Method and system for determining an optimal steering angle in understeer situations in a vehicle

A method for determining an optimal steering angle in understeer situations of a vehicle is described. To assist a driver in reliably stabilizing the vehicle during an understeer situation while driving, a model-based driving traction coefficient factor, a model-based kinematic factor, and a float angle are taken into account in the determination of a steering angle. A limited steering angle δv,lim at which a maximum lateral force is set, is determined by addition of the driving traction coefficient factor, the kinematic factor, and the float angle. A system suitable for implementation of the method is also described.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase application of PCT International Application No. PCT/EP2007/054125, filed Apr. 26, 2007, which claims priority to German Patent Application No. DE102006020279.1, filed Apr. 27, 2006 and German Patent Application No. DE102007020169.0, filed Apr. 26, 2007, the contents of such applications being incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device and method for determining an optimum steering angle in understeer situations of a vehicle.

2. Description of the Related Art

Modern vehicles use electronically controllable motors in the steering train in order, on the one hand, to selectively influence the steering torque to be applied by the driver (power steering systems) and, on the other hand, to selectively set steering angles independently of the driver (superimposition steering systems). In addition to these steering systems which act on the front axle of the vehicle, modern chassis control systems, for example global chassis control (GCC) also use rear axle steering systems for controlling the vehicle dynamics.

In order to influence the steering torque which is to be applied by the driver, various closed-loop and open-loop control structures which are respectively aimed at the specific driving situation are known. For example in the case of oversteering driving situations, closed-loop control on the basis of a yaw rate reference is used (WO 2005/054039 A1) and in the case of braking on μ split open-loop control on the basis of ABS wheel information is used (WO 2005/054040 A1). In the case of understeering driving situations, the steering angle which is present when the situation is detected is “frozen”, i.e. by means of a torque control is to be recommended to the driver that he should not increase the steering angle and as a result make the situation worse. A disadvantage with this concept is that the driver is not provided with any feedback about the maximum possible lateral force.

It would therefore be desirable if the driver could be assisted in such a way that he can set a maximum lateral force at the wheels.

DE 10 2005 036 708 A1 discloses stabilizing means which actuate the steering means as a function of a lateral force coefficient of at least one of the steered wheels in order to set a steering angle which stabilizes the vehicle, in which case the stabilizing means set a slip angle of the steered wheels in such a way that the lateral force coefficient does not substantially exceed the region of the maximum.

SUMMARY OF THE INVENTION

The invention relates to improving a method of the type mentioned previously in such a way that during an understeering driving situation the driver is reliably assisted in stabilizing the vehicle.

The invention makes available a method for determining an optimum steering angle in understeering situations of a vehicle, in which method a first portion which represents the adhesion coefficient in the lateral direction is taken into account in the determination, in which method a second portion which represents a kinematic portion is taken into account, and in which method a third portion which represents the attitude angle is taken into account, and in which method the steering angle δv,limis determined by adding the portion of the adhesion coefficient, the kinematic portion and the attitude angle.

The kinematic portion comprises the proportional velocities from the rotation of the vehicle referred to the velocity of the center of gravity.

The system for controlling electronically controllable motors in the steering train advantageously permits the driver to set the maximum lateral force value in understeering situation by means of power steering. This assistance during steering allows the vehicle to be stabilized in critical driving situations. All wheel steering systems are taken into account here.

During the understeering, the attitude angle can advantageously be estimated according to the relationship β≈0, since the attitude angle is approximately zero at the start of the understeering driving situation.

The coefficient of friction of the underlying surface is advantageously determined at the axles and the center of gravity of the vehicle. The coefficient of friction {circumflex over (μ)}0=max(μVA,μCoG,μHA) of the underlying surface is determined according to at least one of the relationships,

utilizing the adhesion for the front axle

µVA=(ax-lV⁢ψ.2)2+(ay+lV⁢ψ¨)2g,
or
utilizing the adhesion at the center of gravity of the vehicle

µCoG=ax2+ay2g,
or
using the adhesion at the rear axle

The optimum steering angle is advantageously calculated in a model in which the steering angle is determined in terms of absolute value according to the relationship

The steering angle δv,limor a steering angle δv,limwhich is multiplied by a factor k is used as a setpoint value for a steering angle control means or a steering torque control means.

Furthermore, there is advantageously provision that a steering torque control means is activated according to the relationship δv,lim<|δv|

or deactivated according to the relationship δv,lim<|δv|.

Furthermore, the invention makes available an advantageous device for carrying out the method according to aspects of the invention.

