Method for steering an articulated vehicle

Abstract

A method for steering an articulated vehicle traveling on a road, the vehicle comprising a tractor and a trailer, the method comprising determining a position of the tractor in relation to the road, and adjusting the steering in dependence on the determined tractor position. The method further comprises determining an orientation of the trailer in relation to the road, and/or an angular velocity of the trailer in relation to the road, and adjusting the steering in dependence on the determined trailer orientation and/or the determined angular velocity.

Claims

1. A method for steering an articulated vehicle traveling on a road, the vehicle comprising a tractor and a trailer, the method comprising: determining a position of the tractor in relation to the road, and adjusting the steering of the tractor in dependence on the determined tractor position; determining an orientation of the trailer in relation to the road, and/or determining an angular velocity of the trailer in relation to the road, wherein determining the trailer orientation in relation to the road comprises comparing the orientation of the trailer and the direction of the road at the trailer, and wherein determining the trailer angular velocity in relation to the road comprises comparing the angular velocity of the trailer with a rate of change of the direction of the road at the trailer; and adjusting the steering of the tractor in dependence on the determined trailer orientation and/or the determined angular velocity of the trailer.

2. The method according to claim 1, wherein determining the trailer orientation in relation to the road comprises comparing the orientation of the trailer in a fixed coordinate system, and the direction of the road at the trailer in a fixed coordinate system.

3. The method according to claim 1, wherein determining the trailer orientation in relation to the road, and/or the trailer angular velocity in relation to the road, comprises determining a curvature of the road.

4. The method according to claim 1, wherein the trailer orientation in relation to the road, and/or the trailer angular velocity in relation to the road, is determined based on signals from an object sensor mounted on the trailer.

5. The method according to claim 4, wherein the object sensor is arranged to detect spatial features externally of the trailer.

6. The method according to claim 4, wherein the object sensor is a camera, and the signals represent at least one image.

7. The method according to claim 1, wherein the trailer orientation in relation to the road, and/or the trailer angular velocity in relation to the road, is determined based on signals from an object sensor mounted on the tractor, and based on signals from at least one articulation sensor, which articulation sensor signals represent a respective angle of at least one articulated joint between the tractor and the trailer.

8. The method according to claim 7, wherein the object sensor is arranged to detect spatial features externally of the tractor.

9. The method according to claim 7, wherein the object sensor is a camera, and the signals represent at least one image.

10. The method according to claim 1, wherein the vehicle comprises a plurality of trailers, and determining the trailer orientation and/or the trailer angular velocity comprises determining the orientation in relation to the road of the rearmost trailer, and/or the angular velocity in relation to the road of the rearmost trailer.

11. The method according to claim 1, wherein determining a position of the tractor in relation to the road comprises determining rates of changes of angles between a movement direction of the tractor and directions to near and far points ahead of the tractor.

12. A non-transitory computer readable medium carrying a computer program comprising program code for performing the steps of claim 1 when said program code is run on a computer.

13. A control unit configured to perform the steps of the method according to claim 1.

14. A vehicle comprising the control unit according to claim 13.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

(2) In the drawings:

(3) FIG. 1 is a side view of an articulated vehicle with a tractor and two trailers.

(4) FIG. 2 is a diagram schematically depicting the vehicle in FIG. 1, when travelling through a curve of a road, and indicating parameters used in a method according to an embodiment of the invention.

(5) FIG. 3 is a detail of FIG. 2.

(6) FIG. 4 is a diagram depicting steps in said method.

(7) FIG. 5 shows a top view of the vehicle in FIG. 1, and a stretch of a road on which the vehicle is travelling.

(8) FIG. 6 is a diagram depicting steps in a method according to an alternative embodiment of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

(9) FIG. 1 shows an articulated vehicle in the form of a so called A-double combination. The vehicle is herein also referred to as a subject vehicle. The vehicle comprises a tractor 101 in the form of a truck. The vehicle also comprises a forward trailer 102 and a rearmost trailer 103. The rearmost trailer 103 forms part of an A-frame drawbar trailer. The A-frame drawbar trailer comprises the rearmost trailer and a dolly 1031. The dolly 1031 is connected to the forward trailer 102.

