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15731_Sensors and Transducers

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					SENSORS AND TRANSDUCERS




    ELE 101
                    Transducer
   A transducer is a device that converts one type of
    energy to another.
    Energy types include electrical, mechanical, light,
    chemical, acoustic or thermal energy.
   While the term transducer commonly implies the use
    of a sensor/detector, any device which converts
    energy can be considered a transducer.
   Transducers are widely used in measuring
    instruments.
   Example of Transducer




Microphone - a transducer which converts sound to voltage.

Pre-Amplifier - amplifies the small audio signal (voltage) from the microphone.

Tone and Volume Controls - Adjust the nature of the audio signal. (Frequency,
  Strength)

Power Amplifier - increases the strength (power) of the audio signal.

Loudspeaker - a transducer which converts the audio signal to sound.
Classification of Transducer
   Based on the physical phenomenon:
    Primary transducer, Secondary transducer.
   Based on the power type Classification:
    Active transducer, Passive transducer
   Based on the type of output:
    Analog transducer, Digital transducer
   Based on the electrical phenomenon:
    Resistive transducer, Capacitive transducer, Inductive
    transducer, Piezo-electric transducer
Primary & Secondary Transducers
   Primary – Converts a Force or Pressure into
    mechanical displacement.
   Secondary- Converts output of primary transducer
    into electrical voltage.
Active & Passive transducer
   Active – Also known as self generating type,
    develop their own voltage or current from the
    physical phenomenon being measured.
    Example- Piezoelectric transducer.
   Passive - Also Known as externally powered
    transducers, i.e., derive the power required for
    energy conversion from an external power source.
    Example- POT (Potentiometer) used to measure displacement.
Analog & Digital Transducer
   Analog - It converts the input quantity into an
    analog output.
     E.g. LVDT etc
   Digital - Converts input quantity into an electrical
    output which is in the digital form.
   E.g. Digital Weight Machine.
Electrical phenomenon or Principle of
Transduction
   Resistive
   Inductive
   Capacitive etc
      Depending upon how they convert the input
    quantity into resistance, inductance or capacitance
    respectively.
    Introduction to Sensors
   A sensor is a device that produces a measurable response to
    a change in a physical condition, such as temperature or
    thermal conductivity, or to a change in chemical
    concentration.

   Sensors are particularly useful for making     measurements
    such as in industrial process control.

   Sensing is a technique used to gather information about a
    physical object or process, Including the occurrence of events
    (i.e., changes in state such as a drop in temperature or
    pressure).
General Sensing System
Types of Sensors
   Accelerometer is an electromechanical transducer
    which produces at its output terminals, a voltage or
    charge that is proportional to the acceleration to
    which it is subjected. (e.g. RPM)
   Chemical / Gas Concentration sensors for detection
    concentration levels of chemicals and gasses. These
    sensors are critical for safety considerations in many
    industrial applications.
Contd. .
 Force Sensors , which detect the force and converts
  it into electrical energy.
   e.g. piezo-electric crystal, strain gauge etc.
 Sound Sensors, A sensor for detecting sound.

   e.g. Microphones etc.
 Temperature Sensors, which senses the temperature
  from the physical world and converts into an
  electrical form.
 e.g. Air Conditioner, Auto cut Gysers, Iron etc.
Types of Sensors
Classification of Sensor
Resistive Transducer
   In such a transducer resistance between the output
    terminals get changed according to the measurand.
    The resistance of a metal conductor is expressed by
    simple equation :
Devices Based on Resistive Transduction Principle

   Potentiometer resistance Device
   Strain Gauge
   Thermistor
   Wheatstone Bridge
 Potentiometer resistance Device




Here Length changes with sliding of wiper.
Strain Gauge
Thermistors
   It has negative temperature co-efficient.
   With the change in temperature, resistivity changes
    and with the change in resistivity, resistance of the
    material changed.
   With the increase in temperature, resistivity
    decreases.
Resistivity-Temperature characteristics
Wheatstone Bridge
    Contd. .

   It is a simple circuit that can be used to convert a
    physical property into an observable electric effect. In
    this bridge, R1, R2, and R3 are resistors of known
    resistance (where the resistance of R2 is adjustable)
    and Rx is a resistor of unknown value. If the ratio
    R2/R1 is identical to the ratio Rx/R3, the measured
    voltage VOUT will be zero. However, if the resistance
    of Rx changes (e.g., due to changes in temperature),
    there will be an imbalance, which will be reflected by
    a change in voltage VOUT.
  Capacitive Transducers

   The Principle of operation of capacitive transducer is based
   upon the expression of capacitance of parallel plate
   capacitor

Where, A = Area of Plates
       d = Distance between two parallel plates
         = Permittivity
Devices Based on Capacitive
Transduction Principle
   Pressure Gauge
Contd. .
   Capacitive Microphone
Inductive Transducers
    Inductive Transducers are passive transducers
   Works on following principles:
   Variation of Self-inductance of coil.
   Variation of Mutual-inductance of coil.
Variation of Self-inductance of coil
Change in Self Inductance

   By change in any 3 of the Equation parameters i.e.
    ‘N’, Permeability or Geometric Factor(A,lenght), Self
    inductance will vary.

   Examples: Automatic Car Wash, Traffic Light Signal,
    Metal Detectors
By change in Mutual Inductance




 Example: LVDT, As shown in above diagram.
Graph for LVDT
Metal detector

				
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