Types of Strain Gauges
Types of Strain Gauges Posted on March 19, 2012 by llucica Introduction Various means like mechanical, optical, acoustical, pneumatic or electrical can be used to measure deformation (strain) of an object. Earlier mechanical devices such asextension meter (extensiometer) were used to measure strain by measuring the change in length and comparing it to the original length of the object. However, mechanical strain gauges offer certain limitations like low resolutions. Besides they are bulky and difficult to use. Further, capacitance and inductance-based strain gages were introduced but these devices’s sensitivity to vibration, their mounting requirements, and circuit complexity restricted their usage. Next are the photoelectric gauges. These gauges use a light beam, two fine gratings, and a photocell detector to generate an electrical current proportional to strain. A photoelectric gauge can be as short as 1/16 inch but its usage proves to be extremely costly and delicate. In 1938, the first bonded, metallic wire-type strain gage was introduced. The metallic foil-type strain gage is constructed of a grid of wire filament of approximately 0.001 in thickness, bonded directly to the strained surface by a thin layer of epoxy resin. When a load is applied to the surface, it gets strained and experiences a change in length. This resulting change in length is conveyed to the resistor and the corresponding strain is measured in terms of the electrical resistance of the foil wire, which varies linearly with strain. Other types of Strain Gauges are described below. Semiconductor Strain Gauges In the year 1970, the first semiconductor strain gages were developed for the use in automotive industry. Semiconductor strain gauges exhibit following key features:
Unlike other strain gauges, semiconductor strain gages are based upon the piezoresistive effects of silicon or germanium and measure the change in resistance with stress as opposed to strain. The semiconductor bonded strain gage is a wafer with the resistance element diffused into a substrate of silicon. No backing is provided for the wafer element and bonding it to the strained surface needs extra care since only a thin layer of epoxy is used to attach it. Size of a semiconductor strain gauge is much smaller and the cost much lower than for a metallic foil sensor. Advantages include higher unit resistance and sensitivity. Disadvantages: Greater sensitivity to temperature variations and tendency to drift as compared to metallic foil sensors. Also the resistance-to-strain relationship is nonlinear, varying 10-20% from a straight-line equation. However, by means of computer-controlled instrumentation, these limitations can be overcome via software compensation.
Thin-film Strain Gauges Thin-film strain gage is more advanced form of strain gauge as it doesn’t necessitate adhesive bonding. A thin film strain gauge is constructed by first depositing an electrical insulation, usually a ceramic onto the stressed metal surface, and then depositing the strain gage onto this insulation layer. Techniques used to bond the materials molecularly are: Vacuum deposition Sputtering method Advantages 1. Since the thin-film gage is molecularly bonded to the specimen, the installation is very stable and the resistance values experience less drift. 2. The stressed force detector can be a metallic diaphragm or beam with a deposited layer of ceramic insulation.
Diffused Semiconductor Strain Gauges A further improvement in strain gage technology was introduced with the advent of diffused semiconductor strain gages since they purge the need for bonding agents. Main features are listed below: 1. 2.
By eliminating bonding agents, errors due to creep and hysteresis also are eliminated. The diffused semiconductor strain gage employs photolithography masking techniques and solid-state diffusion of boron to molecularly bond the resistance elements. 3. Diffused semiconductors are frequently used as sensing elements in pressure transducers. 4. Limitations include sensitivity to ambient temperature variations, which can be compensated by intelligent transmitter designs. Advantages Small size Inexpensive Accurate and repeatable Available wide pressure range Generate a strong output signal Bonded Resistance Gauges Following are the chief characteristics of bonded resistance strain gauges:
They are reasonably inexpensive. They can pull off overall accuracy of better than +/-0.10%. They are available in a short gauge length and have small physical size. These strain gauges are only moderately affected by temperature changes. They are extremely sensitive and have low mass. Bonded resistance strain gages can be employed to measure both static and dynamic strain. These types of strain gauges are appropriate for a wide variety of environmental conditions. They can measure strain in jet engine turbines operating at very high temperatures and in cryogenic fluid applications at temperatures as low as -452*F (-269*C). Construction of a Bonded resistance strain gauge is shown in the figure below:
Selection of a Proper Gauge Three primary considerations in strain gauge selection are mentioned below: 1. Operating temperature 2. Nature of the strain to be detected
3. Stability requirements In addition, choosing the right carrier material, grid alloy, adhesive, and protective coating plays an important role in the success of the application.
Strain Gauge - Part 1 Contents: 1.0 Definition of stain gauge 2.0 Classification of strain gauges 2.1 Mechanical strain gauges 2.1.1 Berry Strain gauge 2.1.2 Huggenbeger Extensometer 2.1.3 Johansson Extensometer 2.2 Electrical Strain gauges 2.3 Optical strain gauges 2.4 Pneumatic strain gauges 2.5 Acoustical strain gauges 1. Definition : A strain gauge is a device used to measure the strain on a free surface of a structure. Strain gages are the preeminent tool in stress analysis. Strain gauges of all types are essentially employed to measure the linear deformation over a given gauge length. The sense the change in length, Magnify and indicate it in some other form. Strain Gauge is invented by Edward E Simmons and Arthur C Ruge in the year 1938.
2. Classification of Strain Gauges Depending up on the magnification system, the strain gauges are broadly classified as under, a. Mechanical strain gauges b. Electrical strain gauges c. Optical strain gauges d. Pneumatic strain gauges e. Acoustical strain gauges 2.1 Mechanical strain gauges Mechanical strain gauges are also known as Extensometers used to measure static or gradually varying load conditions. These gauges are usually provided with two knife edges which are clamped firmly in contact with the test component by means of a clamping spring at a specific distance of gauge length. When the specimen under testing is strained the knife edges undergoes displacement, this displacement is amplified by a mechanical linkages and the strain is displaced on a calibrated scale.
Types of Mechanical strain gauges 2.1.1 Berry Strain gauge These strain gauges uses a lever magnification with dial indicator to show magnified motion. It consists of one rigid frame and two conically pointed contact pointers. One pointer is rigidly fixed to the frame while the other is pivoted at a point on the frame. The displacement in the lever is magnified and indicated in the dial indicator.
2.1.2 Huggenbeger Extensometer This extensometer has a set of compound levers which are relatively small in size and high magnification factor. These gauges are highly accurate. The movable knife edge rotates the lever at lower pivot, the lever in turn rotates the indicator pointer at upper pivot point with the help of a link.
2.1.3 Johansson Extensometer These extensometers uses tension tape or twisted metal strip between two knife edges. Half of the strip is twisted to one direction and remaining half is twisted to other direction and a pointer is fixed at the center of the strip. On application of load, displacement in the movable knife edge takes place with high amplification due to stretching of twisted metal strip.
