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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical mensuration devices in general, and in
particular to an improved method and apparatus for the optical mensuration
of the surface shape of a three-dimensional object.
2. Brief Description of the Prior Art
Numerous mensuration systems exist in the prior art for sensing the
locations of surface points on three-dimensional solid objects in relation
to a predefined fixed reference frame or coordinate system for input into
an application system, such as a computer or other device for measurement
or analysis. For example, one type of mensuration system that can be used
to determine the location of a single point on the surface of an object
includes the use of a narrow projected beam of light to illuminate a tiny
area or spot on the surface of the object. A lens in the system is
positioned on an optical axis oblique to the axis of the projected beam
and is used to focus the reflected light from the illuminated spot onto a
photoelectric sensor or onto a linear array of sensors. Since the optical
axis of the lens and sensor assembly in that type of system is not
coincident with the axis of the projected beam, the position of the image
of the illuminated spot on the sensor will depend on the location of the
particular illuminated surface point with respect to the illuminating
beam. Therefore, the location of the illuminated point with respect to the
predetermined reference frame can be determined by computing the distance
of the illuminated surface point from the origin of the light beam which,
of course, is known. Examples of such point illumination optical
mensuration systems are found in the following U.S. Pat. Nos. 4,660,970;
4,701,049; 4,705,395; 4,709,156; 4,733,969; 4,743,770; 4,753,528;
4,761,072; 4,764,016; 4,782,239; and 4,825,091.
Of course, to determine the overall shape of an object, numerous individual
surface points, along with their respective locations, must be measured
and recorded. Such optical measurement of multiple surface points of an
object is typically accomplished by mounting the beam projector on a
moveable scanning head capable of being moved from point-to-point with
very high precision, such as the type commonly found on numerically
controlled milling machines. By precisely moving the beam projector
mounted on the scanning head in a raster-like scanning pattern, it is
possible to measure the surface shape of the object being scanned by
measuring the individual locations of surface points individually
illuminated by the point-like scanning beam as it is scanned over the
object's surface. Alternatively, the object itself can be moved while the
scanning head remains stationary. One disadvantage of this type of system
is that only one side of the object may be scanned at any one time, since
other sides of the object are hidden by the side being scanned. Scanning
of these hidden sides can only be accomplished by relocating either the
scanning head or the object to expos the previously hidden surfaces to the
scanning beam. Obviously, such a relocation requires time and precision
equipment to keep track of the changed position of the scanning head, or
the object in relation to the fixed reference frame so that the new
surface data will correspond to the previously obtained surface data.
Helical or three-dimensional scanning heads solve this problem by allowing
the entire object to be scanned at once. However, such helical systems are
relatively expensive, since they require complex mechanical apparatus to
move the scanning head around the object in three-dimensions.
Regardless of the scanning method used, however, deep holes, overhangs,
undercuts, and surfaces nearly parallel to the axis of the scanning beam
reduce the accuracy of the system, since it is difficult to accurately
measure these points, if they can even be illuminated by the scanning beam
at all. For example, such systems cannot completely scan the inside,
outside, and handle details of a coffee cup without requiring the scanning
apparatus to be relocated or the object to be reoriented so that the
inside surfaces or other surfaces previously hidden from the scanning beam
can be illuminated by the beam, thus measured and recorded. As discussed
earlier, such re-locations or re-orientations have the disadvantage of
having to recalibrate the scanning apparatus, or otherwise recorrelate the
new surface points with respect to the original coordinate system.
Moreover, even if such relocations or reorientations are not required,
such as in the case of a helical scanning apparatus, there is still a
severe loss of accuracy when scanning near the top or bottom of a rounded
object, unless the scanning head and detector are relocated to better
illuminate and detect such points. Furthermore, these types of systems are
not very portable or adaptable since they require high precision
electro-mechanical or other apparatus to accurately move the scanning
heads (or the object) and define their positions in relation to the
predetermined reference frames. Therefore, all these prior art scanning
systems will usually require some type of relocation of the scanning
apparatus or reorientation of the object to completely measure and record
all of the surface details.
A variant of the above-described systems projects a thin beam of light in a
single plane which, of course, is incident as a line, as opposed to a
point, on the surface of the object being scanned. The intersection of
this plane of light with the object's surface thus forms a brightly
illuminated contour line. A two-dimensional electronic video camera or
similar device whose optical axis is not coincident with the axis of the
illuminating beam, detects the image of this contour line. Again, since
the optical axis of the camera is not coincident with the axis of the
illuminating light beam, it views the contour line from an oblique angle,
thus allowing location of the contour line to be precisely determined in
relation to the known position of the beam projector. Examples of
inventions using this type of system are found in the following U.S. Pat.
Nos. 4,821,200; 4,701,047; 4,705,401; 4,737,032; 4,745,290; 4,794,262;
4,821,200; 4,743,771; and 4,822,163.
