Single camera system and method for range and lateral position measurement of a preceding vehicle

[0001] 1. Field of the Invention

[0002] The present invention generally relates to a system and method for range and lateral position measurement of a preceding vehicle.

[0003] 2. Description of Related Art

[0004] Radar and stereo camera systems for adaptive cruise control (ACC), have been already introduced into the market. Recently, radar has been applied to for pre-crash safety systems and collision avoidance. Typically, the range and lateral position measurement of a preceding vehicle is accomplished utilizing radar and/or stereo camera systems. Radar systems can provide a very accurate range. However, millimeter wave type radar systems such as 77 Ghz systems are typically quite expensive. Laser radar is low cost but has the drawback of requiring moving parts which scan the laser across a field of view. Furthermore, the laser is less effective in adverse weather conditions which may effect the reflectivity of the preceding vehicle. For example, if mud covers the reflectors of the preceding vehicle, the reflectivity of the preceding vehicle will be less which minimizes the effectiveness of the laser. Finally, radar generally is not well suited to identify the object and give an accurate lateral position.

[0005] Stereo cameras can determine the range and identity of an object. However, these systems are typically difficult to manufacture due to the accurate alignment required between the two stereo cameras and requires two image processors.

[0006] In view of the above, it can be seen that conventional ACC systems typically do not have a high cost-performance ratio even though they may perform to the desired functional requirements. Further, it is apparent that there exists a need for an improved system and method for measuring the range and lateral position of the preceding vehicle.

[0007] In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides a system for determining range and lateral position of a vehicle. The primary components of the system include a single camera, at least one sonar sensor and a processor. The camera is configured to view a first region of interest containing a preceding vehicle and generate an electrical image of the region. The processor is in electrical communication with the camera to receive the electrical image. To analyze the electrical image, the processor identifies a series of windows within the image, each window corresponding to a fixed physical size at a different target range. For example, from the perspective of the camera the vehicle will appear larger when it is closer to the camera than if it is further away from the camera. Accordingly, each window is sized proportionally in the image as it would appear to the camera at each target range. The processor evaluates characteristics of the electrical image within each window to identify the vehicle. For example, the size of the vehicle is compared to the size of the window to create a size ratio. A score is determined indicating the likelihood that certain characteristics of the electrical image actually correspond to the vehicle and also that the vehicle is at the target range for that window.

[0008] The sonar sensor is configured to view a second region of interest. The processor receives data indicating if the preceding vehicle is in the second field of view. If the processor determines that the preceding vehicle is within the second field of view, the processor will calculate the range of the preceding vehicle based on the data generated by the sonar sensor.

[0009] In another aspect of the present invention, the characteristics of electrical image evaluated by the processor includes the width and height of edge segments in the image, as well as, the height, width, and location of objects constructed from multiple edge segments. The position of the window in the electrical image is calculated based on the azimuth angle and the elevation angle of the camera.

[0010] In yet another aspect of the present invention, a method is provided for identifying the vehicle within the electrical image and a sonar signal to determine the vehicle range. To simplify the image, an edge enhanced algorithm is applied to the image. Only characteristics of the electrical image within a particular window are evaluated. The edge enhanced image is trinalized and segmented. The segments are evaluated and objects are constructed from multiple segments. A score is determined for each object based on criteria, such as, the object width, object height position, object height, and segment width. Based on the width ratio, the range and lateral position of the object are estimated on the basis of the target range of the window. However, if the sonar signal indicates that the object is closer than estimated by evaluating the electrical image, the sonar signal will be used to estimate the range of the object.

[0011] Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification.

[0026] Referring now to FIG. 1, a system embodying the principles of the present invention is illustrated therein and designated at 10. As its primary components, the system 10 includes a camera 12, a sonar sensor 15 and a processor 14. The camera 12 is located in the rearview mirror to collect an optical image of a first region of interest 16 including a vehicle 18. The sonar sensor 15 is located near the front bumper of the vehicle to collect data of a second region of interest 17. Typically, the first and second regions of interest 16, 17 partially overlap.

[0027] The optical image received by the camera 12, is converted to an electrical image that is provided to the processor 14. To filter out unwanted distractions in the electronic image and aid in determining the range of the vehicle 18, the processor 14 calculates the position of multiple windows 20, 22, 24 within the first region of interest 16. The windows 20, 22, 24 are located at varying target ranges from the camera 12. The size of the windows 20, 22, 24 are a predetermined physical size (about 4×2 m as shown) and may correspond to the size of a typical vehicle. To provide increased resolution the windows 20, 22, 24 are spaced closer together and the number of windows is increased. Although the system 10, as shown, is configured to track a vehicle 18 preceding the system 10, it is fully contemplated that the camera 12 could be directed to the side or rear the system 10 to track a vehicle 18 that may be approaching from other directions.

