METHOD FOR HAVING A VEHICLE FOLLOW A DESIRED CURVATURE PATH

Abstract

The present invention relates to a method for having a vehicle (100) follow a desired curvature path (C1), said vehicle (100) comprising at least one differential (10, 20, 30) with a differential lock connected to at least one driven wheel axle (40, 50) of said vehicle (100), said method comprising at least the following steps: —providing (S1) information regarding state of said differential lock, said state being either that said differential lock is activated or unlocked, and when said differential lock is activated: —calculating (S2) a yaw moment, M.sub.diff, of said vehicle (100), caused by said differential lock; and —compensating (S3) for a deviation from said desired curvature path (C1) caused by said yaw moment, M.sub.diff, such that a resulting steering angle is equal to or less than a maximum allowed steering angle of said vehicle (100), whereby said compensation is a feed forward compensation. The invention also relates to a control unit, a vehicle, a computer program and a computer readable medium.

Claims

1. A method for having a vehicle follow a desired curvature path, said vehicle comprising at least one differential with a differential lock connected to at least one driven wheel axle of said vehicle, said method comprising at least the following steps: providing information regarding a state of said differential lock, said state being either that said differential lock is activated or unlocked, and when said differential lock is activated: calculating a yaw moment, M.sub.diff, of said vehicle, caused by said differential lock; and compensating for a deviation from said desired curvature path caused by said yaw moment, M.sub.diff, such that a resulting steering angle is equal to or less than a maximum allowed steering angle of said vehicle, whereby said compensation is a feed forward compensation.

2. The method according to claim 1, wherein the method further comprises the step of: calculating a total desired yaw moment, M.sub.z, by calculating a vehicle curvature yaw moment, M.sub.z_curvature, for said desired curvature path, wherein said total desired yaw moment, M.sub.z, is defined as M.sub.z_curvature+M.sub.diff, and wherein the resulting steering angle is provided by the total desired yaw moment M.sub.z.

3. The method according to claim 1, wherein said vehicle is a semi-autonomous vehicle or a fully autonomous vehicle.

4. The method according to claim 1, wherein said compensation is performed in a force generation part of said vehicle, said force generation part being at least used for calculating desired forces and moments of said vehicle for controlling at least one of steering, braking and propulsion of said vehicle.

5. The method according to claim 1, further comprising the step of providing said compensation as a feed forward compensation to a motion support device coordinator of said vehicle, said motion support device coordinator being used for controlling at least one of steering, braking and propulsion of said vehicle.

6. The method according to claim 1, wherein said calculated yaw moment, M.sub.diff, is calculated based on at least one of the following parameters: desired curvature path, preferably said desired curvature path being based on a predicted path for vehicle automation, vehicle speed in a vehicle coordinate system, vehicle speed in a wheel coordinate system, wheel speed of wheels connected to said at least one driven wheel axle, wheel radii of said wheels connected to said at least one driven wheel axle, normal forces exerted on said wheels connected to said at least one driven wheel axle, friction coefficient of said wheels connected to said at least one driven wheel axle, and trackwidth of said vehicle.

7. The method according to claim 1, wherein said method further comprises the step of: activating said at least one differential lock connected to said at least one driven wheel axle when a slip value is identified relating to slip of at least one wheel connected to said at least one driven wheel axle, which slip value is equal to or above a predetermined slip threshold value.

8. The method according to claim 7, wherein when said vehicle is running on a low friction surface having a friction coefficient being below a friction coefficient threshold value and when said at least one differential lock is activated, said at least one differential lock is continued to be activated when an identified slip of at least one wheel connected to said at least one driven wheel axle is lower than a slip limit, which slip limit is preferably larger than a peak slip of said low friction surface.

9. The method according to claim 8, wherein said method further comprises the step of: unlocking said at least one differential lock connected to said at least one driven wheel axle when said identified slip is larger than said slip limit.

