Method for determining the circumferance of a wheel of a motor vehicle

Abstract

The invention relates to a method for determining the circumference of a wheel (2G) of a motor vehicle (1), the vehicle comprising a camera (11G) positioned and oriented so as to observe at least part of a periphery of the wheel for at least one steering angle of the wheel, the method comprising: - a first step (E1) of determining a steering angle of the wheel; - a second step (E2) of acquiring, by means of the camera, a first image (I1) comprising at least part of the periphery of the wheel; then - a third step (E3) of identifying, on said first image, a point cloud (15) belonging to the periphery of the wheel; then - a fourth step of calculating the circumference of the wheel on the basis of the point cloud and on the basis of the steering angle of the wheel.

Description

Description Title of the invention: Method for determining the circumference of a wheel of a motor vehicle. Technical field of the invention

[0001] The invention relates to a method for determining the circumference of a wheel of a motor vehicle. The invention also relates to a motor vehicle comprising a wheel and hardware and software adapted to implement such a method for determining the circumference of the wheel. Prior art

[0002] Knowing the exact position of a motor vehicle is essential for implementing an automatic or semi-automatic parking procedure. During this process, the vehicle performs a series of maneuvers, approaching within very close range of obstacles marking a parking space. To minimize the safety distances from these obstacles, precise knowledge of the vehicle's position is crucial.

[0003] The wheels of motor vehicles are generally equipped with sensors that measure their angular position or angular velocity. By combining information about a wheel's angular position or angular velocity with its circumference, a given vehicle displacement can be calculated. Therefore, to improve the efficiency of automated vehicle parking systems, it is useful to know the circumference of at least one of the vehicle's wheels with high precision.

[0004] To determine the circumference of a vehicle's wheel, it is common practice to enter the tire type mounted on the wheel via an on-board interface. However, this method is very imprecise because there is actually a wide variety of dimensions between different tire models. Furthermore, gradual tire wear and variations in tire inflation pressure also affect their circumference. These variations are not taken into account by current methods of determining wheel circumference. Presentation of the invention

[0005] The object of the invention is to provide a method for determining the circumference of a wheel which remedies the above disadvantages and improves upon the methods known in the prior art.

[0006] More specifically, a first object of the invention is a particularly precise method for determining the circumference of a wheel. Summary of the invention

[0007] To this end, the invention relates to a method for determining the circumference of a wheel of a motor vehicle, the vehicle comprising a camera positioned and oriented so as to observe at least a portion of the wheel's circumference for at least one steering angle of the wheel, the method comprising: - a first step in determining the steering angle of the wheel, - a second acquisition step, using said camera, of a first image including at least part of the circumference of the wheel, then - a third step of identifying, on the said first image, a point cloud belonging to the perimeter of the wheel, then - a fourth step of calculating the circumference of the wheel based on the point cloud and based on the steering angle of the wheel.

[0008] The said camera can be attached to a side of the vehicle, in particular to an external side mirror of the vehicle.

[0009] The second step may include: - a substep of capturing, using said camera, a raw image including at least a part of the wheel's circumference, then - a sub-step of generating said first image by correcting a distortion of the raw image provided by the camera.

[0010] This second step can be executed when the steering angle of the wheel is at its maximum.

[0011] The fourth step may include a sub-step of projecting the point cloud into a plane perpendicular to an axis of rotation of the wheel.

[0012] The fourth step may include: - a step of calculating the position of a wheel center based on the point cloud belonging to the wheel's perimeter, then - a step of calculating the radius of the wheel based on the position of the center of the wheel and based on at least one point in the point cloud belonging to the circumference of the wheel, then - a step of calculating the circumference of the wheel as a function of the radius of the wheel.

[0013] The step of calculating the position of the wheel center may include: - a step of selecting a triplet comprising a first point, a second point and a third point from the point cloud belonging to the perimeter of the wheel, then - a step of calculating a first perpendicular bisector of a first segment defined by the first point and the second point and the calculation of a second perpendicular bisector of a second segment defined by the second point and the third point, then- a step of calculating a point of intersection of the first perpendicular bisector and the second perpendicular bisector.