Strain Gauge - Part 2 2.2 Electrical resistance strain gauge : In electrical resistance strain gauge the displacement or strain is measured as a function of resistance change produced by the displacement in the gauging circuit. When the conductor is stretched, its length will increase and area of cress section will decrease this will result in change in resistance. Change in resistance per unit strain is defined as Gauge Factor. Gauge factor indicates the sensitivity of the strain gauge.
Types of electrical resistance strain gauges Electrical resistance strain gauge with metallic sensing element may be broadly classified in to four groups. a. Un-bonded wire strain gauge b. Bonded wire strain gauge c. Foil strain gauge d. Weldable strain gauge 2.2.1 Un-bonded wire strain gauge : The principal of the un-bonded metallic strain gauge is based on the change in electrical resistance of a metallic wire due to the change in the tension of the wire. This type consists of a stationary frame and a movable platform. Fine wire loops are wounded around the insulated pins with pretension. Relative motion between the platform and the frame increases the tension in two loops, while decreasing tension in the other two loops. These four elements are connected approximately to a four arm Wheat stone bridge. These type strain gauges are used for measurement of acceleration, pressure, force etc.
2.2.2 Bonded Wire Strain Gauge : The bonded metallic type of strain gauge consists of a strain sensitive conductor (wire) mounted on a small piece of paper or plastic backing. In us this gauge is cemented to the surface of the structural member to be tested. The wire grid may be & flat type or wrap-around. In the flat type after attaching the lead wires to the ends of the grids, a second piece of paper is cemented over the wire as cover. in the wrap-around type, the wire is wound around a cylindrical core in the form of a close wound helix. This core is then flattened & cemented between layers of paper for the purpose of protection and insulation. Formerly only wrap-around gauges were available, but generally flat grid gauges are preferred as they are superior to wrap-around gauge in terms of hysterisis, creep, elevated temperature, performance, stability & current carrying capacity.
2.2.3 Foil Strain Gauges: The foil type of strain gauges has a foil grid made up of thin strain sensitive foil. The width of the foil is very large as compared to the thickness (microns) so that larger area of the gauge is for cementing.
2.2.4 Weldable Strain gauge: Weldable strain gauges are easy to install in minutes in any environment compared to bonded type strain gauge. The weldable strain gauge consists of a strain sensitive element, the nickel Chromium or platinum Tungsten, housed within a small diameter stainless steel tube. The strain element is insulated from the tube with highly compacted ceramic insulation. This gauge is subsequenty spot welded to structure under test and provides bonding to transfer the strain. The test specimen which is put into tension or compression, the stress is transmitetd through the weld to mounting flange and in to strain tube. These gauges can be used for static or dynamic applications.
2.3 Optical strain gauges The optical strain gauges are used to measure elongation as well as deflection, following are the two type of optical strain gauges,
a. Marten’s optical gauge b. Tuckerman Optical Gauge 2.3.1. Marten’s optical gauge: These optical stain gauges employs variety of mirror systems to obtain optical magnification. The well known optical system used in a strain gauge on a single mirror system is marten’s optical gauge. The pivoted knife edge carries a mirror and the other end of this arm is fastened to specimen as the specimen elongates the measuring knife edge will rotate about its point there by tilting the mirror. The Reflection of the illuminated scale in this mirror is viewed through the telescope.
2.3.2 Tuckerman Optical Gauge: In this instrument, the relative rotation between the fixed mirror and the movable mirror is measured with autocollimator. The autocollimator consists of a lamp source to produce parallel beam of rays and a scale to measure the deflection of the reflected ray. A tungsten carbide rocker (lozenge) acts as a moving knife; one face of this lozenge is polished to act as a mirror. If the specimen deforms, rotates the lozenge which in turn deflects the incident ray back to the reticule. Actually three images are visible on the reticule one gives the measurement of strain and other two helping alignment of the gauge. The sensitivity of the gauge is 2 micro strains and this gauge is available with a wide range of gauge length of 6mm. it can measure both static and dynamic strains and cyclic strains up to 180 Hz.
2.4 Pneumatic strain gauge : The principal of operation of a pneumatic gauge depends upon the relative discharge of air between a fixed orifice and a variable orifice. Magnification up to 100,000 times and the gauge length as small as 1mm are possible to achieve by these gauges. These gauges are suitable for both Static and dynamic strain measurements. These are sensitive, robust and reliable.
2.5. Acoustic strain gauge : In an acoustic strain gauge the variation in length of a wire stretched between two gauge points is measured which alters the natural frequency of the wire. The magnitude of frequency change for a strain gauge can be increased by decreasing the length of the wire or stress in wire.These gauges are highly accurate and long term reliable. Optical strain gauges are used to measure strains in concrete structure, concrete dams, rock, steel structures etc.
Measuring Strain with Strain Gages 1116 Ratings | 4.08 out of 5 English
Overview This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series will teach you a specific topic of common measurement applications by explaining theoretical concepts and providing practical examples.This tutorial introduces and explains the concepts and techniques of measuring strain with strain gages. For more in-depth guidance on making strain measurements, visit the how-to guide. To find more information on the Measurement Fundamentals series, return to the NI Measurement Fundamentals Main Page. Table of Contents 1. 2. 3. 4. 5. 6.
What Is Strain? The Strain Gage Strain Gage Measurement Signal Conditioning for Strain Gages Data Acquisition Systems for Strain Gage Measurements Relevant NI Products
What Is Strain? Strain is the amount of deformation of a body due to an applied force. More specifically, strain (e) is defined as the fractional change in length, as shown in Figure 1.
Figure 1. Definition of Strain
Strain can be positive (tensile) or negative (compressive). Although dimensionless, strain is sometimes expressed in units such as in./in. or mm/mm. In practice, the magnitude of measured strain is very small. Therefore, strain is often expressed as microstrain (me), which is e x 10 -6. When a bar is strained with a uniaxial force, as in Figure 1, a phenomenon known as Poisson Strain causes the girth of the bar, D, to contract in the transverse, or perpendicular, direction. The magnitude of this transverse contraction is a material property indicated by its Poisson's Ratio. The Poisson's Ratio n of a material is defined as the negative ratio of the strain in the transverse direction (perpendicular to the force) to the strain in the axial direction (parallel to the force), or n = eT/e. Poisson's Ratio for steel, for example, ranges from 0.25 to 0.3. The Strain Gage While there are several methods of measuring strain, the most common is with a strain gage, a device whose electrical resistance varies in proportion to the amount of strain in the device. The most widely used gage is the bonded metallic strain gage. The metallic strain gage consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction (Figure 2). The cross-sectional area of the grid is minimized to reduce the effect of shear strain and Poisson Strain. The grid is bonded to a thin backing, called the carrier, which is attached directly to the test specimen. Therefore, the strain experienced by the test specimen is transferred directly to the strain gage, which responds with a linear change in electrical resistance. Strain gages are available commercially with nominal resistance values from 30 to 3,000 Ω, with 120, 350, and 1,000 Ω being the most common values.
