Method for controlling a steering system of a vehicle

Abstract

The present invention relates to a method for controlling a steering system of a vehicle (100). The steering system comprises individually controllable wheel torque actuators (103, 105) on a respective left (104) and right (106) steerable wheel of the vehicle, wherein the wheel torque actuators (103, 105) are controlled during a turning maneuver of the vehicle.

Claims

1. A method for controlling a steering system of a vehicle, said vehicle comprising a front axle comprising a pair of front steerable wheels on a left and right hand side of the front axle, wherein each of said steerable wheels comprises an individually controllable wheel torque actuator, said method comprising the steps of: determining a required steering angle for operating said vehicle during a turning maneuver; detecting an actual steering angle during the turning maneuver; determining a difference between the required steering angle and the detected, actual steering angle; determining, for the wheel torque actuators of the pair of front steerable wheels, a differential wheel torque required for reducing the difference between the required steering angle and the detected, actual steering angle, the differential wheel torque being determined by the steps of: determining a required additional steering torque of the pair of front steerable wheels for reducing the difference between the required steering angle and the detected, actual steering angle; and determining the differential wheel torque based on the required additional steering torque and a wheel suspension scrub radius of the steerable wheels; and controlling said wheel torque actuators for achieving said differential wheel torque of the steerable wheels.

2. The method of claim 1, further comprising the steps of: determining the turning maneuver by determining an upcoming path ahead of the vehicle; and detecting the actual steering angle when the vehicle is operated at the path.

3. The method of claim 1, wherein the step of determining the differential wheel torque comprises the steps of: determining a front wheel lateral force exposed to the pair of front steerable wheels; and determining said differential wheel torque based on the front wheel lateral force.

4. The method of claim 3, wherein the step of determining the front wheel lateral force comprises the steps of: determining a slip angle of the pair of front steerable wheels for the turning maneuver; and determining the front wheel lateral force based on a cornering stiffness of the pair of front steerable wheels and the slip angle.

5. The method of claim 1, wherein the required additional steering torque is further based on a suspension caster angle of the pair of front steerable wheels.

6. The method of claim 1, further comprising the steps of: determining a desired longitudinal vehicle force of the vehicle during the turning maneuver; determining a change of the longitudinal vehicle force resulting from the applied differential wheel torque of the pair of front steerable wheels; and controlling the vehicle to add propulsion or braking for maintaining the desired longitudinal vehicle force.

7. The method of claim 1, wherein the vehicle further comprises at least one rear axle comprising a pair of rear non-steerable wheels on a left and right hand side of the rear axle, wherein each of said rear non-steerable wheels comprises a rear wheel torque actuator, wherein the method further comprises the step of: inhibiting the rear wheel torque actuator on the pair of rear non-steerable wheels from applying a wheel torque when operating the torque actuators of the front steerable wheels.

8. The method of claim 1, wherein the steering system is a secondary, redundant system able to steer the vehicle in addition to steering by means of a primary steering of the vehicle.

9. A steering system of a vehicle, said vehicle comprising a front axle comprising a pair of front steerable wheels on a left and right hand side of the front axle, wherein each of said steerable wheels comprises an individually controllable wheel torque actuator, and a control unit connected to each of the wheel torque actuators, the control unit being configured to: determine a required steering angle for operating said vehicle during a turning maneuver; detect an actual steering angle during the turning maneuver; determine a difference between the required steering angle and the detected, actual steering angle; determine, for the wheel torque actuators of the pair of front steerable wheels, a differential wheel torque required for reducing the difference between the required steering angle and the detected, actual steering angle, wherein the control unit is configured to determine the differential wheel torque by: determining a required additional steering torque of the pair of front steerable wheels for reducing the difference between the required steering angle and the detected, actual steering angle; and determining the differential wheel torque based on the required additional steering torque and a wheel suspension scrub radius of the pair of front steerable wheels; and control the wheel torque actuators to achieve the differential wheel torque of the pair of front steerable wheels.

10. The steering system of claim 9, wherein the steering system further comprises a path controller arranged to detect an upcoming path for the vehicle, the path controller being connected to the control unit.

