measure of angular velocity

8/9/2019 Measure of Angular Velocity

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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/.

measure of angular velocity

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