Continued on page 8 this section should be read by all users of vibrating wire strain gauges manufactured by mgs geosense. In this chapter, one piezoelectric actuator pzt stack is used to illustrate the measuring principle of strain gauge sensors. How do strain gauge based pressure transducers work. These are available in a variety of shapes and sizes for different applications. In a load cell, the resistors are replaced with strain gauges and arranged in alternating tension and compression formation. The resistance strain gauge is a valuable tool in the field of experimental stress analysis. When force is exerted on the load cell, the structure and resistance of the strain gauges changes and is measured. It also illustrates an approximate mohrs cirlce for the given strain state. When load is acted on a material, elongation or compression is happened. Strain gages have excellent repeatability and linearity over a wide range of strains. By measuring strain with the gage bonded to a material or structure, the strength or safety will be known. A load cell is made by bonding strain gauges to a spring material. The ad8221 gains the signal and an ad630ar synchronously demodulates the waveform. Pdf this article presents an application of strain gauges in measurements of.
Strain can be measured using various types of devices classified depending upon their principle of operation. Explore our resources for the technical support and software updates you need for your madgetech products. Gauge factor is defined as the ratio of fractional change in electrical resistance to the fractional change in length strain. The factor k, also known as the gage factor, is a characteristic of the strain gage and has been checked experimentally 2.
The strain can also be used to carry out stress analysis of the member. In this experiment the strain gage will be utilized. The metallic strain gauge consists of a very fine wire or, more commonly, metallic foil. From the resulting data, can be easily determined using the equation above. The strain gauge is a classic example of a piezoresistive element, a typical strain gauge element shown here on the tip of my finger. However, in instrumental term, it is generally taken to mean the electrical resistance strain gauge, and as the name implies, the strain gauge is an electrical conductor whose resistance varies in proportion to the amount of strain in the device.
Thus, the strain gage is used in various industries including machinery, automobile, electric, civil engineering, medical, and food. Piezoelectric or strain gauge based force transducers. There are three steps in obtaining experimental strain measurements using a strain gage. The length change of the piezoelectric stack alters the resistance of the strain gauge. Given the strain components e x, e y, and e xy, this calculator computes the principal strains e 1 and e 2, the principal angle q p, the maximum shear strain e xy max and its angle q s. Strain gage measurements are often plagued by offset drift, 1f noise, and line noise. Remember to use red wire for positive power connections and black wire for ground connections.
Still symbols normal stress shear stress strain normal micro strain x 10 6 shear strain e modulus of elasticity or youngs modulus poisson ratio gf gage factor rg gage. If a strip of conductive metal is stretched, it will become skinnier and longer, both changes resulting in an increase of electrical resistance. Types of strain gages are classified into foil strain gages, wire strain gages, and semiconductor strain gages, etc. While there are several methods of measuring strain, the most common is with a strain gauge, a device whose electrical resistance varies in.
Introduction to strain gauges and their usage strain gauge applications strain gauges are used to determine or verify component or structure stresses, or by manufacturers of load cells, pressure and torque transducers, etc. In order to be practical, a strain gauge must be glued bonded on to a larger specimen capable of withstanding an applied force stress. The main purpose of a strain gauge is to indirectly determine stress and its variation with time, quantitatively. The working principle of strain gauge based pressure transducers. Lab 8, strain gage lab page 3 suppose a quarter bridge circuit is balanced vo 0 when no load is applied to the beam. The strain gage is used universally by stress analysts in the experimental determination of stresses. A strain gage detects a minute dimensional change strain as an electric signal. The strain gage, which includes two wire leads, has already been attached glued to a bendable ruler. Thus, a strain gage is a device which uses change in electrical resistance to measure strain. A strain gauge sensor is a thin waferlike device that can be attached to a variety of materials to measure applied strain. The strain gauge is bonded to a thin backing carrier, which is attached directly to the load cell enabling the strain of the load cell to be experienced by the strain gauge. Since strain always accompanies vibration, the strain gage or the principle by which it works is broadly applicable in the. This method is simple, fast, and requires no manual adjustments.
Construction and principle of operation the strain gauge measuring grid is manufactured from a copper nickel alloy which has a low and controllable temperature coefficient. The strain gauge amplifier is a purpose designed hybrid, low noise, low drift, linear dc amplifier in a 24 pin dil package, specifically configured for resistive bridge measurement and in particular the strain gauges detailed earlier in this data sheet. Strain gauges are used to measure the strain of a material. It is another type of transducer which is used in various places in analytics of stress and strain. Functions of individual strain gauge s components presents table 1. Strain gauge and rosettes introduction a strain gauge is a device which is used to measure strain deformation on an object subjected to forces. Foil strain gauges when attached to a specimen, produce very small changes in resistance typically. The strain gage is tightly bonded to a measuring object so that the sensing element metallic resistive foil may elongate or contract.