(10) It should be noted that the invention is applicable to a variety of alternative articulated vehicles. Thus, the vehicle may comprise more than two trailers. Alternatively, the vehicle may comprise a single trailer, such as in a tractor semitrailer combination.

(11) The vehicle comprises a control unit 120. The control unit 120 is arranged to control a steering angle of two front wheels 1011 of the tractor 101. The control unit steering angle control may be effected by one or more actuators (not shown). The control unit 120 may be arranged to provide an automatic steering function of the vehicle 1. The tractor may comprise a steering wheel (not shown) arranged to be handled by a driver of the vehicle. Thereby, the control unit 120 may be arranged to provide a supporting steering function of the vehicle 1. Such a supporting steering function may be arranged to adjust the steering control by the steering wheel.

(12) The vehicle is provided with a forward object sensor 12. The forward object sensor 121 mounted on the tractor 101. The control unit 120 is arranged to receive signals from the forward object sensor. The forward object sensor 121 is arranged to detect spatial features in front of the tractor 101. In this embodiment, the forward object sensor 121 is a camera. The object sensor could be any suitable kind of camera, e.g. a stereo camera. In some embodiments, the object sensor may be a radar detector. In some embodiments, the vehicle may be provided with a plurality of forward object sensors; e.g. one of the object sensors may be a camera, and another of the object sensors may be a radar detector.

(13) The vehicle is provided with two rearward object sensors 122. The rearward object sensors 122 are mounted on the rearmost trailer 103. The control unit 120 is arranged to receive signals from the rearward object sensors. The rearward object sensors 121 are arranged to detect spatial features laterally of the rearmost trailer 103. In this embodiment, the rearward object sensors 121 are formed by two cameras 122. The cameras are directed in opposite lateral directions.

(14) The vehicle is provided with a yaw rate sensor 123. The yaw rate sensor 123 is mounted on the rearmost trailer 103. The control unit 120 is arranged to receive signals from the yaw rate sensor. The yaw rate sensor 123 is arranged to detect yaw movements, i.e. rotations around a substantially vertical axis of the rearmost trailer 103.

(15) Below, a method of steering the articulated vehicle, when traveling on a road, is described with reference to FIG. 2-FIG. 4. The method may be used e.g. for lane maintaining maneuvering, lane changes, roadway entrance and exit, ramps, and overtaking. Below, and example involving lane maintaining maneuvering is described.

(16) The method includes determining S1 a line representing the lane centre LC. The lane centre LC is indicated with a broken line in FIG. 2. The lane centre is determined as the vehicle moves. The lane centre may be determined and updated as the vehicle moves. The lane centre LC is determined based at least partly on signals from the forward object sensor 121. Such a determination may include identifying lane edges LEL, LER. Lane edges LEL, LER, indicated in FIG. 2, may be identified e.g. based on image data of the forward object sensor in the form of the camera. Such an identification may include identifying painted lines on the road, curbs, and/or grass or gravel, which indicate lane edges.

(17) The lane centre LC may be determined partly based on one or more predetermined road design features. The road design features may be stored accessible to the control unit 120. The road design features may represent geographical entities based on which the road is designed. The road design features may for example be a straight line, a circular arc, and/or a clothoid. A clothoid is a spiral whose curvature is a linear function of the distance travelled along the spiral.

(18) A road design feature of a stretch of a road on which the vehicle is travelling may be identified based on signals from the forward object sensor 121. A parameter of a road design feature may be determined based on signals from the forward object sensor 121. For example, where a road design feature of a stretch of a road, on which the vehicle is travelling, is assumed to be a circular arc, the radius of the arc may be determined based on signals from the forward object sensor 121.

(19) The method comprises determining S2 a position of the tractor 101 in relation to the road. This determination comprises determining rates of changes of angles θn, θf between a movement direction TM of the tractor 101, and directions sn, sf to near and far points n, f ahead of the tractor 101, indicated in FIG. 2.

(20) The absolute positions of the near point n and far point f are assumed to be located on the lane centre LC. The absolute positions of the near point n and far point f are assumed to be located at the distances sn and sf from the subject vehicle front axle position s1f. The near and far points n, f are ahead of the front axle position s1f.

