DE102012220312A1 - Method for determining adhesion conditions between driven or braked wheel of motor car and road surface in driving slip regulation system, involves determining slip change by measuring change of speed of wheel with respect to force - Google Patents

Abstract

The method involves changing contact force of an accelerated wheel (5) i.e. driven or braked wheel, with frequency of more than 5 Hz. A slip change is determined by measuring change of speed of the wheel with respect to the wheel contact force for determining adhesion conditions. The accelerated wheel is operated with cyclically varying the wheel contact force for determining the adhesion conditions. Torque or rotational speed acting on the driven or braked wheel is reduced or increased if the determined slip change exceeds a threshold value. Independent claims are also included for the following: (1) a control device (2) a motor car (3) a computer program product comprising a set of instructions for executing a method for determining adhesion conditions between accelerated wheel of motor car and road surface (4) a machine-readable medium comprising a set of instructions for executing a method for determining adhesion conditions between accelerated wheel of motor car and road surface.

Description

Field of the invention

The present invention relates to a method for determining adhesion conditions between an accelerated, i. driven or braked wheel of a motor vehicle and a roadway. Moreover, the present invention relates to a method for controlling a moment acting on an accelerated wheel of a motor vehicle. Furthermore, the present application relates to a control unit which is designed to control the proposed method, as well as a motor vehicle equipped with such a control unit and a computer program product, with the aid of which a programmable control unit for implementing the proposed method can be instructed, and with such a computer program product provided computer readable medium.

State of the art

Vehicle dynamics control systems are known for modern motor vehicles, which influence a driving state of a motor vehicle depending on a currently determined slip between a wheel of the motor vehicle and the road via suitable control interventions, for example, the best possible power transmission between the wheel and the road or a stabilization of the driving state of the Reach vehicle. The control actions can be effected, for example, by changing a brake pressure on the wheel or by changing a moment driving the wheel. The control interventions can be performed, for example, by an electronic stabilization program (ESP), a brake slip control (ABS) or by a traction control system (ASR).

In a motor vehicle accelerating, so driving or retarding, wheel must always be a certain slip between the wheel and the road, that is, the wheel must be moved with a slightly higher or lower speed v _wheel , as in a with The speed v _wheel- rotating non-accelerated, that is, freewheeling wheel would be the case. A slip control in an ESP, ABS or ASR can regulate the wheel slip calculated, for example, from (v _Rad - v _Radfrei ) / v _Radfrei . In the case of traction control, the slip should be able to be regulated, for example, in the single-digit percentage range.

In a conventional method of controlling wheel slip, speeds of a free rolling wheel are determined from a speed of the vehicle corrected for the effect of rotational movement of the vehicle along with the vehicle geometry. The rotational movement of the vehicle may be detected by means of a yaw rate sensor or, as the vehicle speed is estimated from the wheel speed signals assuming idle or with defined slip of rolling wheels. It can be seen that the slip and the calculation of the freely rolling wheel speed can influence each other, which can lead to incorrect calculations. In particular, in four-wheel drive vehicles on slippery surfaces such as ice, this can lead to problems because all wheels can accelerate synchronously, without the vehicle is actually faster. A calculated vehicle speed may then be too high or too low, which is also referred to as a "roped-up reference" or "roped-off reference", where the slip controller does not detect slippage and accordingly does not properly control a torque to be transmitted to the wheels.

In order to address the mentioned problems, several different approaches are pursued in current ESP systems. In these approaches, however, it can lead to an impairment of traction or influence on the wheels can be felt by the driver and any noise that can be perceived by the driver as unpleasant. Other approaches suffer from insufficient accuracy to detect mis-calculated vehicle speeds.

In the DE 101 60 049 B4 A conventional system and method for monitoring the driving behavior of a motor vehicle are described.

Disclosure of the invention

Embodiments of the method proposed herein allow for adhesion conditions, and hence slippage, between an accelerated, i. driven or braked to determine wheel of a motor vehicle and a roadway reliably and with high accuracy and thus enable a more accurate control of a force acting on an accelerated wheel of a motor vehicle torque, for example, as part of a caused by a controller slip control in an ESP system for a motor vehicle.

In the proposed method, the Radaufstandskraft an accelerated wheel is varied, that is, a force with which the wheel gets up on a road is selectively changed by a controller temporarily while the wheel is accelerated. The driving or braking torque acting on the wheel may vary during the course of the variation the Radaufstandskraft remain constant or at most slightly and thus negligibly change within the short periods of time to be held. For determining the adhesion conditions, not the slip itself but a slip change is detected by measuring a change in the speed of the wheel in response to the varying wheel contact force.

