Abstract:
An optical device to measure the distance between the device itself and an obstacle/object comprising a unit to emit radiation including an aligned series of sources of radiation coupled to a series of arrangements of lenses to guide the radiation emitted by the sources. An optical acquisition unit comprising a matrix of photodetectors has a field of view including the scene in front of the optical device and including the obstacle. The signals leaving the acquisition unit are sent to a control and processing unit for calculation of the distance from the obstacle. The unit for the emission and conformation of the radiation emits a beam having a transverse section of a form elongated along an axis that is progressively rotated for transverse sections of the beam progressively more distant from the emission unit. In this manner, the distance of the obstacle may be calculated on the basis of the angular position of the section of the beam intercepted by the obstacle, as acquired by said acquisition unit.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to optical methods and devices for measuring the distance from an obstacle. In particular, the invention concerns optical devices of the type comprising:
         a unit to emit radiation and to conform the irradiated beam,   an acquisition unit to acquire optical information relating to the image of the scene in front of the optical device, and   an electronic control unit to receive signals leaving the acquisition unit and to process them for the purpose of determining the distance between said optical device and an obstacle that is part of the scene in front of the optical device.       

   Techniques of the optical type to measure the distance from an obstacle may be subdivided into passive and active techniques. Passive techniques exploit the existing environmental illumination, whereas active techniques illuminate the scene with a source of radiation. 
   The most widely used passive technique, above all in robotics and industrial automation, is the so-called stereo technique, based on the use of two video cameras that observe the same portion of the scene from different angles. 
   Active techniques are however undoubtedly more robust and better adapted to conditions of high background light, or those of poor visibility. 
   Among the active techniques, the triangulation technique has been proposed for numerous different applications, both with a single beam and with structured light, for example in the form of a blade of light, scanning in one or two dimensions and observed from a direction at an angle with regard to the direction of projection. Triangulation enables the processing complexity to be reduced, but there are some limits to the precision that can be achieved. 
   Both the stereo technique and triangulation, indeed, become less precise for large distances between sensor and obstacle, taking into account that the accuracy depends on the relative sensor-sensor or source-sensor distance. Furthermore, measurement depends on calibration of the system, with a high sensitivity with regard to mechanical adjustments; all triangulation systems are extremely sensitive to misalignment. 
   For other applications that require detection in the medium/long range, such as automobile-related applications, active ranging techniques have been used, based on infrared sources (laser), with multiple beams or in some cases with a scanning beam operating in one or two dimensions. Techniques have also been proposed that are based on measuring the (direct or indirect) flight time with pulsed sources. For example, in US2004233416 a device is described to produce a three-dimensional image representing the distance of obstacles. The image is focused on a matrix of CMOS sensors and the distance is calculated by measuring the back-reflected signal of packets of pulses sent, out of phase by a few tenths of a nanosecond. Lastly, techniques have been proposed based on the measurement of the degree of phase displacement between emitted signal and reflected signal, with modulated sources of the sinusoidal type. As an example, a method is described in US2004008394 to acquire and demodulate electromagnetic waves modulated from the temporal standpoint by means of two micro-optical elements capable of focusing the same portion of the scene onto two distinct sensitive areas. In this way it is possible to measure, for example, the phase of a modulated signal and apply this technique in distance measurement systems by calculating the flight time. The sequential nature of the measurements connected to the scanning, the need to work with high frequencies (pulse duration less than a microsecond) and the high degree of complexity of the system are the principal disadvantages of these techniques for vehicle applications, whereas the chief advantage with regard to triangulation techniques lies in the greater ease of installation and calibration, as well as the greater accuracy in distance measurement for long obstacle distances. Prototype 3D vision systems (ranging cameras) are also in the development phase, for robotics and security applications. In such systems, a matrix of photodetectors, with appropriate processing electronics integrated at the pixel level, performs two functions in parallel on an entire image, detecting the scene and measuring the distance of objects. 
