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Chapter-11
School of Electrical Engineering andComputer Science, NUST
1
Helfrick & Cooper
11-3
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INTRODUCTION OF TRANSDUCERS
• A transducer is a device that convert one form of energy toother form.
• A sensor converts a physical parameter to a usable electricalsignal (output).
• An actuator converts an electric signal to a physical output.
• In other word transducer is a device that is capable ofconverting the physical quantity into a proportional electricalquantity such as voltage or current.
Pressure Voltage
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BLOCK DIAGRAM OF TRANSDUCERS
• Transducer contains two parts that are closely related toeach other i.e. the sensing element (sensor) andtransduction element (transducer).
•
The sensing element is called as the sensor. It is device producing measurable response to change in physicalconditions.
• The transduction element convert the sensor output tosuitable electrical form.
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Actuators
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Strain Gages
School of Electrical Engineering andComputer Science, NUST
7
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Basic Definitions related to Strain
Gages
• Stress (s): Applied force per unit area (kg/2)
• Strain (): Elongation per unit length
• Young's modulus (E ): describes tensile elasticity,
or the tendency of an object to deform along an
axis when opposing forces are applied along that
axis; it is defined as the ratio of tensile
stress to tensile strain. It is often referred tosimply as the elastic modulus.
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WHAT IS STRAIN?
Pressure is external applied force per unit area, while stress is the internalresisting force of the material per unit area to the applied force.
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Strain Gages
• It is a Passive Transducer
• Converts Mechanical displacement tochange in resistance
• Wafer like device that can be attachedto variety of material to measureapplied strain
• Basic Principle: Resistance is proportional to applied strain
•
In this Pic, the gage is more sensitiveto strain in the vertical direction thanthe horizontal direction.
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Strain Gages
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Strain Gages
Mechanical strain gauge used to measure thegrowth of a crack in a masonry foundation. This
one is installed on the Hudson-Athens Lighthouse
http://en.wikipedia.org/wiki/Hudson-Athens_Lighthousehttp://en.wikipedia.org/wiki/Hudson-Athens_Lighthousehttp://en.wikipedia.org/wiki/Hudson-Athens_Lighthousehttp://en.wikipedia.org/wiki/Hudson-Athens_Lighthouse
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Gage Factor
• Sensitivity of strain gage is denoted by gage
factor “G” or “K”.
• It is the unit change in resistance per unit
change in length
• It gives the effect of applied stress on the
resistance in a strain gage.
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Gage Factor
Poisson’s ratio is ratio of strain
in lateral to strain in axial dir.
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Gage Factor
• High sensitivity is desirable for strain gage
application
• A large gage factor implies a higher sensitivity
• E.g.
– Constantan Wire, K=2
– Alloy 479, K=4
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Stress Strain and Resistance of a
Copper Wire
• Find the Resistance of the given
wire, where for copper resistivity is
1.68 x 10-8-m, allowable stress
is 200MPa
• Resistance= 2.139
L=1m
D=0.1mm
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Application of Wheatstone Bridge to
measure small Resistances
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Half Bridge Strain Gage Circuit
Temp. Comp.
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Metallic Sensing Element
• Metallic strain gages are formed from – Thin resistance wire
• Small in size
– Etched from thin sheets of metal foil• Relatively Larger
• Different materials used in strain gage are – Constantan (Copper Nickel Alloy)
– Nichrome V (Nickle Chrome Alloy)
– Dynaloy – Stabiloy
– Platinum-Tungsten alloy
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Uniaxial Strain Gage
• Sensitive to strain in the direction of interest
• Sensitivity to transverse strain is low
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Gage Configurations
• Rosette Gages: A Rosette is an arrangement of
two or more closely positioned gage grids,
separately oriented to measure the normal
strains along different directions in the underlyingsurface of the test part.
• Two Element configurations are normally used
when the principle strain directions are known• In multi-element gage configurations numbering
of each gage is done CW or in CCW manner.
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Gage Configurations
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Gage Configurations
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Unbonded Strain Gage
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Typical Applications of Strain Gage
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Typical Applications of Strain gage
• They are used in smart Bridges for
maintenance
• The Tsing Ma Bridge in Hong Kong has 350
sensors to measure it.
http://en.wikipedia.org/wiki/Tsing_Ma_Bridgehttp://en.wikipedia.org/wiki/Tsing_Ma_Bridgehttp://en.wikipedia.org/wiki/Tsing_Ma_Bridgehttp://en.wikipedia.org/wiki/Tsing_Ma_Bridge
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Typical Applications of Strain gage• Strain gauges are used to keep rails in good condition.
• Skin attachable, highly sensitive SGs
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Typical Applications of Strain Gage
• Students in the University of California aretrying to identify what type of car is drivingon the road using only a strain gauge.
• The strain gauges are installed in a bridgedeck to measure how much strain is put onthe sensor. A car will drive over the gaugeand apply strain. The more strain that is puton the sensor, the bigger the vehicle.
• The purpose of the sensor is to find apattern of what types of areas use what carsto classify them, to monitor traffic indifferent areas, and protect the privacy oftransportation users.