennai)
(Accredited with ‗A‘ Grade by NAAC)

 Classification of transducers – Selection of
transducers – Resistive, capacitive & inductive
transducers – Piezoelectric, Hall effect, optical
and digital transducers – Elements of data
acquisition system – A/D, D/A converters –
Smart sensors.

 A transducer is defined as a device that receives energy from one
system and transmits it to another, often in a different form.
 Broadly defined, the transducer is a device capable of being
actuated by an energizing input from one or more transmission
media and in turn generating a related signal to one or more
transmission systems.
 It provides a usable output in response to a specified input
measurand, which may be a physical or mechanical quantity,
property, or conditions. The energy transmitted by these systems
may be electrical, mechanical or acoustical.
 The nature of electrical output from the transducer depends on the
basic principle involved in the design.
 The output may be analog, digital or frequency modulated.

 The input quantity for most instrumentation systems is
nonelectrical. In order to use electrical methods and techniques
for measurement, the nonelectrical quantity is converted into a
proportional electrical signal by a device called transducer.
 Another definition states that transducer is a device which when
actuated by energy in one system, supplies energy in the same
form or in another form to a second system.
 When transducer gives output in electrical form it is known as
electrical transducer.

 Actually, electrical transducer consists of two parts
which are very closely related to Each other.
 These two parts are sensing or detecting element and
transduction element. The sensing or detecting
element is commonly known as sensor.
 Definition states that sensor is a device that produces
a measurable response to a Change in a physical
condition.
 The transduction element transforms the output of
the sensor to an electrical output, as shown in the
Fig.

 Repeatability
When the same input signal is applied to the transducer at different
times under the same environmental conditions, it should give
identical output signals.
 Linearity
The transducers should have linear input-output characteristics.
 Ruggedness
The transducer circuit should have overload protection so that it will
withstand overloads.
 High stability and reliability
The transducers output signal should not get affected by
environmental variations(disturbances) like temperature, vibration
etc. It should give minimum error in measurements.

 Good dynamic response
In real-time applications, the input signal will vary with
time ( ie, the input signal is dynamic in nature). The
transducer should respond as quick as possible for any
change in the input signal.
 Convenient instrumentation
The transducers output signal should be measured either
directly or after suitable amplification.
 Mechanical characteristics
When the transducer is subjected to various mechanical
strains during working conditions, its performance
should not degrade. It should withstand the mechanical
strains.

Transducers may be classified according to their
structure, method of energy conversion and
application. Thus we can say that transducers are
classified
 On the basis of transduction form used
 According to transduction principle
 As primary and secondary transducer
 As active and passive transducer
 As analog and digital transducer
 As transducer and inverse transducer

Principle of transduction
 Resistive , inductance or capacitance
respectively
 Classified as piezoelectric , thermoelectric ,
optical , magnetorestrictive etc.

The transducers can be classified according to
principle used in transduction.
 Capacitive transduction
 Electromagnetic transduction
 Inductive transduction
 Piezoelectric transduction
 Photovoltaic transduction
 Photoconductive transduction

PRIMARY OR SECONDARY
 Some transducers consist of mechanical device along with the
electrical device.
 In such transducers mechanical device acts as a primary
transducer and converts physical quantity into mechanical
signal.
 The electrical device then converts mechanical signal
produced by primary transducer into an electrical signal.
 Therefore, electrical device acts as a secondary transducer.
 For an example, in pressure measurement Bourdons tube acts
as a primary transducer which converts a pressure into
displacement and LVDT acts as a secondary transducer which
converts this displacement into an equivalent electrical signal.

 Active transducers are self-generating type of
transducers.
 These transducers develop an electrical
parameter (i.e. voltage or current) which is
proportional to the quantity under
measurement.
 These transducers do not require any
external source or power for their operation.

 They can be subdivided into the following
commonly used types

Transducers and Data Aquisition Systems.pdf

 Passive transducers do not generate any electrical
signal by themselves.
 To obtain an electrical signal from such transducers,
an external source of power is essential.
 Passive transducers depend upon the change in an
electrical parameter (R, L, or C).
 They are also known as externally power driven
transducers.
 They can be subdivided into the following
commonly used types.

 These transducers convert the input quantity
into an analog output which is a continuous
function of time.
 A strain gauge, LVDT, thermocouples or
thermistors are called analog transducers as
they produce an output which is a continuous
function of time.

 Digital transducers produce an electrical
output in the form of pulses which forms an
unique code.
 Unique code is generated for each discrete
value sensed.

 Transducers convert non-electrical quantity into
electrical quantity whereas inverse transducer
converts electrical quantity into non-electrical
quantity.
 For example, microphone is a transducer which
converts sound signal into an electrical signal
whereas loudspeaker is an inverse transducer
which converts electrical signal into sound
signal.

1. Electrical signal obtained from electrical
transducer can be easily processed (mainly
amplified) and brought to a level suitable for output
device which may be an indicator or recorder.
2. The electrical systems can be controlled with a
very small level of power
3. The electrical output can be easily used,
transmitted, and processed for the purpose of
measurement.

4. With the advent of IC technology, the electronic
systems have become extremely small in size, requiring
small space for their operation.
5. No moving mechanical parts are involved in the
electrical systems. Therefore there is no question of
mechanical wear and tear and no possibility of
mechanical failure.
Electrical transducer is almost a must in this modem
world. Apart from the merits described above, some
disadvantages do exist in electrical sensors.

 The electrical transducer is sometimes less reliable than
mechanical type because of the ageing and drift of the
active components.
 Also, the sensing elements and the associated signal
processing circuitry are comparatively expensive.
 With the use of better materials, improved technology
and circuitry, the range of accuracy and stability have
been increased for electrical transducers.
 Using negative feedback technique, the accuracy of
measurement and the stability of the system are
improved, but all at the expense of increased circuit
complexity, more space, and obviously, more cost.

1. Accuracy: It is defined as the closeness with which the reading
approaches an accepted standard value or ideal value or true value,
of the variable being measured.
2. Ruggedness: The transducer should be mechanically rugged to
withstand overloads. It should have overload protection.
3. Linearity: The output of the transducer should be linearly
proportional to the input quantity under measurement. It should
have linear input - output characteristic. -
4. Repeatability: The output of the transducer must be exactly the
same, under same environmental conditions, when the same
quantity is applied at the input repeatedly.
5. High output: The transducer should give reasonably high output
signal so that it can be easily processed and measured. The output
must be much larger than noise. Now-a-days, digital output is
preferred in many applications

6. High Stability and Reliability: The output of the transducer should be highly stable
and reliable so that there will be minimum error in measurement. The output must
remain unaffected by environmental conditions such as change in temperature,
pressure, etc.
7. Sensitivity: The sensitivity of the electrical transducer is defined as the electrical
output obtained per unit change in the physical parameter of the input quantity. For
example, for a transducer used for temperature measurement, sensitivity will be
expressed in mV/‘ C. A high sensitivity is always desirable for a given transducer.
8. Dynamic Range: For a transducer, the operating range should be wide, so that it can
be used over a wide range of measurement conditions.
9. Size: The transducer should have smallest possible size and shape with minimal
weight and volume. This will make the measurement system very compact.
10. Speed of Response: It is the rapidity with which the transducer responds to
changes in the measured quantity. The speed of response of the transducer should be
as high as practicable.