10. The method according to claim 7, wherein when said vehicle is running on a high friction surface having a specific friction coefficient being above a friction coefficient threshold value and when said at least one differential lock is activated, said at least one differential lock is continued to be activated if the sum of wheel forces of the wheels connected to said at least one driven wheel axle is lower than normal forces of said wheels times said specific friction coefficient.

11. The method according to claim 10, wherein said method further comprises the step of: unlocking said at least one differential lock connected to said at least one driven wheel axle if the sum of wheel forces of the wheels connected to said at least one driven wheel axle is larger than normal forces of said wheels times said specific friction coefficient.

12. A control unit for controlling a vehicle to follow a desired curvature path, said control unit being configured for performing the steps of claim 1.

13. A vehicle comprising at least one differential with a differential lock connected to at least one driven wheel axle of said vehicle, and further comprising said control unit according to claim 12.

14. The vehicle according to claim 13, wherein said vehicle is a semi-autonomous or a fully autonomous vehicle.

15. The vehicle according to claim 13, wherein said vehicle is any one of a truck, a heavy duty truck, a construction equipment vehicle or a bus.

16. The vehicle according to claim 13, comprising at least one differential with at least two respective differential locks connected to least two respective driven wheel axles of said vehicle.

17. A computer program comprising program code means for performing the steps of claim 1, when said program is run on a computer.

18. A computer readable medium carrying a computer program comprising program code means for performing the steps of claim 1, when said program product is run on a computer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0044] In the drawings:

[0045] FIG. 1 depicts a vehicle in the form of a truck;

[0046] FIG. 2 depicts a schematic illustration of a vehicle;

[0047] FIG. 3 depicts a curvature path of a vehicle;

[0048] FIG. 4 depicts a flowchart of a method according to an example embodiment of the present invention;

[0049] FIG. 5 depicts a control unit according to an example embodiment of the present invention;

[0050] FIG. 6 shows an example of a peak slip value; and

[0051] FIG. 7 shows an example of a constant value k.

[0052] The drawings show diagrammatic exemplifying embodiments of the present invention and are thus not necessarily drawn to scale. It shall be understood that the embodiments shown and described are exemplifying and that the invention is not limited to these embodiments. It shall also be noted that some details in the drawings may be exaggerated in order to better describe and illustrate the invention. Like reference characters refer to like elements throughout the description, unless expressed otherwise.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

[0053] FIG. 1 depicts a vehicle in the form of a truck 100 for which the method of the present invention advantageously may be implemented. The truck 100 as shown in FIG. 1 is a towing truck for towing trailers of different kind. Hence, the invention is applicable to vehicles and vehicle combinations, also known as articulated vehicle combinations. It shall however be noted that the present invention is not only usable for a truck, but could likewise be used also for other vehicles, such as buses, construction equipment, passenger cars etc., which vehicles comprise at least one differential with at least one differential lock.

[0054] FIG. 2 depicts a schematic illustration of a vehicle 100, e.g. a truck, seen from above. The vehicle 100 in this particular embodiment comprises two driven rear wheel axles 40 and 50. Two wheels 3 and 4 are connected to the first driven wheel axle 40 and two wheels 5 and 6 are connected to the second driven wheel axle 50. A first differential 10 is provided on the first wheel axle 40, a second differential 20 is provided between the two wheel axles, and a third differential 30 is provided on the second wheel axle 50. The differentials 10, 20 and 30 are provided with differential locks which can be selectively activated and unlocked.

[0055] An x-direction as provided herein corresponds to a longitudinal direction of the vehicle, an y-direction corresponds to a lateral direction of the vehicle and a z-direction corresponds to a vertical direction of the vehicle.

[0056] The vehicle 100 further comprises two front wheels 1 and 2 which can be angled in order to allow the vehicle to follow a curvature path. Further, the vehicle 100 has a certain track width T as seen in the figure.

