Abstract:
A system and method of autonomously steering a vehicle responsively to failure of a lane-centering system through time-dependent steering angle correction to reduce jerk resulting from transition of steering control from a lane-centering system to a lane-keeping system or vise-versa.

Description:
FIELD OF THE PRESENT INVENTION 
       [0001]    The present invention relates to autonomous steering systems, and specifically, relates to the reduction of jerk during transfer of steering control from lane-centering to lane-keeping systems or from lane-keeping to lane-centering systems. 
       BACKGROUND 
       [0002]    Examples of autonomous steering systems include a lane-centering system directed at maintaining vehicle travel centered within a travel lane and a lane-keeping system directed at restoring travel within the lane upon failure of lane-centering system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    The components and their configurations, features, operation, and advantages of the steering-control system may best be understood in reference to the following detailed description in which: 
           [0004]      FIG. 1  is a schematic, perspective view of a vehicle equipped with a steering-control system, according to an example; 
           [0005]      FIG. 2  is a block diagram of a steering-control system, according to an example; 
           [0006]      FIG. 3  is a schematic, top view of a lane environment, as viewed by a sensor array of a steering-control system, according to an example; 
           [0007]      FIG. 4  is a flowchart of a method for reducing jerk when transferring steering control from a lane-centering to a lane-keeping system, according to an example; 
           [0008]      FIG. 4A  is a schematic view of vehicle negotiating a curve depicting multiple steering-angle calculations on the basis of vehicle and road curvature dynamics at given time intervals, according to an example; 
           [0009]      FIG. 4B  is a schematic view of a vehicle dynamics model of the vehicle of  FIG. 4A , according to an example; 
           [0010]      FIG. 5  is a graph depicting several limit functions depicting steering-angle limit as a function of time, according to an example; 
           [0011]      FIG. 5A  is data-plot depicting steering-angle command as a function of time in which no steering-angle limit has been imposed, according to an example; 
           [0012]      FIG. 5B  is a data-plot depicting steering-angle command as a function of time in which 1.0° steering-angle limit has been imposed, according to an example; 
           [0013]      FIG. 5C  is a data-plot depicting steering-angle command as a function of time in which 1.5° steering-angle limit has been imposed, according to an example; 
       
    
    
       [0014]    It should be appreciated that the sake of clarity certain elements in the figures may not be drawn to scale and reference labels may be repeated thought the figures to indicate corresponding or analogous elements. 
       DETAILED DESCRIPTION 
       [0015]    The following description sets forth numerous details to provide a thorough understanding of the jerk reduction system and it may be practiced without these specific details. It should also be appreciated that components and methods well-known to those skilled in the art have been omitted for the sake of clarity. 
         [0016]    The terms “autonomous”, “semi-autonomous”, “automated”, and “automatic” all refer to a steering-control system of a motor vehicle configured to control a path of travel on a road or a lane with reduced driver input. 
         [0017]    The steering-control system includes a primary steering system implemented as a lane-centering system configured to maintain vehicle travel centered within a lane or designated travel zone and a back-up system implemented as a lane-keeping system configured to restore travel to the designated travel zone responsively to vehicle travel straying from the travel zone, according to an example. 
         [0018]    In some examples, the steering-control system is configured to enable a driver to regain either long-term or momentary control of the vehicle from either of the lane-centering or the lane-keeping systems. In some examples, after lane-keeping system succeeds in restoring travel to the proper travel zone, steering control is automatically returned to the lane-centering system. 
         [0019]    Lane-centering system failure may be caused by, inter alia, erroneous deactivation, miscalculation of a corrective steering-angle, and misinterpretation of a travel position. 
         [0020]    The lane-centering or lane-keeping systems, in certain examples, sends corrective steering-angle commands to steering systems like, inter alia, an electrical power steering (EPS) system, or an active front steering (AFS) system or other steering systems. 
         [0021]    Turning now to the figures,  FIG. 1  is a schematic illustration of a vehicle  10  equipped with an autonomous steering-control system  100  including both lane-centering and a lane-keeping systems, sensor arrays  40 , and vehicle-dynamic sensors  30 . 
         [0022]    Road sensor array  40  is configured to capture data that may be used or processed by the steering-control system  100  to indentify vehicle position. Sensor array  40  are implemented as any one or combination of sensors like, inter alia, video cameras, Light Detection And Ranging (LIDAR) sensors, Laser Detection and Ranging (LADAR) sensors, radar, optical remote sensors, or other sensors providing data related to vehicle location with respect to road features like lane markers, road shoulders, median barriers, road edge, and other objects or features. Without diminishing in scope, the present discussion will discuss sensor array  40  implemented as a camera array. 
