Patent Publication Number: US-11654961-B2

Title: Steering assistance control apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2020-143739 filed on Aug. 27, 2020, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The technology relates to a steering assistance control apparatus mountable on a vehicle. 
     Various techniques have been proposed for driving assistance control of a vehicle, such as an automobile. For example, reference may be made to Japanese Unexamined Patent Application Publication No. 2017-65473. 
     SUMMARY 
     An aspect of the technology provides a steering assistance control apparatus to be applied to a vehicle. The steering assistance control apparatus includes a detection unit and a correction unit. The detection unit is configured to detect a road surface cant of a road on which the vehicle is traveling. The correction unit is configured to correct a steering target steering-wheel angle in a steering control system, on the basis of a steering characteristic value for each of left steering and right steering and the road surface cant detected by the detection unit. The steering characteristic value indicates a steering characteristic for each combination of a traveling speed and the road surface cant. 
     An aspect of the technology provides a steering assistance control apparatus to be applied to a vehicle. The steering assistance control apparatus includes circuitry. The circuitry is configured to detect a road surface cant of a road on which the vehicle is traveling. The circuitry is further configured to correct a steering target steering-wheel angle in a steering control system, on the basis of a steering characteristic value for each of left steering and right steering and the detected road surface cant. The steering characteristic value indicates a steering characteristic for each combination of a traveling speed and the road surface cant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate example embodiments and, together with the specification, serve to explain the principles of the technology. 
         FIG.  1    is a diagram illustrating a configuration of a vehicle-mounted system according to one example embodiment of the technology. 
         FIG.  2    is a flowchart illustrating a map data generation process to be executed by a steering assistance control apparatus according to one example embodiment. 
         FIG.  3    is a conceptual diagram illustrating three-dimensional map data indicating correlation between a traveling speed, a road surface cant, and a steering characteristic value according to one example embodiment. 
         FIG.  4    is a conceptual diagram illustrating three-dimensional map data indicating correlation between the traveling speed, the road surface cant, and a characteristic left-right difference value according to one example embodiment. 
         FIG.  5    is a flowchart illustrating a steering target steering-wheel angle correction process to be executed by the steering assistance control apparatus according to one example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A left-right difference is caused in a steering system of a vehicle by variations in shapes, for example, of parts to be assembled in manufacture and aging, such as wear, of the assembled parts. The left-right difference is a difference in characteristic between left and right. Therefore, even if steering is performed with the same steering-wheel angle in a left direction and a right direction by steering assistance control, a difference occurs between left turning performance and right turning performance of the vehicle in some cases. 
     Such a difference in the turning performance of the vehicle can give a feeling of strangeness to an occupant, such as a driver, which can impair comfort and a feeling of security of driving assistance control and automatic driving. 
     It is desirable to reduce an occupant&#39;s feeling of strangeness about a left-right difference in steering assistance control. 
     Some example embodiments of the technology are described below with reference to  FIGS.  1  to  5   . Note that the following description is directed to illustrative examples of the disclosure and not to be construed as limiting to the technology. Each configuration illustrated in the drawings referred to in the following description is partial extraction of a configuration of the example embodiment, and is modifiable in a variety of ways in accordance with a design or any other factor as long as the modification does not depart from the technical idea of the technology. In each of the drawings referred to in the following description, elements have different scales in order to illustrate the respective elements with sizes recognizable in the drawings. Therefore, factors including, without limitation, the number of each of the elements, the shape of each of the elements, a size of each of the elements, a ratio between the elements, and relative positional relationship between the elements are illustrative only and not to be construed as limiting the technology. Further, elements in the following example embodiments which are not recited in a most-generic independent claim of the disclosure are optional and may be provided on an as-needed basis. 
     Configurations once described may be denoted with the same numerals to avoid any redundant description. Moreover, embodiments of the technology are not limited to those described below and encompass modifications, alternations, etc. in a range that allows for achievement of what may be desired by the embodiments of the technology. 
     As steering directions of a steering wheel in the following description, steering in a counterclockwise direction as seen from a driver is referred to as left steering, and steering in a clockwise direction as seen from the driver is referred to as right steering. 
     In the following description, a value related to left steering on the steering wheel is expressed by a value of 0 (zero) or more and a value related to right steering on the steering wheel is expressed by a value of less than 0 (zero), in regard to a value of a vehicle yaw rate [deg/s], an actual steering-wheel angle [deg], a steering target steering-wheel angle [deg], and a corrected steering target steering-wheel angle [deg]. 
     In regard to a value of a road surface cant [deg], a value indicating that the left side as seen from a vehicle is lower is expressed by a value of 0 (zero) or more, and a value indicating that the right side as seen from the vehicle is lower is expressed by a value of less than 0 (zero). 
     Meanings of terms in the example embodiments are as follows.
         The actual steering-wheel angle refers to a steering-wheel angle at which the steering wheel is actually rotated by the driver&#39;s operation on the steering wheel or steering assistance control.   The steering target steering-wheel angle refers to a value to be set to correct be actual steering-wheel angle on the basis of the steering assistance control.   A left-right difference correction steering-wheel angle refers to a value to be used to correct the steering target steering-wheel angle.   The corrected steering target steering-wheel angle refers to a value to be set to correct the actual steering-wheel angle on the basis of the steering assistance control. The corrected steering target steering-wheel angle is obtained by correcting the steering target steering-wheel angle on the basis of the left-right difference correction steering-wheel angle.       

