Method and system for controlling a vehicle tire-to-road friction estimation

Abstract

Method for controlling a vehicle, comprising: identifying an upcoming curve and determining its properties; determining current vehicle speed, v.sub.v, estimating friction, μ.sub.e, between a tire of the vehicle and the road; estimating maximum allowable vehicle speed, v.sub.max_e, when entering the curve based on the properties, speed and friction; if the current vehicle speed is higher than the estimated maximum allowable vehicle speed, determining that a friction measurement is required; if distance, d.sub.v, between the vehicle and a curve entrance is higher than a predetermined threshold distance, d.sub.T, and if a braking action is detected, performing a friction measurement during the braking action to determine current friction, μ.sub.c; if a distance between the vehicle and the curve entrance is lower than the predetermined threshold distance, performing a friction measurement to determine a current friction; and determining a maximum allowable vehicle speed v.sub.max_d based on the curve radius, speed and friction.

Claims

1. A method for controlling a vehicle traveling on a road, the method comprising: identifying an upcoming curve and determining properties of the curve; determining a current vehicle speed, v.sub.v; estimating a friction, μ.sub.e, between a tire of the vehicle and the road; estimating a maximum allowable vehicle speed, v.sub.max_e, when entering the curve based on the curve properties, vehicle speed and estimated friction; if the current vehicle speed is higher than the estimated maximum allowable vehicle speed, determining that a friction measurement is required; if a distance, d.sub.v, between the vehicle and a curve entrance is higher than a predetermined threshold distance, d.sub.T, and if a braking action is detected, performing a friction measurement during the braking action to determine a current friction, μ.sub.c; if a distance between the vehicle and the curve entrance is lower than the predetermined threshold distance, performing a friction measurement to determine a current friction; and determining a maximum allowable vehicle speed, v.sub.max_d, based on the curve radius, vehicle speed and current friction.

2. The method according to claim 1, further comprising, if the current vehicle speed is higher than the determined maximum allowable vehicle speed, braking the vehicle so that the vehicle speed when reaching the curve is lower than or equal to the determined maximum allowable vehicle speed.

3. The method according to claim 1, further comprising, if the current vehicle speed is higher than the maximum allowable vehicle speed, braking the vehicle with a constant deceleration from the current location of the vehicle to the curve entrance so that the vehicle speed is equal to or lower than the determined maximum allowable vehicle speed when the vehicle reaches the curve entrance.

4. The method according to claim 1, further comprising, if the current vehicle speed is higher than the determined maximum allowable vehicle speed, alerting the driver, indicating that there is a risk of leaving the road unless the vehicle speed is reduced to the determined maximum allowable vehicle speed.

5. The method according to claim 1, wherein the predetermined threshold distance is based on a distance required to reduce the vehicle speed from the current speed to the estimated maximum allowable vehicle speed when entering the curve.

6. The method according to claim 1, wherein estimating a friction between a tire of the vehicle and the road comprises acquiring a previously measured friction value for the curve, and estimating the friction to be lower than the previously measured friction.

7. The method according to claim 1, wherein estimating a friction between a tire of the vehicle and the road comprises acquiring a most recently measured friction value for the road on which the vehicle travels, and estimating the friction to be lower than the a most recently measured friction value.

8. The method according to claim 1, wherein an estimated friction is at least two standard deviations lower than an expected friction.

9. The method according to claim 1, wherein determining properties of the curve comprises determining a curve radius.

10. The method according to claim 1, wherein determining properties of the curve comprises determining a curve shape.

11. The method according to claim 1, wherein determining properties of the curve comprises determining at least one of a road width, a lane width and a road camber.

12. The method according to claim 1, wherein determining properties of the curve comprises acquiring curve properties from a remote server.

13. The method according to claim 1, further comprising estimating a vehicle path through the curve based on the curve properties.

14. The method according to claim 1, wherein determining properties of the curve comprises acquiring information from a previously established map.