         [0023]    In some examples, vehicle  10  includes vehicle-dynamic sensors  30  like, inter alia, steering angle sensors, steering torque sensors, wheel speed sensors, inertial measurement units configured to capture vehicle dynamic parameters like, inter alia, steering angle and torque, lateral and longitudinal velocity, lateral and longitudinal velocity changes, yaw-rate, and wheel rotation. 
         [0024]      FIG. 2  is a block diagram of an autonomous steering-control system  100  according to an example, including one or more processors  110 , vehicle-dynamic array  30 , road sensor array  40 , long-term non-transitory storage  130 , memory  120  loaded with a lane-centering module  95 , a lane-keeping module  105 , a database  170  of vehicle dynamic information, digital-road data, or other relevant information. Steering-control system  100  also includes long term storage  170 , user-input device  140  and user-output device  150 . Input device  140  may be implemented as touch-screens, keyboard, microphone, pointer device, or other devices providing such functionality. User output device  150  may be implemented as any one or combination devices like a display screen, speakers or headphones, or other devices providing such functionality. 
         [0025]    Long term storage System  100  may also include a global positioning system (GPS) receiver  180 . 
         [0026]    Memory  120  may be implemented as Random Access Memory (RAM) or other memory types providing such functionality. 
         [0027]    Long-term non-transitory storage  130  may be implemented as a hard disk drive or other suitable non-volatile memory. 
         [0028]    For the purposes of this document, lane-centering functionality provided by the lane-centering module  95  renders the steering-control system  100  into a lane-centering system and, analogously, the lane-keeping functionality provided by the lane-keeping module  105  renders the steering-control system  100  into a lane-keeping system such that the lane-steering system  100  includes both types of steering control, according to an example 
         [0029]      FIG. 3  is a schematic, top view of vehicle  10  traveling in a demarcated lane environment and respective fields of view of a camera array, according to an example. As shown, road  300  is demarcated into a travel lane  305  by lane markers  310  that also define an outer boundary of a safety zone  320  also bound by inner boundary  325 . The inner boundaries  325  and lane center-line  464  are virtually imposed by the steering-control system, according to an example. Center-lane zone  330  between safety zones  320  typically spans between 65-95% of the lane width, in certain example. 
         [0030]    It should be appreciated that the position of vehicle  10  with respect to road features may also be determined in combination with GPS data and vehicle motion sensor data, according to other examples. 
         [0031]    As shown, vehicle  10  frontward camera has a frontward field of vision  340  of about 80 meters and a relatively narrow viewing range of about 40 degrees, for example. Frontward camera is linked to the lane-centering system whereas sideward-viewing and reward-viewing cameras are linked to the lane-keeping system, according to an example. The side and rear viewing fields of vision  360  and  365 , respectively, have a relatively short range from about 10 to 20 meters and a viewing angle of 130-180 degrees, in certain examples. Position data is processed and corrective action taken by the steering-control system every 10 milliseconds, according to an example. It should be appreciated that in other examples, processing timing is set at other set time intervals or changing time intervals. 
         [0032]    In operation, lane-centering system functions as a primary steering control and is configured to maintain vehicle travel centered in lane center-lane zone  330  while lane-keeping system is in standby mode until vehicle  10  drifts out of the safety zone  320  at which time steering control is transferred to the lane-keeping system until vehicle travel is restored to the center-lane zone  330 , as noted above. 
         [0033]      FIG. 4  is a flow diagram depicting steps involved in generating corrective steering-angle commands while minimizing vehicular jerk or lurch and is described in reference to  FIGS. 4A and 4B . 
         [0034]    Generally, in step  400  the lane-keeping system is actuated responsively to failure of the lane-centering system. In step  410 , a corrective steering angle derived from a cost function configured to minimize the lateral and the heading angle deviation of the vehicle predicted path from the lane center. Consequently, the corrective steering angle becomes large as vehicle  10  moves away from lane center  464 . Often this corrective steering angle is so large that it causes motional jerk. 
         [0035]    This cost-function steering-angle may cause excessive lateral jerk and motion discomfort. Therefore, in step  420 , a vehicle-dynamic steering-angle is calculated from road curvature, “ρ” and vehicle motion measurements and is used to temper the cost-function steering-angle. 
         [0036]    In step  440 , the maximum rate at which the corrective-steering angle is applied is calculated. 