   SUMMARY OF THE INVENTION 
   The purpose of the present invention is to produce an optical device for measuring the distance from an obstacle, characterised by extreme simplicity of the device, together with extreme simplicity of the processing algorithms necessary to calculate the distance, as well as, lastly, being of lower cost than the solutions mentioned above. 
   In view of achieving these and further goals, the invention has as its object an optical device for measuring the distance from an obstacle, comprising the characteristics in claim  1 . 
   The present invention also has as its purpose a method according to claim  32 . 
   Thanks to the characteristics indicated above, the invention may be implemented with extremely simple and low-cost means, without thereby prejudicing the possibility of achieving precise and reliable distance measurement. 
   The device according to the invention lends itself to being utilised, for example, in automobile-related applications to measure the distance of a vehicle onto which it is installed from an obstacle, both to provide assistance in parking manoeuvres, and to activate safety devices immediately before a crash. The device according to the invention nevertheless also lends itself to other applications, for example as a portable device or as a stationary device for the surveillance of areas, for road infrastructure, etc. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described and illustrated with reference to the attached drawings, provided as a simple example without limiting intent, in which: 
       FIG. 1  is a plan view, in diagram form, of a first embodiment of the device according to the invention, 
       FIG. 2  is a side view, in diagram form, of the device in  FIG. 1 , 
       FIG. 3  is a three-dimensional view, in diagram form, of an optical simulation that shows the operating principle of the device in  FIG. 1 ; 
       FIG. 4  is a further three-dimensional view, in diagram form, that shows the operating principle of the device in  FIG. 1 , 
       FIG. 5  shows a second embodiment having a substantially point-form source coupled with optical means  40  capable at the same time of collimating the beam and of extending the virtual dimension of the beam along one direction. Furthermore, further optical means  4  are capable of forming an outgoing beam  6  with the characteristics necessary for the device according to the invention, 
       FIG. 6  shows a variant of  FIG. 5  in which the optical means  40  may directly perform the function of collimating the radiation emitted from different points of the source in different directions so as to form an outgoing beam  6  with the characteristics necessary for the device according to the invention, 
       FIG. 7  is a diagram showing the variation of the angle of inclination of the transverse section of the irradiated beam, as a function of the distance from the emitting unit and as the ratio between size of source and size of beam section in correspondence with the plane in which the section is oriented vertically varies, in the case in which the plane is 30 m from the source and the source is 20 cm long: 
       FIG. 8  shows the trend of the error ε in distance measurement as the distance itself varies, in the case in which the segment is of the same length as the source and is inclined by 90°, that is it is vertical, at a distance of 30 m, 
       FIG. 9  is a diagram showing the variation of the angle of inclination of the transverse section of the beam as a function of the distance, for some determinant parameters, 
       FIG. 10  shows a matrix of photodetectors (2×2) on which a segment of light  10  is focused. The signals collected on the four photodetectors are indicated with G 1 , G 2 , G 3  and G 4 , 
       FIG. 11  shows dedicated optics  400 , positioned in front of the matrix of sources  3  with relative collimator optics and beam deviators  4 , such as to multiply the vertical segments of light  10 , 
       FIG. 12  shows, in a simulation, the distribution of segments of light, intercepted by an obstacle at a distance of 30 m from the emission unit, in a field of view of 32°×24°; said segments may be generated for example by using a diffraction grid in front of the source of the single segment, this grid having the effect of multiplying said segments into a number of segments corresponding to the diffraction order of the grid; 
       FIG. 13  is a representation of the same light distribution as  FIG. 12 , in diagram form. 
       FIG. 14  shows, in a simulation, the distribution of segments of light at a distance of 50 m, in a field of view of 32°×24°, generated by diffraction optics; 
       FIG. 15  shows the same light distribution as  FIG. 14 , in diagram form; 
       FIG. 16  shows, in a simulation, the distribution of segments of light at a distance of 100 m, in a field of view of 32°×24°, generated by diffraction optics; 
       FIGS. 17  shows the same distribution of light as  FIG. 16 , in diagram form. 
