Microscopic digital imaging strain gauge

A strain gauge includes an image sensing device having a lens, a magnification lens optically coupled to the lens, a positioning mechanism connected to the image sensing device, an image capture device for receiving an image from the image sensing device, and a processor for mathematically analyzing the image received from the image capture device and to calculate strain.

FIELD OF THE INVENTION 
The present invention relates to strain gauges, and more specifically, to 
microscopic digital imaging for such gauges. 
BACKGROUND OF THE INVENTION 
Strain measurement is of particular importance to automotive designers. In 
the design of automotive vehicles it is often necessary to measure 
hundreds of locations for strain for any given test. Conventional strain 
measurement is often conducted using an electrical strain gauge. 
Resistance strain gauges, extensometers, and capacitor strain gauges are 
examples of such conventional electrical gauges. Electrical strain gauges 
require bonding and wiring which, in an automotive testing environment, is 
a time consuming set up process. Also, once an electrical strain gauge is 
used it must be discarded which can be very costly in automotive testing. 
Further, conventional strain gauges are inaccurate when exposed to high 
temperatures which is an undesirable testing limitation in automotive 
design. 
Efforts have therefore advanced in the automotive strain measurement field 
to develop a noncontacting and nonconsumable method of measuring strain. 
One such method known in the art is shearography. According to this 
method, two laterally-displaced images of the object, which consist of 
random speckle patterns, are made to interfere to form a pattern of 
fringes. The pattern is random, and depends on the characteristics of the 
surface of the object. When the object is deformed, by temperature, 
pressure, or other means, the random interference pattern will change. The 
amount of the change depends on the soundness of the object. A comparison 
of the random speckle patterns for the deformed and undeformed states, and 
their respective fringe patterns, gives information about the structural 
integrity of the object. The method is called shearography because one 
image of the object is laterally-displaced, or sheared, relative to the 
other image. 
Another noncontacting and nonconsumable strain measurement method, which 
was developed with the advent of the laser, is electronic speckle pattern 
interferometry (ESPI). In ESPI, a beam of laser light is directed onto the 
test object and reflected onto an image sensor. At the same time, a 
reference beam is also directed towards the sensor. The reference beam may 
be a "pure" beam or it may be reflected from a "reference" object. Both 
the object beam and the reference beam are nearly parallel when they reach 
the image sensor, so the spatial frequency of the interference speckle 
patterns is relatively low. Thus, the image sensor can be a video camera, 
or its equivalent. 
There are many disadvantages associated with shearography and ESPI. ESPI 
requires an object beam and a reference beam of coherent light. The 
presence of two distinct beams increases the complexity of the optical 
system. The ratio of intensities of the object and reference beams must be 
carefully controlled, and the path lengths of the beams must be matched. 
Also, the use of lasers present safety issues as well as high cost. Both 
ESPI and shearography are full field strain calculation methods and 
require highly complex, and relatively inaccurate, computational methods 
to derive strain. Further, ESPI and shearography are highly sensitive to 
vibration. The slightest movement of either the object or the apparatus 
can ruin the pattern. Thus both methods require special vibration 
isolation precautions, and are not practical for strain measurement in an 
automotive vehicle testing environment. Still further, both methods 
require that the object surface be painted or processed for testing which 
adds cost to the process. Finally, ESPI and shearography methods create 
noise which must be filtered by a noise reduction algorithm, further 
adding to the cumbersome nature of the processes. 
Interferometric point wise, rather than full field, strain measurement is 
also an example of noncontacting strain measurement but is subject to the 
same shortcomings as ESPI. A problem associated with both full field and 
point wise noncontacting strain measurement, which is of great importance 
in automotive design and testing, is uninteruptibility of the method. That 
is, once the particular apparatus is set up to measure strain it can not 
be removed between pre-loading and post-loading. In automotive testing it 
is desired to take an initial, pre-load reading with the testing apparatus 
and then remove the apparatus for cycling. The automobile could, for 
example, be cycled for a predetermined period of time or distance with the 
apparatus being reapplied to the testing area for a post-load reading. 
This technique is impossible with the aforementioned noncontacting strain 
measurement methods. 
Accordingly, a need exists in the art for an automotive vehicle strain 
gauge which is noncontacting, has an uncomplicated strain measurement 
calculation, is not subject to the harsh vibratory environment of an 
automobile, is removable between the pre versus post loading phase, and is 
reusable, accurate, and easy to use. 
