# Patent

## Title
System for providing steering assist torque based on a proportional gain value

## Abstract

A steering system providing an assist torque to a handwheel is provided, and includes a proportional gain module and a torque command module. The proportional gain module determines a proportional gain value. The proportional gain value is scheduled as a function of a lateral position error and at least one of the following: a near field heading angle, a far field heading angle, a curvature of the lane, and a lateral position of the vehicle. The torque command module determines the torque assist based on the proportional gain value.

## Background
FIELD OF THE INVENTION
The present invention relates to a system for providing a steering assist torque, and in particular to a system for providing a steering assist torque based on a proportional gain value.
BACKGROUND OF THE INVENTION
A lane keeping system detects lane markers on the left and right side of the road, and provides an assist torque to a handwheel of a steering system to help keep a vehicle between the lane markers. The lane keeping system may also provide alerts to the driver using visual or audio aids such as, for example, a display, a voice indicator, or chimes. Some lane keeping systems use a lateral position of the vehicle for controlling functions such as the torque assist. The lateral acceleration may also be used during autonomous mode of the lane keeping system.
When the lane keeping system is in autonomous mode, the amount of torque assist that is provided to the handwheel by the lane keeping system may sometimes create more handwheel movement than what a driver typically creates to drive the vehicle. This in turn may create variations in the lateral acceleration of the vehicle, which may be objectionable to the vehicle occupants. However, it should also be noted that sometimes the extra handwheel movement may keep a driver engaged in the driving process, without actually manipulating the handwheel.
SUMMARY OF THE INVENTION
According to one embodiment, a steering system providing an assist torque to a handwheel is provided, and includes a proportional gain module and a torque command module. The proportional gain module determines a proportional gain value. The proportional gain value is scheduled as a function of a lateral position error and at least one of the following: a near field heading angle, a far field heading angle, a curvature of the lane, and a lateral position of the vehicle. The torque command module determines the torque assist based on the proportional gain value.
In another embodiment, a method of determining an assist torque to a handwheel is provided. The method includes determining a proportional gain value. The proportional gain value is scheduled as a function of a lateral position error and at least one of the following: a near field heading angle, a far field heading angle, a curvature of the lane, and a lateral position of the vehicle. The method includes determining a torque assist based on the proportional gain value.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a diagram depicting a vehicle in a lane with lane markers in accordance with one aspect of the invention;
FIG. 2 is a schematic diagram of the vehicle in FIG. 1 having an exemplary system for providing a steering assist torque to a handwheel according to another aspect of the invention;
FIG. 3 is a block diagram illustrating a steering control module and a lane keeping system shown in FIG. 2 according to yet another aspect of the invention; and
FIG. 4 is an alternative embodiment of the block diagram shown in FIG. 3 illustrating another aspect of the invention.



