Strain Gages
Páginas: 2 (320 palabras)
Publicado: 18 de julio de 2012
Electrical Resistance Strain Gage Circuits
In the preceding sections we have examined the various construction schemes and application procedures for electrical resistance strain gages. By thesetechniques and devices, then, we have supposedly succeeded in transferring the extensional strain of the surface of a specimen into a proportional (or equal) strain in the element(s) of a resistivestrain gage. The problem et this juncture is to develop a method suitable for the conversion of resistance changes in the gage element into a more readily observable quantity. We must, in other words,transduce resistance changes. At first consideration, resistance measure appears to be a straightforward operation. It is a basic electrical parameter and ohmmeters for its measure are a common electronicinstruments. In the present application, however, the accuracy and resolution needed for the measure of strain at engineering levels is beyond the capabilities of all but the most refined of theseinstruments. As a result, special circuitry and wiring procedures have been developed to directly and accurately convert the resistance changes into proportional voltage changes. Several techniques willbe discussed in the following sections.
Electrical Properties of the Resistance Gage
The fundamental formula for the resistance of a wire with uniform cross section, A, and resistivity, ρ, can beexpressed as:
L (1) A where L is the wire length. This relation is generally accurate for common metals and many R=ρ
L
∆L
nonmetals at room temperature when subjected to direct or lowfrequency currents*. We consider the gage to be formed from a length of uniform wire and subjected to an elongation as shown: The change in resistance can be expressed from Eq. 1 as
∆R = ρ L L + ∆L − ( ρ+ ∆ρ ) A A + ∆A
where ∆ signifies a change in the quantity. This is a complicated expression in its present form, however, it should be clear that for metallic wires subjected to engineering...
In the preceding sections we have examined the various construction schemes and application procedures for electrical resistance strain gages. By thesetechniques and devices, then, we have supposedly succeeded in transferring the extensional strain of the surface of a specimen into a proportional (or equal) strain in the element(s) of a resistivestrain gage. The problem et this juncture is to develop a method suitable for the conversion of resistance changes in the gage element into a more readily observable quantity. We must, in other words,transduce resistance changes. At first consideration, resistance measure appears to be a straightforward operation. It is a basic electrical parameter and ohmmeters for its measure are a common electronicinstruments. In the present application, however, the accuracy and resolution needed for the measure of strain at engineering levels is beyond the capabilities of all but the most refined of theseinstruments. As a result, special circuitry and wiring procedures have been developed to directly and accurately convert the resistance changes into proportional voltage changes. Several techniques willbe discussed in the following sections.
Electrical Properties of the Resistance Gage
The fundamental formula for the resistance of a wire with uniform cross section, A, and resistivity, ρ, can beexpressed as:
L (1) A where L is the wire length. This relation is generally accurate for common metals and many R=ρ
L
∆L
nonmetals at room temperature when subjected to direct or lowfrequency currents*. We consider the gage to be formed from a length of uniform wire and subjected to an elongation as shown: The change in resistance can be expressed from Eq. 1 as
∆R = ρ L L + ∆L − ( ρ+ ∆ρ ) A A + ∆A
where ∆ signifies a change in the quantity. This is a complicated expression in its present form, however, it should be clear that for metallic wires subjected to engineering...
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