The device for determining an optimum steering angle in understeering situations of a vehicle is based on a determining unit for determining a stabilizing steering angle taking into account a model-based portion of the adhesion coefficient, a model-based kinematic portion and an attitude angle.

These and other aspects of the invention are illustrated in detail by way of the embodiments and are described with respect to the embodiments in the following, making reference to the Figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The description is based on a two-axle, four-wheel motor vehicle having steerable wheels on at least one front axle10and, if appropriate, also on a rear axle12.FIG. 1is a schematic illustration of a vehicle having a steering actuator. A steering wheel20which is attached to a steering column18is connected via a steering gear22to the steered wheels24,26of the vehicle. The steering gear22is preferably embodied as a toothed rack steering system which has a pinion (not illustrated), which is connected to the steering column in a rotationally fixed fashion. A torque sensor14, which determines the driver's steering request here by means of a manual steering toque MHis arranged on the steering column. An electrical EPS servomotor16(EPS=Electric Power Steering) applies an additional steering torque MDSRto the steering train during conventional operation, which steering torque MDSRincreases the steering torque MHapplied by the driver.

In order to set an additional steering torque request MDSR(DSR=Driver Steering Recommendation) to assist the driver, the electric power steering system is used, said electric power steering system being actuated here by a, for example, GCC controller28(GCC=Global Chassis Control), for example via an interface with the CAN bus of the vehicle. The steering wheel angle δLwhich is set by the driver and the rear axle steering angle δH, which are measured with steering angle sensors30,32which are arranged on the steering column18and on the rear axle12, and the manual steering torque MHwhich is determined by the torque sensor14are made available to the controller28as input variables. Furthermore, the controller28is provided with additional variables from the vehicle dynamics controllers and/or driver assistance controllers, as described in more detail in the applications mentioned previously. The controller28determines the additional steering torque MDSRby means of the information which is made available. The EPS servomotor16serves here as an actuator which applies the steering torque MDSR(DSR=Driver Steering Recommendation) to the steering train in correlation with the manual steering torque MHvia the transmission34. Furthermore, the controller28calculates a rear axle steering angle δH,soll, which is transmitted to the rear axle via a rear axle steering unit36.

However, in a similar way, the invention can also be used in vehicles with other steering systems such as, for example, steering systems with hydraulic power steering with an external torque interface (for example APS, Active Power Steering) or a separate torque actuator (for example IPAS, Intelligent Power Assisted Steering).

FIG. 2shows a power steering system with two steering actuators. Identical components and identical blocks have the same reference symbols here. In addition, compared to the embodiment according toFIG. 1, a superimposition transmission40is arranged on the steering column18. The superimposition transmission is generally embodied as a planetary gear mechanism and divides the steering column into two sections18aand18b. By means of the superimposition transmission40it is possible to superimpose a further steering angle on the steering wheel angle δLmeasured by the steering wheel angle sensor32. The composite steering angle δvis measured by the steering angle sensor42which is arranged on the section18bof the steering column. The superimposition transmission40is driven by a steering wheel motor44. The steering wheel motor44is controlled by the controller28whose reference variable is the correction steering wheel angle Δδsoll. For this purpose, the steering angle δvwhich is measured by the front axle steering wheel sensor42is made available to the controller28. As in the control system described inFIG. 1, the controller28is provided with further variables from vehicle dynamics controllers and/or driving assistance controllers.

The wheel steering angle of the front axle δvis included in the additional steering torque MDSRaccording to the relationship

δV=δLiL(2.1)
where iLis the steering transmission ratio. The steering transmission ratio is constant or, in the case of a superimposition steering system, it can also depend on further variables such as, for example, the velocity of the vehicle.

In the case of a steering angle control, the wheel steering angle of the front axle is measured directly.

In the case of the servomotors16which are illustrated inFIGS. 1 and 2, the servomotor is preferably required to receive a setpoint steering torque from the GCC controller and to control it independently in the manner of an “intelligent actuator”. The current manual steering torques MHare acquired by the torque sensor14and fed back to the GCC controller28. The torque sensor14is optional, an IPAS does not contain a torque sensor. The presence of a rear axle steering system is not absolutely necessary for the method. However, the further statements assume that the vehicle is equipped with a rear axle steering unit (for example ARK, Active Rear Axle Kinematics). The method for calculating the maximum steering angle is also suitable for pure superimposition steering as perFIG. 2, in order to apply this value independently of the value specified by the driver.