     A configuration of a vehicle-mounted system  1  according to one example embodiment of the technology is described below with reference to  FIG.  1   . 
     The vehicle-mounted system  1  may include a steering assistance control apparatus  10 , an imaging device  11 L, an imaging device  11 R, a vehicle speed sensor  15 , a yaw rate sensor  16 , a steering wheel rotation angle sensor  17 , an electric motor rotation angle sensor  19 , a memory  20 , and a steering mechanism  30 . 
     The steering assistance control apparatus  10  may be configured as an electronic control unit (ECU), and may include a traveling environment recognizing unit  12 , an arithmetic processing unit  13 , and a steering unit  14 . 
     Thus, the example embodiment describes an example in which the steering assistance control apparatus  10  including the traveling environment recognizing unit  12 , the arithmetic processing unit  13 , and the steering unit  14  is configured as one ECU. In another example, some or all of the traveling environment recognizing unit  12 , the arithmetic processing unit  13 , and the steering unit  14  may be configured as ECUs different from each other. In this case, at least one embodiment of the technology may be implemented by the ECUs performing processing in parallel with each other. 
     In one embodiment, the traveling environment recognizing unit  12  and the arithmetic processing unit  13  may serve as a “detection unit”. In one embodiment, the arithmetic processing unit  13  may serve as a “correction unit”. 
     To the steering assistance control apparatus  10  may be coupled the imaging devices  11 L and  11 R, the vehicle speed sensor  15 , the yaw rate sensor  16 , the steering wheel rotation angle sensor  17 , and the electric motor rotation angle sensor  19 . The imaging devices  11 L and  11 R may be installed in the own vehicle to capture images in a traveling direction of the vehicle, e.g., a frontward direction. The vehicle speed sensor  15  may detect the traveling speed. The yaw rate sensor  16  may detect the vehicle yaw rate. The steering wheel rotation angle sensor  17  may detect a steering-wheel angle. The electric motor rotation angle sensor  19  may detect a rotation angle of a rotor of an electric motor  42  to be described later. 
     The steering assistance control apparatus  10  may be configured to receive detection signals obtained by these sensors. 
     The imaging devices  11 L and  11 R may be disposed, for example, near an upper part of a front windshield of the own vehicle. The imaging devices  11 L and  11 R may be spaced from each other by a predetermined distance in a vehicle width direction to measure distances by a so-called stereo method. The imaging devices  11 L and  11 R may have optical axes parallel to each other and the same focal length as each other. The imaging devices  11 L and  11 R may also have frame periods synchronized with each other and the same frame rate as each other. 
     An electrical signal, or a captured image signal, obtained by an image sensor of each of the imaging devices  11 L and  11 R may be subjected to analog-to-digital (A/D) conversion into a digital image signal, or captured image data. The captured image data may indicate, in units of pixels, a luminance value corresponding to a predetermined gray-scale. 
     The traveling environment recognizing unit  12  may store, in internal memory, pieces of frame image data serving as the captured image data obtained by the imaging devices  11 L and  11 R capturing images in front of the own vehicle. On the basis of two pieces, i.e., a pair, of captured image data serving as each frame, the traveling environment recognizing unit  12  may execute various processes for recognition of an object present in front of the own vehicle as an external environment. For example, the traveling environment recognizing unit  12  may recognize a lane line provided on a road, and a three-dimensional object, such as a preceding vehicle or an obstacle. The lane line may be a line defining traveling lanes, such as a white line or an orange line. The traveling environment recognizing unit  12  may also estimate a traveling path of the own vehicle (hereinafter also referred to as an “own vehicle traveling path”), on the basis of information on the detected lane line. 
     In recognizing the three-dimensional object ahead of the own vehicle, the traveling environment recognizing unit  12  may perform a process on a pair of captured image data, i.e., a stereo image, obtained by the imaging devices  11 L and  11 R. The process may involve obtaining distance information from an amount of shift between corresponding positions on the basis of the principle of triangulation. The traveling environment recognizing unit  12  may generate, on the basis of the distance information, data indicating three-dimensional distance distribution, i.e., a distance image. Thereafter, the traveling environment recognizing unit  12  may perform, on the basis of the data, known grouping and comparison with pre-stored data, such as data on a three-dimensional road shape or data on a three-dimensional object. The traveling environment recognizing unit  12  may thereby extract, for example, data on a lane line, data on a sidewall, such as a guardrail or a curb, present along a road, and data on a three-dimensional object, such as a vehicle. 
     The data on the three-dimensional object may include a distance from the own vehicle to the three-dimensional object, and a temporal change in the distance, i.e., a relative speed with respect to the own vehicle. From the data on the three-dimensional object, a vehicle that is closest to the own vehicle on the own vehicle traveling path and travels at a predetermined speed in substantially the same direction as the own vehicle may be extracted as a preceding vehicle. The predetermined speed may be, for example, 0 km/h or more. Among the preceding vehicles, a vehicle with a speed of substantially 0 km/h may be recognized as a stopped preceding vehicle. 
     An image recognition result of the preceding vehicle, the three-dimensional object, and the lane line, for example, obtained by the traveling environment recognizing unit  12  may be used for various types of driving assistance control, such as steering assistance control. 
     In relation to the steering assistance control according to the example embodiment, for example, information on the detected lane line and information on the preceding vehicle may be inputted to the arithmetic processing unit  13 . Examples of the information on the detected lane line may include information on the own vehicle traveling path. 
     The arithmetic processing unit  13  may perform control for various types of driving assistance, on the basis of input information indicating the image recognition result obtained by the traveling environment recognizing unit  12 . 
     The arithmetic processing unit  13  may be configured to perform the steering assistance control, or steering control, by setting the steering target steering-wheel angle on the basis of, for example, a lane line ahead of the own vehicle. Examples of the steering assistance control may include lane keep control, e.g., active lane keep (ALK), and lane departure prevention control. The lane keep control may keep the own vehicle in the middle of the own vehicle traveling path. The lane departure prevention control may prevent departure from the own vehicle traveling path, i.e., departure of the own vehicle from a lane of the own vehicle traveling path. 
     The arithmetic processing unit  13  may also be configured to perform, as driving assistance control related to steering, follow-up steering control of causing a lateral position of the own vehicle to follow that of the preceding vehicle. 
     The arithmetic processing unit  13  may determine an activation condition for each of various types of steering assistance control, and execute the steering assistance control in a case where the activation condition is satisfied. Whether the steering assistance control is activatable may be determined on the basis of, for example, operation information SD on the driver, information from the traveling environment recognizing unit  12 , and information from the sensors. The operation information SD on the driver may collectively indicate operation information such as on/off operations on an adaptive cruise control (ACC) switch and a steering assistance control execution switch. 
     The arithmetic processing unit  13  calculates, for each combination of the traveling speed and the road surface cant, the steering characteristic value indicating a steering characteristic for each of left steering and right steering. The steering characteristic value may indicate, as the steering characteristic, a degree to which the vehicle turns in response to steering. Examples of the steering characteristic value may include the vehicle yaw rate with respect to the actual steering-wheel angle. 