15. A vehicle control system comprising a vehicle control unit configured to: identify an upcoming curve and determine properties of the curve; determine a current vehicle speed, v.sub.v; estimate a friction, μ.sub.e, between a tire of the vehicle and the road; estimate a maximum allowable vehicle speed, v.sub.max_e, when entering the curve based on the curve properties, vehicle speed and estimated friction; if the current vehicle speed is higher than the estimated maximum allowable vehicle speed, determine that a friction measurement is required; if a distance, d.sub.v, between the vehicle and a curve entrance is higher than a predetermined threshold distance, d.sub.r, and if a braking action is detected, performing a friction measurement during the braking action to determine a current friction μ.sub.c; if a distance between the vehicle and the curve entrance is lower than the predetermined threshold distance, performing a friction measurement to determine a current friction; and determine a maximum allowable vehicle speed v.sub.max_d based on the curve radius, vehicle speed and current friction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein:

(2) FIG. 1 is a flow chart outlining the general steps of a method according to an embodiment of the invention;

(3) FIG. 2 schematically illustrates the method according to an embodiment of the invention performed by a vehicle; and

(4) FIG. 3 schematically illustrates a control system according to an embodiment of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(5) The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This 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, and fully convey the scope of the invention to the skilled person. Like reference characters refer to like elements throughout.

(6) FIG. 1 is a flow chart outlining the general steps of a method of controlling a vehicle 200 according to an embodiment of the invention; and the method will be described with further reference to FIG. 2 schematically illustrating the method performed by a vehicle 200.

(7) The method is applied when a vehicle is travelling on a road 202 and approaching a curve 204 in the road 202. In particular, the method is aimed at determining if and where it is necessary to perform a friction measurement, to thereby determine a maximum allowable vehicle speed in order for the vehicle to be able to travel through the curve 204 without losing grip.

(8) The following notation will be used in the description of the method: v.sub.v=current vehicle speed μ.sub.e=estimated tire-to-road friction v.sub.max_e=estimated maximum allowable vehicle speed d.sub.v=distance from vehicle to curve entrance d.sub.T=threshold distance μ.sub.c=current (measured) tire-to-road friction v.sub.max_d=determined maximum allowable vehicle speed R=curve radius g=gravitational constant d.sub.min_e=estimated minimum braking distance a.sub.max_e=estimated maximum deceleration

(9) The first step of the method involves identifying 100 an upcoming curve 204 and determining 102 properties of the curve. The curve information can for example be acquired from an in-vehicle navigation system or from a remote server as described earlier. A straightforward manner of determining the properties of a curve may be to approximate the curve as having a fixed radius R throughout the curve. However, the curve properties preferably comprise additional information describing a curve shape, a road width, a lane width and a road camber. Moreover, the method can for example be further refined to account for that most roads and vehicle trajectories are shaped as clothoids and not straights and circles. But for the sake of simplicity, the present description use a simple description of the road shape to describe the core ideas of the invention.

(10) Once the curve is identified and curve properties are determined, the current vehicle speed v.sub.v is determined 104 and a friction value μ.sub.e for the curve is estimated 106. The friction value is deliberately underestimated in relation to an expected friction so that the probability that the actual friction is lower than the estimated friction is very low.

(11) When the curve radius R, vehicle speed v.sub.v and estimated friction μ.sub.e is known, an estimated maximum allowable vehicle speed v.sub.max_e can be estimated 108 as
v.sub.max_e=√{square root over (μ.sub.egR)}.

(12) Due to the estimated friction μ.sub.e, assuming that the current vehicle speed v.sub.v is higher than the estimated maximum allowable vehicle speed v.sub.max_e, there is also a limitation on an estimated maximum allowed deceleration a.sub.max_e of the vehicle before the curve. The estimated maximum allowed deceleration a.sub.max_e can be determined as
a.sub.max_e=−μ.sub.eg.

(13) The maximum allowed deceleration a.sub.max_e in turn determines a minimum distance from the curve d.sub.min_e at which braking must be initiated to reach the maximum allowed vehicle speed v.sub.max_e before reaching the curve entrance 206, where the minimum distance from the curve d.sub.min_e is determined as

(14) $d_{\min \_e}=\frac{v_c^2-v_{\min \_e}^2}{2{\mu }_eℊ}.$

(15) Accordingly, if the vehicle speed v.sub.v is higher than the maximum allowable vehicle speed v.sub.max_e, braking of the vehicle must begin at the latest at a distance from the curve entrance corresponding to the estimated minimum distance d.sub.min_e.

(16) Thereby, if the current vehicle speed v.sub.v is higher than the estimated maximum allowable vehicle speed v.sub.max_e, it is determined 112 that a friction measurement is required. The details describing how a friction measurement can be preformed by simultaneously braking on the rear axle and accelerating on the front axle is described in detail in EP3106360A1, hereby incorporated by reference, and will not be discussed in detail in the present disclosure. However, it should be noted that the described method involves measuring the friction without changing the vehicle speed. Accordingly, the braking of the rear axle, which is counteracted by an acceleration of the front axle, does not result in changing the vehicle speed. The cited method may be referred to as a non-braking friction measurement method. This is in contrast to previously known methods involving braking of the vehicle. In addition to the above referenced method, it is also possible to use other non-braking friction measurement methods, such as an optical measurement of the road properties.

