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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for measuring the
profile of a three-dimensional object and, more particularly, to
improvements in a method and apparatus for measuring a profile in a
noncontact manner in which a light beam is projected onto the surface of a
target and scanned over the surface of the target and the profile of the
target is measured from the optical image of the surface of the target.
2. Description of the Prior Art
Applications of a method of measuring the profile of a three-dimensional
object utilizing an optical system have been expected not only in the
fields of CAD/CAM and computer vision and as the eyes of robots, but also
in somatometry and measurement and analysis of natural objects in the
medical and dressmaking fields, and in other various fields including
graphic design.
A stereo system is known as a typical system in several optical methods of
measuring a profile. According to the stereo system, photographs of a
target are taken from a plurality of visual angles by a plurality of
industrial cameras, and a profile is obtained from these images.
This system is based on the principle of binocular stereoscopy, and the
image data of the taken photographs are input in the form of lightness
signal data over the entire imaging plane. In order to extract only a
necessary profile from these data, it is inevitable to detect the
correspondnng points, which requires various kinds of image processings
and, hence, a large amount of memory capacity and a long processing time.
Therefore, this system has not been embodied as a high-speed and simple
apparatus.
Among other conventional systems, a light intersection system is most
general and has been considered to be considerably practical. In this
light intersection system, a spot-like or slit-like light beam is
projected to a target, and image signals based on the optical image of the
surface of the target are input to a computer by an image grabber. Then,
from the positional information on the optical image on the imaging plane
obtained as a result of processing these signals and the relative
positional relationship between the light beam and the image grabber, the
space coordinates of the surface of the target are obtained.
That is, according to the conventional light intersection system, for
example, a light beam deflected for scanning is projected onto the surface
of a target, and the optical image of the surface of the target formed by
the light beam is inputted to a computer in the form of image signals by a
scanning type image input device such as an ITV camera or a CCD camera.
As a result, according to this conventional system, the position of the
optical image of the target is specified by subsequently electrically
scanning the entire imaging plane, and this procedure is repeated for each
light beam deflected for scanning. The profile of the three-dimensional
object is measured from the multiplicity of data obtained in this way.
The conventional light intersection system, however, is disadvantageous in
that it is necessary to scan the entire imaging plane every time each
point of the surface of a target is detected and specified and, hence, it
takes a very long time to measure the profile, thereby making it
impossible to measure the profile in real time. Generally, the time
required for scanning one field is approximately 1/60 to 1/30 second in
the case of an ordinary industrial TV camera, and such slow scanning
operation makes it almost impossible to measure the profile of an object
at real time or to measure a moving object.
Especially, in order to measure a three-dimensional object with a
practicably sufficient resolving power, it is necessary to scan many
fields, so that very slow scanning by deflecting a light beam is necessary
in the conventional light intersection system which requires the
electrical scanning of the entire imaging plane, as described above. It is
therefore impossible to measure the profile with sufficient resolving
power at the real time.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to eliminate the
above-described problems in the prior art and to provide an improved
method and apparatus for measuring the surface profile of a target at a
high speed by using a light beam.
To achieve this aim, namely, to obtain the surface profile of a target at a
high speed and in a noncontact manner, a method and apparatus according to
the present invention are characterized in that the surface of a target is
stroked over with a slit-like light beam (hereinunder referred to as
"slit-ray"), and the positional information on the optical image of the
surface of the target and the identifying information on the slit-ray are
detected without repeating electrical scannings on the entire imaging
plane as in the prior art, thereby enabling the surface profile to be
subsequently measured at the real time.
The process of stroking over the surface of the target by slit-ray
resembles the conventional light intersection system, but a method
according to the present invention is characterized in that while the
surface is stroked over with the slit-ray, the positional information on
the imaging plane of the optical image of the surface of the object
corresponding to the slit-ray is detected without electrical scanning of
the entire imaging plane, and in that all the image addresses
corresponding to each optical image are simultaneously detected.
