Patent Publication Number: US-9896128-B2

Title: Steering assist device

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2015-120563 filed on Jun. 15, 2015 including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to steering assist devices for vehicles, and more particularly to steering assist devices that prevent a running vehicle from deviating from its lane. 
     2. Description of the Related Art 
     If a vehicle deviates from its lane on a highway etc. due to driver&#39;s carelessness or a road surface condition, there is a risk that the vehicle may contact other vehicle(s) or a guardrail. As a solution, lane departure warning systems are developed which obtain road surface information and relative position information between the vehicle and its lane based on an image shot by a camera mounted on the vehicle and warn the driver when the vehicle is about to deviate from its lane. See, e.g., Japanese Patent Application Publication No. 2013-212839 (JP 2013-212839 A), Japanese Patent No. 4292562 (JP 4292562 B), and Japanese Patent Application Publication No. H11-34774 (JP H11-34774 A). 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a steering assist device that can quickly return a vehicle to a target travel line when it deviates from the target travel line. 
     According to one aspect of the present invention, a steering assist device includes: an electric motor that applies a steering driving force to a steering operation mechanism of a vehicle; an information obtaining unit that obtains a lateral deviation of the vehicle from a target travel line, a lateral deviation change rate or a rate of change in the lateral deviation per unit time, and a rate of change in the lateral deviation change rate per unit time; a steering assist current value setting unit that sets a steering assist current value corresponding to a target value of steering assist torque; a lane keep assist current value calculation unit that calculates a lane keep assist current value that makes the lateral deviation and the lateral deviation change rate closer to zero, based on the lateral deviation and the lateral deviation change rate obtained by the information obtaining unit; a correction unit that corrects the lane keep assist current value calculated by the lane keep assist current value calculation unit so that an absolute value of the lane keep assist current value increases, if the lateral deviation change rate and the rate of change in the lateral deviation change rate per unit time obtained by the information obtaining unit have the same sign; a target current value calculation unit that calculates a target current value by using the steering assist current value set by the steering assist current value setting unit and the lane keep assist current value corrected by the correction unit; and a control unit that drivingly controls the electric motor based on the target current value calculated by the target current value calculation unit. 
     The steering assist device of the above aspect can generate lane keep assist torque that makes the lateral deviation and the lateral deviation change rate closer to zero. Since the vehicle is thus guided so as to make the lateral deviation closer to zero, the vehicle can be guided toward the target travel line. Moreover, since the vehicle is guided so as to make the lateral deviation change rate closer to zero, the vehicle can be guided so as to make a lateral centerline of the vehicle parallel to the target travel line when the vehicle is traveling near the target travel line. The vehicle can thus be guided so as to avoid deviating from its lane. 
     In the steering assist device of the above aspect, the lane keep assist current value calculated by the lane keep assist current value calculation unit is corrected so that the absolute value of the lane keep assist current value increases, if the lateral deviation change rate and the rate of change in the lateral deviation change rate per unit time have the same sign. The case where both the lateral deviation change rate and the rate of change in the lateral deviation change rate per unit time have the same sign corresponds to the state where the degree to which the vehicle deviates from the target travel line is increasing. The state where the degree to which the vehicle deviates from the target travel line is increasing refers to the state where both the lateral deviation and the lateral deviation change rate are changing in a direction away from zero. Accordingly, in the present invention, the lane keep assist torque can be increased in the state where the degree to which the vehicle deviates from the target travel line is increasing. The vehicle can therefore be rapidly returned toward the target travel line if it deviates from the target travel line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is a schematic diagram showing a schematic configuration of an electric power steering system to which a steering assist device according to an embodiment of the present invention is applied; 
         FIG. 2  is a block diagram showing an electrical configuration of an electronic control unit (ECU); 
         FIG. 3  is a graph showing an example of setting a steering assist current value Is* with respect to detected steering torque T; 
         FIG. 4  is a schematic diagram illustrating operation of an information obtaining unit; 
         FIG. 5  is a block diagram showing an electrical configuration of a lane keep assist current value calculation unit; 
         FIG. 6A  is a graph showing an example of the relationship of a first lane keep assist current value Ir 1 * to a lateral deviation y; 
         FIG. 6B  is a graph showing another example of the relationship of the first lane keep assist current value Ir 1 * to the lateral deviation y; 
         FIG. 6C  is a graph showing still another example of the relationship of the first lane keep assist current value Ir 1 * to the lateral deviation y; 
         FIG. 7A  is a graph showing an example of the relationship of a second lane keep assist current value Ir 2 * to a lateral deviation change rate dy/dt; 
         FIG. 7B  is a graph showing another example of the relationship of the second lane keep assist current value Ir 2 * to the lateral deviation change rate dy/dt; 
         FIG. 8  is a graph showing an example of setting vehicle speed gain Gv with respect to a vehicle speed V; 
         FIG. 9  is a flowchart illustrating an example of operation of a correction gain setting unit; 
         FIG. 10  is a flowchart illustrating another example of operation of the correction gain setting unit; 
         FIG. 11  is a block diagram showing a modification of the lane keep assist current value calculation unit; 
         FIG. 12  is a graph showing the relationship between the detected steering torque T and control steering torque Ts; 
         FIG. 13  is a block diagram showing another example of the configuration of the ECU; and 
         FIG. 14  is a flowchart illustrating an example of operation of a correction value setting unit. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. 
       FIG. 1  is a schematic diagram showing a schematic configuration of an electric power steering system to which a steering assist device according to an embodiment of the present invention is applied. 
     An electric power steering system (EPS)  1  includes a steering wheel  2  serving as a steering member that steers a vehicle, a steering operation mechanism  4  that steers steered wheels  3  in accordance with the rotation of the steering wheel  2 , and a steering assist mechanism  5  that assists driver&#39;s steering operation. The steering wheel  2  and the steering operation mechanism  4  are mechanically coupled to each other via a steering shaft  6  and an intermediate shaft  7 . 
