Gauging apparatus and method

Disclosed is an apparatus and method for gauging the amount of warpage in a composite panel. In the gauging operation, elongate spacer strips (18) of uniform thickness are placed on the layup surface (14) of a mold (12) to support a panel (10) spaced apart from the mold layup surface (14) by a gap (22) of dimension equal to the thickness of the spacer strips (18). A sensor wand (26) that includes a strain gauge sensor assembly (32) is inserted in the gap (22) to detect and measure gap variations. Instrumentation (66,68) is utilized to operate strain gauges (52) that are included in the strain gauge sensor assembly (32) as a conventional bridge circuit. Measurement error caused by twisting and tilting of sensor wand (26) is minimized by smoothly contoured contact protrusions (54) of the strain gauge sensor assembly (32) that bear against panel surface (16) and layup surface (14). The width of sensor wand (26) and the mounting location of the strain gauge sensor assembly (32) also are established to minimize measurement error caused by twisting and tilting of sensor wand (26). Also included is a calibration fixture (70) for determining the relationship between electrical signals supplied by strain gauges (52) and gap dimension so that variations in gap dimension and hence the contour of panel surface (16) can accurately be determined.

FIELD OF THE INVENTION 
This invention relates to apparatus and methods for determining whether a 
formed part or component exhibits a desired surface contour. More 
specifically, this invention relates to apparatus and method for gauging 
the warpage in panel-like structure that is formed with thermosetting 
composite materials and is fabricated to a desired contour with a surface 
mold or fixture. 
BACKGROUND OF THE INVENTION 
There are numerous situations in which composite material is molded or 
formed to provide high strength, lightweight items or components that 
exhibit a desired geometry. One application of increasing importance is 
the fabrication of aircraft components such as wing and stabilizer panels 
that are formed from thermosetting resins and high strength anisotropic 
fibers such as graphite. Typically, the panels are formed to exhibit a 
desired surface contour by placing layers of uncured composite material on 
a mold having an upper surface that is machined or otherwise shaped to 
match the contour to be established in the finished panel. The laid up 
composite material is then cured by, for example, placing the mold in an 
autoclave. 
To be suitable for use in an aircraft, the surface contour of the 
fabricated panel cannot deviate from the contour of the mold by more than 
a specified amount. Although the surface area of such panels often is 
relatively large, a contour tolerance on the order of a few hundredths of 
an inch may be imposed. 
Since the surface of the mold is machined or otherwise formed to reflect 
the desired contour of the fabricated composite panels, several attempts 
have been made to use the mold for dimensional inspection of the panel 
surface. For example, attempts have been made to use feeler gauges to 
measure gaps between the mold and the surface of the panel that can be 
caused by warpage during the curing process. However, feeler gauges are 
not suitable for inspecting surface contours of large composite panels 
because they only can be used along the periphery of the panel to measure 
gaps between the panel and the mold. 
Another prior technique that has been used in an attempt to measure the 
thickness of gaps between a composite panel and the mold uses a 
conventional eddy current sensor and an ultrasonic thickness gauge. The 
eddy current sensor is positioned at a number of predetermined locations 
on the surface of the composite panel that faces away from the mold and is 
used to determine the distance between that panel surface and the surface 
of the mold that determines the panel contour. The ultrasonic thickness 
gauge also is placed on the surface of the composite panel that faces away 
from the mold at the same locations as the eddy current sensor and 
determines the thickness of the panel. If the local thickness of the 
composite panel is uniform, the difference between the two measurements is 
indicative of gaps (and hence warpage) at the measurement locations. 
Because this technique is based on measurements between the mold and the 
surface of the composite panel that faces away from the mold, it often is 
not usable in situations in which the surface of the composite panel that 
faces the mold must closely conform to the mold contour without a 
corresponding requirement on the contour or smoothness of the other panel 
surface. Further, the eddy current sensor/ultrasonic thickness gauge 
technique cannot be employed in situations in which the entire mold or the 
surface of the mold is formed of a nonconductive material (e.g., a 
composite material). A further disadvantage or drawback with respect to 
relatively large panels, such as wing and stabilizer panels, is that 
mechanical arms or other structure must be used to position the eddy 
current sensor and ultrasonic gauge at locations that cannot otherwise be 
reached by operating personnel. For all these reasons, the eddy current 
sensor/ultrasonic gauge technique often is not desirable from the 
standpoint of complexity, equipment cost, and the amount of time required 
to perform the desired dimensional inspection. 
