Patent ID: 12233974

DETAILED DESCRIPTION

The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. The disclosure 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.

With particular reference toFIG.1, there is provided a vehicle1in the form of a truck. The vehicle1comprises a control unit20, and a steering system500. It should however be understood that the steering system500may be arranged as a control unit. Thus, no separate control unit20may in such case be superfluous. The vehicle1comprises a pair of steerable wheels104,106arranged on a respective left- and right-hand side of a front axle102of the vehicle. The front axle102is thus the foremost located axle of the vehicle1. The vehicle1depicted inFIG.1also comprises a pair of first rear wheels108,110connected to a first rear axle112, and a pair of second rear wheels114,116connected to a second rear axle118. The first rear axle112is arranged in front of the second rear axle118as seen in the longitudinal direction of the vehicle1. As will also be described below with reference toFIG.2, the pair of first rear wheels108,110and/or the pair of second rear wheels114,116can also be arranged as steerable wheels. As is also depicted, the vehicle1comprises a power steering system30. The power steering system30is preferably arranged as a redundant steering system for steer control of the steerable wheels. AlthoughFIG.1illustrates the power steering system30including a steering wheel, the power steering system30may instead form part of a steer-by-wire system, etc.

In order to describe the vehicle1in further detail, reference is made toFIGS.2and3. In detail,FIG.2illustrates an example embodiment of the forces exposed to the vehicle1and its wheels during a turning maneuver, andFIG.3illustrates the vehicle1before initiating a turning maneuver, i.e. before arriving at a road curvature.

Starting withFIG.2, which is a schematic illustration from above depicting the vehicle1inFIG.1exposed to a turning maneuver according to an example embodiment. Hence, the pair of steerable wheels104,106is turning and being exposed to a steering angle δ. The steering angle δ is for simplification inFIG.2illustrated as the same for the left104steerable wheel and the right106steerable wheel and is an angle of the wheels relative to a longitudinal axis of the vehicle1. The vehicle is operated at a vehicle speed indicated as v. The steerable wheels104,106also comprises a respective individually controllable electric machine103,105. As described above, the pair of first rear wheels108,110and/or the pair of second rear wheels114,116can also be arranged as steerable wheels. InFIG.2, the pair of second rear wheels114,116are depicted as steerable and thus also comprises a respective individually controllable electric machine103,105. The first rear axle112can thus be arranged as a steerable pusher axle and the second rear axle118can be arranged as a steerable tag axle. However, the following will for simplicity only describe steering using the front steerable wheels. Also, inFIG.2, the power steering system is illustrated as an electrical power steering system1000.

As also depicted inFIG.2, the individually controllable electric machines103,105are connected to an electrical power supply200for receiving electric power. The electrical power supply200is also arranged to transmit electrical power to the individually controllable electric machines103,105. The electrical power supply200is thus preferably arranged as a vehicle battery. The individually controllable electric machines103,105are thus arranged for propelling the vehicle1, as well as controlling steering of the vehicle1as will be described in further detail below.

The vehicle1comprises, as described above, the pair of steerable wheels104,106arranged on the front axle102, the pair of first rear wheels108,110connected to the first rear axle112, and the pair of second rear wheels114,116connected to the second rear axle118. The front axle102is arranged at a distance11from a center of mass202of the vehicle, the first rear axle112is arranged at a distance12from the center of mass202of the vehicle, and the second rear axle118is arranged at a distance13from the center of mass202of the vehicle. The center of mass202is the position of the vehicle1around which the vehicle rotates during the turning maneuver. The center of mass202is also the position of the vehicle1at which total global forces affecting the vehicle1can be expressed. In the following, the x-axis is the axis extending in the longitudinal direction of the vehicle1, the y-axis is extending in the transversal direction of the vehicle1and the z-axis is extending in the vertical direction of the vehicle1. During the turning maneuver, the vehicle1is exposed to a torque Mz at the center of mass202. Also, the vehicle is exposed to a global longitudinal force Fx and a global lateral force Fy.

Moreover, when the steerable wheels104,106of the front axle102is exposed to the steering angle δ, the steerable wheel104on the left hand side is exposed to a longitudinal force Fx,104and a lateral force Fy,104, while the steerable wheel106on the right hand side is exposed to a longitudinal force Fx,106and a lateral force Fy,106. The sum of the lateral force of the steerable wheels104,106on 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. The steering angle δ can be obtained by either controlling one of the individually controlled electric machines, or controlling both of the individually controlled electric machines to obtain different wheel speeds.

