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Analog Sensors for Motion Measurement

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Title: Analog Sensors for Motion Measurement


1
Analog Sensors for Motion Measurement
  • Presented By
  • Pratyush Singh
  • Naga. C. Pemmaraju
  • Sandeep Panjala
  • Sreenivas
  • Prasanth Bandi
  • March 20, 2006.

2
CONTENT
  • Introduction
  • Motion Transducers
  • Variable Inductance Transducers
  • Permanent Magnet Transducers
  • Eddy Current Transducers
  • Variable Capacitance Transducers
  • Piezoelectric Transducers
  • Other types of Transducers
  • Design Criterion for Control System
  • Conclusion

3
INTRODUCTION
  • Measurement of plant outputs and feedback
    signals is very important
  • Sub measurement system are used for this purpose
    and are made up of transducers and sensors and
    they are used in feedback systems
  • TRANSDUCER Transducer is a device, that converts
    one type of energy to another.
  • Transducers are used for compensation in
    different plants and systems.
  • Help in reducing the sensitivity of a system to
    parameter change.

4
Motion transducers
  • Motion here means the four kinematics variables
  • Displacement
  • Velocity
  • Acceleration
  • Jerk
  • Note That each variable is a time derivation of
    the preceding one.
  • Motion measurement is extremely important for
    systems or plant proper functioning
  • The basic working principle of motion transducer
    have been discussed with illustration examples.

5
Basic Inductive Displacement Sensor.
  • Uses the simple principle of mutual inductance.
  • AC input is applied to coil A output is
    measured across B.
  • Movement of the ferromagnetic core results in
    inductance from A to B

6
Linear Variable Differential Transformer.
  • Used for measurements in the range of mm or cm.
  • Supply is provided to the middle coil and output
    is taken across the other two coils.
  • Differential amplifier is used to measure the
    differential output from the windings.

7
Moire Fringe Detector
  • It converts Light energy to electricity to
    measure displacement.
  • Uses two slides with identical patterns and a
    photocell for the measurement.
  • Displacement is measured as the amount of light
    passing through the slides changes

8
Working principle of Accelerometers
  • Based on the Principle of F Ma.
  • Basic concept of an accelerometer has been shown
    in the diagram.
  • Acceleration is measured by calculating the
    amount of force applied to the restoration
    spring.

9
Magnetic Drag For Car Speedometers
  • Most commonly used technique for speedometers in
    cars.
  • Based on the principal of magnetic inductance.
  • A magnet is connected to the moving shaft and it
    induces a torque in the stationary disc.
  • This torque is measured to compute the speed of
    the cars.

10
VARIABLE-INDUCTANCE TRANSDUCERS
  • Motion transducers that employ the principle of
    electromagnetic induction are termed
    variable-inductance transducers.
  • BASIC PRINCIPLE When the flux linkage through
    an electrical conductor changes, a voltage is
    induced in the conductor. This in turn ,generates
    a magnetic field that opposes the primary field.
    If the change in flux linkage is brought about by
    a relative motion ,the mechanical energy is
    converted in to electrical energy.
  • DIFFERENT TYPES
  • 1.Mutual-induction transducers
  • 2.self-induction transducers
  • 3.permanent-magnet transducers

11
Mutual-induction transducers and Differential
Transformers.
  • The basic arrangement of a Mutual-induction
    transducer constitutes two coils, the primary and
    secondary winding. one of the coils carries an AC
    excitation that induces a steady AC voltage in
    the other coil
  • In Mutual-induction transducers, a change in the
    flux linkage is effected by one of the two common
    techniques. one technique is to move an object
    made of ferromagnetic material within the flux
    path.
  • The other common way to change the flux linkage
    is to move one coil with respect to the other.

12
LINEAR VARIABLE DIFFERENTIAL TRANSFORMER(LVDT)
  • AN LVDT transducer comprises a coil former on to
    which three coils are wound.
  • The primary coil is excited with an AC current,
    the secondary coils are wound such that when a
    ferrite core is in the central linear position,
    an equal voltage is induced in to each coil.
  • The secondary are connected in opposite so that
    in the central position the outputs of the
    secondary cancels each other out.

13
LVDT contd
  • The excitation is applied to the primary winding
    and the armature assists the induction of current
    in to secondary coils.
  • When the armature is in the central position
    there is an equal voltage induced in to both
    secondary coils. The sum of secondary outputs
    cancels each other out resulting in a zero
    output.
  • As the armature moves in to sec.1,the result is
    that sum of sec1 and sec2 favors sec1.
  • As the armature moves in to sec2,the sum favors
    sec2.
  • The out put is an AC waveform .