11. The steering system of claim 9, wherein the steering system further comprises a wheel torque control module connected to the control unit and arranged to control operation of the individually controlled wheel torque actuators, wherein the wheel torque control module is arranged to control the wheel torque actuators to apply the differential wheel torque upon receiving a control signal from the control unit.

12. The steering system of claim 11, wherein the wheel torque control module is a decentralized wheel torque control module arranged in connection with the wheel torque actuators of a respective steerable wheel.

13. The steering system of claim 9, wherein the wheel torque actuator comprises a wheel brake.

14. The steering system of claim 9, wherein the steering system is a secondary, redundant system, wherein the control unit is further arranged to: receive a signal indicative of a primary steering system of the vehicle being unavailable; and control the secondary, redundant system if the primary steering is unavailable.

15. The steering system of claim 9, further comprising a wheel suspension system, wherein the pair of front steerable wheels is connected to the wheel suspension system with a positive scrub radius.

16. A vehicle comprising a front axle comprising a pair of front steerable wheels on a left and right hand side of the front axle, wherein each of said steerable wheels comprises an individually controllable wheel torque actuator, and a steering system, the steering system comprising a control unit connected to each of the wheel torque actuators, the control unit being configured to: determine a required steering angle for operating said vehicle during a turning maneuver; detect an actual steering angle during the turning maneuver; determine a difference between the required steering angle and the detected, actual steering angle; determine, for the wheel torque actuators of the steerable wheels, a differential wheel torque required for reducing the difference between the required steering angle and the detected, actual steering angle, wherein the control unit is configured to determine the differential wheel torque by: determining a required additional steering torque of the pair of front steerable wheels for reducing the difference between the required steering angle and the detected, actual steering angle; and determining the differential wheel torque based on the required additional steering torque and a wheel suspension scrub radius of the pair of front steerable wheels; and control the wheel torque actuators to achieve the differential wheel torque of the pair of front steerable wheels.

17. The vehicle according to claim 16, wherein the vehicle is an autonomous vehicle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of exemplary embodiments of the present invention, wherein:

(2) FIG. 1 is a lateral side view illustrating an example embodiment of a vehicle in the form of a truck;

(3) FIG. 2 is a schematic illustration from above depicting the vehicle in FIG. 1 exposed to a turning maneuver according to an example embodiment;

(4) FIG. 3 illustrates the vehicle in FIG. 1 before entering a curvature of the road;

(5) FIG. 4a-4c are different views illustrating one of the front steerable wheels according to an example embodiment;

(6) FIG. 5 is a schematic illustration of a steering system according to an example embodiment; and

(7) FIG. 6 is a flow chart of a method for controlling a steering system according to an example embodiment.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

(8) The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness. Like reference character refer to like elements throughout the description.

(9) With particular reference to FIG. 1, there is provided a vehicle 1 in the form of a truck. The vehicle 1 comprises a prime mover 2 in the form of an internal combustion engine, and a system (500, see e.g. FIG. 5) for controlling the steering of the vehicle. The vehicle 1 comprises a pair of steerable wheels 104, 106 arranged on a respective left and right hand side of a front axle 102 of the vehicle. The front axle 102 is thus the foremost located axle of the vehicle 1. The vehicle 1 depicted in FIG. 1 also comprises a pair of first rear wheels 108, 110 connected to a first rear axle 112, and a pair of second rear wheels 114, 116 connected to a second rear axle 118. The first rear axle 112 is arranged in front of the second rear axle 118 as seen in the longitudinal direction of the vehicle 1. Preferably, the pair of first rear wheels 108, 110 and the pair of second rear wheels 114, 116 are non-steerable wheels. It should be understood that the rear wheels may also be steerable during normal operation.

(10) In order to describe the vehicle 1 in further detail, reference is made to FIGS. 2 and 3. In detail, FIG. 2 illustrates an example embodiment of the forces exposed to the vehicle 1 and its wheels during a turning maneuver, and FIG. 3 illustrates the vehicle 1 before initiating a turning maneuver, i.e. before arriving at a road curvature.

