Download EXPERIMENT NO. : 1 AIM : To Measure Displacement using LVDT

EXPERIMENT NO. : 1
AIM : To Measure Displacement using LVDT.
Principle and working :
LVDT stands for linear variable differential transformer. It works on the principle
of mutual induction LVDT illustrated in figure consist of three symmetrically spaced
coils bound out and illustrated bobbin. A magnetic core , which moves through the
bobbin, provides a path for magnetic flux linkage between coils. The position of the
magnetic core control the mutual inductance between the primary coils and two
secondary coils.
When a carrier excitation is applied to the primary coil, voltage is induced in the
two secondary coils that are wired in series opposing circuit. When the core is centered
between the two secondary coils, the voltage induced in secondary coils are equal but
cut of phase by 180° with series opposing circuit, the voltage in two secondary coils
contact cancel each other and the O/P voltage is zero. When the core is moved from the
centre position, an in-balance in the mutual inductance between the primary and
secondary coils occurs and an o/p voltage develops. The o/p voltage is linear function of
core position as long as the motion of the core is within the operating range of LVDT.
The direction of motion can be determined from the phase of o/p voltage.
Features :
1. The frequency of voltage applied to the primary winding can range from 50-25000
kHz.
2. Dynamic measurement is possible is the carrier frequency is 10 times greater than
the highest frequency component in dynamic signal.
3. The I/P ranges from 5 – 15 watts / amp i.e. volts.
4. The power required is less than 1 watt.
5. Range of sensitivity is from 0.02 – 0.2 V / nm.
6. Available in operating range ± 2 to ± 150 mm.
7. Like LVDT rotary variable differential transformer (RVDT) is used for angular
measurement.
Advantages :
1. There is no contact between the core and the coils. Hence infinite resolution and no
hysteresis
2. Non-contact ensures long life with no significant deterioration of performance.
3. Sensitivity is high as 40 mV/mm
4. Power consumption is less than 1 W.
5. better linear characteristics.
Disadvantages :
1. The mass of core and the friction limits the capabilities of sensor for dynamic
measurement.
2. Performance of transducer is affected by temperature.
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Procedure :
Make the connection between LVDT sensor and the digital display unit as
directed in manual . Switch ‘ON’ the circuit. Advance the LVDT core on one side. With
the R/n I/P. Record the o/p. Repeat the procedure for possible range i/p in regular steps
on one side. Withdraw the LVDT core to the null position in same steps it was advanced.
Take o/p while drawing i/p. Repeat same on other side of null.
Plot I/P vs O/P graph using least square fit method. Calculate linearity of
instrument.
Result :
From graph, it is clear that O/P varies linearly with I/P in both sense i.e. forward
and backward.
Right hand sensitivity = 0.949 & Linearity = 00453
Left hand sensitivity
= 0.949 & Linearity = 0.0488
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EXPERIMENT NO. : 2
AIM : To Measure Load using Digital Load Indicator.
Apparatus: A Link Type Load Cell, Cables, Weights, Weight Pan, Display Unit.
Introduction:
The various devices used for measurement of force are called load cells. The
typical ways to measure force on can be illustrated as :
i) Balance ii) Hydraulic Load Cells iii) Pneumatic Load Cells iv) Elastic Force devices
Of this, the first three methods are broadly the mechanical way of force
measurement. The elastic force devices employ the elastic members in the form of rings,
diaphragms, strips, cylinder etc.
Through mechanical methods employs dial gauges may be used in combination
with elastic members. The use of strain gauges is more commonly in load cells.
Principle and Working:
The figure shows the constructional details of strain gauges. The form gauges
are mounted on the periphery of cylinder. The two are mounted axially while other two
are circumferentially mounted when the force is applied on the cylinders, it undergoes
strain and hence the gauges depends responds by hanging their resistance. The two
axially mounted gauges are subjected to longitudinal where as other two are subjected
to transverse stresses. Since the two gauges sensing similar stress, are placed in its
opposite arms and the gauges sensing dissimilar strains are placed in adjustment arms.
The budge has the highest sensitivity. The voltage output across the Hodge is measure
of the applied load the voltage signal is further converted to digital mode before
displaced by the display unit.
Features:
1. The sensitivity of load cell depends on v/s area of elastic element, modules of
elasticity of element. The gauges & the I/P applied voltage.
2. High sensitivities are associated with low capacity cell.
3. The typical value of battery are up to 10V and the maximum rated load o/p is 30
mv/v.