         [0037]    In step  460 , the maximum rate-of-angle change calculated in step  440  is modified in accordance with design factors, in a certain example. 
         [0038]    In step  480 , a steering-angle-command is generated in accordance with a time-schedule. 
         [0039]    In step  490 , steering control is returned to lane-centering system 
         [0040]    Specifically, in step  400  the lane-keeping system is actuated responsively to failure of the lane-centering system, as noted above. Such failure is deemed to have occurred when steering-control system  100  identifies a front corner of vehicle  10  will traverse the inner boundary of lane marker  310  within 0.5 seconds, according to an example. 
         [0041]    At step  410  a corrective steering angle “δ” is derived by minimizing a cost function: 
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         [0042]    Wherein: 
         [0043]    t lk  is the time duration of the lane keeping system in operation, e.g, 6 sec. 
         [0044]    y err  is the lateral offset error, e.g., (y desired −y predicted ); 
         [0045]    φ err  is the heading angle error, e.g., (φ desired −φ predicted ); and, 
         [0046]    Q (t) and R (t) are weighting factors derived empirically. 
         [0047]    In  FIG. 4A , a sample, corrective cost-function path  461  having a relatively sharp and uncomfortable corrective steering angle  462  that must be tempered. 
         [0048]    In step  420 , vehicle-dynamic steering-angle is calculated from road curvature and vehicle motion measurements in accordance with the following vehicle dynamic equation: 
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         [0049]    As depicted in  FIG. 4B : 
         [0050]    “y” is lateral deviation from the lane center. 
         [0051]    “φ” is heading angle deviation. 
         [0052]    “v x ” is vehicle longitudinal speed. 
         [0053]    “v y ” is vehicle lateral speed. 
         [0054]    “r” is vehicle yaw rate. 
         [0055]    “δ” is steering angle. 
         [0056]    “a” and “b” are distances to the front and rear axle measured from the vehicle center, respectively. 
         [0057]    “m” is vehicle mass. 
         [0058]    “I” is vehicle inertia. 
         [0059]    “C f ”, “C r ” are front and rear cornering stiffness factors. 
         [0060]    “ρ” is road curvature. 
         [0061]    Values for y, φ, {dot over (φ)}, {dot over (v)} y  {dot over (r)} may be measured from vehicle camera array  40  or vehicle-dynamic sensors  30 , obtained from road data held in a data base  175 , or from GPS data. Their values are plugged into the above vehicle dynamic equation and solved for “δ” at set time intervals that may be fixed anywhere between 10-100 milliseconds, according to examples. 
         [0062]    As shown in  FIG. 4A , vehicle-dynamic travel path  463  defined by the above vehicle-dynamic equation does not guide vehicle  10  to the center of lane  305  along center line  464 ; but rather, it follows the road curvature defined by lane markers  310  and based on the above-noted parameters. Therefore, its corrective steering angles are less than those generated by the cost function. Accordingly, vehicle-dynamic travel path  463  serves as a basis to temper relatively sharp corrective angles of cost-function travel path  461  in the following manner, according to an example. 
         [0063]    Each vehicle-dynamic steering-angle value “δ′” derived from the vehicle dynamic equation is modified by increasing it by about 50% as a safety margin and then compared to the corresponding cost-function steering-angle. If the cost-function steering-angle exceeds this modified, vehicle-dynamic steering-angle, the modified, vehicle-dynamic steering-angle is selected as the corrective steering-angle value rendered into a steering-angle command, according to a certain example. 
         [0064]    For example, if the vehicle-dynamic steering-angle is calculated as 2°, then the value is increased by 50% bringing it 3°. If the steering-angle value calculated from the cost function optimization is 3.5° , only 3° steering angle will be employed, according to an example. It should be appreciated that in other examples, the vehicle-dynamic steering-angle is increased by differing values. 
         [0065]    It should be appreciated that various steering angles and their rates may be rendered into corresponding torques using known relationships between steering angle and steering torque. 
         [0066]    In step  440 , the steering-control system calculates a maximum rate-of-angle change by setting the following derivation matrix equal to an empirically derived, lateral-jerk limit of 0.13 g/200 ms and solving for δ dot. All vehicle parameters are known constant vehicle parameters, V x , V y  may be obtained from vehicle sensors  30 , according to an example. V x  is assumed a constant for a short period of time at the time derivative. 
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         [0067]    At step  460 , steering-control system  100  modifies the maximum rate-of-change in accordance with design parameters, in certain examples. 