U.S. Pat. No. 4,703,648, which is assigned to the assignee of this 
invention, discloses a technique that at least partially overcomes 
disadvantages and drawbacks of prior techniques used to measure the 
thickness of gaps between a composite panel and the mold utilized to form 
the composite panel. In the technique disclosed in U.S. Pat. No. 
4,703,648, the panel is first removed from the mold and elongate, 
inelastically deformable gauging strips are positioned on the surface of 
the mold at locations at which conformity between the mold contour and the 
contour of the panel are to be determined. Also placed on the surface of 
the mold is a series of inflatable tubes that are positioned at locations 
that do not interfere with the positioning of the inelastically deformable 
gauging strips. To perform the gauging operation, the tubes are inflated 
and the composite panel is placed on top of the tubes in a position that 
is spaced apart from and above the position in which the panel was molded. 
The tubes then are deflated to slowly lower the panel so that the molded 
surface thereof presses downwardly on and flattens or crushes the 
inelastically deformable gauging strips. When the panel has settled into a 
position in which it is supported only by the gauging strips, the tubes 
are reinflated to lift the panel off the gauging strips. The panel is then 
removed and the thickness of the gauging strips is measured to thereby 
determine panel warpage at the desired measuring points. 
Although the technique disclosed in U.S. Pat. No. 4,703,648 at least 
partially alleviates disadvantages and drawbacks of prior techniques, a 
need exists for improved warpage gauging methods and apparatus. For 
example, placement of the inelastically deformable gauging strips and the 
inflatable tubes must be carefully executed and, thus, can consume a 
substantial amount of time. Inflation of the tubes, replacement of the 
panel in its proper position and the subsequent steps of deflating the 
tubes and removing the panels also is a time-consuming process. The final 
step of measuring the thickness of the deformed gauging strips also is a 
time-consuming process that must be executed with care. Further, if the 
measurement results are not conclusive (e.g., the results indicate panel 
regions at or very near the allowed contour tolerance), the panel cannot 
be reinspected without repeating the entire measurement process. 
One prior technique that has been utilized in a different dimensional 
inspection application relates to the use of strain gauges to detect and 
measure abnormalities in the inner diameter of rigid, precisely 
dimensioned tubing (e.g., detect and measure dents in the tubing wall). 
More specifically, U.S. Pat. No. 4,235,020 discloses a cylindrical scanner 
or sensor that includes eight spring fingers that are equally spaced about 
the circumference of the scanner and extend radially outward. Located on 
each spring finger is a pair of strain gauges that are positioned so that 
radial deflection of the spring finger results in changes in the 
electrical resistance of the strain gauges. The scanner is dimensioned so 
that the spring fingers bear against and are urged radially inward by the 
inner wall of the tube to be inspected. To detect and measure dents or 
other abnormalities in the inner diameter of the tubing, the sensor is 
pulled through the tube at a predetermined uniform speed by a cable. Dents 
or other irregularities in the inner wall of the tube cause spring fingers 
that contact the irregularity to deflect which, in turn, causes a change 
in the resistance of the associated pair of strain gauges. Since the eight 
pair of strain gauges can be connected in a variety of manners, including 
arranging four strain gauges of two selected spring fingers to form a 
conventional bridge circuit, deviations in the diameter of the tube can be 
measured. Since the speed at which the sensor is pulled through the tube 
is uniform and known, the location of the dimensional irregularity also 
can be detected. 
The sensors of the type disclosed in U.S. Pat. No. 4,235,020 are not 
suitable for use in measuring the gap between two planar surfaces. 
Specifically, those devices are designed to be contained by the inner wall 
of a tube or other structure so that the orientation of the sensor within 
the tube remains constant as the sensor is pulled through the tube. Thus, 
the spring fingers are deflected only by dents or other irregularities in 
the inner wall of the tube. In contrast, if such a sensor were drawn along 
the gap between two contoured planar surfaces, the sensor would be free to 
twist or rotate and free to tilt up or down relative to the direction of 
travel. With a sensor of the type disclosed in U.S. Pat. No. 4,235,020, 
any such changes in physical orientation of the sensor would cause at 
least some deflection of the spring fingers, thus affecting the resistance 
of the strain gauges and causing measurement error. Since warpage of 
composite panels such as aircraft wing and stabilizer panels often is 
limited to a few hundredths of an inch, reliable inspection is not 
possible unless the sensor is accurate within a few thousandths of an 
inch. Sensors of the type disclosed in U.S. Pat. No. 4,235,020, include no 
provision that would result in the required measurement accuracy in the 
composite panel warpage measurement environment. 