Figure 2. Bonded Metallic Strain Gage
It is very important that the strain gage be properly mounted onto the test specimen so that the strain is accurately transferred from the test specimen, through the adhesive and strain gage backing, to the foil itself. A fundamental parameter of the strain gage is its sensitivity to strain, expressed quantitatively as the gage factor (GF). Gage factor is defined as the ratio of fractional change in electrical resistance to the fractional change in length (strain):
The gage factor for metallic strain gages is typically around 2. Strain Gage Measurement In practice, strain measurements rarely involve quantities larger than a few millistrain (e x 10 -3). Therefore, to measure the strain requires accurate measurement of very small changes in resistance. For example, suppose a test specimen undergoes a strain of 500 me. A strain gage with a gage factor of 2 will exhibit a change in electrical resistance of only 2 (500 x 10-6) = 0.1%. For a 120 Ω gage, this is a change of only 0.12 Ω. To measure such small changes in resistance, strain gages are almost always used in a bridge configuration with a voltage excitation source. The general Wheatstone bridge, illustrated in Figure 3, consists of four resistive arms with an excitation voltage, VEX, that is applied across the bridge.
Figure 3. Wheatstone Bridge
The output voltage of the bridge, VO, is equal to:
From this equation, it is apparent that when R1/R2 = R4/R3, the voltage output VO is zero. Under these conditions, the bridge is said to be balanced. Any change in resistance in any arm of the bridge results in a nonzero output voltage. Therefore, if you replace R4 in Figure 3 with an active strain gage, any changes in the strain gage resistance will unbalance the bridge and produce a nonzero output voltage. If the nominal resistance of the strain gage is designated as RG, then the strain-induced change in resistance, DR, can be expressed as DR = R G·GF·e, from the previously defined Gage Factor equation. Assuming that R1 = R2 and R3 = RG, the bridge equation above can be rewritten to express VO/VEX as a function of strain (see Figure 4). Note the presence of the 1/(1+GF·e/2) term that indicates the nonlinearity of the quarter-bridge output with respect to strain.
Figure 4. Quarter-Bridge Circuit
Ideally, you would like the resistance of the strain gage to change only in response to applied strain. However,
strain gage material, as well as the specimen material to which the gage is applied, also responds to changes in temperature. Strain gage manufacturers attempt to minimize sensitivity to temperature by processing the gage material to compensate for the thermal expansion of the specimen material for which the gage is intended. While compensated gages reduce the thermal sensitivity, they do not totally remove it. By using two strain gages in the bridge, you can further minimize the effect of temperature. For example, Figure 5 illustrates a strain gage configuration where one gage is active (R G + DR) and a second gage is placed transverse to the applied strain. Therefore, the strain has little effect on the second gage, called the dummy gage. However, any changes in temperature affect both gages in the same way. Because the temperature changes are identical in the two gages, the ratio of their resistance does not change, the voltage V O does not change, and the effects of the temperature change are minimized. NOTE: In the Wheatstone bridge configuration, the active gage and the dummy gage should be on the same vertical leg of the bridge.
Figure 5. Use of Dummy Gage to Eliminate Temperature Effects
The sensitivity of the bridge to strain can be doubled by making both gages active in a half-bridge configuration. For example, Figure 6 illustrates a bending beam application with one bridge mounted in tension (RG + DR) and the other mounted in compression (RG - DR). This half-bridge configuration, whose circuit diagram is also illustrated in Figure 6, yields an output voltage that is linear and approximately doubles the output of the quarterbridge circuit.
Figure 6. Half-Bridge Circuit
Finally, you can further increase the sensitivity of the circuit by making all four of the arms of the bridge active strain gages in a full-bridge configuration. The full-bridge circuit is shown in Figure 7.
Figure 7. Full-Bridge Circuit
The equations given here for the Wheatstone bridge circuits assume an initially balanced bridge that generates zero output when no strain is applied. In practice, however, resistance tolerances and strain induced by gage application generate some initial offset voltage. This initial offset voltage is typically handled in two ways. First, you can use a special offset-nulling, or balancing, circuit to adjust the resistance in the bridge to rebalance the bridge to zero output. Alternatively, you can measure the initial unstrained output of the circuit and compensate in software. This topic is discussed in greater detail later. The equations given above for quarter-, half-, and full-bridge strain gage configurations assume that the lead wire resistance is negligible. While ignoring the lead resistance may be beneficial to understanding the basics of strain gage measurements, doing so in practice can be a major source of error. For example, consider the 2-wire connection of a strain gage shown in Figure 8a. Suppose each lead wire connected to the strain gage is 15 m long with lead resistance RL equal to 1 Ω. Therefore, the lead resistance adds 2 Ω of resistance to that arm of the bridge. Besides adding an offset error, the lead resistance also desensitizes the output of the bridge. You can compensate for this error by measuring the lead resistance R L and accounting for it in the strain calculations. However, a more difficult problem arises from changes in the lead resistance due to temperature fluctuations. Given typical temperature coefficients for copper wire, a slight change in temperature can generate a measurement error of several microstrain. Using a 3-wire connection can eliminate the effects of variable lead wire resistance because the lead resistance affects adjacent legs of the bridge. As seen in Figure 8b, changes in lead wire resistance, R L2, do not change the ratio of the bridge legs R3 and RG. Therefore, any changes in resistance due to temperature cancel out each other.
Figure 8. 2-Wire and 3-Wire Connections of Quarter-Bridge Circuit
Signal Conditioning for Strain Gages Strain gage measurement involves sensing extremely small changes in resistance. Therefore, proper selection and use of the bridge, signal conditioning, wiring, and data acquisition components are required for reliable measurements. To ensure accurate strain measurements, it is important to consider the following:
Bridge Completion – Unless you are using a full-bridge strain gage sensor with four active gages, you need to complete the bridge with reference resistors. Therefore, strain gage signal conditioners typically provide halfbridge completion networks consisting of high-precision reference resistors. Figure 9a shows the wiring of a halfbridge strain gage circuit to a conditioner with completion resistors R 1 and R2.
