Metro Report

8/2/2019 Metro Report

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Department of Mechanical Engineering

School of Engineering and Technology

Subject-Metrology

Report on Electrical Comparator

Made By-

Vibhor Sharma

100106366

Section-D

Mechanical Engineering

B.Tech, 2nd Year.



Acknowledgement

Any accomplishment requires the effort of many

people and there are no exceptions. The report

being submitted today is a result of collective

effort. Although the report has been solelyprepared by me with the purpose of fulfilling the

requirements of the course of Metrology, there

are innumerous helping hands behind it who have

guided me on my way. First I thank our Metrology

Course developers for creating such an

opportunity for the students to broaden their frame

of skills. I am gratified with their efforts .My sincere

gratitude also goes to my Faculty Mr. P.K.Sinha

as well as to the Assistant

Professors who have helped me to perform

this report and be familiar to the practical aspect

and uses of theoretical knowledge and clarifying

the career goals .I would like to thank



Ms. Raj Laxmi who has provided me the

greatest opportunity to make this report. I am

also grateful to all members of Department of

Mechanical Engineering for providing several

documents, papers, data, figures and services as

well as sharing their experience with me and

teaching me different techniques to understand the

device Electrical Comparator and its uses.

Vibhor Sharma

10106366



DECLARATION

I hereby declare that the REPORT entitled

“ELECTRICAL COMPARATOR

”

Submitted to the Department of Mechanical

Engineering, Sharda University, is a record of an

original work done by me under the guidance of my

respected Faculty Members

, and th is project work is submit ted in the p

art ial fu l f i lment of the requirements for the

course of Metrology. The results embodied in this



thesis have not been submitted to any

other University or Institute for the award of any

course.

Contents

Introduction

Types of Comparators.

Principle of Electrical Comparator.

LVDT

Working of Electrical Comparator.

Uses of Electrical Comparator

Advantages of Electrical Comparator.

Disadvantages of Electrical Comparator.



Introduction

A Comparator gives dimensional differences in

relation to a basic dimension.

It compares the known dimension of a part with

some standard or master setting which represents the

basic size and the dimensional difference from the master

setting are amplified and measured.



Electrical comparators are also known as electro-

mechanical measuring systems as these employ an

electro-mechanical device which converts

a mechanical displacement into electrical signal.

Types of comparators

The most basic classification of comparators is on the

basis of their working principle. The comparators

differ principally in the method used for amplifying

and recording the variation measured. Most

commonly available comparators are of the following

types:

1. Mechanical comparators

2. Optical comparators

3. Electric and electronic comparator machines

4. Pneumatic comparators

5. Fluid displacement comparator machines



6. Projection comparators

7. Multi-check comparator

8. Automatic gauging

As listed above there are mainly 8 types of

comparators, but this report only covers Electrical

Comparator.

Principle of an Electrical comparator

It is based on the principle of application of frequency

modulation or radio oscillation. A special ratio

oscillator is used which produces impulses of a defi-

nite value at a definite frequency. The frequency is

controlled by the measuring head. When the

measuring head is applied to the part to be

measured, the frequency of the instrument oscillator

is modified because of the variation of the dimension

from that of the preset standard. The modified

frequency is annulled by suitable controls to restore it

back to the basic value. The measuring instrument



dial is graduated directly in terms of the physical

standards of length which compare with the

electronic standards of wavelength. By using various

adaptors to suit the work, the comparator can be put

to many applications such as external and internal

gauging, flatness testing, thickness gauging, tube

wall thickness, measurement of electro-plate coating,

gauging on curved or flat surfaces etc., to a very high

limit of measuring precision. In Fig. given, the

comparator is shown being used as a thickness

gauge. With high frequencies, it is extremely

sensitive to such variables, as temperature,

mechanical and electrical changes. Using different

frequencies, the sensitivity of the instrument can be

easily changed and it is achieved by providing a

multi-position switch.



Two of the bridges form a bridge. Initially the

inductors are set so that no resultant current flows

through them. When armature is moved, the balance

of bridge is disturbed and a result the current flows

which is calibrated in terms of length of movement of

the plunger consists of Electrical Gauge Head with

armature and feeler, Ammeter, Power supply, with 2

amps x 2 Nos. dimmer & Specimen Holder.