The imaging plane is constituted by a plurality of photosensors. The
positional information of an optical image on it is detected in real time
on the basis of the output signal of each photosensor individually from
another photosensor when it receives the light caused by the optical
image. The surface profile is subsequently obtained from this positional
information and the identifying information on the slit-ray itself. Thus
high-speed processing is enabled and the scanning time for stroking over
the target with the slit-ray is greatly reduced in comparison with the
conventional system in which electrical scanning of the entire imaging
plane is repeated for each projection of a light beam, so that practically
efficient measurement of a moving object is also enabled.
A method of stroking over the object with a spot-like light beam may also
be considered to be suitable, but in this case, the entire surface of the
target must be scanned two-dimensionally. Such scanning requires a
considerable time and it cannot therefore be expected that the measuring
time is greatly reduced. In contrast, the method of using slit-ray
described above enables one scanning to stroke over the entire surface of
the target, thereby greatly reducing the measuring time. Therefore, the
use of a slit-ray is preferable in the present invention. The slit-ray is
deflected for scanning according to a predetermined scheme. This light
deflecting operation for scanning is preferably carried out by rotating
light source or a mirror. Needless to say, that operation with slit-ray in
the present invention may be carried out by other scanning systems, for
example, by moving the slit-ray in parallel or in a fan shape toward a
predetermined point.
An identification signal on a slit-ray is used to identify the position of
the slit-ray deflected for scanning. For example, if the slit-ray is
fixed, a constant identification signal corresponds to the slit-ray, while
at under scanning, different identification signals correspond to the
different positions of the deflected slit-rays.
To state this more concretely, when the slit-ray is deflected for scanning
by rotating a mirror, the identification signal may be electrically
detected as an rotational angle signal which represents the rotational
angle of the mirror. When the slit-ray is deflected for scanning at a
constant speed, the elapsed time counted from a predetermined reset timing
position may be output as the identification signal. In this way, it is
possible to easily detect and output identifying information on the
slit-ray in real time by using a timer, a counter, and so forth.
When the slit-ray is projected onto the surface of the target in this way,
the fracture of the optical image of the surface of the target caused by
the slit-ray is dependent on the optical system which receives the
reflected light of the slit-ray from the surface of the target.
Accordingly, in the present invention, the optical image is formed on the
imaging plane of a non-scanning type image sensor. The imaging plane of
this non-scanning type sensor is constituted by a multiplicity of mutually
independent arrayed photosensors. Therefore, the positional information on
the optical image on the imaging plane corresponds to the address of the
photosensor. Since each of the photosensors detects the light caused by
the optical image and outputs a signal individually from another sensor,
the above-described merits obtained by the use of the slit-ray are
utilized. When the optical image on the imaging plane moves with the
scanning of the slit-ray over the surface of the target, the positional
information on the moving optical image is detected by means of a light
response output signal from each photosensor constituting the imaging
plane. Thus, in the present invention it is possible to obtain the
positional information on the optical image at the real time without
electrically scanning the entire imaging plane.
If it is assumed that the slit-ray projection device, the non-scanning type
image sensor and the target are fixed respectively at a certain moment,
the position of the optical image of the surface of the target on the
imaging plane produced by the slit-ray is definitely specified by the
surface profile of the target itself. Therefore, it is very easy to
measure the surface profile from the identifying information on the
slit-ray and the positional information on the optical image of the
surface of the target on the imaging plane of the image sensor obtained in
the above-described way.
Thus, according to the method of the present invention, since it is
possible to obtain the desired information at the real time without
electrically scanning the image sensor for each slit-ray, high-speed
measurement of the surface profile of a three-dimensional object is
enabled.
In the embodiments of the present invention, slit-ray is formed by using a
laser or the like and is projected to a target by a light deflector and
scanner. The reflected image from the surface of the target produced by
the slit-ray is formed on the imaging plane which is composed of mutually
independent arrayed photosensors. The identifying information on the
slit-ray is stored in the array of storage cells consisting of analog or
digital memories, flip-flops, shift registers, etc. which correspond to
the respective photosensors, triggered by the light response output signal
of the photosensor supplied when the optical image passes each photosensor
on the imaging plane. Accordingly, the address of each memory element
corresponds to the positional information on the optical image on the
imaging plane and the data which the memory element holds corresponds to
the slit-ray identifying information.
