Inter-vehicle distance measuring device

Abstract: An inter-vehicle distance measuring device which can automatically set a tracing window for tracing an image of a fore-running vehicle to be measured for distance. One of the images is obtained by image sensors (3, 4) and when they pick up an image of a fore-running vehicle (5) through lenses (1, 2). A window setting unit (12) sets plural number of windows at given locations on the one of the images. An estimate member calculates a distance up to objects caught by those windows, detects the symmetry of the image, and estimates presence of a fore-running vehicle and its position on one of images, and the detected distance data as well. A tracing window is automatically set at the position.

Description: The present invention relates to an optical type distance measuring device using image sensors and to a device which continuously measures a distance from a first vehicle to another vehicle running ahead of the first vehicle (hereinafter referred to as "an inter-vehicle distance measuring device", when applicable). In particular, the present invention relates to a distance measuring device installed on a first vehicle for measuring a distance to a second vehicle running ahead of the first vehicle, comprising a pair of image sensors arranged vertically each for picking up an image of an object including the second vehicle and a background. Such a distance measurement device is known from US-A-4 749 848 that discloses an apparatus wherein two-dimensionally arranged light receiving elements are used and the correlation between the illumination distribution of the images received by the light receiving elements is taken to thereby calculate the distance to the object shown on the image. The calculations are effected with respect to a plurality of set portions related to different directions to determine the distances to objects that are present in a plurality of directions. In particular, the apparatus disclosed in US-A-4 749 848 is used in a self-running robot. Further in DE-A-41 09 159 there is described a tracing device for the vehicle that contains an image receiving section to successively generate images of the vehicle to follow a distance measurement section to measure the distance to the vehicle that is traced, a window positioning device to find the image of the vehicle to be followed among different images of other objects and a device to store an image corresponding to the window in a twofold manner in a memory. The actual tracing is achieved through continuously adjusting the window position such that the symmetry of the image within the window is optimized. Further conventional optical type distance measuring devices using image sensors are disclosed in Japanese Patent Publication Nos. Sho. 63-38085, 63-46363, and the like. Any of those devices, as shown in Fig. 10, contains right and left optical systems including lenses 1' and 2', respectively, which are spaced from each other as much as a base width L. Image sensors 3' and 4' are disposed at positions of the focal distance f from the lenses 1' and 2', respectively. A signal processor 51 calculates a distance to an object 52 from the vehicle carrying the distance measuring device (referred to as the measuring-device carrying vehicle) by processing image signals received from the image sensors 3 and 4. Next, the principle of measuring the distance up to the object 52 will be described. The signal processor 51 receives the image signals from the image sensors 3 and 4, which represent images of the object 52 formed on the image sensors 3 and 4 through the lenses 1 and 2, respectively. The processor then electrically superposes those image signals from the image sensors 3 and 4 every shifting them, and calculates, by the following equation, a distance R from the measuring-device carrying vehicle to object 52 by the principle of the trigonometric measurement on the basis of an amount (d) of the shifting when the image signals are most coincident with each other. [Formula 1] R = (f x L)/d On the other hand, a method for tracing the vehicle running ahead picked up by the image sensors, for example, is disclosed in Japanese Patent Publication No. Sho. 60-33352. An feature of the apparatus thereof is recited to trace an image of a target on the screen of a display unit, in which an operator manually sets a tracing gate (window) enclosing the target on the screen, while watching the screen. The conventional distance measuring device is constructed such that the distance from the first vehicle to the object is obtained by comparing the images picked up by the right and left optical systems. Therefore, when measuring the inter-vehicle distance to the vehicle running ahead while the first vehicle runs to carry the distance measuring device, in a situation that other vehicles run on the right or left side of the vehicle running ahead in the same direction, there is a problem that the operator of the first vehicle cannot detect which of the vehicles is under measurement. Further when starting to trace the image of the vehicle running ahead, the operator manually sets a window for tracing image on the display screen by means of a joystick or a trace pole with watching the screen. At this time, his attention tends to be distracted from the forward direction, which may result in the occurrence of a traffic accident. Summary of the Invention The present invention has been made to eliminate the problems as described above. More specifically, an object of the invention is to provide an inter-vehicle distance measuring device which is provided with functions of a conventional device for measuring inter-vehicle distance in an optical type and a conventional device