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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
Our present invention relates to an optical process and apparatus for the
three-dimensional determination of the shape of objects by means of a
calculator.
2. Description of the prior art
Stereoscopy is often used for the determination of the shape of objects by
optical means. A first view of the object and a second are taken, the
latter being at a different angle from the former, and consideration of
these views makes it possible to determine the position of the different
points of the surface of the object, when the locations at which the views
were taken are known.
In reality, this process is a delicate operation and the difficulty is
particularly great when no reference is available, because it is then
impossible to compare the images of a single point on to the two views
without other information.
In order to resolve this difficulty, it has been proposed to compare areas
of the same shape on both images, and to determine the large or small
correlation which exists between, on the one hand, the degrees of
illumination of the different points of an area of one image and, on the
other hand, those of the different points of variously positioned areas of
the same shape of the other image, the correlation obviously being at a
maximum when the area on the other image corresponds to the same region of
the object as the area of the first image.
Usually, the luminosity of each point of an image is made to correspond to
a number expressed in a digital system, and the numbers corresponding to a
multiplicity of pairs of points are processed in a calculator with a view
to studying their correlation.
This processing is long and complicated, all the more so because it is
often necessary to distinguish between several maxima of the correlation.
Analog devices exist, particularly optical devices using rotating mirrors,
which make it possible to save a certain amount of time in the calculation
of the correlation between the degress of illumination of two areas.
However, apart from being delicate to use and adjust, these analog devices
do not overcome the problems posed by the digital devices with reference
to the local maxima, the displacements of the moving area, and the like,
as well as to the dimension and the shape of the areas, which must be
small enough to be "quasi-pinpoint" and large enough to contain the
information required in order to carry out the correlation.
If processes of this kind have found a use for topographical surveys from
the air or by satellite, their application to present-day industrial
purposes is difficult to envisage because of the slow operation which
restricts their use to static scenes and also because of the high cost of
the calculations involved.
It has also been tried, with a view to a more practical use of the
stereoscopy process, to mark the surface of the object whose shape is to
be explored. This marking is time-consuming and expensive as well as
complicated, and cannot always be used.
Furthermore, the known installations of this type use, in order to take
views of the object under observation, two cameras provided with a
conventional sweep system, the volume of data provided by the cameras
resulting in a long treatment time. These installations are also limited
to the representation of a contour line of the surface of the object.
Another installation, which is able to compute a profile line of an object,
is known from the German Offenlegungschrift No. 2,113,522, which describes
the use of the reflection over the surface of the object of a laser pencil
having a variable inclination in a given plane, the reflected pencil being
intercepted by a receiving device. The determination of the profile line
necessitates the computation of the inclination of the emitted and
received pencils with respect to a straight reference line, together with
the knowledge of the position of both the laser emitter and the receiving
device.
This type of installation is limited both in its object and in its use.
OBJECT OF THE INVENTION
Our invention aims at resolving the problem of determining
three-dimensionally the shape of the surface of objects for industrial
purposes, e.g. to guide a gripping device towards a multiplicity of loose
objects with the aim of taking one of the objects under predetermined
conditions, or to direct an instrument towards a certain region of an
object of unknown position, operations of this kind depending on what is
sometimes called robot function.
More particularly, it is our intention to utilize the well-known phenomenon
of parallax i.e. the different angles at which the picture-taking devices
(referred to hereinafter as cameras) see a point of a surface of an object
whose position is to be determined, in a simplified manner allowing the
adaptation of this principle to industrial applications.
The invention takes advantage of the fact that a point of a surface of an
object, whether it is illuminated or not, can be distinguished from the
other points of the surface by selectively subjecting it to a degree of
illumination more intense than that of the other points of the surface.
This technique is equally applicable to objects immersed in darkness as to
objects subjected to ambient illumination.