As described above, in the present invention, a slit-ray is projected onto
the surface of a target to stroke over the surface thereof during scanning
by deflecting the slit-ray. The optical image at the imaging plane caused
by that slit-ray projection moves on the imaging plane in correspondence
with scanning of the slit-ray. The position of this moving optical image
on the imaging plane is detected at the real time by means of the light
response output signal from each photosensor constituting the imaging
plane, without repeating the electrical scanning over the entire imaging
plane as in the prior art. From this positional information and the
slit-ray identifying information which is also detected in real time, the
space coordinates of the surface of the target are specified. Thus, the
present invention enables the configuration of a measuring apparatus which
is sufficiently capable of following high-speed scanning of slit-ray and,
hence, enables high-speed measurement of the surface profile of a
three-dimensional object.
The above and other objects, features and advantages of the present
invention will become clear from the following description of the
preferred embodiments thereof, taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory. view of an embodiment of a measuring apparatus
used in a method of measuring the surface profile of a three-dimensional
object according to the present invention;
FIGS. 2 and 3 are explanatory views of the principle of the method of
measuring the surface profile of a three-dimensional object using the
apparatus shown in FIG. 1;
FIG. 4 is an explanatory view of an example of preferred image sensors
suitable for the apparatus shown in FIG. 1;
FIG. 5 is a more detailed explanatory view of another example of image
sensors which resembles that shown in FIG. 4;
FIG. 6 shows the arrangement of image sensors of an imaging device; and
FIG. 7 is an explanatory view of another example of imaging devices which
use an optical fiber.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be described hereinunder with
reference to the accompanying drawings.
FIG. 1 shows a fucctional structure of a system for measuring the surface
profile of a target by a light beam consisting of slit-ray.
The laser beam from laser source 1 is perpendicularly magnified by a lens
system 2 including a cylindrical lens and constitutes slit-ray, which is
projected onto the surface of a target 4 through a polygonal mirror 3. The
slit-ray is deflected by projection angle sequentially by the rotation of
the polygonal mirror 3, as shown in FIG. 1, whereby the surface of the
target 4 is stroked over. The deflection angle of the slit-ray is
definitely determined by the rotational angle of the polygonal mirror.
Therefore, the slit-ray may be identified by directly measuring the
rotational angle of the polygonal mirror. Alternatively, since the
polygonal mirror is ordinarily controlled to have rotational movement at a
constant angular velocity and the slit-ray is thereby also deflected for
scanning at a constant angular velocity and the slit-ray is thereby also
deflected for scanning at a constant angular velocity .omega., it is
possible to identify the slit-ray by measuring the elapsed time t from a
reset (trigger) signal 6 which is output when the slit-ray passes a
certain reference position. Since the identifying information on the
slit-ray deflected in this way is dependent upon the time t, the time t is
represented as the identifying information on the slit-ray and is used in
the later calculation. The slit-ray itself is represented by n(t) in FIG.
1.
In the example shown in FIG. 1, a photosensor 5 such as a phototransistor
is provided for detecting a reference position, and when the slit-ray
passes across the photosensor 5, the reset (trigger) sigaal 6 is output,
which triggers a timer 7 and a clock counter 8. Thus, the clock counter 8
outputs a signal representing the elapsed time t which gives the
identifying information on the slit-ray n(t) in real time.
It will therefore be understood that according to the present invention,
the deflection angle of the slit-ray n(t) which strokes over the surface
of the target 4 is specified by the identiyying information of the elapsed
time t.
The reflected light which is reflected from the surface of the target 4 is
received by an imaging device 9, and the slit-like optical image of the
surface of the target 4 which corresponds to the slit-ray n(t) is formed
on the imaging plane 11 of an image sensor 10.
In this embodiment, the image sensor 10 is composed of a non-scanning image
sensor such as a picture synchronous image sensor, and the imaging plane
11 is composed of mutually independent one-dimensionally or
two-dimensionally arrayed separate photosensors. Each photosensor
corresponds to each picture element. Since the image information on the
imaging plane 11 is individually detected for each picture element,
parallel processing is enabled. Therefore, when the slit-like optical
image moves on the imaging plane 11, it is possible to detect the
positional information on the entire slit-like optical iaage, for example,
all the addresses of the picture elements corresponding to the slit-like
optical image, simultaneously and in real time, on the basis of the output
signal supplied when each photosensor on the imaging plane 11 receives the
light caused by the optical image.