       FIG. 18  is a diagram showing the variation of said angle of inclination as a function of the distance, for some specific segments of  FIGS. 7-9 , in the case of a vertical line situated at 50 m and of a distribution of segments equal to 16×12. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIGS. 1-3 , the optical device  1  according to the invention comprises an optical unit  2  that includes a device emitting radiation comprising a linear matrix of sources  3 , for example diode lasers or LEDs with an emission peak preferably in the region of the near infrared spectrum (with wavelength between approximately 700 nanometers and 1100 nanometers). The sources  3  are coupled to optical means  4  capable of intercepting the radiation emitted by the sources  3  and of collimating the radiation leaving each individual source in a predetermined direction; the beam  6  produced by said optical means is thus in the form of a superimposition of beams substantially collimated such as to form, on a plane perpendicular to the plane of emission of the sources and at a predetermined distance F from those sources, a distribution of illumination comprising a matrix of spots arranged along a predetermined direction different from the direction of the line along which said sources are arranged (see also  FIG. 4 ). In a limit case, the number of sources may be reduced to two, in such a manner that the beam  6  is comprised of two beams collimated in different directions and such as to form, on a plane situated at a predetermined distance z from the emitting unit  2 , spots arranged along a predetermined direction that is different from the direction of the line along which the emitting sources are arranged. If the intensity of the sources  3  is sufficiently high and/or if the distance between the sources is fully compensated by the divergence of the beams leaving said optical means  4 , the resulting distribution of illumination would be perceived as a continuous distribution rather than as a superimposition of isolated spots. 
   If the collimation of the beams leaving said optical means  4  is sufficiently good and the density of the sources is sufficiently high, the transverse section of the beam may thus be assimilated to a segment of straight line (see  FIG. 4 ); the direction of that straight line also being different from the direction of the line along which said sources are arranged, the distribution of illumination resulting from the intersection of the emitted beam  6  with the plane transversal to the principal direction of the emitted beam  6  will be a straight line that, in transverse sections at progressively greater distances from the emission device  2 , is progressively rotated. 
   With reference to  FIG. 5 , as an alternative to a linear matrix composed of a number of sources, it is possible to provide for a single source of radiation of a linear form from which light collimated in one direction is emitted; this may be achieved for example by using a substantially point-form light source coupled to appropriate optical means  40  capable at one and the same time of collimating the beam and of extending the virtual dimensions of the source along one direction.  FIG. 5  shows the case in which the optical means collimate the radiation in a direction that is the same for all points of the linear source, whereby the linear source produced by said optical means  40  is coupled to further optical means  4  capable of forming an outgoing beam  6  with the above-described characteristics. Alternatively ( FIG. 6 ) said optical means  40  may directly perform the function of collimating the radiation emitted from different points of the source in different directions in such a manner as to form an outgoing beam  6  with the above-described characteristics. 
   In a preferred embodiment, the segment  10  is rotated by 90° with respect to the direction of alignment of the sources  3  at a predetermined distance F. 
   The linear matrix of sources  3  may be extended along the horizontal or vertical direction and generate a vertical or horizontal segment of light at a predetermined distance, as may be seen in  FIGS. 3-6 . 
   The sources  3  may be piloted in a continual modality or may be modulated/pulsed, to increase the ratio between the useful signal (segment of light) and the background signal influenced by the ambient illumination. 
   The device  1  according to the invention also includes an optical acquisition unit  8  ( FIG. 1 ) situated in proximity to the linear matrix of sources  3 , with an optical system having a field of view  7  and such as to form on a detector or on a matrix of detectors an image of the distribution of illumination obtained as the intersection between the beam  6  and an obstacle  5  situated at a distance z that it is required to determine. 
   According to a preferred characteristic, the optical unit  8  is a vision system comprising a matrix of photodetectors, for example with CCD or CMOS technology, coupled to an image formation arrangement of lenses. 
