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Measure of Angular Velocity
The main application of angular velocity measurement is in speed control
systems. They also provide the usual means of measuring translational velocities,
which are transformed into rotational motions for measurement purposes bysuitable gearing. Many different instruments and techniques are available for
measuring rotational velocity.
Some type of sensor is configured to produce a known number of pulses per
revolution of the device being monitored. By counting pulses, the number of
revolutions or angular position may be determined. In conunction with this if time
is also measured angular speed may be measured.
There are three maor types of instruments used for measurement of rotary
motion, namely!
a. "lectrical
b. #ptical
c. Mechanical
$igital tachometers, or to give them their proper title, digital tachometric
generators, are usually non%contact instruments that sense the passage of equally
spaced marks on the surface of a rotating disc or shaft. Measurement resolution is
governed by the number of marks around the circumference. &arious types of sensor are used, such as optical, inductive and magnetic ones. 's each mark is
sensed, a pulse is generated and input to an electronic pulse counter. (sually,
velocity is calculated in terms of the pulse count in unit time, which of course only
yields information about the mean velocity. If the velocity is changing,
instantaneous velocity can be calculated
at each instant of time that an output
pulse occurs. In this circuit, the pulses
from the transducer gate the train of
pulses from a )M*+ clock into a
counter. ontrol logic resets the counter and updates the digital output value after
receipt of each pulse from the transducer.
The measurement resolution of this
system is highest when the speed of
rotation is low.Fig: Scheme to measure angular velocity
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Optical sensing
$igital tachometers with optical sensors are often known as optical
tachometers. #ptical pulses can be generated by one of the two alternative
photoelectric techniques. The pulses are produced as the windows in a slotted disc
pass in sequence between a light source and a detector. 'nother way is to have
both light source and detector mounted on the same side of a reflective disc which
has black sectors painted onto it at regular angular intervals. -ight sources are
normally either lasers or -"$s, with photodiodes and phototransistors being used
as detectors. #ptical tachometers yield better accuracy than other forms of digital
tachometer but are not as reliable because dust and dirt can block light paths.
Variable Reluctance Transducers
Variable reluctance velocity transducers, also known as induction
tachometers, are a form of digital tachometer that uses inductive sensing. They are
widely used in the automotive industry within anti%skid devices, anti%lock braking
systems 'BS/ and traction control. ' more sophisticated and the most recently
used type has a rotating disc that is constructed from a bonded%fibre material
into which soft iron poles are inserted at regular intervals around its periphery. The
sensor consists of a permanent magnet with a shaped pole piece, which carries a
wound coil. The distance between the pick%up and the outer perimeter of the disc is
around 0.1 mm. 's the discrotates, the soft iron inserts on
the disc move in turn past the
pick%up unit. 's each iron
insert moves towards the pole
piece, the reluctance of the
magnetic circuit increases and
hence the flu2 in the pole piece
also increases. Similarly, the
flu2es in the pole piece
decreases as each iron insertmoves away from the sensor.
The changing magnetic flu2
inside the pick%up coil
causes a voltage to be induced
in the coil whose magnitude is
proportional to the rate of
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change of flu2. This voltage is positive whilst the flu2 is increasing and negative
whilst it is decreasing. Thus, the output is a sequence of positive and negative
pulses whose frequency is proportional to the rotational velocity of the disc. The
ma2imum angular velocity that the instrument can measure is limited to about )0
000 rpm because of the finite width of the induced pulses. 's the velocity
increases, the distance between the pulses is reduced, and at a certain velocity, the
pulses start to overlap. 't this point, the pulse counter ceases to be able to
distinguish the separate pulses. The optical tachometer has significant advantages
in this respect, since the pulse width is much narrower, allowing measurement of
higher velocities.
' simpler and cheaper form of variable reluctance transducer also e2ists that
uses a ferromagnetic gear wheel in place of a fibre disc. The motion of the tip of
each gear tooth towards and away
from the pick%up unit causes a
similar variation in the flu2 patternto that produced by the iron inserts
in the fibre disc. The pulses
produced by these means are less
sharp, however, and consequently
the ma2imum angular velocity
measurable is lower.