     The actual steering-wheel angle may be, for example, detected from the steering wheel rotation angle sensor  17 . Note that the actual steering-wheel angle may also be obtained by converting input information from the electric motor rotation angle sensor  19 . 
     The arithmetic processing unit  13  may calculate the steering characteristic value while the own vehicle is traveling, regardless of execution/non-execution of the various types of steering assistance control. A method of calculating the steering characteristic value will be described in detail later. 
     The arithmetic processing unit  13  may cause the memory  20  to record data indicating correlation between the traveling speed, the road surface cant, and the steering characteristic value. The memory  20  may include, for example, a random access memory (RAM), a flash memory, or a hard disk drive (HDD). 
     In the steering assistance control, the arithmetic processing unit  13  corrects the steering target steering-wheel angle in a steering control system, on the basis of the steering characteristic value for each of left steering and right steering, to calculate the corrected steering target steering-wheel angle. A method of calculating the corrected steering target steering-wheel angle will be described in detail later. The steering target steering-wheel angle may be calculated on the basis of input information from, for example, the electric motor rotation angle sensor  19 . 
     The arithmetic processing unit  13  may output the corrected steering target steering-wheel angle to the steering unit  14 . 
     The steering unit  14  may include a microcomputer. The steering unit  14  may control the electric motor  42  on the basis of the corrected steering target steering-wheel angle from the arithmetic processing unit  13 . The steering unit  14  may also execute power steering control of assisting steering by the driver. 
     The steering mechanism  30  to be subjected to the steering control may be configured as follows, for example. 
     The steering mechanism  30  may include a steering shaft  32  rotatably supported by an unillustrated vehicle body frame via a steering column  33 . The steering shaft  32  may have one end extending toward a driver&#39;s seat and the other end extending toward an engine room. A steering wheel  34  may be attached to the one end of the steering shaft  32 . A pinion shaft  35  may be coupled to the other end of the steering shaft  32 . 
     The engine room may have a steering gear box  36  extending in the vehicle width direction. The steering gear box  36  may support a rack shaft  37  inserted therein to allow the rack shaft  37  to reciprocate. The rack shaft  37  may be provided with an unillustrated rack between its ends. An unillustrated pinion provided on the pinion shaft  35  may be in engagement with the rack to configure a rack-and-pinion steering gear mechanism. 
     Left and right ends of the rack shaft  37  may protrude from the steering gear box  36 , and a front knuckle  39  may be coupled to each of the ends via a tie rod  38 . The front knuckle  39  may pivotably support corresponding one of left and right wheels  40 L and  40 R serving as wheels to be steered, and may be steerably supported by the vehicle body frame via an unillustrated kingpin. With this configuration, when the steering wheel  34  is operated to rotate the steering shaft  32  and the pinion shaft  35 , the rotation of the pinion shaft  35  may move the rack shaft  37  in a left-right direction. The movement of the rack shaft  37  may cause the front knuckles  39  to pivot about the respective unillustrated kingpins to steer the left and right wheels  40 L and  40 R in the left-right direction. 
     The electric motor  42  may be coupled to the pinion shaft  35  via an assist transmission mechanism  41 . The electric motor  42  may be controlled on the basis of the steering target steering-wheel angle, or the corrected steering target steering-wheel angle. 
     Description is given below on an example of a map data generation process to be executed by the steering assistance control apparatus  10  according to the example embodiment, with reference to  FIG.  2    to  FIG.  4   . The steering assistance control apparatus  10  may execute a process illustrated in  FIG.  2    while the own vehicle is traveling, regardless of an activation state of the various types of steering assistance control. By executing the process illustrated in  FIG.  2   , the steering assistance control apparatus  10  may generate three-dimensional map data to be used to correct the steering target steering-wheel angle during the steering assistance control. 
     The steering assistance control apparatus  10  may first, in step S 101 , execute reading of various pieces of information. For example, the steering assistance control apparatus  10  may read the traveling speed of the own vehicle detected from the vehicle speed sensor  15 . 
     Thereafter, the steering assistance control apparatus  10  may, in step S 102 , determine whether a traveling data acquisition condition holds. For example, the steering assistance control apparatus  10  may determine whether the traveling data acquisition condition holds on the basis of the traveling speed of the own vehicle. 
     In this case, the steering assistance control apparatus  10  may, for example, set 40 km/h as a predetermined threshold, and determine that the traveling data acquisition condition holds if the traveling speed is 40 km/h or more. One reason for this is that traveling data acquired in a case where the traveling speed is too low is a steering characteristic value not suitable as data for correction of the steering target steering-wheel angle. The traveling data may hereinafter also be referred to as “traveling information”. 
     In another example, the steering assistance control apparatus  10  may determine whether the traveling data acquisition condition holds on the basis of a steering wheel angular velocity. In this case, the steering assistance control apparatus  10  may, for example, set 60 deg/s as a predetermined threshold, and determine that the traveling data acquisition condition holds if the steering wheel angular velocity is 60 deg/s or less. One reason for this is that traveling data acquired in a case where steering is performed on the steering wheel such that the steering wheel angular velocity exceeds 60 deg/s is a steering characteristic value not suitable as data for correction of the steering target steering-wheel angle. 
     In another example, the steering assistance control apparatus  10  may determine whether the traveling data acquisition condition holds on the basis of both the traveling speed of the own vehicle and the steering wheel angular velocity. In this case, the steering assistance control apparatus  10  may, for example, determine that the traveling data acquisition condition holds in a case where the traveling speed of the own vehicle is 40 km/h or more and where the steering wheel angular velocity is 60 deg/s or less. 
     Upon determining that the traveling data acquisition condition does not hold in step S 102  (S 102 : No), the steering assistance control apparatus  10  may cause the process to return to step S 101 , without performing processes of steps S 103  to S 107  to be described later. The steering assistance control apparatus  10  may subsequently execute similar processes. 
     Upon determining that the traveling data acquisition condition holds in step S 102  (S 102 : Yes), the steering assistance control apparatus  10  may cause the process to proceed to step S 103 . 
     The steering assistance control apparatus  10  may execute a traveling data acquisition process in step S 103 . For example, the steering assistance control apparatus  10  may acquire the traveling speed detected from the vehicle speed sensor  15 , the vehicle yaw rate detected from the yaw rate sensor  16 , and the actual steering-wheel angle detected from the steering wheel rotation angle sensor  17 . 
     The steering assistance control apparatus  10  may also acquire the input information indicating the image recognition result obtained by the traveling environment recognizing unit  12 . 
     Thereafter, the steering assistance control apparatus  10  may calculate the steering characteristic value in step S 104 . 
     The steering characteristic value may be calculated by substituting the vehicle yaw rate and the actual steering-wheel angle into [Expression 1] given below.
 