     The steering shaft  6  includes an input shaft  8  coupled to the steering wheel  2  and an output shaft  9  coupled to the intermediate shaft  7 . The input shaft  8  and the output shaft  9  are coupled to each other via a torsion bar  10  so as to be rotatable relative to each other. 
     A torque sensor  11  is disposed around the torsion bar  10 . The torque sensor  11  detects steering torque T applied to the steering wheel  2 , based on the relative rotation displacement between the input shaft  8  and the output shaft  9 . For example, in the present embodiment, the torque sensor  11  detects torque for steering to the right as positive steering torque T and detects torque for steering to the left as negative steering torque T. The larger the absolute value of the detected steering torque T is, the larger the magnitude of the steering torque T is. 
     The steering operation mechanism  4  is a rack and pinion mechanism including a pinion shaft  13  and a rack shaft  14  serving as a steering operation shaft. The steered wheels  3  are each coupled to corresponding one of the ends of the rack shaft  14  via a tie rod  15  and a knuckle arm (not shown). The pinion shaft  13  is coupled to the intermediate shaft  7 . The pinion shaft  13  rotates in accordance with the steering operation of the steering wheel  2 . A pinion  16  is coupled to the tip end (the lower end in  FIG. 1 ) of the pinion shaft  13 . 
     The rack shaft  14  extends linearly in the lateral direction of the vehicle. The rack shaft  14  has a rack  17  in its intermediate portion in the axial direction. The rack  17  meshes with the pinion  16 . The pinion  16  and the rack  17  convert rotation of the pinion shaft  13  to axial movement of the rack shaft  14 . The steered wheels  3  can be steered by moving the rack shaft  14  in the axial direction. 
     When the steering wheel  2  is rotated by driver&#39;s steering operation, this rotation of the steering wheel  2  is transmitted to the pinion shaft  13  via the steering shaft  6  and the intermediate shaft  7 . Rotation of the pinion shaft  13  is converted to axial movement of the rack shaft  14  by the pinion  16  and the rack  17 . The steered wheels  3  are thus steered. 
     The steering assist mechanism  5  includes a steering assist electric motor  18  and a speed reduction mechanism  19 . The electric motor  18  generates a steering assist force (steering assist torque), and the speed reduction mechanism  19  transmits the output torque of the electric motor  18  to the steering operation mechanism  4 . The speed reduction mechanism  19  is a worm gear mechanism that includes a worm shaft  20  and a worm wheel  21  meshing with the worm shaft  20 . The speed reduction mechanism  19  is accommodated in a gear housing  22  serving as a transmission mechanism housing. 
     The worm shaft  20  is rotationally driven by the electric motor  18 . The worm wheel  21  is coupled to the steering shaft  6  so as to be rotatable in the same direction as the steering shaft  6 . The worm wheel  21  is rotationally driven by the worm shaft  20 . 
     When the worm shaft  20  is rotationally driven by the electric motor  18 , the worm wheel  21  is rotationally driven and the steering shaft  6  is rotated accordingly. The rotation of the steering shaft  6  is transmitted to the pinion shaft  13  via the intermediate shaft  7 . Rotation of the pinion shaft  13  is converted to axial movement of the rack shaft  14 , whereby the steered wheels  3  are steered. That is, the steered wheels  3  are steered by rotationally driving the worm shaft  20  by the electric motor  18 . The electric motor  18  is a motor that generates a steering driving force for steering the steered wheels  3 . 
     The vehicle is provided with a vehicle speed sensor  23  that detects the vehicle speed V. A charge coupled device (CCD) camera  24  is also mounted on the vehicle. The CCD camera  24  shoots the road ahead in the direction in which the vehicle is traveling. 
     The steering torque T detected by the torque sensor  11 , the vehicle speed V detected by the vehicle speed sensor  23 , and an image signal output from the CCD camera  24  are input to an electronic control unit (ECU)  12 . The ECU  12  controls the electric motor  18  based on these input signals. 
       FIG. 2  is a block diagram showing an electrical configuration of the ECU  12 . 
     The ECU  12  includes a microcomputer  31 , a drive circuit (inverter circuit)  32 , and a current detection circuit  33 . The microcomputer  31  controls the electric motor  18 . The drive circuit  32  is controlled by the microcomputer  31  to supply electric power to the electric motor  18 . The current detection circuit  33  detects a motor current (actual current value) I flowing in the electric motor  18 . 
     The microcomputer  31  includes a central processing unit (CPU) and a memory (a read only memory (ROM), a random access memory (RAM), a nonvolatile memory, etc.). The microcomputer  31  functions as a plurality of functional processing units by executing a predetermined program. The plurality of functional processing units include a steering assist current value setting unit  41 , an information obtaining unit  42 , a lane keep assist current value setting unit  43 , a target current value calculation unit  44 , a current deviation calculation unit  45 , a proportional-integral (PI) control unit  46 , and a pulse width modulation (PWM) control unit  47 . 
     The steering assist current value setting unit  41  sets a steering assist current value Is* that is a motor current value corresponding to a target value of the steering assist torque. The steering assist current value setting unit  41  sets the steering assist current value Is* based on the steering torque T detected by the torque sensor  11  and the vehicle speed V detected by the vehicle speed sensor  23 .  FIG. 3  shows an example of setting the steering assist current value Is* with respect to the detected steering torque T. For example, the detected steering torque T takes a positive value in the case where it is the torque for steering to the right, and takes a negative value in the case where it is the torque for steering to the left. The steering assist current value Is* is set to a positive value when a steering assist force for steering to the right should be generated by the electric motor  18 , and is set to a negative value when a steering assist force for steering to the left should be generated by the electric motor  18 . 