Hall Effect Sensors
The *all "ffect is the appearance of a
transverse voltage difference on a
conductor carrying a current
perpendicular to a magnetic field. This
voltage is
directly proportional to the magnetic field
strength. 3hen the current is flowing
through a plate because of an emf of
battery/ there is +ero voltage displayed in
the transverse direction. *owever when
there is a magnetic field acting
perpendicular to the current, the electrons are deflected by the magnetic force and
create a negative charge on one end. The other side has accumulation of positive
charge. ' voltage across the transverse ends of plate is displayed on the meter. The
Fig: Variable Reluctance Transducer
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hall effect is present in any conductor carrying current in the magnetic field, but it
is much more predominant in semiconductors than in metals. Thus most hall effect
transducers use a semi conducting material as the conductor.
The rotating element in Hall-effect or
magnetostrictive tachometers has a
very simple design in the form of a
toothed metal gearwheel. The sensor is
a solid%state, *all%effect device that is
placed between the gear wheel and a
permanent magnet. 3hen an intertooth
gap on the gear wheel is adacent to thesensor, the full magnetic field from the
magnet passes through it. -ater, as a
tooth approaches the sensor, the tooth diverts some of the magnetic field, and so
the field through the sensor is reduced. This causes the sensor to produce an output
voltage that is proportional to the rotational speed of the gear wheel.
Tachogenerators
Fig: Hall Efect Sensor
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The measurement of speed can also be done with the help of tachogenerator.
The tachogenerators are of two types, namely! $ 4 '
D.C. Tachogenerator -The armature of the d c tachogenerator is kept in the permanent magnetic field. The armature is coupled with the shaft whose speed is
to be measured. 3hen the shaft revolves, the armature also revolves in the
magnetic field producing an ".M.5, which is proportional to the magnetic flu2
and speed of rotation.
's the field of permanent magnet is fi2ed, the ".M.5 generated is
proportional to speed. The ".M.5 induced is measured using moving coil
voltmeter, the scale of which is calibrated in speed directly. The circuit has
commutator and carbon brush arrangement to receive current from induced
".M.5.
The advantages of $.. Tachogenerator are as follows
The output voltage is small enough to measure it with conventional d.c.
voltmeters
The polarity of output voltage directly indicates the direction of rotation.
$isadvantages
$ue to variation in contact resistances at brushes, considerable error isintroduced in output voltage. 6egular maintenance of commutator 4 brushes
is required.
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A.C. Tachogenerator 7 In this, the coil is wound on stator itself. The magnetic field is thus rotating.
The magnetic field can be obtained using permanent magnet or electromagnet.
$ue to rotation of magnet, an emf is generated in the stator coil, which has
amplitude and frequency directly proportional to the speed of rotation. Thus speedof rotation can be calibrated in terms of either amplitude or frequency of the
induced voltage.
7 In the above circuit shown in fig., the amplitude is used to measure speed.
(sing rectifier and filter '. . voltage is converted to $. and measured with the
help of moving coil voltmeter.
7 'dvantages of '.. Tachometers are as
follows
7 The output can be calibrated in terms of two parameters namely amplitude and
frequency of induced voltage.
7 ommutator and brush contact
resistance problems are eliminated as
the coil is wound on stator.
7 $isadvantages
7 5or very low speeds rotation frequency
of induced voltage is also very low, thus ripples in the output increases. To
overcome this number of poles of stator is increased so that induced voltage
frequency is higher even at low speeds.
7 5or high speeds, the frequency of induced voltage is also very high. Thus the
coil impedance increases and the linearity of output is affected. To maintain
the good linearity the display device should have higher input impedance
than that of the coil.
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Otical Encoders
#ptical instruments for angular velocity measurement include.
7 #ptical Shaft encoder
7 8on contact optical tachometer 7 Stroboscope.
Otical shaft encoders are based on circular binary encoders which are used for
angular displacement measurement can also be used for angular velocity
measurement. Similar to the above disc can be made with optical encoding for
optical tachometer.
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9 A !hoto interruter consists of an -"$ light source often infrared/ and a photo
detector. Mechanical interruption of the light path can be used for various
purposes such as counting, triggering etc.
:hoto interrupter type sensors are used in automobiles to monitor the angular
position and the rate of change of steering angle. The sensor comprises a slit discattached to steering column and rotates with the steering wheel and a fi2ed (
shape photo interrupter. 3hen the column is turned the rotating slotted disc
alternatively e2poses and blocks the light emitting beams directed toward
photodiode receptor, this interruption of the light beam generated a series of
pulses
9 Other non contact otical tacho"eters are available as handheld units. ' light
beam from tachometer is reflected from the moving pattern on the rotating
surface, which may be something as simple as a piece of reflective tape. Bycounting the reflected light pulses, rotary speed may be determined to high
accuracy.
