Unbonded Strain Gauge 6r5w25
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Contents Contents................................................................................................................. 1 Introduction ........................................................................................................... 2 Physics of instrument ............................................................................................2 New concepts and design.......................................................................................5 Advantages and disadvantages..............................................................................5 Applications and limitation.....................................................................................5 Sources of error and minimization techniques........................................................6 References ............................................................................................................ 9
1
Introduction The most popular electrical elements used in force measurements include the resistance strain gage, the semiconductor strain gage, and piezoelectric transducers. The resistance strain gage is a resistive element which changes in length, hence resistance, as the force applied to the base on which it is mounted causes stretching or compression. It is perhaps the most well known transducer for converting force into an electrical variable. Resistance wire strain gauges are used in two forms. These two forms are: I. II.
Unbounded strain gauge and Bonded strain gauge
The strain gage measures force indirectly by measuring the deflection it produces in a calibrated carrier. Pressure can be converted into a force using an appropriate transducer, and strain gage techniques can then be used to measure pressure. Flow rates can be measured using differential pressure measurements which also make use of strain gage technology.
Physics of instrument When the load is applied, the resistance of strain gauges changes causing an unbalance of the bridge circuit which results in an output voltage. This voltage is proportional to the strain. A displacement of 50 μm can be detected with these strain gauges. The typical construction of a gage consists of a thin wire stretched between posts. When the distance between the posts is changed, strain gauge will change in length and cross section. That is, the strain gauge is strained. This strain changes the resistance of the strain gauge and this change in resistance is measured using a wheat stone bridge. This change in resistance when calibrated becomes a measure of the applied force and change in dimensions of the structure under study. In the practical design, several loops of wire usually form arms of the Wheatstone bridge. The bridge circuit is arranged in such a manner that elongation of the two arms is accompanied by shortening of the other two arms.
2
Force acting on the wire (area = A, length = L, resistivity =r) will cause the wire to elongate or shorten, which will cause the resistance to increase or decrease proportionally according to: R = rL/A and DR/R = GF·DL/L, where GF = Gage factor (2.0 to 4.5 for metals, and more than 150 for semiconductors). The dimensionless quantity DL/L is a measure of the force applied to the wire and is expressed in microstrains (1μe = 10–6 cm/cm) which is the same as parts-per-million (ppm). From this equation, larger gage factors result in proportionally larger resistance changes, hence, more sensitivity. Unbonded strain gauge is one in which there is a free filament sensing element where strain is transferred to the resistance wire directly without any backing.
3
Unbonded strain-gage pressure sensor
Unbonded strain gauges are made by stretching wire around an array of posts that are linked to the force-summing device, usually a diaphragm, such that they experience relative motion as the diaphragm deformss under pressure.
(a) Unbonded strain-gage pressure sensor. The diaphragm is directly coupled by an armature to an unbonded strain-gage system. With increasing pressure, the strain on gage pair B and C is increased, while that on gage pair A and D is decreased. (b) Wheatstone bridge with four active elements: R1 = B; R2 = A; R3 = D, and R4 = C when the unbonded strain gage is connected for translational motion. Resistor Ry and potentiometer Rx are used to initially balance the bridge, Vi is the applied voltage, and vo is the output voltage on a voltmeter or similar device with an internal resistance of Ri. Difference between bonded strain gauges and unbonded strain gauges The essential difference is that the bonded strain gauges are bonded on to the specimen whose strain is being measured whereas the unbonded strain gauges are not bonded on to the specimen. 4
As the bonded strain gauges are well bonded on to the specimen, the entire strain being experienced by the specimen is transferred to the strain gauge. However, the bonded strain gauges are affected by temperature changes and also due to transverse strains. For transverse strains and ambient temperature compensations, suitable circuits for compensation can be used using Wheatstone's bridge. The unbonded strain gauges cannot transfer the strain of the specimen to the strain gauge and hence it is used mainly for displacement, or pressure or force transducers. It is least affected by transverse strain and temperature compensation of unbonded gauges is automatically eliminated using Wheatstone's bridge.
New concepts and design Strain-gage technology advanced in the 1960s with the introduction of the semiconductor strain-gage element, which has the advantage of having a high gage factor. However, it is more temperature sensitive and inherently more nonlinear than metal strain gages because the piezoresistive effect varies with strain. Semiconductor elements can be used as bonded, unbonded, or integrated strain-gage units. These integrated devices can be constructed with either silicon or germanium p or n type as the substrate that forms the structural member. The opposite-type material is diffused into the substrate. Opposite signs for the gage factor result for n- and p-type substrate gages. A large gage factor can be attained with lightly doped material.
Advantages and disadvantages Advantages •
The range of this gauge is +/- 0.15% strain.
•
This gauge has a very high accuracy.