Thus, the positional information on the optical image of the surface of the
target 4 is detected without electrically scanning the imaging plane 11
each time as in the prior art.
The identifying information t on the slit-ray n(t) detected in the
above-described way and the corresponding positional information on the
imaging plane 11 on the optical image of the surface of the target are
coordinatingly latched in a memory unit 12 of the image sensor 10, and is
operated by a combine processing part 13 to be converted to the space
coordinates of the surface of the target.
In other words, according to the present invention, it is possible to
coordinatingly detect both the identifying information t on the slit-ray
n(t) and the corresponding positional information on the slit-like optical
image in real time, and thereby to measure the surface profile of the
object 4 at a high speed.
Referring to FIGS. 2 and 3, it is shown that the surface profile of the
objcct 4 in the embodiment shown in FIG. 1 is specified from the
identifying information on the slit-ray and the positional information on
the optical image. In FIGS. 2 and 3, the geo-optical relationship between
the slit-ray n(t) reflected from the center M (xm, ym, zm) of mirror
reflection shown in FIG. 1, the point P (X, Y, Z) on the surface of the
target 4 which is irradiated with the slit-ray n(t), and the optical image
I (x, 0, zi) on the imaging plane is projected on the plane xy (FIG. 2)
and the plane xz (FIG. 3).
The solid lines in FIGS. 2 and 3 denote a state in which the target is
scanned with a single slit beam by using a single light projection device,
and the dot-dashed lines denote a state in which the target is scanned
with a second slit-ray beam by providing a second light projection device
at a different position from that of the first one. By using two light
projection devices in this way, it is possible to greatly reduce the
unmeasurable surface area of a target having a projection or a recess,
which would be in the shade if the target is scanned with a single
slit-ray.
In FIGS. 2 and 3, the origin of the orthogonal coordinate system (x, y, z)
is set, for example, at the center 0 of the imaging plane 11, the axis x
is set in the horizontal direction and in parallel to the imaging plane
11, the axis y is set in coincidence with the optical axis and the axis z
is perpendicular to the xy plane.
Therefore, the coordinate value (X, Y, Z) of the point P on the surface of
the target 4 is specified by the slit-ray n(t) and the reflected light
R(t), as shown in the drawings, and is obtained in the following way by
using the coordinate values of the positions of the lens L and the mirror
M, and the angular velocity .omega. of the slit-ray, all of which are set
as the measuring conditinns, and the identifying information t on the
slit-ray n(t) and the coordinate value (xi, 0, zi) of the optical image I
of the point P, all of which are obtained as the results of measurement:
O: the center of the imaging plane (the origin of the orthogonal coordinate
system)
O (0, 0, 0)
L: the center of the lens of the imaging device 9
L (0, y.sub.1, 0)
M, M': the center of mirror reflection
M (xm, ym, zm)
M' (xm , ym', zm')
P, P': the point on the surface of the target
P (X, Y, Z)
P' (X', Y', Z')
I, I': the optical image of the points P, P'
I (xi, yi, zi)
I' (xi', yi', zi')
I (xi, yi, zi)
I' (xi', yi, zi')
P.D.: the reference point of a light beam (the position of counter set
timing)
.alpha..sub.0, .alpha..sub.0' : the angle between the reference position of
the slit-ray and the axis x
.alpha. , .alpha.'" the angle between the slit-ray and the reference
position of the slit-ray
.omega. ,.omega.': the angular velocity of the slit-ray
t, t': the elapsed time after the slit-ray has passed the reference
position
subscript xy, subscript xz: the projected point of each of the points L, M,
M', P, P', I' I on the plane xy and the plane xz, respectively
##EQU1##
In the equations (2) and (4), .alpha. is a product of the angular velocity
.omega. of the slit-ray and the identifying information t, namely,
.alpha.=.omega. t, and as a result, the equations (2) and (4) are
represented as follows:
##EQU2##
From the equations (3), (5) and (6), it will be understood that the
three-dimensional coordinate value (X, Y, Z) at the point P on the surface
of the target 4 is determined by both the identifying information on the
slit-ray n(t), namely, the elapsed time t from a predetermined reset
timing and the positional information (xi, 0, zi) on the optical image I
on the imaging plane 11.