Figure 9a. Connection of Half-Bridge Strain Gage Circuit
Excitation – Strain gage signal conditioners typically provide a constant voltage source to power the bridge. While there is no standard voltage level that is recognized industry wide, excitation voltage levels of around 3 and 10 V are common. While a higher excitation voltage generates a proportionately higher output voltage, the higher voltage can also cause larger errors because of self-heating. Remote Sensing – If the strain gage circuit is located a distance away from the signal conditioner and excitation source, a possible source of error is voltage drop caused by resistance in the wires connecting the excitation voltage to the bridge. Therefore, some signal conditioners include a feature called remote sensing to compensate for this error. Remote sense wires are connected to the point where the excitation voltage wires connect to the bridge circuit, as seen in Figure 9b. The extra sense wires serve to regulate the excitation supply through negative feedback amplifiers to compensate for lead losses and deliver the needed voltage at the bridge.
Figure 9b. Remote Sensor Error Compensation
Amplification – The output of strain gages and bridges is relatively small. In practice, most strain gage bridges and strain-based transducers output less than 10 mV/V (10 mV of output per volt of excitation voltage). With 10 V excitation, the output signal is 100 mV. Therefore, strain gage signal conditioners usually include amplifiers to boost the signal level to increase measurement resolution and improve signal-to-noise ratios. Filtering – Strain gages are often located in electrically noisy environments. It is therefore essential to be able to eliminate noise that can couple to strain gages. Lowpass filters, when used with strain gages, can remove the high-frequency noise prevalent in most environmental settings. Offset Nulling – When a bridge is installed, it is very unlikely that the bridge will output exactly zero volts when no strain is applied. Slight variations in resistance among the bridge arms and lead resistance will generate some nonzero initial offset voltage. Offset nulling can be performed by either hardware or software: 1. Software Compensation – With this method, you take an initial measurement before strain input is applied, and use this offset to compensate subsequent measurements. This method is simple, fast, and requires no manual adjustments. The disadvantage of the software compensation method is that the offset of the bridge is not removed. If the offset is large enough, it limits the amplifier gain you can apply to the output voltage, thus limiting the dynamic range of the measurement. 2. Offset-Nulling Circuit – The second balancing method uses an adjustable resistance, a potentiometer, to physically adjust the output of the bridge to zero. By varying the resistance of potentiometer, you can control the level of the bridge output and set the initial output to zero volts. Shunt Calibration – The normal procedure to verify the output of a strain gage measurement system relative to some predetermined mechanical input or strain is called shunt calibration. Shunt calibration involves simulating the input of strain by changing the resistance of an arm in the bridge by some known amount. This is accomplished by shunting, or connecting, a large resistor of known value (Rs) across one arm of the bridge, creating a known DR as seen in Figure 9c. The output of the bridge can then be measured and compared to the expected voltage value. The results are used to correct span errors in the entire measurement path, or to simply verify general operation to gain confidence in the setup.
Figure 9c. Shunt Resistor Connected Across R3 Data Acquisition Systems for Strain Gage Measurements Using NI CompactDAQ with Strain Gages NI CompactDAQ hardware provides the plug-and-play simplicity of USB to sensor and electrical measurements. The NI CompactDAQ system consists of an NI cDAQ-9174 4-slot or NI cDAQ-9178 8-slot USB 2.0-compliant chassis that can hold up to eight C Series I/O modules and connect to a PC using a 1.8 m USB cable. NI CompactDAQ delivers fast and accurate measurements with more than 45 self-contained measurement modules available. And with three timing engines, the cDAQ-9174/9178 chassis can run analog modules at up to three different rates. Because all circuitry required for the specific measurement is contained in the C Series I/O module itself, you can connect many different types of sensors, including strain gages, directly to the modules.
Figure 10. NI cDAQ-9174/9178 Chassis with C Series I/O Modules The NI 9219 is a 4-channel universal C Series module designed for multipurpose testing in any NI CompactDAQ or CompactRIO chassis. With the NI 9219, you can measure several signals from sensors such as strain gages, RTDs, thermocouples, load cells, and other powered sensors. The channels are individually selectable, so you can perform a different measurement type on each of the four channels. The NI 9219 uses six-position spring terminal connectors in each channel for direct signal connectivity and contains built-in quarter-, half-, and fullbridge support. For C Series I/O modules specifically designed for the measurement of strain gages, National Instruments offers the NI 9235, NI 9236, and the NI 9237. These bridge modules contain all the signal conditioning required to power and measure bridge-based sensors simultaneously. The NI 9235 and NI 9236 have a higher channel count and include completion for quarter-bridge sensors. The NI 9237 supports up to four full- and half bridge sensors and can measure from quarter bridge strain gages using a completion accessory. The NI 9237 can perform offset/null as well as shunt calibration and remote sense, making the module the best choice for low- to medium-channel-count strain and bridge measurements. Recommended Starter Kit for Strain Gage NI CompactDAQ System: 1. cDAQ-9174 or cDAQ-9178 chassis 2. NI 9237 with an RJ50 cable and an NI 9949 (full and half bridge) or NI 9944/NI 9945 (quarter bridge) 3. Refer to ni.com/sensors for recommended sensor vendors Using PXI with Strain Gages PXI is a rugged PC-based platform that offers a high-performance, low-cost deployment solution for measurement and automation systems. PXI combines the PCI bus with the rugged, modular Eurocard mechanical packaging of CompactPCI and adds specialized synchronization buses and key software features. PXI can integrate a controller and provides up to 18 slots in a single chassis. There are timing and triggering lines on the backplane of the PXI chassis for tight synchronization of the various I/O modules. PXI Express delivers PCI Express data transfer technology to the PXI platform, increasing backplane bandwidth from 132 MB/s to 6 GB/s.
Figure 11. PXI Express Chassis with SC Express Sensor Measurement Modules
The NI SC Express family features PXI Express data acquisition modules with integrated signal conditioning for sensor measurements, such as strain gages and other Wheatstone bridge-based transducers. The NI PXIe4330 8-channel simultaneous bridge input module offers 24-bit resolution, 0.02% accuracy and 25 kS/s per channel sample rate for high-performance strain measurements. The NI PXIe-4330 can perform quarter-, half-, and full bridge-based measurements with automatic synchronization features; the included driver software ensures tight synchronization across multiple modules and chassis with inter-channel skews as low as 5 parts per billion (PPB). This device offers per channel excitation from 0.625 to 10 V with remote sensing to compensate for error caused by resistance in the wires connecting the voltage source to the bridge. For added flexibility, builtin bridge completion (120 Ω, 350 Ω, and 1 kΩ) and shunt calibration (50 kΩ, 100 kΩ) are software-selectable on a per channel basis. The NI PXIe-4330 should be used with the NI TB-4330 front-mounting terminal block for screw terminal connectivity. Recommended Starter Kit for Strain Gage SC Express System: 1. NI PXIe-1073 chassis 2. NI PXIe-4330 universal bridge input module with TB-4330 for connectivity 3. Refer to ni.com/sensors for recommended sensor vendors
Using SCXI with Strain Gages National Instruments SCXI is a signal conditioning system for PC-based instrumentation applications. An SCXI system consists of a shielded chassis that houses a combination of signal conditioning input and output modules, which perform a variety of signal conditioning functions. You can connect many different types of sensors, including strain gages, directly to SCXI modules. The SCXI system operates as a front-end signal conditioning system for PC plug-indata acquisition devices (USB, PCI, and PCMCIA) or PXI data acquisition modules.