An idea of the proposed method can be seen in determining the information necessary for a slip control on currently prevailing liability conditions between a driven wheel and a roadway, and thus on the currently prevailing slip on the basis of the gradient of the so-called μ-S curve , The μ-S curve gives the dependence of the claimed frictional connection μ of the slip S again. It is sometimes referred to as a slip curve. The proposed method makes use of the fact that the gradient of the slip curve, in particular near a transition from stable to critical adhesion conditions, usually changes significantly more clearly than the slip itself as a function of the adhesion stress. In particular, in an unstable region where adhesion between the driven wheel and the roadway threatens to drop below a minimum necessary for safe handling, the change in slip may be, for example, a factor of ten to one hundred higher than in a stable range of the slip curve in which grip conditions between the accelerated wheel and the roadway are sufficiently high for safe transmission of longitudinal and lateral accelerations between the motor vehicle and the roadway, with the same wheel contact force change. A distinction between a stable range of the slip curve and an unstable range, that is, between sufficient grip conditions and insufficient grip conditions, based on the gradient of the slip curve, has generally been found to be more robust against changes in, for example, tire characteristics or the road surface than is possible with the slip value itself would. In particular, it has been recognized that although the value of the currently prevailing slip can be determined, it can not be determined in which region of the slip curve a wheel of the motor vehicle is located, since the slip curve itself can be influenced by external factors. In addition, as already noted, the free rolling wheel speed may be incorrectly determined.

The gradient of the slip curve, the inverse gradient of the slip curve, or another gradient-dependent and easily evaluable magnitude of the slip curve can be determined by applying a small, preferably cyclic wheel contact force change to the wheel and observing the response of the wheels as a slip or speed change ,

The frequency with which the wheel contact force is cyclically varied can be greater than 5 Hz, preferably in a range of 10 Hz to 50 Hz. With a sufficiently high-frequency cyclic variation of the wheel contact force, any change in the position of the body of the vehicle caused thereby for a driver is not or barely noticeable.

For example, an amplitude with which the wheel contact force is varied may be more than 50 N and preferably within a range of 100 N to 1000 N. On the one hand, the change in the wheel contact force should be sufficiently high that a resulting change in the speed of the wheel can be determined simply and with sufficient accuracy. On the other hand, the amplitude of the variation of the wheel contact force should be sufficiently small that there is no noticeable transmission of power to the vehicle and thus a reduction in ride comfort for the driver.

In order to be able to control a moment acting on a driven or braked wheel of a motor vehicle, adhesion conditions between the accelerated wheel and a roadway can be determined with the aid of the method described above, and then the moment acting on the accelerated wheel can be regulated as a function of the determined slip change. In particular, the moment acting on the accelerated wheel can be reduced if the determined slip change exceeds a predetermined limit value. Exceeding the predetermined limit value can be regarded as an indication that the wheel to be monitored threatens to lose sufficient adhesion, so that moments transmitted from a drive or a brake of the motor vehicle to the wheel should be reduced at least temporarily, if possible Loss of liability, and possibly a break-out of the motor vehicle to avoid.

The currently prevailing conditions of adhesion between the accelerated wheel and the road surface, that is, the area of the slip curve in which the wheel is currently located, can preferably be determined periodically during the drive of the vehicle and then the moment acting on the accelerated wheel can be regulated , The described control method can thereby be carried out constantly or at least frequently during the driving and can not be used only for the special case of the roped-up reference or abraded reference on ice. A The probability that the range of the slip curve, in which a wheel of the motor vehicle is currently located, is correctly determined in the case of a suspicion of incorrect free wheel speeds, can be many times higher than in the conventional methods described in the introduction. If the proposed method is used continuously, it can also be detected, in particular, whether a wheel is located in the stable region or near the border to the unstable region. In this way, the available coefficient of friction can be determined more accurately, even if the coefficient of friction has not yet been reached.

By means of the proposed method for determining adhesion conditions between an accelerated wheel of a motor vehicle and a roadway or the method for regulating a torque acting on an accelerated wheel of a motor vehicle, several advantages can be achieved. For example, a distinction between a stable and an unstable region of the slip curve does not depend on a correct determination of a freely rolling wheel speed. Moreover, such a distinction between a stable and an unstable range of the slip curve is more independent of tire characteristics, a road surface, and the like than has been the case with conventional methods. The methods proposed herein allow for improved coefficient of friction estimation and avoid significant limitation of the traction of the motor vehicle, as may occur in conventional methods that address the problems of inaccurately estimated vehicle speed. In addition, the proposed methods are associated with a lower noise than in conventional methods, as a change in the wheel contact force, e.g. can be adjusted via a hydraulically or pneumatically operating adaptable suspension in usually barely audible manner for the driver. With suitably chosen frequency and amplitude of the variation of the wheel contact force, the implementation of the proposed method for a driver is not noticeable or at least not disturbing.