FIG. 4 shows an example of a non-scanning type image sensor for latching
and storing the identifying information on the slit-ray n(t) and the
positional information on the optical image on the imaging plane 11 of the
image sensor 10.
The non-scanning type image sensor is composed of the imaging plane 11 and
the memory unit 12.
In FIG. 4, the imaging plane 11 is composed of a plurality of mutually
independent arrayed phototransistors 15.
As is clear from the above explanation, the light reflected from the target
4 is received by the imaging device 9, and the optical image of the
surface of the target 4 is formed on one of the phototransistors 15 on the
imaging plane 11.
The light response output of the phototransistor 15 is supplied to the
memory unit 12.
The memory unit 12 is composed of storage cells 16 arranged in
correspondence with respective phototransistors 15 on the imaging plane
11. Each of the storage cells 16 of the memory unit 12 is composed of, for
example, D type flip-flops of a plurality of bits or shift registers. The
memrry unit 12 may also be composed of analog storage cells.
In any case, the storage cell 16 is provided in correspondence with the
phototransistor 15, and into its write control input terminal is input a
light response output from the corresponding phototransistor 15. Further,
the identifying information 14 on the slit-ray n(t) is input to a data
input bus 17 of the storage cell 16.
In this example, the identifying information 14 consists of the output of
the counter 8 which starts counting in accordance with the set trigger
signal 6 obtained from the light projection device. As described above,
the counter 8 starts to count from the initial position of the polygonal
mirror 3, and the elapsed time signal t, which is the output of the
counter 8, is supplied from the input bus 17 to each storage cell 16 as
the identifying information on the slit-ray n(t).
Therefore, the phototransistor 15 on which the optical image is formed
outputs a light response signal, and triggers the corresponding storage
cell 16. The thus-triggered storage cell 16 latches the corresponding
elapsed time as the identifying information on the slit-ray n(t).
In other words, when the surface of the target 4 is stroked over by the
slit-ray n(t) deflected for scanning, each identifying information on the
slit-ray n(t) is stored and held in the storage cell 16 corresponding to
each position of the formed image during scanning. At this time, since the
address of each storage cell 16 provides this positional information on
the optical image on the imaging plane 11, the above-described process is
a process for storing and holding the identifying information on the
slit-ray n(t) coordinatingly with the positional information on the
corresponding optical image on the imaging plane.
Since this information storing process does not include any process for
electrically scanning the image sensor unlike the prior art, it is quite
possible for the scanning of the surface of the target to catch up with
high-speed scanning of the surface of the target 4 with the slit-ray.
The data stored in this way are read from each storage cell 16 by a known
method, and is converted to the space coordinates (X, Y, Z) of the surface
of the three-dimensional object on the basis of the above-described
measuring principle.
FIG. 5 shows another example of the image sensors which resembles the one
shown in FIG. 4. In this image sensor, mutually nndependent arrayed
photodiodes are used.
Since the image snnsor 10 is composed of arrayed photodiodes 20, the output
thereof is amplified by transistors 21. Each of the photodiodes 2 and a
resistor 22 are conneoted to the base and the collector of each transistor
21 in series. The emitter of each transistor 21 is grounded.
The memory unit 12 composed of D type flip-flops or resistors, as is the
case that shown in FIG. 4, is coordinatingly connected to each group of
the photodiode and transistor, and the collector terminal of the
transistor 21 is connected to the write control input terminal of each of
the storage cell 16.
Therefore, when an optical image is formed on the receiving surface of the
photodiode 20 on the imaging plane of the imaging device, the transistor
21 which is connected to the photodiode 20 is switched on, and the output
voltage of the collector is changed from the "H" level to the "L" level,
whereby the coreesponding storage cell 16 is triggered and the time data
representing the identifying information of the slit-ray which is
connected to the data input bus 17 is latched and stored, in the same way
as in the case shown in FIG. 4.