SUMMARY OF THE INVENTION 
The apparatus provided by this invention includes an elongate flexible 
wand, which in the currently preferred embodiment of the invention is of 
rectangular cross-sectional geometry. Mounted in an opening near one end 
of the wand is a sensor that includes a pair of leaf springs that extend 
angularly away from one another and are mounted to project outwardly from 
the upper and lower surface of the wand. Included on each of the two leaf 
springs is a pair of strain gauges that are mounted to sense deflection of 
the leaf springs. 
When the sensor-equipped wand is utilized for determining the warpage of 
composite panels, elongate spacer strips that are formed of flexible, 
relatively incompressible material are placed on the surface of the mold 
and the panel is positioned on the spacer strips so that the surface of 
the panel is located directly above the region of the mold that formed the 
panel. The spacer strips are machined or otherwise formed to a uniform 
thickness that is slightly greater than the thickness of the wand. In 
addition, the leaf springs of the sensor are dimensioned and arranged so 
that insertion of the wand into the gap formed between the mold and the 
panel causes the leaf springs to contact the surfaces of the mold and 
panel and be slightly deflected toward one another. 
Various features are incorporated in the wand and the sensor to eliminate 
measurement errors that could be caused by changes in the orientation of 
the wand and sensor as the device is moved between desired measuring 
points. First, the wand is substantially wider than the gap that is formed 
between the composite panel and the mold. Thus, rolling or twisting of the 
wand about its longitudinal centerline is limited. Second, the wand 
extends beyond the sensor to limit sensor tilt or pitch (angular offset 
relative to the longitudinal centerline of the wand). Third, and most 
important, the region of each sensor leaf spring that contacts the mold 
surface is a smoothly contoured rounded protrusion that is of 
substantially hemispherical geometry in the currently preferred 
embodiments of the invention. The curvature of these protrusions is 
established so that there is substantially no deflection of the leaf 
springs for the degree of pitch and roll that can be encountered by the 
sensor for the gap measurement range of interest. Thus, when the two pairs 
of strain gauges are connected to form a conventional resistive 
measurement bridge, an electrical signal is produced that varies only in 
accordance with variation in the gap distance (i.e., panel warpage). 
The currently preferred embodiments of the invention also include features 
and aspects that enhance the measurement accuracy of the sensor equipped 
wand and facilitate its use. For example, in the currently preferred 
embodiments of the invention in which the end region of one of the sensor 
leaf springs is joined to the surface of the second leaf spring, the 
region of the second leaf spring that surrounds the juncture of the spring 
includes a central cutout region. This imparts substantially identical 
flexure characteristics to the leaf springs, thereby equalizing the 
measurement sensitivity of the leaf springs and enhancing sensor accuracy. 
The cutout region also provides for passage of the electrical conductors 
of the strain gauge that is mounted on the second leaf spring. To 
facilitate location of desired measurement points, the currently preferred 
embodiments of the invention include a scale that is embedded in the upper 
surface of the wand. Thus, the depth of insertion easily can be 
ascertained. Further, a sensor calibration fixture is provided that 
includes a rectangular passage into which the wand can be inserted. The 
rectangular passage includes a series of precisely dimensioned steps that 
provide several gap dimensions that are within the sensor range. By using 
the calibration accessory adjust (i.e. "zero") the associated test 
instrumentation prior to each inspection operation, maximum measurement 
accuracy is ensured.

DETAILED DESCRIPTION 
FIGS. 1-5 illustrate the major components of gauging apparatus constructed 
in accordance with this invention. Shown in FIG. 1 is a composite panel 10 
which is suspended above a mold 12 that includes a molding or layup 
surface 14 for establishing desired curvature and surface features in 
surface 16 of composite panel 10. Located on layup surface 14 of mold 12 
are a series of spaced-apart elongate spacers 18 that are used in the 
practice of the invention, but are not present on layup surface 14 when 
composite panel 10 is fabricated. 
The composite panel 10 that is shown in FIG. 1 typifies aircraft wing and 
stabilizer panels, which may be several feet in length and width. When the 
panel is fabricated, layers of composite sheets or strips are applied 
directly to layup surface 14 of mold 12. Typically, the laid-up composite 
material consists of side-by-side strips of fiber roving that are 
impregnated with an uncured thermosetting resin binder. Often the 
composite material layers overlap one another and multiple layers are 
built up in sandwich-like fashion. As is shown in FIG. 1, composite panels 
for use in aircraft commonly include reinforcing stringers 20 that extend 
along the surface of panel 10 that faces away from mold 12 during the 
fabrication process. Stringers 20, which are T-shaped in FIG. 1, are 
joined to the composite panel 10 with additional strips of composite 
material or other means during layup of composite panel 10. When layer of 
composite panel 10 is complete, the panel is cured on top of mold 12 to 
form a lightweight, high strength unitary structure with surface 16 of 
composite panel 10 matching the curvature and any other surface features 
of the mold layup surface 14. Typically, curing takes place in an 
autoclave in which temperature and any other important parameters such as 
humidity are closely controlled. 
