Analog Sensors for Motion Measurement

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Analog Sensors for Motion Measurement

• Presented By
• Pratyush Singh
• Naga. C. Pemmaraju
• Sandeep Panjala
• Sreenivas
• Prasanth Bandi
• March 20, 2006.

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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

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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.

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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.

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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

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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.

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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

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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.

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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.

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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

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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.

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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.

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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 .

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Contd

• Signal conditioning associated with differential
  transformers includes filtering and amplification

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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

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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.

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Self Induction Transducers Contd..

• Self
  Induction proximity sensor

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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.

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Permanent Magnet Transducers Contd..

• Rectilinear velocity transducer

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Permanent Magnet Transducers Contd..

• DC Tachometer-generator

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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.

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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.

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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.

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Eddy Current Proximity Sensor
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Eddy Current Proximity Sensor Contd..

• Schematic diagram of eddy current proximity sensor

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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.

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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.

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Variable Capacitance Transducer Contd..

• Capacitive Displacement Sensor In this a
  transverse displacement of one of the plates
  changes x.

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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.

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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

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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.

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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.

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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.

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OTHERS TYPES OF SENSOR

• Fiber optic sensors and lasers
• Fiber optic Position sensor
• Fiber Optic Gyroscope
• Ultrasonic Sensors
• Gyroscopic sensors

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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

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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.

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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.

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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.

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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

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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

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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.