for tracing a vehicle running ahead and which is operable such that after capturing an image of a target, a distance to the target from the measuring device can be measured continuously so long as it is within a range of an image sensor and such that a target vehicle can be detected among a plurality of vehicles running ahead of the measuring-device carrying vehicle and such that it can automatically set a window for tracing an image of a target vehicle running ahead, without any watching of the monitor display. This object is achieved for a distance measuring device of the type outlined above through the provision of a window setting means for setting a plurality of distance measuring windows at predetermined positions, respectively, in the image picked up by one of the image sensors, and a comparison region setting means for setting a plurality of comparing regions corresponding to the distance measuring windows, respectively, in another image picked up by the other image sensor, a first distance detecting means for comparing image signals contained within the distance measuring windows with image signals contained within the comparing regions corresponding to the distance measuring windows, respectively, to detect each distance to a part of the second vehicle contained within each of the distance measuring windows from the first vehicle, a symmetry detecting gate setting means for setting a symmetry detection gate at a predetermined position in a symmetry detection region lying in the image picked up by one of the image sensors, a symmetry detecting means for detecting symmetry in each of symmetry detection gate, and an estimating means for estimating presence and position of the second vehicle in the image picked up by the one of the image sensors on the basis of data obtained by the symmetry detecting means and the first distance detecting means. An inter-vehicle distance measuring device of the invention comprises a pair of image sensors substantially vertically arranged for picking up an image of a fore-running (second) vehicle, window setting means for setting a plural number of windows on the image, distance detecting means for calculating distances from the measuring-device carrying (first) vehicle to objects caught by the windows, symmetry detecting means for detecting the symmetry of an image signal in a region within an image, and estimating means for estimating presence of a fore-running vehicle and the location of the fore-running vehicle by using the result of symmetry judgement and distance data. The inter-vehicle distance measuring device comprises a tracing window setting means for automatically setting a tracing window of the size dependent on the distance up to the fore-running vehicle at the estimated position of the fore-running vehicle, image tracing means for tracing a fore-running vehicle image with the tracing window, and tracing distance detecting means for detecting a distance up to the fore-running vehicle caught by the tracing window. Namely, the inter-vehicle distance measuring device of the invention measures distances up to targets caught by the windows such that the vertically adjacent image signals picked up by the image sensors, subjecting the image signal within each window to a reference signal, are compared with each other so as to electrically detect displacements of those images, and the result is subjected to the principle of a trigonometric measuring method. On the basis of a result of the measurement, presence or absence of a vehicle running ahead or other obstacles and a rough position of the fore-running vehicle are monitored. The symmetry detecting means detects an area having a good right and left symmetry in the image area, and determines the position of the detected area as a position that the vehicle running ahead exists. Then, the estimating means estimates the presence and position of the vehicle running ahead in the sensor image on the basis of the data thus obtained. After the position of the vehicle running ahead on the image sensor is estimated, a tracing window of the size dependent on the distance up to the fore-running vehicle is determined based on the estimated position, and by the virtue of the determined tracing window, a distance up to the fore-running vehicle is detected during tracing a fore-running vehicle image. Accordingly, where a plurality of fore-running vehicles are present, the system can determine the fore-running vehicle to be measured for distance. Further, an image is traced in a manner that a tracing window enclosing the specified fore-running vehicle at the position where the fore-running vehicle is present anew, is set and the distance from the measuring-device carrying vehicle to the fore-running vehicle caught by the tracing window is detected. Therefore, the tracing window for tracing the fore-running vehicle image can be automatically set. This ensures a safety driving. Brief Description of the Drawings Fig. 1 is a diagram showing the construction of a inter-vehicle distance measuring device according to an embodiment of the present invention. Fig. 2 is a diagram showing a plural number of distance measuring windows displayed on the screen of a display unit. Fig. 3 is a diagram showing the window-contained display on the screen in which a fore-running vehicle image comes out. Fig. 4 is a diagram showing the comparison reasons of the image signals to be compared with the reference image signals within the distance measuring windows. Fig. 5 is a diagram showing the symmetry detecting gate and the symmetry detecting region within which the symmetry detecting gate is shifted. Fig. 6 is a diagram showing the symmetry detecting gate of large size and the symmetry detecting