Linear variable differential transformer

(LVDT)

Linear variable differential transformer (LVDT) is the most

popular electro-mechanical

device used to convert mechanical displacement

into Electrical signal. It, in effect, is a

transformer consisting of three symmetrically spaced coils

carefully wound on an insulated

bobbin. It works on mutual inductance principle and



consists of a primary coil wound on an

insulating form (bobbin) and two identical secondary

symmetrically spaced from the primary.

AC carried excitation is applied to the primary and two

secondary are connected externally in a series opposition

circuit. The lead wires exit through an opening in the outer

shield, usually in the end cover washers. A cylindrical

shield of ferromagnetic material is spun over the metallic

end washer after the windings have been vacuum-

impregnated with a potting compound suitable for the

application environment. The finished transformer there-

after becomes quite impervious to humidity or ordinary

magnetic influences. The device thus also becomes

extremely rugged and reliable. There is a non-contacting

magnetic core, made from a uniformly dense cylinder of

nickel-iron alloy, carefully annealed to improve and

homogenize its magnetic permeability, which moves in the



centre of these coils wound on the insulating form and the

motion of this core varies the mutual inductance of each

secondary to the primary, which

determines the voltage induced from the primary to each

secondary. If the core is centered in

the middle of the two secondary windings, then voltage

induced in each secondary winding

will be identical and 180° out-of-phase and the net output

will be zero. If the core is moved off

middle position, then the mutual inductance of the primary

with secondary will be greater

than the other, and a differential voltage will appear

across the secondaries in series. For off

centre displacements within linear range of operation, the

output is essentially a linear

function of core displacement. The various unique

features of LVDT are : Due to no physical



contact between the core and coil its mechanical

components do not wear out; as a result the

friction is absent and true infinite resolution with no

hysteresis is obtained. The small core

mass and the lack of friction enhance response

capabilities for dynamic measurements and

thus it becomes very suitable for taking measurements for

on-line machining. Further as it is

not affected by overload, its reliability is high.

Fig. 2 above. Schematic Arrangement of LVDT

(core-shown in the middle position).

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Fig. 3 above. Phase referenced output voltage of

LVDT.

Fig. 4 above. Absolute value of output voltage of LVDT.

Working of an Electrical Comparator

If alternating current is applied to the bridge, the

inductance and capacitance of the arms must also

be accounted for along with resistance. The pair of

coils forms a pair of inductance. The movement of

the plunger displaces an armature thus causing a

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variation in the inductance in the coils. The amount

of unbalance caused by movement of measuring

plunger is amplified and shown on a linear scale

magnifications of about 30,000 are possible with this

system. Zero setting arrangement is provided.

The degree of magnification is adjustable and other

examples of electrical comparators are electricator,

electric gage and sigma electronic comparator.



Advantages of Electrical Comparators

Remote indication is possible

High magnification with smaller number of moving

parts

Insensitive to vibration and mechanism carrying the

pointer is high

The cyclic vibration reduces errors due to sliding

friction on an AC supply

Smaller measuring unit and several magnifications is

possible with same instrument.



Uses of Electrical Comparator

The various ways in which the comparators

can be used are as follows:

i) In mass production, where components are to

be checked at a very fast rate.

ii) As laboratory standards from which

working or inspection gauges are set and

correlated.

iii) For inspecting newly purchased gauges.



iv) Attached with some machines,

comparators can be used as working

gauges to prevent work spoilage and to

maintain required tolerances at all stages

of manufacturing.

(v) In selective assembly of parts, where

parts are graded in three or more groups

depending upon their tolerance.

Disadvantages of Electrical

Comparator



i) It requires an external agency to

operate i.e., the A.C. electrical supply.

Thus the vibrations in voltage or frequency

of electric supply may affect the accuracy.

ii) Heating of coils in the measuring unit

may cause zero drift and alter the

calibration.

iii) If only a fixed scale is used with a

moving pointer then with high

magnifications a very small range is

obtained.

iv) This is usually more expensive than

mechanical instrument.

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