Furthermore, the pair of first rear wheels108,110is exposed to a respective lateral force Fy,108and Fy,110, and the pair of second rear wheels114,116is exposed to a respective lateral force Fy,114and Fy,116. In the exemplified embodiment ofFIG.2, the longitudinal force of the pair of first rear wheels108,110and the pair of second rear wheels114,116is set to zero, i.e. the respective wheel is not exposed to propulsion or braking.

Turning now toFIG.3, which is an illustration of the vehicle before entering a curvature302of the road, i.e. before the turning maneuver takes place. As can be seen inFIG.3, the vehicle1is currently driving straight ahead at the vehicle speed v. Thus, before entering the curvature302, the steering angle δ is zero. The curvature has a radius denoted as rroad. 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 (seeFIG.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 inFIG.3. On the contrary, the turning maneuver may also relate to e.g. a lane change operation of the vehicle.

Turning now toFIGS.4a-4cwhich illustrate different views of the left steerable wheel104according to an example embodiment. In detail,FIG.4ais a side view of the left steerable wheel104,FIG.4bis a rear view of the left steerable wheel104, andFIG.4cis a top view of the left steerable wheel104during the turning maneuver.

Starting withFIG.4a, which is a side view of the left steerable wheel104. The suspension (not shown) of the wheel104is arranged such that the wheel104is provided with a suspension caster angle γ which is defined as the angular displacement of a steering axis402from a vertical axis404of the left steerable wheel104, measured in the longitudinal direction of the vehicle1. The distance between the intersection of the road surface401and the steering axis402, and the intersection of the road surface401and the vertical axis404is denoted as tm. With the suspension of the wheel, the point of force application of the contact patch406between the wheel104and the road surface401will be located slightly offset in the longitudinal direction from the intersection of the road surface401and the vertical axis404. This offset is denoted as tp. The contact patch is thus the area of the tire in contact with the ground surface. The point of force application of the contact patch406between the wheel104and the road surface401is dependent on the suspension caster angle γ.

Turning toFIG.4b, which is a rear view of the left steerable wheel104. As can be seen, the effective wheel radius R is indicated as the distance between the front axle102and the road surface401, and the wheel104is connected to the suspension by an inclined king pin axis408, which inclination is indicated as T. Thus, the wheel104is rotated around the king pin axis408during the turning maneuver. Furthermore, the point of force application of the contact patch406between the wheel104and the road surface401is located at the intersection between the vertical axis404and the road surface401. The vehicle1, and in particular the steerable wheels104,106are provided with a positive wheel suspension scrub radius rs. The wheel suspension scrub radius rsis defined as the distance between the point of force application of the contact patch406and the intersection403between the king pin axis408and the road surface401. A positive wheel suspension scrub radius rsis generated when the intersection between the king pin axis408and the road surface401is located on an inner side of the vertical axis404as seen in the longitudinal direction depicted inFIG.4b. When e.g. decelerating the individually controllable electric machine103of the left steerable wheel104, the wheel will rotate around the king pin axis408due to the positive scrub radius rscausing the vehicle to turn to the left. Hereby, an additional steering torque Msteercan be generated.

Turning toFIG.4c, which is a simplified illustration of a combined left and right front wheel seen from above. InFIG.4c, a differential wheel speed of the left103and right105individually controllable electric machines is provided for causing the vehicle to turn to the left. In particular, an increased wheel speed on the right-hand side wheel is provided compared to the wheel speed of the left-hand side wheel to cause the vehicle to turn left. As can be seen, the vehicle1is operated at the road curvature described above in relation toFIG.3, where the road curvature has the radius rroad. The steerable wheel104thus has a steering angle δ. However, the steerable wheel104will 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.

The front wheel lateral force may be determined by determining the slip angle of the steerable wheels for the turning maneuver. In particular, the front wheel lateral force can be determined based on a cornering stiffness of the steerable wheels and the slip angle. The slip angle should thus be construed as an angle defining the difference between angular position of the wheel and the actual angular direction of movement of the wheel. For example, if the steerable wheels are steered 15 degrees relative to a longitudinal axis, but the actual movement of the steerable wheels is 12 degrees relative to the same longitudinal axis, then the slip angle is 3 degrees. The cornering stiffness of the tires on the other hand is the stiffness of the steerable wheels in the lateral direction. The cornering stiffness is a tire parameter defined as a factor between the slip angle (or side slip angle) and lateral tire force. The cornering stiffness may be considered constant for small slip angles, for a given tire at a given normal load.