 Operating range: Chosen to maintain range
requirements and good
 Sensitivity: Chosen to allow sufficient output.
 Frequency response and resonant
frequency: Flat over the entire desired range.
 Environmental compatibility: Temperature
range, corrosive fluids, pressure, shocks,
interaction, size and mounting restrictions.

 Minimum sensitivity: To expected stimulus, other
than the measurand.
 Accuracy: Repeatability and calibration errors as well
as errors expected due to sensitivity to other
stimuli.
 Usage and ruggedness: Ruggedness, both of
mechanical and electrical intensities versus size and
weight.
 Electrical parameters: Length and type of cable
required, signal to noise ratio when combined with
amplifiers, and frequency response limitations.

 Resistive Transducer Definition are those in which the
resistance changes due to a change in some physical
phenomenon.
 The change in the value of the resistance with a change
in the length of the conductor can be used to measure
displacement.
 Strain gauges work on the principle that the resistance of
a conductor or semiconductor changes when strained.
This can be used for the measurement of displacement,
force and pressure.
 The resistivity of materials changes with changes in
temperature. This property can be used for the
measurement of temperature.

 A resistive potentiometer (pot) consists of a
resistance element provided with a sliding
contact, called a wiper.
 The motion of the sliding contact may be
translatory or rotational.
 Some have a combination of both, with
resistive elements in the form of a helix, as
shown in Fig. (c). They are known as helipots.

 Translatory resistive elements, as shown in Fig. (a),
are linear (straight) devices. Rotational resistive
devices are circular and are used for the
measurement of angular displacement, as shown in
Fig. (b).
 Helical resistive elements are multi turn rotational
devices which can be used for the measurement of
either translatory or rotational motion.
 A potentiometer is a passive transducer since it
requires an external power source for its operation.

 They are inexpensive.
 Simple to operate and are very useful for
applications where the requirements are not
particularly severe.
 They are useful for the measurement of large
amplitudes of displacement.
 Electrical efficiency is very high, and they provide
sufficient output to allow control operations.

 When using a linear potentiometer, a large
force is required to move the sliding contacts.
 The sliding contacts can wear out, become
misaligned and generate noise.

 Strain Gauge is an example of an passive
transducer that uses the variation in electrical
resistance in wires to sense the strain produced
by a force on the wires.
 It is well known that stress (force/unit area) and
strain (elongation or compression/unit length) in
a member or portion of any object under
pressure is directly related to the modulus of
elasticity.

 Since strain can be measured more easily by using
variable resistance transducers, it is a common practice to
measure strain instead of stress, to serve as an index of
pressure.
 Such transducers are popularly known as strain gauges.
 If a metal conductor is stretched or compressed, its
resistance changes on account of the fact that both the
length and diameter of the conductor changes.
 Also, there is a change in the value of the resistivity of the
conductor when subjected to strain, a property called the
piezo-resistive effect.

 The following types of Strain Gauge Factor
 Wire Strain Gauge
 Foil Strain Gauge
 Semiconductor Strain Gauge

 Resistance wire gauges are used in two basic forms, the unbounded
type, and the bonded type.
Unbounded Resistance Wire Strain Gauge:

 An unbounded strain gauge consists of a wire
stretched between two points in an insulating
medium, such as air.
 The diameter of the wire used is about 25 μm. The
wires are kept under tension so that there is no
sag and no free vibration.
 Unbounded Strain Gauge Factor Derivation are
usually connected in a bridge circuit.
 The bridge is balanced with no load applied as
shown in Fig.

 When an external load is applied, the
resistance of the Strain Gauge Factor
Derivation changes, causing an unbalance of
the bridge circuit resulting in an output
voltage.
 This voltage is proportional to the strain.
 A displacement of the order of 50μm can be
detected with these strain gauges.

 A metallic bonded Strain Gauge Derivation is
shown in Fig

 A fine wire element about 25 μm (0.025 in.) or less in
diameter is looped back and forth on a carrier (base) or
mounting plate, which is usually cemented to the
member undergoing stress.
 The grid of fine wire is cemented on a carrier which may
be a thin sheet of paper, Bakelite, or Teflon.
 The wire is covered on the top with a thin material, so
that it is not damaged mechanically.
 The spreading of the wire permits uniform distribution
of stress. The carrier is then bonded or cemented to the
member being studied. This permits a good transfer of
strain from carrier to wire.

 This class of strain gauges is an extension of the resistance
wire strain gauge. The strain is sensed with the help of a metal
foil.
 The metals and alloys used for the foil and wire are nichrome,
constantan (Ni + Cu), isoelastic (Ni + Cr + Mo), nickel and
platinum.
 Foil gauges have a much greater dissipation capacity than wire
wound gauges, on account of their larger surface area for the
same volume.
 For this reason, they can be used for a higher operating
temperature range.
 Also, the large surface area of foil gauges leads to better
bonding.

 The advantage of foil type Strain Gauge Transducer Types is
that they can be fabricated on a large scale, and in any shape.
 The foil can also be etched on a carrier.
 Etched foil gauge construction consists of first bonding a
layer of strain sensitive material to a thin sheet of paper or
Bakelite.
 The portion of the metal to be used as the wire element is
covered with appropriate masking material, and an etching
solution is applied to the unit.
 The solution removes that portion of the metal which is not
masked, leaving the desired grid structure intact.

 To have a high sensitivity, a high value of gauge
factor is desirable.
 A high gauge factor means relatively higher change in
resistance, which can be easily measured with a good
degree of accuracy.
 Semiconductor strain gauges are used when a very
high gauge factor is required.
 They have a gauge factor 50 times as high as wire
strain gauges.
 The resistance of the semiconductor changes with
change in applied strain.