     The steering assist current value Is* takes a positive value when the detected steering torque T has a positive value, and takes a negative value when the detected steering torque T has a negative value. The steering assist current value Is* is set to zero when the detected steering torque T has a very small value in the range of −T 1  to T 1  (torque dead band) (e.g., T 1 =0.4 N·m). In the case where the detected steering torque T is out of the range of −T 1  to T 1 , the steering assist current value Is* is set so that its absolute value increases as the absolute value of the detected steering torque T increases. The steering assist current value Is* is set so that its absolute value decreases as the vehicle speed V detected by the vehicle speed sensor  23  increases. A large steering assist force can thus be generated when the vehicle is traveling at low speeds, and the steering assist force can be reduced when the vehicle is traveling at high speeds. 
     As shown in  FIG. 4 , the information obtaining unit  42  recognizes a pair of lane marking lines (white lines) Ll, Lr indicating a lane in which a vehicle  100  is traveling, and recognizes the lane in which the vehicle  100  is traveling, based on an image shot by the CCD camera  24 . The information obtaining unit  42  sets a target travel line Ls of the vehicle  100  within the recognized lane. In the present embodiment, the target travel line Ls is set in the middle of the width of the vehicle&#39;s lane. The information obtaining unit  42  obtains a lateral deviation y of the vehicle  100  from the target travel line Ls, a lateral deviation change rate dy/dt, or a rate of change in lateral deviation y per unit time, and a change rate d 2 y/dt 2 , or a rate of change in lateral deviation change rate dy/dt per unit time. Hereinafter, the change rate d 2 y/dt 2  (the rate of change in lateral deviation change rate dy/dt per unit time) is sometimes referred to as the “second derivative value d 2 y/dt 2  of the lateral deviation y.” 
     The lateral deviation y of the vehicle  100  represents the distance from a reference position C of the vehicle  100  to the target travel line Ls as viewed in plan. The reference position C of the vehicle  100  may be the position of the center of gravity of the vehicle  100  or may be the position where the CCD camera  24  is placed in the vehicle  100 . In the present embodiment, the lateral deviation y is set so that the sign of the lateral deviation y is positive if the reference position C of the vehicle  100  is located on the right side of the target travel line Ls, and is negative if the reference position C of the vehicle  100  is located on the left side of the target travel line Ls, as viewed in the direction in which the vehicle  100  is traveling. 
     The lateral deviation change rate dy/dt may be the deviation (y(t)−y(t−Δt)) between a lateral deviation y(t) obtained this time and a lateral deviation y(t−Δt) obtained a predetermined unit time Δt ago. The lateral deviation change rate dy/dt may be the deviation (y(t+Δt)−y(t)) between a predicted lateral deviation y(t+Δt) after the predetermined unit time Δt and the lateral deviation y(t) obtained this time. The predicted lateral deviation y(t+Δt) may be obtained in view of the vehicle speed, the yaw angle, etc. 
     The lateral deviation change rate dy/dt may be the deviation (y(t+Δtx+Δt)−y(t+Δtx)) between a predicted lateral deviation y(t+Δtx) at time t 1  after a predetermined time Δtx and a predicted lateral deviation y(t+Δtx+Δt) at time t 2  that is the predetermined unit time Δt after time t 1 . The predicted lateral deviations y(t+Δtx), y(t+Δtx+Δt) may be obtained in view of the vehicle speed, the yaw angle, etc. Since a method for calculating or predicting the lateral deviation y of a vehicle by shooting the road ahead in the direction in which the vehicle is traveling is known in the art, as described in patent documents such as JP 2013-212839 A, JP 4292562 B, and JP H11-34774 A, description thereof will be omitted. 
     For example, the second derivative value d 2 y/dt 2  of the lateral deviation y may be the deviation ((dy/dt)(t)−(dy/dt)(t−Δt)) between a lateral deviation change rate (dy/dt)(t) obtained this time and a lateral deviation change rate (dy/dt)(t−Δt) obtained a predetermined unit time Δt ago. 
     Referring back to  FIG. 2 , the lane keep assist current value setting unit  43  sets a lane keep assist current value Ir* based on the vehicle speed V, the lateral deviation y, the lateral deviation change rate dy/dt, and the second derivative value d 2 y/dt 2  of the lateral deviation y. The lane keep assist current value Ir* is a value that is used to cause the vehicle  100  to travel along the target travel line Ls. The lane keep assist current value setting unit  43  will be described in detail later. 
     The target current value calculation unit  44  calculates a target current value I* by adding the lane keep assist current value Ir* set by the lane keep assist current value setting unit  43  to the steering assist current value Is* set by the steering assist current value setting unit  41 . The current deviation calculation unit  45  calculates the deviation between the target current value I* obtained by the target current value calculation unit  44  and the actual current value I detected by the current detection circuit  33  (current deviation ΔI=I*−I). 
     The PI control unit  46  generates a drive command value by performing a PI operation on the current deviation ΔI calculated by the current deviation calculation unit  45 . The drive command value is a value that is used to control the current I flowing in the electric motor  18  toward the target current value I*. The PWM control unit  47  generates a PWM control signal having a duty cycle corresponding to the drive command value and supplies the PWM control signal to the drive circuit  32 . Electric power corresponding to the drive command value is thus supplied to the electric motor  18 . 
     The current deviation calculation unit  45  and the PI control unit  46  form a current feedback controller. The current feedback controller serves to control the motor current I flowing in the electric motor  18  toward the target current value I*. 
     The lane keep assist current value setting unit  43  will be described in detail. As shown in  FIG. 2 , the lane keep assist current value setting unit  43  includes a lane keep assist current value calculation unit  51 , a correction gain setting unit  52 , and a gain multiplication unit  53 . 
