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Dr. Dayananda Pai

Dept. of Aero & Auto Engg.

M.I.T., Manipal

Man is a tool-using animal. Weak in himself and small stature, he stands on a basis of some half-square foot, has to straddle out his legs lest the very winds supplant him. Nevertheless he can use tools, can devise tools with these the granite mountain melts in to light dust before him; seas are his smooth highway, winds and fire his unwearying steeds. Nowhere do you find him without tools. Without tools he is nothing. With tools he is all.

Thomas Carlyle (1795-1881)

A comparator works on relative measurements, i.e. to say, it gives only dimensional differences in relation to a basic dimension. So a comparator compares the unknown dimensions of a part with some standard or master setting which represents the basic size, and dimensional variations from the master setting are amplified and measured.

Advantages of comparators: Not much skill is required on the part of operator in its

use.

The calibration of instrument over full range is of no importance as comparison is done with a standard end length.

Since range of indication is very small, being the deviation from set value, a high magnification resulting into great accuracy is possible.

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Characteristics of Comparators The instrument must be of robust design and construction

so as to withstand the effect of ordinary usage without impairing its measuring accuracy.

The indicating device must be such that readings are obtained in least possible time and for this, magnification system used should be such that the readings are dead beat. The system should be free from backlash, and wear effects and the inertia should be minimum possible.

Provision must be made for maximum compensation for temperature effects.

The scale must be linear and must have straight line characteristic

Characteristics of Comparators Indicator should be constant in it return to zero.

Instrument, though very sensitive, must withstand a reasonable ill usage without permanent harm.

Instrument must have the maximum versatility, i.e., its design must be such that it can be used for a wide range of operations.

Measuring pressure should be low and constant.

Uses of Comparators In mass production, where components are to be checked

at a very fast rate.

As laboratory standards from which working or inspection gauges are set and correlated.

For inspecting newly purchased gauges.

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.

In selective assembly of parts, where parts are graded in three or more groups depending upon their tolerances

Mechanical Comparators Dial Indicator

Johnson Mikrokator

Read type Mechanical Comparator

Sigma Comparator

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Dial Gauge Consist a base with rigid

column.

An adjustable arm caries the dial gauge.

An anvil & worktable are mounted on the base.

Operation:

The indicator is set to zero by slip gauge representing the basic size of the part.

The gear & pinion arrangement magnifies the plunger movement.

The variation in size of the part is indicated on the dial.

Johnson Mikrokator It uses a twisted strip to

convert linear movement of plunger to large circular movement of a pointer.

The twisted strip carries a pointer at its centre, one end is connected to adjustable cantilever strip and other end is anchored to the spring elbow which acts as bell crank lever.

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The upward movement of the plunger makes the bell crank-lever to rotate applying tension on the strip.

This tension causes the strip to untwist resulting in the movement of pointer.

The magnification of the instrument is ~ the ratio of rate of change of pointer movement to the rate of change in length of strip. (dQ/dL)

Magnification of instrument is [(dQ/dL) X (L/w2n)]

Where w is width of strip and n is the number of turns.

Sigma Comparator

Cross strip used in sigma Comparator

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Sigma Comparator It consist of a plunger mounted on two flat springs.

The plunger holds a knife edge which bears upon the face of the moving block of the cross strip hinge.

The displacement in vertical direction of the plunger makes moving block of the cross strip lever to pivot. This cause rotation of arm ‘y’, the metallic band attached to the arm make the driving drum and the pointer to rotate.

The ratio of effective length (Y) of the arm and distance (x) of the knife edge from the pivot gives the first stage magnification.

The ratio of pointer length (l) and radius (r) of the driving drum gives 2nd stage magnification.

Thus the total magnification is [(Y/x) X (l/r)]

A magnification of upto 5000 can be obtained.

Advantages of mechanical comparators

1. These instruments are usually cheaper in comparison to other devices of amplifying.

2. These instruments do not require any external agency such as electricity or air and as such the variations in outside supplies do not affect the accuracy.