[0057] Vehicle speed v.sub.x,i at each respective wheel can be calculated according to the two following equations, depending on if the wheel is provided on the left or right side of the vehicle:

[00001]$v_{x,i}^{\mathrm{left}}=v_x\left(\frac{R-T\text{/}2}{R}\right)v_{x,i}^{\mathrm{right}}=v_x\left(\frac{R+T\text{/}2}{R}\right)$

where R is the curvature radius of the vehicle's path.

[0058] When the differential locks are activated, the angular speed of each driven wheel, 3 to 6, is the same, i.e. the following relation between the angular speed (ω) of wheels can be expected:

ω.sub.w,3=ω.sub.w,4=ω.sub.w,5=ω.sub.w,6

[0059] Traction slip λ.sub.w,i each wheel may in turn be calculated by the following equation:

[00002]${\lambda }_{w,i}=\frac{{\omega }_{w,i}.Math.R_w-v_{x,i}}{{\omega }_{w,i}.Math.R_w}$

where R.sub.w is the wheel radius for each wheel.

[0060] From this, force contribution from the differential lock at each wheel may be calculated according to the following equation:

F.sub.x,i.sup.diff=min(k.Math.λ.sub.w.sub.2.sub.i,μ.sub.i.Math.F.sub.z,i)

where k is a constant defined as surface friction coefficient divided by traction slip, as shown in e.g. FIG. 7, where μ.sub.i is the friction coefficient at the respective wheel and where F.sub.z is the normal force acting in the z-direction. μ.sub.i may be defined by F.sub.x/F.sub.zi.

[0061] The yaw moment, M.sub.diff, caused by the at least one differential lock may be calculated as the sum of wheel forces times half the track width T, i.e.:

[00003]$M^{\mathrm{diff}}=\underset{i=3}{\overset{6}{.Math.}}{F}_{x,i}.Math.T\text{/}2$

[0062] Desired forces and moments for controlling the vehicle 100 may be defined as

V.sub.req=[F′.sub.x,F′.sub.y,M.sub.z]

where F.sub.x may be defined as:

F.sub.x=m.Math.α.sub.x.sup.req

where m is the vehicle's mass and a.sub.x,req is the acceleration in the x-direction;
where F.sub.y may be defined as:

F.sub.y=δ.sub.f.sup.path.Math.2c.sub.α

where δ.sub.f,path is the actual steering angle and C.sub.α is tire cornering stiffness, also known as lateral slip stiffness of the vehicle; and
where the total desired yaw moment M.sub.z may be defined as:
M.sub.z=M.sub.z_curvature+M.sub.diff. In more detail, the total desired yaw moment M.sub.z may be defined as:

M.sub.z=δ.sub.f.sup.path.Math.2C.sub.α.Math.l.sub.f+M.sup.diff

where l.sub.f is the distance between the vehicle's centre of gravity on the x-axis and the front wheel axle of the vehicle where the wheels 1 and 2 are provided. Hence, M.sub.z_curvature is here defined as:

δ.sub.f.sup.path.Math.2C.sub.α.Math.l.sub.f

[0063] By calculating the forces and moments, F.sub.x, F.sub.y and M.sub.z, as in the above, the deviation from the desired curvature path caused by the yaw moment M.sub.diff can be compensated, preferably in the force generation part, such that a resulting steering angle is equal to or less than a maximum allowed steering angle of the vehicle. The maximum allowed steering angle may of course vary depending on the type of vehicle. For example, the maximum allowed steering angle may correspond to that the front wheels can be angled about ±75 degrees with respect to a forward direction of the vehicle. The force generation may take into account the yaw moment M.sub.diff caused by a locked, i.e. activated, differential, and hence compensate for the yaw to follow a desired curvature path by at least one of steering, braking at least one wheel and propulsion of the vehicle.