When a composite panel such as panel 10 of FIG. 1 is to be inspected in 
accordance with this invention, the cured panel is removed from layup 
surface 14 of mold 12 and spacer strips 18 are placed on layup surface 14 
in the spaced-apart manner depicted in FIG. 1. Each spacer strip 18 is 
formed of a material flexible enough to allow the spacer strips to exactly 
follow the contour of layup surface 14. Further, each spacer strip 18 is 
machined or otherwise formed to a uniform predetermined thickness. In 
addition, the spacer strips 18 must bear the weight of composite panel 10 
and any weight (i.e., loading) that is applied to the upper surface of 
composite panel 10 during the hereinafter described gauging procedure. In 
the currently preferred embodiments of the invention, spacer strips 18 are 
formed of acrylonitrile-butadiene-styrene copolymer (ABS). In one 
realization of the invention, the spacer strips 18 are machined to a 
thickness of 0.125 inch.+-.0.001 inch. 
Regardless of the material and thickness of the spacer strips 18, the 
strips are positioned on layup surface 14 at locations that will maintain 
surface 16 of panel 10 in uniform spaced-apart juxtaposition with layer 
surface 14 when panel 10 is lowered into a position that corresponds to 
the position it occupied when the panel was fabricated (e.g., spacer 
strips 18 are placed beneath reinforcing stringers 20 or other panel 
features such as rib stations). Thus, as is indicated more clearly in FIG. 
4, a gap 22 is formed between layer surface 14 of mold 12 and surface 16 
of composite panel 10. Since surface 16 of panel 10 rests on the upper 
surface of spacer strips 18, gap 22 will be constant, except for regions 
in which panel warpage has occurred. If desired or necessary, weights 24 
(FIG. 4), such as bags filled with shot, can be placed on top of composite 
panel 10. When used, weights 24 are placed directly over spacer strip 
locations and are distributed along the edges of composite panel 10 to 
urge the panel into contact with spacer strips 18 without deforming 
composite panel 10 downwardly into gap 22. 
Referring now to FIGS. 2-4, in accordance with the invention, regions of 
warpage are located and measured by a sensor wand 26 that is inserted into 
and moved along gap 22 in the manner indicated in FIG. 4. As is shown in 
FIGS. 2 and 3, sensor wand 26 includes an elongate plate-like wand 28, 
which is of rectangular cross-sectional geometry in the depicted 
embodiment. To allow sensor wand 26 to be inserted into and moved through 
gap 22, wand 28 is constructed of relatively flexible material (e.g., ABS) 
and the thickness of wand 28 is slightly less than the thickness of spacer 
strips 18. For example, in the realization of the invention in which 
spacer strips 18 are 0.125 inches, the thickness of wand 28 is 0.110 inch. 
Located in a rectangular cutout region 30 that is near the end of wand 28 
that is inserted into gap 22 is a strain gauge sensor assembly 32. A small 
cable 34 for interconnecting strain gauge sensor assembly 32 with 
appropriate test instrumentation is routed along and recessed in a groove 
36 that is formed in the upper surface of wand 28. As is shown in FIG. 2, 
groove 36 and cable 34 extend between strain gauge sensor 32 and an 
electrical connector 38 that is located at the opposite end of wand 28. To 
assist in positioning sensor wand 26 for measurement of warpage at desired 
locations in gap 22, graduation marks indicating the distance between the 
edge of composite panel 10 and the insertion position of strain gauge 
sensor assembly 32 are included along one edge of wand 28. In the depicted 
embodiment, a scale 40 that is graduated in inches is recessed in the 
upper surface of wand 28. 