14
Contd
  • Signal conditioning associated with differential
    transformers includes filtering and amplification

15
POTIENTIOMETERS
  • The potentiometer is a displacement transducer.
  • The active transducer consists of resistive
    material whose resistance is proportional to its
    length.
  • A fixed voltage is applied across the coil using
    an external voltage supply.
  • The output signal is the DC voltage between the
    movable contact sliding on the terminal of the
    coil.
  • Slider displacement x is proportional to the
    output voltage

16
Self Induction Transducers
  • Based on the principle of self induction.
  • Only a single coil is employed.
  • Self Induction transducers are usually
    variable-reluctance devices.
  • This can be used as a displacement sensor.

17
Self Induction Transducers Contd..
  • Self
    Induction proximity sensor

18
Permanent Magnet Transducers
  • A permanent magnet is used to generate a uniform
    and steady magnetic field.
  • Permanent magnet transducers are used in
    measuring speed.
  • Two types of speed are measured.
  • Rectilinear speed
  • Rectilinear velocity transducer is used to
    measure rectilinear speed.
  • Angular speed.
  • DC tachometer-generator and AC
    tachometer-generator are used in measuring
    angular speed.

19
Permanent Magnet Transducers Contd..
  • Rectilinear velocity transducer

20
Permanent Magnet Transducers Contd..
  • DC Tachometer-generator

21
Permanent Magnet Transducers Contd..
  • AC Permanent magnet Tachometers
  • Contains a permanent magnet rotor and 2 sets of
    stator windings.
  • One set of stator windings is energized using AC
    reference voltage and the other set of windings
    is used to measure output.
  • When
  • Rotor is stationary gt output is a constant
    voltage.
  • Rotor is moving at finite speed gt induced
    voltage proportional to speed is generated at
    secondary winding.

22
Permanent Magnet Transducers Contd..
  • AC Induction Tachometers
  • Similar in construction to two-phase induction
    motors.
  • Stator arrangement is similar to AC permanent
    magnet tachometer and rotor windings are shorted
    and not energized by any external source.
  • Voltage is induced in rotor windings which has 2
    components, one due to the transformer action of
    AC supply and the other due to the speed of
    rotation of rotor.
  • Voltage induced in the output stator windings is
    due to both the primary stator winding and rotor
    winding.

23
Eddy Current Transducers
  • Eddy current
  • The current generated when a conducting medium
    is subjected to fluctuating magnetic field.
  • Eddy currents are circular induced currents
    generated by an AC current in the nearby coil.
  • Eddy currents generate their own magnetic fields.
  • This principle is used in Eddy current proximity
    sensor.

24
Eddy Current Proximity Sensor
25
Eddy Current Proximity Sensor Contd..
  • Schematic diagram of eddy current proximity sensor

26
Variable Capacitance Transducer
  • VARIABLE CAPACITOR
  • Capacitance may be changed either mechanically
    or electronically.
  • Capacitance C KA/x.
  • The variable capacitance used to convert a
    physical phenomena into electrical signals is
    called a Variable Capacitance Transducer.
  • Example capacitor microphone.

27
Variable Capacitance Transducer Contd..
  • Capacitance transducers are commonly used to
    measure small transverse displacements, large
    rotations, fluid levels and angular velocities.
  • Capacitive rotation sensor
  • The angular displacement of one of the plates
    causes a change in A.

28
Variable Capacitance Transducer Contd..
  • Capacitive Displacement Sensor In this a
    transverse displacement of one of the plates
    changes x.

29
Variable Capacitance Transducer Contd..
  • Capacitive Liquid level sensor
  • A change in k is produced as the fluid level
    between the capacitor plates changes.
  • The advantage of capacitance transducer is
    negligible loading effects.
  • The capacitance bridge compensates the errors.

30
Piezoelectric Transducers
  • Piezoelectricity is the electricity produced by
    applying pressure.
  • Piezoelectric materials Barium titanate, single
    crystal quartz.
  • Piezoelectric Effect
  • When mechanical stress or strain is applied to
    the piezoelectric material, generates an electric
    charge and associated potential difference.
  • The direct application of piezoelectric effect is
    used in pressure and strain measuring devices

31
Piezoelectric Sensor
  • It may be represented as a charge source with a
    series capacitive impedance Z.
  • Z 1/jwc
  • Piezoelectric sensors have a limitation on the
    useful lower frequency.