DETAILED DESCRIPTION
Referring now to FIG. 1, where the invention will be described with reference to specific embodiments without limiting same, an exemplary schematic diagram of a vehicle 10 driving within a lane 12 is illustrated. The lane 12 includes lane markers 14 on the left and right hand sides of the lane 12. The lane 12 includes a lane center 20 (the lane center is a calculated value, as there is no lane marker for the lane center 20) and the vehicle 10 includes a vehicle centerline 22. A heading angle θ (not shown in FIG. 1) is measured between the lane center 20 and the vehicle centerline 22, where a positive sign for the heading angle θ (e.g., θ+) is provided if the vehicle centerline 22 is to the right of the lane center 20, and a negative sign (e.g., θ−) for the heading angle θ is provided if the vehicle centerline 22 is to the left of the lane center 20. A lateral position Δ of the vehicle 10 is also illustrated in FIG. 1, where the lateral position Δ includes a positive sign if the vehicle centerline 22 is to the right of the lane center 20, and a negative sign if the vehicle centerline 22 is to the left of the lane center 20. In the embodiment as shown in FIG. 1, the lane 12 also includes a curvature ρ. In the embodiment as shown, the curvature ρ is positive, as the curvature ρ is oriented to the right side of the lane 12. If the curvature ρ is oriented to the left side of the lane 14, then the curvature ρ includes a negative sign.
FIG. 1 also illustrates a near field heading angle θhnear and a far field heading angle θhfar. The near field heading angle θhnear is observed relatively close to the vehicle 10 (e.g., generally within about 40 meters of the vehicle 10). The lateral position Δ is also observed relatively close to the vehicle 10 as well (e.g., generally within about 40 meters of the vehicle 10). The near field heading angle θhnear, the far heading angle θhfar, and the lateral position A are then projected directly in front of the vehicle 10 (e.g., at zero meters). The curvature ρ of the lane 12 is generally calculated as an average value over a distance (where the distance is usually between about 5 to about 25 meters in front of the vehicle 10). The far field heading angle θhfar is located in an area labeled as a look ahead distance D. The look ahead distance D is generally in the range of between about 35 to about 70 meters. The far field heading angle θhfar is determined by calculating a tangent line T of the lane center 20 at the look ahead distance D. The far field heading angle θhfar is measured between the vehicle centerline 22 and the tangent line T at the look ahead distance D.
Turning now to FIG. 2, the vehicle 10 includes a steering system 30. The steering system 30 includes a handwheel 34 coupled to a steering shaft 36. The steering system 30 is an electric power steering (EPS) system that further includes a steering assist unit 38 that couples to the steering shaft 36 of the steering system 30 and to tie rods 40, 42 of the vehicle 10. The steering assist unit 38 includes, for example, a rack and pinion steering mechanism (not shown) that may be coupled through the steering shaft 36 to a steering actuator motor and gearing. During operation, as the handwheel 34 is turned by a vehicle operator, the motor of the steering assist unit 38 provides the assistance to move the tie rods 40, 42 which in turn moves steering knuckles 44, 46, respectively, coupled to roadway wheels 48, 50, respectively of the vehicle 10.
As shown in FIG. 2, the vehicle 10 further includes various sensors that detect and measure observable conditions of the steering system 30 and/or of the vehicle 10. In one example, a torque sensor 50, a vehicle speed sensor 52, and a steering angle sensor 56 are provided. A steering control module 60 controls the operation of the steering system 30 and the steering assist unit 38 based on one or more of the signals from the sensors 50, 52 and 56 and a lane keeping system that are included in the vehicle 10, and determines an torque assist command Tassist. In various embodiments, the steering control module 60 can include one or more sub-modules and datastores. As used herein the terms module and sub-module refer to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.
In the exemplary embodiment as shown in FIG. 2, an integrated camera and processor 62 are illustrated. Of course in other embodiments, the camera and processor 62 may be separate components. In one embodiment, the camera and processor 62 may be part of the lane keeping system. The lane keeping system generates feedback for a corrective input into the handwheel 34 in the event a path deviation with respect to the lane markers 14 (shown in FIG. 1) is detected. The camera and processor 62 may calculate the lateral position Δ, the near field heading angle θhnear, the far field heading angle θhfar, and the curvature ρ based on the lane markers 14 that are detected by the camera. Specifically, the camera detects the presence of the lane markers 14, and the processor includes control logic for determining the lateral position Δ, the near field heading angle θhnear, the far field heading angle θhfar, and the curvature ρ based on the lane markers 14 that are detected by the camera. The camera and processor 62 is in communication with the steering control module 60.
FIG. 3 is an exemplary block diagram of the steering control module 60 and the camera and processor 62. In the embodiment as shown, the camera and processor 62 may include image processing blocks 72 and 72 (in one embodiment, block 72 may be sent from block 70). Image processing block 70 includes control logic for determining the near field heading angle θhnear and the far field heading angle θhfar based on the lane markers 14 that are detected by the camera. The image processing block 70 is in communication with the image processing block 72. Image processing block 72 determines the lateral position Δ and the curvature ρ based on the lane markers 14 that are detected by the camera. The image processing block 72 is in communication with a proportional gain module 76, a lateral position derivative block 78, and a junction 80, which are each part of the steering control module 60.
The junction 80 receives as input a servo lateral position command 82 and the lateral position Δ as calculated by the image processing block 72. The junction 80 determines the difference between the servo lateral position command 82 and the lateral position Δ to determine a lateral position error εA. The lateral position error εΔ is sent to the proportional gain module 76 to calculate a proportional gain KP. The proportional gain KP is scheduled as a function of lateral position error εΔ as well as at least one of the near field heading angle θhnear, the far field heading angle θhfar, the curvature ρ, and the lateral position Δ.
The torque assist command Tassist (which is an output of the summing junction 88) is based on the proportional gain KP. Specifically, the amount or magnitude of the torque assist command Tassist, as well as the sign or direction of the torque assist command Tassist may be modified based on the proportional gain KP. That is, the torque assist command Tassist depends on the sign of the lateral position error c (the sign may be positive or negative), the sign of the far field heading angle θhfar, and the sign of the curvature p of the lane 12 (shown in FIG. 1). Table 1 illustrates one example of how the torque assist command Tassist may be modified based on the proportional gain KP.










