Measuring weight with a strain gauge.
Extracts from this document...
Introduction
Measuring weight with a strain gauge
A strain gauge is a wire which is used to measure strain by the
change in its resistance when it gets either longer and thinner or
shorter and thicker. They are attached to a surface for which the
strain is wanted, and need to be able to move as if they are part of
the surface. Modern strain gauges are etched onto foil because its
thin and flexible, and therefore able to move with the surface.
Gauges are glued onto the test object with superglue so that they
move as if they are a part of the object.
Elastic modulus = stress/strain (When stress is a linear tensile or
compressive stress, the elastic modulus is called Young’s modulus).
A tensile strain will be accompanied by a reduction (and
compressive strain by an increase) in lateral dimensions. The ratio of
the lateral strain to the longitudinal strain is called Poisson’s ratio1.
For most materials the value is between 0.25 and 0.4, and written as
a positive number although the signs of the lateral and longitudinal
strain are always opposite. The gauge factor of a strain gauge (G) =
(?R/R)/(?l/l) where R = resistance and l = length. Since ?l/l is the
strain (e) in the object which the gauge is attached to this can be
written as ?
Middle
R1(1+x)(1+y), and Vo = VsRg/(Rg+R’g)-Vs/2 ?
VsGe/4. This method also has drawbacks, though: that an unstrained
specimen of the original material has to be provided and that the
dummy gauge is not necessarily at the same temperature as the
active one. These problems can be solved by mounting the dummy
gauge on the same member as the active one and at right angles to
the direction of strain, so that the gauges are unlikely to have a
measurable difference in temperature. The dummy gauge will be
strained at right angles to its active axis, which will make it slightly
shorter along its active axis, as explained above, which means that
the resistance will decrease by an amount proportional to Poisson’s
ratio (v). If Rg = R1(1+x)(1+y) as before, and R’g = R1(1-vx)(1+y),
then Vo = VsR1(1+x)(1+y)/[R1(1+x)(1+y)+ R1(1-vx)(1+y)]- Vs/2 ?
Vs(1+v)Ge/4.
This method can be used to give a measurement of strain in a
member under tensile stress, but I’m planning to use a cantilever, the
end of which I will be putting weights on. In this situation I will be
able to increase my output readings by putting active gauges on both
sides of the cantilever, because as the cantilever bends the gauge on
Conclusion
results. Even so, the gauges would not stick at all the proper way
round, with the plastic backing stuck directly to the cantilever, so I
had to glue them on with the foil on the metal, and hope that the
superglue would insulate the gauge from the cantilever, which it
seems as though it did.
The choice of what to use as a cantilever is a very important one,
as if it is too rigid it will not bend much and so not give a big enough
reading, but if it is too flexible it will bend too much and the gauges
limit will be exceeded. I think that the metal I used as the cantilever
was too thick, and gave too small a reading, but I was restricted in
my choice by the materials available to me
I think that my technique was a suitable one, as it gave accurate
results without causing major problems or being too complicated:
the most complicated part of the project was assembling the bridge
itself. It is also an adaptable one, with different cantilevers being
used for different weight ranges, and different arrangments of the
bridge available.
Bibliography
Strain gauges supplied by RS (http://rswww.com)
1 Instrumentation units 1 and 2 by the instrumentation course team
(The Open University Press, Walton Hall, Milton Keynes, MK7
6AA)
2 Advanced Design and Tecnology by Eddie Norman, Joyce
Cubitt, Syd Urry, Mike Whittaker (Addison Wesley Longman
Limited, Edinburgh Gate, Harlow, Essex CM20 2JE)
3 RS Electronic Catalogue data sheets (http://rswww.com)
This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.
Found what you're looking for?
- Start learning 29% faster today
- 150,000+ documents available
- Just £6.99 a month