 Semiconductor strain gauges depend for their action upon the
piezo resistive effect, i.e. change in value of the resistance due
to change in resistivity, unlike metallic gauges where change in
resistance is mainly due to the change in dimension when
strained.
 Semiconductor materials such as germanium and silicon are
used as resistive materials.
 A typical strain gauge consists of a strain material and leads
that are placed in a protective box, as shown in Fig.
 Semiconductor wafer or filaments which have a thickness of
0.05 mm are used.
 They are bonded on suitable insulating substrates, such as
Teflon.

 Semiconductor strain gauges have a high gauge
factor of about + 130. This allows measurement
of very small strains, of the order of 0.01 micro
 Hysteresis characteristics of semiconductor strain
gauges are excellent, e. less than 0.05%.
 Life in excess of 10 x 106 operations and a
frequency response of 1012 HZ.
 Semiconductor strain gauges can be very small in
size, ranging in length from 0.7 to 7.0 mm.

 They are very sensitive to changes in
temperature.
 Linearity of semiconductor strain gauges is
poor.
 They are more expensive.

 The resistance of a conductor changes when its
temperature is changed.
 This property is utilized for the measurement of
temperature.
 The Resistance Thermometer Transducer is an
instrument used to measure electrical resistance
in terms of temperature, i.e. it uses the change in
the electrical resistance of the conductor to
determine the temperature.

 The main part of a resistance thermometer is its sensing
element. The characteristics of the sensing element
determines the sensitivity and operating temperature range
of the instrument.
 (There are three common types of temperature sensitive
resistive elements in use, the wire wound resistance,
the Thermistor and the PTC semiconductor resistance.)
 The sensing element may be any material that exhibits a
relatively large resistance change with change in temperature.
 Also, the material used should be stable in its characteristics,
i.e. neither its resistance nor its temperature coefficient of
resistance should undergo permanent change with use or
age.

 To maintain the calibration of a resistance thermometer, it is
necessary to consider its stability.
 The need for stability frequently limits the temperature range
over which the sensing element may be used.
 Another desirable characteristic for a sensing element is a
linear change in resistance with change in temperature.
 The speed with which a resistive element responds to
changes in temperature is important when the measured
temperature is subjected to rapid variations.
 The smaller a given sensing element, the less heat required
to raise its temperature, and the faster its response.

 Platinum, nickel and copper are the metals most
commonly used to measure temperature.
 Figure (a) shows an industrial platinum resistance
thermometer. The changes in resistance caused by
changes in temperature are detected by a Wheatstone
bridge, as shown in Fig. (b).
 Hence, the temperature sensing element, which may
be nickel, copper or platinum contained in a bulb or
well, along with the balancing bridge, form the
essential components of a temperature measuring
system based upon this principle.

 The sensing element Rs is made of a material having a high temperature
coefficient, and R1, R2, and R5 are made of resistances that are practically
constant under normal temperature changes.
 When no current flows through the galvanometer, the normal principle of
Wheatstone‘ s bridge states the ratio of resistance is
 Now it resistance Rs changes, balance cannot be maintained and the
galvanometer shows a deflection, which can be calibrated to give a
suitable temperature scale.

 In normal practice, the sensing element is
away from the indicator, and its leads have a
resistance, say R3, R4.
 Therefore,

 The measurement is very accurate.
 It has a lot of flexibility with regard to choice of measuring
equipment.
 Indicators, recorders or controllers can also be operated.
 More than one resistance element can be clubbed to the
same indicating/ recording instrument.
 The temperature sensitive resistance element can be easily
installed and
 The accuracy of the measuring circuit can be easily checked
by substituting a standard resistor for the resistive element.
 Resistive elements can be used to measure differential
temperature.

 Resistance thermometers have a wide working range without loss of
accuracy, and can be used for temperature ranges (-200°C to +
650°C).
 They are best suited for remote indication.
 The resistive element response time is of the order of 2 to lOs
 The limits of error of a resistive element are ± 0.25% of the scale
reading.
 The size of the resistive element may be about 6 — 12 mm in
diameter and 12 — 75 mm in length.
 Extremely accurate temperature sensing.
 No necessity of temperature compensation.
 Stability of performance over long periods of time.

 High cost
 Need for bridge circuit and power source
 Possibility of self-heating
 Large bulb size, compared to a thermocouple

 Thermistor Circuit – The electrical resistance of
most materials changes with temperature.
 By selecting materials that are very temperature
sensitive, devices that are useful in temperature
control circuits and for temperature
measurements can be made.
 Thermistor (THERMally sensitive resISTOR) are
non-metallic resistors (semiconductor material),
made by sintering mixtures of metallic oxides
such as manganese, nickel, cobalt, copper and
uranium.

 Figure shows a graph of resistance vs temperature for
a Thermistor. The resistance at room temperature
(25°C) for typical commercial units ranges from 100 Ω
to 10 Ω
 They are suitable for use only up to about 800°C.
 In some cases, the resistance of Thermistor at room
temperature may decrease by 5% for each 1°C rise in
temperature.
 This high sensitivity to temperature changes makes
the Thermistor extremely useful for precision
temperature measurements, control and compensation

 The smallest Thermistor are made in the form of
beads. Some are as small as 0.15 mm (0.006 in.)
in diameter.
 These may come in a glass coating or sealed in
the tip of solid glass probes. Glass probes have a
diameter of about 2.5 mm and a length which
varies from 6 — 50 mm.
 The probes are used for measuring the
temperature of liquids.
 The resistance ranges from 300 Ω to 100 Ω.

 Typical Thermistor configurations are as shown in Fig.
(a). Figure (b) shows a bush type Thermistor.
 A Thermistor in one arm of a Wheatstone
bridge provides precise temperature information.
 Accuracy is limited, in most applications, only by the
readout devices.
 Thermistor are non-linear devices over a temperature
range, although now units with better than 0.2% linearity
over the 0-100°C temperature range are available.
 The typical sensitivity of a Thermistor is approximately 3
mV/°C at 200°C.

 Small size and low cost.
 Fast response over narrow temperature range.
 Good sensitivity in the NTC region.
 Cold junction compensation not required due to
dependence of resistance on absolute
temperature.
 Contact and lead resistance problems not
encountered due to large Rth (resistance).

 Non-linearity in resistance vs temperature
characteristics.
 Unsuitable for wide temperature range.
 Very low excitation current to avoid self-
heating.
 Need of shielded power lines, filters, etc. due
to high resistance.

 Inductive Transducer Definition may be either of
the self generating or the passive type.
 The self generating type utilizes the basic
electrical generator principle, i.e. a motion
between a conductor and magnetic field induces
a voltage in the conductor (generator action).
 This relative motion between the field and the
conductor is supplied by changes in the
measured.