(steering characteristic value [(deg/s)/deg])=(vehicle yaw rate [deg/s])/(actual steering-wheel angle [deg])  [Expression 1]
 
     It is possible for the steering assistance control apparatus  10  to calculate the steering characteristic value, regardless of the activation state of the various types of steering assistance control. Accumulating the steering characteristic value regardless of activation/non-activation of the steering assistance control makes it possible to correct the steering target steering-wheel angle to minimize influence of a change in the steering characteristic value due to alignment adjustment and part replacement. 
     In the following description, the steering characteristic value calculated on the basis of left steering is referred to as a “left steering characteristic value”, and the steering characteristic value calculated on the basis of right steering is referred to as a “right steering characteristic value”, if these steering characteristic values are to be distinguished from each other. 
     The steering assistance control apparatus  10  may calculate the road surface cant on the basis of the input information indicating the image recognition result obtained by the traveling environment recognizing unit  12 . The road surface cant may be calculated by any of various existing methods. For example, the steering assistance control apparatus  10  may calculate the road surface cant on the basis of an unillustrated acceleration sensor provided in the vehicle. 
     The steering assistance control apparatus  10  may cause the memory  20  to record correlation between the steering characteristic value, the traveling speed, and the road surface cant. For example, the steering assistance control apparatus  10  may cause the memory  20  to record the correlation in regard to a preset, predetermined distance traveled by the own vehicle in the past. In this case, the steering characteristic value may be a value calculated on the basis of the traveling information within the predetermined distance traveled by the own vehicle in the past. 
     The steering assistance control apparatus  10  may cause the correlation between the steering characteristic value, the traveling speed, and the road surface cant to be recorded by, for example, generating three-dimensional map data in which the X axis indicates the traveling speed, the Y axis indicates the road surface cant, and the Z axis indicates the steering characteristic value as illustrated in  FIG.  3   . 
     In this case, the steering assistance control apparatus  10  may generate the three-dimensional map data for the correlation for each of the left steering characteristic value and the right steering characteristic value. 
     The steering assistance control apparatus  10  may, in step S 105 , perform averaging for the three-dimensional map data for each of the left steering characteristic value and the right steering characteristic value. The averaging may be performed by, for example, a method of leveling time-series data, such as a moving average method. For example, any of simple moving average, weighted moving average, and exponential moving average may be used. To remove value variations serving as noise, filtering may be performed by setting any threshold. 
     Thereafter, the steering assistance control apparatus  10  may, in step S 106 , calculate a characteristic left-right difference value indicating ease with which the vehicle turns in the left-right direction in response to steering. 
     The steering assistance control apparatus  10  may acquire, on the basis of the three-dimensional map data of  FIG.  3    subjected to the averaging, the left steering characteristic value and the right steering characteristic value for each combination of the traveling speed and the road surface cant. 
     The steering assistance control apparatus  10  may calculate the characteristic left-right difference value for each combination of the traveling speed and the road surface cant, by substituting the acquired left steering characteristic value and right steering characteristic value into [Expression 2] given below.
 