3. Usually the mechanical comparators have linear scale.

4. These are usually robust and compact and easy to handle.

5. For day to day workshop works, these instruments are very suitable and being portable can be issued from the store.

Disadvantages of mechanical comparators 1. These instruments usually have more moving linkages as

compared to other types. Due to more moving parts, the friction is more and ultimately the accuracy comes down.

2. Any slackness in moving parts reduces the accuracy considerably.

3. The mechanisms in mechanical comparators have more inertia and this may cause them to be sensitive to vibrations.

4. Any wear, play, backlash or dimensional faults in the mechanical devices used will also be magnified.

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

6. Error due to parallax are more likely with these instruments as the pointer moves over a fixed scale.

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Mechanical optical comparators In mechanical optical comparators small displacements of

the measuring plunger are amplified first by a mechanical system consisting of pivoted levers. The amplified mechanical movement is further amplified by a simple optical system involving the projection of an image. The usual arrangement employed is such that the mechanical system causes a plane reflector to tilt about an axis and the image of an index is projected on a scale on the inner surface of a ground-glass screen. Optical magnifications provide high degree of measuring precision due to reduction of moving members and better wear resistance qualities. Optical magnification is also free from friction, bending, wear etc.

In this system, Mechanical amplification = 20 /1 , And, Optical amplification 50 /1 x 2 It is multiplied by 2, because if mirror is tilted by an angle δθ, then image will be tilted by 2 x δθ. Thus overall magnification of this system = 2 x (20/1) ( 50/1 =2000 units)

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Optical Projectors

Tool maker’s microscope

Tool makers microscope

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Advantages of Optical Comparators 1. It has small number of moving parts and hence a

higher accuracy.

2. In the optical comparators, the scale can be made to move past a datum line and thus have high range and no parallax errors.

3. It has very high magnification.

4. Optical lever is weightless

Disadvantages 1. As the instrument has high magnification, heat from

the lamp, transformer etc. may cause the setting to drift.

2. An electrical supply is necessary.

3. The apparatus is usually large and expensive.

4. When the scale is projected on a screen, then it is essential to use the instrument in a dark room in order to take the readings easily.

5. The instruments in which the scale is viewed through the eyepiece of a microscope are not convenient for continuous use

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

Electrical and Electronic Comparators ELECTRIC COMPARATORS These comparators depend for their operation on

Wheatstone bridge circuit. In d.c. circuit, a change of balance of the electrical resistance in each arm of the bridge is caused by the displacement of an armature relative to the arm under the action of the measuring plunger. Once out of balance is caused in the bridge, it is measured by a galvanometer graduated to read in units of linear movement of plunger.

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Electrical and Electronic Comparators In actual measuring instruments, one pair of

inductances is formed by a pair of coils in the measuring head of the instrument. The movement of the plunger displaces an armature, thus causing a variation in the inductance of a pair of coils forming one arm of a.c. bridge. The arm carries the armature and the inductance in the coils is dependent upon the displacement of the armature relative to the coils.

ELECTRIC COMPARATORS In general, two important applications of electrical

comparators are of the greatest interest:

the use of electrical comparators as measuring heads,

the use of electrical gauging heads to provide visual indication as to whether a dimension is within the limits laid down.

Electrolimit Gauge Vertical movements of the

plunger are transmitted to an armature, which is suspended, as shown in the diagram, on thin metal strips. At the left-hand side of the armature it will be seen that it lies between two electromagnetic coils A and B. These coils form two arms of an a.c. bridge circuit.

Any movement of the armature between the two electromagnetic coils sets up out-of-balance effects, which are recorded.

A great advantage possessed by this electrical comparator is the dual magnification available. A simple switching arrangement enables a second magnification to be obtained, exactly double the first.

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Visual Gauging Heads Gives visual indication, using small coloured signal

lamps, of the acceptability of an engineering component with regard to the dimension under test.

with reference to Fig. Vertical displacement of an interchangeable plunger causes movement of the rod C either to the left or right, as shown in the diagram. A and B are electrical contacts, capable of precise adjustment in the direction of the arrows.

In the position shown, that is to say with the rod in mid-position between the contacts A and B, the dimension under test is within the limits. If the dimension is oversize, the rod C moves to the right and makes contact with B. Immediately the top red lamp is illuminated. Likewise if the dimension is undersize the rod moves to the left, making contact with A and illuminating the yellow lamp.

