Strain gauge Introduction

INTRODUCTION

 Strain gauge transforms an input energy (mechanical energy in the form of strain resulting from an applied force) into an energy output (electrical energy in the form of an induced voltage difference).  The induced voltage can be amplified, converted to a digital signal and read into a computer.  With the appropriate software and knowledge of the workings of a load cell, the resulting signal can be converted into a digital readout of the initial load.

By using this instrument, the strain of an object can be determined. It is possible only when any object is placed on load cell, the strain is obtained and it is displayed in LCD display through Arduino Uno.

1.1 Physical Discoveries

A strain gauge is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, which is related to the strain by the quantity known as the gauge factor.

1.2 Physical Operations

A strain gauge takes advantage of the physical property of electrical conductance and its dependence on the conductor's geometry. When an electrical conductor is stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance end-to-end. Conversely, when a conductor is compressed such that it does not buckle, it will broaden and shorten changes that decrease its electrical resistance end-to-end. From the measured electrical resistance of the strain gauge, the amount of induced stress may be inferred. A typical strain gauge arranges a long, thin conductive strip in a zig-zag pattern of parallel lines such that a small amount of stress in the direction of the orientation of the parallel lines results in a multiplicatively larger strain measurement over the effective length of the conductor surfaces in the array of conductive lines and hence a multiplicatively larger change in resistance than would be observed with a single straight-line conductive wire.

Fig: 1.1 Strain gauge

Typical foil strain gauge. The gauge is far more sensitive to strain in the vertical direction than in the horizontal direction. The markings outside the active area help to align the gauge during installation.

1.3 Gauge Factor

The gauge factor is defined as:

  GF= △R/Rg*ε

Where

△R is the change in resistance caused by strain.
Rg is the resistance of the non deformed gauge and

ε is strain.

For common metallic foil gauges, the gauge factor is usually a little over 2. For a single active gauge and three dummy resistors of the same resistance about the active gauge in a Wheatstone bridge configuration, the output  from the bridge is:

           ૭=(BV*GF*ε)/2

Where BV is the bridge excitation voltage

             GF is the Gauge factor and

             ε is the mechanical strain.

Foil gauges typically have active areas of about 2–10 mm² in size. With careful installation, the correct gauge and the correct adhesive, strains up to at least 10% can be measured.

1.4 Working

An excitation voltage is applied to input leads of the gauge network and a voltage reading is taken from the output leads. Typical input voltages are 5 V or 12 V and typical output readings are in mill volts.

Foil strain gauges are used in many situations. Different applications place different requirements on the gauge. In most cases the orientation of the strain gauge is significant.

Gauges attached to a load cell would normally be expected to remain stable over a period of years, if not decades; while those used to measure response in a dynamic experiment may only need to remain attached to the object for a few days, be energized for less than an hour and operate for less than a second.

Strain gauges are attached to the substrate with a special glue. The type of glue depends on the required lifetime of the measurement system. For short term measurements (up to some weeks) cyanoacrylate glue is appropriate, for long lasting installation epoxy glue is required. Usually epoxy glue requires high temperature curing (at about 80-100 °C). The preparation of the surface where the strain gauge is to be glued is of the utmost importance. The surface must be smoothed (e.g. with very fine sand paper), deoiled with solvents, the solvent traces must then be removed and the strain gauge must be glued immediately after this to avoid oxidation or pollution of the prepared area. If these steps are not followed the strain gauge binding to the surface may be unreliable and unpredictable measurement errors may be generated.

Strain gauge based technology is utilized commonly in the manufacture of pressure sensor. The gauges used in pressure sensors themselves are commonly made from silicon, polysilicon, metal film, thick film and bonded foil.

Fig: 1.2 Strain gauge working

Visualization of the working concept behind the strain gauge on a beam under exaggerated bending

1.5 Variations in Temperature

Variations in temperature will cause a multitude of effects. The object will change in size by thermal expansion, which will be detected as a strain by the gauge. Resistance of the gauge will change and resistance of the connecting wires will change.