Procedure :
As shown in figure, mount the pans to the load cell hook. Adjust the display to
show ‘0’ . For the pan of there is an initial bias record it, add weights to the pan in steps
of 0.5 kg up to 10 kg. Note down the readings. Repeat the procedure for downscale
readings.
Plot the I/P – O/P graph using least square fit method. Calculate the linearity of
instruments.
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Result :
1. From the graph, it is clear that output varies linearity with input
2. Linearity is found to be.
Precautions :
i)
ii)
iii)
Check that all the connection are tight.
See that the reading doubt fluctuate in the display.
Check for proper placement of weight in the pan.
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EXPEIMENT NO. : 3
AIM : To measure the Torque using Digital Torque Indicator and Torque Meter.
Apparatus : A torque cell, cable for connection, display unit, weights, etc.
Principle and working :
Torque are very similar to load cell, they consist of mechanical element (usually a
shaft with circular c/s) and a sensor (usually electric resistance strain gauges).
A general configuration of a strain gauge budge circuit widely employed for
torque measurement from a rotating shaft as shown in set up figure. Four bounded wire
strain gauges are mounted on a 45° helix with axis of rotation and are placed in pairs
diametrically opposite. If the gauges are accurately placed and have matched
characteristic, the system is temperature compensated and is sensitive to bending and
thrust or pull effects. Any change in gauge circuit, then results only for tensional
deflection. When the shaft is under torsion gauges ‘I’ & ‘4’ will elongate as a result of
tensile component of a pure shear stress on one diagonal axis, while gauges ‘2’ and ‘3’
will contact owing to compressive component on shaft between two flanges. A deflection
of shaft may be read one directly, if the shaft is stationary, however, this is not possible
in case of rotating shaft.
A flashing light of stroboscope is directed other diagonal axis. This tensile and
compressive principle strain can be measured and shaft torque can be calculated.
In a solid shaft of diameter d, subjected to torque T, the following relation exist.
T = fs
∏ d3
---------16
G θ ∏ d4
T = ------------32 L
Where fs :
G :
θ :
Shear stress induced in shaft.
Shear modules (rigidity)
Angle of twist in radians over a shaft length L.
Following are various ways to measure torque:
i)
Stroboscope method: A torque can be measured by means of measuring angular
displacement of two attachments set on shaft at some distance apart depending
upon convenience. This relative deflection may be measured by utilizing a
stroboscope.
For torque measurement two slotted flanges A & B are mounted on shaft. Flange A
carries a scale while flange B carries a pointer, when torque is applied to a shaft, it
causes displacement of pointer relative to scale on account of angular twist of length
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of shaft between two flanges. A deflection of shaft may be read once directly, if the
shaft is stationary, however, this is not possible in case of rotating shaft.
A flashing light of stroboscope is directly on the scale and the flashing frequency or is
adjusted till a stationary image is obtained. Scale reading can now be taken. Since
deflection is small the accuracy of measurement is poor. But this method is simple
and inexpensive.
ii) Inductance torque transducer : In order to get the high sensitivity, the angular
displacement between two flange must be measured with the help of linear
displacement transducer which are specially designed for measurement of small
displacement.
Flange A carries a coil & Flange B an iron core . This core moves in and out of coil
according to relative displacement of two flanges. Therefore inductance of the coil is
altered on account of relative displacement. Since the displacements depend on the
torque and hence the bridge output can be directly calibrated to load the torque.
Advantages :
i) Accurate and sensitive arrangements for torque measurement.
ii) It cam be employed for rotating shaft by using strip rings for single transmission
from rotating shaft to stationary budge circuit.
Procedure : As shown in figure, mount the pan into the holes of arm of load cell. If the
digital display unit shows sense value adjusts in to zero if possible otherwise note down
the initial bias. Measure the distance of hole and a pan from centre of torque cell. Add
weights to the pan in steps. Calculate the torque applied. Note down the torque display.
Repeat the procedure by varying distance of pan from load centre.
Result : The output values of torque varies linearly with input torque or applied torque.
The linearity is found to be loss.
Precaution : Reading of output strain should be noted only where reading reaches
steady state.
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EXPERIMENT NO. : 4
AIM : To measure Strain using Strain Gauge.
Principle and Working :
Electrical resistance strain gauges are thin metals foils, grids, that can be adhesively
bonded to the surface of a component or structure is load, the strain develops and are
transmitted to the foil grid changes in proportion to the body strain.
The resistance are of uniform metallic conductor can be expressed as.
ρL
R = -----A
And where
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ρ = Specific resistance of metal.
L = Conductor length
A = c/s area of conductor.
