Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OPTICAL TRACXING SYSTEM
FIELD OF THE lNv~NllON
The invention relates to an optical system and method for
tracking a moving target. The system may also function to
search for a target prior to tracking it.
Further, the system may be used to assess the shape or
profile of the target surface.
BACKGROUND OF THE INVENTION
There are many applications for tracking systems, such as
the locating and trac~ing of missiles, satellites, and other
objects in space that reflect optical pulses, namely so-called
LIDAR (light radar) systems. There are other, shorter-range
applications, such as the docking or repair of satellites in
space.
In the present invention, the wavelength of light that
can be used for the optical beam need not necessarily be in
the visible portion of the spectrum, and can extend from
ultraviolet light with a wavelength as short as O.l~m, to
light in the far infrared portion of the spectrum with a
0 wavelength as long as lOO~m.
SUMMARY OF THE lN V ~:N'l'lON
The object of the present in~ention is to pro~ide an
optical trac~ing system and method that perform more rapidly
or more reliably tor both) than prior systems.
In particular, it is an object of the preferred
embodiment of the invention to achieve a system and method
that are especially insensitive to optical interference or
noise from extraneous sources.
,~
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The invention provides for transmitting an outgoing
optical beam towards a target and receiving a return optical
signal reflected by a point on the target, while determininq
from such beam and return signal the range of such target
point. This ranging can be achieved either by triangulation
or by time measurement. If the distance to the target is
greater than about 20 to 30 meters, it is usually preferred to
use radar type range finding, i.e. based on elapsed time
between transmission and reception, in which case the
transmitted beam could be pulsed, or the phase difference
between transmission and reception in the case of amplitude
modulation can be used to give the range information. In the
case of frequency modulation the beat frequency will give the
range information. If a triangulation system is to be used
for ranging, one of the numerous systems described in U.S.
patent no. 4,627,734 issued December 9, 1986 to M. Rioux can
be adopted.
Other 3-D (range finding) scanning systems are disclosed
in "Laser range finder based on synchronized scanners" by M.
Rioux published in Applied optics Vol. 23, No. 21, November
1984, pp 3837 - 3844, and in "Practical Considerations `for a
Design of a High Precision 3-D Laser Scanner System" by
F. Blais et al published in SPIE Vol. 959 Optomechanical and
Electro-Optical Design of Industrial Systems (1988)
pp 225-246.
The invention achieves its objective by scanning the
outgoing beam in a Lissajous pattern, the pattern being of
such shape and amplitude that at least one of its portions
extends across the target. Using at least three and usually
more of the return signals, it is possible to calculate the
extent to which a reference location on the pattern, e.g. the
center of the pattern, deviates from a selected location on
the target, again typically (but not necessarily) the center
of the target. This calculated deviation can then be applied
to the scanning process to modify the position of the pattern
in such a way as to move its reference location towards the
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selected target location and hence effectively track the
target.
Subsidiary features of preferred forms of the invention
that will be more fully explained below are:
1. An ability to expand and contract the pattern to
adjust the pattern on the target or to search for a
target prior to tracking it.
2. An ability to determine the profile, shape and/or
identity and/or attitude of the target.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a diagram of a circular target and a
Lissajous pattern traced out by an optical beam symmetrically
arranged on the target to pass beyond the edges of the target
and to intersect such edges as it passed over the target, in
accordance with a first embodiment of the invention;
Figure lA is a fragment of Figure 1 on a larger scale
showing this symmetry distur~ed by movement of the target;
Figure 2 is a diagram of electrical signals produced by
reflections received from the target;
Figure 3 shows diagrammatically a system for scanning an
optical beam in a Lissajous pattern;
Figure 4 shows diagrammatically one embodiment of a
driving and detecting system for use in the arrangement of
Figure 3;
Figure 5 shows diagrammatically another embodiment of a
driving and detecting system;
Figure 6 illustrates how the system can be used in a
search mode;
Figure 7A to 7F show alternative Lissajous patterns that
can be used;
Figure 8 shows an alternative to Figure 6;
Figure 9 shows a manner of use of the complex Lissajous
pattern of Figure 8; and
Figures 10 and 11 show further alternative applications
of the invention.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 shows a target J (here assumed to be a flat
disc) towards which a light beam is transmitted while being
scanned in a simple Lissajous pattern L. Lissajous patterns
are generated by two sine (or cosine) waves of different
frequencies. In the case of Figure 1, the frequencies bear
the ratio 2:1 in the X and Y directions. There is also a
phase difference of 90.