[0028] Now referring to FIG. 2, an electronic image of the first region of interest 16 as viewed by the camera 12 is provided. The windows 20, 22, 24 are projected into their corresponding size and location according to the perspective of the camera 12. The vehicle 18 is located between windows 22 and 24, accordingly, the size of the vehicle 18 corresponds much more closely to the height and width of windows 22 and 24 than window 20. As can be seen from FIG. 1, although the size and width of the windows are physically constant at each target range, the window sizes appear to vary from the perspective of the camera 12. Similarly, the height and width of the preceding vehicle 18 will appear to vary at each target range. The perspective of the camera 12 will affect the apparent size and location of the preceding vehicle 18 within the electrical image based on the elevation angle and the azimuth angle of the camera 12. The processor 14 can use the location and size of each of the windows 20, 22, 24 to evaluate characteristics of the electrical image and determine a score indicating the probability the vehicle 18 is at the target range associated with a particular window.

[0029] Now referring to FIG. 3, a side view of the system 10 is provided illustrating the use of the elevation angle in calculating the height and position of the window 20 within the electrical image. The elevation angle is the angle between the optical axis of the camera 12 and the surface of the road. The lower edge of window 20 is calculated based on Equation (1).
θ₁=a tan(−r1/hc)  (1)
Where hc is the height of the camera 12 from the road surface, r1 is the horizontal range of window 20 from the camera 12, and the module is [0, π].

[0030] Similarly, the upper edge of the first window is calculated based on Equation (2).
θ_1h=a tan(rc/(hw−hc))  (2)
Where hw is the height of the window, hc is the height of the camera 12 from the road surface, r1 is the range of window 20 from the camera 12, and the module is [0, π]. The difference, Δθ₁=θ₁−θ_1h, corresponds to the height of the window in the electronic image.

[0031] Now referring to FIG. 4, the horizontal position of the window in a picture corresponds to the azimuth angle. The azimuth angle is the angle across the width of the preceding vehicle from the perspective of the camera 12. The right edge of the range window 20 is calculated according to Equation (3).
φ₁=a tan(−width₁₃ w/(2*r1))+(π/2)  (3)

[0032] Similarly, the left edge of the range window 20 is calculated according to Equation (4).
φ_1h=a tan(width₁₃ w/(2*r1))+(π/2)  (4)
Where window w is the distance from the center of the window 20 to the horizontal edges, r1 is the horizontal range of the window 20 from the camera 12, and the module is [−π/2, π/2].

[0033] The window positions for the additional windows 22, 24 are calculated according to Equations (1)-(4), substituting their respective target ranges for r1.

[0034] Now referring to FIG. 5A, the electronic image is shown relative to window 20. Notice the width of the object 26 is about 30% of the width of the window 20. If the window width is set at a width of 4 m, about twice the expected width of the vehicle 18, the estimated width of the object 26 at a distance of r1 would equal 4×0.3=1.2 m. Therefore, the likelihood that the object 26 is the vehicle 18 at range r1 is low. In addition, the processor 14 evaluates vertical offset and object height criteria. For example, the distance of the object 26 from the bottom of the processing window 20 is used in determining likelihood that the object 26 is at the target range. Assuming a flat road, if the object 26 were at the range r1, the lowest feature of the object 26 would appear at the bottom of the window 20 corresponding to being in contact with the road at the target range. However, the object 26 in FIG. 5A, appears to float above the road, thereby decreasing the likelihood it is located at the target range. Further, the extracted object 26 should have a height of 0.5 m or 1.2 m. The processor 14 will detect an object height of 0.5 m if only the bottom portion of the vehicle 18 is detected or 1.2 m if the full height of the vehicle 18 is detected. The closer the height of the object 26 is to the expected height the more probable the object 26 is the vehicle 18 and the more probable it is located at the target range r1. The vertical offset, described above, may also affect the height of the object 26, as the top of the object, in FIG. 5A, is chopped off by the edge of the window 20. Therefore, the object 26 appears shorter than expected, again lowering the likelihood the object is the vehicle 18 at the range r1.

[0035] Now referring to FIG. 5B, the electronic image is shown relative to window 22. The width of the object 27 is about 45% of the window 22. Therefore, the estimated width of the object 27 at range r2 is equal to 4×0.45−1.8 m much closer to the expected size of the vehicle 18. In this image, the object 27 is only slightly offset from the bottom of the window 22, and the entire height of the object 27 is still included in the window 22.

[0036] Now referring to FIG. 5C, the electronic image is shown relative to window 24. The width of the object 28 is about 80% of the width of the window 24. Accordingly, the estimated width of the object 28 at range r3 is equal to 4×0.08=3.2 m. Therefore, the object width is significantly larger than the expected width of vehicle 18, usually about 1.75 m. Based on the object width, the processor 14 can make a determination that object 27 most probably corresponds to vehicle 18 and r2 is the most probable range. The range accuracy of the system 10 can be increased by using a finer pitch of target range for each window. Using a finer pitch between windows is especially useful as the vehicle 18 is closer to the camera 12, due to the increased risk of collision.