region. Fig. 7 is a diagram showing a state that a tracing window is set on a fore-running vehicle image. Fig. 8 is a diagram showing a state that a large tracing window is set on a fore-running vehicle image. Fig. 9 is a flow chart showing the operation of the inter-vehicle distance measuring device. Fig. 10 is a diagram showing the construction of a conventional inter-vehicle distance measuring device. Detailed Description of the Preferred Embodiment Embodiment 1 In Fig. 1, lenses 1 and 2 which are separated by a base line L constitute upper and lower optical systems, respectively. Two-dimensional image sensors 3 and 4 are disposed in association with the lenses 1 and 2, respectively. A fore-running motor vehicle 5 is traced. Analog-to-digital (A/D) converters 6 and 7 convert the analog signals outputed from the image sensors 3 and 4 into digital signals. Memories 8 and 9 store the signals from the A/D converters 6 and 7, respectively. A microcomputer 10 is connected to memories 8 and 9, a display screen 11 and a window setting unit 12 described bellow. The display 11 is provided to display an image picked up by the upper image sensor 3 under control of the microcomputer 10. The window setting unit 12 which is connected to the display 11 sets a window to specify an object under distance measurement on the display. The window setting unit 12 is connected to an image-trace instruction switch 13 operated by a driver. In the inter-vehicle distance measuring device thus arranged, when a driver operates the image-trace instruction switch 13, the window setting unit 12, as shown in Fig. 2, sets a plurality of distance measuring windows 21, 22, 23, 24, 25, and 26 on the display 11 visually presenting an image of a view lying ahead of the device-carrying vehicle, which is picked up by the upper image sensor 4. As shown in Fig. 2, those windows which is located at predetermined positions on the screen are horizontally and equidistantly arrayed in line. As shown in Fig. 3, when a fore-running vehicle image 5a comes out on the screen, the vehicle image is caught by those windows 21 to 26. The windows measure the distances up to the object caught by the windows themselves. The distance measuring method will be described by typically using the window 24 catching the fore-running vehicle image 5a. To start with, the microcomputer 10 reads an image signal within the window 24 from the memory 9, and uses the image signal as a reference image signal for a distance calculation. As seen from the illustration of Fig. 4, the area in the memory 8 relative to the distance calculation is a comparison area 30 corresponding to the location of the distance measuring window 24. The area 30 is an area resulting from vertical expansion of the area corresponding to the window 24. Namely, the microcomputer 10 selects the memory area 30 in the memory 8 storing the image signal of the lower image sensor 3 corresponding to the window 24, to shift the image signal of the memory area 30 with respect to the reference image signal 24, pixel by pixel, to accumulate the differences between the corresponding pixels in the images in one shift, thereby obtaining a total sum of the absolute values of the differences. That is, a position of the image in the memory area 30 being most resemblant to the image of the window 24 is obtained by shifting the image signals pixel by pixel. Assuming that pixel number n is a shifting amount when the corresponding pixels are successively compared and the total sum of the absolute values of the difference signals takes the minimum value, further assuming the pixel pitch P, the base length L of the optical systems and the focal distances f of the lenses 1 and 2, a distance R from the measuring-device carrying vehicle to the fore-running vehicle 5 is expressed by the following equation. In this way, the distance R from the device-carrying vehicle to the fore-running vehicle 5 can be measured. [Formula 2] R = (f × L)/(n × P) With respect to the distance measuring windows 23 and 25 catching the fore-running vehicle image 5a, the distance R is measured by the similar system. And with respect to the distance measuring windows 21, 22 and 26 catching an object without the fore-running vehicle 5, each distance to the object caught by the windows is measured. Thus, the objects caught by the distance measuring windows 21 to 26 on the display 11 as shown in Fig. 4 are compared with the image within the comparison regions 27 to 32 corresponding to the respective windows, whereby measuring the distances to the objects caught by the windows 21 to 26. The microcomputer 10 compares the measuring values of the windows 21 to 26 with each other, and selects the adjoin windows containing a same measuring distance which is relatively short. In other words, the distance measuring windows 23, 24 and 25 which catch the objects located in a shorter distance than that of a background are selected. It is estimated that the fore-running vehicle 5 would be present at the locations of the windows 23, 24 and 25 unless measuring values of those windows abruptly change with time. In a case where an object to be measured for distance is a motor vehicle, the silhouette of the vehicle is generally symmetrical. By utilizing such a feature, the system detects the symmetry within the area in which the distance measuring windows 21 to 26 are set, and determines the presence and position of the fore-running vehicle image 5 on the display 11, in addition to the distance information. As shown in Fig. 5, a symmetry detecting gate 33 having the size dependent on the distance R is formed on the basis of the distance data obtained by the windows 23, 24 and 25 catching the fore-running vehicle 5 as shown in