(21) Similarly to as suggested in pages 125-128 in said PhD thesis “Steering control characteristics of human driver coupled with an articulated commercial vehicle”, (S. Taheri), for a straight-line road segment, the near and far points n, f may be located at respective intersection of boundaries of near and far visual fields with the lane centre LC. For a given forward speed, the distances sn, sf from the subject vehicle front axle position s1f to the near and far points may be constant for both straight-line and curved paths. Alternatively, in curved paths the far point f may be located by projecting a tangent line from the subject vehicle front axle position s1f to an inside edge of the lane, the intersection of the tangent line with the lane centre LC being considered as the far point f, similarly to as suggested said PhD thesis.

(22) The rates of changes of the angles θn, θf between the movement direction TM of the tractor 101, and the directions sn, sf to the near and far points n, f are herein also referred to as angular velocities {dot over (θ)}.sub.n, {dot over (0)}.sub.f of the directions sn, sf to the near and far points n, f.

(23) Below, an example of a calculation of the angular velocity {dot over (θ)}f of the direction to the far point sf is provided. A calculation of the angular velocity {dot over (θ)}n of the direction to the near point sn may be similar.

(24) The absolute velocity {dot over (s)}f of the far point f can be described with a vector vf as
vf=({dot over (s)}f.Math.cos(θf+ψ1−ΔψRf)).Math.es1f′−({dot over (s)}f.Math.sin(θf+ψ1−ΔψRf)).Math.en1f′  (1)
ΔψRf=ψRf−ψR1f  (2)
where es1f′ and en1f′ are unit vectors, and ψR1f and ψRf are the road angles at the subject vehicle front axle position s1f and the far point position f, respectively. ψ1 is the yaw angle of the tractor 101, relative to the road.

(25) The absolute velocity of point f may be described in the moving coordinate frame (s1f′; n1f′; b1f′) as
vf=(({dot over (s)}1f−{dot over (Ψ)}R1f−d1f).Math.cos(θf+ψ1)+{dot over (d)}1f.Math.sin(θf+ψ1)+{dot over (r)}f/o′)es1f′+(−({dot over (s)}1f−R1f.Math.d1f).Math.sin(θf+ψ1)+{dot over (d)}1f.Math.cos(θf+ψ1)+({dot over (θ)}f+{dot over (Ψ)}1).Math.rf/o′).Math.en1f′  (3)
where {dot over (s)}1f and {dot over (d)}1f are the velocities of the subject vehicle front axle, in the road coordinate frame (s1f; n1f; b1f). The centre o of the road coordinate frame is on the lane centre LC, and moves along with the vehicle. {dot over (Ψ)}R1f and {dot over (Ψ)}1 are the angular velocity of the road at the subject vehicle front axle position s1f, and the angular velocity of the tractor 101, respectively. d1f is the subject vehicle front axle position perpendicular to the road tangent. {dot over (r)}f/o′ and rf/o′ are the velocity and position of the far point f in the moving coordinate frame.

(26) By combining (1)-(3), the velocity {dot over (r)}f/o′ of point f and the angular velocity {dot over (0)}f of the direction sf to the far point f can be described as
{dot over (0)}f=(−{dot over (s)}f.Math.sin(θf+ψ1−ΔψRf)+({dot over (s)}1f−{dot over (Ψ)}R1f.Math.d1f).Math.sin(θf+ψ1))/rf/o′−{dot over (d)}1f.Math.cos(θf+ψ1)/rf/o′−{dot over (Ψ)}1  (4)
{dot over (r)}f/o′={dot over (s)}f.Math.cos(∂f+ψ1−ΔψRf)−(({dot over (s)}1f−{dot over (Ψ)}R1f.Math.d1f).Math.cos(θf+ψ1)−{dot over (d)}1f.Math.sin(θf+ψ1)  (5)

