Three-dimensional vision system
BACKGROUND OF THE INVENTION 
     The present invention relates to a three-dimensional vision system which recognizes the three-dimensional shape of an object by the use of slit-like light segmentation when the object is subjected to the operations of assembly etc. in a robot, an automaton etc. 
     Prior-art vision sensors which apply light segmentation are, for example, CONSIGHT published by General Motors in U.S. (literature: M. R. Ward et al, &#34;CONSIGHT An Adaptive Robot with Vision&#34;, Robotics Today, pp. 26-32 (Summer, 1979)) and a compact robot vision published by Hitachi Ltd. (literature: Nakagawa et al, Structured light method for inspection of solder joints and assembly robot vision system: Proc. of 1st Int. Symp. of Robots Res. Bretton Woods, Aug. 28-Sept. 2, &#39;83). In a case where the surface of an object to be seen is liable to irregularly reflect light and where external light conditions are stable, such a vision sensor based on the light segmentation system can receive a clear image and is therefore a very excellent system capable of three-dimensional image inputing. However, in a case where the object to be handled has a blackish surface liable to absorb light or where it has a reflectivity close to the total reflection, the quantity of light to enter the image sensor is comparatively small, and the extraction of a light segmentation line has been difficult under the influences of a background image etc. The reason why the extraction of the light segmentation line becomes difficult when the reflected light entering the image sensor lessens due to the properties of the surface of the object to-be-handled, is that a background object brighter than reflected light from the light segmentation line on the object to-be-handled becomes prone to appear in an image screen due to illumination based on external light. 
     Moreover, in a case where the fluctuation of external light is great, a segmentation decision threshold value in a light segmentation line extracting circuit fluctuates greatly, and stable light segmentation line extraction has been difficult. The reason is that, since the input image from the image sensor has a brightness obtained by adding up the light segmentation illumination and the external light illumination, the threshold value for deciding the segmentation line must be sharply changed on account of the fluctuation of the external light. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a three-dimensional vision system which can extract a light segmentation line stably without being affected by the surface condition of an object to-be-handled, the fluctuation of external light, etc. and can precisely recognize a three-dimensional shape. 
     The present invention for accomplishing the object consists in a three-dimensional vision system comprising a three-dimensional vision sensor which projects slit-like light on an object to-be-handled and which reads out a resulting slit image, an image input portion which controls the slit-like to be lit up and put out and also controls the read-out of the slit image and which performs extraction processing of differential signals as to image signals read out, and an image processor which performs required controls concerning said image input portion and which processes image data of said image input portion; an image with the projected slit-like light and an image without it being sequentially sampled on an identical scanning line of the read-out image so as to obtain the differential image between them, thereby to extract a light segmentation line based on the projected slit-like light and to recognize an attitude and position of the object to-be-handled in three dimensions from the extracted light segmentation line. 
     That is, according to the present invention, when a light segmentation line is input, the projected image of the light segmentation line and a background image without projecting the light segmentation line are input in two operations, and the background image is subtracted from the projected image, whereby the image of only the light segmentation line is extracted, and the background image formed by external light and a fluctuant component attributed to the external light are removed from the light segmentation image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a system arrangement diagram of an embodiment of a light segmentation extraction circuit according to the present invention; 
     FIGS. 2A and 2B are parts of a folded detailed block diagram of the embodiment; 
     FIGS. 3(a)-3(d) are diagrams for explaining the basic principle of the light segmentation extraction circuit according to the present invention; and 
     FIG. 4 is a conceptual diagram showing the three-dimensional positions of three points on a two-dimensional image which is detected by a three-dimensional vision sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now, the present invention will be described in conjunction with an embodiment shown in the drawings. 
     FIG. 1 is a view which shows the schematic arrangement of a three-dimensional vision system according to the present invention. In the arrangement of the system, an image input from a three-dimensional vision sensor (range finder) 11 mounted on the hand of a robot 13 is fed into an image processing apparatus 12, in which the position and attitude of an object to-be-handled 15 are calculated by image processing. The results are transferred to a robot control device 14, and the robot 13 performs, e.g., the operation of gripping the object under the control of the control device. 