(characteristic left-right difference value [(deg/s)/deg])=(left steering characteristic value [deg/s)/deg])−(right steering characteristic value [(deg/s)/deg])  [Expression 2]
 
     A case where the characteristic left-right difference value is 0 (zero) or more indicates that the vehicle yaw rate caused by left steering is larger than the vehicle yaw rate caused by right steering. This indicates a state in which, if steering is performed with the same steering-wheel angle for left and right by the driver or the steering assistance control, it is more difficult for the vehicle to turn in the right direction than in the left direction. 
     A case where the characteristic left-right difference value is less than 0 (zero) indicates that the vehicle yaw rate caused by right steering is larger than the vehicle yaw rate caused by left steering. This indicates a state in which, if steering is performed with the same steering-wheel angle for left and right by the driver or the steering assistance control, it is more difficult for the vehicle to turn in the left direction than in the right direction. 
     The steering assistance control apparatus  10  may cause the memory  20  to store correlation between the characteristic left-right difference value, the traveling speed, and the road surface cant. 
     The steering assistance control apparatus  10  may cause the correlation between the characteristic left-right difference value, the traveling speed, and the road surface cant to be recorded by, for example, generating three-dimensional map data in which the X axis indicates the traveling speed, the Y axis indicates the road surface cant, and the Z axis indicates the characteristic left-right difference value as illustrated in  FIG.  4   . 
     The steering assistance control apparatus  10  may, in step S 107 , perform averaging for the three-dimensional map data illustrated in  FIG.  4   . The averaging may be performed by a method similar to the method described above. 
     After executing the process of step S 107 , the steering assistance control apparatus  10  may cause the process to return to step S 101 , and subsequently repeatedly execute similar processes while the vehicle is traveling. 
     This enables generation of the three-dimensional map data, illustrated in  FIG.  3    and  FIG.  4   , to be used to correct the steering target steering-wheel angle. 
     Description is given next on an example of a steering target steering-wheel angle correction process to be executed by the steering assistance control apparatus  10  according to the example embodiment, with reference to  FIG.  5   . The steering assistance control apparatus  10  may execute the process illustrated in  FIG.  5   , for example, while the steering assistance control is working. Examples of the steering assistance control may include lane keep control, e.g., active lane keep, of keeping the own vehicle in the middle of the own vehicle traveling path, and lane departure prevention control of preventing departure from the own vehicle traveling path. 
     The steering assistance control apparatus  10  may first, in step S 201 , execute reading of various pieces of information. For example, the steering assistance control apparatus  10  may read the traveling speed of the own vehicle detected from the vehicle speed sensor  15  and the actual steering-wheel angle detected from the steering wheel rotation angle sensor  17 . 
     The steering assistance control apparatus  10  may also acquire the input information indicating the image recognition result obtained by the traveling environment recognizing unit  12  to be used to calculate the road surface cant. 
     Thereafter, the steering assistance control apparatus  10  may, in step S 202 , calculate the road surface cant on the basis of the input information indicating the image recognition result obtained by the traveling environment recognizing unit  12 . 
     The steering assistance control apparatus  10  may acquire, from the three-dimensional map data illustrated in  FIG.  4   , the characteristic left-right difference value corresponding to the current traveling speed and road surface cant. 
     The steering assistance control apparatus  10  may, in step S 203 , determine whether a condition that “the characteristic left-right difference value is 0 (zero) or more and the steering target steering-wheel angle is less than 0 (zero)” is satisfied at the current traveling speed and road surface cant. 
     “The characteristic left-right difference value being 0 (zero) or more” indicates a state in which, in a case where steering is performed with the same steering-wheel angle for left and right, it is more difficult for the traveling vehicle to turn in the right direction than in the left direction. “The steering target steering-wheel angle being less than 0 (zero)” indicates a state in which the steering assistance control is working to cause the vehicle to turn in the right direction. 
     This means that a state in which the condition of “the characteristic left-right difference value is 0 (zero) or more and the steering target steering-wheel angle is less than 0 (zero)” is satisfied is a state in which it is difficult for a turning path of the vehicle in the right direction based on the steering assistance control to reach a turning path of the vehicle assumed by the steering assistance control apparatus  10 . 
     If the condition of “the characteristic left-right difference value is 0 (zero) or more and the steering target steering-wheel angle is less than 0 (zero)” is satisfied in step S 203  (S 203 : Yes), the steering assistance control apparatus  10  may cause the process to proceed to step S 204 . 
     The steering assistance control apparatus  10  may, in step S 204 , calculate the left-right difference correction steering-wheel angle for correction of the steering target steering-wheel angle. 
     The steering assistance control apparatus  10  may acquire, from the three-dimensional map data illustrated in  FIG.  3   , the left steering characteristic value and the right steering characteristic value at the current traveling speed and road surface cant. 
     The steering assistance control apparatus  10  may calculate a left-right difference correction factor by substituting the acquired left steering characteristic value and right steering characteristic value into [Expression 3] given below.
 