 Inductive Transducer Definition are mainly
used for the measurement of displacement.
 The displacement to be measured is arranged
to cause variation in any of three variables
 Number of turns
 Geometric configuration
 Permeability of the magnetic material or
magnetic circuits

 The output may be caused by a change in the
number of turns.
 Figures (a) and (b) are transducers used for,
the measurement of displacement of linear
and angular movement respectively.
 In both cases, as the number of turns are
changed, the self inductance and the output
also changes.

 Figure shows an Inductive Transducer Definition
which works on the principle of the variation of
permeability causing a change in self inductance.
 The iron core is surrounded by a winding. If the iron
core is inside the winding, its permeability is
increased, and so is the inductance.
 When the iron core is moved out of the winding, the
permeability decreases, resulting in a reduction of the
self inductance of the coil. This transducer can be
used for measuring displacement.

 A transducer of the variable type consists of a
coil wound on a ferromagnetic core.
 The displacement which is to be measured is
applied to a ferromagnetic target.
 The target does not have any physical contact
with the core on which it is mounted. The
core and the target are separated by an air
gap, as shown in Fig. (a)

 The differential transformer is a passive inductive transformer.
It is also known as a Linear Variable Differential Transducer
(LVDT)
 The transformer consists of a single primary winding P1 and two
secondary windings S1 and S2 wound on a hollow cylindrical
former.
 The secondary windings have an equal number of turns and are
identically placed on either side of the primary windings.
 The primary winding is connected to an ac source.
 An movable soft iron core slides within the hollow former and
therefore affects the magnetic coupling between the primary
and the two secondaries.

 The output voltage of the secondary windings
S1 is Es1 and that of secondary winding S2 is Es2.
 In order to convert the output from S1 to S2 into a
single voltage signal, the two secondaries S1 and
S2 are connected in series opposition, as shown
in Fig.
 Hence the output voltage of the transducer is the
difference of the two voltages. Therefore the
differential output voltage Eo=Es1~Es2.

 When the core is at its normal position, the flux linking with
both secondary windings is equal, and hence equal emfs are
induced in them. Hence, at null position Es1 = Es2.
 Since the output voltage of the transducer is the difference of
the two voltages, the output voltage Eo is zero at null position.
 Now, if the core is moved to the left of the null position, more
flux links with winding S1 and less with winding S2.
 Hence, output voltage Es1 of the secondary winding S1 is
greater than Es2 .
 The magnitude of the output voltage of the secondary is then
Es1 — Es2, in phase with Es1 (the output voltage of secondary
winding S1).

 Similarly, if the core is moved to the right of
the null position, the flux linking with
winding S2 becomes greater than that linked
with winding S1.
 This results in Es2 becoming larger than Es1.
The output voltage in this case is Eo = Es2 —
Es1 and is in phase with Es2.

 The amount of voltage change in either secondary winding is
proportional to the amount of movement of the core.
 Hence, we have an indication of the amount of linear motion.
 By noting which output is increasing or decreasing, the
direction of motion can be determined.
 The output ac voltage inverts as the core passes the centre
position.
 The farther the core moves from the centre, the greater the
difference in value between Es1 and Es2 and consequently the
greater the value of Eo.
 Hence, the amplitude is function of the distance the core has
moved, and the polarity or phase indicates the direction of
motion, as shown in Fig.

 As the core is moved in one direction from the null position, the difference
voltage, i.e. the difference of the two secondary voltages increases, while
maintaining an in-phase relation with the voltage from the input source.
 In the other direction from the null position, the difference voltage
increases but is 180° out of phase with the voltage from the source.
 By comparing the magnitude and phase of the difference output voltage
with that of the source, the amount and direction of the movement of the
core and hence of the displacement may be determined.
 The amount of output voltage may be measured to determine the
displacement. The output signal may also be applied to a recorder or to a
controller that can restore the moving system to its normal position.
 The output voltage of an Linear Variable Differential Transducer is a linear
function of the core displacement within a limited range of motion (say 5
mm from the null position).

 Figure (d) shows the variation of the output voltage against
displacement for various position of the core. The curve is
practically linear for small displacements (up to 5 mm). Beyond this
range, the curve starts to deviate.
 The diagram in Figs (a), (b) and (c) shows the core of an Linear
Variable Differential Transducer at three different positions.
 In Fig. (b), the core is at 0, which is the central zero or null position.
Therefore, Es1 = Es2, and Eo = 0.
 When the core is moved to the left, as in Fig. 13.21(a) and is at A,
Es1 is more than Es2 and Eo is positive.
 This movement represents a positive value and therefore the phase
angle, is Φ = 0°.

 When the core is moved to the right towards B, Es2 is greater than
Es1 and hence Eo is negative.
 Therefore, S2 the output voltage is 180° out of phase with the
voltage which is obtained when the core is moved to the left. The
characteristics are linear from 0 — A and 0 — B, but after that
they become non-linear.
 One advantage of an Linear Variable Differential Transducer over,
the inductive bridge type is that it produces higher output
voltage for small changes in core position.
 Several commercial models that produce 50 mV/mm to 300
mV/mm are available. 300 mV/mm implies that a 1 mm
displacement of the core produces a voltage output of 300 mV.

 Linearity: The output voltage of this transducer is practically linear for
displacements up to 5 mm (a linearity of 0.05% is available in commercial
LVDTs).
 Infinite resolution: The change in output voltage is stepless. The effective
resolution depends more on the test equipment than on the
 High output: It gives a high output (therefore there is frequently no need
for intermediate amplification devices).
 High sensitivity: The transducer possesses a sensitivity as high as 40
V/mm.
 Ruggedness: These transducers can usually tolerate a high degree of
vibration and shock.
 Less friction: There are no sliding contacts.
 Low hysteresis: This transducer has a low hysteresis, hence repeatability is
excellent under all conditions.
 Low power: consumption Most LVDTs consume less than 1 W of power

 Large displacements are required for appreciable
differential output.
 They are sensitive to stray magnetic fields (but
shielding is possible).
 The receiving instrument must be selected to
operate on ac signals, or a demodulator network
must be used if a dc output is required.
 The dynamic response is limited mechanically by
the mass of the core and electrically by the applied
voltage.
 Temperature also affects the transducer.

 A symmetrical crystalline materials such as Quartz,
Rochelle salt and Barium titanate produce an emf when
they are placed under stress.
 This property is used in the Working Principle of
Piezoelectric Transducer, where a crystal is placed
between a solid base and the force-summing member,
as shown in Fig.
 An externally applied force, entering the transducer
through its pressure port, applies pressure to the top
of a crystal.
 This produces an emf across the crystal proportional
to the magnitude of applied pressure.

