COMPARATORS

CONTENTS- 6 hours

Introduction to comparators,

Comparators-Characteristics & Classification

Mechanical comparators, Sigma, Dial comparators Optical Comparators-

Principles, Zeiss ultra optimeter

Electrical & Electronic Comparator,

LVDT, Pneumatic Comparator, Back pressure gauges Solex Comparators Multi-Check comparators

COMPARATORS• The general principle of comparator is to indicate the

differences in size between the standard and the work being measured by means of some pointer on a scale with sufficient magnification

• All comparators consist of three basic features1)A sensing device which faithfully senses the input signal2) A magnifying or amplifying system to increase the signal

to suitable magnitude. Mechanical, Optical, Pneumatic, hydraulic and electronic methods are used for this purpose.

3) A display system (usually a scale and pointer) which utilizes the amplified signal to provide a suitable readout.

Need for a comparator• A comparator is used in mass production to inspect the

components to close tolerances with high degree of precision and speed. Ex: Piston

• Use of line standards such as vernier and micrometer calipers require considerable skill

• Many dimensions can be checked in a very short timeComparators can be classified as 1. Mechanical 2. Optical 3. Electrical & Electronic

4. Pneumatic 5. Fluid displacement comparators 6. Mechanical –optical comparator

7. Electro-mechanical comparator8. Multi-check comparator.

Design ConsiderationDesign Consideration

1.Ability to record variation of at least 0.0025 mm2.Linear recording scale3.Variance free (Instrument readings should not vary when repeated measurements of given quantity are made4.Scale indicators clear and free from oscillations5.Free from backlash, unnecessary friction & clearances inertia should be low.6.Measuring pressure should be low and constant7.Supporting frame to be rigid and withstand reasonable ill usage without permanent damage8.Capable of measuring wide range.

Mechanical Comparators• In Mechanical type, the required magnification is

obtained by using mechanical linkages, levers, gears and other mechanical devices.

• Magnification (M): It is the ratio between the movement of the plunger and the resultant movement of the pointerM= P(pointer movement)/p Plunger movement

Mechanical comparators types:1. Dial test indicator.2. Johansson mikrokator.3. Sigma mechanical comparator

PointerScale

ReedsFixed link

SpindleSpindle displacement

Movable link

Flexure strips

REED COMPARATOR

REED COMPARATOR

• As shown in fig, the movable link is constrained by thin metal flexure strips to move vertically relative to fixed link attached to housing.

• The change of orientation of the flexure strips due to relative motion, the reeds provide a large angular movement of the pointer.

• The scale may be calibrated to indicate any deviation from an initial setting.

Attachment screws

Measuring plungerPlunger tip

Cantilever strip

Twisted stripScaleLight pointer

Slit washer

Spring elbow

JOHANSSON MIKROKATER

JOHANSSON MIKROKATOR• It works on the principle of a button spinning in a

loop of string.

• A twisted thin metal strip carries a very light pointer made of thin glass at the center of its length.

• The two halves of the strip from the centre are twisted in opposite directions so that any pull on the strip will cause the centre to rotate.

• One end of the strip is fixed to an adjustable cantilever and the other is anchored to the spring elbow.

• One arm of spring elbow carries the measuring plunger.

JOHANSSON MIKROKATOR (contd…)• As the measuring plunger moves either upwards or

downwards, the elbow causes the twisted strip to change its length thus making it to further twist or untwist.

• Thus the pointer at the center of the twisted strip rotates by an amount proportional to the change in length of the strip and hence to the plunger movement.

• The strip is perforated along the center line to prevent excessive stress on the center portion.

• A slit washer is used for lower mounting of the plunger.

JOHANSSON MIKROKATOR (contd…)• The amplification of this comparator is given by

turnsofnumber theisn

axis neutral its along measured strip twisted theoflength theis L

ends, therespect towith

strip theofpoint mid at the twist theis where2

n

L

dL

d

The amplification increases with the number of turns. The magnification is of the order 5000

The Mikrokator is an instrument for measuring differences in length. The Mikrokator principle greatly magnifies any deviation in size so that even small deviations produce large deflections of the pointer over the scale.

The transference of movement from the measuring tip to the pointer is mechanical, completely frictionless and free from backlash.

The CE Johansson Mikrokator was introduced in 1938 and is still today the finest mechanical comparator in the world. A wide range of models are available, which gives graduations varying between .0001” (.005 mm) and .00002” (.0001 mm). The unique “Twisted Strip” principle on which all Mikrokators are based, provides many features which are not available on any other measure instrument.