     FIGS. 2A and 2B are parts of a folded a detailed block diagram corresponding to FIG. 1. The three-dimensional vision sensor 11 comprises a projector portion for slit-like light beams 28, which is composed of laser luminous diodes 21, slit-like light patterns 22, and projective lenses 23; and an image detector portion in which a linear image sensor 26 is combined with a rotating mirror 25, an imaging lens 24, and a sensor signal reading circuit 27 for the image sensor 26. Such arrangement affords a three-dimensional image sensor which can receive a slit image when the luminous diodes 21 are lit up and a background image when they are put out. 
     The internal structure of the image processing apparatus 12 is such that an image input portion 41 which is composed of various circuits 30-40 to be described later, an image processor 42 which is constructed of a microcomputer by way of example, and a data input/output interface circuit 43 which serves to communicate with the robot control device 14 are connected by a bus line 44. Here, the image input portion 41 is generally controlled by the scanning control circuit 30 thereof. 
     The scanning of one line of the image is performed as stated below. 
     First, the scanning control circuit 30 supplies the rotating mirror control circuit 32 with a positioning signal, by which the rotating mirror 25 is positioned. Thereafter, the laser luminous diode driving circuit 31 is turned &#34;on&#34; (brought into a closed state) to light up the luminous diodes 21. The slit image is applied from the linear image sensor 26 by the sensor signal reading circuit 27, and it is digitized by the A/D converter 33. While the slit image is input, the signal distribution circuit 34 is sending a video signal to the shift register 35. This shift register 35 can store data of the same number of pixels as that of the linear image sensor 26, and can hold the data until the next scanning cycle. 
     When the slit image has been input by one scanning line, the scanning control circuit 30 puts out the luminous diodes 21 and starts the sensor signal reading circuit 27 again so as to receive the background image. While the background image is input, the signal distribution circuit 34 sends a video signal to the differential signal extracting circuit 36 in accordance with a signal from the scanning control circuit 30. The differential signal extracting circuit 36 is simultaneously supplied with the slit-like light segmentation image data from the shift register 35. It finds the differences of the background image signals from the slit image signals for the same pixels, and stores the results into the differential signal memory 39. At the same time, the position P max  of the maximum signal level within the scanning operation is detected by the peak signal position detecting circuit 37 (which is concretely disclosed in U.S. Pat. No. 4343553), and it is recorded in the peak signal position memory 40 after the end of the scanning operation. 
     Further, the differential signals f i  in the vicinity of the position of the maximum signal level [the range of (P max  -n)-(P max  +n)] are read out from the differential signal memory 39, and the operation of ##EQU1## is executed. Then, the position P max  &#39; of the maximum signal level can be precisely obtained. This position P max  &#39; of the maximum signal level is stored in the peak signal position memory 40. 
     When the image input operation corresponding to one line has ended in the above way, the scanning control circuit 30 positions the rotating mirror 25 to the next scanning line so as to repeat the image input operation of the next line. 
     When one image frame has been input in the above-stated manner, image inputing ends. 
     When the processing of one image frame has thus ended, a gradational image free from the slit-like light is stored in the image memory 38, and the positions of the brightest points found on the scanning lines struck by the slit-like light are stored in the peak signal position memory 40. 
     When the contents of the peak signal position memory 40 are utilized, a distance from the three-dimensional vision sensor 11 to the peak position of the slit-like light can be detected on the basis of the principle of a light segmentation type range finder which is described in, for example, U.S. patent application Ser. No. 482730 (EP Application No. 83103402.0) now U.S. Pat. No. 4,575,304, U.S. patent application Ser. No. 537095 (EP Application No. 83109972.6) now U.S. Pat. No. 4,611,292, or Nakagawa et al, Structured light method for inspection of solder joints and assembly robot vision system: Proceeding of 1st International Symposium of Robotics Research Bretton Woods, NH, U.S.A., Aug. 28-Sept. 2, 1983. When the detecting operations are performed as to the right and left slit-like light beams, the spatial position of the surface of the object 15 struck by the slit-like light beams can be determined. 