   According to a further preferred characteristic, the acquisition unit includes means to filter the radiation reflected from the obstacles and incident on said acquisition means so as to transmit only the components of the spectrum corresponding to the radiation emitted from said emission unit, in order to increase the ratio between useful signal and background signal, as well as that between signal and noise 
   According to a further preferred characteristic, the optical unit  8  consists of a single detector of linear form coupled to a focussing lens. The response of said detector will be maximum when the linear image of the distribution of illumination formed by the focussing lens on the plane of the detector is substantially aligned to the direction of extension of the detector, and will be minimum when said linear image is orthogonal to the direction of extension of the detector. The use of a single detector of linear form makes it possible to obtain direct information concerning the distance without needing to employ image processing techniques such as in the case of a CCD or CMOS matrix. The same effect may be achieved with a sensor of square or rectangular shape appropriately masked with a diaphragm having a linear slit, as will be explained in detail below. 
   Lastly, an electronic control and processing unit  9  synchronises the emission from the sources, in the case of modulated or pulsed operation, with the acquisition by the optical unit  8 , and processes the image determining the distance measurement. 
   With reference to  FIGS. 3 and 4 , the operating principle of the optical device according to the invention is based on the fact that the sources  3 , arranged for example horizontally, form at a predetermined distance F a distribution of light, with the conformation of a segment of light  10 , rotated by 90°. As the distance  z  between the sources of radiation  3  and the obstacle  5  varies, the intersection of the beams  6  emitted by the sources  3  with the obstacle forms a segment  10  inclined with an angle α with regard to the vertical that progressively varies as the distance from the obstacle varies. 
   The relation between the angle of inclination α and the distance of the obstacle depends on the relationship between the dimension of the segment  10  and the linear dimension of the source (or enlargement factor) at a predetermined distance F. 
   That is, if: 
   α is the angle of inclination, 
   F is the value of the predetermined distance, 
   z is the distance at which the angle of inclination α is calculated, and 
   M is the relationship between the dimension of the segment  10  and the linear dimension of the source (or enlargement factor), 
   then the formula that connects these parameters is given by:
 
 Tg (α( z ))= Mz /( F - z )  [1]
 
     FIG. 7  shows the variation of the angle of inclination α as a function of the distance, as the relationship between the dimension of the source and that of the segment of light varies, where this latter is vertical. This figure relates in particular to the case in which the dimension of the source is 20 cm and the segment of light becomes vertical at a distance of 30 m from the source. 
   As is clear in  FIG. 7 , linearity is maximum when said enlargement factor is equal to 1 and for distances of the obstacle between 0 and said predetermined distance F. It is equally interesting to note that the angle of inclination α reaches an asymptotic value. 
   If the distance F for which the inclination of the segment  10  reaches a predetermined value is known beforehand (in the examples in the figures mentioned above the predetermined value is 90°, corresponding to a vertical segment at a predetermined distance) and measuring the inclination α of the segment  10  in correspondence with the obstacle  5 , the distance of the obstacle can be determined by resolving formula 1 for z:
 
 z=Ftg (α)/( M+tg (α))  [2]
 
   The error ε in the distance measurement that can be detected depends on the minimum variation Δα of the angle of inclination α that the optical unit  8  coupled to the relative electronic control unit  9  is capable of measuring; this error varies in its turn with the distance z, since α does not depend in a linear manner on z (see  FIG. 5 ) . The relationship that connects ε to the distance is as follows:
 
ε= dα/dz=FM/ ( F−z ) 2 +( Mz ) 2   [3]
 
   Since the angle of inclination α reaches an asymptotic value, the error ε increases rapidly for distance values above the predetermined value F.  FIG. 8  shows the trend of the error in the distance measurement in the case of M=1, supposing an uncertainty in measuring the angle of inclination α of 3 degrees. 
