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BACKGROUND OF THE INVENTION
Vision guided robotic systems have been employing the principle of optical
triangulation to measure surface points on objects to be worked on by a
robot. In particular, one arrangement employs the projection of a plane of
light that falls upon the object to be measured and forms a line of light
on the object surface where the surface intersects the light plane. A TV
camera views the line of light from a known angle away from the plane of
light, and this forms the basis for computing the location of each point
on the line relative to the camera/projector sensor. The TV camera
provides a video output signal that is processed to provide these
measurements.
In using a sensor of this type, mounted on a robot arm to guide the robot
along an edge or seam on an object, it has been necessary to anticipate
the sudden loss of measurement data when the corner of the edge is reached
or the seam reaches an edge. Since data is only obtained when the plane of
light intersects the surface, the data disappears when the plane is
carried beyond the corner or edge. The robot is usually fitted with a
tool, such as a welding gun, which is being guided by the vision data and
which must trail the line of intersection to avoid interfering with the
vision measurement. Thus, the tool still has some distance to travel when
the guidance data is lost.
SUMMARY OF THE INVENTION
It is an object of the invention to overcome the prior art disadvantages. A
more particular object is to provide an improved arrangement for 3-D
optical measurement systems employing scanned light beams or planes. In
keeping with this object, and with still others which will become
apparent, one aspect of the invention resides in an arrangement for
providing a beam of light that is not shaped as a plane. By projecting a
beam of light that tends to partially surround the working tool from a
vision sensor mounted on a robot arm for guidance, a greater amount of
information becomes available to the guidance system than if the sensor
projects a plane of light. When tracking along an edge, and a corner is
reached, the portion of a non-planar light beam in the direction of travel
will go beyond the corner and lose data while a portion of the beam will
remain on the object surface providing continuous data around the corner
and along the intersecting edge. This allows the guidance system to
continue a smooth track around the corner, a very desirable capability.
When the robot brings the tool around the corner, the attached vision
sensor will gradually return to where the center of the light beam is
again placed along the edge to be tracked. This enables continuous
guidance data to be obtained without requiring any auxiliary mechanism to
move the sensor relative to the end of the robot arm carrying the working
tool and sensor.
A further benefit is derived from this arrangement in that it provides
additional measurement information sufficient to enable the determination
of one additional degree of freedom in the orientation of the sensor and
the robot arm to which it may be attached. A sensor that projects a plane
of light onto a flat surface can measure surface points and from these
points the angle of the surface relative to the sensor can be determined
for the angle contained within the plane (in-plane angle). The angle of
the surface contained in a plane orthogonal to the light plane
(cross-plane angle) cannot be determined. However, by projecting a light
beam that does not lie wholly within a plane, this deficiency is
eliminated.
The invention will hereafter be described with reference to an exemplary
embodiment, as illustrated in the drawings. However, it is to be
understood that this embodiment is illustrated and described for the
purpose of information only, and that nothing therein is to be considered
limiting of any aspect of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view and illustrates a prior art 3-D measurement
sensor using optical triangulation, mounted on a robot arm and obtaining
measurements from a surface;
FIG. 2a shows the relationship between the prior art sensor light plane
intersection line and tool work area;
FIG. 2b shows the same relationship when a corner is encountered;
FIG. 2c shows the improved sensor relationship;
FIG. 3a shows in cross section a plane of light reflecting from a plane
mirror;
FIG. 3b shows a plan view of FIG. 3a;
FIG. 3c shows in cross section a plane of light reflecting from a folded
mirror;
FIG. 3d shows a plan view of FIG. 3c;
FIG. 3e shows an end view of the folded mirror;
FIG. 3f shows an end view of a mirror with circular shape;
FIG. 4a shows an end view of a V-shaped light beam;
FIG. 4b shows a side view of FIG. 4a;
FIG. 5a is a plan view of optics forming a plane of light from a point
source;
FIG. 5b is a side view of FIG. 5a; and
FIG. 6 is a perspective view and shows the use of a V-shaped light beam in
accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The principles involved in making 3-D measurements with a projected light
beam or plane (not restricted to the visible spectrum) and light sensitive
detector are well described in U.S. Pat. No. 4,238,147. FIG. 1 illustrates
a sensor 11 based on these principles and mounted on a robot arm 10 for
the purpose of measuring how far surface 13 lies from the robot arm,
orientation of the surface, relative to the arm, and location of features
such as seams or edges. Sensor 11 projects a scanning beam or a plane of
light 12 which is viewed along a portion of its length 14, 15 by a
light-sensitive detector such as a TV camera within sensor 11 via path 16.
With the separation between the projector and detector well known, and the
angle of light projection and light detection known, the distance to
surface 13 can be computed. In fact, the three spatial coordinates of
every resolvable point along the intersection 17 of light plane 12 and
surface 13 can be computed relative to sensor 11. From this data, the
angle whereby sensor 11 views surface 13, as measured in the plane of
light 12, can also be computed. On flat featureless surfaces four degrees
of freedom of the surface relative to the sensor cannot be determined
without scanning the sensor or surface in a controlled manner.
FIG. 2a illustrates the intersection pattern 22 generated by a prior art
sensor which projects a plane of light. Surface 24, having edges 20 and
23, represents, for example, the corner of a box to be welded along the
edges. A vision sensor, as described above, can be attached to the arm of
a welding robot to provide a means of sensing inexact placement of the box
and variations in edge contour caused by normal manufacturing tolerances.