SIGMA COMPARATOR

• It has magnifications in the range of 300 to 5000.

• It has a plunger attached to a rectangular bar which is supported at its upper and lower ends by flexure plates. A knife edge is fixed to the side of rectangular bar which bears on a moving block.

• The moving block and the fixed block are connected by flexible strips at right angles to each other.

SIGMA COMPARATORScale

Drum

Bronze band

Forked arm

Plunger

Flexure plate

Fixed block

Flexible strips

Flexure plate

Pointer

Y X

Knife edge

Moving block

SIGMA COMPARATOR (contd…)• If an external force is applied to the moving block, it

would pivot about the hinge .• A forked arm or Y-arm attached to the moving block

transmits the rotary motion to the indicator driving drum through a bronze band wrapped around the drum.

• Magnification: If Y is the length of forked arm and X is the distance from the knife edge to hinge, then first stage magnification is Y/X

• If the pointer length is R and the radius of the drum is r the second stage magnification is R/r such that the total magnification is (Y/X)* (R/r)

DIAL INDICATOR

DIAL INDICATOR

Plunger

PinionRack

Dial Scale

Pointer

Pillar

Base

Plunger

Cylindrical

component

V block

DIAL INDICATOR (contd..)• It consists of a robust base whose surface is

perfectly flat and a pillar carrying a bracket.• The bracket incorporates a spindle fitted with

a pinion & dial scale.• The linear movement of the plunger is

magnified by means of a rack & pinion arrangement into sizeable rotation of the pointer on the dial scale.

• The dial scale is set to zero by use of slip gauges representing the basic size of the part.

Advantages of Mechanical Comparators• They are cheaper compared to other

amplifying devices• Do not require electricity or air and such the

variations in the outside sources do not affect the accuracy.

• They have a linear scale, robust and easy to handle.

Disadvantages of Mechanical Comparators• They have more linkages due to which friction

is more and accuracy is less.• Mechanisms used have more inertia and

hence they become sensitive to vibrations.• Any wear, backlash, play or dimensional faults

in the mechanical devices used will also be magnified.

• The range of the instruments is limited as the pointer moves over a fixed scale.

• It is also difficult to incorporate arrangement for adjusting magnification.

O

d

C

AScreenLamp

Pivot

Mirror

Plunger

NormalO

C

A

Screen

Lamp

Mirror

Normal

d

2

h angleof tilt

PRINCIPLE OF OPTICAL LEVER

OPTICAL COMPARATORS

OPTICAL COMPARATORS• An optical comparator works on one of the

following two main principles:1)Use of optical lever 2) Use of enlarged image– If a beam of light AC is directed on to a mirror as

shownin fig, it will be reflected onto the screen at O as a dot.The angle of incidence = angle of reflection– When the plunger moves up, the mirror tilts by

Then the reflected beam moves through 2 i.e. the illuminated dot moves to B.

– Also the increase in distance CO of the screen from the tilting mirror will increase the magnification.

L1 L2 L3

L4

PivotMeasuringPlunger

Mecahnicallever

Screen with scale

Light source

Mirror pivot

Mirror

Projection lensGlass plate carrying indexline

Collimating lens

D

Mechanical-Optical Comparator

Mechanical Optical Comparator

• In this comparator, small displacements of the measuring plunger are first amplified by a mechanical lever. It is equal to (L2/L1)

• The amplified mechanical movement is further amplified by optical system due to tilting of mirror by .

• The reflected ray D (image of index line) will be turned through an angle 2and hence optical amplification =2(L4/L3)

• The overall magnification = (L2/L1)* 2(L4/L3)

Zeiss Ultra Optimeter

Fixed mirrorM2

MovableMirror M1

Plunger

Work piece

Index lens

Condenser

Green Filter

Lamp

Eye piece

Objective lens

Zeiss Ultra Optimeter• The optical system of this instrument involves

double reflection of light and hence gives higher degree of magnification.

• The green filter filters all rays except green (to reduce fatigue to eye) and green light passes through a condenser.

• The light then passes through an index lens and the index mark is projected on to a movable mirror M1.

• It is then reflected on to another fixed mirror M2 and then back to the first movable mirror.

