EMI

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EXPERT SYSTEMS AND SOLUTIONS Email
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PAIYANOOR, OMR, CHENNAI Call For Research
Projects Final year students of B.E in EEE,
ECE, EI, M.E (Power Systems), M.E (Applied
Electronics), M.E (Power Electronics) Ph.D
Electrical and Electronics. Students can
assemble their hardware in our Research labs.
Experts will be guiding the projects.
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EI1361-MEASUREMENT AND INSTRUMENTATION

• -C.KAMALA KANNAN
• Lecturer/EEE

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UNIT 5

• TRANSDUCERS AND DATA
• ACQUISITION SYSTEMS

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• TRANSDUCERS AND DATA
• ACQUISITION SYSTEMS
• Classification of transducers
• Selection of transducers
• Resistive transducers
• Capacitive transducers
• Inductive transducers
• Piezoelectric transducers
• Optical transducers
• Digital transducers
• Elements of data acquisition system - A/D, D/A
  converters.

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Reference

• TEXT BOOKS
• 1. E.O. Doebelin, Measurement Systems
  Application and Design, Tata McGraw Hill
  publishing company, 2003.
• 2. A.K. Sawhney, A Course in Electrical
  Electronic Measurements Instrumentation,
  Dhanpat Rai and Co, 2004.

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• REFERENCE BOOKS
• 1. A.J. Bouwens, Digital Instrumentation, Tata
  McGraw Hill, 1997.
• 2. D.V.S. Moorthy, Transducers and
  Instrumentation, Prentice Hall of India Pvt Ltd,
  2003.
• 3. H.S. Kalsi, Electronic Instrumentation, Tata
  McGraw Hill, 1995.
• 4. Martin Reissland, Electrical Measurements,
  New Age International (P) Ltd., Delhi, 2001.
• 5. J. B. Gupta, A Course in Electronic and
  Electrical Measurements, S. K. Kataria Sons,
  Delhi, 2003.

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TRANSDUCERS

• Its a device which convert one form of energy to
  another form
• Non electrical quantity is converted into an
  electrical form by a transducer.
• Another name is pick up

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Advantage of Electrical Transducers

• Electrical amplification and attenuation can be
  done easily.
• Mass inertia effects are minimized.
• The effect of friction is minimized.
• The electrical or electronic systems can be
  controlled with a very small power level.

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Conti..

• The electrical output can be easily used ,
  transmitted and processed for the purpose of
  measurement.
• Telemetry (aerospace remote indication /
  recording)
• Miniaturization on account of use of ICs.

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Two parts/element of transducer

• Sensing or detector element.
• A detector or a sensing element is that part
  of a transducer which responds to a physical
  phenomenon or a change in a physical phenomenon.
• Transduction element.
• A transduction element transforms the output of
  a sensing element to an electrical output. The
  transduction element in a way acts as a secondary
  transducer.

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Classification of Transducers

• On the basis of transduction form used.
• As primary and secondary transducers
• As passive and active transducer.
• As analog and digital transducer.
• As transducers and inverse transducers.

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Classification based upon principle of
transduction

• Resistive
• Inductive
• Capacitive etc
• Depending upon how they convert the input
  quantity into resistance, inductance or
  capacitance respectively.
• Eg piezoelectric, thermoelectric, magneto
  restrictive, electro kinetic and optical

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Primary and Secondary Transducers
LVDT (Linear Variable Differential Transformer)

• Primary- Pressure to displacement (bourdon tube)
• Secondary-Displacement into analogous voltage
  (LVDT).

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Passive and Active Transducer

• Active Transducer
• Also known as self generating type, develop
  their own voltage or current from the physical
  phenomenon being measured.
• Velocity , temperature , light intensity and
  force can be transduced with the help of active
  transducer.

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Conti..

• Passive Transducer
• Also known as externally powered transducers,
  i.e., derive the power required for energy
  conversion from an external power source.
• e.g. POT (Potentiometer)-used for the
  measurement of displacement .

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Analog and Digital Transducer.

• Analog Transducers It converts the input
  quantity into an analog output which is a
  continuous function of time.
• E.g. LVDT, Thermocouple or a thermistor
• (gives output which is continuous function of
  time)

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Conti..

• Digital Transducer Converts input quantity into
  an electrical output which is in the form of
  pulse.

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Transducers and Inverse Transducers

• Transducer Non electrical to electrical quantity
• Inverse transducer Electrical quantity into non
  electrical quantity.

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Characteristics and Choice of Transducer

• Input Characteristics
• Transfer Characteristics
• Output Characteristics.

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Input Characteristics

• Type of Input and Operating Range
• Loading effect.
• Type of Input The type of input, which can be
  any physical quantity, is generally determined in
  advance .
• Operating Range Choice of transducer depends
  upon the useful range of input quantity.

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Conti..

• Loading Effect The transducer, that is selected
  for a particular application should ideally exact
  NO force, power or energy from the quantity under
  measurement in order that is measured accurately.

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Transfer Characteristics

1. Transfer function.
2. Error.
3. Response of transducer to environmental
   influences.

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Transfer function.