[0064] In the case when the vehicle 100 is running on a low friction surface having a friction coefficient being below a friction coefficient threshold value and when the at least one differential lock is activated, the at least one differential lock may be continued to be activated when an identified slip of at least one wheel connected to the at least one driven wheel axle is lower than a slip limit, which slip limit is larger than a peak slip of the low friction surface. Thereby, in such situation, the differential lock is continued to be activated when the following is fulfilled:

λ.sub.w,i<λ.sub.lim

[0065] For example, λ.sub.lim, may be a value such as 0.1-0.6, or 0.2-0.6, or 0.3-0.6, where λ.sub.lim, is set to be larger than a peak slip of the low friction surface, and where 0 corresponds to no slip and 1 corresponds to full slip. An example of a peak slip is shown in FIG. 6, where it can be seen that the peak slip 42 occurs at a slip ratio of about 0.2. As mentioned in the above, the low friction surface may be a surface comprising ice, snow, gravel or the like. Further, the method may further comprise the step of unlocking the at least one differential lock connected to the at least one driven wheel axle when the identified slip is larger than the slip limit.

[0066] Further, in the case when the vehicle is running on a high friction surface having a specific friction coefficient being above a friction coefficient threshold value and when the at least one differential lock is activated, the at least one differential lock is continued to be activated if the sum of wheel forces of the wheels connected to the at least one driven wheel axle is lower than normal forces F.sub.z of the wheels times the specific friction coefficient. This means that the differential lock may be allowed to be activated for a longer time until there is a risk for rotational windup of the wheel axles. Thereby, in such situation, the differential lock is continued to be activated when the following is fulfilled:

[00004]$\underset{i=3}{\overset{6}{.Math.}}\mu F_{z,i}>\underset{i=3}{\overset{6}{.Math.}}k{\lambda }_{w,i}$

where μ is the specific friction coefficient of the surface and F.sub.z is the normal force. Further, the method may further comprise the step of unlocking the at least one differential lock connected to the at least one driven wheel axle if the sum of wheel forces of the wheels connected to the at least one driven wheel axle is larger than normal forces of the wheels times the specific friction coefficient.

[0067] FIG. 3 depicts the desired curvature path C.sub.1 for the vehicle 100. It has been found that by controlling the resulting steering angle of the vehicle as disclosed herein, the vehicle will more closely follow the desired curvature path, indicated by the curvature C.sub.2, rather than the curvature C.sub.3 which shows the path the vehicle may take if no compensation was performed when the differential lock(s) is/are activated.

[0068] FIG. 4, depicts a flowchart of the method of the present invention. Step S0 is optional, which is indicated in that the box is provided with dashed lines. The method may hence comprise the following steps: [0069] S0, activating the at least one differential lock connected to the at least one driven wheel axle when a slip value is identified relating to slip of at least one wheel connected to the at least one driven wheel axle, which slip value is equal to or above a predetermined slip threshold value; [0070] S1, providing information regarding state of the differential lock, the state being either that the differential lock is activated or unlocked, and when the differential lock is activated: [0071] S2, calculating a yaw moment, M.sub.diff, of the vehicle 100, caused by the differential lock; and [0072] S3, compensating for a deviation from the desired curvature path C1 caused by the yaw moment, M.sub.diff, such that a resulting steering angle is equal to or less than a maximum allowed steering angle of the vehicle 100, whereby the compensation is a feed forward compensation.

[0073] FIG. 5 depicts a control unit 200 according to an example embodiment of the present invention. In this embodiment, the control unit is provided in an autonomous vehicle and comprises a force generation part 210 and a motion control device coordinator 220. Even though FIG. 5 indicates that the control unit 200 is provided as one unit, the same functionality could of course also be provided with several units located proximate to or distanced from each other. The control unit may for example comprise a processing unit and/or a memory unit. Still further, the control unit may comprise a computer program and/or a computer readable medium according to the invention. In this example, the compensation is provided as feed forward compensation from the force generation part to the motion support device coordinator.

[0074] 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.