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

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.

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.

DETAILED DESCRIPTION

Referring now toFIG. 1, where the invention will be described with reference to specific embodiments without limiting same, an exemplary schematic diagram of a vehicle10driving within a lane12is illustrated. The lane12includes lane markers14on the left and right hand sides of the lane12. The lane12includes a lane center20(the lane center is a calculated value, as there is no lane marker for the lane center20) and the vehicle10includes a vehicle centerline22. A heading angle θ (not shown inFIG. 1) is measured between the lane center20and the vehicle centerline22, where a positive sign for the heading angle θ (e.g., θ+) is provided if the vehicle centerline22is to the right of the lane center20, and a negative sign (e.g., θ−) for the heading angle θ is provided if the vehicle centerline22is to the left of the lane center20. A lateral position Δ of the vehicle10is also illustrated inFIG. 1, where the lateral position Δ includes a positive sign if the vehicle centerline22is to the right of the lane center20, and a negative sign if the vehicle centerline22is to the left of the lane center20. In the embodiment as shown inFIG. 1, the lane12also 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 lane12. If the curvature ρ is oriented to the left side of the lane14, then the curvature ρ includes a negative sign.

FIG. 1also illustrates a near field heading angle θhnearand a far field heading angle θhfar. The near field heading angle θhnearis observed relatively close to the vehicle10(e.g., generally within about 40 meters of the vehicle10). The lateral position Δ is also observed relatively close to the vehicle10as well (e.g., generally within about 40 meters of the vehicle10). The near field heading angle θhnear, the far heading angle θhfar, and the lateral position A are then projected directly in front of the vehicle10(e.g., at zero meters). The curvature ρ of the lane12is 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 vehicle10). The far field heading angle θhfaris 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 θhfaris determined by calculating a tangent line T of the lane center20at the look ahead distance D. The far field heading angle θhfaris measured between the vehicle centerline22and the tangent line T at the look ahead distance D.

Turning now toFIG. 2, the vehicle10includes a steering system30. The steering system30includes a handwheel34coupled to a steering shaft36. The steering system30is an electric power steering (EPS) system that further includes a steering assist unit38that couples to the steering shaft36of the steering system30and to tie rods40,42of the vehicle10. The steering assist unit38includes, for example, a rack and pinion steering mechanism (not shown) that may be coupled through the steering shaft36to a steering actuator motor and gearing. During operation, as the handwheel34is turned by a vehicle operator, the motor of the steering assist unit38provides the assistance to move the tie rods40,42which in turn moves steering knuckles44,46, respectively, coupled to roadway wheels48,50, respectively of the vehicle10.

As shown inFIG. 2, the vehicle10further includes various sensors that detect and measure observable conditions of the steering system30and/or of the vehicle10. In one example, a torque sensor50, a vehicle speed sensor52, and a steering angle sensor56are provided. A steering control module60controls the operation of the steering system30and the steering assist unit38based on one or more of the signals from the sensors50,52and56and a lane keeping system that are included in the vehicle10, and determines an torque assist command Tassist. In various embodiments, the steering control module60can 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 inFIG. 2, an integrated camera and processor62are illustrated. Of course in other embodiments, the camera and processor62may be separate components. In one embodiment, the camera and processor62may be part of the lane keeping system. The lane keeping system generates feedback for a corrective input into the handwheel34in the event a path deviation with respect to the lane markers14(shown inFIG. 1) is detected. The camera and processor62may 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 markers14that are detected by the camera. Specifically, the camera detects the presence of the lane markers14, 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 markers14that are detected by the camera. The camera and processor62is in communication with the steering control module60.

FIG. 3is an exemplary block diagram of the steering control module60and the camera and processor62. In the embodiment as shown, the camera and processor62may include image processing blocks72and72(in one embodiment, block72may be sent from block70). Image processing block70includes control logic for determining the near field heading angle θhnearand the far field heading angle θhfarbased on the lane markers14that are detected by the camera. The image processing block70is in communication with the image processing block72. Image processing block72determines the lateral position Δ and the curvature ρ based on the lane markers14that are detected by the camera. The image processing block72is in communication with a proportional gain module76, a lateral position derivative block78, and a junction80, which are each part of the steering control module60.

The junction80receives as input a servo lateral position command82and the lateral position Δ as calculated by the image processing block72. The junction80determines the difference between the servo lateral position command82and the lateral position Δ to determine a lateral position error εA. The lateral position error εΔis sent to the proportional gain module76to calculate a proportional gain KP. The proportional gain KPis scheduled as a function of lateral position error εΔas 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 junction88) 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 Tassistmay be modified based on the proportional gain KP. That is, the torque assist command Tassistdepends 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 lane12(shown inFIG. 1). Table 1 illustrates one example of how the torque assist command Tassistmay be modified based on the proportional gain KP.