     The lane keep assist current value calculation unit  51  calculates a lane keep assist current value Iro* based on the lateral deviation y and the lateral deviation change rate dy/dt which are obtained by the information obtaining unit  42 . The lane keep assist current value Iro* corresponds to lane keep assist torque that makes the lateral deviation y and the lateral deviation change rate dy/dt closer to zero. 
       FIG. 5  is a block diagram showing an electrical configuration of the lane keep assist current value calculation unit  51 . The lane keep assist current value calculation unit  51  includes a first current value calculation unit  61 , a second current value calculation unit  62 , an addition unit  63 , a vehicle speed gain setting unit  64 , and a multiplication unit  65 . 
     The first current value calculation unit  61  calculates a first lane keep assist current value Ir 1 * based on the lateral deviation y. The second current value calculation unit  62  calculates a second lane keep assist current value Ir 2 * based on the lateral deviation change rate dy/dt. The addition unit  63  calculates a third lane keep assist current value Ir 3 *(=Ir 1 *+Ir 2 *) by adding the first lane keep assist current value Ir 1 * calculated by the first current value calculation unit  61  and the second lane keep assist current value Ir 2 * calculated by the second current value calculation unit  62 . The vehicle speed gain setting unit  64  sets vehicle speed gain Gv according to the vehicle speed V. The multiplication unit  65  calculates the lane keep assist current value Iro*(=Gv·(Ir 1 *+Ir 2 *)) by multiplying the third lane keep assist current value Ir 3 *(=In 1 *+Ir 2 *) calculated by the addition unit  63  by the vehicle speed gain Gv set by the vehicle speed gain setting unit  64 . The lane keep assist current value Iro* is applied to the gain multiplication unit  53 . 
     The first current value calculation unit  61 , the second current value calculation unit  62 , and the vehicle speed gain setting unit  64  will be more specifically described. 
     The first current value calculation unit  61  calculates the first lane keep assist current value Ir 1 * based on a map or an arithmetic expression that represents the relationship of the first lane keep assist current value Ir 1 * to the preset lateral deviation y. The second current value calculation unit  62  calculates the second lane keep assist current value Ir 2 * based on a map or an arithmetic expression that represents the relationship of the second lane keep assist current value Ir 2 * to the preset lateral deviation change rate dy/dt. 
     It is preferable that the first current value calculation unit  61  and the second current value calculation unit  62  calculate the first lane keep assist current value Ir 1 * and the second lane keep assist current value Ir 2 * as follows, where a 1  and a 2  represent constants of the same sign, b 1  represents a degree of a natural number of two or larger, and b 2  represents a degree of a natural number smaller than b 1 . 
     In the case where b 1  is set to an odd number, it is preferable that the first current value calculation unit  61  calculate the first lane keep assist current value Ir 1 * based on the relationship between y and Ir 1 * as given by the function Ir 1 *=a 1 ·y b1 . In the case where b 1  is set to an even number, it is preferable that the first current value calculation unit  61  calculate the first lane keep assist current value Ir 1 * based on the relationship between y and Ir 1 * as given by the function Ir 1 *=a 1 ·y b1  for y≧0 and given by the function Ir 1 *=−a 1 ·y b1  for y&lt;0. 
     In the case where b 2  is set to an odd number, it is preferable that the second current value calculation unit  62  calculate the second lane keep assist current value Ir 2 * based on the relationship between dy/dt and Ir 2 * as given by the function Ir 2 *=a 2 ·(dy/dt) b2 . In the case where b 2  is set to an even number, it is preferable that the second current value calculation unit  62  calculate the second lane keep assist current value Ir 2 * based on the relationship between dy/dt and Ir 2 * as given by the function Ir 2 * a 2 ·(dy/dt) b2  for dy/dt≧0 and given by the function Ir 2 *=−a 2 ·(dy/dt) b2  for dy/dt&lt;0. 
     As described above, in the present embodiment, the steering assist current value Is* is set to a positive value when a steering assist force for steering to the right should be generated by the electric motor  18 , and is set to a negative value when a steering assist force for steering to the left should be generated by the electric motor  18 . The lateral deviation y is set so that the sign of the lateral deviation y is positive if the reference position of the vehicle is located on the right side of the target travel line Ls, and is negative if the reference position of the vehicle is located on the left side of the target travel line Ls, as viewed in the direction in which the vehicle is traveling. In the case where the sign of the lateral deviation y is set in this manner, the constants a 1 , a 2  are set to negative values. 
     In the case where both the sign of the steering assist current value Is* and the sign of the lateral deviation y are set in the opposite manner to the present embodiment, the constants a 1 , a 2  are also set to negative values. 
     On the other hand, in the case where the sign of the steering assist current value Is* is set in a manner similar to the present embodiment and the sign of the lateral deviation y is set in the opposite manner to the present embodiment, or in the case where the sign of the steering assist current value Is* is set in the opposite manner to the present embodiment and the sign of the lateral deviation y is set in a manner similar to the present embodiment, the constants a 1 , a 2  are set to positive values. 
     The reason why it is preferable that the first current value calculation unit  61  and the second current value calculation unit  62  calculate the first lane keep assist current value Ir 1 * and the second lane keep assist current value Ir 2 * in the manner described above will be described. 
     In general, in the case where a is a constant in the function given by f(x)=ax b  (b represents a degree of a natural number), the absolute value of f(x) increases as the absolute value of x increases. In the case where the value of b is two or larger, the average rate of change increases as the absolute value of x increases. The average rate of change is the amount of change in f(x) divided by the amount of change in x. 
     In the case where the value of b 1  is two or larger, the absolute value of the first lane keep assist current value Ir 1 * increases as the absolute value of the lateral deviation y increases, and the average rate of change (rate of increase in absolute value of the first lane keep assist current value Ir 1 *) increases as the absolute value of the lateral deviation y increases. The vehicle can therefore be more rapidly guided toward the target travel line (in the present embodiment, toward the middle of the width of the vehicle&#39;s lane). 