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. Also, in order to determine the most power efficient mode of operating the vehicle during the turning maneuver, a first power utilization value is compared to a second power utilization value. In further detail, the first power utilization value is a value defining the power efficiency of operating the vehicle during the turning maneuver by controlling the individually controllable electric machines, while the second power utilization value is a value defining the power efficiency of operating the vehicle during the turning maneuver by controlling the power steering system.

Parameters described above will not be given any further detailed description unless indicated. The wheel speed, and in turn the wheel torque can be determined by determining a required differential longitudinal force ΔFx, which is the difference between Fx,104and Fx,106, and the wheel radius R.

According to a non-limiting example, the first power utilization value can be determined to be greater than the second power utilization value when individually controllable electric machines are able to generate a sufficient steering torque Msteer, such that the vehicle sufficiently follows curvature during the turning maneuver. Hence, the steering torque Msteeris sufficient to obtain the required steering angle. The required steering torque Msteercan be determined according to:
Msteer=ΔFx·rs=(Fy,104+Fy,106)·t(1)where:Fy, 104and Fy, 106=the front wheel lateral force of the steerable wheels104,106
t=tm+tp

Equation (1) can be rewritten according to:
Msteer=−2Cα·α·(tm+tp)  (2)where:Cα=lateral stiffness of the tire;Fy,i=Cα·α=the front wheel lateral force; and

α=(δ-l1v⁢ω)(3)wherev=the longitudinal vehicle speed; andω=rotational speed of the vehicle during the turning maneuver.

Furthermore, the global vehicle torque Mzat the center of rotation202can be determined according to:

Mz=l1((Fx,104+Fx,106)·δ+2⁢Cα·α)+w2⁢(-Fx,104+Fx,106)-2⁢Cα·l22·ωv+w2·(-Fx,108+Fx,110)-2⁢Cα·l32·ωv+w2·(-Fx,114+Fx,116)(4)where:⁢Δ⁢Fx=Fx,104-Fx,106⁢Fx,108=Fx,110=Fx,114=Fx,116=0⁢β=0⁢w=track⁢width⁢of⁢the⁢vehiclewhere 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.

Furthermore, the slip angle of the steerable wheels can be determined according to:
(Fx,104−Fx,106)·rs+(Fy,104+Fy,106)·(tm+tp)=D{dot over (α)}−J{umlaut over (α)}(5)
Fy=Fy,104+Fy,106=2*C∞·α  (6)
ΔFx·rs+2Cα·α·(tm+tp)=D{dot over (α)}—J{umlaut over (α)}(7)

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

Δ⁢Fx=2⁢Cα·α·(tm+tp)rs(8)α=Δ⁢Fx·rs2⁢Cα·(tm+tp)(9)

Furthermore, with the assumption that

rroad≈vω
and t=tm∓tp, the following expressions can be made:

0=-Δ⁢Fx·(l1·rst+ω2)-2⁢Cα·α⁡(1rroad·(l22+l32))(10)=>Δ⁢Fx=2⁢Cα·α⁢-1rroad·(l22+l32)l1·rst+ω2(11)

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

The above may be controlled by assigning control allocations, whereby the following expression can be formulated:

uopt=arg⁢minumin≤u≤umin[Wu(u-ud)22+γ⁢Wv(Bu-v)22]⁢where:(12)v=Bu(13)in which the following matrices and vectors are defined as

[FxFyMzMsteer]=[1R1R1R1R1R1R00000002⁢Cα-w12⁢Rw12⁢R-w22⁢Rw22⁢R-w32⁢Rw32⁢Rl1*2⁢Cα-rsRrsR00000][T104T106T108T110T114T116δ](14)wherein:R is the effective radius; andT is the wheel torque for the respective wheel.

If the power steering system is deactivated, the steering angle δ can be set to the actual value of the steering angle, i.e. δactual. Hence, during a steering maneuver, capabilities for the steering actuator is substantially limited to the actual steering angle.

Reference is now made toFIG.5, which illustrates the steering system500according to an example embodiment. As can be seen inFIG.5, the steering system500comprises an actuator control module502, a vehicle motion control module504and a traffic situation controller506. The actuator control module502comprises an electric machine control module508and a power steering system controller512. The vehicle motion control module504comprises a motion controller514, a utilization comparison controller515and an actuator coordinator module516. Finally, the traffic situation controller506comprises a path controller518, a vehicle stability control module520and a motion request module522.