As is most clearly shown in FIG. 3, strain gauge sensor assembly 32 
includes a lower leaf spring 42 and an upper leaf spring 44. One end of 
lower leaf spring 42 is secured by screws 48 or other conventional 
fastening means to a shelf-like recessed region 46 that is located at one 
end of rectangular cutout 30 of wand 28. Beginning from the point at which 
it is secured to wand 28, lower leaf spring 42 initially extends into 
rectangular cutout 30 with the upper surface of lower leaf spring 42 being 
substantially parallel to the surface of wand 28. Lower leaf spring 42 
then projects angularly downward so as to project below the lower surface 
of wand 28. One end of upper leaf spring 44 is joined to lower leaf spring 
42 by spotwelds 50 or other conventional techniques. In the depicted 
embodiment, the end of upper leaf spring 44 that is joined to lower leaf 
spring 42 is spaced away from the end of lower leaf spring 42 that is 
joined to wand 28. Upper leaf spring 44 is configured similarly to lower 
leaf spring 42, first extending into rectangular cutout 30 and then 
extending angularly upward so that the distal or free end of upper leaf 
spring 44 projects above the upper surface of wand 28. The outwardly 
projecting regions of lower leaf spring 42 and upper leaf spring 44 are 
substantially identical in configuration so that the leaf springs are 
symmetrically disposed about the longitudinal centerline of sensor wand 
26. 
Located on and bonded to the upper surface of upper leaf spring 44 is a 
pair of strain gauges 52. One of the strain gauges is arranged so that its 
axis of sensitivity extends parallel to the longitudinal centerline of 
upper leaf spring 44. The axis of sensitivity of the second strain gauge 
is perpendicular to the axis of sensitivity of the first strain gauge (and 
hence the longitudinal centerline of upper leaf spring 44). As is known in 
the electrical arts, pairs of strain gauges that are arranged with 
perpendicular sensitivity axes are commercially available. For example, in 
a current realization of strain gauge sensor assembly 32, a strain gauge 
pair is used that is supplied by Micromeasurements Division of 
Measurements Group, Inc. Raleigh, N.C., and is identified by part no. 
CEA-06-062WT-350. Although not shown in FIG. 3, a second pair of strain 
gauges is similarly mounted to the lower surface of lower leaf spring 42 
with conductors of the previously mentioned electrical cable 34 being 
electrically connected to each pair of strain gauges 52. 
With continued reference to FIG. 3, located near the free end of both lower 
leaf spring 42 and upper leaf spring 44 is a contact protrusion 54 that 
extends outwardly away from wand 28. Contact protrusions 54 bear against 
surface 16 of panel 10 and layup surface 14 of mold 12 when sensor wand 26 
is inserted in gap 22 in the manner shown in FIG. 4. When sensor wand 26 
is not inserted in gap 22, the distance between the portions of contact 
protrusions 54 that contact panel surface 16 and layup surface 14 slightly 
exceeds the maximum gap distance that can be measured with sensor wand 26. 
Thus, when sensor wand 26 is inserted in gap 22, the free ends of lower 
leaf spring 42 and upper leaf spring 44 are deflected toward one another. 
The deflection causes changes in the resistance of the strain gauges that 
have sensitivity axes that extend longitudinally along the leaf springs 
(i.e., one strain gauge of each pair of strain gauges 52). 
As is shown in FIG. 3, each contact protrusion 54 is of a rounded, smoothly 
contoured configuration. In the currently preferred embodiments, contact 
protrusions 54 are substantially hemispherical in geometry. Thus, only 
small surface regions of contact protrusions 54 contact layup surface 14 
of mold 12 and panel surface 16 of composite panel 10 when sensor wand 26 
is inserted into gap 22 in the manner shown in FIG. 4. As shall be 
described relative to FIGS. 8 and 9, preferably the radius of each contact 
protrusion 54 is established in view of the gap dimensions to be measured 
or examined with sensor wand 26 to prevent deflection of lower leaf spring 
42 and upper leaf spring 44 as a result of twisting or tilting of the 
portion of sensor wand 26 that includes strain gauge sensor assembly 32. 
As also shall be described relative to FIGS. 8 and 9, the distance between 
contact protrusions 54 and the end of wand 28 and the width of wand 28 
preferably are established to virtually eliminate sensor error caused by 
twisting or tilting of strain gauge sensor assembly 32 within gap 22. 
There are two other notable provisions of strain gauge sensor assembly 32 
of FIG. 3. First, the terminal portion of the free end of upper leaf 
spring 44 tapers to a substantially rectangular tab 56 that extends 
angularly toward wand 28. The terminal portion of the free end of lower 
leaf spring 42 also extends angularly toward wand 26 is inserted in gap 22 
(FIG. 4), the leaf springs are deflected toward one another and tab 56 
extends into notch 58. Thus, as can be seen in FIGS. 6 and 9, when the 
leaf springs are deflected into their normal operating position, the free 
ends of the lower leaf spring 42 and upper leaf spring 44 are within 
rectangular cutout 30 of wand 28. This ensures that the free ends of the 
leaf springs will not come into contact with layup surface 14 of mold 12 
or surface 16 of panel 10 thereby preventing possible damage to strain 
gauge sensor assembly 32 and/or erroneous gap indication. 