[0014] The step of calculating the position of the center of the wheel on said first image may include a plurality of repetitions of the step of selecting a triplet, of the step of calculating the first perpendicular bisector and the second perpendicular bisector, and of the step of calculating the point of intersection of the first perpendicular bisector and the second perpendicular bisector, then a step of calculating an average between the points of intersection of the first perpendicular bisector and the second perpendicular bisector calculated at each repetition of the step of calculating the point of intersection of the first perpendicular bisector and the second perpendicular bisector.

[0015] The invention also relates to a computer program comprising instructions which, when the program is executed by a computer, lead the computer to implement the determination process as defined above.

[0016] The invention also relates to a motor vehicle comprising - a wheel, in particular a steering wheel, - a camera positioned and oriented so as to observe at least part of the wheel's circumference for at least one wheel steering angle, and - a computing unit connected to the camera, the computing unit comprising a microprocessor and a memory comprising instructions which, when executed by the microprocessor, cause it to implement the determination process as defined above. Presentation of the figures

[0017] These objects, features and advantages of the present invention will be described in detail in the following description of a particular embodiment, given by way of non-limiting example, with reference to the accompanying figures, among which:

[0018] Fig. 1 is a schematic top view of a motor vehicle according to one embodiment of the invention.

[0019] Figure [Fig. 2] is a synoptic diagram of a method for determining the circumference of a wheel of the vehicle in Figure [1].

[0020] The [Fig.3] is a raw image provided by a camera mounted in the vehicle.

[0021] [Fig.4] is an image obtained by correcting a distortion of the raw image of [Fig.2],

[0022] [Fig.5] is a schematic representation of part of the circumference of the vehicle's wheel. Detailed description

[0023] Figure 1 schematically illustrates, in top view, a motor vehicle 1 according to an embodiment of the invention. The vehicle 1 may, for example, be a passenger car, a commercial vehicle, a truck, or a bus. The vehicle 1 comprises two steering wheels 2G, 2D at the front. The vehicle 1 is also equipped with a steering system for controlling the steering angle of the steering wheels 2G, 2D. Conventionally, the steering system comprises a steering wheel 3 for operation by a driver of the vehicle 1 and a transmission mechanism mechanically connecting the steering wheel 3 to the steering wheels 2G, 2D. The transmission mechanism comprises a steering column 4, a rack 5, and two tie rods 6G, 6D. The steering column 4 extends from the steering wheel 3 to the rack 5, with which it cooperates via a pinion.Each steering tie rod 6G, 6D connects one end of the rack 5 to a steering wheel 2G, 2D. The steering system may also include an actuator 7 configured to control the steering of the steering wheels 2G, 2D autonomously, that is, independently of any action by a driver on the steering wheel 4.

[0024] The X-axis represents the longitudinal axis of vehicle 1. When moving forward in a straight line, vehicle 1 progresses from rear to front in a direction parallel to its longitudinal axis. The X-axis is oriented from the front to the rear of vehicle 1, that is, in the direction of reverse. The Y-axis represents the transverse axis of vehicle 1. The Y-axis is oriented from left to right, with left and right defined from the perspective of a driver of vehicle 1. The Z-axis represents the axis perpendicular to the X-axis and the Y-axis. Vehicle 1 is considered to be resting on a horizontal surface. The Z-axis is a vertical axis, oriented from bottom to top. The X, Y, and Z axes form an orthogonal coordinate system.

[0025] Typically, when the two steering wheels are pivoted to turn right or left, the steering angle of the right steering wheel (2D) can differ from the steering angle of the left steering wheel (2G). Hereafter, "steering wheel angle" will refer to the average of the steering angles of the two steering wheels.

