Strain Gauge 5y1n4k

Content Page 1: What are strain gauges? 2. How does a strain gauge work? 3. What are the factors affecting accuracy of strain gauges 4. What is Gauge Factor? 5. Examples of applications of strain gauges. 6. How are strain gauges attached to objects? 7. What are lead wire effects? 8. How does temperature affect strain measurement? 9. What are strain rosettes? 10. Factors to consider before selecting strain rosettes.

1 Invented in 1938 by Edward E Simmons and Arthur C Ruge, a Strain gauge is a sensor whose resistance varies with applied force. An example of strain gauges being used are in buildings are aging or beginning to buckle, these gauges can be used to monitor a buildings condition on changes that are minute. There are several types of strain gauges, and they are chosen based on the application, an example of strain gauges in existence are the semi-conductor gauge, carbon resistive gauge, bonded metallic wire, foil resistance gauges, mechanical strain gauges, optical strain gauges and pneumatic strain gauges. 2 The most common type of strain gauge that can be found today consists of an insulating flexible backing which s a metallic foil pattern; the gauge is attached to the object by cyanoacrylate or other suitable adhesives. A strain gauge converts weight, pressure, tension and force into a difference in electrical resistance which can be measured. When the object is under a lot of such forces, the material can slowly fatigue and begin to bend, this increases its electrical resistance. The change in electrical resistance is commonly measured with a Wheatstone bridge; the change in resistance measured can be converted to strain using the gauge factor (strain factor). 3 The factors that affect a strain gauge are the temperature, electromagnetic interference, humidity, and the environment. There are many ways in which temperature can affect the readings of a strain gauge, an increase or decrease in the temperature will cause the object to either expand or contract, this change in size will be detected as a strain on the object by the strain gauge. Another way the temperature can affect a strain gauge reading is the change in resistance of the gauge and the wiring in the strain gauge. A type of string gauge known as the vibrating wire strain gauge operates between a frequency of 600 to 1500 Hz, they are easily affected by electromagnetic interference. This compromises the accuracy of the readings by introducing noise into the raw data which is difficult to separate from the real data and will therefore be processed and calculated as load. Humidity can affect the accuracy by the breakdown of insulation between the gauge and the ground point. It can also cause electro-chemical corrosion of gauge wire due to electrolysis.

The environment can affect the readings of a string gauge as well, if the strain gauge is used in a construction environment. The accuracy of the instrumental readings can be easily and accidentally influenced by countless construction activities, leading to misinterpretation of the readings. Welding is commonly used in construction sites and can cause erratic and significantly high loads to be ed by the strain gauges, high heat generated from welding can result in a high and sudden increase in the strut loads. The testing environment can greatly affect the strain gauge accuracy through hydrostatic pressure, nuclear radiation and high humidity. 4 A change in capacitance, inductance or resistance is proportional to the strain experienced by the sensor. If a wire is held under tension, it changes in length and its cross-sectional area is also affected. This changes its resistance in proportion to the strain sensitivity of the wire’s resistance. When a strain is introduced, the resistance of the strain gauges changes. This change in electrical resistance can be converted to the mechanical strain using the gauge factor. Gauge factor is also defined as unit charge in resistance per unit change in length of strain gauge wire. 5 Bridges are an example of where strain gauges can be used. In order to keep the bridges in good condition, sensors have been placed on bridges to monitor the weaker areas of the bridge. Multiple sensors are placed on various areas of the bridge to get different readings, the sensors measure the stresses on the bridge so if sudden change in readings take place, and someone can fix the problem before if fails. There are many sensors to measure the type of strain on different parts of the bridge, photonic sensors measure the strain on the cables, thermistors measure the temperature, and vibrating wire strain gauges measure vibrations on the bridge. An example is the Tsing Ma Bridge in Hong Kong where 350 sensors are used. Strain gauges are also used to monitor the conditions of train tracks. The gauges are placed on the rails in either a vertical, longitudinal or lateral positions to measure the different forces acting on the rails. These gauges are placed on the top, bottom and sides of a rail, like bridges, these gauges are used to keep track of the conditions of the rails and allow the workers to replace the tracks if necessary.