Principle of LVDT

LVDT works under the principle of mutual induction, and

the displacement which is a non-electrical energy is

converted into an electrical energy.

LVDT consists of a cylindrical former where it is

surrounded by one primary winding in the centre of the

former and the two secondary windings at the sides. The

number of turns in both the secondary windings are equal,

Iron core is placed in the centre of the cylindrical former

which can move in to and fro motion as shown in the

figure

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This is a LVDT. Linear movement of the core changes the Impedance. The electrical output changes in proportion to the core movement.

LVDT Components

Signal conditioning circuitry

Primary coil

Secondary coil

Secondary coil

Bore shaft

Ferrous core

Cross section of a LVDT

Epoxy encapsulation

Stainless steel end caps

High density glass filled coil forms

Magnetic shielding

Underlying Principle Electromagnetic Induction:

Primary Coil (RED) is connected to power source

Secondary Coils (BLUE) are connected in parallel but with opposing polarity

Primary coil’s magnetic field (BLACK) induces a current in the secondary coils

Ferro-Metallic core (BROWN) manipulates primary’s magnetic field

Working of LVDT

Case 1: During displacement, if the core remains in the null

position itself without providing any movement then the voltage

induced in both the secondary windings are equal which results

in net output is equal to zero i.e., Esec1-Esec2=0

Case 2:When an external force is applied and if the steel iron

core tends to move in the left hand side direction then the emf

voltage induced in the secondary coil 1 is greater compared to

the secondary coil 2. Therefore the net output will be Esec1-

Esec2

Case 3:When an external force is applied and if the steel iron

core moves in the right hand side direction then the emf

induced in the secondary coil 2 is greater compared to the

secondary coil 1. therefore the net output voltage will be Esec2-

Esec1.

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Advantages of Electrical Comparators 1. The electrical comparators have got small number of

moving parts. 2. It is possible to have a very high magnification and the

same instrument may have two or more magnifications. Thus the same instrument can be used for various ranges.

3. The mechanism carrying the moving core is very light and not sensitive to vibrations.

4. As the instrument is usually operated on A.C. supply, the cyclic vibration substantially reduces errors due to sliding friction.

5. The measuring unit can be made very small and it is not necessary that the indicating instrument be close to the measuring unit, rather it can be remote also.

Disadvantages 1. It requires an external agency to operate i.e., the A.C.

electrical supply. Thus the variations in voltage or frequency of electric supply may affect the accuracy.

2. Heating of coils in the measuring unit may cause zero drift and alter the calibration.

3. If only a fixed scale is used with a moving core then with high magnifications a very small range is obtained.

4. This is usually more expensive than mechanical instrument.

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Pneumatic Comparators Air gauging has rapidly increased during some past

time due to the following important characteristics.

Very high amplifications are possible.

No physical contact is made either with the setting gauge or the part being measured.

Internal dimensions and forms can be readily measured

Pneumatic Comparators It is independent of operator skill.

High pressure air gauging can be done.

Gauging pressures can be kept sufficiently low to prevent part deflection.

Out of roundness, taperness, concentricity, regularity can be measured

Not only it measures the actual size, but it can also be used to salvage oversized pieces for rework or to sort out for selective assembly.

Pneumatic Comparators The total life cost of the gauging heads in much less.

It is accurate, flexible, reliable, universal and speedy device for inspecting parts in mass production.

It is best suited for checking multiple dimensions and conditions on a part simultaneously in least possible time. It can be used for parts from 0.5 mm to 900 mm diameter having tolerance of 0.05 mm or less. It can be easily used for on line measurement of parts as they are being machined and take corrective actions.

Principle of Pneumatic gauge Pneumatic comparators work on the

principle that, an air-jet is in close proximity with a surface, the flow of air out of that jet is restricted.

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Fig. shows a curve between the air flow and the clearance between the part and the orifice in gauge head.