The pattern L intersects the edges of the target J at
successive edge points P1, P2, P3 and P4. Assuming that
whenever the transmitted beam strikes the target J there is
reflected light by which the range T of the target can be
judged, whereas when the transmitted beam misses the target
there is no reflected light (infinite range T') or light is
reflected from some object more remote than the target from
the transmitter and hence distinguishable by reason of its
different range (somewhere between T and T'). Resulting
electrical signals are shown in Figure 2, i.e. square pulses
Sl and S2 beginning respectively at points Pl and P3 and
ending at points P2 and P4. The point of intersection R of
the two portions of the Lissajous pattern that extend across
the target will coincide with a selected location on the
target, for example, its center Q. In this case the point R
represents the reference location on the pattern. In
practice, as the target moves, a situation such as illustrated
in Figure lA develops, with the intersection point R deviating
from the location Q by distances oX and ~Y at an angle e. If
N is the number of points around the pattern L starting from a
point O, then the values of ~X and ~Y are given by:
AX=(~P12+~P34)cos~,
and
~Y=(AP34-AP12)sin~
where
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~P12= Pl +P2 _ N
A p3 4 = P3 + P4 5 N
The angle e is known from ~X and ~Y
For a selected reference amplitude A of the pattern L,
the deviation of the actual amplitude from this reference is
given by
/~A= (P2-Pl ) + (Pg-P3) -A
Figure 3 shows a driving and detecting system for
generating and scanning transmitted and reflected optical
beams. The system 10 directs a beam 11 to a first mirror 12
that is rotatable about an axis 13 by a motor or galvanometer
14 (hereinafter referred to as a motor) to produce a beam 15
that travels to a second mirror 16 that is rotatable by a
motor or galvanometer 17 (hereinafter referred to as a motor)
about an axis 18 that is perpendicular to the axis 13. The
final beam (not shown, since it extends perpendicular to the
plane of Figure 3) is transmitted to the target. The
reflected signal is assumed to follow essentially the same
path in reverse and is received in the system 10.
The motor 14 which controls scanning in the X direction
is oscillated by an output 20 of a first sinewave generator
21, multiplied in multiplier 22 by an output 23 of an
amplitude setting circuit 24 that is basically set to generate
the reference amplitude A and receives the ~ value as an
input. The output 25 from the multiplier 22 is received in an
adder 26 which receives a signal 27 from an offset circuit 28
that receives the ~X value as an input. The output 29 of the
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adder 26 controls the motor 14 and hence the angle of the
mirror 12.
A similar system for controlling scanning in the Y
direction by the mirror 16, consists of a second sinewave
generator 30, an amplitude setting circuit 31, a multiplier
32, an offset circuit 33 (that receives the ~Y value), and an
adder 34.
The two waveform generators 21 and 30 are controlled by
respective frequency selectors 35 and 36, through multipliers
37 and 38, controlled by a clock 39. The selected phase
difference between the outputs of the sinewave generators will
be set by means of a phase adjustment circuit 40.
Because of the inertia of the mirrors 12 and 16 and the
motors 14, 17 driving them, in order to achieve a high
oscillation rate of these mirrors, it is necessary to drive
the motors 14, 17 with high amplitude outputs from the
circuits 24, 31. The result is that the actual oscillation of
each mirror lags behind the input signal to its motor, but
nevertheless remains sinusoidal, and, by virtue of the large
amplitude, attains the full travel required. The lag can be
compensated for by adjusting the phase in the circuit 40. For
example, a mirror/motor assembly that has an inherent band
width of 40 Hz can be caused to oscillate as high 500 Hz by
these means. This ability to achieve high-speed scanning is
an advantage of the Lissajous figure operation because such
figure is made up solely of sinusoidal components. Scanning
in any figure not solely comprising sinusoidal components
would be distorted by the lag which could not be compensated
for by a simple phase shift and a simple amplitude change.
There are various ways in which the driving and detecting
system 10 can be constructed. As shown in Figure 4, the
transmitted beam can be generated by a laser 41 and the return
signal detected by a detector 42, these devices being arranged
closely side-by-side. The laser 41 and detector 42 will be
controlled by a microprocessor 43 that will determine the
range as well as generating the values of ~X, ~Y, and ~ in
accordance with the equations above.
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Figure 5 shows a system based on Figure 2 of the Rioux
patent mentioned above, a light source 50 (preferably, but not
necessarily, a laser) emitting a beam 51 to a first scanning
mechanism 52 that forwards a beam 53 to a point 54 on a target
55. A reflected signal 56 is passed through a scanning
mechanism 57 and a lens 58 to strike a position sensitive
detector 59 at a point 60, the location of which is indicative
of the range of the point 54. The two scanning mechanisms 52,
57 can be synchronised to follow the same Lissajous pattern by
0 a circuit such as in Figure 3, shown in Figure 5 as a scanning
control 61. Alternatively, a single scanning mechanism can be
used for both the outgoing and return beams as demonstrated in
any one of Figures 4-6 or 9-15 of the Rioux patent, preferably
the system shown in Figure 12 of this patent.