The above-described embodiments of methods according to the invention and the functionalities and advantages that can be achieved therewith can be implemented by a control unit provided in a motor vehicle. The control unit can be designed to control signals from a chassis control unit, which can provide information about current settings of the chassis via suitable interfaces, for example, control signals of, for example, an engine control unit, which can provide information about the currently applied to a wheel drive or braking torque , as well as sensors that can provide information about a current speed of the wheel to receive. In addition, the controller may be adapted to output via suitable interfaces control signals, for example to an engine control unit and / or a chassis control unit, in particular to be able to effect the desired variation of the wheel contact force by suitable control of the chassis of the motor vehicle.

A motor vehicle equipped with such a control device allows increased driving safety due to the determination of liability conditions between wheels of the vehicle and a currently traveled roadway which can be carried out continuously and with great reliability.

In this case, the motor vehicle may, for example, have an adaptable chassis, by means of which a wheel contact force of one of the wheels of the vehicle can be varied. In this case, the control unit can be designed to control the chassis for effecting the preferably cyclically varying wheel contact force on one or more wheels of the motor vehicle. For this purpose, for example, a hydraulics or a pneumatics of the adaptable chassis can be suitably timed or modified, so that sets a cyclically varying Radaufstandskraft. Alternatively, for example, spring travel or spring stiffnesses in the chassis can be varied over time or e.g. in the chassis existing stabilizers are rotated to effect the desired varying Radaufstandskraft.

A controller may implement embodiments of the method proposed herein as well as, for example, any informational evaluation of sensor signals in hardware and / or in software. It may be advantageous to program a programmable controller for carrying out the method described above. For this purpose, a computer program product may have computer-readable instructions instructing the programmable controller to perform the steps of the respective method. The computer program product may be stored on a computer readable medium such as a CD, a DVD, a flash memory, a ROM, an EPROM, or the like. In order to be able to correctly control the variation of the wheel contact force to be achieved, in addition to the processing of further sensor data, information stored in a database or in the form of characteristic curves can be used, for example, about the vehicle, its tires, operating conditions, chassis settings, etc.

It is noted that possible features and advantages of embodiments of the invention herein described in part with reference to an inventive method for determining adhesion conditions between wheel and roadway, a method according to the invention for controlling a moment or a wheel speed acting on a wheel, a control device according to the invention or a vehicle equipped with such a control device. A person skilled in the art will recognize that the individual features can be suitably combined or exchanged with one another in order to be able to obtain further embodiments and possibly synergy effects in this way.

Brief description of the drawings

Hereinafter, embodiments of the invention will be described with reference to the accompanying drawings. Neither the description nor the drawings are intended to be construed as limiting the invention.

1 shows a motor vehicle with a control unit for carrying out a method according to an embodiment of the present invention.

2 shows curves to illustrate how a driven wheel can change its speed with variation of the wheel contact force depending on different adhesion conditions.

3 shows a slip curve and the absolute value of an inverse gradient of the slip curve.

The figures are only schematic and not to scale.

Embodiments of the invention

1 shows a motor vehicle 1 with a control unit 3 , which is adapted to the method proposed herein for determining adhesion conditions between a driven wheel 5 of the motor vehicle 1 and a roadway 7 perform and based on the driven wheel 5 acting moment or its speed to regulate. The vehicle 1 has a drive motor 9 on top of a powertrain 11 the wheel to be driven 5 acted upon to drive the motor vehicle. The vehicle 1 also has an adaptable chassis 13 by means of which a wheel-up force, with which a wheel 5 of the vehicle 1 on a roadway 7 weighs, can be influenced. The control unit 3 Can both with the drive motor 9 be connected to control its drive torque or speed, as well as with the chassis 13 be connected to the wheel contact force varying with time, as for determining the adhesion conditions between the wheel 5 and the roadway 7 is needed.

In 2 (I) the slip behavior of an accelerated wheel is shown as a function of the forces acting on the wheel. The y-axis indicates the quotient of the slip change and the change of the used coefficient of friction. The x-axis gives the quotient of drive torque and normal force on the wheel, ie a measure of the friction coefficient used. In a first range (a), the adhesion conditions between the wheel and the road are stable and the slip is small. However, here is a power transmission between the wheel and the roadway, as it is required for accelerating or decelerating the motor vehicle, also low. In a second area (b), the vehicle can be optimally accelerated while holding conditions remain stable and there is almost the nominal slip. If a drive torque is further increased to the vehicle, the driven or braked wheel in a region (c) comes into a so-called slip, in which adhesion conditions can be critical and in the worst case a liability between the wheel and the road is lost and the vehicle is thus lost can slip.

In 2 (II) is the cyclic variation of the wheel contact force 16 shown on the wheel of the motor vehicle. The wheel-upforce 16 changes in time periodically, for example with a sinusoidal course. The amplitude with which the wheel contact force is varied is in 2 (I) with the arrow 15 illustrated.

