05 Strain Gage

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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

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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.

05 Strain Gage

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