TABLE 1εΔΘhfarFar FieldCurvature (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 Tassistis a relatively large negative command (e.g., between about −3 Nm to about −7 Nm). If the lateral position error εΔand the sign of the far field heading angle θhfarare positive and if the curvature ρ is negative, then the torque assist command Tassistis a relatively large negative command (e.g., between about −3 Nm to about −7 Nm). If the lateral position error εΔand the sign of the far field heading angle θhfarare negative and if the curvature ρ is positive, then the torque assist command Tassistis a relatively large positive command (e.g., between about 3 Nm to about 7 Nm). If the lateral position error εΔand the curvature ρ are positive and the far field heading angle θhfaris negative, then the torque assist command Tassistis a relatively moderate positive command (e.g., between about 1 Nm to about 5 Nm).

The proportional gain KPis sent to a junction90. Junction90is also in communication with a derivative gain block92. In one embodiment, the lateral position Δ as calculated by the image processing block72is sent to the lateral position derivative block78. The lateral position derivative block78determines 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 block92. A derivative gain value KDis multiplied by the derivative of the lateral position {dot over (Δ)}.

The product of the derivative gain value KDand the lateral position {dot over (Δ)} is sent to the junction90. The product of the derivative gain value KDand the lateral position {dot over (Δ)} is subtracted from the proportional gain KPto determine a value93. The product of the derivative gain value KDand the lateral position {dot over (Δ)} represents a damped value that is provided. That is, the product of the derivative gain value KDand the lateral position {dot over (Δ)} is subtracted from the proportional gain KP, which in turn provides damping to the steering system30(shown inFIG. 2). Subtracting the product of the derivative gain value KDand the lateral position A from the proportional gain KPwill in turn reduce the amount of handwheel activity (e.g., turning of the handwheel34shown inFIG. 2) if the lane keeping system is operating in an autonomous mode.

The value93is multiplied by a lateral position gain GcΔat block94. The lateral position gain GcΔis the gain or compensation to insure stability that operates on the lateral position error εΔ, the lateral position derivative block78, and the derivative gain block92. The product of the lateral position gain GcΔand the value93are sent to the junction88, which produces the torque assist command Tassist. The torque assist command Tassistis based on the proportional gain KP, as well as the product of the derivative gain value KDand 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 system30of the present disclosure schedules the proportional gain KPon the lateral position error εΔas 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 KPis now based on various characteristics of the lane12(shown inFIG. 1). This in turn results in reduced handwheel activity in the event the lane keeping system is operating in the autonomous mode.

FIG. 4is an alternative embodiment of an exemplary block diagram of a steering control module160and a camera and processor162that are also used to calculate a torque assist command T′assist. In the embodiment as shown, the camera and processor162may include image processing block172. In the embodiment ofFIG. 4, the steering control module160calculates a handwheel angle of the handwheel34(shown inFIG. 2) based on the curvature ρ of the lane12(shown inFIG. 1). The curvature of the lane12is based on the lane markers14that are detected by the camera.

The curvature ρ of the lane12is determined by the imaging processing block172, and is sent to a steering angle calculation block210. The steering angle block210converts the curvature ρ into a calculated handwheel angle212of the handwheel34(shown inFIG. 2). Specifically, in one embodiment, the calculated handwheel angle212is determined by the following equation:
δf=L/R+Kus*V2/(g)*1/R
or
δf=L/R+Kus*ay
where δfis the calculated handwheel angle, L is the wheelbase of the vehicle10, R is the turn radius, Kus is an understeer coefficient, V is the forward vehicle velocity, ayis lateral acceleration, and g is the acceleration due to gravity. The turn radius R is measured from the camera and processor162(e.g., where R=1/ρ).

The calculated handwheel angle212is sent to junction214, which compares a measured handwheel angle that is measured by the steering angle sensor56(shown inFIG. 2) with the calculated handwheel angle to determine to determine a curvature error ερ. The curvature error ερis sent to proportional gain module176. In the embodiment as shown inFIG. 4, the proportional gain KP′ is based on the lateral position error εΔas well as the curvature error ερ.

In one embodiment, the derivative gain value KD(shown in block192) and the lateral position {dot over (Δ)} (shown in block178) are included (however, in various embodiments the blocks178and192may be omitted as well). The product of the derivative gain value KDand the lateral position {dot over (Δ)} is sent to a junction190. The product of the derivative gain value KDand the lateral position {dot over (Δ)} is subtracted from the proportional gain KPto determine a value192. The value192is multiplied by the lateral position gain GcΔat block94. The product of the lateral position gain GcΔand the value192are sent to the junction188, which provides the torque assist command T′assist. In one embodiment, the steering torque generated by the algorithm as discussed inFIG. 4may require a sign change, as packaging requirements for the steering system30cause 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 ερis sent to a block216, which multiplies the curvature error ερwith a closed loop gain Gcρ. The closed loop gain Gcρoperates on a closed loop portion of the block diagram shown inFIG. 4(e.g., the closed loop portion is defined by the junction214and the block210). The closed loop gain Gcρmay 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 εΔare sent to the junction188, which determines the torque assist command T′assist. Gcρmay 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 inFIG. 4schedules the proportional gain KPon the lateral position error εΔas well as the curvature error ερ. This in turn will modify the torque assist command T′assist, which results in improved handing when the vehicle10is being driven along a curved lane12(e.g., the curved lane shown inFIG. 1) if the lane keeping system is operating in autonomous mode.