Fig. 5. In the memory 9, the symmetry detecting gate 33 is shifted within a symmetry detecting region 34 containing the distance measuring windows 21 to 26. Through the shifting motion of the symmetry detecting gate 33, the equation (3) is operated for the image signal within the symmetry detecting gate 33 in order to evaluate the symmetry of the silhouette of the vehicle. In the equation (3), S(i, j) represents an image signal within the symmetry detecting region 34, and expresses an optical density in 256 gray levels. Pixel numbers m and n represent the width and height of the symmetry detecting gate 33. Pixel number w indicates the width of the symmetry detecting region 34. k = 1, 2, ..., w-m. [Equation 3] The best symmetry is obtained at the position where the value of A k is the smallest. Actually, some silhouettes having symmetry are present except the fore-running vehicle 5. Accordingly, the result of calculation of the equation (3) have some minimum values. The positions of the minimum values are candidates for the position of the fore-running vehicle 5. The fore-running vehicle 5, or the position of it, can be selected from those candidate points on the basis of the stability data with respect to time passing and the distance data by the windows 21 to 26. In this way, the position of the fore-running vehicle image 5a is determined on the display 11. A tracing window 35, as shown in Fig. 7, is set at the determined position on the display 11 by the window setting unit 12. The tracing window 35 has the same size as that of the symmetry detecting gate 33 that is set according to the measuring values of the distance measuring windows 23, 24, and 25. The size of the fore-running vehicle image 5a varies according to the distance R up to the fore-running vehicle 5. When the distance measuring value is large, the tracing window 35 of small size is set. When the measuring value is small, a symmetry detecting gate 33a is set to a large size as shown in Fig. 6. In this case, the symmetry detecting region 34a is as shown in Fig. 6, so that a large tracing window 35a is set as shown in Fig. 8. After the tracing window 35 enclosing the fore-running vehicle image 5a is set as described above, the tracing window 35 starts to trace the fore-running vehicle image 5a The image tracing operation is similar to that of the conventional device disclosed in Japanese Patent Laid-Open Publication No. Sho. 60-33352 and Japanese Patent Laid-Open Publication No. Hei. 1-35305. No further explanation of it will be given. Furthermore, a distance between the fore-running vehicle 5 caught by the tracing window 35 and the measuring-device carrying vehicle is detected. As the detection process, an image signal within the tracing window 35 tracing the fore-running vehicle image 5a is used as a reference image signal for the calculation of inter-vehicle distance. Further, the corresponding pixels of the vertical image signals are compared successively and the principle of the trigonometric measurement is used as in the manner described in connection with the equation (2). The tracing window 35 traces the fore-running vehicle image 5a even if the vehicle image 5a moves to depict any trace on the display 11, and continuously detects the inter-vehicle distance between the fore-running vehicle 5 and the measuring-device carrying vehicle. The above operation can be explained by a flow chart as shown in Fig. 9. When a fore-running vehicle 5 appears in a forward place of the measuring-device carrying vehicle, a driver operates the image-trace instruction switch 13 (step 101). The microcomputer 10, when receiving an image-trace instruction signal from the switch, controls the window setting unit 12, causing it to set the plural number of distance measuring windows 21 to 26 at given positions on the display 11 (step 102). Then, it measures the distances from the measuring-device carrying vehicle to the objects caught by the windows 21 to 26 (step 103). The measured distance data and the window positions are stored into the memory (step 104). The adjacent windows which are for the relatively short distance and provide substantially the same measuring values are selected from those windows 21 to 26. Presence of the fore-running vehicle 5 is estimated on the basis of the selected windows (step 105). Then, the symmetry detecting gate 33 having the size dependent on the measuring distance is provided, and detects the symmetry of the silhouette of the fore-running vehicle within the symmetry detecting region 34. And the candidate regions of the fore-running vehicle 5 are picked up (step 106). The system determines the fore-running vehicle image 5a by using the symmetry stability and distance data (step 107). The distance measuring windows 21 to 26 are erased on the display 11 (step 108). The tracing window 35 having the size dependent on the inter-vehicle distance is set at the thus selected position (step 109). Thereafter, the distance between the fore-running vehicle 5 caught by the tracing window 35 and the measuring-device carrying vehicle is continuously detected (step 110: the system returns to the step 109 after executing the step 110). As described above, a fore-running vehicle running ahead of the measuring-device carrying vehicle is picked up through a pair of upper and lower optical systems. One of the images picked up by the image sensor is displayed, while a plural number of distance measuring windows are set at given positions on the screen. Distances from the measuring-device carrying vehicle to the objects caught by the distance measuring windows are measured. The symmetry of the image is detected. In this way, the presence and position of the fore-running vehicle are estimated.