• The transfer function of a transducer defines a
  relationship between the input quantity and the
  output. The transfer function is
• Where are respectively output
  and input of the transducer.

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Conti..

• Sensitivity,
• Scale Factor, Inverse of sensitivity.

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Error

• The error in transducer occur because they do not
  follow, the input output relationship.
• Example.. Instead of qo, we might get a output as
  qo, then the error of the instrument is

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Three components of error

1. Scale error.
2. Dynamic error
3. Error on account of noise and drift.

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Scale error.

• Zero error.
• Sensitivity error
• Non conformity.
• Hysteresis.

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Zero error

• Output deviates from the correct value by a
  constant factor over the entire range of the
  transducer.

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Sensitivity Error

• Observed output deviates from the correct value
  by a constant value.

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Non conformity

• Transfer function deviates from the theoretical
  transfer function for almost every input.

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Hysteresis
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Response of transducer to environmental
influences.

• It should not be subjected to any disturbances
  like stray electromagnetic and electrostatic
  fields, mechanical shocks and vibrations
  temperature changes, pressure and humidity
  changes, changes in supply voltage and improper
  mechanical mountings.

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Output Characteristics

• Type of Electrical Output.
• Output Impedance
• Useful Range.

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Type of Electrical Output.

• The type of output which may be available from
  the transducers may be a voltage, current ,
  impedance or a time function of these amplitudes.

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Output Impedance

• Ideally the value of output impedance should be
  zero if no loading effects are there on the
  subsequent stage.
• Since zero output impedance is not possible , it
  should be kept as low as possible, since it
  determines the amount of power that can be
  transferred to the succeeding stages of the
  instrumentation system.

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Useful Output Range

• The output range of a transducer is limited at
  the lower end by noise signal.
• The upper limit is set by the maximum useful
  input level.

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Factors Influencing the choice of Transducer.

1. Operating Principle
2. Sensitivity
3. Operating Range
4. Accuracy
5. Cross sensitivity
6. Errors
7. Transient and frequency response

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Conti..

1. Loading effects.
2. Environmental compatibility
3. Insensitivity to unwanted signals
4. Usage and Ruggedness
5. Electrical aspects
6. Stability and Reliability
7. Static characteristics.

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• Operating Principle The transducer are many
  times selected on the basis of operating
  principle used by them. The operating principle
  used may be resistive, inductive, capacitive ,
  optoelectronic, piezo electric etc.
• Sensitivity The transducer must be sensitive
  enough to produce detectable output.
• Operating Range The transducer should maintain
  the range requirement and have a good resolution
  over the entire range.

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• Accuracy High accuracy is assured.
• Cross sensitivity It has to be taken into
  account when measuring mechanical quantities.
  There are situation where the actual quantity is
  being measured is in one plane and the transducer
  is subjected to variation in another plan.
• Errors The transducer should maintain the
  expected input-output relationship as described
  by the transfer function so as to avoid errors.

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1. Transient and frequency response The transducer
   should meet the desired time domain specification
   like peak overshoot, rise time, setting time and
   small dynamic error.
2. Loading Effects The transducer should have a
   high input impedance and low output impedance to
   avoid loading effects.

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1. Environmental Compatibility It should be assured
   that the transducer selected to work under
   specified environmental conditions maintains its
   input- output relationship and does not break
   down.
2. Insensitivity to unwanted signals The transducer
   should be minimally sensitive to unwanted signals
   and highly sensitive to desired signals.

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1. Usage and Ruggedness The ruggedness both of
   mechanical and electrical intensities of
   transducer versus its size and weight must be
   considered while selecting a suitable transducer.
2. Electrical aspects The electrical aspects that
   need consideration while selecting a transducer
   include the length and type of cable required.
3. Stability and Reliability The transducer should
   exhibit a high degree of stability to be
   operative during its operation and storage life.

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1. Static Characteristics Apart from low static
   error, the transducer should have a low non-
   linearity, low hysteresis, high resolution and a
   high degree of repeatability.

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Resistive Transducers
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• Any method of varying one of the quantities
  involved in the above relationship can be the
  design basis of an electrical resistive
  transducer.
• The translational and rotational potentiometers
  which work on the basis of change in the value of
  resistance with change in length of the conductor
  can be used for measurement of translational or
  rotary displacement.

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• Strain gauge work on the principle that the
  resistance of the conductor or a semiconductor
  changes when strained. This property can be used
  for measurement of displacement, force and
  pressure.
• The resistivity of the material changes with
  change of temperature thus causing a change of
  resistance. This property may be used for
  measurement of temperature.

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Potentiometers

• POT
• Resistive potentiometer used for the purposes of
  voltage division is called POT.
• Resistive potentiometer consist of a resistive
  element provided with a sliding contact.
• Sliding Contact-Wiper

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POT

• Its a Passive Transducer.
• Linear Pot Translational Motion
• Rotary Pot-Rotational Motion
• Helipots- Combination of the two motions
  (translational as well as rotational).
• In Electrical Measurement , Standard
  potentiometer are used to measure the unknown
  voltage by comparing it with a standard known
  voltage.

