Note: Descriptions are shown in the official language in which they were submitted.
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MIJLTIPLE C~ANNEL OPTICAL FLYING SPOT
TRIANGLATION RANGER SYSTEM
Background of the Invention
This invention relates to a time-based optical
triangulation system and especially to improvements
to increase the speed of acquiring range valves.
A swept aperture flying spot optical ranging instru-
05 ment has been developed and was offered as a product
known as the MIG TRAgTM welding seam tracker. It is
described in patent 4,645,917 - Penney, Roy and Thomas
and patent 4,701,031 - Penney and Thomas. Although
it provides the highest quality data among developed
ranging systems, the maximum rate at which this type
of system can acquire data is limited by the beam de-
flecting acousto-optic cell to less than 100,000 range
elements per second, and in fact runs at about 20,0C0
range measurements per second. To our knowledge no
other developed system runs significantly faster.
However, for applications such as a three-dimensional
camera a much higher range data rate is needed; for
example the present goal for printed circuit board
inspection is 4,000,000 range elements per second.
What is needed is a faster way to obtain range data
while preserving the other high performance features
of the MIG TRA~ approach.
Summary of the Invention
The speed of measuring range points in a swept
aperture flying spot triangulation ranger is increased
by adding additional detector channels, each of which
observes the surface at a different sensitive zone
along the line which forms the intersection of the
swept beam with the surface. Since all of the channels
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make use of the same swep~ beam, the beam source and
deflector are used more efficiently, providing N range
points per beam sweep, where N is the number of channels,
rather than only one.
05 According to one aspect of the invention, the
improved flying spot triangulation ranger system is
comprised of: means for providing a light beam and
rapidly sweeping this beam across a surface in a given
direction; multiple light detection channels optically
coupled to separate detectors; and receiving lens means
for focusing images of the light spot onto the entrance
faces of the adjacent detection channels. As the light
spot generated by the scanned beam crosses a small
sensitive zone on the surface from which a detection
channel can receive light, the respective detector
generates an electrical signal pulse. These pulses
are output at times that are a function of range to
the surface; for every sweep of the light beam there
are a plurality of range measurements equal to the
number of detection channels.
The detection channels are preferably separate
noncoherent fiber optic bundles, but may be solid glass,
and typically there are ten to twenty such channels.
The entrance faces of the detection channels may be
rectangular or square, and the sensitive zones "visible"
to the channels have corresponding shapes. The detectors
may be photomultipliers.
Another aspect of the invention is a multiple
channel flying spot triangulation ranger system comprised
of a laser beam source and means for deflecting and
sweeping the beam across the surface in the X direction;
multiple fiber optic light detection channels each
receiving li~ht from a different sensitive zone along
a scanned line; and a receiving lens. Separate detectors
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sense the light delivered by the fiber optic channels
and generate signal pulses, one per channel, at times
respectively related to range to the surface. Neans
are provided for processing these signal pulses in
05 parallel a~d calculating multiple range values.
Other features ~f the system are an aperture stop
plate between the receiving lens and light detection
channels having a rectangular slit to limit passage
of light and increase depth of focus. Mirror means,
such as polygons, may be provided to scan the laser
beam in the Y direction coordinated with the X direction
sweep. ~he processing means calculates range values
after each beam sweep, covering a strip of the surface,
before stepping a table to scan adjacent strips.
Brief Description of the Drawings
Fig. 1 shows the prior art swept aperture flying
spot ranging system.
Fig. 2 (prior art) are graphs illustrating the
X direction scan and electrical signal pulses generated
by the scanned beam at two surface heights.
Fig. 3 shows the improved flying spot triangulation
ranger having a multiple channel light detector.
Fig. 4 is a perspective view of the multichannel
light detector assembly.
Figs. 5 and 6 show two embodiments of the light
detector respectively having fiber optic and solid
glass channels.
Fig. 7 illustrates the scan motions used in scanning
an object.
Fig. 8 is a perspective view of an embodiment
of the improved range camera to inspect printed circuit
boards.
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Fig. 9 is a simplified block diagram showing
parallel ~rocessing of the channel light detector
signals to obtain range values.
Fi~s. lOa and lOb show the X direction acousto-
05 optic deflector scan and the electrical pulses generated
by each channel at times dependent on range.
Fig. 11 shows a portion of the multiple channel
swept aperture ranger and the required increase in
scan length.
Detailed DescriPtion of the Invention
Referring to prior art Figs. l and 2, a beam of
light L is swept by beam deflectox 10 along a line
across the observed surface 11 over the time to to
t1. At time tz within this period, the intersection
point of the beam with the surface, a spot of light,
passes through the region 12 viewed by the lens 13
and aperture 14; the latter is at the focal point of
the receiving lens. Conseguently, the scattered light
from the point reaches the detector, photomultiplier 15,
which puts out a sharp signal pulse at the time tz.