Applications and limitation Application of Unbonded strain gauge: • Unbonded strain gauge is used in places where the gauge is to be detached and used
again and again. • Used in force, pressure and acceleration measurement. 5
• Measurement of small motions as small as a few microns can be made.
Limitation of unbonded strain gauges It occupies more space.
Sources of error and minimization techniques Systematic errors: These types of errors are classified into three categories. 1. Instrumental errors 2. Environmental errors and 3. Observational errors. Instrumental errors: There are many factors in the design and construction of instruments that limit the accuracy attainable. Assembly errors (Because of bend or distorted pointers, non uniform division of the scale, or displaced scale that does not coincide with the actual zero position) come under this category of errors. These types of errors do not alter with time, but it can be easily discovered and corrected. •
Improper selecting and poor maintenance of the instrument.
•
Faults of construction resulting from finite width of knife edges.
•
Mechanical friction and wear, backlash, yielding of s, and hystersis of elastic due to aging.
•
Unavoidable physical phenomenon due to friction, capillary attraction and imperfect rarefaction.
Environmental errors: These types of errors are more dangerous as they change with time in an unpredictable manner. The instrument would have been assembled and calibrated in one environment. For measurement it would have been carried to a different place and because of this change, error
6
occurs. The change may be due to different temperatures, pressures, humidity and altitude etc. These errors can be eliminated or reduced by the following precautions mentioned. •
Using the instrument in controlled condition of pressure, temperature, in which it was assembled and calibrated.
•
If this is not possible, then the deviations in local conditions from the calibrated value are measured and suitable correction to the instrument readings is applied.
•
Automatic compensation using sophisticated devices is possible and is usually applied.
•
Make a new calibrations based on the local conditions.
Observational errors: Even when the instruments are properly selected, carefully installed and calibrated, short coming in the measurement occur due to certain mistakes on the part of the observer. These types of errors may be due to •
Parallax apparent displacement when the line of vision is not normal to the scale.
•
Inaccurate estimates of average reading, lack of ability to interpolate properly between graduations.
•
Non simultaneous observation of independent quantities.
•
Personal bias - a tendency to read high or low, or anticipate a signal and read too soon.
•
Wrong scale reading and wrong recording of data. Modern instruments use digital systems that eliminate the possibilities of errors due to
human observations. These errors can be eliminated by careful training and by taking independent readings of each item by two or more observers. Possible sources of error and their minimization techniques in strain gauge 1. Cross-sensitivity Because a strain gauge has width as well as length, a small proportion of the resistance element lies at right angles to the major axis of the gauge, at the points where the conductor reverses direction at the ends of the gauge. So as well as responding to strain in the direction of its major axis, the gauge will also be somewhat responsive to any strain there may be at right angles to major axis.
7
2. Hysteresis If a strain gauge installation is loaded to a high value of strain and then unloaded, it may be found that the gauge element appears to have acquired a permanent set, so that resistance values are all slightly higher when unloading when loading-up. The same effect continues when the direction of loading is reversed. To manipulate this problem, repeating cycles of loading/unloading should cause the hysteresis loop to narrow to negligible proportions. Severe hysteresis, which cannot be reduced in this way indicates faulty bonding of the gauge. 3.
Effects of moisture
The gauges or the bonding adhesive may absorb water. This can cause dimensional changes which appear as false strain values. Another effect when moisture connections forms high resistance connected in parallel with the gauge. To prevent this, gauges should be bonded in dry condition or a suitable electrically insulating water repellent, such as a silicone rubber compound. 4.
Temperature change
One possible source of temperature difference is the heat produced by the current through a strain gauge. When the bridge is first switched on, the gauges may warm up, so the bridge should not be used for measurement until sufficient time for temperature to stabilize.
8
References •
SECTION 4 STRAIN, FORCE, PRESSURE, AND FLOW MEASUREMENTS Walt Kester
•
Aero Ltd, P.O. Box 483, Farnham, Surrey, GU9 8WS, U.K. 4-202 Strain Gauge Accelerometer
•
BASIC SENSORS AND PRINCIPLES Robert A. Peura and John G. Webster
•
http://books.google.com.np/books? id=yqULL_uu5EAC&pg=PA274&lpg=PA274&dq=unbonded+strain+gauge&source=b l&ots=8Gmv9QkyFm&sig=rGCr5672bOTHY2fu6SBaR2OJKlE&hl=en&sa=X&ei=7 QMZUPH5OMPprAfytYDgCw&redir_esc=y#v=onepage&q=unbonded%20strain %20gauge&f=false
•
http://www.jms-se.com/catalog/File0109.PDF
•
http://www.analog.com/static/importedfiles/seminars_webcasts/927412250sscsect4.PDF
•
http://wiki.answers.com/Q/Difference_between_unbonded_strain_gauge_and_bonded_ strain_gauge#ixzz1zdv07Zqa
•
http://instrumentationandcontrollers.blogspot.com/2010/11/unbonded-straingauges.html
9
1
Introduction The most popular electrical elements used in force measurements include the resistance strain gage, the semiconductor strain gage, and piezoelectric transducers. The resistance strain gage is a resistive element which changes in length, hence resistance, as the force applied to the base on which it is mounted causes stretching or compression. It is perhaps the most well known transducer for converting force into an electrical variable. Resistance wire strain gauges are used in two forms. These two forms are: I. II.