         [0068]    The modification of the maximum rate-of-change, in a certain example, is implemented as follows: 
         [0000]      Modified Maximum Rate-of-Angle-Change (MMRAC)(° /sec.)=(δ dot)( k   speed )( k   curvature )
 
         [0069]    Wherein: 
         [0070]    t: time elapsed in second after activation of the lane-keeping controller 
         [0071]    k speed : weighting factor for speed dependent on speed ranges 
         [0072]    k curvature : weighting factor for curvature 
         [0073]    For example: 
         [0074]    k speed  is set at a value of “1.1” for a speed range, V x , between 0-30 m.p.h. and 
         [0075]    k speed  is set at a value of “1” for a speed range, V x , between 30-65 m.p.h. and 
         [0076]    k speed  is set at a value of “0.9” for a speed range, V x , exceeding 65 m.p.h. 
         [0077]    Thus, as the speed, V x , increases, the rate of steering-angle-change decreases as depicted in  FIG. 5 . 
         [0078]    Similarly, k curvature  may set in accordance with ranges of road curvatures, ρ. 
         [0079]    For example: 
         [0080]    k curvature  is set at a value of “1.1” for curvatures, ρ, less than 0.001 (1/m) 
         [0081]    k curvature  is set at a value of “1.0” for curvatures, ρ, greater than 0.001 and less than 0.0005 (1/m) 
         [0082]    k curvature  is set at a value of “0.9” for curvatures, ρ, greater than 0.0005 (1/m) 
         [0083]    Weighting factors k curvature  and k curvature  are derived empirically and in certain examples are stored in database  170  of steering-control system  100 . Weighting factors k curvature  and k curvature  are design parameters selected in accordance with the vehicle design concept. For example, k curvature  can be set at 1.2, 1.0, and 0.9 in the above ranges for a sport vehicle or 1.05, 10.0, and 0.95 for a luxury sedan. 
         [0084]    In step  480 , the steering-control system generates steering-angle-command includes the above-described corrective steering-angle derived from the cost function and vehicle dynamics and an application rate defined by the Modified Maximum Rate-of-Angle Change (MMRAC) in a time sequence in accordance with the following example: 
         [0000]      Applied Steering limit=(MMRAC)( t− 1)+1°
 
         [0085]    Wherein: “t” is time in seconds between 1.0 and 6.0 seconds after activation of the lane-keeping controller, for example. It should be appreciated that various time increments and time periods may also be employed. 
         [0086]    In step  490  steering control is returned to lane-centering system responsively to detection of travel within the lane-center zone  330  by steering-control system, according to an example. 
         [0087]      FIG. 5  depicts several time-dependent, steering-limit functions, “A”, “B”, and “C” of a steering-angle limit (degrees)  500  as a function of time (seconds)  502  during a response period commencing one second after the lane-keeping system assumes steering control to the end of the response period when proper vehicle travel has been restored and the lane-centering system has resumed steering control, according to an example. The six-second sample is merely an example of a time period in which a path of travel of a vehicle is deemed to have been restored. As shown, the slope of each steering-limit function represents a limit on the rate of steering angle-change as described above. 
         [0088]    As shown, steering-limit function “A” no steering angle limits versus  5 C depict various steering angle responses in accordance with the degree of steering. 
         [0089]    Steering-limit function “A” achieves a maximum steering limit of 2.5° at six seconds at a maximum rate-of-angle-change of 0.5 as represented by the slope, according to an example. 
         [0090]    Steering-limit function “B” achieves a maximum steering limit of 1.5° at six seconds at a maximum rate-of-angle-change of 0.3 as represented by the slope, according to an example. 
         [0091]    Steering-limit function “C” achieves a maximum steering limit of 1.0° at six seconds at a maximum rate-of-angle-change of 0.2 as represented by the slope, according to an example. 
         [0092]      FIG. 5A  is a data plot of steering angle command (degrees)  504  as a function of time (seconds)  506  and is shown within the first six seconds; the steering level command never exceeds 2.5°.  FIGS. 5B and 5C  are also data plots of steering-angle command (degrees)  504  as a function of time (seconds)  506  and depict a maximum steering-angle command within the first six seconds of 1.5° and 1.0° degrees, respectively; according to examples. 
         [0093]    It should be appreciated that additional examples may be combinations of various features set froth in the above examples explicitly. 
         [0094]    The foregoing description of the examples of the invention has been presented for the purposes of illustration and description and is not exhaustive or limiting. It should be appreciated by persons skilled in the art that modifications, variations, substitutions, changes, and equivalents are possible in light of the above teaching.