The second additional feature of strain gauge sensor assembly 32 that 
should be noted is a longitudinally extending rectangular cutout 60 in 
lower leaf spring 42. As is shown in FIG. 3, rectangular cutout 60 extends 
beneath the end of upper leaf spring 44 that is joined to lower leaf 
spring 42. The size of rectangular cutout 60 is established to impart 
substantially identical flexure characteristics to lower leaf spring 42 
and upper leaf spring 44. That is, rectangular cutout 60 ensures 
substantially equal deflection of lower leaf spring 42 and upper leaf 
spring 44 for all gap dimensions within the measurement range of strain 
gauge sensor assembly 32. This equalization enhances sensor accuracy. 
Rectangular cutout 60 also provides for passage of cable 34, which 
interconnects the strain gauge pair that is mounted on lower leaf spring 
42 with electrical connector 38. 
FIG. 5 is a block diagram that schematically depicts the manner in which 
strain gauge sensors 52 of sensor wand 26 are electrically connected to 
test instrumentation for measuring deviations in gap 22 (FIG. 4) and, 
hence, warpage in surface 16 of composite panel 10. In FIG. 5, the pair of 
strain gauges 52 that is mounted on lower leaf spring 42 is represented by 
resistors 62-1 and 64-1, and the pair of strain gauges that is mounted on 
upper leaf spring 44 is represented by 62-2 and 64-2. In the depicted 
arrangement, resistors 62-1 and 62-2 respectively correspond to the strain 
gauges that are mounted so that the strain gauge axes of sensitivity are 
substantially parallel to the longitudinal centerlines of the respective 
leaf springs. Resistors 64-1 and 64-2 respectively correspond to the 
strain gauges of lower leaf spring 42 and upper leaf spring 44 whose axes 
are perpendicular to the longitudinal centerlines of the leaf springs. 
With this arrangement, the resistance values of resistors 62-1 and 62-2 
are a function of both ambient temperature and deflection of the leaf 
springs to which the corresponding strain gauges are mounted. On the other 
hand, the resistance value of resistor 64-2 and resistor 64-1 are a 
function only of ambient temperature. 
When the resistive elements of the strain gauges are connected in a 
conventional bridge arrangement of the type shown in FIG. 5, the strain 
gauges in effect form voltage dividers which produce a 
temperature-compensated signal that indicates the difference in the strain 
applied to the two pairs of strain gauges (e.g., the combined deflection 
of lower leaf spring 42 and upper leaf spring 44 in the arrangement of 
strain gauge sensor assembly 32). More specifically, in the arrangement of 
FIG. 5, a strain gauge supply 66 establishes a predetermined potential 
(voltage) across each pair of resistors that correspond to each pair of 
strain gauges 52. A differential detector 68 measures the difference in 
potential that is developed at the juncture between the resistors that 
correspond to each strain gauge pair. Since the potential at each of these 
junctures is proportional to the deflection of the associated leaf spring 
(strain induced at the strain gauge), the potential measured by 
differential detector 68 indicates total deflection of the leaf springs 
and, hence, gap dimension. 
It should be recognized that the block diagram shown in FIG. 5 is 
simplified for purposes of discussion. In actual practice, the terminals 
of each strain gauge pair are each connected to apparatus that serves the 
function of strain gauge supply 66 in FIG. 5. This permits initial 
balancing of the measurement bridge to compensate for strain gauge 
resistance variation that results from manufacturing tolerances and other 
factors. It also should be recognized that various strain gauge 
instrumentation devices are available. For example, device identified as 
the Daytronic mode 3270 Conditioner Indicator and the DataMyte model 753 
Data Collector have been successfully used in the practice of this 
invention. 
Regardless of the specific type of instrumentation employed in the practice 
of the invention, precise gap measurement (and, hence, warpage 
measurement) requires knowledge of the relationship between gap dimension 
and the electrical signal provided by strain gauges sensor assembly 3. 