     Here, as disclosed in U.S. patent application Ser. No. 582555 (corresponding to Japanese Patent Application Laying-open Nos. 59-154574 and 59-154578), two-dimensional image processing is performed on the gradational image stored in memory 38 to extract the contour shape of the two-dimensional surface of the object 15, and the extracted contour shape of the two-dimensional surface is coordinate-transformed on the basis of the spatial position data of the three-dimensional surface extracted by utilizing the slit-like light as described before, whereby a three-dimensional surface shape can be obtained. 
     More specifically, assuming that the positions (i 1 , j 1 ), (I 2 , j 2 ) and (i 3 , j 3 ) of respective points P1, P2 and P3 on an image screen and the corresponding coordinates (x 1 , y 1 , z 1 ), (x 2 , y 2 , z 2 ) and (x 3 , y 3 , z 3 ) viewed from the three-dimensional vision sensor 11 have been known from the slit-like light segmentation, the coordinates (x, y, z) of a contour point P (i, j) are given by the following equations: 
     
         x=x.sub.1 +{(i-i.sub.1)a.sub.i +(j-j.sub.1)a.sub.j }z/z.sub.1 (1) 
    
     
         y=y.sub.1 +{(i-i.sub.1)b.sub.i +(j-j.sub.1)b.sub.j }z/z.sub.1 (2) 
    
     
         z=z.sub.1 +{(i-i.sub.1)c.sub.i +(j-j.sub.1)c.sub.j }z/z.sub.1 (3) 
    
     Here, ##EQU2## b i , b j  and c i , c j  are respectively obtained by replacing x in Eqs. (4) and (5) with y and z. 
     In the above way, the three-dimensional coordinates of the contour line can be found. 
     Thus, the center and attitude of the surface can be found by utilizing diagrammatical primary and secondary moments, and the position and attitude of the object can be determined. 
     When the data are sent to the robot control device 14 through the input/output interface circuit 43 and then given to the robot 13 as position data, the robot 13 can perform an operation such as gripping for the object 15. 
     Referring now to FIGS. 3(a)-3(d), the basic principle of the slit-like light segmentation extraction according to the present invention will be described. In an image in FIG. 3(a), the sensor output 4 of one scanning line 3 is schematically graphed as shown in FIG. 3(b). On the other hand, the sensor output 6 of the scanning line produced without projecting slit-like light as shown in FIG. 3(c). As illustrated in FIG. 3(d), when the difference 7 is obtained between the two image outputs, a background image part almost consists of only noise components, and only a signal component based on the slit-like light segmentation can be clearly taken out. 
     Although, in the above embodiment, the shift register 35 has a storage capacity corresponding to one raster, it may well be constructed so as to store signals of one image frame and to be fed with the signals of one image frame by revolving the rotating mirror 25 with the luminuous diodes 21 put out. In this case, the signal distribution circuit 34 is changed-over, and the luminous diodes 21 are lit up, whereupon the slit-like light segmentation image corresponding to one image frame is input. Then, the background image read out from the shift register 35 is subtracted from the input light segmentation image by the differential signal extracting circuit 36. Thus, the differential signals can be extracted. 
     As described above in detail, according to the present invention, in an image input apparatus which applies the slit-like light segmentation system, components possessed by a background image can be removed from an image of slit-like segmentation, so that segmentation extraction without the influence of the background image can be performed. In addition, even when the brightness of external light has fluctuated, the fluctuant component can be eliminated, so that the threshold value of the slit-like light segmentation extraction, etc. need not be changed, and stable extraction is possible. Accordingly, remarkable effects are attained for enhancing the precision, efficiency and stability of the image recognition processing of a vision sensor in a robot or a automaton.