In the image sensor shown in FIG. 5, both the identifying information on
the slit-ray and the positional information on the optical image are also
stored and held without delay and, as a result, it is possible to control
the speed of scanning the surface of the target 4 by the slit-ray n(t) at
a high speed.
The non-scanning type image sensors shown in FIGS. 4 and 5 can be
integrated into IC chips in the present art, and there is every reason to
expect a miniaturized image pickup device.
However, in the case of constituting a non-scanning type image sensor
composed of two-dimensionally arrayed photosensors by combining ordinary
individual elements, signal processing elements and storage cells occupy a
large area, so that the image sensor may actually become large-sized. The
configuration shown in FIG. 6 will solve such a problem.
In FIG. 6, the image sensor includes a group of M.times.N picture element
photosensors which are two-dimensionally arrayed to constitute the desired
imaging plane. These photosensor group are provided with M row addresses
i=0, 1, 2, . . . M-1, and N column addresses j=0, 1, 2, . . . N-1, as
shown in FIG. 6.
These photosensor group is divided into sub-groups in accordance with the
row address i, and are selectively energized by a switching element 31
such as a multiplexer. When the divided sub-groups of photosensors are
selectively energized by the switching element 31, they are controlled by
a row address signal which is supplied from a row address signal line 32
to a switching element 31.
On the other hand, the output terminals of the photosensors of a sub-group
having the same column address j are connected to a common output line 33.
In other words, N sub-groups of photosensors having the same column
address j are formed by N common output lines 33.
The outputs of the photosensor sub-groups fetched from the respective
common output lines 33 are supplied to the write control input terminals
of the respective memory units 35 through respective signal processing
elements 34. The number of the signal processing elements 34 and the
memory units 35 provided are respectively N in correspondence with the
common output lines 33.
A row address signal is supplied from the row address signal line 32 to the
address bus of the memory unit 35, and a slit-ray identifying signal is
suppled to a data bus 36.
In the imaging device shown in FIG. 6, as soon as the desired address group
is selectively energized on the basis of a row address signal, the storage
cell which corresponds to the selected row address in the memory unit 35
is energized.
In this state, when the surface of the target is scanned once by the
slit-ray, the slit-ray identifying signal, which is output at the moment
when the optical image has passed the picture element photosensor of the
row address group which is selectively being energized, is stored and held
in the memory unit 35, as described above.
Accordingly, all information necessary for measuring the profile of the
target is obtained by repeating this cycle of scanning M times with
respect to each row address group.
In the imaging device shown in FIG. 6, since the signal processing element
and the memory unit are not required for each picture element photosensor
unlike the example shown in FIG. 4, and the number of the output lines is
greatly reduced, it is possible to greatly reduce the size of the device,
which is very useful for a device which is composed of individual
elements.
FIG. 7 shows an example of imaging devices adopting an optical fiber which
are suitable for the present invention. One end surface of each of the
optical fibers 51 is arrayed on an imaging plane 50, on which the optical
image is formed. The other end of the optical fiber 51 is led to the
arranging surface of arrayed photosensors 52. The arrayed photosensors 52
are actuated by the light introduced through the optical fibers 51.
Accordingly, in the example shown in FIG. 7, even if the arrayed
photosensors 52 are large, it is possible to make the imaging plane 50
itself compact, thereby enabling the imaging part of the device to be
miniaturized.
As described above, according to the present invention, the surface of a
target is stroked over by slit-ray, and the positional information on the
optical image which moves on the imaging plane with the movement of the
scanned light is detected at the real time on the basis of an output
signal supplied when each photosensor which constitutes the imaging plane
receives the light caused by the optical image, without the need for
repeated electrical scanning on the entire imaging plane as in the prior
art. It is possible to measure the surface profile of the target at a high
speed from this positional information and the identifying information on
the slit-ray which is also detected at the real time. Such high-speed
scanning of the surface of a target with the slit-ray also enables very
accurate profile measurement with respect to a moving target.
While there has been described what are at present considered to be
preferred embodiments of the invention, it will be understood that various
modifiaations may be made thereto, and it is intended that the appended
claims cover all such modifications as fall within the true spirit and
scope of the invention.
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