FR2739196A1 - Accurate three dimensional measurement of luminous intensity - Google Patents

Abstract

The equipment stands on a large cylindrical box (1) which also houses electronic cards. A first step by step motor (2) provides rotation about a vertical axis (Y) and is connected by a rubber bush (3) to an angled aluminium frame (4). The aluminium frame supports a second step by step motor (5) rotating about a horizontal axis (X) but eccentric to the vertical axis (Y). This motor drives a box (13) through a rubber bush (6) to reduce friction. The box (13) contains a photodiode, a pinhole and a supporting a lens (9).

Description

 The present invention relates to a passive device which allows the capture of a couple of images from 2 different viewpoints with a field of view of up to 3600 vertically and horizontally. The essential components are a mechanical sweep and an optoelectronic device for measuring illumination. The digital images obtained undergo computer processing, to find the three-dimensional coordinates of objects. The basic principle used is called stereovision and allows the calculation of distances between the device and objects using 2 offset images from the same scene. The constitution of the present invention allows a simplification as well as a reduction of the material means (sensors, computer) necessary for the implementation of stereovision.

There are several types of three-dimensional sensors involving a vision system. We can distinguish active sensors using a light source (such as a laser) in addition to ambient lighting, and passive sensors which do not involve an additional source. First of all, the active 3D sensors can be subdivided into 2 categories:
- The first category is based on use
a scanning laser rangefinder that establishes the distance
material points of a scene by measuring time
propagation of the light beam, or by phase shift
in amplitude between the emitted and reflected beam (case
amplitude modulated laser).

- The second category uses one or more cameras
videos and allows by highlighting a number of
scene points to determine their coordinates
three-dimensional by triangulation. All
the scene can be studied by moving the source
from light.

Active sensors using additional light sources (laser), are sensitive to ambient lighting and are not suitable for reflective objects. In addition, they have the disadvantages of posing problems with regard to human beings and of being limited in terms of distances and field of vision.

The other methods are passive techniques and therefore do not involve an additional light source:
- Passive three-dimensional sensors are made up
usually several video cameras (2 or 3) which
capture the scene from several points of view, the relief
of objects is reconstructed by image processing. The
method for providing a description in 3
dimensions is called stereovision. The problem is
in the image matching, that is to say
determining the pairs of points from the same
real points of the scene. Knowledge of disparity
between each pair of points and the characteristics of the
optical system of shooting allow the calculation of
three-dimensional coordinates of objects.

For passive sensors, the complexity of image processing requires considerable computing resources (computer with parallel architecture, cards specialized in image processing, etc.). They also have a reduced field of vision since only part of the scene is common on the cameras' electronic retinas.

The device according to the invention overcomes these drawbacks. According to a first characteristic, it comprises a mechanical scanning on 2 axes provided by 2 precision stepping motors. A first stepper motor fixed to a large cylindrical housing supports and drives in rotation a second stepper motor via an aluminum bracket. This second motor is coupled to a cylindrical housing of small dimensions, which contains an opto-electronic device. This device consisting of a lens, a diaphragm and a photodiode provided with a pinhole camera allows a point measurement of illumination
The smallest box is offset from the axis of rotation of the first stepper motor, moreover the axes between these 2 motors form a right angle. The structure of the present invention allows the capture of 2 images from 2 different points of view. using a single photodiode. In addition, the captured images can cover field angles of 3600 vertically and horizontally.

 According to a second characteristic, the (minute) angle of the scene taken into account by the measurement photodiode is equal to the angle between 2 consecutive steps of a stepping motor. This fundamental characteristic of the present invention makes it possible to faithfully reproduce a scene from multiple point measurements of light intensity.

This structure also allows a simplification of the calculation means to be used for the development of stereovision. Indeed, since the points to be paired to reconstruct the relief of a scene are located on the same horizontal lines in each of the 2 images. For example, the points of line n04 of the first image correspond only to points of line n04 of the second image.

This is valid for each pair of horizontal lines.

 According to a third claim, the present invention has an optical assembly with fixed focus. A reduced focal lens and a small aperture diaphragm give the image a very large depth of field.

 According to a fourth, fifth and sixth claim, a photodiode with a cylindrical metal case and a flat window incorporating a color correction filter receives a light flux to be measured through a pinhole camera. This opto-electronic device for measuring illumination has a second photodiode with the same characteristics as the previous photodiode, but receiving no light radiation. In addition, the photodiodes operate in photovoltaic mode, without an external polarization source. An important feature of this operating mode is an increased reduction in SCHOTTKY noise (random current fluctuation) and thus the possibility of measuring very low luminous fluxes. Finally, each photodiode is connected to a current-voltage converter, consisting of a precision operational amplifier with FET (Field Effect Transistor) input and a resistor of very high ohmic value. After a bandwidth reduction by passive low pass filters, the potential difference is amplified hundreds of times by a high precision instrumentation amplifier. To eliminate electromagnetic interference and coupling with the electrical network (50 Hz), a thin conductive sheet surrounds the housing containing the 2 photodiodes and the amplification circuit on all sides.