(11) Starting with FIG. 2, which is a schematic illustration from above depicting the vehicle 1 in FIG. 1 exposed to a turning maneuver according to an example embodiment. Hence, the pair of steerable wheels 104, 106 is turning and being exposed to a steering angle δ. The steering angle δ is for simplification in FIG. 2 illustrated as the same for the left 104 steerable wheel and the right 106 steerable wheel and is an angle of the wheels relative to a longitudinal axis of the vehicle 1. The vehicle is operated at a vehicle speed indicated as v. The steerable wheels 104, 106 also comprise a respective wheel torque actuator 103, 105.

(12) The vehicle 1 comprises, as described above, the pair of steerable wheels 104, 106 arranged on the front axle 102, the pair of first rear wheels 108, 110 connected to the first rear axle 112, and the pair of second rear wheels 114, 116 connected to the second rear axle 118. The front axle 102 is arranged at a distance I.sub.1 from a center of mass 202 of the vehicle, the first rear axle 112 is arranged at a distance I.sub.2 from the center of mass 202 of the vehicle, and the second rear axle 118 is arranged at a distance I.sub.3 from the center of mass 202 of the vehicle. The center of mass 202 is the position of the vehicle 1 around which the vehicle rotates during the turning maneuver. The center of mass 202 is also the position of the vehicle 1 at which total global forces affecting the vehicle 1 can be expressed. In the following, the x-axis is the extending in the longitudinal direction of the vehicle 1, the y-axis is extending in the transversal direction of the vehicle 1 and the z-axis is extending in the vertical direction of the vehicle 1. During the turning maneuver, the vehicle 1 is exposed to a torque M.sub.z at the center of mass 202. Also, the vehicle is exposed to a global longitudinal force F.sub.x and a global lateral force F.sub.y.

(13) Moreover, when the steerable wheels 104, 106 of the front axle 102 is exposed to the steering angle δ, the steerable wheel 104 on the left hand side is exposed to a longitudinal force F.sub.x, 104 and a lateral force F.sub.y, 104, while the steerable wheel 106 on the right hand side is exposed to a longitudinal force F.sub.x, 106 and a lateral force F.sub.y, 106. The sum of the lateral force of the steerable wheels 104, 106 on the left and right hand sides can be expressed as a front wheel lateral force. The sum of the front wheel longitudinal forces may be increased/reduced when e.g. propelling the vehicle or braking the vehicle, whereas the differential front wheel forces is used for controlling the steering angle.

(14) Furthermore, the pair of first rear wheels 108, 110 is exposed to a respective lateral force F.sub.y, 108 and F.sub.y, 110, and the pair of second rear wheels 114, 116 is exposed to a respective lateral force F.sub.y, 114 and F.sub.y, 116. In the example of FIG. 2, the longitudinal force of the pair of first rear wheels 108, 110 and the pair of second rear wheels 114, 116 is set to zero, i.e. the respective wheel is not exposed to propulsion or braking. This will be described further below.

(15) Turning now to FIG. 3, which is an illustration of the vehicle before entering a curvature 302 of the road, i.e. before the turning maneuver takes place. As can be seen in FIG. 3, the vehicle 1 is currently driving straight ahead at the vehicle speed v. Thus, before entering the curvature 302, the steering angle δ is zero. The curvature has a radius denoted as r.sub.road. Hereby, the vehicle may detect the curvature of the road ahead by means of a suitable sensor. According to an example embodiment, the vehicle may comprise a path controller (see FIG. 5) arranged to detect the road ahead, i.e. the upcoming turning maneuver. It should however be understood that the below described system and method may also be implemented during the turning maneuver, i.e. when the turning maneuver takes place. Also, the turning maneuver does not necessarily have to relate to a road curvature as depicted in FIG. 3. On the contrary, the turning maneuver may also relate to e.g. a lane change operation of the vehicle.

(16) Turning now to FIGS. 4a-4c which illustrate different views of the left steerable wheel 104 according to an example embodiment. In detail, FIG. 4a is a side view of the left steerable wheel 104, FIG. 4b is a rear view of the left steerable wheel 104, and FIG. 4c is a top view of the left steerable wheel 104 during the turning maneuver.