Figure 12. SCXI Signal Conditioning System The NI SCXI-1520 is an 8-channel universal strain gage input module that offers a variety of features for strain measurements. With this single module, signals from strain, force, torque, and pressure sensors can be easily read. The SCXI-1520 also offers a programmable amplifier and programmable four-pole Butterworth filter on each channel, and simultaneous sampling with track-and-hold circuitry. In addition, the SCXI-1520 system offers a half-bridge completion resistor network in the module and a socketed 350 W quarter-bridge completion resistor. Table 1 summarizes some additional features of the SCXI-1520 that relate to strain gage measurements.
Table 1. SCXI-1520 Features for Strain Gages Number of channels
Multiplexer scan rate
Up to 333 kS/s1
1 to 1,000
Excitation voltage source
0.0 to 10.0 V in 0.635 V increments
Excitation current drive
29 mA throughout excitation voltage range
Remote excitation sensing
Multiplexer scan rate depends on the data acquisition device.
Recommended Starter Kit for Strain Gage SCXI Data Acquisition System: 1. USB-1600 USB Data Acquisition and Control Module for SCXI 2. NI SCXI-1000 chassis 3. SCXI-1520 with NI SCXI-1314 terminal block 4. Refer to ni.com/sensors for recommended sensor vendors
When external forces are applied to a stationary object, stress and strain are the result. Stress is defined as the object's internal resisting forces, and strain is defined as the displacement and deformation that occur. For a uniform distribution of internal resisting forces, stress can be calculated (Figure 2-1) by dividing the force (F) applied by the unit area (A):
Strain is defined as the amount of deformation per unit length of an object when a load is applied. Strain is calculated by dividing the total deformation of the original length by the original length (L):
Typical values for strain are less than 0.005 inch/inch and are often expressed in micro-strain units:
Strain may be compressive or tensile and is typically measured by strain gages. It was Lord Kelvin who first reported in 1856 that metallic conductors subjected to mechanical strain exhibit a change in their electrical resistance. This phenomenon was first put to practical use in the 1930s.
Figure 2-1: Definitions of Stress & Strain Fundamentally, all strain gages are designed to convert mechanical motion into an electronic signal. A change in capacitance, inductance, or resistance is proportional to the strain experienced by the sensor. If a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This changes its resistance (R) in proportion to the strain sensitivity (S) of the wire's resistance. When a strain is introduced, the strain sensitivity, which is also called the gage factor(GF), is given by:
The ideal strain gage would change resistance only due to the deformations of the surface to which the sensor is attached. However, in real applications, temperature, material properties, the adhesive that bonds the gage to the surface, and the stability of the metal all affect the detected resistance. Because most materials do not have the same properties in all directions, a knowledge of the axial strain alone is insufficient for a complete analysis. Poisson, bending, and torsional strains also need to be measured. Each requires a different strain gage arrangement. Shearing strain considers the angular distortion of an object under stress. Imagine that a horizontal force is acting on the top right corner of a thick book on a table, forcing the book to become somewhat trapezoidal (Figure 2-2). The shearing strain in this case can be expressed as the angular change in radians between the vertical y-axis and the new position. The shearing strain is the tangent of this angle.
Figure 2-2: Shearing Strain Poisson strain expresses both the thinning and elongation that occurs in a strained bar (Figure 2-3). Poisson strain is defined as the negative ratio of the strain in the traverse direction (caused by the contraction of the bar's diameter) to the strain in the longitudinal direction. As the length increases and the cross sectional area decreases, the electrical resistance of the wire also rises.
Figure 2-3: Poisson Strain
Bending strain, or moment strain, is calculated by determining the relationship between the force and the amount of bending which results from it. Although not as commonly detected as the other types of strain, torsional strain is measured when the strain produced by twisting is of interest. Torsional strain is calculated by dividing the torsional stress by the torsional modulus of elasticity. Sensor Designs The deformation of an object can be measured by mechanical, optical, acoustical, pneumatic, and electrical means. The earliest strain gages were mechanical devices that measured strain by measuring the change in length and comparing it to the original length of the object. For example, the extension meter (extensiometer) uses a series of levers to amplify strain to a readable value. In general, however, mechanical devices tend to provide low resolutions, and are bulky and difficult to use.
Figure 2-4: Strain Gage Designs Optical sensors are sensitive and accurate, but are delicate and not very popular in industrial applications. They use interference fringes produced by optical flats to measure strain. Optical sensors operate best under laboratory conditions. The most widely used characteristic that varies in proportion to strain is electrical resistance. Although capacitance and inductance-based strain gages have been constructed, these devices' sensitivity to vibration, their mounting requirements, and circuit complexity have limited their application. The photoelectric gage uses a light beam, two fine gratings, and a photocell detector to generate an electrical current that is proportional to strain. The gage length of these devices can be as short as 1/16 inch, but they are costly and delicate. The first bonded, metallic wire-type strain gage was developed in 1938. The metallic foil-type strain gage consists of a grid of wire filament (a resistor) of approximately 0.001 in. (0.025 mm) thickness, bonded directly to the strained surface by a thin layer of epoxy resin (Figure 2-4A). When a load is applied to the surface, the resulting change in surface length is communicated to the resistor and the corresponding strain is measured in terms of the electrical resistance of the foil wire, which varies linearly with strain. The foil diaphragm and the adhesive bonding agent must work together in transmitting the strain, while the adhesive must also serve as an electrical insulator between the foil grid and the surface. When selecting a strain gage, one must consider not only the strain characteristics of the sensor, but also its stability and temperature sensitivity. Unfortunately, the most desirable strain gage materials are also sensitive to temperature variations and tend to change resistance as they age. For tests of short duration, this may not be a serious concern, but for continuous industrial measurement, one must include temperature and drift compensation. Each strain gage wire material has its characteristic gage factor, resistance, temperature coefficient of gage factor, thermal coefficient of resistivity, and stability. Typical materials include Constantan (copper-nickel alloy), Nichrome V (nickel-chrome alloy), platinum alloys (usually tungsten), Isoelastic (nickel-iron alloy), or Karma-type alloy wires (nickel-chrome alloy), foils, or semiconductor materials. The most popular alloys used for strain gages are copper-nickel alloys and nickel-chromium alloys. In the mid-1950s, scientists at Bell Laboratories discovered the piezoresistive characteristics of germanium and silicon. Although the materials exhibited substantial nonlinearity and temperature sensitivity, they had gage factors more than fifty times, and sensitivity more than a 100 times, that of metallic wire or foil strain gages. Silicon wafers are also more elastic than metallic ones. After being strained, they return more readily to their original shapes.