[0026] The transmission mechanism mechanically linking the steering wheel 3 to the steering wheels 2G, 2D also includes mechanical stop means that mechanically limit the steering angle of the steering wheels 2. The absolute maximum steering angle of the steering system refers to the maximum steering angle permitted by the mechanical stop means. The absolute maximum steering angle can be between 35° and 45°, for example, approximately 40°.

[0027] The vehicle 1 may also include a steering angle sensor 8 for the steering wheels 2G, 2D. The steering angle sensor 8 may be arranged on the steering column 4 as illustrated in [Fig. 1], or alternatively at another location on the steering system. The steering angle sensor 8 is configured to provide data for calculating the steering angle of each steering wheel 2G, 2D.

[0028] Vehicle 1 can also include two rotation sensors 9G, 9D for each steering wheel 2G, 2D. Typically, each rotation sensor 9G, 9D can work with a magnetic encoder attached to each steering wheel 2G, 2D. Each rotation sensor 9G, 9D can be configured to provide the rotational speed and / or angular position of the respective steering wheel 2G, 2D.

[0029] Furthermore, the vehicle includes two external side mirrors 10G and 10D, arranged respectively on the left and right sides of the vehicle. The left external side mirror 10G is equipped with a camera 1IG for observing the environment on the left side of the vehicle. The camera 1IG can be used to identify obstacles and / or the presence of other road users on the left side of the vehicle. The camera 1IG can thus be part of a system for observing the environment all around the vehicle. The camera 1IG is attached to the left external side mirror 10G, preferably at one of the mirror's outer edges. The camera 1IG is positioned and oriented so that the left steering wheel 2G (hereafter simply referred to as wheel 2G) is at least partially within the field of view of the camera 1IG for at least one steering angle of the wheel.In particular, camera 1 IG is positioned and oriented so that wheel 2G enters at least partially into the field of vision of camera 1 IG when wheel 2G is turned completely to the left and / or completely to the right according to the absolute maximum steering angle of the steering system.

[0030] Advantageously, the 1IG camera has a wide field of view (FOV), preferably greater than or equal to 120°, or even greater than or equal to 150°, and preferably greater than or equal to 180°. The 1IG camera can be equipped with a fisheye or ultra-wide-angle lens. Unlike conventional lenses that minimize distortion, fisheye lenses exaggerate it, producing images where straight lines far from the center appear curved. The camera's FOV can be fixed relative to the vehicle's frame of reference. Alternatively, the camera's FOV could be variable. In this case, the camera's viewing axis can be determined using a suitable sensor.

[0031] The vehicle 1 also includes a processing unit 12 equipped with a memory 13 and a microprocessor 14. The processing unit 12 is connected directly or indirectly to the camera 1 IG. In particular, the processing unit 12 is intended to receive digitally encoded images captured by the camera 1 IG. Where applicable, the processing unit 12 is also connected to the steering angle sensor 8 and / or the rotation sensors 9G, 9D.

[0032] The memory 13 of the computing unit 12 is a data storage medium on which is stored a computer program comprising program code instructions for implementing a method of determining the circumference of wheel 2G according to an embodiment of the invention. The microprocessor 14 is capable of executing this computer program.

[0033] We assume that vehicle 1 is in the configuration illustrated in [Fig. 1]. Wheel 2G is turned fully to the left, meaning that the steering angle of wheel 2G is equal to the absolute maximum steering angle. The absolute maximum steering angle is known by design of the steering system, and the value of this angle is stored in memory 13 of the processing unit 12. In this configuration, the front half of wheel 2G protrudes from the left side of the vehicle. This configuration is optimal because it allows for the best possible observation of wheel 2G by camera 1IG.

[0034] According to another embodiment, the 2G wheel could also be turned completely to the right provided that the 1 IG camera manages to observe at least partially the outline of the 2G wheel when the 2G wheel is turned to the right.