(17) Starting with FIG. 4a, which is a side view of the left steerable wheel 104. The suspension (not shown) of the wheel 104 is arranged such that the wheel 104 is provided with a suspension caster angle γ which is defined as the angular displacement of a steering axis 402 from a vertical axis 404 of the left steerable wheel 104, measured in the longitudinal direction of the vehicle 1. The distance between the intersection of the road surface 401 and the steering axis 402, and the intersection of the road surface 401 and the vertical axis 404 is denoted as t.sub.m. With the suspension of the wheel, the point of force application of the contact patch 406 between the wheel 104 and the road surface 401 will be located slightly offset in the longitudinal direction from the intersection of the road surface 401 and the vertical axis 404. This offset is denoted as t.sub.p. The contact patch is thus the area of the tire in contact with the ground surface. Thus, the point of force application of the contact patch 406 between the wheel 104 and the road surface 401 is dependent on the suspension caster angle γ.

(18) Turning to FIG. 4b, which is a rear view of the left steerable wheel 104. As can be seen, the effective wheel radius R is indicated as the distance between the front axle 102 and the road surface 401, and the wheel 104 is connected to the suspension by an inclined king pin axis 408, which inclination is indicated as T. Thus, the wheel 104 is rotated around the king pin axis 408 during the turning maneuver. Furthermore, the point of force application of the contact patch 406 between the wheel 104 and the road surface 401 is located at the intersection between the vertical axis 404 and the road surface 401. The vehicle 1, and in particular the steerable wheels 104, 106 are provided with a positive wheel suspension scrub radius r.sub.s. The wheel suspension scrub radius r.sub.s is defined as the distance between the point of force application of the contact patch 406 and the intersection 403 between the king pin axis 408 and the road surface 401. A positive wheel suspension scrub radius r.sub.s is generated when the intersection between the king pin axis 408 and the road surface 401 is located on an inner side of the vertical axis 404 as seen in the longitudinal direction depicted in FIG. 4b. When e.g. applying a brake torque on the left steerable wheel 104, the wheel will rotate around the king pin axis 408 due to the positive scrub radius r.sub.s causing the vehicle to turn to the left. Hereby, an additional steering torque M.sub.steer can be generated.

(19) Turning to FIG. 4c, which is a simplified illustration of a combined left and right front wheel seen from above. In FIG. 4c, the steerable wheel is exposed to an increased wheel brake torque causing the vehicle to turn to the left. As can be seen, the vehicle 1 is operated at the road curvature described above in relation to FIG. 3, where the road curvature has the radius r.sub.road. The steerable wheel 104 thus has a steering angle δ. However, the steerable wheel 104 will move at a speed v in the direction a relative to the steering angle δ. This angle α is referred to as a slip angle α.

(20) By means of the above description, it is possible to control the motion of the vehicle by determining the required steering angle for operating the vehicle at the specific road curvature, and to compare such required steering angle with an actual steering angle. Parameters described above will not be given any further detailed description unless indicated. Hereby, the wheel torque of the steerable wheels 104, 106 can be added to reduce the difference between the required steering angle and the actual steering angle. The wheel torque can be determined by determining a required differential longitudinal force ΔF.sub.x, which is the difference between F.sub.x, 104 and F.sub.x, 106, and the wheel radius R.

(21) The required additional steering torque M.sub.steer can be determined according to:
M.sub.steer=ΔF.sub.x.Math.r.sub.s=(F.sub.y,104+F.sub.y,106).Math.t  (1)

(22) where:

(23) F.sub.y, 104 and F.sub.y, 106=the front wheel lateral force of the steerable wheels 104, 106 t=t.sub.m+t.sub.p

(24) Equation (1) can be rewritten according to:
M.sub.steer=−2C.sub.α.Math.α.Math.(t.sub.m+t.sub.p)  (2)

(25) where:

(26) $\begin{array}{cc}{C}_{\alpha }=\mathrm{lateral}\mathrm{stiffness}\mathrm{of}\mathrm{the}\mathrm{tire};& \left(3\right)\\ F_{y,i}=C_{\alpha }.Math.\alpha =\mathrm{the}\mathrm{front}\mathrm{wheel}\mathrm{lateral}\mathrm{force};\mathrm{and}& \\ \alpha =\left(\delta -\frac{l_1}{v}\omega \right)& \end{array}$

(27) where

(28) v=the longitudinal vehicle speed; and

(29) ω=rotational speed of the vehicle during the turning maneuver.