32
Piezoelectric accelerometer
  • Accelerometers are acceleration measuring
    devices.
  • The piezoelectric accelerometer is a
    piezoelectric motion transducer.
  • It is based on d Alemberts principle which
    states that
  • If a force of magnitude Ma were applied to the
    accelerating mass in the direction opposing the
    acceleration, then the system could be analyzed
    using static equilibrium considerations.
  • Advantage Light weight and High frequency
    response.

33
Piezoelectric accelerometer Contd
  • Piezoelectric velocity transducer
  • It uses piezoelectric accelerometer and an
    integrating amplifier along with impedance
    matching amplifier.
  • Piezoelectric displacement transducer
  • It is obtained by using a double integration of
    piezoelectric accelerometer.

34
OTHERS TYPES OF SENSOR
  • Fiber optic sensors and lasers
  • Fiber optic Position sensor
  • Fiber Optic Gyroscope
  • Ultrasonic Sensors
  • Gyroscopic sensors

35
Fiber optic sensors and lasers
  • The Characteristic component in a fiber optic
    sensor is a bundle of glass fiber (Typically a
    few hundred ) that can carry light.
  • Basically Two types fiber optic sensors
  • Indirect type the optical fiber acts only as
    the medium in which sensed light is transmitted
    . In this type the sensing element itself does
    not consists of optical fibers. Ex Fiber Optic
    motion sensor and Tactile sensor .
  • Direct type The optical fiber bundle acts as
    sensing element. When the conditions of the
    sensed medium change, the light propagation
    properties of the optical fiber change, providing
    a measurement of the change in conditions. Ex
    Fiber optic gyroscope and fiber optic hydrophones

36
Fiber optic Position sensor
  • The Optical fiber bundle is divided into two
    groups.
  • Transmitting fiber
  • Receiving fiber
  • Light from the light source is transmitted along
    the first bundle of fibers to the target object
    whose position is being measured. Light reflected
    onto the receiving fibers by the surface of the
    target object is carried to a photo detector. The
    intensity of the light received by the photo
    detector will depend on position x of the target
    object.

37
Fiber Optic Gyroscope
  • This is an angular speed sensor that uses fiber
    optics.
  • Two loops of optical fibers wrapped around a
    cylinder are used in this sensor.
  • One loop carries a monochromatic light (or laser)
    beam in the clockwise direction, Other loop
    carries a beam from the same source in counter
    clockwise direction. Since the laser beam
    traveling in the direction of rotation of the
    cylinder has a higher frequency than that of the
    other beam, the difference in the frequencies of
    the two lasers beams received at a common
    location will measure the angular speed of the
    cylinder. Angular displacement can be measured
    with the same sensor simply by counting the
    number of cycles and clocking fractions of the
    cycles. Acceleration can be determined by
    digitally determining the rate of change of speed.

38
Ultrasonic Sensors
  • In distance measurement using ultrasonic, aburst
    of ultrasound is projected at the target object,
    and the time taken for the echo to be recived is
    clocked. A signal processor computes the position
    of the target object ,possibly compensating for
    environmental conditions . alternatively velocity
    of the target object can also be measured, using
    the Doppler effect, by measuring the change in
    frequency between the transmitted wave and the
    received wave.

39
DESIGN CRITERION FOR CONTROL SYSTEM
  • Accuracy is affected by parameter changes in the
    control system components and by the influence of
    external disturbances
  • Consider general feedback control system to
    parameter changes and to external disturbances.
  • GP(s) Transfer function of the plant ( Of the
    system to be controlled)
  • G c(s) Transfer function of the controller (
    Including Compensators)
  • H(s) Transfer function of the output feedback
    system ( Including the measurement system)
  • u System input command
  • ud External disturbance input
  • y system output

40
Contd..
  • After analyzing the feedback back control
    system we can stipulate the fallowing design
    criterion for the system.
  • Make the measurement system (H) very accurate and
    stable
  • Increase the loop gain to reduce the sensitivity
    of the control system to changes in the plant and
    controller .
  • Increase the gain of Gc H to reduce the
    influence of external disturbances

41
Conclusion
  • Analog Transducers play a very important part in
    insuring proper functioning of the systems.
  • They are simple, User friendly and reliable.
  • But with increase in complexity and need for
    accuracy in modern day plants use of analog
    transducers is very limited. Newly developed
    Digital and Optical Transducers are more apt for
    use in these plants.
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