[0035] According to yet another embodiment, the steering angle of wheel 2G could be strictly less than the absolute maximum steering angle. In this case, the value of the effective steering angle of wheel 2G could be calculated using information provided by the steering angle sensor 8. Preferably, the determination method is implemented with the largest possible steering angle to improve the observation of wheel 2G by camera 1IG. However, if camera 1IG is sufficiently offset from a plane in which an outer face of wheel 2G extends, the method could also be implemented when the steering angle of wheel 2G is zero. Thus, the method could also be applied to determining the circumference of a non-steering wheel of the vehicle. Advantageously, the steering angle of a non-steering wheel is known precisely since this angle is zero.Precise knowledge of the (zero) steering angle of a non-steering wheel allows, to some extent, to compensate for the loss of precision associated with observing this wheel with a very small angle.

[0036] We also assume that vehicle 1 is stationary. This allows us to obtain a particularly clear image of the 2G wheel using camera 11. Alternatively, the determination process could also be implemented while the vehicle is moving forward or backward.

[0037] A diagram of a method for determining the circumference of wheel 2G according to an embodiment of the invention is illustrated in [Fig.2],

[0038] In a first step El, we determine the steering angle of wheel 2G. As mentioned previously, the steering angle of wheel 2G can be equal to the absolute maximum steering angle stored in memory 13 and / or be provided by the steering angle sensor 8.

[0039] In parallel with the first step E1, in a second step E2, a first image II is acquired by means of camera 1 IG, comprising at least a portion of the wheel's circumference. More precisely, the second step may include a substep E21 of capturing, by means of said camera, a raw image 10 comprising at least a portion of the circumference of wheel 2G, and then a substep E22 of generating the first image II by correcting a distortion of the raw image provided by the camera. This correction operation is particularly useful if camera 1 IG includes an ultra-wide-angle lens and / or if wheel 2G appears near an edge of the raw image 10. An example of a raw image 10 is shown in [Fig. 3]. An example of the first image II is shown in [Fig. 4].4], Note that the expression "first image" does not mean that the process requires the use of a second image but aims only to identify said first image II, and to distinguish it from the raw image 10. Alternatively, the process could also be implemented on the basis of the raw image 10 but would in this case lead to a slightly less precise result.

[0040] In a third step E3, on the said first image II, a cloud of points 15 belonging to the perimeter of the wheel 2G is identified, that is to say a cloud of points 15 belonging to the tread of a tire of the wheel 2G. This identification can be achieved, in particular, by running shape and / or color recognition algorithms. It can also be achieved by identifying a tire contour in the first image II.

[0041] Note that the process does not require identification of the entire circumference of the wheel. Depending on the camera's position relative to wheel 2G and the steering angle of wheel 2G, only a portion of the wheel's circumference can be observed by camera 1IG. Advantageously, this portion is nevertheless as large as possible, preferably greater than or equal to 50% of the wheel's circumference, or even greater than or equal to 75% of the wheel's circumference.

[0042] Next, in a fourth step E4, the circumference of the wheel is calculated. 2G is determined based on the point cloud 15 identified during the third step E3 and the steering angle of the wheel determined during the first step E1. The invention therefore proposes to determine the circumference of wheel 2G using a camera mounted in the vehicle. The camera 1 IG, whose integration into the vehicle is initially intended to observe the vehicle's surroundings, is thus ingeniously used to observe at least a portion of a wheel of the vehicle and to determine its circumference.

[0043] Several methods can be used to calculate the circumference of wheel 2G. These methods can be empirical or theoretical. For example, according to one empirical method, during vehicle development, wheel 2G can be fitted with different tires of varying sizes, inflation pressures, and / or wear levels, and the wheel can be observed using camera 1 IG. For each configuration, an image obtained by camera 1 IG can be associated with a circumference obtained through measurement. This creates a database containing multiple images and their corresponding circumferences. Then, to calculate the circumference of wheel 2G, the database can be used to search for an image that most closely matches an image provided by camera 1 IG, thus determining the value of the wheel's circumference.