(left-right difference correction factor)=(left steering characteristic value [(deg/s)/deg])/(right steering characteristic value [(deg/s)/deg])  [Expression 3]
 
     Thereafter, the steering assistance control apparatus  10  may calculate the left-right difference correction steering-wheel angle. The steering assistance control apparatus  10  may calculate the left-right difference correction steering-wheel angle by substituting values of the calculated left-right difference correction factor and the current steering target steering-wheel angle into [Expression 4] given below.
 
(left-right difference correction steering-wheel angle [deg])={(left-right difference correction factor)−1}×(steering target steering-wheel angle Nee  [Expression 4]
 
     The steering assistance control apparatus  10  may execute range limiting of comparing, for example, the absolute value of the left-right difference correction steering-wheel angle with a preset limit value, and setting the limit value as a value of the left-right difference correction steering-wheel angle in a case where the absolute value exceeds the limit value. This makes it possible to prevent promotion of slipping of the vehicle down the cant when the road surface cant switches, and to prevent runaway of the correction process due to accumulation of inappropriate data, for example. 
     The steering assistance control apparatus  10  may, in step S 205 , calculate the corrected steering target steering-wheel angle on the basis of the calculated left-right difference correction steering-wheel angle. The steering assistance control apparatus  10  may calculate the corrected steering target steering-wheel angle by substituting the calculated left-right difference correction steering-wheel angle into [Expression 5] given below.
 
(corrected steering target steering-wheel angle [deg])=(steering target steering-wheel angle [deg])−(left-right difference correction steering-wheel angle [deg])  [Expression 5]
 
     Thus subtracting the left-right difference correction steering-wheel angle from the steering target steering-wheel angle makes the vehicle yaw rate in a right turn based on the steering assistance control equivalent to the vehicle yaw rate in a left turn. 
     Thereafter, the steering assistance control apparatus  10  may execute, on the electric motor  42 , control based on the corrected steering target steering-wheel angle. The electric motor  42  may be coupled to the pinion shaft  35  via the assist transmission mechanism  41 , as illustrated in  FIG.  1   . 
     After the process of step S 205 , the steering assistance control apparatus  10  may cause the process to return to step S 201 , and subsequently execute similar processes. 
     If the condition of “the characteristic left-right difference value is 0 (zero) or more and the steering target steering-wheel angle is less than 0 (zero)” is not satisfied in step S 203  (S 203 : No), the steering assistance control apparatus  10  may cause the process to proceed to step S 206 . 
     The steering assistance control apparatus  10  may, in step S 206 , determine whether a condition that “the characteristic left-right difference value is less than 0 (zero) and the steering target steering-wheel angle is 0 (zero) or more” is satisfied at the current traveling speed and road surface cant. 
     “The characteristic left-right difference value being less than 0 (zero)” indicates a state in which, in a case where steering is performed with the same steering-wheel angle for left and right, it is more difficult for the traveling vehicle to turn in the left direction than in the right direction. “The steering target steering-wheel angle being 0 (zero) or more” indicates a state in which the steering assistance control is working to cause the vehicle to turn in the left direction. 
     This means that a state in which the condition of “the characteristic left-right difference value is less than 0 (zero) and the steering target steering-wheel angle is 0 (zero) or more” is satisfied is a state in which it is difficult for a turning path of the vehicle in the left direction based on the steering assistance control to reach a turning path of the vehicle assumed by the steering assistance control apparatus  10 . 
     If the condition of “the characteristic left-right difference value is less than 0 (zero) and the steering target steering-wheel angle is 0 (zero) or more” is satisfied in step S 206  (S 206 : Yes), the steering assistance control apparatus  10  may cause the process to proceed to step S 207 . 
     The steering assistance control apparatus  10  may, in step S 207 , calculate the left-right difference correction steering-wheel angle for correction of the steering target steering-wheel angle. 
     The steering assistance control apparatus  10  may acquire, from the three-dimensional map data illustrated in  FIG.  3   , the left steering characteristic value and the right steering characteristic value at the current traveling speed and road surface cant. 
     The steering assistance control apparatus  10  may calculate a left-right difference correction factor by substituting the acquired left steering characteristic value and right steering characteristic value into [Expression 6] given below.
 