During operation of the exemplified system500inFIG.5, the path controller518detects an upcoming path for the vehicle1and transmits a steering angle δpathrequired to maintain the path to the motion request module522. The signal is based on path curvature, and in some implementation on vehicle speed. Furthermore, the vehicle stability control module520transmits a maximum allowable rotational velocity for the vehicle at the upcoming path to the motion request module522. The motion request module522evaluates the received signals and transmits a requested steering angle δref, a requested rotational velocity ωreq, and a requested longitudinal vehicle acceleration ax, reqto the motion controller514.

The motion controller514evaluates the received parameters and transmits a vector comprising a longitudinal vehicle force Fx, lateral vehicle force Fy, global vehicle torque Mz, as well as the above described additional steering torque Msteer. The utilization comparison controller515thereafter determines if the power utilization of the vehicle1, by controlling steering using the individually controllable electric machines during the turning maneuver, is better compared to the power utilization by controlling steering using the power steering system. Thus, a first power utilization value of the vehicle arrangement obtained by operating the individually controllable electric machines is compared to a second power utilization value of the vehicle arrangement obtained by operating the power steering system to obtain the steering angle.

It may be determined that at least one of the individually controllable electric machines103,105is able to regenerate electrical power to the electrical power supply during the turning maneuver. In such case, the utilization comparison controller515can determine that the first power utilization value is higher than the second power utilization value. Preferably, the state of charge level of the electrical power supply200should be at such level as to be able to receive electrical power. According to another alternative, the utilization comparison controller515may determine that the first power utilization value is higher than the second power utilization value when the individually controllable electric machines are solely able to sufficiently control the vehicle to obtain the required steering angle during the turning maneuver. In such case, the operational capacity of the power steering system can be reduced, such as preferably be deactivated and, for example, control systems such as hydraulic systems, pneumatic systems or electrical systems connected to the power steering system can be deactivated whereby the overall power utilization of the vehicle increases.

The utilization comparison controller515transmits a signal to the actuator coordinator module516with information relating to the best option for controlling the steering of the vehicle during the turning maneuver. Based on the received signal from the utilization comparison controller515, the actuator coordinator module516transmits signals to one or more of the electric machine control module508and/or the power steering system controller512. In detail, the actuator coordinator module516receives a signal for determining to control steering of the vehicle using the individually controllable electric machines and/or the power steering system. If the first power utilization value is equal to, or greater than the second power utilization value, the actuator coordinator module516transmits a control signal to the electric machine control module508to control steering of the vehicle1using the individually controllable electric machines103,105. However, if the second power utilization value is greater than the first power utilization value, the actuator coordinator module516transmits a control signal to the power steering system controller512to control steering of the vehicle1using the power steering system. In a case where the operating conditions are such that the individually controllable electric machines103,105are unable to solely obtain the required steering angle during the turning maneuver, the actuator coordinator module516can transmit a control signal to the electric machine control module508as well as to the power steering system controller512. Hereby, steering of the vehicle1during the turning maneuver is controlled by the individually controllable electric machines103,105as well by the power steering system. When controlling steering using the individually controllable electric machines, the optimization of the steering system is preferably restricted to a steering angle corresponding the actual steering angle as also described above. Hence, the operational capacity of the power steering system is reduced, preferably deactivated and unable to control the steering angle.

In order to sum up, reference is made toFIG.6, which is a flow chart of a method for controlling the steering system500according to an example embodiment. During operation, a required steering angle δreqfor operating the vehicle during a turning maneuver is determined S1. A differential wheel speed, for the individually controllable electric machines, required for obtaining the required steering angle δreqis determined S2. The required steering angle δreqcan be determined in advance based on a signal received from e.g. a path follower or from an operator turning the steering wheel. A first power utilization value of the vehicle arrangement1obtained by operating the individually controllable electric machines with the differential wheel speed is determined S3. Various options for determining the first power utilization value are described above. In order to determine if it is power efficient to control the steering using the individually controllable electric machines103,105, also a second power utilization value of the vehicle arrangement obtained by operating the power steering system to obtain the steering angle is determined S4.

The first and second power utilization values are thereafter compared to each other. When the first power utilization value is equal to, or greater than the second power utilization value, the steering of the vehicle arrangement1during the turning maneuver is controlled S6by applying the differential wheel speed by at least one of the individually controllable electric machines and reducing the operational capacity of the power steering system. However, when the second power utilization value is greater than the first power utilization value, steering of the vehicle arrangement1during the turning maneuver is controlled S7by controlling the power steering system, either alone or in combination with the individually controllable electric machines103,105.

It is to be understood that the present disclosure 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.