Around 1970, the first semiconductor (silicon) strain gages were developed for the automotive industry. As opposed to other types of strain gages, semiconductor strain gages depend on the piezoresistive effects of silicon or germanium and measure the change in resistance with stress as opposed to strain. The semiconductor bonded strain gage is a wafer with the resistance element diffused into a substrate of silicon. The wafer element usually is not provided with a backing, and bonding it to the strained surface requires great care as only a thin layer of epoxy is used to attach it (Figure 2-4B). The size is much smaller and the cost much lower than for a metallic foil sensor. The same epoxies that are used to attach foil gages also are used to bond semiconductor gages. While the higher unit resistance and sensitivity of semiconductor wafer sensors are definite advantages, their greater sensitivity to temperature variations and tendency to drift are disadvantages in comparison to metallic foil sensors. Another disadvantage of semiconductor strain gages is that the resistance-to-strain relationship is nonlinear, varying 10-20% from a straight-line equation. With computer-controlled instrumentation, these limitations can be overcome through software compensation. A further improvement is the thin-film strain gage that eliminates the need for adhesive bonding (Figure 2-4C). The gage is produced by first depositing an electrical insulation (typically a ceramic) onto the stressed metal surface, and then depositing the strain gage onto this insulation layer. Vacuum deposition or sputtering techniques are used to bond the materials molecularly. Because the thin-film gage is molecularly bonded to the specimen, the installation is much more stable and the resistance values experience less drift. Another advantage is that the stressed force detector can be a metallic diaphragm or beam with a deposited layer of ceramic insulation. Diffused semiconductor strain gages represent a further improvement in strain gage technology because they eliminate the need for bonding agents. By eliminating bonding agents, errors due to creep and hysteresis also are eliminated. The diffused semiconductor strain gage uses photolithography masking techniques and solid-state diffusion of boron to molecularly bond the resistance elements. Electrical leads are directly attached to the pattern (Figure 2-4D). The diffused gage is limited to moderate-temperature applications and requires temperature compensation. Diffused semiconductors often are used as sensing elements in pressure transducers. They are small, inexpensive, accurate and repeatable, provide a wide pressure range, and generate a strong output signal. Their limitations include sensitivity to ambient temperature variations, which can be compensated for in intelligent transmitter designs. In summary, the ideal strain gage is small in size and mass, low in cost, easily attached, and highly sensitive to strain but insensitive to ambient or process temperature variations.
Figure 2-5: Bonded Resistance Strain Gage Construction
Bonded Resistance Gages The bonded semiconductor strain gage was schematically described in Figures 2-4A and 2-4B. These devices represent a popular method of measuring strain. The gage consists of a grid of very fine metallic wire, foil, or semiconductor material bonded to the strained surface or carrier matrix by a thin insulated layer of epoxy (Figure 2-5). When the carrier matrix is strained, the strain is transmitted to the grid material through the adhesive. The variations in the electrical resistance of the grid are measured as an indication of strain. The grid shape is designed to provide maximum gage resistance while keeping both the length and width of the gage to a minimum. Bonded resistance strain gages have a good reputation. They are relatively inexpensive, can achieve overall accuracy of better than +/-0.10%, are available in a short gage length, are only moderately affected by temperature changes, have small physical size and low mass, and are highly sensitive. Bonded resistance strain gages can be used to measure both static and dynamic strain.
Typical metal-foil strain gages. In bonding strain gage elements to a strained surface, it is important that the gage experience the same strain as the object. With an adhesive material inserted between the sensors and the strained surface, the installation is sensitive to creep due to degradation of the bond, temperature influences, and hysteresis caused by thermoelastic strain. Because many glues and epoxy resins are prone to creep, it is important to use resins designed specifically for strain gages. The bonded resistance strain gage is suitable for a wide variety of environmental conditions. It can measure strain in jet engine turbines operating at very high temperatures and in cryogenic fluid applications at temperatures as low as -452*F (269*C). It has low mass and size, high sensitivity, and is suitable for static and dynamic applications. Foil elements are available with unit resistances from 120 to 5,000 ohms. Gage lengths from 0.008 in. to 4 in. are available commercially. The three primary considerations in gage selection are: operating temperature, the nature of the strain to be detected, and stability requirements. In addition, selecting the right carrier material, grid alloy, adhesive, and protective coating will guarantee the success of the application.
Measuring Circuits In order to measure strain with a bonded resistance strain gage, it must be connected to an electric circuit that is capable of measuring the minute changes in resistance corresponding to strain. Strain gage transducers usually employ four strain gage elements electrically connected to form a Wheatstone bridge circuit (Figure 2-6). A Wheatstone bridge is a divided bridge circuit used for the measurement of static or
dynamic electrical resistance. The output voltage of the Wheatstone bridge is expressed in millivolts output per volt input. The Wheatstone circuit is also well suited for temperaturecompensation.
Figure 2-6: Wheatstone Bridge Circuit Schematic In Figure 2-6, if R1, R2, R3, and R4 are equal, and a voltage, VIN, is applied between points A and C, then the output between points B and D will show no potential difference. However, if R4 is changed to some value which does not equal R1, R2, and R3, the bridge will become unbalanced and a voltage will exist at the output terminals. In a so-called G-bridge configuration, the variable strain sensor has resistance Rg, while the other arms are fixed value resistors. The sensor, however, can occupy one, two, or four arms of the bridge, depending on the application. The total strain, or output voltage of the circuit (VOUT) is equivalent to the difference between the voltage drop across R1 and R4, or Rg. This can also be written as:
For more detail, see Figure 2-6. The bridge is considered balanced when R1/R2 = Rg/R3 and, therefore, VOUT equals zero. Any small change in the resistance of the sensing grid will throw the bridge out of balance, making it suitable for the detection of strain. When the bridge is set up so that Rg is the only active strain gage, a small change in Rg will result in an output voltage from the bridge. If the gage factor is GF, the strain measurement is related to the change in Rg as follows:
The number of active strain gages that should be connected to the bridge depends on the application. For example, it may be useful to connect gages that are on opposite sides of a beam, one in compression and the other in tension. In this arrangement, one can effectively double the bridge output for the same strain. In installations where all of
the arms are connected to strain gages, temperature compensation is automatic, as resistance change due to temperature variations will be the same for all arms of the bridge. In a four-element Wheatstone bridge, usually two gages are wired in compression and two in tension. For example, if R1 and R3 are in tension (positive) and R2 and R4 are in compression (negative), then the output will be proportional to the sum of all the strains measured separately. For gages located on adjacent legs, the bridge becomes unbalanced in proportion to the difference in strain. For gages on opposite legs, the bridge balances in proportion to the sum of the strains. Whether bending strain, axial strain, shear strain, or torsional strain is being measured, the strain gage arrangement will determine the relationship between the output and the type of strain being measured. As shown in Figure 2-6, if a positive tensile strain occurs on gages R2 and R3, and a negative strain is experienced by gages R1 and R4, the total output, VOUT, would be four times the resistance of a single gage.
