FUCK1

1. 05-09-2014
1
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 maximumcompensation for
temperatureeffects.
 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 laboratorystandards from which working or inspection
gauges are set and correlated.
 For inspecting newly purchased gauges.
 Attached with some machines, comparatorscan 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
pointerat 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 electricityor 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 accuracycomes 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 membersand 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-Metalliccore (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 mechanismcarrying 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)
dp
b
P dM C
dp P
or 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
M
b
C
M b
C 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 indicatorsensitivity 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
dM
M 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
500
0.40 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 orificeas
designed. Therefore, in order to avoid the error of
manufacture, we need a needle valve so that area may be
adjusted accordingly.