Gage Factor F of the conductor is defined by (dR/ R) / (dL/L) = (dR/ R) / ε
dR
dρ
------ = ----R
ρ
+
dL
----- ( 1 + 2V)
L
equation - 2
The strain sensitivity of metal is expressed as
dR
--- / εa = ( d ρ / ρ) / εa + (1 + 2v)
R
equation - 3
It is evident from the above equation that the sensitivity of a metal or alloy is due
to two factors namely, the changes in the dimensions of conductor as expressed by the
term ( 1 + 2v) & the change in specific resistance.
The Wheatstone Bridge Circuit :
The output DR / R of strain gauge us usually converted to a voltage signal with
Wheatstone bridge. The bridge is initially balanced and the o/p occurs across the point
B – C is zero. For this bridge null position, the. Condition,.
R1 X R3 = R2 X R4 is satisfied
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As the arms of the bridge changes resistance the o/p across B – C is given
expression.
∆ Eo = r / ( 1 – r )2 X
∆R1 ∆R2
∆R3 ∆R4
---- - ----- - ----- - ---R1
R2
R3 R4
Where, r = R2 / R1
The gauge can be placed in R1, R1 and R2 and all the arms of the bridge
accordingly referred as quarter wave half wave or full wave bridge.
When all the resistances of strain gages are same, R1 = R2 = R3 = R4 The current
flow through the galvanometer is zero and the bridge is said to be balance. When the
resistance of strain gages changes then the bridge becomes imbalance and the current
through galvanometer is given by
Ig = - E dR1 / 4R1(R1 + Rg) = - E F ε / 4 (R1 + Rg)
Features :
1. Though these are various types of gauge, most commonly used gauges are
metal foil gauges.
2. The metal foil gauges are very fragile & easy to distort, wrinkle or tear. Hence
metal grid is bounded to a thin plastic film. This also provides the electrical
insulation between the gauge and the component, when the gauge is mounted.
3. The shortest available is 0.20 mm and the largest is 102 mm.
4. The gauges can sense the strain as well as temp change. The effect of temp.
change by comported by using proper bridge arrangement.
5. The gauges can be manufactured in various combinations for special transducer
applications.
Advantages :
1. It gives electrical signals.
2. Easy to mount, can be bounded on component on which strain measurement is
desired.
3. Sensitivity can be improved by using more no of bridges & proper combination.
Disadvantages :
1. It need special care to achieve temp. compensation.
2. Ti cannot be reused once removed from component.
3. It o/p is usually small and need amplifier.
Procedure :
1. Measure the length, width & thickness of cantilever beam.
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2. Make the connection between the strain gauge. Terminal and the digital display
unit as directed in the manual.
3. For the two gauge half wave bridge, connect the common terminal of the two
gauges in position A and the other two in C & D positions.
4. Switched ON the circuit. Add then two weights to the weight pan.
5. Calculate the strain at the given c/s.
6. Wait for sufficient time so that the reading becomes stable.
7. Note down the o/p strain shown by the instrument. Increase the I/P in steps, note
down the reading.
Note down observations while decreasing the I/P in manual steps.
Plot the I/P - O/P using least square fit method. Calculate linearity.
Result :
O/P of strain ≈ I/P strain, linearity =.
Hence use of such gauges reduces error in the measurement.
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EXPERIMENT NO. : 5
AIM : To measure the pressure using the pressure cell.
Apparatus : A Pressure Cell, Cables for connection, display unit, pressure gauges etc.
Principle and Working of Pressure Cell :
Diaphragm gauges is a thin plate of circular shape clamped firmly around its
edges. The diaphragm gets deflected in accordance with the pressure differential across
the sides, deflection being towards the law pressure side. The deflection can be sensed
by an appropriate displacement transducer i.e. it may be converted into electrical signal
or may undergo in mechanical amplification to permit display of the O/P of an indicator
dial.
These are two basic types of diaphragms element design.
i) Metallic diaphragm which depends upon its own resilience for its operation.
ii) Non-metallic or stuck diaphragm which employs a soft-flexible material with no
elastic characteristics. The movements of the diaphragm are opposed by a spring
which determines the deflection for given pressure.
The general requirement of the diaphragm are :
1. Dimensions and total load must be comparable with physical properties of
intermediate used.
2. Flexibility must be such as to provides the sensitivity required by secondary
transducer.
3. Volume of displacement should be minimize to provide the reasonable dynamic
response.
4. Natural frequency of diaphragm should be sufficiently high to provide satisfactory
frequency response.
5. The O / P should be linear.
6. The diaphragm to response linearly its maximum deflection ‘y’ should be less than
1/3 of its thickness.
y
Where
P
V
E
R
r
=
3
(1 - V2)
-- P - ------16
Et3
( R2 - r2)2
= Pressure
= Poisson’s ratio
= Modules of elasticity of diaphragm.