 Since the transducer has a very good HF response, its principal use is in HF
accelerometers.
 In this application, its output voltage is typically of the order of 1 — 30 mV
per gm of acceleration.
 The device needs no external power source and is therefore self generating.
 The disadvantage is that it cannot measure static conditions.
 The output voltage is also affected by temperature variation of the crystal.
 The basic expression for output voltage E is given by
where
Q = generated charge
Cp = shunt capacitances

 This transducer is inherently a dynamic responding sensor
and does not readily measure static conditions. (Since it is a
high impedance element, it requires careful shielding and
compensation.)
 For a Piezoelectric Transducer element under pressure, part
of the energy is, converted to an electric potential that
appears on opposite faces of the element, analogous to a
charge on the plates of a capacitor.
 The rest of the applied energy is converted to mechanical
energy, analogous to a compressed spring.
 When the pressure is removed, it returns to its original
shape and loses its electric charge.

 From these relationships, the following formulas have been derived for the
coupling coefficient K.

 An alternating voltage applied to a crystal causes
it to vibrate at its natural resonance frequency.
 Since the frequency is a very stable quantity,
Working Principle of Piezoelectric Transducer
are used in HF accelerometers.
 The principal disadvantage is that voltage will be
generated as long as the pressure applied to the
piezo electric element changes.

 Thermocouple Circuit – One of the most commonly used
methods of measurement of moderately high temperature
is the thermocouple effect.
 When a pair of wires made up of different metals is joined
together at one end, a temperature difference between the
two ends of the wire produces a voltage between the two
wires as illustrated in Fig.
 Temperature measurement with Thermocouple Circuit is
based on the Seebeck effect.
 A current will circulate around a loop made up of two
dissimilar metal when the two junctions are at different
temperatures as shown in Fig.

 When this circuit is opened, a voltage appears that is
proportional to the observed seebeck current.
 There are four voltage sources, their sum is the
observed seebeck voltage. Each junction is a voltage
source, known as Peltier emf.
 Furthermore, each homogenous conductor has a self
induced voltage or Thomson emf.
 The Thomson and Peltier emfs originate from the fact
that, within conductors, the density of free charge
carriers (electrons and holes) increases with
temperature.

 Conductors made up of different materials have different free-
carriers densities even when at the same temperature.
 When two dissimilar conductors are joined, electrons will diffuse
across the junction from the conductor with higher electron
density.
 When this happens the conductor losing electrons acquire a
positive voltage with respect to the other conductor.
 This voltage is called the Peltier emf.)
 When the junction is heated a voltage is generated, this is known
as seebeck effect.
 The seebeck voltage is linearly proportional for small changes in
temperature.
 Various combinations of metals are used in Thermocouple‘s.

 A Thermocouple Circuit, therefore consists of a pair of dissimilar
metal wires joined together at one end (sensing or hot junction)
and terminated at the other end (reference or cold junction), which
is maintained at a known constant temperature (reference
temperature).
 When a temperature difference exists between the sensing junction
and the reference junction, an emf is produced, which causes
current in the circuit.
 When the reference end is terminated by a meter or a recording
device, the meter indication will be proportional to the
temperature difference between the hot junction and the reference
junction.
 The magnitude of the thermal emf depends on the wire materials
used and in the temperature difference between the junctions.

 It has rugged construction.
 It has a temperature range from —270 °C-2700 °C.
 Using extension leads and compensating cables, long distances
transmission for temperature measurement is possible.
 Bridge circuits are not required for temperature measurement.
 Comparatively cheaper in cost.
 Calibration checks can be easily performed.
 Thermocouples offer good reproducibility.
 Speed of response is high compared to the filled system
thermometer.
 Measurement accuracy is quite good.

 Cold junction and other compensation is essential for
accurate
 They exhibit non-linearity in the emf versus
temperature characteristics.
 To avoid stray electrical signal pickup, proper
separation of extension leads from thermocouple
wire is essential.
 Stray voltage pick-up are possible.
 In many applications, the signals need to be
amplified.

 By the use of a digital code, it is possible to identify the
position of a movable test piece in terms of a binary
number.
 The position is converted into a train of pulses.
 This is achieved by a digital transducer and is also termed
as encoder.
 Since the binary system uses only two states, 0 or 1, it can
be easily represented by two different Types of Encoders in
Digital Electronics systems, namely
 Optical Encoder
 Resistive Electric Encoder
 Shaft Encoder

 A sector may be designed as shown in Fig., with a pattern of
opaque and translucent areas.
 A photo sensor and a light source is placed on the two sides of
the sector.
 The displacement is applied to the sector and therefore changes
the amount of light falling on the photo electric sensor.
 The pattern of the illuminated sensor then carries the
information to the location of the sector.
 Figure shows a possible pattern on sector of opaque and
translucent areas.
 The number of levels in the encoder determines the accuracy
with which the device operates.

Advantages:
 They give a true digital readout
 No mechanical contact is involved and
therefore problems of wear and tear and
alignment are not present
Disadvantages:
 Light sources burn out. (However, the life of
the light is about 50,000 hours.)

 Another Types of Encoders in Digital Electronics in which a
pattern may be used is the resistive electric encoder.
 The shaded areas are made of conducting material and the
unshaded areas of insulating material.
 Sliding contacts are used for making the contacts.
 Circuits of the sliding contacts which come in contact with
the conducting areas are completed, while those which
make contact with insulated areas are not completed.
 The encoder gives a digital readout which is an indication
of the position of the device, and hence determines the
displacement.

Advantages:
 It is relatively inexpensive.
 It can be made to any degree of accuracy desired,
provided the sector is made large enough to
accumulate the required number of rows for binary
The sectors are quite adequate for a slowly moving
system.
Disadvantages:
 Wear and tear of the contacts causes error.
 There is often an ambiguity of 1 digit in LSB

 A spatial encoder is a mechanical converter that translates the
angular position of a shaft into a digital number.
 It is therefore an A/D Converter.
 An increasing number of measuring instruments are being used to
communicate with digital computers for measurement and control
applications.
 There are two ways of generating digital signals.
 The first converts the analog variable to a shaft rotation (or
translation in linear measurements) and then uses many types of
shaft angle encoders to generate digital voltage signals.
 The other form converts the analog variable into an electrical
analog signal and then converts this into digital form.
 These two forms are very close to a true digital transducers.

