Velocity Measurement

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					     Ch. 11

Velocity Measurement




                       1
             Applications
• Measuring the approach speed of a
  robotic tool onto its target.
• Monitoring the speed of a generator in an
  electric power station.
• Measuring an automobile’s wheel speed in
  order to provide feedback to an antilock
  brake system.


                                          2
Measurement of Linear Velocity




                             3
    Measurement of Linear Velocity
•   Average speed is:
                                      y 2 − y1 ∆y
                             Vavg =            =
                                      t 2 − t1   ∆t
•   As the time interval becomes small, the average speed becomes
    the instantaneous speed Vy,
                                            ∆y dy
                             V y = lim         =
                                      ∆t →0 ∆t   dt

                                            t
                             V y (t ) = Vi − a y (t )dt
                                            ti
    where ay(t) is the acceleration in the y direction


                                                                    4
Reference-Based Measurement
• Velocity = displacement / time taken.
• To measure the displacement, there are 2
  pickups by displacement sensors.
• Measuring the time interval with an
  electronic counter or displaying the output
  of the pickups from displacement sensors
  on an oscilloscope.


                                                5
Reference-Based Measurement




       •   Velocity transducers (LVT)
                                        6
Reference-Based Measurement

                    ei ∝ BLV
where ei = induced voltage
      B = magnetic flux density
      L = length of wire in the coil
      V = speed of the coil relative to the magnet




                                                     7
              Doppler Shift
• When the source and observer are in
  motion relative to each other, there is
  Doppler Shift.
• It is applicable to waves, e.g. sound, light,
  microwaves, etc.
• Application: radar.



                                                  8
            VISAR System
• Velocity Interference System for Any
  Reflector
• Can be used with either specularly or
  diffusely reflecting surfaces, and is quite
  insensitive to tilting of the target.
• It was developed for shock wave research
  work
• Useful for measurement of very high
  speeds.
                                                9
VISAR System




      •   Illustrate how fiber optic
          components, available from
          Valyn, can guide laser light to
          and from a shock experiment,
          minimizing any laser light
          beam hazards.
                                        10
 Angular Velocity Measurement
• Often applied to rotating machinery such
  as pumps, engines, and generators.
• Most familiar unit: revolutions per minute
  (rpm)




                                               11
 Electrical (dc and ac) Tachometer
              Generator
• A rotating generator produces a voltage signal
  proportional to the rotational velocity of the input shaft.




          •   Permanent-magnet dc tach-generator
                                                                12
                  Counter Types
• Rotating Magnet Sensors—passive speed sensors
  convert mechanical motion to ac voltage without an
  external power source. These self-contained magnetic
  sensors produce a magnetic field that, when in the
  proximity of ferrous objects in motion, generates a
  voltage.
• Applications for these types of sensors:
   –   Transmission speed
   –   Engine rpm
   –   Pump shaft speed
   –   Computer peripheral speeds



                                                         13
           Counter Types




•   Magnetic speed sensor output voltage against speed
                                                         14
            Optical Sensors




•   A slotted disk provides one pulse output for each rotation
                                                                 15
                        Stroboscope
• An oscillator produces a pulse wave of a
  known frequency. This is then used to
  drive a bright LED, which can cope with
  the fast rate of flashing.
• Note: a bulb cannot be used since when it
  is driven at a high frequency, the filament
  remains hot when the power goes off, and
  the light that is not flashing at all, but is
  permanently on.
(http://homepages.which.net/~paul.hills/Circuits/Stroboscope/Stroboscope.html)
                                                                            16
                Stroboscope
• A mark is made on the object.
• If the rotational velocity ω of the object is not
  matched with the frequency f of the oscillator,
  random appearance of the mark is seen.




                                                      17
              Stroboscope
• If ω = nf (where n=1, 2…) , the mark becomes
  stationary.




                                                 18
                Stroboscope
• If ω is slightly lower than nf (where n=1, 2…) , the
  mark creeps forward .




                                                    19
                Stroboscope
• If ω is slightly higher than nf (where n=1, 2…) ,
  the mark creeps backward .




                                                      20
               Hall Effect
• It describes the potential difference that
  develops across the width of a current-
  carrying conductor.
• Application—the wheel speed sensor for
  antilock braking systems in automobiles.




                                               21
 Hall Effect (negative charge
           carriers)




(http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/hall.html)
                                                                  22
                 Hall Effect
• If a current flows through a conductor in a
  magnetic field, the magnetic field will exert a
  lateral force on the moving charge carriers.
• A buildup of charge at the sides of the conductor
  will balance this magnetic influence, producing a
  measurable voltage between the two sides of
  the conductor. This measurable lateral voltage is
  called the Hall effect.


                                                 23
                 Hall Effect
                         IB
                   VH =
                        ned

             n = density of mobile charges
             e = electron charge

• The Hall effect can be used to measure
  magnetic fields with a Hall probe.


                                             24
Hall Effect (positive charge carriers)




    (http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/hall.html)
                                                                      25
                  Hall Effect
• Fm = evdB, where vd is the drift velocity of the
   charge.
• I = neAvd
                        eIB
                   Fm =
                        neA
 • In equilibrium,
                            eIB
                  Fm = Fe =
                            neA

                           IB
                     VH =
                          ned                        26
Hall Effect Sensors




 •   Two magnet hall sensor
                              27
Hall Effect Sensors




 •   Hall-effect gear tooth sensor
                                     28
Hall Effect Sensors




•   Hall-effect gear tooth sensor circuit
                                            29
            Wiegand Effect
• It employs unique magnetic properties of
  specially processed, small-diameter
  ferromagnetic wire.
• By causing the magnetic field of this wire
  to suddenly reverse, a sharp, uniform
  voltage pulse is generated.
• Wiegand pulse.


                                               30
             Wiegand Effect
• It is useful for proximity sensing,
  tachometry, rotary shaft encoding, and
  speed sensing.
• Application:
  – Electronic indexing for water, gas, and electric
    meters.
  – Measuring shaft speed in engines.
  – Tachometers, speedometers, and other
    rotational counting devices.

                                                  31
        Angular Rate Sensors—
             Gyroscopes
• Many absolute angular rate-measuring devices
  fall under the designation of gyroscope.
• It consists of a spinning mass mounted on a
  base so that its axis can turn freely in one or
  more directions.
• Angular velocity gyros are used to measure
  motion and as signal inputs to stabilization
  systems.
• Rate-integrating gyros are used as the basis
  for highly accurate inertial navigation systems.

                                                     32
         Angular Rate Sensors—
              Gyroscopes




•   A vibrating quartz tuning fork uses the Coriolis effect to sense
    angular velocity
                                                                       33
Gyroscopes




             34
   Inductive Sensors—Linear and Rotary
      Variable-Reluctance Transducer
• Based on change in the reluctance of a
  magnetic flux path.
• Application—displacement, velocity and
  acceleration measurements.
• Use Inductive Displacement Sensor to
  measure the displacement. Then, divide it
  with the time taken to find the velocity.


                                          35
Inductive Displacement Sensors—Microsyn




                                      36
    Inductive Displacement Sensors—Linear
    Variable-Differential Transformer (LVDT)




•   It is a passive inductive transducer. The 2 secondaries are having
    equal sizes, shapes, and no. of turns.                               37

				
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