An electronic system analyzes this signal to derive
a second signal encoding the time of the pulse. Since
there is an angle, typically about 30, between the
directions of the swept beam and observed beam, the
time at which the pulse is observed is a function of
the range to the surface, and in fact varies nearly
linearly with this range. Thus this system provides
range information in the form of an encoded pulse time.
If the surface is lowered by an amount ~Z, then the
position in time of the corresponding detected electrical
signal pulse will- shift to the left in time as is seen
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by comparing lines (b) and (c) in Fig. 2. ~y measuring
the time shift ~T the flying spot triangulation system
can measure the change in range.
In practice, the light beam is swept across the
05 surface very rapidly at constant velocity, usually
by an acousto-optic modulator beam deflector, such
that range measuremen~s can be obtained very rapidly,
at rates at least up to 20,000 range measurements per
second. Nevertheless, even this high speed is insuffi-
cient for some applications, including many that are
motivating the development of 3-D cameras.
Fig. 3 shows the multiple channel optical flying
spot triangulation ranger of this invention. A signifi-
cant increase in speed is achieved by adding additional
detector channels, each of which observes the surface
at a different point along the line which forms the
intersection of the swept beam with the surface. Four
light detection channels are illustrated, but ten to
twenty channels are feasible and a reasonable upper
limit is thirty to forty channels. All of the detection
channels make use of the szme swept beam L, but there
are now N range points per bea~ sweep, where N is the
number of channels. Only the receiver channels must
be duplicated to achieve the increased speed.
A laser (not shown) generates a narrow laser beam
that is presented to acousto-optic beam deflector 16
and rapidly scanned across the surface 17 in the X
direction. A single spherical receiving lens 18 or
a compound lens serves all of the light detection channels
19-l to 19-4, the ends of which are encapsulated in
a plastic plug 20. The ends of the light trans~itting
channels extend out of the plug 20 and deliver light
to four separate ligh~ detectors 21-l to 21-4. These
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~ay be photomultipliers, certain types of photodiodes,
or other solid state light detectors. Photomultipliers
are a good choice because they have ideal light detection
qualities, i.e. excellent sensitivity and wide dynamic
05 range. Optionally there is an aperture stop plate
22 between the receiving lens 18 and the light detection
channels, having a rectangular slit 23 to pass light.
This slit is relatively large, say 0.125" x 1.000~,
and functions to increase the depth of focus and limit
passage of light, much like the f/stop on a camera.
This slit does not serve the same purpose as aperture
14 in the prior art single channel instrument, and
is not an essential part of the system. Light detection
channel 19-1 receives light only from the sensitive
zone 24-1 on surface 17, and the other three channels
respectively from the sensitive zones 24-2, 24-3, and
24-4. The shape of these sensitive zones is approximately
the same as the entrance faces of the light detection
channels and the size of the zones is governed by the
maqnification of the optical system. A square or rec-
tangular entrance face geometry is ideal; a small aperture
is desired in the X direction and the longer Y dimension
in the rectangular case gives freedom to align.
The detector head 20 is illustrated in perspective
in Fig. 4. Fig. 5 shows the use of fiber optic bundle
technology to create the adjacent fiber optic channels
19-1 to 19-4. Each channel is comprised of a noncoherent
fiber optic bundle, rectangular at the entrance and
gathered together toward the other end to make a flexible,
roughly circular bundle that can be routed to the photo-
multiplier box. There is an optical separator 25 between
the channel bundles, which is an optically opaque barrier
to prevent cross channel modulation and to adjust the
sensitive area geometry. Fig. 6 shows solid glass
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light transmitting channels 26-1 to 26-4. In practice,
it is reiterated, there are a greater number of light
detection channels than are here illustrated, and a
flying spot triangulation (FST) ranging system was
oS built having twelve channels. Each channel at the
entrance had dimensions of 2 mils by 50 mils, separated
by approximately 2 mils of opaque filler. The sensitive
zones 24-1 to 24-4 on the surface observed by the channels
(Fig. 3) had dimensions of about 1 mil by 5 mils.
The scanning of a larger area on an object by
coordinated X and Y scans, followed by indexing of
a table on which the object rests to scan other strips
on the object surface, is illustrated in Fig. 7. The
sensitive zones viewed by the multiple light detection
channels, which are stationary as is the receiving
lens and aperture stop, are shown as circles 27 in
this figure. The beam deflector 16 in Fig. 3 sweeps
the laser beam L along a line in the X direction as
was previously described. As the laser beam L enters
and crosses the four sensitive zones 27, the respective
light detection channels generate output signal pulses
at times that are a function of range to the surface.
Thus there are four range points per beam scan. Then
a Y scan mirror 28 shown schematically in Fig. 3 scans
the beam in the Y direction and a second X scan beam
scan is made to obtain four more range measurements.