     In the function given by f(x)=ax b , the average rate of change in the range where the absolute value of x is smaller than one decreases as the value of b increases. The average rate of change in the range where the absolute value of x is equal to or larger than one increases as the value of b increases. 
     When a 1  is equal to a 2  and b 1  is larger than b 2 , the average rate of change in first lane keep assist current value Ir 1 * in the range where the absolute value of the lateral deviation y is smaller than one is lower than that in second lane keep assist current value Ir 2 * in the range where the absolute value of the lateral deviation change rate dy/dt is smaller than one. The average rate of change in first lane keep assist current value Ir 1 * in the range where the absolute value of the lateral deviation y is larger than one is higher than that in second lane keep assist current value Ir 2 * in the range where the absolute value of the lateral deviation change rate dy/dt is larger than one. 
     Accordingly, when the reference position of the vehicle is located in an area away from the target travel line, the function to make the lateral deviation y closer to zero by the first lane keep assist current value Ir 1 * tends to be stronger than that to make the lateral deviation change rate dy/dt closer to zero by the second lane keep assist current value Ir 2 *, even if the sign of the second lane keep assist current value Ir 2 * is opposite to that of the first lane keep assist current value Ir 1 *. The vehicle can therefore be guided toward the target travel line (in the present embodiment, toward the middle of the width of the vehicle&#39;s lane) even if the sign of the second lane keep assist current value Ir 2 * is opposite to that of the first lane keep assist current value Ir 1 *. 
     The second lane keep assist current value Ir 2 * according to the magnitude of the lateral deviation change rate dy/dt is obtained regardless of the value of the lateral deviation y. The vehicle can therefore be guided so as to make the lateral centerline of the vehicle parallel to the target travel line, even if the reference position of the vehicle is located in an area close to the target travel line. 
     In the present embodiment, the first current value calculation unit  61  calculates the first lane keep assist current value Ir 1 * based on a map storing the relationship of the first lane keep assist current value Ir 1 * to the lateral deviation y, as shown in  FIG. 6A , or an arithmetic expression representing this relationship. In the example of  FIG. 6A , the first lane keep assist current value Ir 1 * is represented by the cubic function Ir 1 *=a 1 ·y 3 , where a 1  is a negative constant. That is, this function corresponds to the case where a 1  is negative and b 1  is three. 
     For example, the first current value calculation unit  61  may calculate the first lane keep assist current value Ir 1 * based on a map storing the relationship of the first lane keep assist current value Ir 1 * to the lateral deviation y, as shown in  FIG. 6B , or an arithmetic expression representing this relationship. The curve shown in  FIG. 6B  is created by translating the curve in the region where Ir 1 * is zero or larger in  FIG. 6A  in the direction of the abscissa by −A (A&gt;0) and translating the curve in the region where Ir 1 * is smaller than zero in  FIG. 6A  in the direction of the abscissa by +A. In the curve of  FIG. 6B , a dead band where the first lane keep assist current value Ir 1 * is zero is set in the range where the lateral deviation y is −A (A&gt;0) to A. 
     For example, the first current value calculation unit  61  may calculate the first lane keep assist current value Ir 1 * based on a map storing the relationship of the first lane keep assist current value Ir 1 * to the lateral deviation y, as shown in  FIG. 6C , or an arithmetic expression representing this relationship. In the example of  FIG. 6C , the first lane keep assist current value Ir 1 * is given by the quadratic function Ir 1 *=a 1 ·y 2  for y≧0 and is given by the quadratic function Ir 1 *=−a 1 ·y 2  for y&lt;0, where a 1  is a negative constant. This function corresponds to the case where a 1  is negative and b 1  is two. 
     In the present embodiment, the second current value calculation unit  62  calculates the second lane keep assist current value Ir 2 * based on a map storing the relationship of the second lane keep assist current value Ir 2 * to the lateral deviation change rate dy/dt, as shown in  FIG. 7A , or an arithmetic expression representing this relationship. In the example of  FIG. 7A , the second lane keep assist current value Ir 2 * is represented by the linear function Ir 2 *=a 2 ·dy/dt, where a 2  is a negative constant. That is, this function corresponds to the case where a 2  is negative and b 2  is one. A dead band where the second lane keep assist current value Ir 2 * is zero may be set in the range where the absolute value of the lateral deviation change rate dy/dt is close to zero. 
     For example, the second current value calculation unit  62  may calculate the second lane keep assist current value Ir 2 * based on a map storing the relationship of the second lane keep assist current value Ir 2 * to the lateral deviation change rate dy/dt, as shown in  FIG. 7B , or an arithmetic expression representing this relationship. In the example of  FIG. 7B , the second lane keep assist current value Ir 2 * is given by the quadratic function Ir 2 *=a 2 ·(dy/dt) 2  for dy/dt≧0 and is given by the quadratic function Ir 2 *=−a 2 ·(dy/dt) 2  for dy/dt&lt;0, where a 2  is a negative constant. This function corresponds to the case where a 2  is negative and b 2  is two. 
     Referring back to  FIG. 5 , the vehicle speed gain setting unit  64  sets the vehicle speed gain Gv based on the vehicle speed V detected by the vehicle speed sensor  23 .  FIG. 8  shows an example of setting the vehicle speed gain Gv with respect to the vehicle speed V. In the example of  FIG. 8 , the vehicle speed gain Gv is fixed to zero in the range where the vehicle speed V is close to zero, and is fixed to one in the range where the vehicle speed V is higher than a predetermined value. When the vehicle speed V is in the intermediate range, the vehicle speed gain G is set according to the characteristics in which the vehicle speed gain Gv increases from zero to one with the vehicle speed V. 