 To understand the operation of a shaft encoder, let us consider a
translational encoder (a linear displacement transducer) shown in Fig.
 The encoder shown has four tracks (bits) and is divided into conducting
and insulating positions, with a smallest increment of 0.01 mm.
 As the scale moves under the brushes, the lamp circuits are made or
broken, so that the number shown on the readout lamps is at every
instant equal to one hundredth mm.
 For angular displacements, the pattern given in Fig. is changed or
modified, so that the length of the scale becomes the circumference of a
circle on a flat disc.
 The brushes are then placed along a radial line on the disc, as shown in
Fig.

 The disc is divided into concentric circular tracks, each of which is then
divided into segments in a manner depending upon the code being
used.
 For pure binary code, the inner most track is halved, the next quartered,
the next divided into eight parts, and so on.
 Each track has twice as many segments as the adjacent one near the
centre.
 The detection method determines the treatment of the disc. Alternate
segments on each track are made transparent and opaque, if
transmitted light and photo cells are used.
 If the segments are made reflecting and non-reflecting, reflected lights
and photo cells are used. Electrical methods are used for detection in
case the segments are made alternately conducting and non-
conducting.

 A typical Data Acquisition System consists of
individual sensors with the necessary signal
conditioning, data conversion, data processing,
multiplexing, data handling and associated
transmission, storage and display systems.
 In order to optimize the characteristics of the system
in terms of performance, handling capacity and cost,
the relevant sub systems can be combined together.
 Analog Data Acquisition System is generally acquired
and converted into digital form for the purpose of
processing, transmission, display and storage.

 Processing may consist of a large variety of
operations, ranging from simple comparison to
complicated mathematical manipulations.
 It can be for such purposes as collecting information
(averages, statistics), converting the data into a
useful form (e.g., calculations of efficiency of motor
speed, torque and power input developed), using
data for controlling a process, performing repeated
calculations to separate signals buried in the noise,
generating information for display, and various other
purposes.

 Data acquisition generally relates to the
process of collecting the input data in digital
form as rapidly, accurately, and economically
as necessary.
 The basic instrumentation used may be a
DPM with digital outputs, a shaft digitizer, or
a sophisticated high speed resolution device.

 For converting analog information from more
than one source, either additional
transducers or multiplexers are employed.
 To increase the speed with which information
is accurately converted, sample-hold circuits
are used.
 (In some cases, for analog signals with extra-
wide range, logarithmic conversion is used.)

 The characteristics of the data acquisition
system, depend on both the properties of the
analog data and on the processing carried out.
 Based on the environment, a broad
Classifications of data acquisition system into
two categories.
 Those suitable for favorable environments
(minimum RF interference and electromagnetic
induction)
 Those intended for hostile environments

 The former category may include, among other,
laboratory instrument applications, test systems
for collecting long term drift information on
zeners, high calibration test instruments, and
routine measurements in research, as mass
spectrometers and lock-in amplifiers.
 In these, the systems are designed to perform
tasks oriented more towards making sensitive
measurements than to problems of protecting
the integrity of analog data.

 The Classifications of data acquisition system specifically
includes measure, protecting the integrity of the analog
data under hostile conditions.
 Such measurement conditions arise in aircraft control
systems, turbo viscous in electrical power systems, and in
industrial process control systems.
 Most of these hostile measurement conditions require
devices capable of a wide range of temperature
operations, excellent shielding, redundant paths for
critical measurements and considerable processing of the
digital data acquisition system.

 The important Factors to Consider When Setting
Up a Data Acquisition System are as follows.
 Accuracy and resolution
 Number of channels to be monitored
 Analog or digital signal
 Single channel or multichannel
 Sampling rate per channel
 Signal conditioning requirements of each channel
 Cost

The various general Configuration of Data Acquisition System are
Single channel possibilities
 Direct conversion
 Pre-amplification and direct conversion
 Sample and hold, and conversion
 Pre-amplification, signal conditioning and any of the above
Multi channel possibilities
 Multiplexing the outputs of single channel converters
 Multiplexing the output of sample-hold circuits
 Multiplexing the inputs of sample-hold circuits
 Multiplexing low level data

 It must acquire the necessary data, at correct speed and at the
correct
 Use of all data efficiently to inform the operator about the state of
the
 It must monitor the complete plant operation to maintain on-line
optimum and safe operations.
 It must provide an effective human communication system and be
able to identify problem areas, thereby minimizing unit availability
and maximizing unit through point at minimum cost.
 It must be able to collect, summaries and store data for diagnosis of
operation and record purpose.
 It must be able to compute unit performance indices using on-line,
real-time data.
 It must be flexible and capable of being expanded for future require
 It must be reliable, and not have a down time greater than 0.1%.

 A Single Channel Data Acquisition System
consists of a signal conditioner followed by an
analog to digital (A/D) converter, performing
repetitive conversions at a free running,
internally determined rate.
 The outputs are in digital code words including
over range indication, polarity information and a
status output to indicate when the output digits
are valid

 A Single Channel Data Acquisition System is shown in Fig. The
digital outputs are further fed to a storage or printout device, or
to a digital computer device, or to a digital computer for
analysis.
 The popular Digital panel Meter (DPM) is a well known example
of this. However, there are two major drawbacks in using it as a
DAS.
 It is slow and the BCD has to be changed into binary coding, , if
the output is to be processed by digital equipment.
 While it is free running, the data from the A/D converter is
transferred to the interface register at a rate determined by the
DPM itself, rather than commands beginning from the external
interface.

 Analog to digital converters used for DAS
applications are usually designed to receive
external commands to convert and hold.
 For dc and low frequency signals, a dual slope
type converter is often used.
 The advantage is that it has a linear averaging
capability and has a null response for frequencies
harmonically related to the integrating period.

 Many low resolution (8/10 bit) A/D converters are
constructed with a single ended input and have a
normalized analog input range of the order of 5-10 V,
bipolar or unipolar.
 For signal levels which are low compared to input
requirements, amplification may be used in order to
bring up the level of the input to match converter
input requirements, so that optimum use can be made
in terms of accuracy and resolution.
 The amplifier used has a single ended input or a
differential input, as shown in Fig.

 Pre-amplifiers can be coupled with active
filters before processing of data, in order to
minimize the effect of noise carriers and
interfering high frequency components.
 They effective compensate for
transmission sensitivity loss at high
frequency and hence enable measurements
over an enhanced dynamic frequency range.

 The Multi Channel Data Acquisition System
can be time shared by two or more input
sources.
 Depending on the desired properties of the
multiplexed system, a number of techniques
are employed for such time shared
measurements.

 The individual analog signals are applied directly or after
amplification and/or signal conditioning, whenever
necessary, to the multiplexer.
 These are further converted to digital signals by the use of
A/D converters, sequentially.
 For the most efficient utilization of time, the multiplexer is
made to seek the next channel to be converted while the
previous data stored in the sample/hold is converted to
digital form.
 When the conversion is complete, the status line from the
converter causes the sample/hold to return to the sample
mode and acquires the signal of the next channel.