2 (III) gives the speed changing with time 18 the wheel in response to the varying Radaufstandskraft 16 again. This is a measure of the currently prevailing slip or the effective range of the slip curve. The amplitude with which the speed 18 the wheel changes is in 2 (I) with the arrow 17 illustrated.

As in 2 clearly recognizable causes in the areas (a) and (b), that is, at low slip or near the target slip, a temporal variation of the Radaufstandskraft at most a small corresponding change in the speed of the driven wheel. In the stable range (a), a slip control effected by the control unit can remain passive, the wheel can be used reliably for calculating the vehicle speed and it can be concluded that the available friction coefficient has not yet been reached and the target slip can thus be increased. If the wheel is operated in the region of the setpoint slip, that is to say in the region (b), a desired slip control can be prepared thereon, that to the wheel lower applied acceleration torque. The wheel can only be used conditionally to calculate the vehicle speed. The available coefficient of friction is reached so that it can be accurately determined in this state.

In the area (c) of the slip causes the same variation of the wheel contact force 16 but a very strong corresponding variation of the wheel speed 18 , which can be used as an indication that the bike is under critical liability conditions and threatens to lose liability. Upon detection of such a critical condition, the drive or braking torque acting on the wheel should be lowered. In addition, this wheel should not be used to calculate the vehicle speed and the estimated available friction value should be corrected downwards.

3 illustrates a slip curve 19 and the course of their inverse gradient 21 in absolute values. The x-axis indicates the slip S in this case. The y-axis gives the quotient of a slip change Δμ and a change of the traction load Δ (F / FN) for the slip curve 19 at.

In order to ensure sufficient driving safety, the aim is always to operate the vehicle within the stable range (a), in which the slip curve is largely linear, or in the range (b) close to the nominal slip. An operation in the area (c) of the slip should be avoided as far as possible.

As in 3 However, it is precisely in the region at the transition between the nominal slip region (b) and the critical region (c) that the slip curve is clearly visible 19 relatively flat. In this boundary region, the gradient or the inverse values of the gradient changes 21 The slip curve is much clearer than the slip itself as a function of traction. The use herein of this gradient for determining adhesion conditions between a driven wheel and the roadway thus makes it much more accurate than conventional methods to detect whether the wheel is already operating at the limit of critical slip and thus a further increase of the drive or braking torque should be avoided in the present driving situation. In other words, a slip variation caused by a varying wheel contact force, which greatly increases in the transition to driving situations with imminent loss of liability, can be used as a control variable for regulating the driving or braking torque for the vehicle.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10160049 B4 [0006]

Claims (12)

Method for determining adhesion conditions between an accelerated wheel ( 5 ) of a motor vehicle ( 1 ) and a roadway ( 7 ), characterized in that a wheel contact force ( 16 ) of the accelerated wheel ( 5 ) and to determine the adhesion conditions a slip change by measuring a change in a velocity ( 18 ) of the wheel ( 5 ) in response to the varying wheel contact force ( 16 ) is determined. Method according to claim 1, wherein the accelerated wheel ( 5 ) for determining the adhesion conditions with a cyclically varying wheel contact force ( 16 ) is operated. Method according to claim 2, wherein the wheel contact force ( 16 ) is varied with a frequency of more than 5 Hz. Method according to one of claims 1 to 3, wherein the wheel contact force ( 16 ) is varied by more than 50 N. Method of regulating an accelerated wheel ( 5 ) of a motor vehicle ( 1 ) or its rotational speed, characterized in that the adhesion conditions between the accelerated wheel ( 5 ) and a roadway ( 7 ) are determined by a method according to one of claims 1 to 4 and that on the accelerated wheel ( 5 ) acting moment is regulated in dependence on the determined slip change. Method according to claim 5, wherein the said wheel is driven on the driven or braked wheel ( 5 ) acting moment or its speed is reduced or increased when the determined slip change exceeds a predetermined limit. A method according to claim 5 or 6, wherein determining the conditions of adhesion and controlling the accelerated wheel ( 5 ) acting moments or its speed can be performed periodically repeated. Control unit ( 3 ) for a motor vehicle ( 1 ), whereby the control unit ( 3 ) is adapted to control a method according to one of claims 1 to 7. Motor vehicle ( 1 ) with a control device ( 3 ) according to claim 8. Motor vehicle according to claim 9, wherein the motor vehicle ( 1 ) has an adaptable chassis, by means of which a wheel contact force of one of the wheels of the vehicle can be varied, and wherein the control unit ( 3 ) is adapted to the adaptable suspension to effect a varying Radaufstandskraft ( 16 ) on a wheel ( 5 ) of the motor vehicle ( 1 ) head for. Computer program product comprising computer readable instructions comprising a programmable controller ( 3 ) to perform a method according to any one of claims 1 to 7. A computer readable medium having a computer program product stored thereon according to claim 11.

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