SUMMARY OF THE INVENTION 
Responsive to the deficiencies in the prior art, the present invention 
provides a strain gauge including an image sensing device having a lens, a 
magnification lens optically coupled to the lens, a positioning mechanism 
connected to the image sensing device, an image capture device for 
receiving an image from the image sensing device, and a processor for 
mathematically analyzing the image received from the image capture device 
and to calculate strain. 
An advantage of the present invention is that the present apparatus 
utilizes a microscopic lens which does not require a complex and sensitive 
optical system, therefore the present invention may be used in a harsh 
vibrating environment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to the drawings, and in particular to FIG. 1 thereof, an 
automotive vehicle 10 has positioned adjacent thereto a microscopic 
digital imaging strain gauge 12. The gauge 12, of FIG. 2, includes a 
positioning mechanism 14, an image sensing device 16, a magnification lens 
18, and a processor 20. The gauge 12 is positioned to image a micro/nano 
indentation, or micro/nano lithographic mark pattern 26 on an object 
surface for determining strain. The mark pattern 26 may also be produced 
on a thin metal or composite sheet and then adhere to the object surface. 
A dynamic loading device 28 with a trigger system may also be included, as 
well as coupled to the processor 20, for dynamic load strain testing. 
As shown in FIG. 2, the image sensing device 16 is preferably a digital 
video recorder. The recorder may operate in either a color or gray scale. 
The sensing device 16 has a lens 19. The lens 19 is preferably a 
telecentric lens so that the size of the mark pattern will not be affected 
by the view angle and focus distance, especially in the case of a curved 
object surface. The sensing device 16 may further include a shutter 
mechanism, not shown, for taking "snap-shot" images of the object surface 
mark pattern 26 under dynamic loading. 
A magnification lens 18, preferably a microscopic magnification lens, is 
preferably optically coupled to the lens 19 and is positioned intermediate 
the sensing device 16 and the lens 19. However, the lens 19 may be 
intermediate the magnification lens 18 and the image sensing device 16 or 
may be formed integral with the magnification lens 18. Further, an 
optical, scanning electron, transmission electron or scanning probe 
microscope may be used in place of the microscopic magnification lens 
without departing from the scope of the herein described invention. 
A positioning mechanism 14 is attached to the sensing device 16. The 
positioning mechanism 14 preferably has three equidistantly spaced legs 
22. The legs 22 are adapted to be longitudinally adjusted and locked in to 
place at a predetermined elevation above the object surface. 
A fiber optic light source 24 may be attached to the apparatus 12 to 
illuminate a dimly lit object surface. 
A processor 20 is coupled to the sensing device 16 and is adapted to 
receive a digital image. The processor has a digital imaging board with 
preferably a 30 Hertz digitizing rate. However, a high speed imaging board 
may be coupled with a high speed imaging device if the operating 
environment of the object surface so requires. The processor further has a 
computer for receiving information from the imaging board and for 
calculating the strain associated with a given object surface. The 
calculation utilizes a Young's fringe phase shift technique, as explained 
below and as known in the art, to interpret pre-load and post-load object 
surfaces. The Young's fringes are preferably processed using a low pass 
filter. The processor 20 may further be utilized to coordinate the trigger 
system of the dynamic loading device 28 with the shutter mechanism of the 
image sensing device 16 so that upon each triggered incremental dynamic 
load application, a "snap-shot" of the mark pattern 26 is taken. 
In use, the positioning mechanism 14 is placed over a mark pattern 26 on an 
object surface and the distance between the imaging sensing device 16 and 
the mark pattern 26 is set by the adjustable legs and locked into place. 
The lens 19 is focused and the mark pattern image is taken by the image 
sensing device 16 before and after loading. The gauge 12 may be removed 
from the object surface if desired between pre and post-loading. The 
images are digitized into the computer via the digital imaging board. A 
digital Fourier transformation is then applied to the images of the marks 
before and after loading to produce Young's fringes. The number of fringes 
are related to the degree of distance between marked points. The 
displacement, as derived from the fringe patterns, between points of the 
mark pattern 26, pre and post deformation, are used to calculate strain at 
that region. 
Only one embodiment of a microscopic digital imaging strain gauge of the 
present invention has been described. Those skilled in the strain gauge 
arts will appreciate that others may be possible without departing from 
the scope of the following claims.