The gauge factor for metallic strain gauges is typically around 2. Instruction bulletin b1298 surface preparation for. This force results in minimal deformation of the spring element. The discovery of the principle upon which electrical resistance strain gage is based was made in 1856 by lord kelvin, who loaded copper. Pdf application of strain gauges in measurements of. There is a linear relationship between the strain of. The latter form of strain gauge is represented in the previous illustration. Build the analog circuit shown in figure 3 on an electronic breadboard. This change in resistance is a measure of its mechanical motion.
Basic principle of strain gauge when stress is applied to the metal conductor, its resistance changes owing to change in length and crosssectional area of the conductor. We will introduce considerations that affect the accuracy of this measurement and suggest procedures for. A minimum of four strain gauges are connected into a wheatstone bridge circuit. What results is a dc output proportional to the strain on the bridge. Laboratory 3 strain gage sensors national instruments. The change in stress is determined by multiplying the measured strain by the modulus of elasticity. Strain gauges and load cells issued march 1997 strain gauges two ranges of foil strain gauges to cover general. Strain gauge based force transducers always comprise a spring element to which force is applied. Lets briefly learn about stress and strain and strain gages.
The electric resistance of a metal changes proportionally to the mechanical deformation caused by an external force applied. A fundamental parameter of the strain gauge is its sensitivity to strain, expressed quantitatively as the gauge factor gf. Brief history the electrical resistance strain gage in the basic form known today was first used in 1936. Stress is defined as the objects internal resisting forces, and. Strain gauges installed at appropriate points are extended and therefore show a change in resistance. A strain gauge is an electrical transducer which is used for measuring mechanical surface strain. Alternatively, strain gauge conductors may be thin strips of metallic film deposited on a nonconducting substrate material called the carrier. Types of strain gages include foil strain gage, wire strain gage and semiconductor strain. For varied straingage bridge formation systems, refer to bridge. The change in resistance measured by a single strain gauge is extremely small, around 0. In this article, i am going to describe you strain gauge working principle, gauge factor and strain gauge load cell working. Below, the wheatstone bridge circuit will only be considered with respect to its application in.
To efficiently detect the strain, strain gauges are bonded to the position on the spring material where the strain will be the largest. The exact value is specified on each strain gage package. In general, the gage factor for metal strain gages is about 2. Strain gauges are an important geotechnical tools that measure strain in underground cavities, tunnels, buildings, concrete, masonry dams, bridges, embedment in soilconcrete. The foil strain gage has metal foil on the electric insulator of the thin resin, and gage leads attached, as shown in fig. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. Pdf strain gauge principle, types, features and applications. For varied strain gage bridge formation systems, refer to bridge. When the metallic member to which the strain gauges are attached, is stressed by the application of a force, the resulting strain leads to a change in resistance in one or more of the resistors. A strain gauge figure 1 is a sensor used tomeasure strain.
The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. This change in resistance results in a change in output voltage. Strain gauge sensors are widely applied to measure the piezoelectric displacement. Strain gages are very stable and transducers retain their calibration and performance over extended periods of time. This small change in output voltage usually about 20 mvolt of total change in response to full load can be measured and.
A strain gauge is a passive transducer, that converts mechanical displacement into the change of resistance. Whenever any material is subjected to high loads, they come under strain, which can be measured easily with the strain gauges. Printiciple, types and structure of strain gage kyowa. In this clip we will discuss that how does a strain gauge works.
A diaphragm is frequently used as the pressuresensitive measuring body in the lower pressure range, while. Wilson introduction a strain sensitive material is one whose electrical resistance is proportional to the. One solution is to use an ac signal to excite the bridge, as shown in figure 1. When external forces are applied to a stationary object, stress and strain are the result. When environmental temperature changes, the resistance of the strain gauge changes accordingly. The electricalresistance strain gage is the most widely used device for strain measurement, its operation is based on the principle that electrical resistance of a conductor changes when its subjected to mechanical deformation. Singlesupply strain gauge in a bridge configuration. Pressure transducers and pressure sensors often consist of a spring element on which multiple strain gauges are installed. Strain gauge working principle instrumentation tools. A strain gauge is a sensor whose resistance varies with applied force. A strain gauge, in mechanical term, is a device for measuring mechanical strain. The strain gauge utilizes this principle and detects a strain by changes in resistance.