     The lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51  is applied to the gain multiplication unit  53  (see  FIG. 2 ). 
     Referring back to  FIG. 2 , the correction gain setting unit  52  sets correction gain G based on the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y which are obtained by the information obtaining unit  42 . The correction gain G is used to correct the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51 . Operation of the correction gain setting unit  52  will be described in detail later. 
     The gain multiplication unit  53  corrects the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51  by multiplying the lane keep assist current value Iro* by the correction gain G set by the correction gain setting unit  52 . The corrected lane keep assist current value G·Iro* is applied as the final lane keep assist current value Ir* to the target current value calculation unit  44 . The correction gain setting unit  52  and the gain multiplication unit  53  form a correction unit that corrects the lane keep assist current value Iro*. 
       FIG. 9  is a flowchart illustrating an example of operation of the correction gain setting unit  52 . The process of  FIG. 9  is repeatedly performed in predetermined calculation cycles. 
     The correction gain setting unit  52  obtains the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 1 ). The correction gain setting unit  52  determines if the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 2 ). For example, the correction gain setting unit  52  may determine that the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y when the product of the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y is positive, and may determine that the sign of the lateral deviation change rate dy/dt is different from that of the second derivative value d 2 y/dt 2  of the lateral deviation y when the product of the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y is zero or negative. 
     If the correction gain setting unit  52  determines that the sign of the lateral deviation change rate dy/dt is different from that of the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 2 : NO), it sets the correction gain G to one (step S 3 ). In this case, the gain multiplication unit  53  outputs the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51  as it is. The correction gain setting unit  52  thus ends the process of the current calculation cycle. 
     If the correction gain setting unit  52  determines in step S 2  that the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 2 : YES), it sets the correction gain G to a predetermined value α that is larger than one (step S 4 ). In this case, the gain multiplication unit  53  outputs the lane keep assist current value Ir* whose absolute value is larger than that of the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51  and whose sign is the same as that of the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51 . The correction gain setting unit  52  thus ends the process of the current calculation cycle. 
     The reason why the correction gain setting unit  52  sets the correction gain G to the predetermined value α that is larger than one if it determines that the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y will be described. 
     The state where both the lateral deviation y and the lateral deviation change rate dy/dt are changing in a direction away from zero corresponds to the state where the degree to which the vehicle deviates from the target travel line is increasing. In the state where both the lateral deviation y and the lateral deviation change rate dy/dt are changing in the direction away from zero, the sign of the lateral deviation change rate dy/dt or a first derivative value of the lateral deviation y is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y. Accordingly, if the sign of the lateral deviation change rate dy/dt or the first derivative value of the lateral deviation y is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y, it can be determined that the degree to which the vehicle deviates from the target travel line is increasing. 
     The correction gain setting unit  52  thus sets the correction gain G to the predetermined value α that is larger than one if the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y. Accordingly, in the state where the degree to which the vehicle deviates from the target travel line is increasing, the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51  can be corrected so that its absolute value increases. The lane keep assist torque can thus be increased in the state where the degree to which the vehicle deviates from the target travel line is increasing. The vehicle can therefore be rapidly returned toward the target travel line if it deviates from the target travel line. 
       FIG. 10  is a flowchart illustrating another example of operation of the correction gain setting unit  52 . The process of  FIG. 10  is repeatedly performed in predetermined calculation cycles. 
     The correction gain setting unit  52  obtains the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 11 ). The correction gain setting unit  52  calculates first correction gain G 1  based on the following formula (1) (step S 12 ).
 
 G 1=|( dy/dt )· K 1×( d   2   y/dt   2 )· K 2|  (1)
 
     where K 1  and K 2  represent preset positive coefficients. 
     The correction gain setting unit  52  then determines if the first correction gain G 1  is larger than one (step S 13 ). 
     If the first correction gain G 1  is one or less (step S 13 : NO), the correction gain setting unit  52  sets the first correction gain G 1  to a predetermined value γ larger than one (step S 14 ) and proceeds to step S 15 . If the correction gain setting unit  52  determines in step S 13  that the first correction gain G 1  is larger than one (step S 13 : YES), it proceeds to step S 15 . 
     In step S 15 , the correction gain setting unit  52  determines if the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y. For example, the correction gain setting unit  52  may determine that the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y when the product of the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y is positive, and may determine that the sign of the lateral deviation change rate dy/dt is different from that of the second derivative value d 2 y/dt 2  of the lateral deviation y when the product of the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y is zero or negative. 
     If the correction gain setting unit  52  determines that the sign of the lateral deviation change rate dy/dt is different from that of the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 15 : NO), it sets the final correction gain G to one (step S 16 ). The correction gain setting unit  52  thus ends the process of the current calculation cycle. 
     If the correction gain setting unit  52  determines in step S 15  that the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 15 : YES), it sets the final correction gain G to the value of the first correction gain G 1  (step S 17 ). The correction gain setting unit  52  thus ends the process of the current calculation cycle. 
     In this modification, the correction gain G that is set if it is determined that the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y is set according to the magnitudes of the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y. Since the correction gain G can thus be set according to the degree to which the vehicle deviates from the target travel line, the lane keep assist current value can be more appropriately corrected. 
       FIG. 11  is a block diagram showing a modification of the lane keep assist current value calculation unit  51 . 
     The lane keep assist current value calculation unit  51  includes a first current value calculation unit  61 , a second current value calculation unit  62 , an addition unit  63 , a vehicle speed gain setting unit  64 , a multiplication unit  65 , a control steering torque setting unit  66 , and a switch unit  67 . 