When the system is to be used to locate a target J not
initially intersected by the pattern L (Figure 6), the
amplitude of the pattern can be increased, as shown at L1,
until intersection takes place. The system can then revert to
the normal tracking mode, bringing the Lissajous pattern onto
0 the target and reducing its amplitude to a value such as shown
in Figure 1. In the embodiment of Figure 1 in which the
pattern L has portions that extend beyond the edges of the
target, it is usually preferred that the distance from P1 to
P2 (and the distance from P3 to P4) be about~25~ of the total
length of the Lissajous pattern, so that the beam strikes the
target about 50~ of the time and misses it about 50~ of the
time, although these values can be varied to suit different
circumstances, especially irregularly shaped targets.
In addition to, or instead of, searching for a target by
0 this expansion of the Lissajous pattern, the entire pattern
can be moved either in a random manner or in a regulated scan,
such as a raster type scan, to bring the pattern near enough
to the target to achieve at least initial contact.
It will be appreciated that Figure 2 shows a series of
signals that determine range. It is also possible to add some
intensity measurements to the range measurements. For
example, if the target J were to have a portion, such as a
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central portion F (Figure 1), of a different colour from the
remainder of the target surface, the return signals from the
target as the beam pattern passes onto and off from the
portion F will provide intensity signals that appear basically
the same as those in Figure 2. In this case, while the
primary assessment of the target is still based on range, it
is possible to simultaneously employ intensity data to provide
additional information about the nature of the target surface.
To this point, a simple, so-called 1:2 Lissajous pattern
has been discussed. In practice, it may often be desirable to
employ one of the more complex Lissajous patterns that will
produce more than one intersection. Figures 7A to 7F
respectively show patterns generated by frequency ratios of
1:3, 2:3, 3:4, 3:5, 4:5 and 5:6, all with the two sine waves
out of phase with each other. Although not illustrated ratios
of 6:7 or 7:8 or higher can be used. The preferred phase
re`lationship is either 90 or 180. While one of these pha,e
relationships is not essential, it is desirable, since it
produces fully symmetrical patterns, whereas other phase
relationships tend to produce patterns with less symmetry.
The phase relationship referred to is the initial phase
relationship at the beginning of a scan. Since the
frequencies differ, the phase relationship will differ during
a scan.
A much faster method of operating the system in the
search mode is shown in Figure 8. Here the pattern of Figure
7E is used to cover a large portion of the field of view. If
the target J falls within one of the spaces of this pattern,
it can quickly be detected, either by a small movement of the
pattern in any direction or by a contraction of the pattern.
Thus, in the search mode the pattern can be scanned over the
entire search area or it can be repeatedly contracted and
reexpanded to cover the entire search area. Or a combination
of these techniques can be used.
Similarly, the complex Lissajous patterns such as those
in Figures 7A to 7F can be used to improve the accuracy of the
tracking by having more points to calculate the error signals
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~, ~X, and ~Y, such as in Figure 9. Figure 9 shows the
pattern contracted so as to produce a large number of edge
points P where portions of the pattern cross onto or leave the
target J.
The system can be used to track a number of targets
simultaneously using the same driving and detecting system and
the same mirrors. To achieve this result with two targets,
the arrangement of Figure 3 would be augmented to have two of
each of the elements 24, 28, 31, 33, 35, 36 and 40, with
switching means for switching over simultaneously and
repeatedly from one set of these elements to the other, so
that the targets would be scanned alternately. One set would
produce a first Lissajous pattern tracking the first target,
and the other set would produce a different Lissajous pattern
tracking the second target. A number of targets larger than
these could be tracked simultaneously in this ~ay by providing
a corresponding number of such sets.
An important characteristic of Lissajous patterns is
their relatively uniform density. Rosette patterns have in
the past been used for tracking systems, because they have a
very high center density. A Rosette pa~tern is especially
s-~itable for detecting a relatively sr.lall target at a long
dist~nce, but is less efficient at tracking the target once it
has bee.' found because so ~uch of the pattern is concentrated
at or n~_ar i~ cent~~. Because a Lissajous pattern is more
uniformly distributed than a Rosette pattern, it has faster
performance, a feature that can be very important when
track ng a fast moving target. A Lissajous pattern scan is
a~so ~nore accurate than a raster scan or a vector scan.
,~n important adv.lntage of a laser scanner over a
cor,v~ntional TV camera approach for tracking satellites or
S~ace shuttles is its i,~sensitivity to background
illumination, such as light coming from the earth, and
especially from the sun, and its reflections, Good immunity
from background noise can be obtained, due to the fact that
laser light is emitted over a very narroW band of fre~uencies
which can be tuned by filters, and the possibility of having a
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small instantaneous field of view looking only at the target.