Zeiss Ultra Optimeter (contd…)• The objective lens brings the reflected beam from the

movable mirror to focus at a transparent graticule.• The graticule contains a precise scale which is viewed

by an eye piece.• The projected image of the index line on the graticule

can be adjusted to set initial zero reading by a screw.• Any movement of the plunger will tilt the movable

mirror, which causes a shift in the reflected index line on the eye piece graticule scale.

• This in turn measures the displacement of the plunger

Advantages & Disadvantages of Optical comparators• Advantages:

1) Few moving linkages and hence no friction & wear.2) High range of measurements and no parallax error3) Magnification is usually high.

• Disadvantages:1) Heat from the source of light, transformers etc., may cause the setting to drift.2) An electric supply is required to operate these comparators.3) The size is large and costly.4) Use of eyepiece to view is inconvenient for prolonged use. Also a dark room is essential to take readings.

Electrical& Electronic ComparatorsR1 R2

R3 R4

Battery

Coils

Arm

MeasuringPlunger

Coils

IronArmature

Electrical& Electronic Comparators (Contd…)• These comparators depend on the principle of

balancing the Wheatstone bridge, (R1/R2) = (R3/R4) applicable for only to direct

current obtained from a battery.• In actual instruments, one pair of inductances takes

the form of a pair of coils in the measuring head of the instrument.

• An iron armature inside these coils moves along with the measuring plunger, and upsets the balance of the circuit and causes the deflection of the pointer meter which is calibrated directly in linear units.

• Magnifications of the order of 30,000 are possible.

Electrolimit gauge

Electromagneticcoils

A

B

Armature

SpringThin steel strips

Spring

Plunger

Electrolimit gauge (contd…)• Fig shows the principle of Electrolimit gauge or

measuring head.• Vertical movements of the plunger are transmitted

to an armature, which in turn is suspended by thin metal strips.

• At the left end, the armature between two electromagnetic coils form the two arms of an AC bridge circuit.

• Any movement of the armature sets up out of balance effects which are recorded by a micrometer from which direct reading of plunger movements can be obtained.

Linear Variable Differential Transformer

• LVDT is the most popular electro mechanical device used to convert mechanical displacement into electrical signal.

• It is a differential transformer which provides an A.C voltage output proportional to displacement of a core passing through the windings.

• Of the three symmetrically placed coils, the center coil is energized from an A.C source and the two end coils connected together in phase opposition are used as pick up coils.

Linear Variable Differential Transformer

Input displacement

Fig.b

InputVoltage

OutputVoltage

displacement

Fig.a

Linear Variable Differential Transformer (contd…)• Based on the position of the core, a voltage is

induced in each of the pick up coils.• A position of the core at which the voltages

induced in the pick up coils are same, but cancel each other due to phase opposition, is called the “null position”.

• The fig shows the typical differential transformer characteristics obtained by plotting output voltages vs. core displacements.

• All commercial devices are designed to operate only in the linear range and hence the name LVDT.

• Supply requirements 6.3 V at 60 Hz, Range of displacement-25 mm.

Linear Variable Differential Transformer (contd…)

Fig c

Linear range

CoreDisplacement

Output Voltage

Null position

Commercial LVDTs

Sectional View of LVDT

Advantages of LVDT

• It can be used as a primary detector transducer, as it does not require assistance of another element such as a strain gauge to convert mechanical displacement into electrical voltage.

• The core is completely separable from the rest of the equipment and hence no mechanical losses.

• It is insensitive to temperature changes.• Provides a comparatively high output and does

not require amplification.• It is of reasonable cost.

Disadvantages of LVDT

1) It cannot be used for dynamic measurements due to its appreciable mass compared to strain gauge.2) Requirement of 60 cps supply voltage becomes a limiting factor for dynamic measurements.3) The direction from the null point is to be indicated which makes the circuit arrangement complex.

Pneumatic comparators• In Pneumatic comparators air is used as a means of

magnification and hence they use principle of air jet.• A chamber is fitted with control orifice C and a

gauging orifice G through which air flows from a supply at a constant pressure P1.

• If the size of the control orifice C remains constant, any variation in size of G will cause alteration of pressure P2 in the chamber.

• This variation is measured by a suitable pressure gauge

graduated to read in linear units.

To pressure gaugePressure P2

Workpiece

G (Gauging orifice)

Chamber

Air @ constant Pr P1

C (Control orifice)

Principle of Pneumatic comparator

Systems of Pneumatic comparators• Based on the physical phenomenon, Pneumatic

comparators are classified as;(a) Flow or velocity type (b) Back pressure type

• Flow types operate by sensing & indicating the momentary rate of flow.