(30) Furthermore, the global vehicle torque M.sub.z at the center of rotation 202 can be determined according to:

(31) $\begin{array}{cc}{M}_z=l_1\left(\left(F_{x,104}+F_{x,106}\right).Math.\delta +2C_{\alpha }.Math.\alpha \right)+\frac{w}{2}\left(-F_{x,104}+F_{x,106}\right)-2C_{\alpha }.Math.l_2^2.Math.\frac{\omega }{v}+\frac{w}{2}.Math.\left(-F_{x,108}+F_{x,110}\right)-2C_{\alpha }.Math.l_3^2.Math.\frac{\omega }{v}+\frac{w}{2}.Math.\left(-F_{x,114}+F_{x,116}\right)& \left(4\right)\end{array}$

(32) where:

(33) ΔF.sub.x=F.sub.x,104−F.sub.x,106

(34) F.sub.x,108=F.sub.x,110=F.sub.x,114=F.sub.x,116=0

(35) β=0

(36) w=track width of the vehicle

(37) where β is the side slip angle of the vehicle. Hereby, an assumption is made that the velocity is pointing in the same direction as the longitudinal axis of the vehicle.

(38) Furthermore, the slip angle of the steerable wheels can be determined according to:
(F.sub.x,104−F.sub.x,106).Math.r.sub.s+(F.sub.y,104+F.sub.y,106).Math.(t.sub.m+t.sub.p)=D{dot over (α)}−J{umlaut over (α)}  (5)
F.sub.y=F.sub.y,104+F.sub.y,106=C.sub.∝.Math.α  (6)
ΔF.sub.x.Math.r.sub.s+2C.sub.α.Math.α.Math.(t.sub.m+t.sub.p)−D{dot over (α)}−J{umlaut over (α)}  (7)

(39) For a steady state operation: {dot over (α)}={umlaut over (α)}=0:

(40) $\begin{array}{cc}\Delta F_x=\frac{2C_{\alpha }.Math.\alpha .Math.\left(t_m+t_p\right)}{r_s}& \left(8\right)\\ \alpha =\frac{\Delta F_x.Math.r_s}{2C_{\alpha }.Math.\left(t_m+t_p\right)}& \left(9\right)\end{array}$

(41) Furthermore, with the assumption that

(42) $r_{\mathrm{road}}\approx \frac{v}{\omega }\mathrm{and}t=t_m-t_p,$
the following expressions can be made:

(43) $\begin{array}{cc}0=-\Delta F_x.Math.\left(\frac{l_1.Math.r_s}{t}+\frac{\omega }{2}\right)-2C_a.Math.\alpha \left(\frac{1}{r_{\mathrm{road}}}.Math.\left(l_2^2+l_3^2\right)\right)& \left(10\right)\\ \text{=}\text{>}\Delta F_x=2C_{\alpha }.Math.\alpha \frac{-\frac{1}{r_{\mathrm{road}}}.Math.\left(l_2^2+l_3^2\right)}{\frac{l_1.Math.r_s}{t}+\frac{\omega }{2}}& \left(11\right)\end{array}$

(44) Hereby, the differential wheel torque of the steerable wheels can be determined based on the effective wheel radius R.