Shown in FIG. 6 is a calibration fixture 70 that can be used to determine 
the relationship between gap dimension and electrical signal output and, 
further, can be used to adjust (i.e., "zero") the measurement system when 
each gap measurement procedure is initiated. Calibration fixture 70 of 
FIG. 6 includes a rectangular passage 72 into which sensor wand 26 can be 
inserted. Passage 72 is formed to provide a number of precisely 
dimensioned steps 74 that provide a corresponding number of gap dimensions 
that are with the gap measurement range of sensor wand 26. For example, in 
the previously mentioned realization of the invention that employs spacer 
strips 18 which are 0.125 inch thick, the narrowest gap formed in 
calibration fixture 70 is approximately equal to 0.125 inch, with the 
exact gap dimension being known (e.g., a gap of 0.1245 inch). The 
remaining gaps that are formed in passage 70 by steps 74 are nominally 
dimensioned to provide a series of increasing gap dimensions, with the 
greatest gap dimension being approximately equal to the maximum gap that 
can be measured with sensor wand 26. For example, strain gauge sensor 
assembly 32 of the previously mentioned realization of the invention is 
capable of measuring gaps as wide as 0.250 inch. The calibration fixture 
70 for this particular realization of the invention included five 
different gap dimensions ranging between nominal gap dimensions of 0.125 
inch and 0.250 inch in increments of approximately 0.030 inch. One 
calibration fixture 70 that was machined to these nominal dimensions 
provide gap dimensions of 0.1245 inch, 0.1540 inch, 0.1850 inch, 0.2020 
inch and 0.2440 inch. 
It will be recognized by those skilled in the art that calibration fixture 
70 can be constructed in various manners. One manner of constructing 
calibration fixtures 70 that has proven satisfactory is joining together 
two metal plates that are identically configured to define the upper and 
lower stepped boundary regions of passage 72. More specifically, an 
identical series of stepped regions of a width equal to the desired width 
of passage 72 is machined in the surface of two metal plates. The two 
plates then are securely fastened to one another by bolts or other 
conventional fasteners that are positioned beyond the edge boundaries of 
passage 72. After assembly, the gap dimensions defined by the steps are 
measured by ball gauges or other suitable instruments to determine the 
exact gap dimensions. 
FIG. 7 illustrates a calibration curve (i.e., gap dimension-electric signal 
output relationship) for a particular sensor wand 26 that is configured in 
the previously described manner and is connected to appropriate 
instrumentation in the manner described relative to FIG. 5. In generating 
the calibration curve of FIG. 7, the sensor wand was inserted in 
calibration fixture 70 of FIG. 6 so that contact protrusions 54 of lower 
leaf spring 42 and upper leaf spring 44 were deflected toward one another 
by the stepped region of calibration fixture 70 that nominally provides a 
gap dimension of 0.125 inch. The instrumentation then was adjusted to zero 
the electrical output obtained from strain gauges 52. Next, sensor wand 26 
was moved through passage 72 of calibration fixture 70 with electrical 
output readings being taken for each of the stepped calibration regions 
(indicated by calibration points 76 in FIG. 7). 
In viewing FIG. 7, it can be noted that the invention provides an output 
signal that is substantially linear function of gap dimension. This linear 
relationship, which ensures highly accurate gap determination throughout 
the entire range of sensitivity, results from the previously described 
configuration of strain gauge sensor assembly 32 and the manner in which 
it is mounted and arranged in wand 28. Further, the linear characteristics 
are achieved in part by utilizing appropriate materials for the 
construction of lower leaf spring 42 and upper leaf spring 44. For 
example, in the currently preferred embodiments of the invention, both 
lower leaf spring 42 and upper leaf spring 44 are constructed of type 17-7 
PH CRES steel that is 0.010 inch thick. The leaf springs are trimmed to 
the desired shape and dimensions and all openings (e.g. rectangular cutout 
60) are formed by a laser beam cutter. After the leaf springs have been 
bent in an annealed state to the previously described configuration, the 
leaf springs are heat treated to condition CH900 by maintaining the leaf 
springs at a temperature of 900.degree. for one hour and then allowing the 
leaf springs to cool at ambient air temperature. 
It will be noted by those skilled in the art that the linear gap 
dimension-signal output relationship of FIG. 7 also permits the invention 
to be embodied so that gap dimension is directly indicated. Specifically, 
strain gauge instrumentation of the type discussed relative to FIG. 5 
often includes range control that in effect multiplies the sensor output 
signal by a desired constant. By appropriate setting of such a range 
control the signal levels produced by the system can be established equal 
to panel warpage. For example, in the realization of the invention that 
provided the signal levels shown in FIG. 7 suitably setting such a range 
control would provide output readings ranging between 0 and 0.125, in 
accordance with panel warpage. 
As was previously mentioned, certain dimensions of wand 28 and the 
dimensions of contact protrusions 54 of lower leaf spring 42 and upper 
leaf spring 44 are established to eliminate deflection of lower and upper 
leaf springs (42,44) that otherwise could be caused by twisting the 
tilting of wand 28 as it is passed through gap 22. This aspect of the 
invention can be understood with reference to FIGS. 8 and 9, which 
respectively illustrate twisting or rolling of sensor wand 26 in a gap 22 
and tilting or pitching of sensor wand 26 in a gap 22. 