Unbounded strain gauge and Bonded strain gauge
The strain gage measures force indirectly by measuring the deflection it produces in a calibrated carrier. Pressure can be converted into a force using an appropriate transducer, and strain gage techniques can then be used to measure pressure. Flow rates can be measured using differential pressure measurements which also make use of strain gage technology.
Physics of instrument When the load is applied, the resistance of strain gauges changes causing an unbalance of the bridge circuit which results in an output voltage. This voltage is proportional to the strain. A displacement of 50 μm can be detected with these strain gauges. The typical construction of a gage consists of a thin wire stretched between posts. When the distance between the posts is changed, strain gauge will change in length and cross section. That is, the strain gauge is strained. This strain changes the resistance of the strain gauge and this change in resistance is measured using a wheat stone bridge. This change in resistance when calibrated becomes a measure of the applied force and change in dimensions of the structure under study. In the practical design, several loops of wire usually form arms of the Wheatstone bridge. The bridge circuit is arranged in such a manner that elongation of the two arms is accompanied by shortening of the other two arms.
2
Force acting on the wire (area = A, length = L, resistivity =r) will cause the wire to elongate or shorten, which will cause the resistance to increase or decrease proportionally according to: R = rL/A and DR/R = GF·DL/L, where GF = Gage factor (2.0 to 4.5 for metals, and more than 150 for semiconductors). The dimensionless quantity DL/L is a measure of the force applied to the wire and is expressed in microstrains (1μe = 10–6 cm/cm) which is the same as parts-per-million (ppm). From this equation, larger gage factors result in proportionally larger resistance changes, hence, more sensitivity. Unbonded strain gauge is one in which there is a free filament sensing element where strain is transferred to the resistance wire directly without any backing.
3
Unbonded strain-gage pressure sensor
Unbonded strain gauges are made by stretching wire around an array of posts that are linked to the force-summing device, usually a diaphragm, such that they experience relative motion as the diaphragm deformss under pressure.
(a) Unbonded strain-gage pressure sensor. The diaphragm is directly coupled by an armature to an unbonded strain-gage system. With increasing pressure, the strain on gage pair B and C is increased, while that on gage pair A and D is decreased. (b) Wheatstone bridge with four active elements: R1 = B; R2 = A; R3 = D, and R4 = C when the unbonded strain gage is connected for translational motion. Resistor Ry and potentiometer Rx are used to initially balance the bridge, Vi is the applied voltage, and vo is the output voltage on a voltmeter or similar device with an internal resistance of Ri. Difference between bonded strain gauges and unbonded strain gauges The essential difference is that the bonded strain gauges are bonded on to the specimen whose strain is being measured whereas the unbonded strain gauges are not bonded on to the specimen. 4
As the bonded strain gauges are well bonded on to the specimen, the entire strain being experienced by the specimen is transferred to the strain gauge. However, the bonded strain gauges are affected by temperature changes and also due to transverse strains. For transverse strains and ambient temperature compensations, suitable circuits for compensation can be used using Wheatstone's bridge. The unbonded strain gauges cannot transfer the strain of the specimen to the strain gauge and hence it is used mainly for displacement, or pressure or force transducers. It is least affected by transverse strain and temperature compensation of unbonded gauges is automatically eliminated using Wheatstone's bridge.
New concepts and design Strain-gage technology advanced in the 1960s with the introduction of the semiconductor strain-gage element, which has the advantage of having a high gage factor. However, it is more temperature sensitive and inherently more nonlinear than metal strain gages because the piezoresistive effect varies with strain. Semiconductor elements can be used as bonded, unbonded, or integrated strain-gage units. These integrated devices can be constructed with either silicon or germanium p or n type as the substrate that forms the structural member. The opposite-type material is diffused into the substrate. Opposite signs for the gage factor result for n- and p-type substrate gages. A large gage factor can be attained with lightly doped material.
Advantages and disadvantages Advantages •
The range of this gauge is +/- 0.15% strain.
•
This gauge has a very high accuracy.
Applications and limitation Application of Unbonded strain gauge: • Unbonded strain gauge is used in places where the gauge is to be detached and used
again and again. • Used in force, pressure and acceleration measurement. 5
• Measurement of small motions as small as a few microns can be made.
