Electrical resistors – Strain gauge type
Patent
1994-03-11
1996-04-16
Walberg, Teresa J.
Electrical resistors
Strain gauge type
73862629, G01L 122
Patent
active
055086764
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The present invention relates to a strain gauge on a flexible support, as well as to a transducer equipped with said gauge.
Strain gauges on a flexible support are used for measuring the deformations of mechanical parts. They have numerous applications. They are in particular used for weighing (e.g. commercial and bathroom scales), for pressure measurements, for stress measurements on mechanical parts such as transmission shafts, different portions of an aircraft wing or concavities. They are also used in extensometry for the inspection of concrete structures, such as dams or bridges. Finally, these gauges can also be used for performing torsion, torque, vibration or acceleration measurements.
In its most simple form, the gauge 1 is constituted by a very thin strand 3 bonded to a thin support 5 and arranged in the form of loops shown in FIG. 1, i.e. most of its length is located parallel to a fixed direction (arrow X). Larger strands 7 are used for welding the outlets for cables for connecting to instruments.
When it is wished to know the elongation of a structure in a given direction, bonding takes place of the gauge 1 with the strands parallel to said direction. The gauges 1 are also used for producing transducers 9, such as is illustrated in the attached FIG. 2. A transducer is a mechanical device for transforming a random physical quantity A (pressure, force, acceleration, etc.) into a deformation of a part 11 known as the test body. Gauges bonded to test bodies detect its deformations with a view to the measurement of the physical quantity A or for acting on regulating devices.
In the case illustrated in FIG. 2, the two gauges 1c operate in compression and the two gauges 1e in extension. This makes it possible to obtain very accurate measurements of approximately 10.sup.-4 of the extent of the measurement. The same arrangement can be used on all types of structures, as described in "L'encyclopedie Vishay d'analyse des contraintes", Vishay-Micromesures, Malakoff, France, 282-284.
Finally, the gauges can be arranged in the form of a Wheatstone bridge, as is described e.g. in EP-A-53 059.
No matter what the arrangement of the strain gauges, the measurements performed are all based on resistance variations of the strand 3, said variations being a function of the type of material forming it or its length and section formations.
EP-A-53 059 discloses a strain gauge, obtained by the vacuum deposition of a metal alloy coating of 50 to 500 nm on a 100 to 250 .mu.m thick glass substrate. However, glass is extremely brittle and is difficult to use in certain cases. Therefore this gauge can be used only in compression and not in tension due to the ultimate strength of the glass.
The prior art also discloses strain gauges to be used in compression and extension and which for this purpose are fixed to a flexible support. These gauges have a thin film of polyimide or phenolic epoxy resin with a thickness of 25 .mu.m and to this is bonded a very thin sheet of approximately 5 .mu.m of a laminated resistive material such as an alloy of nickel-chromium, nickel-copper or platinum-tungsten.
In this case, the resistive layer is then etched in fine strip form in order to obtain a resistor, whose shape can be gathered from the enclosed FIG. 1. The resulting resistors can have values between 120 and 6000 ohms.
This type of gauge suffers from a certain number of disadvantages. It is difficult to manufacture the gauges, because the bonding of very thin sheets of resistive material is a complicated operation, which it is difficult to reproduce. The manufacture of 5 .mu.m thick metal sheets also takes a long time and is difficult, because it requires a succession of rolling and annealing operations for stabilizing the material between each rolling stage. Finally, the thickness of the 5 .mu.m resistive metal coating limits the values of the resistors obtained, which generally do not exceed 6000 ohms.
U.S. Pat. No. 4,786,887 discloses a gauge having a flexible substrate covered with a polymeric insulating layer and a r
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Grange Hubert
Maeder Catherine
Commissariat a l''Energie Atomique
Valencia Raphael
Walberg Teresa J.
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