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Resistive potentiometer
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Translational, rotational and helipots
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• Consider a translational potentiometer
• If the distribution of the resistance with
  respect to translational movement is linear, the
  resistance per length is

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• The output voltage under ideal conditions is
• For Rotational Motion

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Strain Gauges

• If a metal conductor is stretched or compressed
  , its resistance changes on account of the fact
  that both length and diameter of conductor
  change.
• Also there is a change in the value of
  resistivity of the conductor when strained and
  this property is called piezoresistive effect.
• Resistive strain gauges are also known as
  piezoresistive gauges.

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Let a tensile stress s be applied to the wire.
Divide equation (2) by
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• From (3) , per unit change in resistance is due
  to
• Per unit change in length
• Per unit change in Area
• Per unit change in resistivity
• Area

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Equation (3) can be written as
Poissons ratio ,
Or
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For small variation , the above relationship ,
can be written as
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The gauge factor is defined as the ratio of per
unit change in resistance to per unit change in
length.
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Resistance change due to change in length
Resistance change due to change in area
Resistance change due to change in piezoresistive
effect.
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Types of strain gauges

• Unbonded metal strain gauge
• Bonded metal wire strain gauge
• Bonded metal foil strain gauge
• Vacuum deposited thin metal film strain gauges.
• Sputter deposited thin metal film strain gauge.
• Bonded semiconductor strain gauges.
• Diffused metal strain gauge.

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Unbonded metal strain gauge
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• Used almost exclusively in transducer
  applications.
• At initial preload , the strains and resistances
  of the four arms are normally equal, with the
  result the output voltage of the bridge, e00.
• Application of pressure produces a small
  displacement , the displacement increases
  tension in 2 wires and decreasing the resistance
  of the remaining 2 wires.
• This causes an unbalance of the bridge producing
  an output voltage which is proportional to the
  input displacement and hence to the applied
  pressure.

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Bonded metal wire strain gauge
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• It consist of a grid of fine resistance wire of
  diameter of about 0.025mm.
• The wire is cemented to a base.
• The base thin sheet of paper or bakelite.
• Wire is covered with a thin sheet of material so
  that it is not damaged mechanically.
• The spreading of wire permits a uniform
  distribution of stress over a grid.

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Bonded metal foil strain gauge
Extension of the bonded metal wire strain
gauge. The bonded metal wire strain gauge have
been completely superseded by bonded foil strain
gauge.
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Metal foil strain gauge
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Semiconductor strain gauge.
The semiconductor strain gauge depends for their
action upon piezo resistive effect. i.e. the
change in the value of the resistance due to
change in resistivity.
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Rosettes
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Resistance Thermometers

• The resistance of the conductor changes when its
  temperature is changed. This property is utilized
  for measurement of temperature.
• The variation of resistance R with temperature
  T(ok) can be represented by the following
  relationship for most of the metals as
• RR0(1?1T ?2T2 ?nTn)
• Where R0resistance at temperature T0 and
• ?1 ,?2,?n are constants.
• Platinum as it can withstand high temperatures
  while maintaining excellent stability.

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Requirements of a conductor material to be used
in RTDs are

• The change in resistance of material per unit
  change in temperature should be as large as
  possible.
• The material should have a high value of
  resistivity so that minimum volume of material is
  used for the construction of RTD.
• The resistance of material should have a
  continuous and stable relationship with
  temperature.

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Thermistors

• Contraction of a term thermal resistors
• Its composed of semiconductor materials.
• Used in applications which involve measurements
  in the range of -60oC to 15oC
• The resistance of thermistors ranges from 0.5? to
  0.75 M?

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Thermistors
Composed of sintered mixture of metallic oxides
such as manganese, nickel , cobalt, copper, iron
and uranium.
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Thermocouple

• When two metals having different work functions
  are placed together, a voltage is generated at
  the junction which is nearly proportional to the
  temperature. This junction is called a
  thermocouple.

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Variable Inductance Transducer

• Change in self Inductance
• Change in Mutual Inductance.
• Production of eddy currents.

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Transducers working on principle of change of
Self Inductance.

• Self inductance of a coil

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Transducers working on principle of change of
Mutual Inductance.

• Uses multiple coils.
• The mutual inductance between two coils is

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Transducers working on principle of production of
eddy currents

• If a conducting plate is placed near a coil
  carrying alternating current, eddy currents are
  produced in the conducting plate.

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Linear Variable Differential Transformer (LVDT)

• The transformer consists of single primary
  winding P and two secondary windings S1 and S2
  wound on a cylindrical former.
• The secondary windings have equal number of turns
  and are identically placed on either side of the
  primary winding.
• The primary winding is connected to an
  alternating current source.

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LVDT
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• A movable soft iron is placed inside the former.
• The displacement to be measured is applied to the
  arm attached to the soft iron core.
• Since the primary winding is excited by an
  alternating magnetic field which in turn induces
  alternating current voltages in the secondary
  windings.
• The output voltage of secondary , S1is Es1 and
  that of secondary, S2is Es2

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Capacitive Transducer

• The Principle of operation of capacitive
  transducer is based upon the familiar equation
  for capacitance of a parallel plate capacitor.

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