Semiconductor Strain GagesBrandon WithersECE 5320 MechatronicsAssignment #1

OutlineMajor ApplicationsStrain TheoryStrain Gage FactorsUnbonded and Bonded Strain GagesSemiconductor Strain GagesPrinciple of Measurement and Measurement CircuitsStrain Gage Specifications for the HBM SLB-700A/06

Reference 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.htm

To 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.htm

Major 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 , and length L the resistance is given byWhen 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:= fractional resistance change= Poissons ratio= fractional change in length= fractional change in resistivity

Strain 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 asHigher 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 Properties

Small 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 compensation

Principle 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 CircuitsCommon Wheatstone bridge circuit, null when R1/R4=R2/R3All 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.Output doubles if two identical variable elements can be usedTwo 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 bridge, and it is linear with x.Uses a zero-centered potentiometer to constitute two adjacent arms.Op amp forces the bridge to be balanced. It has good linearity and very low output impedance, thus making the output measurement easier and more accurate.The Mechatronics Handbook

Strain Gage SpecificationsHBM SLB-700A/06Key FeaturesFor Monitoring strain in statically and dynamically loaded units such as cranes, presses and roll standsSimply bolted into placeStainless steelProtected from harmful environmental effectsInexpensiveStrain gauge full bridgeSLB-700A/06 Strain Transducer

HBM SLB-700A/06 SpecificationsSLB-700A/06 Strain Transducer

Type SLB700A/06 Nominal strain m/m 500 Nominal sensitivity mV/V 1.5 "0.15 Restoring force N approx. 3110 Zero point deviation % 5 Temperature effect on the sensitivity, per 10 K % 0.2 Temperature effect on the zero point, per 10 K % 0.5 Compensated temperature range C 10...+40 Input resistance > 1000 Output resistance 1000 "3 Isolation resistance G 5 Nominal range of the excitation voltage V 2...15 Max. operating strain % 150 Breaking strain % 300 Nominal displacement mm approx. 0.038 Cable length m 6 Cable diameter mm approx. 3 Weight (complete with 6 m cable) kg 0.16 Degree of protection acc. to EN 60 529 IP65

HBM SLB-700A/06 SpecificationsSLB-700A/06 Strain Transducer

HBM SLB-700A/06MountingThe SLB700A strain transducer is attached to the measurement object bymeans of four normal M6 hexagon socket screws (e.g. DIN 912). Werecommend screws of resistance class 12.9, which should be tightened in asequence of diagonal opposites, using a tightening torque of 16 Nm.Alternatively, use screws of resistance class 8.8 and a tightening torque of8 Nm. The strain transducer must not be mounted in the central, offset areaand it must be installed free from distortion.

SLB-700A/06 Strain Transducer