If strip 1 has 500 X beam sweeps and there are 4 range
points per sweep, the total from strip 1 is 2000 range
points. Tbe table is now stepped in the X direction
and strip 2 on the object surface is scanned, and so
on.
Fig. 8 is a perspective drawing of the entire
range camera and X-Y table arrangement to inspect a
printed wiring board. These boards consist of a thin
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planar sheet on which are mounted electronic components
such as integrated circuits, resistors and capacitors.
These components are interconnected by conductors on
- the board surfaGe or by sets of conductors within the
05 plane of the board. Within a few years, the complexity
will be such that testing of the printed wiring board
assemblies can be done only through the use of automated,
visually directed inspection machines. ~he X-Y table
28 holds the printed circuit board 29 with the bottom,
lead side facing down towards the optical head and
parallel to the X-Y plane. The table scans the active
measurement region over the entire surface of the printed
circuit board. The polygon mirrors assembly 30 rotates
at a constant velocity and operates to Y scan the laser
beam and as a Y descanner for the receiving optics.
Briefly reviewing, the narrow beam generated by a laser
31 is deflected by the acousto-optic modulator 32 to
perform the X scan. The swept beam is reflected by
folding mirrors onto the first polygon mirror 33 and
hence sweeps across the surface of board 29. The reflected
beam is descanned by the second polygon mirror 34,
focused by lens 50, and reflected by a mirror to the
multichannel fiber optic bundles 35 which deliver light
to the photomultiplier box 36.
The signal processing electronics block diagram
in Fig. 9 illustrates parallel processing of the detector
output signals and computation of range values. Here
the multichannel light detector assembly 37 has three
channel~ and, looking at one channel, the others being
the same, the signal pulse from photomultiplier 38
is converted to a voltage and conditioned in amplifier
39 and digitized at 40. Digital signal processor 41
produces the best estimate of the time signal after
rejecting any possible range clutter and noise impulses,
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accounts for variations in surface reflectivity and
luminance, and calculates range or height of the surface.
The three computed range values are presented to buffer
- 42 and hence read out to the range computer 43. When
05 all the range measurements are stored, a range image
may be output. The range co~puter coordinates the
timing of the system and acousto-optic beam deflector
44, and may have other functions such as coordinate
conversion.
Referring also to Figs. 10a and 10b, the period
T of one acousto-optic sweep is, for instance, 50-60
microseconds and is divided into time segments corres-
ponding to beam angle. The start of sweep (S0S) and
end of sweep ~EOS) are sent to digital processor 41
as well as the sample clock (SC). In the three channels
the detector signal pulse occurs at times tl, t2 and
t3, respectively, measured from the start of sweep.
These times are a function of range to the surface
at those range points. As is explained more fully in
patent 4,645,917, this time determines the angle of the
swept beam and knowing the angle of the reflected beam,
optical triangulation yields the range. For avery beam
sweep, N range values are determined where N is the
number of detection channels.
The use of multiple channels provides a
practical increase in data rate by at least a
factor of 10. A practical upper limit to the total
number of channels may be in the order of 30-40
channels. One consideration is the complexity
introduced by a large num~er of detectors and time
analysis electronics. Another limit is the required
increase in scan length. This derives from the
relationship between scan length and stripe length.
The stripe length (in the X direction here) is given
.~ _g_
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by S=s*N, where s is the separation between measurement
points on .he surface. The scan length for a single
channel is given by: Ll = ~Z * tan ~, where ~Z is
the range over which the instrument is to provide range
05 ~ata. When ~ channels are used, the scan length must
be increased to approximately
LN = Ll + S (1)
where the approximation is good if Z is much greater
than ~Z. This increase insures that all of the channels
will be illuminated at some time during the sweep for
all surface ranges within ~Z.
~his situation is illustrated in Fig. 11, where
a portion of the ranger system including focusing lens
45, multiple light detection channel head 46, and fiber
optic bundles leading to separate light detectors 47
are shown (N = 6). For simplicity the light beam L
is shown scanned parallel and swept over the surface
from time to time to tl. The image of the multiple
detection channels on the surface at the closest design
range is shown at 48, and the image at farthest design
range at 49. For small values of N (typically less
than 20) S is much smaller than L, such that an insig-
nificant change in sweep length is required to accommodate
the additional channels. But as N is increased, the
sweep length must be increased substantially, according
to equation (1). This spreads out the light, weakening
the signal to each channel. Furthermore, fast scanners
such as acousto-optic cells provide a limited number
of resolution spots. I~ the scan is spread out too
far, the scanned spot will have an ins~antaneous width
greater than the resolved spot, further weakening the
signal. Despite these limitations a very large improvement
in data rate is realized within the scanned aperture
configuration.
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While the invention has been particularly shown
and described with reference to several preferred embodi-
ments, it will be understood by those skilled in the
- art that the foregoing and other changes in form and
05 details ~ay be made without departing from the spirit
and scope of the invention as defined in the appended
claims.
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