# 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.

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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 views: 11 posted: 5/31/2012 language: English pages: 37