A strain gauge can be attached to a dynamometer (dyno) to measure the torque on an engine. The dyno is connected to a flywheel or a crankshaft and a strain gauge is attached to the frame of the dyno to measure the torque. When the engine starts, the strain gauge measures the spinning wheel and converts it to torque. CT scan machines require highly repeatable table positioning as well as equal weight distribution of the patient and precise movment of the CT scan imaging device. Precision is required in order to perform high-accuracy imaging functions while preventing over-travel of the patient lying on the scanning tube. Within this environment the incorporation of a multi-axis strain gauge subassembly has proven to be an effective means of better ensuring the smooth, consistent movement and positioning of the table while adjusting for better weight distribution. These design enhancements; created by the incorporation of strain gauge technology has facilitated the manufacture of medical diagnostic imaging machinery with higher accuracy. Strain gauges can also be used to monitor and control the fluid flow of intravenous medication in a medical infusion pump. The device allows uniform fluid delivery without coming into with the fluid itself. The complex assembly of the medical infusion pumps mimics the behavior of a spring, returning to zero when liquid stops flowing, for highly repeatable performance. Medical scale assemblies, such as those required for pediatrics, veterinary medicine, home health monitoring and pharmaceutical use are all examples of medical applications that incorporate the use of strain gauge technologies. Kidney dialysis machines incorporate strain gauge sensing technologies to ensure uniform fluid flow and circulation of proper rate, proportion and frequency according to the parameters set by its accompanying electronic controller device. Timing within this flow measurement system is of critical importance as it has to ensure that as the blood leaves the body and is filtered, the recirculation process occurs in a synergistic timeframe. In these applications, strain gauged parts are used to monitor not only medical pumps, but also the weight of canisters of both blood and waste and flow of hanging intravenous fluids, all with the goal of ensuring consistent therapy deliver. 6 One of the more common methods of attaching string gauges to objects requires the use of some tape. Firstly, the surface where the string gauge will be attached to must be cleaned. The gauge is then taped to on one side to the surface, the un-taped side of the gauge is then lifted up to apply a bonding agent and

adhesive is applied to the surface, pressure is then applied while the bonding agent dries up. The tap is then removed and the gauge lead wires are cut lead wires are then soldered together. 7 Strain gauges are sometimes are mounted at a distance from the measuring equipment, this increases the possibility of errors due to temperature variations, lead desensitization and lead-wire resistance changes. To correct the lead-wire effects, an additional third lead can be introduced into the Wheatstone bridge. This is the minimum acceptable method of wiring strain gauges to a bridge to reduce the effects of extension wire errors. 8 Strain-sensing materials, such as copper, change their internal structures at high temperatures. Temperature can alter the properties of a strain gauge element and the properties of the base material to which to strain gauge is attached. Differences in the expansion coefficient of both the electrical wiring in the strain gauge and the attached surfaces may cause dimensional changes in the sensor element which are inaccurate, such errors are extremely difficult to correct. A change in the resistivity or in the temperature coefficient of resistance of the strain gauge element changes the zero reference used to calibrate the unit. By using either two or four strain gauges in a Wheatstone bridge, the effects of temperature can be reduced. With all strain gauges in a bridge at the same temperature and mounted on the same material, any changes in temperature affect all gauges in the same way. As these temperature changes are identical in the gauges, the ratio of their resistance does not change, and the output voltage of the gauge does not change. This is one of the simpler ways to correct the effects on the strain gauge caused by temperature variations. 9 A strain gauge rosette, by definition is an arrangement of two or more closely positioned gauge grids, separately orientated to measure the normal strains along different directions in the underlying surface of the test part. Rosettes are designed to perform a very practical and important function in experimental stress analysis. Similar to single-element strain gauges, rosettes are manufactured from different combinations of grid alloy and backing material to meet varying application requirements. A wire strain gauge can effectively measure strain in only one direction. To determine the three independent components of plane strain, three linearly independent strain gauges are needed. These three gauges are position in a rosette like layout, hence the name.

10 Strain-sensitive alloys, backing materials, self-temperature-compensation number, gauge length and other strain gauge characteristics suitable to the expected application should be considered before selecting a rosette. The tee rosette should only be used when the principal strain directions are known in advance from other considerations. Cylindrical pressure vessels and shafts in torsion are two classical examples of the latter condition. However, care must be exercised in all cases that extraneous stresses are not present, since these will affect the directions of the principal axes. Attention must also be given to nearby geometric irregularities such as holes, ribs or shoulders which can alter the principal directions. If there is uncertainty about the principle directions, a three-element rectangular or delta rosette is preferable. When necessary, the tee rosette can be installed with its axes at any precise known angle from the principal axes; but greater accuracy can be achieved by alignment along the principal directions. In the latter case, except for the readily corrected error due to transverse sensitivity, the two gauge elements in the rosette indicate the corresponding principal strains directly. The three-element rosette can be installed without regard to orientation.

REFERENCES http://www.ehow.com/how_7886280_attach-strain-gauge.html http://www.hbm.com/en/menu/applications/industry-solutions/medicalindustry/application-reports/oem-medical-devices/ http://engfac.cooper.edu/pages/tzavelis/s/StrainGage%20instrumentation0.pdf http://www.omega.com/prodinfo/StrainGages.html http://www.ehow.com/how-does_5008692_strain-gauge-work.html http://www.pcm-uk.com/straingauge-whatisastraingauge.html http://www.eng.nus.edu.sg/civil/csge/UGS/2007/papers/s1-1.pdf http://www.omega.com/literature/transactions/volume3/strain2.html http://www.ni.com/white-paper/3432/en/#t http://www.engineer.tamuk.edu/departments/ieen/faculty/DrLPeel/Courses/Meen 5330/strain-gage-rosette-theory_Davidson.pdf http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/strain_gage_r osette.cfm