Claims: Distance measuring device installed on a first vehicle for measuring a distance to a second vehicle running ahead of the first vehicle, comprising: a) a pair of image sensors (3, 4) arranged vertically each for picking up an image (5a) of an object including the second vehicle (5) and a background, characterized in that b) a window setting means (12) is provided for setting a plurality of distance measuring windows (21-26) at predetermined positions, respectively, in the image (5a) picked up by one of the image sensors (3, 4), and c) a comparison region setting means (10) for setting a plurality of comparing regions (27-32) corresponding to the distance measuring windows (21-26), respectively, in another image picked up by the other image sensor (4, 3), d) a first distance detecting means (10) for comparing image signals contained within the distance measuring windows (21-26) with image signals contained within the comparing regions (27-32) corresponding to the distance measuring windows (21-26), respectively, to detect each distance (R) to a part of the second vehicle (5) contained within each of the distance measuring windows (21-26) from the first vehicle, e) a symmetry detecting gate setting means (10) for setting a symmetry detection gate (33) at a predetermined position in a symmetry detection region (34) lying in the image (5a) picked up by one of the image sensors (3, 4), f) a symmetry detecting means (10) for detecting symmetry in each of symmetry detection gate (33), and g) an estimating means (10) for estimating presence and position of the second vehicle (5) in the image (5a) picked up by the one of the image sensors (3, 4) on the basis of data obtained by the symmetry detecting means (10) and the first distance detecting means (10). Distance measuring device according to claim 1, characterized in that the estimating means (10) selects adjoining distance measuring windows (21-26) having the same and shortest distance as distance measuring windows (21-26) including the second vehicle (5) and that the estimating means (10) determines the symmetry detection gate (33) of the symmetry detection region (34) for the symmetry detection corresponding to the detected distance (R) of the adjoining distance measuring windows (21-26). Distance measuring device according to claim 1, characterized in that the estimating means (10) determines the position of the second vehicle (5) in the image (5a) picked up by one of the image sensors (3, 4) on the basis of a stability of the symmetry and the detected distance (R). Distance measuring device according to claim 1, characterized in that it further comprises a tracing window setting means (12) for automatically setting a tracing window (35) including the second vehicle and having the size corresponding to the distance (R) up to the second vehicle (5), tracing means (10) for tracing the second vehicle (5) in the image (5a) picked up by one of the image sensors (3, 4) with the tracing window (35), and second distance detecting means (10) for detecting a distance (R) up to the second vehicle (5) included in said tracing window (35) . Distance measuring device according to claim 4, characterized in that the size of the tracing window (35) corresponds to the size of the symmetry detecting gate (33). Distance measuring device according to claim 1, characterized in that is further comprises display means (11) for displaying the image (5a) picked up one of the image sensors (3, 4) and for displaying the distance measuring windows (21-26).

Inventors: TATSUJI IRIE, TATSUJI, IRIE,, Tatsuji, Irie, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: G01S11
IPC codes: G06T1, B60R21, G01C3, B60W30, G01S11