Semiconductor Strain GagesOutlineReference ListTo Explore FurtherMajor ApplicationsStrain TheoryStrain Theory (cont.)Slide 8Strain Gage FactorsUnbonded and Bonded Strain GagesSlide 11Ideal Strain Gage vs. Diffused Strain GagePrinciple of MeasurementMeasurement CircuitsStrain Gage SpecificationsHBM SLB-700A/06 SpecificationsSlide 17HBM SLB-700A/06 MountingSemiconductor Strain GagesBrandon WithersECE 5320 MechatronicsAssignment #1OutlineMajor ApplicationsStrain TheoryStrain Gage FactorsUnbonded and Bonded Strain GagesSemiconductor Strain GagesPrinciple of Measurement and Measurement CircuitsStrain Gage Specifications for the HBM SLB-700A/06Reference List W. Tompkins, J. Webster, Interfacing Sensors To The IBM PC. Englewood Cliffs, NJ.:Prentice Hall Inc., 1988. National Instruments, (2004, Feb. 21). “Measuring Strain with Strain Gauges” (2004) [Online]. Available: http://zone.ni.com/devzone/conceptd.nsf/webmain/C83E9B93DE714DB08625686600704DB1?OpenDocument Omega Engineering, (2004, Feb. 21). “The Strain Gage” (2003) [Online]. Available: http://www.omega.com/literature/transactions/volume3/strain.html HBM, (2004, Feb. 21). “SLB-700A/06 – Strain Transducer” (2003) [Online]. Available: http://www.ae.utexas.edu/courses/ase389/midterm/larry/sircxsar.htmTo Explore FurtherStrain Gage Tutorials -- http://www.omega.com/techref/strain-gage.html -- http://www.vishay.com/company/brands/measurements- group/guide/indexes/tn_index.htmManufacturers of Strain Gages and Accessories -- http://www.hbm.com/en/ -- http://www.smdsensors.com/ -- http://www.omega.com -- http://www.vishay.comUseful Site With Many Strain Gage Links -- http://users.telenet.be/educypedia/electronics/sensorsdis.htmMajor ApplicationsForce, Torque and Pressure TransducersLoad Cells in Commercial Scales, Tank and Vessel WeighingBathroom and Kitchen ScalesStress Analysis TestsMeasurements of deflection angles of control surfaces (helicopter blades, wing flaps, elevators, etc.)Strain Theory Strain is the amount of deformation of a body due to an applied force or in other words strain is the fractional change in length shown in the figure above. Strain can be positive (tensile) or negative (compressive).“Measuring Strain with Strain Gauges”Strain Theory (cont.) For a wire of cross-sectional area A, resistivity ρ, and length L the resistance is given byLRAr=When the wire is stretched, the cross-sectional area A is reduced, which causes the total wire resistance to increase. In addition, since the lattice structure is altered by the strain, the resistivity of the material may also change, and this, in general, causes the resistance to increase further. Both effects are included in the following equation:(1 2 )R LR LD + s D Dr= +r= fractional resistance change= Poisson’s ratio= fractional change in length= fractional change in resistivityRRDsLLDDrrStrain Theory (cont.) To provide a means of comparing the performance of various gage materials, the gage factor, or strain sensitivity, of a gage is defined as/1 2/ /R RGL L L LD Dr /r= = + s +D DHigher gage factors are generally more desirable because the higher the gage factor the higher the resolution of the strain gage.Strain Gage FactorsSemiconductor materials such as Silicon and Germanium are used for strain gages because of their high gage factors.“The Mechatronics Handbook”Unbonded and Bonded Strain Gages The unbonded strain gage consists of a wire stretched between two points in an insulating medium such as air. Four gages are normally connected in a Wheatstone bridge circuit and arranged so that two gages are lengthened and two shortened by the displacement.A bonded strain-gage element, consisting of a metallic wire, etched foil, vacuum-deposited film, or semiconductor bar, is cemented to the strained surface.“The Mechatronics Handbook”Semiconductor Strain GagesStrain-gage technology advanced in the 1960s with the introduction of the semiconductor strain-gage elementsSilicon gages are formed from single-crystal silicon whose orientation and doping are the most important design parameters. The gage factor depends on the resistivity (determined by the doping) and the crystal orientation.Bonded semiconductor gages are made by slicing sections from specially processed silicon crystals and are available in both n and p types. The high gage factor is accompanied by high-temperature sensitivity, nonlinearity, and mounting difficulties.Diffused semiconductor gages utilize the diffusion process employed in integrated-circuit manufacture. This type of construction may allow lower manufacturing costs in some designs, since a large number of devices can be made on a single silicon wafer. The deviation from linearity is approximately 1%Ideal Strain Gage vs. Diffused Strain GageIdeal Strain Gage PropertiesSmall in size and massLow in costAccurate and repeatableEasily attachedPerfect output signalInfinite pressure rangeHighly sensitive to strain but insensitive to ambient or process temperature variationsDiffused Strain Gage Properties Small in size and mass Low in cost Accurate and repeatable Tricky to attach High output signal Wide pressure range Highly sensitive to strain but limited to moderate-temperature applications and requires temperature compensationPrinciple of MeasurementMechanical loading produces a change of length in the measurement object, which is conveyed to the strain gauge. Because there is a change in length, the electrical resistance of the applied strain gauge also changes in proportion to the strain. If there is excitation voltage, the circuit supplies an output signal proportional to the change in resistance and therefore also proportional to the change in length. A carrier frequency or DC amplifier suitable for strain gauges enables measurement signal evaluation to continue.Measurement Circuits(a) Common Wheatstone bridge circuit, null when R1/R4=R2/R3(b) All resistances equal but one is variable by a factor, (1+x), where x is a fractional deviation around zero, as a function of strain. Sufficiently linear for small values of x.(c) Output doubles if two identical variable elements can be used(d) Two variable resistors increase while two decrease. Commonly used with two identical two-element strain gages attached to opposite faces of a thin beam to measure bending. The output is four times the output for a single-element
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