Limitation of unbonded strain gauges It occupies more space.
Sources of error and minimization techniques Systematic errors: These types of errors are classified into three categories. 1. Instrumental errors 2. Environmental errors and 3. Observational errors. Instrumental errors: There are many factors in the design and construction of instruments that limit the accuracy attainable. Assembly errors (Because of bend or distorted pointers, non uniform division of the scale, or displaced scale that does not coincide with the actual zero position) come under this category of errors. These types of errors do not alter with time, but it can be easily discovered and corrected. •
Improper selecting and poor maintenance of the instrument.
•
Faults of construction resulting from finite width of knife edges.
•
Mechanical friction and wear, backlash, yielding of s, and hystersis of elastic due to aging.
•
Unavoidable physical phenomenon due to friction, capillary attraction and imperfect rarefaction.
Environmental errors: These types of errors are more dangerous as they change with time in an unpredictable manner. The instrument would have been assembled and calibrated in one environment. For measurement it would have been carried to a different place and because of this change, error
6
occurs. The change may be due to different temperatures, pressures, humidity and altitude etc. These errors can be eliminated or reduced by the following precautions mentioned. •
Using the instrument in controlled condition of pressure, temperature, in which it was assembled and calibrated.
•
If this is not possible, then the deviations in local conditions from the calibrated value are measured and suitable correction to the instrument readings is applied.
•
Automatic compensation using sophisticated devices is possible and is usually applied.
•
Make a new calibrations based on the local conditions.
Observational errors: Even when the instruments are properly selected, carefully installed and calibrated, short coming in the measurement occur due to certain mistakes on the part of the observer. These types of errors may be due to •
Parallax apparent displacement when the line of vision is not normal to the scale.
•
Inaccurate estimates of average reading, lack of ability to interpolate properly between graduations.
•
Non simultaneous observation of independent quantities.
•
Personal bias - a tendency to read high or low, or anticipate a signal and read too soon.
•
Wrong scale reading and wrong recording of data. Modern instruments use digital systems that eliminate the possibilities of errors due to
human observations. These errors can be eliminated by careful training and by taking independent readings of each item by two or more observers. Possible sources of error and their minimization techniques in strain gauge 1. Cross-sensitivity Because a strain gauge has width as well as length, a small proportion of the resistance element lies at right angles to the major axis of the gauge, at the points where the conductor reverses direction at the ends of the gauge. So as well as responding to strain in the direction of its major axis, the gauge will also be somewhat responsive to any strain there may be at right angles to major axis.
7
2. Hysteresis If a strain gauge installation is loaded to a high value of strain and then unloaded, it may be found that the gauge element appears to have acquired a permanent set, so that resistance values are all slightly higher when unloading when loading-up. The same effect continues when the direction of loading is reversed. To manipulate this problem, repeating cycles of loading/unloading should cause the hysteresis loop to narrow to negligible proportions. Severe hysteresis, which cannot be reduced in this way indicates faulty bonding of the gauge. 3.
Effects of moisture
The gauges or the bonding adhesive may absorb water. This can cause dimensional changes which appear as false strain values. Another effect when moisture connections forms high resistance connected in parallel with the gauge. To prevent this, gauges should be bonded in dry condition or a suitable electrically insulating water repellent, such as a silicone rubber compound. 4.
Temperature change
One possible source of temperature difference is the heat produced by the current through a strain gauge. When the bridge is first switched on, the gauges may warm up, so the bridge should not be used for measurement until sufficient time for temperature to stabilize.
8
References •
SECTION 4 STRAIN, FORCE, PRESSURE, AND FLOW MEASUREMENTS Walt Kester
•
Aero Ltd, P.O. Box 483, Farnham, Surrey, GU9 8WS, U.K. 4-202 Strain Gauge Accelerometer
•
BASIC SENSORS AND PRINCIPLES Robert A. Peura and John G. Webster
•
http://books.google.com.np/books? id=yqULL_uu5EAC&pg=PA274&lpg=PA274&dq=unbonded+strain+gauge&source=b l&ots=8Gmv9QkyFm&sig=rGCr5672bOTHY2fu6SBaR2OJKlE&hl=en&sa=X&ei=7 QMZUPH5OMPprAfytYDgCw&redir_esc=y#v=onepage&q=unbonded%20strain %20gauge&f=false
•
http://www.jms-se.com/catalog/File0109.PDF
•
http://www.analog.com/static/importedfiles/seminars_webcasts/927412250sscsect4.PDF
•
http://wiki.answers.com/Q/Difference_between_unbonded_strain_gauge_and_bonded_ strain_gauge#ixzz1zdv07Zqa
•
http://instrumentationandcontrollers.blogspot.com/2010/11/unbonded-straingauges.html
9
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