In a correctly designed pneumatic devices the ratio of the orifice areas is so proportioned that within a limited range of restriction the rate of change of p is uniform. i.e., dp/dL=constant. The effective area M of air escapement from the measuring jet is M=πDL

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Investigations have shown that when the ratio p/P and the area ratio (M/C) are plotted over a wide range of supply pressure (15 to 500kN/m2). That portion on all curves where p/P lies between 0.6 and 0.8 is linear to within 1% and the intercept on the p/P axis is almost 1.10 in all cases

Experimentally determined characteristics for different operating pressures

The general linear equation may be

p/P=k-b(M/C) --------(1)

k =intercept on the p/P axis and is found to be 1.10.

b = slope (0.4 < b < 0.6)

M = area of the measuring orifice i.e. πD L

C = area of control orifice

Mmax-Mmin=(1/2)Mavg

Pneumatic sensitivity As p and M are the only variables in eq.(1)

differentiating p with respect to M the pneumatic sensitivity is obtained as

1 1

(2)

dpb

P dM C

dp Por b

dM C

The important factor governing pneumatic sensitively is the size of the control orifice i.e. C and it can be seen from eq. that the pneumatic sensitively is inversely proportional to C.

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However, it must be remembered that M is dependent upon C since if we work in the range 0.6 <p/P < 0.8, then the average value of M corresponds to p/P = 0.7. Substituting this in eq. (1), gives

0.7 1.1

0.4

0.4

avg

avg

avg

Mb

C

M bC b or

C M

Substituting in eq 2

Ignoring negative sign

0.4

avg

dp P

dM M

0.4(3)

avg

dp P

dM M

From above it is obvious that pneumatic sensitivity is seen to vary directly as P and inversely as Mavg. Increase in P decrease indicator sensitivity. Hence a high pneumatic sensitivity requires that the average escapement i.e. Mean clearance between the measuring orifice and the surface must be small.

Overall magnification of a pneumatic system

The overall magnification is the rate of change of output with respect to input.

The output variable is a pressure gauge or water column reading and the input variable is surface displacement. Three factors combine to produce the overall magnification in a pneumatic system;

(a) The pneumatic sensitivity: dp/ dM

(b) The indicator sensitivity i.e. output gauge magnification where G is the Gauge reading: dG/dp

(c) The measuring head sensitivity i.e. the rate of change of M with respect to the displacement of the restricting surface: dM/dL.

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Overall magnification

Lmax - Lmin = (½)Lave

If the linear scale of length R provides readings of p over the range zero to P then

(dG/dp)=R/P

Hence

(4)dG dp dG dM

dL dM dp dL

dMM DL and D

dL

0.40 (5)avg

dG R

dL L

For example in a pneumatic measuring system in which the scale length is 500 mm and the linear range is 0.02 mm, the overall magnification will be

5000.40 0.4 5000

0.04avg

dG R

dL L

Equations (3) and (5) indicate that increased magnification could be obtained by increased operating pressure and scale length but in practice this is inconvenient if the scale is to be kept to a reasonable length. But the increased magnification can be obtained by reducing the linear range and this is achieved by careful proportioning of the orifice areas.

Solex Pneumatic Comparator The well known "Solex" pneumatic

comparator consists of a reverse acting, indirect measuring head, mounted to a rigid vertical post upon which it may be adjusted for height.

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Solex Pneumatic Comparator Solex Pneumatic Comparator Any variation in the dimension changes the value of h,

e.g. Change in dimension of 0.002 mm changes the value of h from 3 to 20 mm. Moderate and constant supply pressure is required to have the high sensitivity of the instrument.

The advantage of the reverse acting head is that a measured part larger than the setting standard causes the liquid column to rise against the scale.

Applications of pneumatic comparators Applications of pneumatic comparators

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Applications of pneumatic comparators Applications of pneumatic comparators

Differential comparator Advantages of Differential Circuit over Single Channel

Circuit. (i) Effect of change of operating pressure P: The operating

pressure may vary slightly from the designed value. In case of differential circuit the error would be 0.1 times the change in pressure.

(ii) Zero setting of master gauge is an extra advantage. (iii) Rectification for control orifice: In a single channel

system, the practical limitations may not give the perfectly correct and accurate dimension of the control orifice as designed. Therefore, in order to avoid the error of manufacture, we need a needle valve so that area may be adjusted accordingly.