In accordance with our invention, a generally punctiform area of a body
surface to be explored is selectively subjected to distinctive
illumination -- advantageously with the aid of a laser beam -- to give off
reflected light rays. Certain of these light rays, passing along two
divergent paths, are intercepted and focused into a pair of image points
projected upon a receiving surface or a pair of such surfaces. From the
location of each of these image points with respect to a reference point,
which may be different for each image point or common to both of them, the
position of the distinctively illuminated punctiform area relative to the
receiving surface or surfaces is determined through mathematical
evaluation.
To carry out the aforedescribed method, our invention provides optical
means including a pair of projection objectives or a single such
objective. In the latter instance the apparatus also comprises
light-guiding means for deflecting the intercepted light rays from
different directions toward that objective, advantageously by way of a
parallax-increasing reflector assembly. The position evaluator may include
coding means for converting the locations of the projected image points,
relative to their reference point or points, into numerical values on
which an associated calculator can operate. The evaluation could be based,
however, on analog rather than digital values.
BRIEF DESCRIPTION OF THE DRAWING
The above and other features of our invention will now be described in
detail with reference to the accompanying drawing in which:
FIG. 1 is a schematic view of an apparatus according to the invention;
FIG. 2 is a schematic view of a modification of the apparatus shown in FIG.
1;
FIG. 3 is a schematic view of another embodiment;
FIGS. 4 and 5 are detail views of a component of the apparatus of FIG. 3;
and
FIG. 6 is a diagram of an installation according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
In order to determine the shape of a body C, shown in FIG. 1, a pinpoint or
quasi-pinpoint region Z of body C is illuminated and that body is observed
by means of a first camera 11.sub.1 and a second camera 11.sub.2 whose
relative position is known. The camera 11.sub.1 projects with its
objective 12.sub.1, on its photosensitive surface 13.sub.1, and image
z.sub.1 of the quasi-pinpoint region Z image Z.sub.1 lies at a distance
d.sub.1 from the point of intersection c.sub.1 of the photosensitive
surface 13.sub.1 with the axis a.sub.1 of the objective 12.sub.1.
The camera 11.sub.2 projects, with its objective 12.sub.2, an image Z.sub.2
of the quasi-pinpoint region Z on its photosensitive surface 13.sub.2, at
a distance d.sub.2 from a point c.sub.2 which is the intersection of the
photosensitive surface 13.sub.2 with the axis a.sub.2 of the objective
12.sub.2.
A knowledge of the coordinates of the images z.sub.1 and z.sub.2 formed on
the photosensitive surfaces 13.sub.1 and 13.sub.2 makes it possible, by
elementary geometric and/or trigonometric considerations, to measure the
parallax of the pinpoint or quasi-pinpoint region Z in relation to the two
cameras, more precisely in relation to the centers O.sub.1 and O.sub.2 of
their objectives 12.sub.1 and 12.sub.2.
In the embodiment shown in FIG. 2, the photographs are taken by a single
camera 14 comprising two objectives 12.sub.1 and 12.sub.2 and a single
support for the photosensitive surface 15. The difficulty of the precise
alignment of one camera with respect to another is thus eliminated.
In the embodiment shown in FIG. 3, a camera 21 comprises a single objective
or lens assembly 22 placed in front of a single support for the
photosensitive surface 23. This camera comprises a ray-transposing device
24 consisting of two crossed mirrors 25 and 26 (see also FIG. 4)
including, for example, an angle of 90.degree. with each other. Mirrors 25
and 26 are of the partly transparent or semireflective type, their line of
intersection 27 coinciding with the axis 28 of the lens assembly 22.
According to a modification (FIG. 5), a ray-transposing device 24' is used
which is made up of two ordinary reflective mirrors 25' and 26', which
again cross at an angle of, for example, 90.degree. but are relatively
offset along the line 27'.
The mirrors shown in the drawing could be replaced by prisms and/or lenses.
The device 24 or 24' is bracketed by two mirrors 31 and 32 which may be
parallel, respectively, to the mirrors 26 and 25. The combination of this
ray-transposing device and its two flanking mirrors magnifies the
parallactic angle, as will be apparent from the drawing.