= radius of diaphragm
= radius at point of interest
The deflection for diaphragm is given by above equation.
The natural frequency of diaphragm should be high enough for good dynamic
response.
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Advantages of Diaphragm :
1. Relatively small size and moderate cost.
2. Capability to withstand high over pressure and maintain good linearity over over a
wide range.
3. Availability of gauges for absolute and differential pressure.
4. Minimum of hysteretic k no permanent zero shift.
Disadvantages of pressure cell :
1. Ned protection from shock and vibration.
2. Cannot be used to measure high pressure & is difficult to repair.
Application of diaphragm pressure cell :
Typical applications are low pressure absolute pressure gauges and many types
of recorders and controllers operating in low range of direct or diff. pressure.
Procedure :
Connect the Pressure Cell with the compressor and note down the initial pressure by
releasing valve. Then Take corresponding o/p reading from digital display to pressure
cell by releasing the pressure valve and note down the valves for difficult different
readings. Plot the graph between I/P and O/P pressure.
Result : The linearity is found to be and the slope calculated is
& graphically is.
Conclusion : From graph, it is clear that output varies linearly with input.
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EXPERIMENT NO. : 6
AIM : To measure speed using
i) Magnetic Pickup.
ii) Photoelectric Pickup.
iii) Stroboscope
Principle and working of
i) Magnetic Pickup :
The voltage is induced in the coil around to a magnetic subjected to change in
magnetic field, this is principle of electromagnetic induction.
The arrangement is shown in fig. As shaft rotates, the teeth pass in front of pick
up and produce a change in field resistance of magnetic circuit. The density of a
magnetic field increases and decreases as each tooth approaches and leaves away
from the end of the bar magnet per second.
Let T be no. of teeth on the rotor, N be be the revolutions per second and P be
the no. of pulse per sec. Then,
Speed N =
Pulses per sec.
----------------- =
No. of teeth
P
--- x 60 rpm
T
Procedure : Connect the motor to the dimmerstat to vary the I/P supply, by which
speed changes. Connect the motor connection to speed measurement motor changing
the speed take the corresponding reading of motor and note down on the observation
table. Find out % error from the graph.
ii) Photoelectric pickup :
The photoelectric pick up tachometer utilize a rotating shaft to the intercept a
beam of light flow on a photoelectric or photo-conductive cell. The shaft has an
intermittent reflecting (white) and non reflecting (blade) surface. When a beam of light
the reflecting surface on a rotating shaft, light pulses are obtained and reflected light is
focused on the photoelectric cell. The frequency of the light pulses is proportional to
shaft speed and so will be frequency of electrical o/p pulses from photoelectric.
Another similar method consist in mounting an opaque disc on shaft. The
rotating component the disc has a no. of evenly spaced peripheral. Holes. A light source
on the side of disc and there is light sensor on opaque portion of disc between the lights
source & the light sensor, no light falls on sensor and consequently on o/p results. But a
pulse of voltage is produced every time a hole appear between the two is light
illuminates the sensor. The frequency of pulse generations is determined by no. of holes
in disc and its speed of rotations. Since no of holes is fixed, pulse of repetitions,
frequency of transducers o/p is proportional to angular velocity of moving member.
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Procedures: Connect the motor to dimmerstat and dimmerstat to supply source. Switch
the lights and passes through the light from hole and rotor. Switch the photo sensor
and take reading from motor.
III) Stroboscope: The phenomenon used in stroboscope depends upon the imperfect
dynamic response of human eye. It consist of an electronic stroboscopic lamp which
flashes at known and adjustable rate. The device has source of variable frequency
flashing light. The disc with a mark attached to a motor is rotate in front of the flashing
light of stroboscope,
When the speed of the disc is adjusted with the flashing light, the mark on the
disc appears stationary.
The disc speed =speed of shaft = frequency of flashing light / no of images \
N = f / No of images
Procedures : Connect the motor to dimmerstat and dimmerstat to supply source.
Switch the lights of the stroboscope and keep it in front of the disc. Adjust the constant
speed of the motor and adjust the frequency of flashing light of the stroboscope such
that the image on the disc appears stationary. Note the speed and frequency. Repeat
the procedure by changing the speed of the motor.
Result : From graph, it is seen that input and output are linear.
Precautions :
1. Always start the motor with help of zero speed., operate dimmerstat slowly.
2. Connect motor to earthing wire.
3. When experiment is over disconnect the photoelectric and magnetic pick up.
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