(left-right difference correction factor)=(right steering characteristic value [(deg/s)/deg])/(left steering characteristic value [(deg/s)/deg])  [Expression 6]
 
     Thereafter, the steering assistance control apparatus  10  may calculate the left-right difference correction steering-wheel angle. The steering assistance control apparatus  10  may calculate the left-right difference correction steering-wheel angle by substituting values of the calculated left-right difference correction factor and the current steering target steering-wheel angle into [Expression 4] as described above. 
     The steering assistance control apparatus  10  may execute range limiting of comparing, for example, the absolute value of the left-right difference correction steering-wheel angle with a preset limit value, and setting the limit value as a value of the left-right difference correction steering-wheel angle in a case where the absolute value exceeds the limit value. 
     The steering assistance control apparatus  10  may, in step S 208 , calculate the corrected steering target steering-wheel angle on the basis of the calculated left-right difference correction steering-wheel angle. The steering assistance control apparatus  10  may calculate the corrected steering target steering-wheel angle by substituting the calculated left-right difference correction steering-wheel angle into [Expression 7] given below.
 
(corrected steering target steering-wheel angle [deg])=(steering target steering-wheel angle [deg])+(left-right difference correction steering-wheel angle [deg])  [Expression 7]
 
     Thus adding the left-right difference correction steering-wheel angle to the steering target steering-wheel angle makes the vehicle yaw rate in a left turn based on the steering assistance control equivalent to the vehicle yaw rate in a right turn. 
     Thereafter, the steering assistance control apparatus  10  may execute, on the electric motor  42  illustrated in  FIG.  1   , control based on the corrected steering target steering-wheel angle. 
     After the process of step S 208 , the steering assistance control apparatus  10  may cause the process to return to step S 201 , and subsequently execute similar processes. 
     If the condition of “the characteristic left-right difference value is less than 0 (zero) and the steering target steering-wheel angle is 0 (zero) or more” is not satisfied in step S 206  (S 206 : No), the steering assistance control apparatus  10  may cause the process to proceed to step S 209 . 
     The steering assistance control apparatus  10  may, in step S 209 , set the acquired steering target steering-wheel angle as the corrected steering target steering-wheel angle as expressed by [Expression 8] given below, without calculating the left-right difference correction steering-wheel angle.
 
(corrected steering target steering-wheel angle [deg])=(steering target steering-wheel angle [deg])  [Expression 8]
 