To measure more than one contour line of an object, either the measuring
apparatus or the object is panned along (or rotated about) an axis through
the object. While these line scanning devices share similar drawbacks with
the point scanning devices previously described, they do operate much
faster, gathering a larger number of sample points during a given scanning
interval. Unfortunately, the accuracy of each surface sample point is
limited by the relatively low resolution of the two-dimensional charge
coupled device (CCD) sensors found in most video cameras, which is
typically in the range of 1 part in 512. Even worse, these systems still
suffer the disadvantages of the point scanning systems in that either the
scanning head or the object must be relocated or re-oriented to completely
and accurately record all of the surface details of an object.
Still other mensuration systems track the positions of specific points in
three-dimensional space by using small radiating emitters which move
relative to fixed receiving sensors, or vice versa. Such radiation
emitters may take the form of sound, light, or nutating magnetic fields.
Another mensuration system uses a pair of video cameras plus a computer to
calculate the position of homologous points in the pair of stereographic
video images. See, for example, U.S. Pat. Nos. 4,836,778 and 4,829,373.
The points tracked by this system may be passive reflectors or active
light sources. The latter simplifies finding and distinguishing the
points.
Additional prior art relevant to this patent application are found in the
following references:
Burton, R. P.; Sutherland, I. E.; "Twinkle Box--a three dimensional
computer input device", National Computer Conference, AFIPS Proceedings, v
43, 1974, p 513-520;
Fischer, P.; Mesqui, F.; Kaeser, F.; "stereometric measurement system for
quantification of object forms", SPIE Biostereometrics 602, 1985, p 52-57;
Fuchs, H.; Duran, J.; Johnson, B.; "Acquisition and Modeling of Human Body
Form Data", Proc. SPIE, v 166, 1978, p 94-102;
Macellari, V.; "A Computer Peripheral Remote Sensing Device for
3-Dimensional; Monitoring of Human Motion", Med. & Biol. Eng. & Comput.,
21, 1983, p 311-318;
Mesqui, F.; Kaeser, F.; Fischer, P.; "real-time, noninvasive recording and
3-d display of the functional movements of an arbitrary mandible point",
SPIE Biostereometrics 602, 1985, p 77-84;
Yamashita, Y.; Suzuki, N.; Oshima, M.; "Three-Dimensional Stereometric
Measurement System Using Optical Scanners, Cylindrical Lenses, and Line
Sensors", Proc. SPIE, v. 361, 1983, p. 67-73.
In particular, the paper by Fuchs, et al, (1978) describes a basic method
of tracking a light source in three-dimensional space. The method is based
on using three or more one-dimensional sensors, each consisting of a
cylindrical lens and a linear array of photodetectors, such as charge
coupled devices (CCDs), to determine the location of the currently
radiating source.
Numerous other methods have been devised and patented for determining the
position of a point along a line, within a plane, or in three-dimensional
space. Devices employing these methods include photographic camera
rangefinders, tablet digitizers, coordinate measuring machines, and
surveying tools. Some exploit sound, magnetic fields, or mechanical
apparatus for mensuration, and there are other devices employing x-rays,
nuclear magnetic resonance, radar, sonar, and holography to sense the
shapes of objects.
Unfortunately, each of the above mensuration systems has its own set of
drawbacks, which include high cost, poor accuracy, poor resolution,
awkward or difficult use, limitations on geometrical complexity, excessive
numerical computation, or slow measurement speed. Experience has shown
that no single prior art system best suits all three-dimensional
measurement applications. For example, there is no existing mensuration
device that can perform even straightforward anatomical measurements of a
person without significant drawbacks.
Thus, there remains a need for a non-contact, three-dimensional optical
mensuration system which is capable of accurate, speedy, convenient, and
inexpensive sensing of three-dimensional geometric shapes or objects.
Ideally, the scanning head of such an improved system should be hand-held
to allow the operator to easily move the scanning beam over some of the
more complex surface details of the object while dispensing with the need
for the expensive, cumbersome, and high precision scanning head
positioning apparatus currently required. Such a hand-held scanner must
also provide the accuracy and precision associated with currently
available optical mensuration systems, that is, it must be able to
accurately measure and precisely locate the surface details of the object
in relation to the predetermined reference frame.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an
improved, non-contact, three-dimensional optical mensuration system
capable of accurately sensing the surface shapes of three-dimensional
objects without the numerous drawbacks associated with the prior art
systems.
It is another object of this invention to provide an optical mensuration
system that is inexpensive, portable, and easy to use.
It is a further object of this invention to provide a three-dimensional
optical mensuration system which can quickly scan the surface of the
object without the need for expensive, complicated, and high precision
mechanical positioning apparatus to position either the scanning head or
the object being scanned.
A still further object of this invention is to provide a portable,
hand-held, and hand-maneuverable scanner for the three-dimensional,
non-contact shape-scanning and/or mensuration of three-dimensional
objects.
Additional objects, advantages, and novel features of this invention shall
be set forth in part in the description that follows, and in part will
become apparent to those skilled in the art upon examination of the
following or may be learned by the practice of the invention. The objects
and the advantages of the invention may be realized and attained by means
of the instrumentalities and in combinations particularly pointed out in
the appended claims.
To achieve the foregoing and other objects and in accordance with the
purposes of the present invention, as embodied and broadly described
herein, the apparatus for three-dimensional, non-contact shape sensing of
this invention may compris | | |