The attached figures illustrating the invention are as follows:
FIGURE 1 shows the device according to the invention in
front view.

FIGURE 2 shows the device according to the invention in
left view.

FIGURE 3 shows in section the box containing the
opto-electronic measuring device as well
as its amplification package.

DIAGRAM 4 represents the amplifier of the opto device
electronic.

FIGURE 5 shows the functional conditions at
respect to reconstruct an image from
occasional lighting measurements.

 In introduction and with reference to Figures 1, 2 and 3 the device according to 1 invention is composed of 2 essential elements. On the one hand, a photodiode (17) housed in the housing (13), this provides a point measurement of light flux. On the other hand, a mechanical scan on 2 axes, ensuring a description in rows and columns of the scene taken into account by the photodiode (17). This scanning is ensured by 2 stepping motors (2) and (5) of precision (800 steps per revolution) on a field of vision which can reach 360 "vertically and horizontally.

 Referring to Figures 1 and 2, the device comprises a large cylindrical housing (1) serving as a frame for the system, as well as housing the electronic cards thereof.

On its upper part is fixed the stepping motor (2) for scanning in lines (along the Y axis) of the housing (13) containing the device for measuring illumination. A brass barrel wheel (3) is screwed on the one hand on the axis of the stepping motor (2) and on the other hand on an aluminum bracket (4). This assembly supports a second stepping motor (5) ensuring column sweeping (along the X axis) of the housing (13). The angle formed by these 2 motor axes is 90, so the eccentricity of the stepping motor (5) relative to the Y axis allows the capture of a couple of images from 2 different points of view. The spacing between each image is 200 millimeters and thus ensures a compromise between the precision of the distance calculations (the larger the gap and the more the calculations are precise) and the speed of the scanning due to the inertia of the moving masses. (the smaller the difference, the faster the scan can be). A brass stop ring (6) fixed on the axis of the stepping motor (5) and on the aluminum bracket (7), rigidly and frictionlessly couples the housing (13) to the motor (5 ).

The cylindrical housing (13), opaque to light, is provided with a cover (8) provided in its center with an optical device with fixed focusing constituted by a single lens (9) and a diaphragm (14) (visible in Figure 3 only). The electrical connections (10) consist of flexible sheet cable (7 conductors) with a pitch of 2.54 millimeters and a shielded connection of small diameter (3 millimeters).

These connections (10) supply the windings of the stepping motor (5) (4 conductors), supply the amplifier of the lighting measurement device (3 conductors), as well as the transmission of the amplified signal. (shielded wire). Although each motor axis can rotate on 3600 (1 turn only), the links (10) have anchor points formed by wire passes (11) and (12) placed so as to minimize their deformations and especially to distribute them over all their lengths. Similarly, the position of their anchor points as well as their semicircle shapes are studied so that they do not touch any mechanical part, even for a complete rotation of the stepper motors (2) and (5).

 The operation of the present invention is as follows: the image of a scene is reconstructed by scanning in rows and columns of a measuring device. Each column is cut into a certain number of steps and at each advance of one step of the motor (5), an opto-electronic assembly performs a point measurement of illumination. Then at the end of a column, the stepping motor (2) for line scanning advances by one step, the cycle begins again until all the desired space has been entered. In addition to obtain the second image from another point of view essential for a relief vision, the 2 stepper motors (2) and (5) rotate by 1800 (this angle varies according to the dimensions chosen for the images) and the cycle described above begins again.

Note in Figures 1 and 2
- The fastening elements (screws, nuts, washers) have
been omitted for better readability of the drawings,
however their locations are visualized by
short dotted lines.

- For a better understanding the drawings show
the device, where each mechanical axis of the motors pitch
with steps (2) and (5) pivoted by an angle equal to 900 by
relative to their rest position (00).

 With reference to FIG. 3, the housing (13) contains an opto-electronic device for point-in-time illumination measurement, mainly composed of a photodiode (17).