TABLE 1





εΔ
Θhfar Far Field
Curvature (ρ)
Torque assist command Tassist







+
+
+
↓↓ (large negative command)


+
+
−
↓↓ (Large Negative command)


−
−
+
↑↑ (Large Positive command)


+
−
+
↑ (moderate Positive command)





Note:


negative torque would be steer to the left, positive steer to right.






For example, Table 1 illustrates that if the lateral position error εΔ, the sign of the far field heading angle θhfar, and the sign of the curvature p are all positive, then the torque assist command Tassist is a relatively large negative command (e.g., between about −3 Nm to about −7 Nm). If the lateral position error εΔ and the sign of the far field heading angle θhfar are positive and if the curvature ρ is negative, then the torque assist command Tassist is a relatively large negative command (e.g., between about −3 Nm to about −7 Nm). If the lateral position error εΔ and the sign of the far field heading angle θhfar are negative and if the curvature ρ is positive, then the torque assist command Tassist is a relatively large positive command (e.g., between about 3 Nm to about 7 Nm). If the lateral position error εΔ and the curvature ρ are positive and the far field heading angle θhfar is negative, then the torque assist command Tassist is a relatively moderate positive command (e.g., between about 1 Nm to about 5 Nm).

The proportional gain KP is sent to a junction 90. Junction 90 is also in communication with a derivative gain block 92. In one embodiment, the lateral position Δ as calculated by the image processing block 72 is sent to the lateral position derivative block 78. The lateral position derivative block 78 determines the derivative of the lateral position (e.g., the lateral velocity), which is denoted as {dot over (Δ)}. The derivative of the lateral position {dot over (Δ)} may be sent though a low-pass filter (not illustrated), and to the derivative gain block 92. A derivative gain value KD is multiplied by the derivative of the lateral position {dot over (Δ)}.
The product of the derivative gain value KD and the lateral position {dot over (Δ)} is sent to the junction 90. The product of the derivative gain value KD and the lateral position {dot over (Δ)} is subtracted from the proportional gain KP to determine a value 93. The product of the derivative gain value KD and the lateral position {dot over (Δ)} represents a damped value that is provided. That is, the product of the derivative gain value KD and the lateral position {dot over (Δ)} is subtracted from the proportional gain KP, which in turn provides damping to the steering system 30 (shown in FIG. 2). Subtracting the product of the derivative gain value KD and the lateral position A from the proportional gain KP will in turn reduce the amount of handwheel activity (e.g., turning of the handwheel 34 shown in FIG. 2) if the lane keeping system is operating in an autonomous mode.
The value 93 is multiplied by a lateral position gain GcΔ at block 94. The lateral position gain GcΔ is the gain or compensation to insure stability that operates on the lateral position error εΔ, the lateral position derivative block 78, and the derivative gain block 92. The product of the lateral position gain GcΔ and the value 93 are sent to the junction 88, which produces the torque assist command Tassist. The torque assist command Tassist is based on the proportional gain KP, as well as the product of the derivative gain value KD and the lateral position {dot over (Δ)}, which is a damped value. Some other steering systems currently available schedule the proportional gain based on only the lateral position error εΔ. In contrast, the steering system 30 of the present disclosure schedules the proportional gain KP on the lateral position error εΔ as well as at least one of the near field heading angle θhnear, the far field heading angle θhfar, the curvature ρ, and the lateral position Δ. This in turn will modify or improve the accuracy of the torque assist command Tassist, as the proportional gain KP is now based on various characteristics of the lane 12 (shown in FIG. 1). This in turn results in reduced handwheel activity in the event the lane keeping system is operating in the autonomous mode.
FIG. 4 is an alternative embodiment of an exemplary block diagram of a steering control module 160 and a camera and processor 162 that are also used to calculate a torque assist command T′assist. In the embodiment as shown, the camera and processor 162 may include image processing block 172. In the embodiment of FIG. 4, the steering control module 160 calculates a handwheel angle of the handwheel 34 (shown in FIG. 2) based on the curvature ρ of the lane 12 (shown in FIG. 1). The curvature of the lane 12 is based on the lane markers 14 that are detected by the camera.
The curvature ρ of the lane 12 is determined by the imaging processing block 172, and is sent to a steering angle calculation block 210. The steering angle block 210 converts the curvature ρ into a calculated handwheel angle 212 of the handwheel 34 (shown in FIG. 2). Specifically, in one embodiment, the calculated handwheel angle 212 is determined by the following equation:

δf=L/R+Kus*V2/(g)*1/R 

or

δf=L/R+Kus*ay 

where δf is the calculated handwheel angle, L is the wheelbase of the vehicle 10, R is the turn radius, Kus is an understeer coefficient, V is the forward vehicle velocity, ay is lateral acceleration, and g is the acceleration due to gravity. The turn radius R is measured from the camera and processor 162 (e.g., where R=1/ρ).

The calculated handwheel angle 212 is sent to junction 214, which compares a measured handwheel angle that is measured by the steering angle sensor 56 (shown in FIG. 2) with the calculated handwheel angle to determine to determine a curvature error ερ. The curvature error ερ is sent to proportional gain module 176. In the embodiment as shown in FIG. 4, the proportional gain KP′ is based on the lateral position error εΔ as well as the curvature error ερ.
In one embodiment, the derivative gain value KD (shown in block 192) and the lateral position {dot over (Δ)} (shown in block 178) are included (however, in various embodiments the blocks 178 and 192 may be omitted as well). The product of the derivative gain value KD and the lateral position {dot over (Δ)} is sent to a junction 190. The product of the derivative gain value KD and the lateral position {dot over (Δ)} is subtracted from the proportional gain KP to determine a value 192. The value 192 is multiplied by the lateral position gain GcΔ at block 94. The product of the lateral position gain GcΔ and the value 192 are sent to the junction 188, which provides the torque assist command T′assist. In one embodiment, the steering torque generated by the algorithm as discussed in FIG. 4 may require a sign change, as packaging requirements for the steering system 30 cause positive torque to turn left or right. Some examples of items that may cause the sign change include, for example, knuckle steer arm ahead vs. behind the steer axle, and a pinion on steering rack (either above or below the rack).
The curvature error ερ is sent to a block 216, which multiplies the curvature error ερ with a closed loop gain Gcρ. The closed loop gain Gcρ operates on a closed loop portion of the block diagram shown in FIG. 4 (e.g., the closed loop portion is defined by the junction 214 and the block 210). The closed loop gain Gcρ may also be scheduled based on the lateral position error ερ. The product of the closed loop gain Gcρ, the curvature error ερ, and the lateral position error εΔ are sent to the junction 188, which determines the torque assist command T′assist. Gcρ may also include a frequency based compensation to insure stability.
Some other steering systems currently available base the proportional gain only on the lateral position error εΔ. In contrast, the embodiment as shown in FIG. 4 schedules the proportional gain KP on the lateral position error εΔ as well as the curvature error ερ. This in turn will modify the torque assist command T′assist, which results in improved handing when the vehicle 10 is being driven along a curved lane 12 (e.g., the curved lane shown in FIG. 1) if the lane keeping system is operating in autonomous mode.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.