At close range, the two techniques (laser scanner and TV
camera) are comparable in terms of range accuracy. At longer
range, photogrammetry is still more accurate but suffers from
background light interference. Because the laser scanner has
the capability to identify and locate a satellite, even with
the sun shining directly into the sensor, integration of these
techniques enables them to benefit from one another.
In addition to tracking a target and determining its
range, the present invention can also be used to assess the
shape of the target.
Figure 10 shows a hemispherical target J' and Figure 11
shows a pyramidal target J" each scanned with a Lissajous
pattern L" of the type shown in Figure 7B. In addition to the
central intersection point P0 of this pattern, the micro-
processor 43 can be employed to measure the range of each of a
relatively large number of points on the target, such as
points PA, PB, PC, these points not necessarily being at
intersections of portions of the Lissajous pattern L". When,
as in Figures 10 and 11, the shape of the target is such that
changes of range will occur between almost every adjacent pair
of measured points, there is no need for the pattern to have
portions that extend beyond the edges of the target, and it
will be noted that the pattern L" is fully within the targets
J' and J". The range data will both enable identification of
the deviation of the central intersection point P0 from the
selected location on the target, such as the pinnacle Q' of
the hemisphere J' or the apex Q" of the pyramid J" to cause
the target to be tracked in a lik~ fashion to that already
described, and enable the microcomputer to assess the three
dimensional surface shape of the target. The more measurement
points that are used, the more refined this shape measurement
will be.
The invention can be practiced in some circumstances,
e.g. when the target is of a shape such as shown in Figure 10,
with a 1:1 Lissajous pattern with an initial phase relation-
ship of 180, namely a circular pattern. Another example of a
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11
situation in which a circular pattern would be appropriate is
when the target has a generally cruciform shape, each of four
segments of the circle intersecting a respective arm of the
target. Hence in this application including the claims, the
term "Lissajous pattern" includes a circle.
In the patterns of Figures 7A to 7F, the chosen reference
location on the pattern need not necessarily be a point where
two portions of the pattern intersect. For example, in Figure
7A or 7D, the reference location could be the geometrical
center of the pattern, even though such center is blank. On
the other hand, in Figure 7D for example the reference
location on the pattern could be one of the intersections,
preferably one of the intersections near the geometric center
in order that the system should track the target with the
center of the pattern reasonably close to the selected
location on the target. When the Lissajous pattern is a
circle, i~ will be appropriate to chose its center as the
reference location.
The minimum number of measurement points from the
viewpoint of tracking is three, but since this relatively
small number of points will yield only very crude data on the
target shape, a much larger number will usually be used in
practice when shape assessment is required. In a case where
the shape of the target is known, but its dimensions are
unknown, as in Figure 1, the minimum number of measurement
points is four. If the dimensions are known, then this number
can be reduced to three by using the range information to
evaluate the necessary amplitude of the scan.
As well as assessing the surface shape, the system can
assess the profile of the target, i.e. the edge outline as
seen from the transmitting and receiving location. For this
purpose the Lissajous pattern will have its amplitude adjusted
to extend beyond such edge, as in Figure 1, a relatively
complex Lissajous pattern preferably being employed in order
to provide a relatively large number of points where the
pattern intersects the edge and hence provide sufficient data
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12
to be able to achieve a comparatively detailed assessment of
the profile.
If the target profile is generally circular, it may be
useful to fit the scanning pattern to the target using one of
the minimum least square methods or the Robust Fitting method.
See, for example, "Segmentation of Geometric Signals using
Robust Fitting" by G. Roth et al published in Proceedings of
the Tenth International Conference on Pattern Recognition,
Atlantic City, N.J., June 16-l9, 1990 pp 826-832. If the
target profile is irregular, either the Robust Fitting or the
Hough Transform technique is preferred. These techniques are
valid for either 2D or 3D data, i.e. when either only the
edges of the target or all the range data of the visible
surface of the target are measured.
The microprocessor can compare the calculated profile
and/or surface shape of the target with stored data on the
- shapes of known objects to achieve identification. Assuming
that data on the three dimensional shape of the target, say a
satellite, is known, the profile and/or surface shape seen by
the system can be compared with such data to indicate the
attitude of the target.
When a satellite is the target, a Lissajous pattern can
be employed to surround the entire satellite, or alterna-
tively, it can be directed at only one part of the satellite,
such as a corner of a solar panel where the edges of the
target will be especially well defined, i.e. such corner will
become the selected target location. If desired, the target
can be provided with reflectors of known shape mounted at
known locations on the target to enhance the return signals,
although it is one of the features of the present invention
that reflectors are not essential, and for many targets would
be unnecessary.