• Compressed air after filtering & pressure regulation flows through a glass tube with a small metal float.

• The air then passes through a plastic tube to the gauge head with two diametrically opposite orifices for the air to escape.

Pneumatic comparators (contd…)

Air supply

Filter

Pr regulator

Scale

Float

Plastic tube Bore to bemeasured

Gauging head

Flow or Velocity type Comparator

Flow or velocity type (contd…)

• The position of the float depends upon the amount of air flowing through the gauging head, which in turn depends upon the clearance between the bore to measured and the gauging head.

• These types when assembled side by side, can measure multiple inter related dimensions with great ease, accuracy & speed.

Back pressure type Pneumatic Comparators

Constantpressuresource

P1 P2To atmosphere

Oc mO

Principle of back pressure type Pneumatic comparator

Back Pressure Circuit

Air supply

FilterRegulator

Bourdon tube

Scale

Work piece

Measuring head

Back pressure type Pneumatic Comparators• The principle of back pressure gauges is that when

the orifice Om is blocked, the upstream pressure P1 becomes equal to pressure P2 between the two orifices.

• When the orifice opening Om is increased indefinitely, the pressure P2 tends to become zero.

• In the basic back pressure unit shown in fig, a bourdon tube deflects according to back pressure changes built up in the circuit when work piece is placed over the measuring head.

• The deflection is amplified by gear & lever and indicated on a dial.

Solex Pneumatic gauge• Solex type of gauge employs a water manometer

for the indication of back pressure.• It consists of a water tank filled up to a certain level

and a dip tube immersed into it upto required depth.

• Air, sent at higher pressure than required, bubbles to the top of the water tank.

• The air at the desired constant pressure then passes through the control orifice and escapes through the measuring jets.

• The back pressure in the circuit is indicated by the head of water displaced in the manometer tube.

Solex Pneumatic gauge

Air

Air filter Control orifice

Scale Plug withmeasuring jaws

workpieceManometer

Water tank

Dip tube

Excess airbubbling at top

Solex Pneumatic gauge

Solex Pneumatic gauge (contd…)• The pressure in the manometer is regulated by the

relative rates of escape of air through the control orifice and the measuring jets.

• The manometer tube is graduated linearly to show changes in pressure resulting from changes in internal diameter of the work being measured.

• By revolving the workpiece around the measuring gauge, the roundness or concentricity of the bore can be checked.

• By moving the workpiece along the length of the measuring gauge, taper of the bore, if any, can be checked.

• Best suited for checking roundness and taper of cylinder bores & gun barrels.

Advantages & Disadvantages of Pneumatic Comparators

Advantages:(1) No physical contact between gauge & work and hence practically no wear take place.(2) High magnification & accuracy possible.(3) Suitable for checking small bores.(4) Measuring pressure is very small and jet of air cleans the dust, if any, from the workpiece.

Disadvantages:(1) Requires elaborate auxiliary equipment such as pressure regulator, compressor, etc.(2) Parallax error in glass tubes which is an indicating device.(3) Scale is generally not linear and apparatus in not portable.

Brooke’s level Comparator

Spirit level

ScaleBubble

Ball feet

Column

Bracket

Spirit level

Rotatable disc

Base

Levelling scews

Brooke’s level Comparator

• It is a mechanical comparator which uses a sensitive spirit level and a rotatable disc.

• The rotatable disc is mounted on a massive base with leveling screws.

• A bracket supports the spirit level and allows it to be raised or lowered along the main column of the instrument.

• The upper surface of the rotatable disc is a plane lapped surface while the lower face has a lapped annular ring allowing easy rotation of the base.

Brooke’s level Comparator

GSG S

(c) (d)

Brooke’s level Comparator (Contd…)• In operation, the bubble is first brought to the center of the

scale by allowing the ball feet to rest on the surface of the rotatable disc and adjusting the leveling screws of the base.

• The standard S and the gauge to be compared G are now wrung to the disc, and the ball feet are made to rest on them as shown in fig.

• The position of the bubble is noted, the spirit level is then raised, and disc rotated through 1800.

• A second reading is taken in the position shown. • The displacement of the bubble along the scale represents

twice the difference in height between the two gauges.• Thus, H= ½(L) where H= difference in height between

gauges, L = bubble dispalcement.