The components and interfaces of the GCC controller28are represented inFIG. 3. Only the portions which relate to the steering are illustrated. Controller portions for other actuators such as, for example a brake, internal combustion engine, stabilizer etc. are not illustrated. The steering angle controller50and the steering torque controller52are either alternatively present or are present together for steering systems as illustrated inFIG. 2. The steering angle controller50generates steering angle setpoint values Δδsoll, δH,sollfor the front axle10and the rear axle12. The steering torque controller52generates the additional steering torque MDSRwhich, as a driver steering recommendation (DSR, Driver Steering Recommendation), constitutes a haptic feedback for the driver. The following variables are made available as input variables to the steering wheel controller50and the steering torque controller52:axlongitudinal acceleration, measured with a longitudinal acceleration sensor or estimated from wheel speed signalspBbrake pressure, measured with a pressure sensor (1× driver) or estimated at the wheel brakes of the respective wheels24,26or in a model for the four wheel brakes of the wheels24,26dψ/dt yaw rateaylateral accelerationvxvelocity of vehicle, estimated from wheel speed signalsδLsteering wheel angleδvwheel steering angle front axleδHwheel steering angle rear axle

In addition, the driver manual torque MHwhich is determined by the torque sensor14is also supplied as an input variable to the steering torque controller52. If the steering angle controller50is also present, the change in setpoint in the wheel steering angle Δδsollis additionally fed as an input variable to the steering torque controller52.

An exemplary embodiment of the steering torque controller52in understeering situations is illustrated inFIG. 4. An exemplary embodiment of the steering angle controller50in understeering situations is shown byFIG. 5.

Both controller50,52have the following basic design of the steering train control system for determining the steering torque request MDSRor the steering angle request Δδsoll. Driving situations in which an understeering driving state of the vehicle is present are detected in the blocks60and62. Said blocks make use, in particular, of information which is made available by a vehicle dynamics controller. The driving state controller can be, for example, an ESP system and/or an ABS system. Critical driving situations in which the vehicle understeers are preferably detected in the block60by means of an ESP understeering detection means. As an alternative, understeering of the vehicle is detected in the block62by means of a slip angle understeering detection means.

The detection of an understeering situation is carried out here at both controllers50,52according to two alternatives. An understeering detection means which is expanded with the rear axle steering portion and consists in the ESP uses the linear steady-state single-track model.

The model (3.1) supplies a reference for the front axle steering angle in the form

Understeering is detected if the difference
|δV|−|δV,ref|>Sδ(3.3)
exceeds a predefined threshold value Sδ. The second possible way of detecting understeering is based on the slip angle at the front axle.

αV=-δV+β+lVvx⁢ψ..(3.4)
and the slip axle at the rear axle, cf.FIG. 7,

The detection requires not only the individual slip angles but also the difference

Understeering is detected as a function of a threshold value for the difference (3.6) between the slip angle and the sign of the yaw rate if the following is true
dψ/dt>0 and Δα<−Sα
or
dψ/dt<0 and Δα>Sα

The threshold value Sαis between 2 and 10 degrees, and is preferably 5 degrees. If an understeering situation is detected in one of the blocks60,62from the upward transgression of the threshold values Sδor Sα, the understeer flag64, which is the output signal of the block60or62, is set to the value 1. The understeer flag is reset from the value 1 to the value 0 if the aforesaid conditions are no longer met. However, relatively small threshold values are preferably used as the basis so that the control is steadied by a hysteresis.

The threshold values may be dependent on further variables of the vehicle dynamics such as, for example, the velocity vxof the vehicle or the coefficient of friction μ of the underlying surface. As the velocity decreases, the threshold values are increased, and as the coefficient of friction of the underlying surface decreases they are correspondingly decreased.

The blocks60,62are connected via an OR element66to an activation logic68for activating the control system. The wheel steering angle δvof the front axle, the limited wheel steering angle δv,limof the front axle, the determination of which will be described later, and the understeer flag64are input into the activation logic68as the input signal.

If the following conditions are met
δV,lim<|δV|and understeer flag=1
the steering torque control52is activated by an understeer active flag, which represents the output signal of the activation logic68, being set to the value 1.

The torque control52is terminated and the output signal understeer active flag of the activation logic68is set to 0 if the following conditions apply:

or understeer flag=0
or after termination conditions which provide for a termination after a predetermined time has expired.

Each of the controllers50,52contains a determining unit70for limiting the steering angle, to which determining unit70the yaw rate dΨ/dt, the longitudinal acceleration ax, the lateral acceleration ay, and the velocity vx, of the vehicle are added as input variables.