This housing (13) is surmounted by a cover (8), on this is fixed a convex lens (9) of small dimensions and focal length 35 millimeters. It is associated with a diaphragm (14) of opening f / ll maintained by bonding thereto. This assembly constitutes an optical device with fixed focus, forming a sharp image at the pinhole (18) for objects located from 20 centimeters to infinity. The diaphragm (14) of small aperture as well as the short focal length of the lens (9) allow a large depth of field in the image. In accordance with the optical axis, the measuring photodiode (17) is positioned, with a metallic cylindrical housing. Likewise, a simple element which is essential for the point measurement of lighting is made up of a pinhole (18) .This pinhole is a very small diameter hole (0.3 millimeter) drilled in the center of a metal plate (thickness 0, 1 millimeter) and located at the focal distance of the lens (9). The pinhole diameter (18) is calculated so that the illumination measurement takes place over a very small angle equal to one step of the motors (2) or (5), i.e. 0.450 (360/800 steps ). The presence of a second photodiode (15) receiving no light radiation allows a differential measurement to overcome any variations in temperature and supply voltages. Light-opaque adhesive tape (16) completely covers the top of the photodiode (15) and allows the pinhole (18) to be fixed and positioned on the transparent window of the photodiode (17) .The two photodiodes (15) and (17) are held in place by a tight fit between their housings and a profile (20) of thin aluminum (1.2 mm). The point measurement of illumination carried out by the photodiode (17) is amplified by an electronic device (19). The assembly (19) consists of operational amplifiers in integrated circuit and of some passive elements (resistors and capacitors). The electronic circuit (19) is located as close as possible to the photodiodes (15) and (17) to limit the length of their electrical connections. An essential element for the proper functioning of the assembly is an electrical shielding. It consists of a thin sheet of aluminum (26) completely surrounding the housing (13), as well as the edges of the cover (8). This shielding considerably attenuates the noises of radioelectric origins and the coupling with the electrical network (50 Hz) taking into account the very weak power of the signal collected by the photodiode (17). The shield (26) is at OV potential via the metal boxes of the photodiodes (15) and (17) (their cathodes are connected to ground), in contact with the support (20) made of aluminum. Then via the conductive washer (22), itself in connection with the bottom of the housing covered with the aluminum shielding sheet (26). This sheet (26) being reflective and in order to avoid parasitic light reflections inside the case (13) liable to modify the measurements, it is covered with black mat paper (27) except for the bottom of the case (13) .The threaded rod (24), the washer (25) and one nut (23) simultaneously maintains the support (7) and the bracket (20) with the housing (13).

Referring to Figure 4, the essential building blocks of the amplifier are:
- 2 BPW21 PHOTODIODES (15) and (17) with silicon
sensitivity 7 nA per lux, in metal case (cathode
housing) with flat window incorporating a filter
color correction (visible spectrum).

- 2 AD795JN OPERATIONAL AMPLIFIERS (28) and (30)
precision with FET input specifically designed for
photodiodes preamplification and conversion
current-voltage.

- 1 AMP02FP INSTRUMENT AMPLIFIER (32) high
precision with adjustable voltage gain from 1 to 10,000
by adding a simple resistance.

 The photodiode (17) receives a light flux to be measured through a pinhole camera (18) and transforms it into a current proportional to the illumination received on its sensitive surface.

The electronic assemblies constituted by the operational amplifiers (28) and (30) and resistances of 10 Mohms allow the operation of the photodiodes (15) and (17) in photovoltaic mode. No external bias source is associated with these photodiodes which then become current generators. An essential characteristic of this operating mode for the present invention is the absence of dark current. This results in a large reduction in SCHOTTKY noise (random current fluctuation) and therefore the possibility of measuring very low lighting values. Taking into account the measurement current (tens of pA) brought into play by the photodiode (17 ), the operational amplifiers (28) and (30) were chosen for their very low values of input current (1 pA typical), noise and offset voltage (100 pV typical).

The presence of a second photodiode (15) receiving no illumination and its preamplifier (30), allow a differential measurement to overcome temperature variations as well as supply voltages, and to compensate for offset voltages . At the output of the operational amplifiers (28) and (30) filters pass passive low (29) and (31) cutoff frequency 150 Hz reduce the bandwidth of the preamplified signal. They reduce unwanted electrical noise generated by electronic components. Then the potential difference is greatly amplified by a high precision instrumentation amplifier (32), having a voltage gain fixed at 500 by a resistance of 100 ohms. Taking into account the low voltages (a few mV) coming from the operational amplifiers ( 28) and (30), the instrumentation amplifier (32) has a high common mode rejection rate (115 dB typical) as well as a low offset voltage (40 MV typical) and excellent linearity (0.006 %). A shield (26) made of aluminum foil isolates the electronic components from all electromagnetic interference capable of drowning the signal collected by the photodiode (17). On leaving the instrumentation amplifier (32), the amplified signal is sent to the housing (1) (see FIGS. 1 and 2) via a shielded link (10) to an analog-digital converter to be digitized there. This 8-bit converter allows the differentiation of 256 gray levels. A computer (external to the present invention) based on a 68000 microprocessor collects images in the form of matrices where each light intensity is represented by a number from O to 255.