     FIG. 6  shows the variation of the angle of inclination α as a function of the distance for a case in which the segment of light is vertical at a distance of 30 m from the source, the horizontal dimension of the source is 20 cm and the dimension of the vertical segment at a distance of 30 m is 50 cm. 
   With regard to the acquisition system  8 , the operating principle for measuring the angle of inclination α of the segment  10  with regard to the vertical in general consists in acquiring, with a matrix of photodetectors, the image of the obstacle  5  so as to detect the segment of light  10 , emitted by the matrix of sources  3  and projected onto the obstacle, and calculating the angle of inclination α with respect to the vertical, using image processing algorithms. This method may be actuated with a matrix of photodetectors, for example with a video camera based on CCD and CMOS technology, utilising an arrangement of lenses of appropriate focal length to focus the radiation. Furthermore, for the means of acquisition, different alternative solutions are provided for. 
   In a first solution, a photodetector (pixel) is used above which are situated a diaphragm and optics capable of resolving the shape of the segment of light, and thus also the angle of inclination α. The diaphragm has the form of a slit reproducing the segment of light with a predetermined angle of inclination α. In this way, whenever the segment of light has an angle of inclination equal to that reproduced by the diaphragm, there is maximum signal with regard to that which would be obtained with different angles of inclination. This method presents marked advantages with regard to the computational load for image acquisition and processing and to the electronics; however, not knowing the reflectance of the obstacle beforehand, it is not possible to distinguish a segment with an angle of inclination equal to that of the diaphragm, reflected by an obstacle with a poorly-reflecting surface, from a segment with angle of inclination different from that of the diaphragm, reflected by an obstacle having a highly reflective surface (“co-operative” obstacle). A solution to eliminate this ambiguity consists in having a measure of the reflectance of obstacles that may be obtained for example utilising an additional source, with emission wavelength the same as that of the matrix of sources  3 , to project a substantially collimated and very narrow beam in the angular direction corresponding to the centre of the obstacle and acquiring from it a signal with a photodetector, said signal being proportional to the reflectance of the obstacle  5 . 
   In a second solution, a matrix of photodetectors comprising a Region of Interest (RoI) is used for acquisition, above which is situated optics (an arrangement of lenses) for focusing the image. The angle of incidence α of the segment  10  is calculated by means of image processing algorithms. The minimum number of photodetectors is 2×2, whereas a value of 10×10 may be considered typical; in general, the larger is the number of photodetectors, the greater is the accuracy with which it is possible to extrapolate the inclination of the segment. The number of photodetectors determines the accuracy in calculating the angle of incidence α. In the case in which the number of photodetectors is 2×2, the incident signals respectively on the photodetectors  1 ,  2 ,  3  and  4  constituting the matrix 2×2 being G 1 , G 2 , G 3  and G 4 , a possible method to calculate the angle of inclination α consists in calculating the relative weights of these signals so as to extrapolate the inclination of the straight line. To discriminate the lack of uniformity of the signals G 1 , . . . , G 4  due to the difference in illumination incident on the photodetectors  1 , . . .  4 , one proceeds, with a differential technique, to subtract the background signal when the segment  10  is not projected onto the obstacle  5 ; in this way the relative weight of the signals G 1 , . . . G 4  is exclusively due to the portions of segment of light  10  that fall on the different detectors comprising the 2×2 matrix. With reference to  FIG. 10 , the angle of inclination is determined as follows: G 2 /G 1 =G 4 /G 3  segment  10  is to be considered vertical. G 2 /G 1 &gt;0 and G 4 /G 3 &lt;0 segment  10  is inclined to the right. G 2 /G 1 &lt;0 and G 4 /G 3 &gt;0 segment  10  is inclined to the left. From the exact values of these ratios it is possible to extrapolate quantitative information on the inclination of the segment  10 . 
   All the methods described above may be generalised to the case in which a plurality of segments of light  10  are projected, as described below, provided that the single photodetector and the single RoI are replaced with a matrix of photodetectors and/or a matrix of RoIs. 