Most strain gauges are made from a constantan alloy. Various constantan alloys and Karma alloys have been designed so that the temperature effects on the resistance of the strain gauge itself cancel out the resistance change of the gauge due to the thermal expansion of the object under test. Because different materials have different amounts of thermal expansion, self-temperature compensation (STC) requires selecting a particular alloy matched to the material of the object under test.

Strain gauges that are not self-temperature-compensated (such as isoelastic alloy) can be temperature compensated by use of the dummy gauge technique. A dummy gauge (identical to the active strain gauge) is installed on an unstrained sample of the same material as the test specimen. The sample with the dummy gauge is placed in thermal contact with the test specimen, adjacent to the active gauge. The dummy gauge is wired into a Wheatstone bridge on an adjacent arm to the active gauge so that the temperature effects on the active and dummy gauges cancel each other. (Murphy's Law was originally coined in response to a set of gauges being incorrectly wired into a Wheatstone bridge.)

Temperature effects on the lead wires can be cancelled by using a "3-wire bridge" or a "4-wire ohm circuit" (also called a "4-wire Kelvin connection").

In any case, it is a good engineering practice to keep the Wheatstone bridge voltage drive low enough to avoid the self-heating of the strain gauge. The self-heating of the strain gauge depends on its mechanical characteristic (large strain gauges are less prone to self-heating). Low voltage drive levels of the bridge reduce the sensitivity of the overall system.  
1.6 Errors and Compensation

Zero Offset – If the impedance of the four gauge arms are not exactly the same after bonding the gauge to the force collector, there will be a zero offset which can be compensated by introducing a parallel resistor to one or more of the gauge arms.

·         Temperature coefficient of gauge factor (TCGF) is the change of sensitivity of the device to strain with change in temperature. This is generally compensated by the introduction of a fixed resistance in the input leg, whereby the effective supplied voltage will increase with temperature, compensating for the decrease in sensitivity with temperature.

·         Zero shift with temperature – If the TCGF of each gauge is not the same, there will be a zero shift with temperature. This is also caused by anomalies in the force collector. This is usually compensated with one or more resistors strategically placed in the compensation network.

·         Linearity is an error whereby the sensitivity changes across the pressure range. This is commonly a function of the force collection thickness selection for the intended pressure and the quality of the bonding.

·         Hysteresis is an error of return to zero after pressure excursion.

·         Repeatability – This error is sometimes tied-in with hysteresis but it is across the pressure range.

·         EMI induced errors – As strain gauges output voltage is in the mV range, even μV if the Wheatstone bridge voltage drive is kept low to avoid self heating of the element, special care must be taken in output signal amplification to avoid amplifying and also the superimposed noise. A solution which is frequently adopted is to use "carrier frequency" amplifiers which convert the voltage variation into a frequency variation (as in VCOs) and have a narrow bandwidth thus reducing out of band EMI.

·         Overloading – If a strain gauge is loaded beyond its design limit (measured in micro strain) its performance degrades and cannot be recovered. Normally good engineering practice suggests not to stress strain gauges beyond ±3000 micro strain.

·         Humidity – If the wires connecting the strain gauge to the signal conditioner are not protected against humidity, such as bare wire and corrosion leading to parasitic resistance. This can allow currents to flow between the wires and the substrate to which the strain gauge is glued or between the two wires directly, introducing an error which competes with the current flowing through the strain gauge. For this reason, high-current, low-resistance strain gauges (120 ohm) are less prone to this type of error. To avoid this error it is sufficient to protect the strain gauges wires with insulating enamel (e.g., epoxy or polyurethane type). Strain gauges with unprotected wires may be used only in a dry laboratory environment but not in an industrial one.

·         In some applications strain gauges add mass and damping to the vibration profiles of the hardware they are intended to measure. In the turbo machinery industry, one used alternative to strain gauge technology in the measurement of vibrations on rotating hardware is the Non-Intrusive Stress Measurement System, which allows measurement of blade vibrations without any blade or disc-mounted hardware.