The computer processes the images (smoothing, edge detection, etc.) to extract the three-dimensional coordinates of the objects in a scene.

FIG. 5 represents the functional conditions to be respected in order to faithfully capture an image of a scene from point measurements of illumination. In this figure the value of the angles, and the width L have been very exaggerated for better readability. This drawing represents on a single plane 2 punctual lighting measurements taken for 2 consecutive steps of the engine (5). t corresponds to the angle taken into account for the illumination measurement, it is related to the distance between the lens (9) and the sensitive surface of the photodiode (17) (see FIG. 3) as well as to the useful width
L of its sensitive surface. The width L corresponds to the pinhole diameter (18) or 0.3 millimeter. On the other hand, the angle q comprised between 2 consecutive steps is linked to the number of steps of the motors, thus for 800 steps per revolution i = (360/800) = 0.45. One of the important conditions is to respect the equality between the angle e and to faithfully reproduce a scene. Thus, as can be seen in the drawing in FIG. 5, there is no overlap or difference between each point measurement of illumination. The presence of an area of constant width (hatched) not taken into account by the photodiode (17) is negligible, since it is equal in width to L or 0.3 millimeters. In addition, equality = allows the pinhole calculation ( 18) (see figure 3) to be installed on the sensitive surface of the photodiode (17) by the following formula: L = 2.DF.tan (/ 2) with: DF = focal distance = 35 millimeters
Angle between 2 steps = 0.45 "we get: L = 0.3 millimeter
The angle taken into account by the photodiode (17) is 0.45, which justifies the term of point measurement of illumination.

The second condition to be observed in close connection with the first, is related to the position of the axes of rotation of the motors (2) and (5) relative to the pinhole (18). Thus the axes of rotation must be located on the right n so that the first condition is verified for each of the stepping motors (2) and (5). On the present invention, with the aid of FIGS. 1, 2 and 3 , the axis of rotation of the motor (5) is located exactly in coincidence with the pinhole (18). On the other hand, the motor (2) has its axis of rotation offset by 100 millimeters from the center of the pinhole (18).

 The device according to the invention can be intended for automatic inspection and identification of 3-dimensional shapes.

Claims (6)

 1) Passive device which allows to capture a couple of images of a scene from 2 different points of view and with a field of vision which can cover angles of 3600 vertically and horizontally. The digital images obtained undergo computer processing (external to the present invention) to reconstruct a 3-dimensional vision of objects. This device is characterized by a mechanical scanning on 2 axes provided by 2 stepping motors (2) and (5) of precision. This scanning allows a description in rows and columns of the scene to a photodiode (17) located inside the housing (13). The stepper motor (2) fixed to the housing (1) supports and rotates the second stepper motor (5) via a bracket (4). The stepper motor (5) is coupled to a cylindrical housing (13) of small dimensions and its cover (8) by means of a bracket (7). The housing (13) is offset from the axis of the motor (2), moreover the axes of these 2 motors (2) and (5) form a right angle. An optical device consisting of a lens (9), a diaphragm (14) and a pinhole camera (18) allows a point measurement of illumination by the photodiode (17).  2) Device according to claim 1 characterized in that the angle of consideration of the light flux by the photodiode (17) is equal to the angle between 2 consecutive steps of the motors (2) and (5). This angle of illumination measurement is adjusted by a pinhole camera (18) placed on the window where the sensitive surface of the photodiode (17) is located.  3) Device according to claim 1 characterized by an optical assembly with fixed focus, consisting of a lens (9) with reduced focal length and a diaphragm (14) of small aperture. A sharp image is formed exactly at the pinhole (18) for objects located from a few tens of centimeters to infinity. The very small pinhole pinhole (18) is located at the focal distance of the lens (9) and coincides with its optical axis.  4) Device according to claim 1 characterized by a photodiode (17) with flat window, metal housing and incorporating a color correction filter, it receives a light flux to be measured through the circular orifice of the pinhole (18). A second photodiode (15) with the same characteristics as the previous photodiode (17) receives no light radiation.  5) Device according to claim 4 characterized by an operation of the photodiodes (15) and (17) in photovoltaic mode without external polarization source. Each is connected to a current-voltage converter, consisting of an operational amplifier (28) and (30) of precision (with FET input) and of a resistance of very high ohmic value. After a bandwidth reduction by passive low pass filters (29) and (31), the potential difference is greatly amplified by a high precision instrumentation amplifier (32).  6) Device according to claim 4 characterized by an electromagnetic shield connected to the OV potential and consisting of a thin aluminum sheet (26) which surrounds on all sides the housing (13) and the edges of the cover (8). The metal housings of the photodiodes (15) and (17) are connected to ground (OV) via their cathodes.