   Lastly, a further acquisition method, applicable only to the case of projection of a single segment  10 , consists in utilising a matrix of single photodetectors (pixels) onto which diaphragms with slits reproducing the segment of light are placed, each having a different angle of inclination α. 
   The angle of inclination of the segment  10  is determined by comparing the signal acquired by the single photodetectors: the strongest signal is that equipped with a diaphragm whose slit reproduces the angle of inclination  10 . 
   This method, being of the comparative type, has the advantage, with regard to the method (I), of not requiring the reflectance of the obstacle  5  to be estimated. The method described above requires the use of image formation optics of the type with a matrix of microlenses, each microlens being associated to a single pixel and producing on the respective pixel the same image of the segment of light  10 . 
   In order to apply the measurement principle described above to automobile-related applications that provide for monitoring the scene with a significant field of view, for example with a FOV of 32°×24°, and with a resolution (or instantaneous field of view, IFOV) for example of 0.1 degrees (for example necessary to resolve an object of 20 cm at a distance of 50 m), it is necessary to have a distribution of many vertical segments of light, at a distance one from the other so as to uniformly cover the field of view at the maximum measurable distance. For this purpose, dedicated optics  400  may be used ( FIG. 11 ), positioned in front of the matrix of sources  3  with the relative beam collimator and deviator optics ( 4  or  40 ), so as to multiply the vertical segments of light  10 . The dedicated optics may be of the holographic or diffraction type. 
   With reference to this latter methodology, the angular resolution and the wavelength are determined so as to fix the period of the grid univocally, whereas the overall field of view and the angular resolution are fixed so as to determine univocally the number of orders of diffraction into which it is intended to concentrate the light, appropriately distributed in a uniform manner. 
     FIGS. 12 ,  14  and  16  show the simulated distribution of segments of light with diffraction optics, in which the number of orders of the matrix is  16  horizontally and  12  vertically, the field of view is 32°×24°, the length of the matrix of sources is 40 cm, the dimension of the vertical line is 40 cm at a distance of 50 m. As may be seen in  FIGS. 13 ,  15  and  17 , to determine the inclination of the segments, it must be taken into account that, for a given distance  17 , as the distance from the central axis increases the lines become inclined as a function of the displacement angle, a phenomenon known as conical diffraction. This means that, in order to calculate the distance, this phenomenon, introduced by the diffraction optics, must be taken into account. As may be seen in  FIG. 18 , the trend of the angle of inclination α of the segments varies as a function of the distance and is different for segments of different orders, but can be deduced beforehand. The diagram illustrates the variation for the segment corresponding to order 0,0, positioned on the axis of the matrix of sources, as well as the variations for the segments corresponding to orders 16,12 and −16,12, which are generated at the extremities of the field of view. 
   All of the above is based on the presumption that the matrix of sources has a single dimension extended horizontally and that the transverse section of the beam irradiated from the sources has a vertical dimension at a predetermined distance. A similar reasoning may be applied considering a matrix of sources with a single dimension extending vertically capable of producing segments of light with horizontal dimension at a predetermined distance. 
   Furthermore, depending on the type of application, it is possible to adapt the technology subject of this patent. For example, in the case of automobile-related applications, the device may be integrated into the front part of the vehicle, above the bumper, for example in the radiator grill, with a number of matrices of sources with dimension extending horizontally (for a total for example of one meter) capable of generating a number of vertical segments of light distant one from the other in such a manner as to uniformly cover, at 50 m, a specific field of view. 
   The beam of light may be conformed such that it gives rise to a transverse section including of a different shape than that of a segment, for example an elliptical or similar shape, that is in any case extended along a principle axis, or any other shape that enables its orientation to be determined. 
   Naturally, the principle of the invention holding good, construction details and embodiments may vary widely with regard to what is described and illustrated as a simple example, without thereby departing from the sphere of the present invention.