(17) Next, if the distance d.sub.v between the vehicle 200 and the curve entrance is higher than a predetermined threshold distance d.sub.T and if a braking action is detected, a friction measurement is performed 116 during the braking action to determine a current friction μ.sub.c. The predetermined threshold distance d.sub.T is set to be higher than the estimated minimum distance d.sub.min_e so that there is sufficient time to perform a friction measurement. The amount by which the threshold distance exceeds the minimum distance d.sub.min_e may for example be based on the current vehicle speed v.sub.v, the estimated friction μ.sub.e and the estimated time/distance required for performing the friction measurement. A braking action of the vehicle sufficiently long for performing a braking friction measurement may for example be in the range of 1-2 seconds.

(18) Accordingly, if the vehicle 200 is at a safe distance form the curve entrance 206 and if the driver brakes the vehicle, a friction measurement is performed during the diver initiated braking. Thereby, the current friction is known and there is no need for an additional friction measurement. If no braking action is detected, the distance from the vehicle 200 to the curve 204 will eventually be less than the threshold distance d.sub.T.

(19) Next, if the current distance d.sub.v between the vehicle 200 and the curve entrance 206 is lower than the predetermined threshold distance d.sub.T, and no friction measurement has been performed since the curve 204 was identified, a friction measurement is performed 118 to determine a current tire-to-road friction μ.sub.c. At minimum, the threshold distance d.sub.T is the estimated minimum braking distance d.sub.min_e plus the distance required for performing the friction measurement.

(20) Based on the determined, i.e. measured, current tire-to-road friction μ.sub.c a maximum allowable vehicle speed v.sub.max_d can be determined 120 based on the curve radius, vehicle speed and current friction as.
v.sub.max_d=√{square root over (μ.sub.cgR)}.

(21) Thereby a more informed determination of the maximum allowable vehicle speed v.sub.max_d is performed and if the current vehicle speed is higher than the determined maximum allowable vehicle speed v.sub.max_d, the vehicle is braked so that the vehicle speed when reaching the curve is lower than or equal to the determined maximum allowable vehicle speed v.sub.max_d. The vehicle 200 may be braked automatically or by driver initiated braking. In a non-automated system, the safety margins are advantageously increased such that the driver both has time to react and to perform the required braking in a safe manner. In other words, the threshold distance d.sub.T is preferably higher in a non-automated system.

(22) The vehicle control system 300 comprises a control unit 208 configured control the vehicle to perform the described steps of the method. The control unit 208 may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The control unit 208 may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the control unit 208 includes a programmable device such as the microprocessor, microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device.

(23) Moreover, the control unit 208 may be embodied by one or more control units, where each control unit may be either a general purpose control unit or a dedicated control unit for performing a specific function.

(24) FIG. 3 schematically illustrates a control system 300 where the control unit is connected to the various sub-systems responsible for performing the various functions of the method.

(25) As illustrated in FIG. 3, the control system 300 comprises an environmental sensor unit 302 which uses e.g. GPS, camera, radar or the like to determine the vehicle position, a motion planner 304 which performs strategic and tactic motion planning, e.g. determines an acceleration/deceleration profile. The control system 300 further comprises a vehicle state estimator 306 which determines the motion states of the vehicle, e.g. vehicle speed over ground and tire-to-road friction, a friction measurement monitor 308 which determines when to start the a friction measurement, a vehicle motion controller 310 which receives motion requests and outputs requests to actuators e.g. powertrain and brakes to execute the motion requests, and finally a friction measurement executer 312 which determines actuation, e.g. wheel torques, to perform a friction measurement.

(26) The present description is based on a simplified model of the curve and it should be noted that it is further possible to modify the described method and system to control the vehicle to have one vehicle speed when entering the curve, and to modify the vehicle speed while in the curve. This may for example be desirable for curves having a complex shape deviating from a simple arc, for long curves or for multiple consecutive curves.

(27) Accordingly, the described method is intended to maximize the probability that the vehicle will be able to stay on the road under the assumption that the friction in the curve is not significantly lower than the friction just before the curve, given the current speed of the vehicle, the shape of the forward roadway and a priori knowledge of the minimum available tire-to-road friction.

(28) Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Also, it should be noted that parts of the method and system may be omitted, interchanged or arranged in various ways, the method and system yet being able to perform the functionality of the present invention.

(29) Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.