Figure 2-7: Chevron Bridge Circuit Schematic The Chevron Bridge The Chevron bridge is illustrated in Figure 2-7. It is a multiple channel arrangement that serves to compensate for the changes in bridge-arm resistances by periodically switching them. Here, the four channel positions are used to switch the digital voltmeter (DVM) between G-bridge (one active gage) and H-bridge (two active gages) configurations. The DVM measurement device always shares the power supply and an internal H-bridge. This arrangement is most popular for strain measurements on rotating machines, where it can reduce the number of slip rings required.
Figure 2-8: Four-Wire Ohm Circuit Schematic Four-Wire Ohm Circuit Although the Wheatstone bridge is one of the most popular methods of measuring electrical resistance, other methods can also be used. The main advantage of a four-wire ohm circuit is that the lead wires do not affect the measurement because the voltage is detected directly across the strain gage element. A four-wire ohm circuit installation might consist of a voltmeter, a current source, and four lead resistors, R1, in series with a gage resistor, Rg (Figure 2-8). The voltmeter is connected to the ohms sense terminals of the DVM, and the current source is connected
to the ohms source terminals of the DVM. To measure the value of strain, a low current flow (typically one milliampere) is supplied to the circuit. While the voltmeter measures the voltage drop across Rg, the absolute resistance value is computed by the multimeter from the values of current and voltage. The measurement is usually done by first measuring the value of gage resistance in an unstrained condition and then making a second measurement with strain applied. The difference in the measured gage resistances divided by the unstrained resistance gives a fractional value of the strain. This value is used with the gage factor (GF) to calculate strain. The four-wire circuit is also suitable for automatic voltage offset compensation. The voltage is first measured when there is no current flow. This measured value is then subtracted from the voltage reading when current is flowing. The resulting voltage difference is then used to compute the gage resistance. Because of their sensitivity, four-wire strain gages are typically used to measure low frequency dynamic strains. When measuring higher frequency strains, the bridge output needs to be amplified. The same circuit also can be used with a semiconductor strain-gage sensor and high speed digital voltmeter. If the DVM sensitivity is 100 microvolts, the current source is 0.44 milliamperes, the strain-gage element resistance is 350 ohms and its gage factor is 100, the resolution of the measurement will be 6 microstrains.
Figure 2-9: Constant Current Circuit Schematic Constant Current Circuit Resistance can be measured by exciting the bridge with either a constant voltage or a constant current source. Because R = V/I, if either V or I is held constant, the other will vary with the resistance. Both methods can be used. While there is no theoretical advantage to using a constant current source (Figure 2-9) as compared to a constant voltage, in some cases the bridge output will be more linear in a constant current system. Also, if a constant current source is used, it eliminates the need to sense the voltage at the bridge; therefore, only two wires need to be connected to the strain gage element. The constant current circuit is most effective when dynamic strain is being measured. This is because, if a dynamic force is causing a change in the resistance of the strain gage (Rg), one would measure the time varying component of the output (VOUT), whereas slowly changing effects such as changes in lead resistance due to temperature variations would be rejected. Using this configuration, temperature drifts become nearly negligible. Application & Installation The output of a strain gage circuit is a very low-level voltage signal requiring a sensitivity of 100 microvolts or better. The low level of the signal makes it particularly susceptible to unwanted noise from other electrical devices. Capacitive coupling caused by the lead wires' running too close to AC power cables or ground currents are potential error sources in strain measurement. Other error sources may include magnetically induced voltages when the lead wires pass through variable magnetic fields, parasitic (unwanted) contact resistances of lead wires, insulation failure, and thermocouple effects
at the junction of dissimilar metals. The sum of such interferences can result in significant signal degradation. Shielding Most electric interference and noise problems can be solved by shielding and guarding. A shield around the measurement lead wires will intercept interferences and may also reduce any errors caused by insulation degradation. Shielding also will guard the measurement from capacitive coupling. If the measurement leads are routed near electromagnetic interference sources such as transformers, twisting the leads will minimize signal degradation due to magnetic induction. By twisting the wire, the fluxinduced current is inverted and the areas that the flux crosses cancel out. For industrial process applications, twisted and shielded lead wires are used almost without exception. Guarding Guarding the instrumentation itself is just as important as shielding the wires. A guard is a sheet-metal box surrounding the analog circuitry and is connected to the shield. If ground currents flow through the strain-gage element or its lead wires, a Wheatstone bridge circuit cannot distinguish them from the flow generated by the current source. Guarding guarantees that terminals of electrical components are at the same potential, which thereby prevents extraneous current flows. Connecting a guard lead between the test specimen and the negative terminal of the power supply provides an additional current path around the measuring circuit. By placing a guard lead path in the path of an error-producing current, all of the elements involved (i.e., floating power supply, strain gage, all other measuring equipment) will be at the same potential as the test specimen. By using twisted and shielded lead wires and integrating DVMs with guarding, common mode noise error can virtually be eliminated.
Figure 2-10: Alternative Lead-Wire Configurations Lead-Wire Effects Strain gages are sometimes 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. In a two-wire installation (Figure 2-10A), the two leads are in series with the strain-gage element, and any change in the lead-wire resistance (R1) will be indistinguishable from changes in the resistance of the strain gage (Rg). To correct for lead-wire effects, an additional, third lead can be introduced to the top arm of the bridge, as shown in Figure 2-10B. In this configuration, wire C acts as a sense lead with no current flowing in it, and wires A and B are in opposite legs of the bridge. This is the minimum acceptable method of wiring strain gages to a bridge to cancel at least part of the effect of extension wire errors. Theoretically, if the lead wires to the
sensor have the same nominal resistance, the same temperature coefficient, and are maintained at the same temperature, full compensation is obtained. In reality, wires are manufactured to a tolerance of about 10%, and three-wire installation does not completely eliminate two-wire errors, but it does reduce them by an order of magnitude. If further improvement is desired, four-wire and offset-compensated installations (Figures 2-10C and 2-10D) should be considered. In two-wire installations, the error introduced by lead-wire resistance is a function of the resistance ratio R1/Rg. The lead error is usually not significant if the lead-wire resistance (R1) is small in comparison to the gage resistance (Rg), but if the lead-wire resistance exceeds 0.1% of the nominal gage resistance, this source of error becomes significant. Therefore, in industrial applications, lead-wire lengths should be minimized or eliminated by locating the transmitter directly at the sensor.