     The first current value calculation unit  61  calculates a first lane keep assist current value Ir 1 * by an operation similar to that of the first current value calculation unit  61  in  FIG. 5 . The second current value calculation unit  62  calculates a second lane keep assist current value Ir 2 * by an operation similar to that of the second current value calculation unit  62  in  FIG. 5 . The addition unit  63  calculates a third lane keep assist current value Ir 3 *Ir 1 *+Ir 2 *) by adding the first lane keep assist current value Ir 1 * and the second lane keep assist current value Ir 2 *. 
     The vehicle speed gain setting unit  64  sets vehicle speed gain Gv according to the vehicle speed V by an operation similar to that of the vehicle speed gain setting unit  64  in  FIG. 5 . The multiplication unit  65  calculates a fourth lane keep assist current value Ir 4 *(=Gv·(Ir 1 *+Ir 2 *)) by multiplying the third lane keep assist current value Ir 3 * (Ir 1 *+Ir 2 *) by the vehicle speed gain Gv. 
     The control steering torque setting unit  66  sets control steering torque Ts based on the detected steering torque T detected by the torque sensor  11 .  FIG. 12  shows an example of setting the control steering torque Ts with respect to the detected steering torque T. A dead band where the control steering torque Ts is zero is set in the region where the absolute value of the detected steering torque T is equal to or smaller than a predetermined value T 2  (e.g., T 2 =0.4 N·m). The control steering torque Ts is set to have the same value as the detected steering torque T in the region where the detected steering torque T is larger than T 2  and in the region where the detected steering torque T is smaller than −T 2 . 
     The fourth lane keep assist current value Ir 4 * calculated by the multiplication unit  65  is input to a first input terminal of the switch unit  67 . Zero is input to a second input terminal of the switch unit  67 . The control steering torque Ts calculated by the control steering torque setting unit  66  and the fourth lane keep assist current value Ir 4 * calculated by the multiplication unit  65  are applied to the switch unit  67  as switch control signals. The switch unit  67  selects one of the fourth lane keep assist current value Ir 4 * received at the first input terminal and zero received at the second input terminal based on the sign of the control steering torque Ts and the sign of the fourth lane keep assist current value Ir 4 *, and outputs the selected one of the fourth lane keep assist current value Ir 4 * and zero as a lane keep assist current value Iro*. 
     The sign of the control steering torque Ts represents the steering direction in which the driver steers the vehicle. The sign of the fourth lane keep assist current value Ir 4 * represents the steered direction in which the steered wheels are steered, which corresponds to the fourth lane keep assist current value Ir 4 * (lane keep assist torque). If the steering direction represented by the sign of the control steering torque Ts is different from the steered direction represented by the sign of the fourth lane keep assist current value Ir 4 *, the switch unit  67  selects the fourth lane keep assist current value Ir 4 * received at the first input terminal, and outputs the fourth lane keep assist current value Ir 4 * as the lane keep assist current value Iro*. 
     If the steering direction represented by the sign of the control steering torque Ts is the same as the steered direction represented by the sign of the fourth lane keep assist current value Ir 4 *, the switch unit  67  selects zero received at the second input terminal, and outputs zero as the lane keep assist current value Iro*. This is because it is considered that the driver is steering the vehicle toward the target travel line when the steering direction represented by the sign of the control steering torque Ts is the same as the steered direction represented by the sign of the fourth lane keep assist current value Ir 4 *. As described above, the dead band where the control steering torque Ts is zero is set in the region where the absolute value of the detected steering torque T is equal to or smaller than the predetermined value T 2 . This can restrain frequent inversions of the sign of the control steering torque Ts in the region where the detected steering torque T is small. This can restrain frequent switching between the fourth lane keep assist current value Ir 4 * and zero and can therefore restrain frequent fluctuations in motor torque generated by the electric motor  18 . 
     In this modification, if the sign of the control steering torque Ts is different from that of the fourth lane keep assist current value Ir 4 *, the switch unit  67  selects the fourth lane keep assist current value Ir 4 * and outputs the fourth lane keep assist current value Ir 4 * as the lane keep assist current value Iro*. For example, the switch unit  67  may determine that the sign of the control steering torque Ts is different from that of the fourth lane keep assist current value Ir 4 * when the product of the control steering torque Ts and the fourth lane keep assist current value Ir 4 * is equal to or smaller than zero (zero or a negative value). 
     If the sign of the control steering torque Ts is the same as that of the fourth lane keep assist current value Ir 4 *, the switch unit  67  selects zero and outputs zero as the lane keep assist current value Iro*. For example, the switch unit  67  may determine that the sign of the control steering torque Ts is the same as that of the fourth lane keep assist current value Ir 4 * when the product of the control steering torque Ts and the fourth lane keep assist current value Ir 4 * is larger than zero (a positive value). 
     When the steering direction represented by the sign of the control steering torque Ts is the same as the steered direction represented by the sign of the fourth lane keep assist current value Ir 4 *, it is considered that the driver is steering the vehicle toward the target travel line. If the lane keep assist torque is generated even though the driver is steering the vehicle toward the target travel line, a steering response (steering reaction force) may be significantly reduced, whereby a steering feel may be degraded or the vehicle may return too much toward the target travel line. In this modification, the lane keep assist current value Iro* is zero when the steering direction represented by the sign of the control steering torque Ts is the same as the steered direction represented by the sign of the fourth lane keep assist current value Ir 4 *. This can appropriately give the driver a steering response when he/she is steering the vehicle toward the target travel line. This can improve a steering feel and can also restrain the vehicle from returning too much toward the target travel line. 
       FIG. 13  is a block diagram showing another example of the configuration of the ECU. In  FIG. 13 , the portions corresponding to the portions shown in  FIG. 2  are denoted by the same reference characters. 
     An ECU  12 A in  FIG. 13  is different from the ECU  12  in  FIG. 2  in the configuration of a lane keep assist current value setting unit  43 A. The lane keep assist current value setting unit  43 A includes a lane keep assist current value calculation unit  51 , a correction value setting unit  52 A, and a correction value addition unit  53 A. The lane keep assist current value calculation unit  51  has the same composition as that of the lane keep assist current value calculation unit  51  in  FIG. 2 . The lane keep assist current value calculation unit  51  may have the same configuration as that in the modification shown in  FIG. 11 . 