[0044] Another method, this time theoretical, is described below. This method has the advantage of requiring no prior calibration. First, in a first substep E41, the point cloud 15 identified during the third step E3 is projected onto a plane perpendicular to an axis of rotation of the wheel 2G. This projection is performed virtually, by applying geometric projection rules, based on the steering angle of the wheel, the position of the camera relative to the wheel, and the orientation of the camera's field of view. The position of the camera relative to the wheel and the orientation of the camera's field of view can be predetermined data, stored in memory 13. This projection allows the problem of determining the wheel diameter to be transposed to a two-dimensional problem, which is simpler to solve.This projection therefore makes it easier to perform the calculations that will follow.

[0045] Next, in a second substep E42, the position of a center 16 of the wheel 2G is determined as a function of the point cloud 15 belonging to the circumference of the wheel. Figure 5 illustrates the point cloud 15 after the first substep E41. It is assumed that only a portion of the circumference of the wheel was observable by means of camera 1IG. The points of the point cloud 15 are therefore distributed along an arc of a circle.

[0046] In a first substep E421, a triplet is selected comprising a first point PI, a second point P2, and a third point P3 belonging to the point cloud 15. Then, in a second substep E422, a first perpendicular bisector DI of a first segment defined by the first point PI and the second point P2 is calculated. Similarly, a second perpendicular bisector D2 of a second segment defined by the second point P2 and the third point P3 is calculated. Then, in a third substep E423, a point of intersection of the first perpendicular bisector DI and the second perpendicular bisector D2 is calculated. A mathematical formula previously coded and stored in memory 13 can be used to directly calculate the coordinates (Xc, Yc) of the point of intersection of the perpendicular bisectors as a function of the coordinates (XI, YI) of point PI, the coordinates (X2, Y2) of point P2, and the coordinates (X3, Y3) of point P3. This mathematical formula can be written as follows: <

[0047] In a simplified embodiment, steps E421, E422, and E423 are performed only once, and the center 16 of wheel 2G is defined as the point of intersection of the first perpendicular bisector DI and the second perpendicular bisector D2. In a more precise embodiment, steps E421, E422, and E423 are repeated N times. The center 16 of wheel 2G is then calculated, in a fourth substep E424, as the average of the N points of intersection of the first perpendicular bisector DI and the second perpendicular bisector D2 previously determined. The number of repetitions N can be determined according to the required precision and / or the precision of camera 1 IG. For example, N can be chosen to be at least 10, or even at least 50, or even at least 100.

[0048] At the end of the second substep E42, the position of the center 16 of the wheel 2G was determined. Then, in a third substep E43, a radius R of the wheel is calculated based on the position of the center 16 and on at least one point in the point cloud 15, specifically one of the points P1, P2, or P3 used previously. This simply requires calculating the distance between these two points. As before, this calculation can advantageously be repeated M times by considering several points in the point cloud 15, and then averaging the results obtained.

[0049] Next, in a fourth substep E44, the circumference P of wheel 2G is calculated as a function of the wheel's radius. For this, the well-known formula is used: P = 2 x Pi x R.

[0050] Following the procedure just described, we therefore have a particularly reliable estimate of the circumference of wheel 2G. Of course, this procedure could be repeated to determine the circumference of another wheel of the vehicle, in particular the right steering wheel 2D, by equipping the right external side mirror 10D with a second camera.

[0051] The circumference values ​​of wheels 2G and 2D can be advantageously used when implementing an automatic vehicle parking procedure. The data provided by the rotation sensors 9G and 9D can be advantageously combined with the previously determined circumferences to calculate the vehicle's movement during automatic parking with high precision. This allows for a parking procedure in which the vehicle approaches even closer to the obstacles defining a parking space, thus improving the efficiency of the parking process. The vehicle parking procedure can include autonomous control of the steering of the steerable wheels 2G and 2D using actuator 7.