The limitation of the steering angle serves to determine a limitation of the wheel steering angle at the front axle. For this purpose the following polynomial model of the lateral force is used

If the lateral force Fyis related to the vertical force Fz, the characteristic curve of the adhesion coefficient in the lateral direction which is illustrated inFIG. 8is obtained from the model (3.7). The adhesion coefficient reaches its maximum value at the slip angle

The steering angle corresponding to the maximum adhesion coefficient can be determined, from (3.4) using (3.8), as

The coefficient of friction μ0of the underlying surface and the attitude angle β cannot be acquired economically in terms of measuring technology in the vehicle. In the case of understeering, the following applies approximately to the attitude angle
β≈0.  (3.10)

An estimation of the coefficient of friction of the underlying surface on the basis of the accelerations of the center of gravity of the vehicle (CoG Center of Gravity) or of the front and rear axles yields
{circumflex over (μ)}0=max(μVA,μCoG,μHA),  (3.11)
using the adhesion for the front axle

μV⁢⁢A=(ax-lV⁢ψ.2)2+(ay+lv⁢ψ¨)2g,(3.12)
using the adhesion at the center of gravity of the vehicle

μCoG=ax2+ay2g(3.13)
and using the adhesion at the rear axle

Taking into account the relationship of the signs of the slip angle and lateral acceleration
sign(αlim)=−sign(ay)  (3.15)
the aimed-at limitation of the wheel steering angle at the front axle is obtained, in terms of absolute value, as

The parameter C∞may be dependent on the coefficient of friction of the underlying surface and has to be applied in the driving trial.

A small attitude angle is assumed for the calculation of the limitation of the steering angle according to (3.10). It has to be assumed that the vehicle initially veers in and the attitude angle therefore increases. For this reason, the limitation should be performed only for a certain time (preferably 4 s). In a haptic system, the increase in the steering torque should then be cancelled. In a superimposition steering system, the additional steering angle is reduced again after this time.

The limited wheel steering angle δv,limwhich is calculated in accordance with 3.16 is made available to the activation logic68which activates or terminates the steering torque control52on the basis of the previously described conditions.

In order to control the steering torque or steering angle, the current wheel steering angle δvof the front axle passes with reversed sign through a transmission element72with a dead zone. The dead zone is defined between the positive and negative values of the current limitation for the wheel steering angle (3.16). The output variable of the dead zone transmission element72is the control error

Within the dead zone, the control error is zero, and outside it said value is the value of the wheel steering angle δvwhich is reduced by the value of the limitation. The control error eδis fed to a controller74. The controller74can be embodied as a simple P controller or as a dynamic controller. If a superimposition steering system (FIG. 5) is present, the set point change Δδsollin the wheel steering angle cannot be used as a pilot control in the sense of applying different variables for the steering torque control of the steering torque controller52, in accordance with the illustration inFIG. 6. The controller output variable uMor uΔδis, if appropriate, restricted in terms of its value and its increase by the limiting element76. The parameters of the controller74and of the limiting element76should be set as a function of the vehicle. A limitation taking into account the current manual torque of the driver is also possible.

While preferred embodiments of the invention have been described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. It is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.

LIST OF REFERENCE SYMBOLS

axLongitudinal acceleration, if appropriate estimated from wheel speed signalspBBrake pressure, 1× driver, 4× wheels, if appropriate estimated dΨ/dt, Ψ Yaw rateayLateral accelerationvxVelocity of vehicle, estimated from wheel speed signalsδLSteering wheel angleδvWheel steering angle, front axleδv,limLimitation of wheel steering angle, front axleδv,refReference value wheel steering angle, front axleΔδsollSetpoint wheel steering angle change, front axleδHWheel steering angle, rear axleδH,sollSetpoint wheel steering angle, rear axleMHDriver's manual torque on steering wheelMDSRSetpoint steering torqueβ Attitude angleα Slip angleαlimSlip angle at maximum angle of lateral force or maximum value of adhesion coefficientΔα Difference in slip angle between front axle and rear axle αv-αHFyLateral forceFzVertical forceμ0Coefficient of friction of underlying surfaceμyAdhesion coefficient in lateral directionμy,maxMaximum value of cohesion of adhesion coefficientCα0Initial rise in adhesion coefficient/slip angle curveEG Intrinsic steering gradientI Wheel baseIvDistance between center of gravity of vehicle and front axleIHDistance between center of gravity of vehicle and rear axle
eδControl error