module 1.pptx

7/27/2019 Module 1.pptx

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Measurement of Force, Torque, Velocity : Electric balance

different types of load cells magnets elastics load cell-

strain gauge load cell-different methods of torque

measurement, strain gauge, relative regular twist-speed

measurement-revolution counter- capacitive tacho-drag up

type tacho D.C and A.C tacho generators stroboscope.


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Contact Forces

Frictional Force

Tension Force

Normal Force

Air Resistance Force

Applied Force

Spring Force

Action at a distance forces

Gravitational Force

Magnetic Force

Electrical Force


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FORCE


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One Newton is theamount of force

required to give a 1-kg

mass an acceleration of1 m/s2.


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How to measure FORCE?

FORCE

OUTPUT


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Hydraulic Load Cells

Pneumatic Load Cells

Magnetic Load Cells

Strain Gauge Load Cells


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Full load deflection of

about 0.05 mm Range up to 20 tonnes.

(500 tonnes also available)


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Advantages:

Trouble free over a long period of use.

They require no outside force of power for indication.

Responds quickly to load changes.

Relatively insensitive to temperature changes.Well suited for hazardous areas as they do not require

electrical power.

Well suited for high impact loads.

Can withstand high overloads.High natural frequency.

Fast response rate and is suitable for torque

measurement.


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Similar to hydraulic type.

Flapper-Nozzle arrangementact as feed back as well as

amplifier.

Output pressure,

p=W/(ks/kf+ )

ks=diaphragm thickness;kf=flapper-nozzle gain;

=effective diaphragm areaW = p


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Range 50 tonnes with 0.1%

accuracy (49,000 N)Nominal deflection level of

0.25mm

Disadvantage: only static

measurements possible.


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Advantages: Suitable for hazardous and explosive areas.

No special transmitting system is required.

They cannot cause product contamination due to leakage

or hydraulic fluid. Relatively free from temperature related error.

Disadvantages & Limitations:

The operating gas must be dry enough to prevent

condensation of vapour and subsequent freezing attemperatures below 00 C.

Respond slowly to sudden (dynamic) changes.


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Permeability of a

Ferromagnetic material

changes with applied

stress/strain. This load cell is also known

as pressductor.

Magnetic material sheets are

bonded together to form

transducer.

Primary and secondary

windings are wounded at 900

apart.


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When no load, secondary

winding unaffected by the

excited primary.

When loaded, stress changesthe angle between 10 and 20

thereby inducing emf in the

secondary.

Range 0.1 to 100 tonnes.


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Advantages:

Extremely robust transducer.

Produces relatively high output signal levels.

Overload ratings are high as 15 times rated loads.

Disadvantages:

Excessive stress and ageing may cause permanent

changes.

Zero drift and sensitivity changes due to temperature

variation.

It has hysteresis errors.


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Made in the form of a disc cut from a quartz crystal.

Output of 3 to 4 pC/N.


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Advantages:

Small & Compact in size.

Electrical output signal.

Rapid response to load variations.

Relatively maintenance free, and if properly sealed canbe used in any environment.

Inexpensive.

Disadvantages:

They have to be protected from angular or non-axialloads as they have no way to discriminate between

bending and axial loads.

Overloads above rated loads should be avoided.


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A twisting force that tends to cause rotation or rather a

simple rotational force.


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T () = D x F

F = Force

D = perpendicular distance from the axis of

rotation of the line of action of the force.


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TYPES

In-line rotating torque sensors

In-line stationary torque sensors


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Torque measurement

of rotating bodies.

Uses strain gauges in

bridge configuration.


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Utilizes Wiedemann

Magnetostriction effect.

Change in permeability

due to tensile orcompressive stress.

Permeability increases

under tensile stress and

decreases under

compressive stress.


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Under no load, flux

induces in both

secondaries are equal

and opposite; henceoutput is zero.

When shaft is under

torsion, permeability

changes, linking flux

changes, output voltagechanges.


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Torque is determined

by measuring the

angular displacement

between the two

sections of a rotatingshaft.

Consists of two

identical toothed

wheels (gears) fixedon a shaft.

Proximity sensors

(magnetic pick ups)

used to pick up the

angular movement.


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Alternating voltages

are produced at two

pick ups whose phase

difference is

proportional toapplied torque.


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Revolutions per minute of some piece of a rotating

equipment.


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Revolutions per minute of some piece of a rotating

equipment.


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Consists of a worm gear which is

driven by the speed source.

The worm gear drives the spur

gear, on which a scale is inscribed.

Advantages:

Being a direct measuring device, it is quite accurate and is

unaffected by environmental variations.

Disadvantages:

Method is unsuitable if the speed does not remain constant for

a sufficiently long time.


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A transducer produces an analog signal in the form of a

continuous drag due to eddy currents induced in an

aluminum cup.


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Rotor of tachometer is attached to the machine.

A permanent magnet is attached to the rotor which rotates

within an aluminium cup along with rotor.

A spindle is attached to the aluminium cup, to which a

pointer is fixed.

As the magnet rotates within the aluminium cup along with

the shaft of the machine, eddy currents are induced in the

cup which result in a torque or drag that tends to turn the

cup against the spring. The deflection of the cup is proportional to the induced

emf, which in turn proportional to the speed of the shaft.

These type of tachometers are used in automobiles (cars).


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These type of tachometers are used in automobiles (cars)

which measure the angular speed of wheels.

In industrial applications speed up to 12,000 rpm can be

measured by indirect means.


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An electromechanical device that generates a voltage

output proportional to the shaft speed.

TYPES

AC Tachometer Generator

DC Tachometer Generator


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Similar to a 2 phase induction motor.

Consists of primary and secondary windings placed 900

apart.

When rotor is stationary and the primary winding is excited

by an ac input voltage, the induced voltage in the

secondary is zero.

As the rotor turned, a voltage is induced in the secondary

winding whose magnitude is proportional to the rotor

speed. 500 10,000 rpm can be measured.


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Consists of a permanent magnet to provide the magnetic

flux and an output winding placed on the rotor.

when the rotor is stationary, there is no relative motion

between magnetic field and winding, and the output

voltage is zero. As rotor speed increases, the relative motion between

magnetic field and winding also increases, and hence

output voltage is induced in the winding which is sinusoidal

and whose magnitude is proportional to the rotor speed.


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ROTOR

Electronic Stroboscopic Lamps incidentlight at adjustable flash rates on the rotor.

Rotor should have some spokes, gear

teeth, or some similar features.

Frequency of lash is adjusted so that rotorappears motionless.

Flashing rates adjusted to give

synchronization at the largest flashing

rates (r1

).

Then decrease the flash rate to get

synchronization at another rate (r2).


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Then rotor speed, n is given by

n = r1r2/(r1-r2)

When synchronization is obtained N times, then speed is

given by

n = r1rN(N-1)/(r1-rN)


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Uses the principle ofcharging a capacitor and

discharging through a

meter alternately.

If the charging anddischarging is controlled by

the speed of the

equipment, the average

discharge current would be

proportional to the speed.

If is the speed of

rotation in rpm, then

I = CR


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With reversed direction of rotation the direction of

module 1.pptx

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