 On completion of acquisition, either immediately
or upon command, the S/H is switched to the
hold mode, a conversion begins again and the
multiplexer selects the next channel.
 This method is relatively slower than
systems where S/H outputs or even A/D
converter outputs are multiplexed, but it has the
obvious advantage of low cost due to sharing of
a majority of sub-systems.

 When a large number of channels are to be monitored
at the same time (synchronously) but at moderate
speeds, the technique of multiplexing the outputs of
the S/H is particularly attractive.
 An individual S/H is assigned to each channel as
shown in Fig. , and they are updated synchronously
by a timing circuit.
 The S/H outputs are connected to an A/D converter
through a multiplexer, resulting in a sequential
readout of the outputs.

 It is now economically feasible to employ an A/D
converter for each analog input and multiplex the
digital outputs.
 Since each analog to digital converter (A/D) is
assigned to an individual channel, the conversion rate
of the A/D need only be as fast as is needed for that
channel, compared to the higher rates that would be
needed if it were used as in a multi channel analog
multiplexed system.

 The parallel conversion scheme shown in Fig. provides additional
advantages in industrial data acquisition systems where many strain
gauges, thermocouples and LVDTs are distributed over large plant
areas.
 Since the analog signals are digitized at the source, the digital
transmission of the data to the data centre (from where it can go on
to a communication channel) can provide enhanced immunity
against line frequency and other ground loop interferences.
 The data converted to digital form is used to perform logic
operations and decisions.
 Based on the relative speed at which changes occur in the data, the
scanning rate can be increased or decreased.

 A low level data multiplexing system, as shown in Fig.,
enables the use of a single high quality data amplifier
for handling multichannel low level inputs.
 Individual amplifiers are used for each low level signal.
 Low level multiplexing can be attractive when a large
number of channels (25), all having low level outputs,
need to be used at moderate speeds.
 The use of individual channels is possible because of
the availability of high quality amplifiers at moderate
cost.

 Several factors have to be considered to accomplish low
level multiplexing successfully.
 Guarding may have to be employed for every channel, and
each individual guard may have to be switched, so that the
appropriate guard is driven by the common mode
pertaining to that channel.
 Problems of pickup gets more complicated and have to be
taken care of, to preempt the possibility of signal-to-
signal, and even common mode-to differential mode
signal cross-talk.

 Aerospace application
 Biomedical field
 Telemetry industries
 Industries

 Principles of ADC – The input signal is compared
with an internally generated voltage which is
increased in steps starting from zero.
 The number of steps needed to reach the full
compensation is counted.
 A simple compensation type is the staircase
ramp.

 The basic principle is that the input signal Vi is compared
with an internal staircase voltage, Vc generated by a series
circuit consisting of a pulse generator (clock), a counter
counting the pulses and a digital to analog converter,
converting the counter output into a dc signal.
 As soon as Vc is equal to Vi, the input comparator closes a
gate between the clock and the counter, the counter stops
and its output is shown on the display.
 The basic block diagram is shown in Fig.

 The clock generates pulses continuously.
 At the start of a measurement, the counter is reset to 0 at
time t1 so that the output of the digital to analog converter
(DAC) is also 0.
 If Vi is not equal to zero, the input comparator applies an
output voltage that opens the gate so that clock pulses are
passed on to the counter through the gate.
 The counter starts counting and the DAC starts to produce
an output voltage increasing by one small step at each
count of the counter. The result is a staircase voltage
applied to the second input of the comparator, as shown in
Fig.

 The successive approximations principle can be easily
understood using a simple example; the determination of
the weight of an object.
 By using a balance and placing the object on one side and
an approximate weight on the other side, the weight of the
object is determined.
 If the weight placed is more than the unknown weight, the
weight is removed and another weight of smaller value is
placed and again the measurement is performed.

 Now if it is found that the weight placed is less than that
of the object, another weight of smaller value is added to
the weight already present, and the measurement is
performed.
 If it is found to be greater than the unknown weight the
added weight is removed and another weight of smaller
value is added.
 In this manner by adding and removing the appropriate
weight, the weight of the unknown object is determined.

 At the beginning of the measurement cycle, a start pulse is applied to
the start-stop multivibrator.
 This sets a 1 in the MSB of the control register and a 0 in all bits
(assuming an 8-bit control) its reading would be 10000000.
 This initial setting of the register causes the output of the D/A converter
to be half the reference voltage, i.e. 1/2 V.
 This converter output is compared to the unknown input by the
comparator.
 If the input voltage is greater than the converter reference voltage, the
comparator output produces an output that causes the control register
to retain the 1 setting in its MSB and the converter continues to supply
its reference output voltage of 1/2 Vref

 The ring counter then advances one count, shifting a 1 in the second MSB of
the control register and its reading becomes 11000000.
 This causes the D/A converter to increase its reference output by 1 increment
to 1/4 V, i.e. 1/2 V + 1/4 V, and again it is compared with the unknown input.
 If in this case the total reference voltage exceeds the unknown voltage, the
comparator produces an output that causes the control register to reset its
second MSB to 0.
 The converter output then returns to its previous value of 1/2 V and awaits
another input from the SAR. When the ring counter advances by 1, the third
MSB is set to 1 and the converter output rises by the next increment of 1/2 V
+ 1/8 V. The measurement cycle thus proceeds through a series of successive
approximations.
 Finally, when the ring counter reaches its final count, the measurement cycle
stops and the digital output of the control register represents the final
approximation of the unknown input voltage.

 Digital transducers are defined as transducers with a
digital output.
 Transducers available at large are primary analogue at
nature, and some form of conversion is needed to
convert to transform them into digital form.
 Analogue transducers with A/D convertors can serve
the purpose of digital transducers.
 However, this introduces an additional uncertainty, that
of the converter. Inconsequence, overall accuracy and
resolution are likely to be affected.

 Mechanical disks (or bar) with optical receivers and
transmitters can act as digital displacement transducers.
 This type of transducers called optical encoder.
 Optical encoders can be used to measure linear and
angular displacements.
 Therefore, optical encoders can be classified as:
Rotary encoders
Linear encoders

Optical Rotary Encoders
 An optical rotary encoder produces angular position data directly in digital form,
eliminating any need for the ADC converter.
 The concept is illustrated in following figure, which shows a slotted disk attached
to a shaft.
 A light source (LED) and light receiver (phototransistor or photodiode)
arrangement are mounted so that the slots pass the light beam as the disk
rotates.
 The angle of the shaft is deduced from the output of the photocell.
 There are two types of optical rotary encoders: the absolute encoder and the
incremental encoder.