     The correction value setting unit  52 A calculates a correction value β based on the lateral deviation change rate dy/dt, the second derivative value d 2 y/dt 2  of the lateral deviation y, and the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51 . Operation of the correction value setting unit  52 A will be described in detail later. The correction value addition unit  53 A corrects the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51  by adding the correction value β set by the correction value setting unit  52 A to the lane keep assist current value Iro*. The corrected lane keep assist current value (Iro*β) is applied as a final lane keep assist current value Ir* to the target current value calculation unit  44 . 
       FIG. 14  is a flowchart illustrating an example of operation of the correction value setting unit  52 A. The process of  FIG. 14  is repeatedly performed in predetermined calculation cycles. 
     The correction value setting unit  52 A obtains the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 21 ). The correction value setting unit  52 A determines if the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 22 ). For example, the correction value setting unit  52 A may determine that the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y when the product of the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y is positive, and may determine that the sign of the lateral deviation change rate dy/dt is different from that of the second derivative value d 2 y/dt 2  of the lateral deviation y when the product of the lateral deviation change rate dy/dt and the second derivative value d 2 y/dt 2  of the lateral deviation y is zero or negative. 
     If the correction value setting unit  52 A determines that the sign of the lateral deviation change rate dy/dt is different from that of the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 22 : NO), it sets the correction value β to zero (step S 23 ). In this case, the correction value addition unit  53 A outputs the lane keep assist current value Ira* calculated by the lane keep assist current value calculation unit  51  as it is. The correction value setting unit  52 A thus ends the process of the current calculation cycle. 
     If the correction value setting unit  52 A determines in step S 22  that the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y (step S 22 : YES), it proceeds to step S 24 . In step S 24 , the correction value setting unit  52 A determines if the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51  is a positive value, a negative value, or zero. 
     If the correction value setting unit  52 A determines that the lane keep assist current value Iro* is a positive value (Iro*&gt;0), it sets the correction value β to a predetermined positive value D (D&gt;0) (step S 25 ). In this case, the correction value addition unit  53 A outputs the sum of the lane keep assist current value Iro* (Iro*&gt;0) calculated by the lane keep assist current value calculation unit  51  and the correction value β (=D). The correction value setting unit  52 A thus ends the process of the current calculation cycle. 
     If the correction value setting unit  52 A determines in step S 24  that the lane keep assist current value Iro* is a negative value (Iro*&lt;0), it sets the correction value β to a predetermined negative value −D (step S 26 ). In this case, the correction value addition unit  53 A outputs the sum of the lane keep assist current value Iro* (Iro*&lt;0) calculated by the lane keep assist current value calculation unit  51  and the correction value β (=−D). The correction value setting unit  52 A thus ends the process of the current calculation cycle. 
     If the correction value setting unit  52 A determines in step S 24  that the lane keep assist current value Iro* is zero (Iro*=0), it sets the correction value β to zero (step S 23 ). In this case, the correction value addition unit  53 A outputs the lane keep assist current value Iro* (Iro*=0) calculated by the lane keep assist current value calculation unit  51  as it is. The correction value setting unit  52 A thus ends the process of the current calculation cycle. 
     In this modification as well, if the sign of the lateral deviation change rate dy/dt is the same as that of the second derivative value d 2 y/dt 2  of the lateral deviation y, the lane keep assist current value Iro* calculated by the lane keep assist current value calculation unit  51  can be corrected so that its absolute value increases. The vehicle can therefore be rapidly returned toward the target travel line if it deviates from the target travel line. 
     Although the embodiment of the present invention is described above, the present invention may be carried out in other forms. For example, a limiter that limits the absolute value of the third lane keep assist current value Ir 3 * (=Ir 1 *+Ir 2 *) to a predetermined range may be provided between the addition unit  63  (see  FIGS. 5 and 11 ) and the multiplication unit  65 . 
     Although the above embodiment includes the multiplication unit  65  (see  FIGS. 5 and 11 ), the multiplication unit  65  may be omitted. The vehicle speed gain setting unit  64  is not required in the case where the multiplication unit  65  is omitted. 
     In the above embodiment, the steering assist current value setting unit  41  sets the steering assist current value Is* by using the steering torque T (specifically, based on the steering torque T and the vehicle speed V). However, the steering assist current value setting unit  41  may set the steering assist current value Is* by using a steering angle. 
     Although the modification of the lane keep assist current value calculation unit  51  (see  FIG. 11 ) includes the control steering torque setting unit  66 , the control steering torque setting unit  66  may be omitted. In the case where the control steering torque setting unit  66  is omitted, the detected steering torque T detected by the torque sensor  11  is applied to the switch unit  67  instead of the control steering torque Ts. In this case, when the steering direction represented by the sign of the detected steering torque T is different from the steered direction represented by the sign of the fourth lane keep assist current value Ir 4 *, the switch unit  67  selects the fourth lane keep assist current value Ir 4 * received at the first input terminal and outputs the fourth lane keep assist current value Ir 4 * as the lane keep assist current value Iro*. When the steering direction represented by the sign of the detected steering torque T is the same as the steered direction represented by the sign of the fourth lane keep assist current value Ir 4 *, the switch unit  67  selects zero received at the second input terminal and outputs zero as the lane keep assist current value Iro*. 
     Although the above embodiment is described with respect to an example in which the present invention is applied to an electric power steering system, the present invention is also applicable to other vehicle steering systems such as a steer-by-wire system. 
     The present invention is also applicable in an autonomous driving mode in which the steering wheel  2  is not steered by the driver.