     Thus, the steering target steering-wheel angle may be set as the corrected steering target steering-wheel angle, i.e., correction of the steering target steering-wheel angle may not be performed, in a case where a condition of “the characteristic left-right difference value is 0 (zero) or more and the steering target steering-wheel angle is 0 (zero) or more” is satisfied. This case indicates a state in which, although it is more difficult for the traveling vehicle to turn in the right direction than in the left direction, the steering assistance control is working to cause the vehicle to turn in the left direction. 
     The steering target steering-wheel angle may be set as the corrected steering target steering-wheel angle, i.e., correction of the steering target steering-wheel angle may not be performed, in a case where a condition of “the characteristic left-right difference value is less than 0 (zero) and the steering target steering-wheel angle is less than 0 (zero)” is satisfied. This case indicates a state in which, although it is more difficult for the traveling vehicle to turn in the left direction than in the right direction, the steering assistance control is working to cause the vehicle to turn in the right direction. 
     Thereafter, the steering assistance control apparatus  10  may execute, on the electric motor  42  illustrated in  FIG.  1   , control based on the corrected steering target steering-wheel angle. The corrected steering target steering-wheel angle in this case may be the uncorrected steering target steering-wheel angle based on [Expression 8] given above. 
     After the process of step S 208 , the steering assistance control apparatus  10  may cause the process to return to step S 201 , and subsequently execute similar processes until the steering assistance control ends. 
     The above-described processes performed by the steering assistance control apparatus  10  enables the example embodiment to be implemented. 
     The steering assistance control apparatus  10  according to one example embodiment of the technology includes the detection unit and the correction unit. The detection unit is configured to detect the road surface cant of the road on which the own vehicle is traveling. The correction unit is configured to correct the steering target steering-wheel angle in the steering control system, on the basis of the steering characteristic value for each of left steering and right steering and the road surface cant detected by the detection unit. The steering characteristic value indicates the steering characteristic for each combination of the traveling speed and the road surface cant. For example, see steps S 202 , S 205 , and S 208  in  FIG.  5   . 
     This enables the steering assistance control to be executed on the basis of the steering characteristic value corresponding to the traveling speed and the road surface cant. 
     This makes it possible to reduce an occupant&#39;s feeling of strangeness about a left-right difference in the steering assistance control, i.e., the occupant&#39;s feeling of strangeness about the steering assistance control in the left and right directions. This helps to implement driving assistance control and automatic driving comfortable and secure for the occupant. 
     Examples of the left-right difference in the steering assistance control may include a left-right difference in an amount of departure, i.e., a left-right difference in performance, in the lane departure prevention control, wandering in the lane keep control, and a left-right difference in followability and responsiveness to a target path, i.e., a left-right difference in feel. 
     In the steering assistance control apparatus  10  according to one example embodiment, the steering characteristic value may include a value calculated on the basis of the traveling information within the predetermined distance traveled by the own vehicle in the past. For example, see step S 104  in  FIG.  2   . 
     This enables the steering target steering-wheel angle to be corrected on the basis of the steering characteristic value calculated from the traveling information actually measured in the past in the own vehicle. This makes it possible to improve accuracy of the steering target steering-wheel angle correction. 
     In the steering assistance control apparatus  10  according to one example embodiment, the steering characteristic value may include a value calculated on the basis of the traveling information and subjected to averaging. The steering target steering-wheel angle may be corrected on the basis of the steering characteristic value subjected to the averaging. For example, see step S 105  in  FIG.  2   . 
     The above averaging suppresses variations in the steering characteristic value. This makes it possible to improve the accuracy of the steering target steering-wheel angle correction, which helps to reduce the occupant&#39;s feeling of strangeness about the steering assistance control in the left and right directions. 
     In the steering assistance control apparatus  10  according to one example embodiment, the steering characteristic value may include a value calculated on the basis of the traveling information acquired in a case where the traveling speed of the own vehicle is a predetermined threshold or more. For example, see step S 102  in  FIG.  2   . 
     Thus, the traveling information acquired in case where the traveling speed is too low is not reflected in the steering characteristic value. 
     This makes it possible to improve the accuracy of the steering target steering-wheel angle correction, which helps to reduce the occupant&#39;s feeling of strangeness about the steering assistance control in the left and right directions. 
     In the steering assistance control apparatus  10  according to one example embodiment, the steering characteristic value may include a value calculated on the basis of the traveling information acquired in a case where the steering wheel angular velocity is a predetermined threshold or less. For example, see step S 102  in  FIG.  2   . 
     Thus, the traveling information acquired while the driver is performing steering different from normal driving, such as abrupt steering, is not reflected in the steering characteristic value. 
     This makes it possible to improve the accuracy of the steering target steering-wheel angle correction, which helps to reduce the occupant&#39;s feeling of strangeness about the steering assistance control in the left and right directions. 
     Note that the effects described in the disclosure are merely illustrative and non-limiting. Any other effect may be achieved, or part of the effects described in the disclosure may be achieved. 
     Although some embodiments of the technology have been described in the foregoing by way of example, the technology is by no means limited to the embodiments described above. It should be appreciated that modifications and alterations may be made in accordance with a factor such as a design without departing from the technical idea of the embodiment of the technology, other than the example embodiments described above. Further, it may not be necessary to provide all of the combination of the configurations described above in the example embodiments to solve an issue. 
     Each of the traveling environment recognizing unit  12 , the arithmetic processing unit  13 , and the steering unit  14  illustrated in  FIG.  1    is implementable by circuitry including at least one semiconductor integrated circuit such as at least one processor (e.g., a central processing unit (CPU)), at least one application specific integrated circuit (ASIC), and/or at least one field programmable gate array (FPGA). 
     At least one processor is configurable, by reading instructions from at least one machine readable non-transitory tangible medium, to perform all or a part of functions of each of the traveling environment recognizing unit  12 , the arithmetic processing unit  13 , and the steering unit  14 . Such a medium may take many forms, including, but not limited to, any type of magnetic medium such as a hard disk, any type of optical medium such as a CD and a DVD, any type of semiconductor memory (i.e., semiconductor circuit) such as a volatile memory and a non-volatile memory. The volatile memory may include a DRAM and an SRAM, and the nonvolatile memory may include a ROM and an NVRAM. The ASIC is an integrated circuit (IC) customized to perform, and the FPGA is an integrated circuit designed to be configured after manufacturing in order to perform, all or a part of the functions of each of the traveling environment recognizing unit  12 , the arithmetic processing unit  13 , and the steering unit  14  illustrated in  FIG.  1   .