(45) The above may be controlled by assigning control allocations, whereby the following expression can be formulated:
u.sub.opt=arg min.sub.u.sub.min.sub.≤u≤u.sub.min[∥W.sub.u(u−u.sub.d)∥.sub.2.sup.2+γ∥W.sub.v(Bu−v)∥.sub.2.sup.2]  (12)
where:
v=Bu  (13)

(46) in which the following matrices and vectors are defined as

(47) $\begin{array}{cc}\left[\begin{array}{c}{F}_x\\ F_y\\ M_z\\ M_{\mathrm{steer}}\end{array}\right]=\left[\begin{array}{cccccc}\frac{1}{R}& \frac{1}{R}& \frac{1}{R}& \frac{1}{R}& \frac{1}{R}& \frac{1}{R}\\ 0& 0& 0& 0& 0& 0\\ -\frac{w_1}{2R}& \frac{w_1}{2R}& -\frac{w_2}{2R}& \frac{w_2}{2R}& -\frac{w_3}{2R}& \frac{w_3}{2R}\\ -\frac{r_s}{R}& \frac{r_s}{R}& 0& 0& 0& 0\end{array}\right]\left[\begin{array}{c}{T}_{104}\\ T_{106}\\ T_{108}\\ T_{110}\\ T_{114}\\ T_{116}\end{array}\right]& \left(14\right)\end{array}$

(48) wherein:

(49) R is the effective radius

(50) T is the wheel torque for the respective wheel

(51) Reference is now made to FIG. 5, which illustrates the steering system 500 according to an example embodiment. As can be seen in FIG. 5, the steering system 500 comprises an actuator control module 502, a vehicle motion control module 504 and a traffic situation controller 506. The actuator control module 502 comprises a wheel torque control module 508, a propulsion controller 510 and a steer controller 512. The vehicle motion control module 504 comprises a motion controller 514 and an actuator coordinator module 516. Finally, the traffic situation controller 506 comprises a path controller 518, a vehicle stability control module 520 and a motion request module 522.

(52) During operation of the exemplified system 500 in FIG. 5, the path controller 518 detects an upcoming path for the vehicle 1 and transmits a steering angle δ.sub.path required to maintain the path to the motion request module 522. The signal is based on path curvature, and in some implementation on vehicle speed. Furthermore, the vehicle stability control module 520 transmits a maximum allowable rotational velocity for the vehicle at the upcoming path to the motion request module 522. The motion request module 522 evaluates the received signals and transmits a requested steering angle δ.sub.ref, a requested rotational velocity ω.sub.req, and a requested longitudinal vehicle acceleration a.sub.x, req to the motion controller 514.

(53) The motion controller 514 evaluates the received parameters and transmits a vector comprising a longitudinal vehicle force F.sub.x, lateral vehicle force F.sub.y, global vehicle torque M.sub.z, as well as the above described additional steering torque M.sub.steer to the actuator coordinator module 516.

(54) Based on the received signal from the motion controller 514, the actuator coordinator module 516 transmits signals to one or more of the wheel torque control module 508, the propulsion controller 510 and the steer controller 512. In detail, the wheel torque control module 508 receives a signal indicative of a requested wheel torque, whereby the wheel torque control module 508 controls the individually controllable wheel torque actuator 103, 105 to apply the desired differential wheel torque. The propulsion controller 510 receives a signal indicative of a requested propulsion and control the vehicle to provide such propulsion. Finally, the steer controller 512 receives a signal indicative of a requested steer angle δ.sub.req. The steer controller 512 is thus mainly used in case the primary steering system is functioning as desired/intended. Each of the wheel torque control module 508, the propulsion controller 510 and the steer controller 512 can also transmit control signals back to the actuator coordinator module 516 for indicating e.g. the status of the respective parameters, etc.

(55) In order to sum up, reference is made to FIG. 6, which is a flow chart of a method for controlling the steering system according to an example embodiment. During operation, a required steering angle δ.sub.req for operating the vehicle during a turning maneuver is detected S1. The required steering angle δ.sub.req can be determined in advance based on a signal received from e.g. a path follower or from a an operator turning the steering wheel. An actual steering angle is thereafter detected S2 when operating the vehicle 1 during the turning maneuver. Hereby, a difference between the required steering angle and the detected, actual steering angle can be determined S3. By means of this determined difference, a differential wheel torque can be determined S4. The wheel torque actuators 103, 105 of the steerable wheels 104, 106 can then be controlled S5 for reducing such difference. The wheel torque actuators can be controlled to apply a differential braked force on the steerable wheels, or add a differential propulsion to the steerable wheel, depending on the type of wheel torque actuator used.

(56) 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.