The intersection pattern 22 would be placed forward of the weld arc 21 in
the direction of travel. The portion of the light plane not intersecting
the surface 24 is shown dashed. The pattern 22 is placed forward of arc 21
to avoid the interfering glare of arc 21 and to provide guidance signals
to the robot in advance of bringing arc 21 to a point on edge 20. The
robot needs signals in advance to enable it to accurately position arc 21.
This is particularly true as the travel rates are increased and dynamic
lag errors become significant.
When a corner is encountered, a illustrated in FIG. 2b, the sensor of the
prior art suddenly loses its ability to provide guidance data since the
intersection pattern 22 reaches the end of edge 20 in advance of arc 21.
The robot can then continue welding to the corner and swing out and around
to bring pattern 22 onto edge 23, or the robot can attempt to blindly
navigate the corner for a continuous weld from edge 20 to edge 23.
The condition that neither choice is completely satisfactory, has prompted
the present invention, which replaces the projected plane of light 12 with
a non-planar sheet of light. By bending the light plane around the working
area, for example, as shown in FIG. 2c, where a "V"-shaped pattern 22 is
projected upon surface 24--we obtain continuous data, even at the corner.
As "V" pattern 22 travels along edge 20, data is obtained as in the prior
art, and weld arc 21 can be properly guided. When a corner is encountered,
pattern 22 continues to provide data on the newly encountered edge 23,
regardless of whether it is to the left or right of edge 20. Thus, the
weld arc 21 may be continuously guided around the corner and along the
edge 23.
Although pattern 22 in FIG. 2c can be generated from two light sources, it
is advantageous to use a single light source. FIG. 3a indicates in a side
view how a plane of light 30 incident on surface 31, will reflect along
path 32. FIG. 3b provides a plan view of this situation, with plane of
light 30 emanating from light source 34 and reflecting along line of
incidence 33 where surface 31 is a flat mirror-like surface. The reflected
light beam 32 will be a plane if incident beam 30 is a plane.
However, if surface 31 is "V"-shaped as shown in side view in FIG. 3c, in
plan view in FIG. 3d, and in end view in FIG. 3e, then light from source
34 in the form of a plane 30 will form a "V"-shaped beam 32 after
reflecting from surface 31 at line of incidence 33. If surface 31 is
elliptical, then reflected beam 32 will have the form of a circular
cylinder. Conversely, if surface 31 is a circular cylinder as shown in
FIG. 3f which is an end view, the reflected beam 32 will have the shape of
an elliptical cylinder. In similar manner other desirable shapes may be
formed.
It should be noted that when a plane of light intersects a flat surface at
an angle, the line of intersection is a straight line which yields no
information about the angles formed by the surface and plane. However,
when a "V"-shaped light beam or other non-planar beam intersects a flat
surface, the line of intersection varies as the angle of the beam to
surface varies, and this produces a unique measure of the in-plane and
cross-plane angles. Further, when tracking along an edge such as 20 of
FIG. 2c, these unique angles can be measured, even though only a part of
the light pattern 22 intersects surface 24, provided that the part of
pattern 22 intersecting surface 24 is not planar (e.g. elliptical or
circular). This additional measurement value is obtained without resorting
to two parallel light beams separated by a small distance that could
potentially introduce ambiguity into the measurement if the reflected
light cannot be correlated to the correct light beam.
The non-planar light beam may also be generated using conventional flying
spot technology. A narrow pencil beam of light can be deflected over the
non-planar path using mechanical or electronic deflection means.
Alternately, a light plane derived from a point source can be formed into a
non-planar beam as illustrated in FIGS. 4a and 4b. Point source 44 emits
rays of light forming a plane of light 40 as seen in a side view in FIG.
4b, and in an end view in FIG. 4a. The rays within plane 40 striking
reflecting surface 41 reflect and form plane 42. Planes 40 and 42 form a
"V"-shaped light beam. In practical implementations surface 41 ends a
significant distance from the target surface upon which the "V"-shaped
beam is projected for making measurements. Also point source 44 must be
located at least a small amount above surface 41 to avoid eclipsing some
of the rays which are generally derived from an optical arrangement or to
provide clearance for a physical light source. Therefore some rays coming
from source 44 will not reflect from surface 41 even though they lie below
a plane through source 44 and parallel to surface 41. For example, ray 46
will just miss surface 41 at end 49, and reach the target surface a
distance 48 below the plane of surface 41. A ray slightly steeper than ray
46 will reflect along ray 47, and have a gap to the plane of surface 41 at
the target of distance 45. Gap 45 is minimized by locating source 44 as
close to surface 41 as practical.
FIGS. 5a and 5b show an arrangement for generating a line source of light
that may be used as the point source of light 44. FIG. 5b shows a side
view of the system with point light source 51 emitting light that is
collected by spherical lens 52 which is positioned to image the point
source at the target surface. FIG. 5a shows a plan view of the system. A
cylindrical lens 53 focuses the rays into line 54. These rays then spread
into a vertical fan beam 50.
In using a line source 54 for point source 44, the length of line 54 must
be kept short to obtain a small gap 45 at the target surface. Since line
54 is perpendicular to plane 50 (40 in FIG. 4), at least part of the line
will be separated substantially from surface 41 and cause a gap 45.
However, the gap 45 may not be of great importance in some applications.
The invention has been described and illustrated with reference to an
exemplary embodiment. It is not to be considered limited thereto, inasmuch
as all modifications and variations which might offer themselves are
intended to be encompassed within the scope of the appended claims.
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