Figure 2-11: Gage-Factor Temperature Dependence Temperature and the Gage Factor Strain-sensing materials, such as copper, change their internal structure at high temperatures. Temperature can alter not only the properties of a strain gage element, but also can alter the properties of the base material to which the strain gage is attached. Differences in expansion coefficients between the gage and base materials may cause dimensional changes in the sensor element. Expansion or contraction of the strain-gage element and/or the base material introduces errors that are difficult to correct. For example, a change in the resistivity or in the temperature coefficient of resistance of the strain gage element changes the zero reference used to calibrate the unit. The gage factor is the strain sensitivity of the sensor. The manufacturer should always supply data on the temperature sensitivity of the gage factor. Figure 2-11 shows the variation in gage factors of the various strain gage materials as a function of operating temperature. Copper-nickel alloys such as Advance have gage factors that are relatively sensitive to operating temperature variations, making them the most popular choice for strain gage materials.
Figure 2-12: Apparent Strain Variation with Temperature Apparent Strain Apparent strain is any change in gage resistance that is not caused by the strain on the force element. Apparent strain is the result of the interaction of the thermal coefficient of the strain gage and the difference in expansion between the gage and the test specimen. The variation in the apparent strain of various strain-gage materials as a function of operating temperature is shown in Figure 2-12. In addition to the temperature effects, apparent strain also can change because of aging and instability of the metal and the bonding agent. Compensation for apparent strain is necessary if the temperature varies while the strain is being measured. In most applications, the amount of error depends on the alloy used, the accuracy required, and the amount of the temperature variation. If the operating temperature of the gage and the apparent strain characteristics are known, compensation is possible. Stability Considerations It is desirable that the strain-gage measurement system be stable and not drift with time. In calibrated instruments, the passage of time always causes some drift and loss of calibration. The stability of bonded strain-gage transducers is inferior to that of diffused strain-gage elements. Hysteresis and creeping caused by imperfect bonding is one of the fundamental causes of instability, particularly in high operating temperature environments. Before mounting strain-gage elements, it should be established that the stressed force detector itself is uniform and homogeneous, because any surface deformities will result in instability errors. In order to remove any residual stresses in the force detectors, they should be carefully annealed, hardened, and stress-relieved using temperature aging. A transducer that uses force-detector springs, diaphragms, or bellows should also be provided with mechanical isolation. This will protect the sensor element from external stresses caused either by the strain of mounting or by the attaching of electric conduits to the transducer. If stable sensors are used, such as deposited thin-film element types, and if the forcedetector structure is well designed, balancing and compensation resistors will be sufficient for periodic recalibration of the unit. The most stable sensors are made from platinum or other low-temperature coefficient materials. It is also important that the transducer be operated within its design limits. Otherwise, permanent calibration shifts can result. Exposing the transducer to temperatures outside its operating limits can also degrade performance. Similarly, the transducer should be protected from vibration, acceleration, and shock.
Figure 2-13: Strain Gage Installation Alternatives Transducer Designs Strain gages are used to measure displacement, force, load, pressure, torque or weight. Modern strain-gage transducers usually employ a grid of four strain elements electrically connected to form a Wheatstone bridge measuring circuit. The strain-gage sensor is one of the most widely used means of load, weight, and force detection. In Figure 2-13A, a vertical beam is subjected to a force acting on the vertical axis. As the force is applied, the support column experiences elastic deformation and changes the electrical resistance of each strain gage. By the use of a Wheatstone bridge, the value of the load can be measured. Load cells are popular weighing elements for tanks and silos and have proven accurate in many other weighing applications. Strain gages may be bonded to cantilever springs to measure the force of bending (Figure 2-13B). The strain gages mounted on the top of the beam experience tension, while the strain gages on the bottom experience compression. The transducers are wired in a Wheatstone circuit and are used to determine the amount of force applied to the beam. Strain-gage elements also are used widely in the design of industrial pressure transmitters. Figure 2-13C shows a bellows type pressure sensor in which the reference pressure is sealed inside the bellows on the right, while the other bellows is exposed to the process pressure. When there is a difference between the two pressures, the strain detector elements bonded to the cantilever beam measure the resulting compressive or tensile forces. A diaphragm-type pressure transducer is created when four strain gages are attached to a diaphragm (Figure 2-13D). When the process pressure is applied to the diaphragm, the two central gage elements are subjected to tension, while the two gages at the edges are subjected to compression. The corresponding changes in resistance are a measure of the process pressure. When all of the strain gages are subjected to the same temperature, such as in this design, errors due to operating temperature variations are reduced. Installation Diagnostics All strain gage installations should be checked using the following steps:
1. Measure the base resistance of the unstrained strain gage after it is mounted, but before wiring is connected. 2. Check for surface contamination by measuring the isolation resistance between the gage grid and the stressed force detector specimen using an ohmmeter, if the specimen is conductive. This should be done before connecting
the lead wires to the instrumentation. If the isolation resistance is under 500 megaohms, contamination is likely. 3. Check for extraneous induced voltages in the circuit by reading the voltage when the power supply to the bridge is disconnected. Bridge output voltage readings for each strain-gage channel should be nearly zero. 4. Connect the excitation power supply to the bridge and ensure both the correct voltage level and its stability. 5. Check the strain gage bond by applying pressure to the gage. The reading should be unaffected. References & Further Reading Omegadyne Pressure, Force, Load, Torque Databook, OMEGADYNE, Inc., 1996 The Pressure, Strain, and Force Handbook, Omega Press LLC, 1996. Instrument Engineers' Handbook, Bela Liptak, CRC Press LLC, 1995. Marks' Standard Handbook for Mechanical Engineers, 10th Edition, Eugene A. Avallone, and Theodore Baumeister, McGraw-Hill, 1996. McGraw-Hill Concise Encyclopedia of Science and Technology, McGraw-Hill, 1998. Process/Industrial Instruments and Controls Handbook, 4th Edition, Douglas M. Considine, McGrawHill, 1993. Van Nostrand's Scientific Encyclopedia, Douglas M. Considine and Glenn D. Considine, Van Nostrand, 1997.