In FIG. 3, the path of the reflected back-scattered rays is schematically
shown starting from the generally punctiform Z.
The mirror 31 gives an image Z.sub.1 of the region Z which is reproduced by
the mirror 26 as an image Z.sub.1.1 projected by the lens assembly 22 at
z.sub.1 on the receiving surface 23; the position of this image on surface
23 is numerically coded.
In the same way, the mirror 32 gives an image Z.sub.2 of the pinpoint
region Z which is reproduced by the mirror 25 as an image Z.sub.2.2. The
lens assembly 22 projects the image Z.sub.2.2 on the receiving surface at
a point z.sub.2 whose position can again be numerically coded.
The distance between points z.sub.1 and z.sub.2 is a measure of the
parallax and thus of the distance of area Z from the receiving surface.
The position of the quasi-pinpoint region Z is determined by evaluating the
numerical codes which represent the positions of the images z.sub.1 and
z.sub.2. Since each of these image positions uniquely defines a principal
ray passing through the center of objective 12.sub.1, 12.sub.2 or 22, the
point Z can be found geometrically or analytically as the intersection of
these principal rays.
By directing the illumination pencil onto different quasi-pinpoint or
generally punctiform regions of the body C, may thus successively
determine the locations of these different regions and, finally, the shape
of the body C as a whole.
In the installation shown in FIG. 6, a laser beam 51 is provided by a laser
generator 52 working into by a beam deflector 53 which can be a mirror
system controlled by galvanometric means or a solid-phase device
controlled piezoelectrically or ultrasonically. A device of this latter
type is sold, for example, by the firm SORO ELECTRO-OPTICS, under the name
"ACOUSTO-OPTIC LASER DEFLECTOR, MODEL D". The deflected beam 54 issuing
from the device 53 illuminates a quasi-pinpoint region Z of the body C. At
56.sub.1 and 56.sub.2 we show laser rays reflected by the region Z which
traverse two objectives of a television camera 55 comprising a tube or a
pair of tubes of the vidicon type or of the image-dissector type, using a
"random" read-out, which is very suitable for following a moving point. A
device of this type is sold, for example, by the firm EMR SCHLUMBERGER,
under the name "OPTICAL DATA DIGITIZER, TYPE 658A". We may also use the
"EMR PHOTO-ELECTRIC MODEL S75 IMAGE DISSECTOR" sold by the same firm.
The output 57 of the camera 55 is linked to the input of a coder 58 which
provides at its output 59 the coordinates of the illuminated point or
points on the image surface of the camera. These numerical codes are fed
to the input of a computer 61 which comprises a device 62 for calculating
the spatial coordinates of the point Z, the calculator 62 being part of a
logic network 63 of this computer. A programmer 64, linked to the
calculator 62 by a circuit 60, determines the direction of progress of the
luminous beam 54 as a function of the points of the body C which are of
interest. The latter can be the points of contour lines, which have the
same parallax, or peaks, i.e. points of maximum height which are obtained
by starting from any point and progressing according to a line of greatest
slope, or points of contours which are characterized by a discontinuity of
the parallax.
The above operations are preferably carried out by reading out only one
randomly chosen point, without using a scanning sweep.
The information provided by the device 64 is applied through a connection
65 to a device 66 for the digital control of the displacement of the beam;
the information present at the output 67 of controller 66 is applied via a
digital/analog, converter 68 to the control input of the deflecting device
53.
A memory 69 included in the computer 61 stores a summary of the information
acquired. This memory may be a specially wired device or part of a logic
system using a universal calculator.
The laser emitter 52, 53 may be replaced by a mobile light source.
Alternatively, the light pencil would be emitted by several sources
alternately, thus allowing the intensified illumination of points of a
body which could not be reached by a single pencil.
An apparatus according to the invention can form the pick-up of a
three-dimensional perception mechanism which is part of a device, such as
a telemanipulator, serving for the gripping, positioning or assembly of
objects or for the delivery of loosely heaped articles to a machine.
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