## Claims

1. A steering system providing an assist torque to a handwheel, the control system comprising:
a proportional gain module for determining a proportional gain value, the proportional gain value scheduled as a function of a lateral position error, a near field heading angle, a far field heading angle and at least one of the following: a curvature of a lane and a lateral position of the vehicle; and
a torque command module that determines the torque assist based on the proportional gain value, thereby providing the assist torque to the handwheel.

2. The steering system as recited in claim 1, comprising a lane keeping system that calculates the near field heading angle, the far field heading angle, the curvature of the lane, and the lateral position of the vehicle based on a plurality of lane markers of the lane.

3. The steering system as recited in claim 2, wherein the lane keeping system includes a camera and a processor, wherein the camera detects the presence of the lane markers and the processor includes control logic for determining the near field heading angle, the far field heading angle, the curvature of the lane, and the lateral position of the vehicle.

4. The steering system as recited in claim 1, wherein a magnitude and a direction of the assist torque is dependent on a sign of the lateral position error, a sign of the far field heading angle, and a sign of the curvature of the lane.

5. The steering system as recited in claim 4, wherein if the sign of the lateral position error, the sign of the far field heading angle, and the sign of the curvature of the lane are all positive, then the assist torque is negative.

6. The steering system as recited in claim 4, wherein if the sign of the lateral position error and the sign of the far field heading angle are negative, and the sign of the curvature of the lane is positive, then the assist torque is positive.

7. The steering system as recited in claim 4, wherein if the sign of the lateral position error and the curvature of the lane is positive, and the far field heading angle is negative, then the assist torque is positive.

8. The steering system as recited in claim 1, comprising a derivative gain module and a lateral position module, wherein the lateral position module determines a derivative of the lateral position and the derivative gain module multiples the derivative of the lateral position by a derivative gain value.

9. The steering system as recited in claim 8, wherein a product of the derivative of the lateral position and the derivative gain value are subtracted from the proportional gain to determine a damped value.

10. The steering system as recited in claim 9, wherein the damped value is multiplied by a lateral position gain to determine a value, and the torque assist is based on the value.

11. The steering system as recited in claim 1, comprising a steering angle calculation block that converts the curvature of the lane into a calculated handwheel angle.

12. The steering system as recited in claim 11, wherein the calculated handwheel angle is based on one of the following equations:


δf=L/R+Kus*V2/(g)*1/R, δf=L/R+Kus*ay 


wherein δf is the calculated handwheel angle, L is a wheelbase, R is a turn radius, Kus is an understeer coefficient, V is a forward vehicle velocity, ay is lateral acceleration, and g is an acceleration due to gravity.

13. The steering system as recited in claim 12, wherein the calculated handwheel angle is compared to a measured handwheel angle to determine a curvature error.

14. The steering system as recited in claim 12, wherein the calculated handwheel angle is compared to a measured handwheel angle to determine a curvature error.

15. The steering system as recited in claim 14, wherein the proportional gain value is based on the curvature error.

16. The steering system as recited in claim 14, wherein the curvature error is multiplied by a closed loop gain, and wherein the torque assist is based on a product of the curvature error and the closed loop gain.

17. A method of determining an assist torque to a handwheel, comprising:
determining a proportional gain value, the proportional gain value scheduled as a function of a lateral position error, a near field heading angle, a far field heading angle, a curvature of a lane, and a lateral position of the vehicle; and
determining a torque assist based on the proportional gain value to provide the assist torque to the handwheel.

18. The method as recited in claim 17, comprising calculating the near field heading angle, the far field heading angle, the curvature of the lane, and the lateral position of the vehicle based on a plurality of lane markers of the lane.

19. The steering system as recited in claim 18, comprising detecting the presence of the lane markers by a camera, and determining the near field heading angle, the far field heading angle, the curvature of the lane, and the lateral position of the vehicle by a processor.

20. The steering system as recited in claim 17, wherein a magnitude and a direction of the assist torque is dependent on a sign of the lateral position error, a sign of the far field heading angle, and a sign of the curvature of the lane.