Electrically insulated strain gage

Details Electrically insulated strain gage Electrically insulated strain gage

1999-02-18

2003-10-14

Noori, Max (Department: 2855)

Measuring and testing

Specimen stress or strain, or testing by stress or strain...

Specified electrical sensor or system

Reexamination Certificate

active

06631646

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of strain measurement. More particularly, the present invention relates to using semiconductive strain gages to measure strain on an object.
2. Discussion of the Related Art
When force is applied to an object, this results in stress on the object. Stress is the force per unit area acting on the object. When an object experiences stress, the object will experience deformation, which is the change in shape of the object in any dimension. Strain is a measurement of the intensity of this deformation. More specifically, strain is the deformation per unit length of the object in any dimension resulting from stress.
Devices employing a variety of techniques are available to measure strain on an object. Typically, these devices translate the mechanical strain on an object into an electrical signal. Strain gages are commonly used in such devices.
One type of conventional strain gage consists of a monolith of conductive or semiconductive material whose resistance changes when the gage deforms. Typically, this type of conventional strain gage is mechanically bonded to a mechanical sensor with an adhesive to form a strain gage assembly. Specifically, as illustrated in
FIG. 1
, an adhesive is applied to the mechanical sensor
2
to form an adhesive layer
3
, the strain gage
4
is pressed against the adhesive layer
3
, and the adhesive layer
3
is allowed time to cure. It should be noted that
FIG. 1
is not drawn to scale. The adhesive may be an epoxy, paste or other suitable bonding compound or agent.
In operation, when a force impacts on the mechanical sensor
2
, the resultant deformation of the mechanical sensor
2
causes the strain gage
4
to similarly deform, with a resultant change in electrical resistance. This change in resistance is measured and used to provide a measurement of the strain on the mechanical sensor
2
. This change in resistance may also be used to determine the stress or pressure on the mechanical sensor
2
. Force cells, loads cells, pressure transducers and accelerometers are typical devices that make use of this principle.
The material of the strain gage
4
can be a conductive metal or a semiconductive material. Semiconductive materials have the advantage of providing a larger change in resistance for a given change in strain than do conductive metals. In a strain gage assembly
1
, attaching a strain gage
4
of semiconductive material to a mechanical sensor
2
of metal material may result in electrical shorts or electrical leakage during operation. Specifically, shorts and leakage result when the adhesive layer
3
is too thin or unevenly applied. Subsequently, when the strain gage
3
is pressed against the mechanical member
2
and the adhesive is allowed to cure, the strain gage
4
may actually contact the mechanical member
2
at points, resulting in a short circuit between the mechanical sensor
2
and the strain gage
4
.
If the adhesive layer
3
is too thin at a certain point, current will leak across the adhesive layer
3
when a sufficient voltage potential exists between the mechanical sensor
2
and the strain gage
4
. The voltage at which leakage will occur is the dielectric breakdown voltage of the adhesive layer
4
.
To prevent electrical shorts and electrical leakage, the strain gage
4
must be better insulated from the mechanical sensor
2
. Initially, this involves choosing an insulating adhesive as opposed to a non-insulating adhesive. Conventionally, to achieve improved insulation, a filled adhesive is chosen to make the adhesive layer. The filler of a filled adhesive is typically a granular substance such as a fine powder. The purpose of using a filled adhesive is to increase the thickness of the adhesive layer
3
. Increasing the thickness of the adhesive layer
3
produces a higher dielectric breakdown voltage of the adhesive layer
3
. Since the breakdown voltage is higher, there is less likelihood of electrical leakage across the adhesive layer
3
.
Although using a filled adhesive produces a higher dielectric breakdown voltage, the application of only one coat of filled adhesive does not provide a high enough dielectric breakdown voltage in the adhesive layer to avoid electrical shorts and electrical leakage.
Conventionally, to further improve insulation, two coats of the filled adhesive are applied to produce the adhesive layer
3
.
FIG. 2
is a perspective side view of a section
9
of the strain gage assembly
1
of
FIG. 1
that shows in further detail the adhesive layer
3
. The adhesive layer
3
includes a pre-coat
5
and a gage coat
6
. During manufacturing, the pre-coat
5
of filled adhesive is applied to the mechanical sensor
2
and allowed time to cure. Next, the gage coat
6
of filled adhesive is applied to the pre-coat
5
, the strain gage
4
is pressed against the gage coat
6
, and the gage coat
6
is allowed time to cure. Adding the extra coat assures that the adhesive layer
3
is sufficiently thick to void electrical shorts and electrical leakage. Typically, the pre-coat
5
and gage coat
6
are of the same or similar adhesive material. The thickness of the adhesive layer
3
is represented in
FIG. 2
by t
a
.
A conventional strain gage assembly
1
of
FIG. 2
that uses a strain gage
3
of semiconductive material has an adhesive layer
3
with a thickness t
a
of approximately 1.0 mil (25.4 &mgr;m). The dielectric strength of a material is the voltage potential at which dielectric breakdown will occur per unit length of the material. For filled adhesives typically used for the strain gage assembly
3
of
FIG. 2
, the dielectric breakdown of the filled adhesive is approximately 250 Volts per mil (250 V/mil; 9.84 V/&mgr;m). Therefore, the typical dielectric breakdown voltage of the adhesive layer
3
is approximately 250 Volts (250 V/mil×1.0 mil).
Using a filled adhesive and adding a second coat of adhesive increases the thickness of the adhesive layer
3
. Although increasing the thickness produces a higher dielectric breakdown of the adhesive layer
3
, as the thickness of the adhesive layer
3
increases, mechanical performance can decrease. Furthermore, the fillers of filled adhesives can reduce the strength of the adhesive. The filler within the adhesive may have inconsistent granule size and this can make it more difficult for the adhesive layer
3
to bond the strain gage
4
to the mechanical member
2
, create high stress points in the strain gage
4
, and introduce possible voids between the strain gage
4
and the adhesive layer
3
.
When choosing an adhesive, one wants an adhesive with the best combination of performance parameters, for example, highest strength, highest dielectric breakdown, and broadest temperature range. The need to use a filled adhesive, however, limits the choices of adhesives for use in the adhesive layer
3
of a strain gage assembly
1
. Furthermore, as discussed above, as thickness of the adhesive layer
3
increases, mechanical performance of the adhesive layer
3
can decrease. Consequently, when choosing an adhesive, tradeoffs are made between the adhesive's strength, temperature range, and dielectric breakdown.
Therefore, the benefits of increasing the thickness of the adhesive layer
3
must be weighed against the drawbacks caused by such an increase. This results in a tradeoff between the electrical insulation provided by the adhesive layer
3
and the mechanical performance of the adhesive layer
3
. The filled adhesive, the amount and granule sizes of the filler in the filled adhesive, and the thickness of the adhesive layer
3
are chosen in light of these tradeoffs. Typically, the adhesive layer
3
used in the strain gage assembly
1
of
FIG. 2
has a shear strength of approximately 3,000 p.s.i. and an operating temperature range from approximately −60° F. to 250° F.
Thus, for the strain gage assembly
1
where the strain gage
4
is made of semiconductive material, it is desirable to eliminate the need for a filled adhes

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