Absolute Rotary Encoder
 The output of the absolute rotary encoder is in the form
of a binary word which is proportional to the angle of the
shaft.
 The absolute encoder does not need to be homed
because when it is energized, it simply outputs the shaft
angle as a digital value.
 Absolute optical encoders use a glass or plastic disk
marked off with a pattern of concentric tracks as shown
in the figure.

 A separate light beam is sent through each track to
individual photo sensors.
 Each photo sensor contributes 1 bit to the output digital
word.
 The encoder in the figure outputs a 4-bit word with the LSB
coming from the outer track (note that this is for illustrative
purposes only and a 4-bit encoder is of little practical use).
 The disk is divided into 16 sectors, so the resolution in this
case is 360°/16 = 22.5°.

 The absolute angle of the encoder shaft can be found by multiplying the
binary output of the encoder times the resolution.
 For example, assume our 4-bit encoder has an output of 1101 (decimal
13).
 The encoder shaft would therefore be at an angle of 13 x 22.5 degrees =
292.5 degrees.
 Because of the relatively poor resolution of this encoder, the shaft could be
at some angle between 292.5 degrees and 292.5+22.5 degrees.
 For better resolution, more tracks would be required.
 For example, eight tracks (providing 256 states) yield 360°/256 =
1.4°/state, and ten tracks (providing 1024 states) yield 360°/1024 =
0.35°/state.

 An advantage of this type of encoder is that the
output is in straightforward digital form and, like a
pot, always gives the absolute position.
 This is in contrast to the incremental encoder that, as
will be shown, provides only a relative position.
 A disadvantage of the absolute encoder is that it is
relatively expensive because it requires that many
photocells be mounted and aligned very precisely

 If the absolute optical encoder is not properly aligned, it may occasionally
report completely erroneous data.
 The following figure illustrates this situation, and it occurs when more than
1 bit changes at a time, say, from sector 7 (0111) to 8 (1000).
 In the figure, the photo sensors are not exactly in a straight line.
 In this case, sensor B1 is out of alignment (it‘s ahead) and switches from a
1 to a 0 before the others.
 This causes a momentary erroneous output of 5 (0101).
 If the computer requests data during this ―transition‖ time, it would get the
wrong answer.

Absolute Rotary Encoder Digital

 One inherent problem that is encountered with binary output absolute
encoders occurs when the output of the encoder changes its value.
 Consider our 4-bit binary encoder when it changes from 7 (binary 0111)
to 8 (binary 1000).
 Notice that in this case, the state of all four of its output bits change
value.
 If we were to capture the output of the encoder while these four outputs
are changing state, it is likely that we will read an erroneous value.
 The reason for this is that because of the variations in slew rates of the
photo-transistors and any small alignment errors in the relative
positions of the phototransistors, it is unlikely that all four of the
outputs will change at exactly the same instant.

 For this reason, all binary output encoders include one
additional output line called data valid (also called data
available, or strobe).
 This is an output that, as the encoder is rotated, goes false for
the very short instant while the outputs are changing state.
 As soon as the outputs are settled, the data valid line goes
true, indicating that it is safe to read the data.
 This is illustrated in the timing diagram in the following
figure.

 The second solution is to use the Grey code on the disk instead of the
straight binary code as shown in the following figure.
 Gray code requires the same number of bits to achieve the same resolution
as a binary encoder equivalent.
 However, the counting pattern is established so that, as the angle increases
or decreases, no more than one output bit changes at a given time, i.e. only
1 bit changes between any two sectors.
 If the photo sensors are out of line, the worst that could happen is that the
output would switch early or late.
 Put another way, the error can never be more than the value of 1 LSB when
using the Grey code.

Converting Binary to Gray:
 Write the binary number to be converted and add a leading zero (on the left side).
 Exclusive-OR each pair of bits in the binary number together and write the
resulting bits below the original number.
Converting Gray to Binary:
 Write the gray code number to be converted and add a leading zero (on the left
side).
 Beginning with the leftmost digit (the added zero), perform a chain addition of all
the bits, writing the "running sum" as you go.

Incremental Rotary Encoder
 The incremental optical encoder has one track of equally spaced slots.
Position is determined by counting the number of slots that pass by a
photo sensor, where each slot represents a known angle.
 This system requires an initial reference point, which may come from a
second sensor on an inner track or simply from a mechanical stop or
limit switch.
 In many applications, the shaft being monitored will be cycling back-
and-forth, stopping at various angles.
 To keep track of the position, the controller must know which direction
the disk is turning as well as the number of slots passed.
 A single photo sensor cannot convey which direction the disk is rotating;
however, a clever system using two sensors can

Incremental Rotary Encoder Digital

Incremental Rotary Encoder
 In the following , the two sensors, V1 and V2, are located slightly apart from each other on the
same track.
 For this example, V1 is initially off (well, almost you can see it is half-covered up), and V2 is on.
 Now imagine that the disk starts to rotate CCW.
 The first thing that happens is that V1 comes completely on (while V2 remains on).
 After more rotation, V2 goes off, and slightly later V1 goes off again. Figure (b) shows the
waveform for V1 and V2.
 Now consider what happens when the disk is rotated in the CW direction [starting again from the
position shown in Figure (a)].
 This time V1 goes off immediately, and V2 stays on for half a slot and then goes off. Later V1
comes on, followed by V2 coming on. Figure (c) shows the waveforms generated by V1 and V2.
 Compare the two sets of waveforms, notice that in the CCW case V2 leads V1 by 90°, whereas for
the CW case V1 is leading V2 by 90°. This difference in phase determines which direction the
disk is turning.

 The optical transducer convert light into electrical
quantity.
 They are also called as photoelectric transducers. The
optical transducer can be classified as photo
emissive, photoconductive and photovoltaic
transducers.
 The photo emissive devices operate on the principle
that radiation falling on a cathode causes electrons to
be emitted from the cathode surface.

 The photoconductive devices operate on the
principle that whenever a material is illuminated,
its resistance changes.
 The photovoltaic cells generate an output voltage
that is proportional to the radiation intensity.
 The radiation that is incident may be x-rays,
gamma rays, ultraviolet, infrared or visible light.

 An optical transducer converts light rays into
an electronic signal.
 The purpose of an optical transducer is to
measure a physical quantity of light and,
depending on the type of transducer, then
translates it into a form that is readable by an
integrated measuring device.

Transducers and Data Aquisition Systems.pdf

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Transducers and Data Aquisition Systems.pdf