In viewing FIG. 8, it first can be noted that if contact protrusions 54 
collectively defined a sphere of a diameter exactly equal to the dimension 
of gap 22, strain gauge sensor assembly 32 could be rolled or twisted 
through relatively large roll angles without further deflection of lower 
and upper leaf springs 42 and 44. That is, under such conditions, the 
geometry of contact protrusions 54 in effect would correspond to the 
geometry of a ball gauge. In view of this characteristic of contact 
protrusions 54, in the practice of the invention the radius of each 
contact protrusion 54 is dimensioned in accordance with the gap dimensions 
to be measured to substantially reduce undesired deflection of lower and 
upper leaf springs 42 and 44 is the strain gauge sensor assembly 32 is 
twisted or rolled when gap measurements are being taken. By way of 
example, in the previously mention realization of the invention that is 
configured for measurement of gaps in the range of 0.125 inch and 0.250 
inch, substantial reduction in roll induced sensor deflection (and, hence, 
measurement error) can be obtained by utilizing contact protrusions 54 
that are substantially hemispherical in geometry with each contact 
protrusion 54 having a radius that is greater than 0.0625 inch and less 
than 0.125 inch. 
It also can be noted in FIG. 8 that the maximum angle through which wand 28 
can be rolled or twisted is determined by the width of wand 28. Thus, by 
appropriately selecting the width of dimension of wand 28 in view of the 
sensor gap range and the radii of contact protrusions 54, roll induced 
measurement error can be further reduced. For example, the previously 
discussed embodiment of the invention that is configured for measurement 
of gaps 22 in the range of 0.125 inch to 0.250 inch utilizes a wand width 
of 1.5 inch with each contact protrusion having a radius of 0.087 inch. In 
that realization of the sensor wand 26, the maximum roll induced error was 
limited to 0.00025 inch, throughout the entire gap measurement range. 
Thus, that particular realization of the invention was fully suited for 
gauging the surface contour of composite panels 10 to a required contour 
tolerance of .+-.0.03 inch. 
Dimensional considerations important to controlling pitch or tilt induced 
sensor measurement error can be understood with reference to FIG. 9, which 
illustrates sensor wand 26 positioned in a region of gap 22 that exhibits 
a radius of curvature. The type of gap measurement situation depicted in 
FIG. 9 commonly is encountered in gauging composite aircraft panels since 
such panels often exhibit contoured surface regions (the radius of which 
may be one the order of 30 inches). In viewing FIG. 9, it can be seen that 
the contact protrusion 54 dimensional considerations discussed relative to 
sensor wand roll (FIG. 8) also apply to reducing pitch induced measurement 
error. Further, it also can be seen that the maximum pitch angle that can 
be experienced by strain gauge sensor assembly 32 is determined by the 
distance between the end of wand 28 and the regions of contact protrusions 
54 that contact layup surface 14 of mold 12 and surface 16 of composite 
panel 10. Thus, by suitably dimensioning and arranging the sensor region 
of wand 28, pitch induced measurement error can be minimized to an 
acceptable value. For example, in the previously discussed realization of 
sensor wand 26 in which the radius of each contact protrusion 54 is 0.087 
inch, the distance between the end of sensor wand 26 and the center of 
each contact protrusion 54 is 3.22 inch. Dimensioned in this manner, pitch 
induced error was limited to no more than 0.00025 inch throughout the gap 
measurement range of 0.125 inch to 0.250 inch. 
While the invention has been described in relation to a specific 
embodiment, it is to be understood that various changes, substitutions and 
alterations can be made without departing from the scope and spirit of the 
invention. For example, in some realizations of strain gauge sensor 32, 
rectangular cutout 60 may be of a different configuration. Similarly, it 
may be possible to utilize smoothly contoured contact protrusions other 
than the substantially hemispherical protrusions disclosed herein. 
It also is contemplated that a gauging arrangement formed in accordance 
with this invention can be used where a mold is configured with at least a 
portion of the mold extending upwardly toward the vertical direction to, 
for example, define an upwardly projected curved portion of a panel that 
is employed in the leading or trailing edge of an aircraft wing. In such a 
situation, suitable spacer strips would be used to establish a gap of 
known dimension between all surfaces of the mold and the panel being 
inspected. Depending upon the angular orientation of the upwardly 
projecting mold and panel sections, it might be necessary to insert a 
sensor wand that is configured in accordance with the invention first from 
one edge of the mold and then insert the sensor wand from the oppositely 
disposed edge of the mold.