[0052] As a side note, accurately determining a wheel's circumference can also be useful for detecting underinflation or overinflation of a tire, or for assessing tire wear. If the vehicle has another method for determining wheel circumference, this method could also be used to detect wheel alignment issues, since misalignment would lead to discrepancies between the circumference measurements obtained using the two methods.

Claims

Demands

1. A method for determining the circumference of a wheel (2G) of a motor vehicle (1), the vehicle comprising a camera (11G) positioned and oriented so as to observe at least a portion of a circumference of the wheel for at least one steering angle of the wheel, the method comprising: - a first step (El) of determining a steering angle of the wheel, - a second acquisition step (E2), using said camera, of a first image (II) comprising at least a part of the circumference of the wheel, then - a third step (E3) of identification, on said first image, of a cloud of points (15) belonging to the perimeter of the wheel, then - a fourth step of calculation of the circumference of the wheel as a function of the cloud of points and as a function of the steering angle of the wheel.

2. Method of determination according to the preceding claim, characterized in that said camera (1 IG) is fixed to a lateral side of the vehicle, in particular to an external side mirror (10G) of the vehicle.

3. A method for determining according to any one of the preceding claims, characterized in that the second step (E2) comprises: - a substep (E21) of capturing, using said camera (1 IG), a raw image (10) comprising at least a part of the circumference of the wheel, then - a sub-step (E22) of generating said first image (II) by correcting a distortion of the raw image supplied by the camera.

4. A method of determination according to any one of the preceding claims, characterized in that said second step (E2) is carried out when the steering angle of the wheel (2G) is at its maximum.

5. Method of determination according to any one of the preceding claims, characterized in that the fourth step (E4) comprises a substep (E41) of projecting the point cloud (15) into a plane perpendicular to an axis of rotation of the wheel.

6. A method of determination according to any one of the preceding claims, characterized in that the fourth step (E4) comprises: - a step (E42) of calculating the position of a center of the wheel as a function of the cloud of points belonging to the circumference of the wheel, then - a step (E43) of calculating a radius of the wheel as a function of the position of the center of the wheel and as a function of at least one point of the cloud of points (15) belonging to the circumference of the wheel, then - a step (E44) of calculating the circumference of the wheel as a function of the radius of the wheel.

7. Method of determination according to the preceding claim, characterized in that step (E42) of calculating the position of the center of the wheel comprises: - a step (E421) of selecting a triplet comprising a first point (PI), a second point (P2) and a third point (P3) from the point cloud (15) belonging to the perimeter of the wheel, then - a step (E422) of calculating a first perpendicular bisector (D1) of a first segment defined by the first point (PI) and the second point (P2) and the calculation of a second perpendicular bisector (D2) of a second segment defined by the second point (P2) and the third point (P3), then - a step of calculating a point of intersection of the first perpendicular bisector (D1) and the second perpendicular bisector (D2).

8. Method of determination according to the preceding claim, characterized in that the step of calculating the position of the center of the wheel on said first image comprises a plurality of repetitions of the step (E421) of selecting a triplet, of the step (E422) of calculating the first perpendicular bisector and the second perpendicular bisector, and of the step (E423) of calculating the point of intersection of the first perpendicular bisector and the second perpendicular bisector, then a step (E424) of calculating an average between the points of intersection of the first perpendicular bisector and the second perpendicular bisector calculated at each repetition of the step of calculating the point of intersection of the first perpendicular bisector and the second perpendicular bisector.

9. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to implement the determination method according to one of the preceding claims.

10. Motor vehicle (1) comprising - a wheel (2G), in particular a steering wheel, - a camera (1IG) positioned and oriented so as to observe at least a part of a circumference of the wheel for at least one steering angle of the wheel, and - a computing unit (12) connected to the camera, the computing unit comprising a microprocessor (14) and a memory (13) comprising instructions which, when executed by the microprocessor, cause the latter to implement the determination method according to one of claims 1 to 8.

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