Transducers

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					Transducers
     ME 3251
Thermal Fluid Systems


                        1
             Transducers
• Transform values of physical variables into
  equivalent electrical signals
• Converts a signal from one form to
  another form




                                            2
         Types of Transducers
• Variable-Resistance        • Photovoltaic Transducer
  Transducer                 • Ionization Transducer
• Differential Transformer   • Magnetometer Search
  (LVDT)                       Coil
• Capacitive Transducer      • Hall-Effect Transducer
• Piezoelectric Transducer   • Digital Displacement
• Photoelectric Transducer     Transducer
• Photoconductive
  Transducer



                                                         3
Variable-Resistance Transducer
• Resistance Potentiometer, rheostat
• Converts linear or angular displacement into an electric
  signal
• Can be used to convert force and pressure to a
  displacement through mechanical methods
• Moving contact on a slide-wire or moving contact that
  moves through an angular displacement on a solid
  conductor (e.g. graphite)
• Fairly cheap – some can be expensive, though




                                                             4
    Differential Transformer (LVDT)
•   Coverts displacement into electric signal
•   Three coils placed in a linear relationship with a magnetic core free
    to move inside the coils
•   Alternating input voltage impressed in center coil, and output voltage
    from two end coils depends on magnetic coupling between the core
    and coils
•   The coupling is dependent on the position of the core
•   If the core remains near the center of the coils the output is very
    nearly linear.
•   Most differential transformers are only operated in this linear range,
    and these are called linear variable differential transducers (LVDT)




                                                                         5
LVDT Continued




                 6
              Capacitive Transducer
•   Measures a change in capacitance through a change in distance between 2 plates, d
•   Measures capacitance change through a change in overlapping area, A, resulting
    from plate movement or a change in dielectric constant, ε
                    A
    C = 0.225 ⋅ ε ⋅
                                                                 2
•                          0.225 is   in 2   use 0.0885 for cm
                    d
    ε = 1 for air
    ε = 1 for plastics




                                                                                    7
     Capacitive Transducer cont.
•   For liquid level measurements two electrodes are placed in the
    liquid, and the dielectric constant varies between them according to
    the liquid level
•   Thus, capacitance between electrodes is a direct indication of liquid
    level




                                                                            8
           Piezoelectric Transducer
•   A force applied to a piezoelectric crystal
    between two electrodes produces a stress
    and corresponding deformation in the
    crystal
•   Deformation produces potential difference
    at crystal surface
•   The stress on the crystal produces an
    output voltage:
                      E = gtp

    where t is crystal thickness in m
    p is impressed force in N
                             m2

    g is voltage sensitivity        d
                               g=
                                    ε
•   Piezoelectric crystals used as pressure
    transducers for dynamic measurements

                                                 9
         Photoelectric Transducer
•   Converts a light beam into a usable electric signal
•   Light strikes cathode and releases electrons
•   Electrons are attracted toward anode, thereby producing an electric
    current in external circuit
•   Cathode and anode enclosed in glass or quartz, which is evacuated
    or filled with inert gas
•   Photoelectric tubes useful for measuring light intensity




                                                                      10
  Photoconductive Transducers
• Voltage impressed on semiconductor material
• Light strikes semiconductor material and a decrease in
  resistance occurs, thereby producing an increase in
  current
• Useful for measurement of radiation at all wavelengths




                                                           11
      Photovoltaic Transducers
• Sandwich design of a metal base plate and a thin
  transparent metallic layer with a semiconductor material
  in between
• Light strikes barrier between transparent metal layer and
  semiconductor material, and a voltage is generated
• Most widely used application of photovoltaic cell is the
  light exposure meter in photographic work




                                                          12
         Ionization Transducer
• Converts displacement to a voltage through a
  capacitance change
• As tube moves horizontally between the two plates, a dc
  potential difference is created between electrodes 1 and
  2




                                                         13
       Magnetometer Search Coil
•   Transforms a magnetic
    field signal into a voltage
•   A flat coil with a
    specified number of
    turns is placed in a
    magnetic field, and a
    voltage is generated
•   To measure a steady
    magnetic field, the
    search coil must be
    moving. Typically a
    rotating coil is used in
    this situation.


                                  14
          Hall-Effect Transducers
•   An external current passes
    through a semiconductor
    plate.
•   When a magnetic field is
    impressed on a plate in a
    direction perpendicular to
    the plate surface, a voltage
    is generated
•   This voltage is the Hall
    voltage
• E = K ⋅ IB
   H   H
               t
    where K H is the Hall
    coefficient

                                    15
    Digital Displacement Transducers
•   Used to measure angular and linear displacements
•   For an angular measurement, a wheel rotates and light is alternately
    transmitted and stopped through small cutouts
•   The signal is sent to the photodetector, amplified, then sent to a
    counter.
•   The number of counts is proportional to angular displacement.
•   Sensitivity can be improved by increasing the number of cutouts




                                                                      16
 Digital Displacement Transducers
• For a linear measurement, small reflecting strips are
  installed on a motion device.
• Light is then alternately reflected and absorbed with
  linear motion presenting a signal to the photodetector
• Calibration with a known displacement standard must be
  performed




                                                       17

				
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Description: Capacitive sensors are often known as the capacitance level meter, capacitance level meter capacitive sensing element is a cylindrical capacitor according to the principle of work, by the two capacitor plates insulated coaxial cylindrical inner and outer electrodes electrodes, between the two cylinders filled with a dielectric constant e of the electrolyte, the capacitance between two cylinders for the C = 2ΠeL/lnD/d, where L is the overlapping part of two tube length; D electrode diameter for the outer tube; d is the diameter of the inner cylinder electrode; e is the dielectric constant of the intermediate medium. In the actual measurement D, d, e is basically unchanged, so you can know the level measured at the level of C, which is easy to use with capacitive sensors, a simple structure and high sensitivity, low price and so one of the reasons .