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Stroboscoe
9 "arly version of mechanical stroboscope is shown in fig. above. which willhelp understanding the principle/. It consists of a whirling disc and a stationary
mask having opening of same shape 4 si+e. 3hen the two openings on the disc
4 stationary mask coincide, an observer can catch glimpses of the obect behind
the mask.
9 If the obect is rotating and the speed of disc is synchroni+ed with the motion of
obect. The obect would appear to be motionless. 'lso if the disc is rotating at
slightly lesser speed than obect the obect would appear to creep forward and if
the disc is rotating at slightly faster speed the rotating obect would appear tocreep backward.
9 Modern stroboscopes operate with slightly different setup. Instead of the
whirling disc, a controllable, intense flashing light source is used. 6epeated
short duration )0%;0 ms/ light flashes of adustable frequency are supplied by
the light source. The required flashing rate frequency for synchroni+ation with
the motion of obect is read as the angular speed of the obect. These devices
are often called strobe lights.
9 To properly view the rotating obect in the flash light it is required to place a
distinctive mark on the rotating obect e.g. mark on one of the spokes of
moving wheel/. This mark is taken as reference to look for one%on%one
synchroni+ation.
9 Stroboscopes are used to check speeds ranging from
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9 The stroboscope is highly versatile= it uses no power from the moving obect
that is being measured.
9 It has an accuracy as close as 0.)>
9 Most stroboscopes consist of an oscillator, a reed and a flasher as shown
above. The oscillator provides trigger pulses to flasher mechanism to control
the flashing rate. The vibrating reed serves as a reference for accurately
calibrating the stroboscope. The reed is driven from ac lines and vibrates at
?@00 times per minute. This steady rate is used to calibrate the flasher.
5lasher produces illumination required for the measurements. The flasher
lamp can be a very bright -"$ or an inner gas filled tube which gives
illumination on ionisation of the gas.
9 3hen the frequency of rotation of obect e2actly matches the stroboscope
frequency the moving mark on the obect is viewed stationery as it is at the
same position every time when flash light illuminates. Such stationery image
also appears when speed of rotation is some e2act multiple of the
stroboscope frequency. The highest of the strobe frequency that produces
stationery image is the fundamental frequency.
e.g. when obect is rotating at )@00 rpm and the flashing rate is
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Thus it is not necessary to always obtain one%to%one synchroni+ation. 'lso
note that above procedure makes it possible to e2tend the upper limit of the
device beyond stroboscopes normal frequency range. 5lash rates range from
))0 to )10,000 cpm enabling measuring speeds upto ),000,000 rpm.
Mechanical Tacho"eters
9 Mechanical Tachometers are either
)/ $irect counters #6
@/ entrifugal speed indicators
9 Mechanical counters may be of the
direct counting digital type or maybe
counters with a gear reducer. In thelatter case, angular motion available at
shaft end is reduced by a worm and
gear, and the output is indicated by
rotating scales.
9 In both e2amples rpm is measured by
simply counting the revolutions for
length of time as measured with stop
watch and calculating the rpm.
Sometimes timing mechanism is also
incorporated in the counter. The timer is
used to actuate a internal clutch that
controls the time interval during which
count is measured.
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Centrifugal r" indicators
entrifugal tachometer. entrifugal rpm indicators / use the Cfly%ball
governorD principle, which balance centrifugal force against a mechanical
spring. 'n appropriate mechanism transmits the resulting displacement to a
pointer, which indicates speed on a calibrated scale.
$escription of working !
"ach portable centrifugal tachometer has a small rubber%covered wheel and
a number of hard rubber tips. Eou fit the appropriate tip or wheel on the end
of the tachometer drive shaft, and hold it against the shaft to measure speed
of rotation. 's the drive shaft begins to rotate, the fly weights rotate with it.
entrifugal force tends to pull the fly weights away from the center, causing
the lower collar to rise and compress the spring. The lower collar is attached
to a pointer, and its upward motion, restricted by the spring tension, causes
an increase in the indication on the dial face. 3hen properly used, a
centrifugal tachometer will indicate correct shaft speed as long as it is in
contact with the machine shaft under test. ' portable centrifugal tachometer
has three ranges! low 10 to 100 rpm/, medium 100 to 1,000 rpm/, and high
1,000 to 10,000 rpm/.