(27) In some embodiments, equation (4) may be simplified. By assuming a circular road section and equal velocities {dot over (s)}f, {dot over (s)}1f of the far point f and the subject vehicle front axle s1f, the length of the vector rf/o′, indicating the position of the far point f in the moving coordinate frame, becomes constant. Thereby,
ΔψRf=sf.Math.ch  (6)
where ch is the horizontal curvature of the road. Assuming θf, ψ1 and ΔψRf are small angles, and ignoring products of vehicle states (vyv1f.Math.ψ1 and d1f ∩ψ1), equation (4) can be written as
{dot over (0)}f=(−vyv1f−vxv1f.Math.(ψ1+sf.Math.ch))/rf/o′−{dot over (Ψ)}1  (7)
vyv1f and vxv1f are velocity components of the vehicle front axle, along the lane centre LC and transverse to the lane centre LC, respectively. {dot over (0)}f represents the rate of change of the angle θf between the tractor movement direction TM, and the direction sf to the far point f. As mentioned, the rate of change {dot over (θ)}n of the angle θn between the tractor movement direction TM, and the direction sn to the near point n may be determined in a similar manner. In this embodiment, the position of the tractor 101 in relation to the road is represented by the angle θn between the tractor movement direction TM, and the direction sn to the near point n, and the determined rates of changes {dot over (θ)}f, {dot over (θ)}n of the angles θn, θf between the tractor movement direction TM, and the directions sn, sf to the near and far points n, f.

(28) The method comprises determining S3 an orientation θr, indicated in FIG. 2, of the rearmost trailer 103, in relation to the road. The method comprises determining an angular velocity of the rearmost trailer 103 in relation to the road.

(29) Determining the trailer orientation θr in relation to the road, and the trailer angular velocity in relation to the road, comprises determining the yaw angle ψ4 and the yaw rate {dot over (Ψ)}4 of the rearmost trailer, illustrated in FIG. 2. The yaw angle ψ4 and the yaw rate {dot over (Ψ)}4 of the rearmost trailer may be determined based on signals from the yaw rate sensor 223 in the rearmost trailer.

(30) Alternatively, the yaw angle ψ4 and the yaw rate {dot over (Ψ)}4 of the rearmost trailer may be determined based on signals from a yaw rate sensor in the tractor 101, and based on signals from articulation sensors in articulated joints connecting the rearmost trailer 103 to the dolly 1031, the dolly 1031 to the forward trailer 102, and the forward trailer 102 to the tractor 101.

(31) Determining the trailer orientation r in relation to the road, and the trailer angular velocity in relation to the road {dot over (Ψ)}R4r′−{dot over (Ψ)}4, comprises determining a direction ψR4r′ of the road at the trailer 103. As illustrated in FIG. 2, the angle of the road ψR4r′, at a rear point r, in relation to a fixed coordinate system, and the rate of change {dot over (Ψ)}R4r′ of the angle of the road {dot over (ψ)}R4r′ in relation to the fixed coordinate system, are determined. The rear point r is determined, by means of the rear object sensor, to be positioned on an inner edge LEL of the lane, and to move along with the vehicle.

(32) The orientation r of the rearmost trailer 103, in relation to the road, is determined as
θr=ψR4r′−ψ4  (8)

(33) The angular velocity of the rearmost trailer 103 in relation to the road is determined as
{dot over (θ)}r={dot over (Ψ)}R4r′−{dot over (Ψ)}4  (9)

(34) The steering angle δ, indicated in FIG. 2, of the tractor front wheels 1011 are adjusted based on the position of the tractor 101 in relation to the road {dot over (θ)}f, {dot over (θ)}n, θn, as well as the orientation θr of the rearmost trailer 103 in relation to the road, and the angular velocity {dot over (θ)}r of the rearmost trailer 103 in relation to the road. In this embodiment, a desired rate of change {dot over (δ)} of the steering angle δ is determined S4 as
{dot over (δ)}=kf.Math.{dot over (0)}f+kn.Math.{dot over (θ)}n+knl.Math.θn+kr.Math.{dot over (θ)}r+krl.Math.θr  (10)
where kf, kn, knl, kr and krl are gain factors.