[0037] Now referring to FIG. 6, a typical electronic image as seen by the camera 12 is provided and will be used to further describe the method implemented by the processor 14 to determine the range and lateral position of the vehicle 18. The electronic image includes additional features that could be confusing for the processor 14 such as the lane markings 30, an additional car 32, and a motorcycle 34.

[0038] FIG. 7 shows a vertically edge enhanced image. The electronic image is comprised of horizontal rows and vertical columns of picture elements (pixels). Each pixel contains a value corresponding to the brightness of the image at that row and column location. A typical edge enhancement algorithm includes calculating the derivative of the brightness across the horizontal rows or vertical columns of the image. However, many other edge enhancement techniques are contemplated and may be readily used. In addition, the position and size of the window 36 is calculated for a given target range. Edge information located outside the window 36 is ignored. In this instance, much of the edge enhanced information from the car 38 and the motorcycle 40 can be eliminated.

[0039] Now referring to FIG. 9, the edge enhanced image is then trinarized, meaning each of the pixels are set to a value of −1, +1, or 0. A typical method for trinarizing the image includes taking the value of each pixel value and applying an upper and lower threshold value, where if the brightness of the pixel value is above the upper threshold value, the pixel value is set to 1. If the brightness of the pixel value is below the lower threshold value, the pixel value is set to −1. Otherwise, the pixel value is set to 0. This effectively separates the pixels into edge pixels with a bright to dark (negative) transition, edge pixels with a dark to bright (positive) transition, and non-edge pixels. Although, the above described method is fast and simple, other more complicated thresholding methods may be used including local area thresholding or other commonly used approaches. Next, the pixels are grouped based on their relative position to other pixels having the same value. Grouping of these pixels is called segmentation and each of the groups is referred to as a line-segment. Height, width and position information is stored for each line-segment.

[0040] Relating these segments back to the original image, Segment 42 and 43 represent the lane marking on the road. Segment 44 represents the upper portion of the left side of the vehicle. Segment 46 represents the lower left side of the vehicle. Segment 48 represents the left tire of the vehicle. Segment 50 represents the upper right side of the vehicle. Segment 52 represents the lower right side of the vehicle while segment 54 represents the right tire.

[0041] Still referring to FIG. 9, objects may be constructed from two segments having different polarity. Segment 42 and segment 44 are combined to construct object 56. Segment 42 and segment 46 are combined to construct object 58. In segment 46 and segment 52 are combined to construct object 60. Each of the objects are then scored based on the width of the object, the height of the object, the position of the object relative to the bottom edge of the window, the segment width, and the segment height. The above process is repeated for multiple windows with different target ranges. The object with the best score is compared with a minimum score threshold. If the best score is higher than the minimum score threshold the characteristics of the object are used to determine the object's range and lateral position.

[0042] Now referring to FIG. 10, a sideview of the system 10 is provided illustrating the use of the sonar sensor 15 in estimating the range of a large vehicle 19, such as a semi truck having a trailer. In certain low speed situations, such as a traffic backup, it is preferable to follow the preceding vehicle at a close distance. As shown in FIG. 10, the larger vehicle 19 is within the first and second regions of interest 16, 17.

[0043] Now referring to FIG. 11, a typical electronic image as seen by the camera 12 is provided and will be used to further describe the method implemented by the processor 14 to determine the range and lateral position of the larger vehicle 19. When following closely to a larger vehicle 19, only a portion of the larger vehicle 19 will be within view of the camera 12. The electronic image as seen in FIG. 11 will be vertically edge enhanced and then trinalized as described previously.

[0044] Referring to FIG. 12, a trinalized image of the electronic image is shown. However, because of the size of the larger vehicle 19 and the closeness of the larger vehicle 19 to the camera 12, only the edge of the larger vehicle, as represented by segment 64, is shown. The processor 14 will be unable to determine if the edge 64 represents a vehicle, such as the larger vehicle 19, a lane marker, a tree or other object.

[0045] In order to take into account situations as described above, the processor 14 will receive data from the sonar sensor 15. The sonar sensor 15 has a shorter range than the camera 12 as illustrated by the second region of interest 17. The inherent characteristics of sonar sensor 15, which are well known in the art, allow the sonar sensor 15 to detect large objects, such as the larger vehicle 19, that would otherwise be undetectable to the processed image captured by camera 12. If the processor 14 received data from the sonar sensor 15 indicating that a vehicle is present and undetected by the camera 12 in the first region of interest, the processor 14 will use the data generated by the sonar sensor 15 to calculate the range of the larger vehicle 19.

[0046] As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.