(35) Thus, θr is an error which provides, with an increasing size, an increased desired rate of change {dot over (δ)} of the steering angle. {dot over (θ)}r is an error which provides, with an increasing size, an increased desired rate of change {dot over (δ)} of the steering angle. Thereby, the steering is adjusted S5 in dependence on the determined trailer orientation and/or angular velocity. In some embodiments, either the orientation r of the trailer in relation to the road, or the angular velocity {dot over (θ)}r of the trailer in relation to the road may be determined.

(36) The desired steering angle change rate {dot over (δ)} may be used in a fully automatic steering function. Alternatively, the desired steering angle change rate {dot over (δ)} may be used for correcting manual steering actions by a driver of the vehicle. For example, when the actual steering angle change rate, provided as a result of the driver's manual steering actions, differ from the desired steering angle change rate {dot over (δ)} by more than a threshold differentiation, an automatic steering function may override the manual steering actions.

(37) In some embodiments, the orientation θr of the rearmost trailer 103 in relation to the road, and the angular velocity {dot over (θ)}r of the rearmost trailer 103 in relation to the road may be determined based on a determined curvature of the road.

(38) Above, embodiments of the invention have been described as used for maintaining the vehicle in a lane of a road. As suggested, the invention may also be applicable to other types of driving situations. For example, embodiment of the invention may be used to assist a vehicle driver in a lane changing manoeuvre, or to automatically perform such a manoeuvre.

(39) Reference is made to FIG. 5. For a lane changing manoeuvre, the method described above with reference to FIG. 1-FIG. 4 may include identifying boundaries LEL, LEL2 of a lane, adjacent to a lane in which the vehicle is travelling. Such an identification may be done based, at least partly, on signals from the forward object sensor 121, (FIG. 1). The identification may include identifying lane edges of the adjacent lane. Lane edges may be identified e.g. based on image data of the camera formed by the forward object sensor. The method may include determining a line representing the centre LC2 of the adjacent lane. The actual track of the vehicle front axle during the lane change is in FIG. 5 indicated with the broken lime DP.

(40) The method may comprise determining, during the lane change, a position of the tractor 101 in relation to the centre LC2 of the adjacent lane. This determination may comprise determining rates of changes of angles between a movement direction of the tractor, and directions to a near point n and a far point f ahead of the tractor. The absolute positions of the near point n and far point f may be assumed to be located on the centre LC2 of the adjacent lane.

(41) The method may advantageously comprise determining, during the lane change, an orientation of the rearmost trailer 103 in relation to the road, and determining an angular velocity of the rearmost trailer 103 in relation to the road. Such determinations may involve determining an orientation of the rearmost trailer 103 in relation to the road. Thereby, similarly to what has been described above, the yaw angle and the yaw rate of the rearmost trailer are determined, e.g. based on signals from a yaw rate sensor in the rearmost trailer. Further, a direction of the road at the trailer 103 is determined.

(42) In this example, the angle of the road, at a rear point r, in relation to a fixed coordinate system, and the rate of change of the angle of the road in relation to the fixed coordinate system, are determined. The rear point r is determined, by means of a rear object sensor (not shown), mounted to the trailer, to be positioned on one of the boundaries LEL2 of the adjacent lane, and to move along with the vehicle.

(43) The orientation of the rearmost trailer 103, in relation to the road, is determined based on the determined yaw angle of the rearmost trailer, and on the determined direction of the road at the trailer 103, similarly to what has been described above with reference to FIG. 2. The angular velocity of the rearmost trailer 103 in relation to the road is determined based on the determined yaw rate of the rearmost trailer, and on the determined rate of change of the angle of the road in relation to the fixed coordinate system, similarly to what has been described above with reference to FIG. 2.

(44) FIG. 6 depicts steps in a method according to a further embodiment of the invention. The method provides for steering an articulated vehicle traveling on a road. The articulated vehicle could be of any kind, e.g. as described above with reference to FIG. 1. The vehicle comprises a tractor and a trailer. The method comprises determining S2 a position of the tractor 101 in relation to the road. The method also comprises determining S3 an orientation of the trailer in relation to the road, and/or an angular velocity of the trailer in relation to the road. The steering of the articulated vehicle is adjusted S5 in dependence on the determined tractor position, and in dependence on the determined trailer orientation and/or the determined angular velocity.

(45) It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.