Abstract:
A system and method for stabilizing a motor vehicle. The system includes a speed sensor for sensing a longitudinal speed and a transversal speed of the motor vehicle. The system also includes a calculation unit designed to calculate an attitude angle of the motor vehicle from the longitudinal speed and the transversal speed. The system also includes an evaluation unit designed to compare the attitude angle with a threshold value and to detect an oversteering situation if the attitude angle exceeds the threshold value. The system also includes an actuation unit which influences the driving behavior of the motor vehicle. The actuation unit is designed to actuate an actuator in an oversteering situation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase application of PCT International Application No. PCT/EP2008/060771, filed Aug. 15, 2008, which claims priority to German Patent Application No. 10 2007 038 850.2, filed Aug. 16, 2007 and German Patent Application No. 10 2008 038 642.1, filed Aug. 12, 2008, the contents of such applications being incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to a system for stabilizing a motor vehicle. In addition, the invention relates to a method for stabilizing a motor vehicle which the system is suitable for carrying out. 
     BACKGROUND OF THE INVENTION 
     Vehicle movement dynamics control systems such as the known ESP system, which is described for example in DE 195 15 051 A1, which is incorporated by reference, serve to stabilize a motor vehicle in critical travel situations. 
     Such systems are usually based on yaw rate control. In this context, the current yaw rate of the vehicle is sensed by means of a yaw rate sensor and compared with a setpoint yaw rate. The setpoint yaw rate is acquired by means of a vehicle model using the vehicle speed and the wheel lock angle set by the driver at the steerable wheels of the vehicle. The vehicle speed and the wheel lock angle are sensed with appropriate sensors. If the difference between the setpoint yaw rate and the measured yaw rate exceeds a threshold value in terms of absolute value, an unstable driving state is detected and the vehicle is stabilized by interventions into the driving behavior. 
     The stabilizing interventions into the driving behavior comprise braking interventions at individual wheels of the vehicle. In vehicles with hydraulic brake systems, a hydraulic unit which is integrated into the brake system and which permits brake pressure to be built up on a wheel-specific basis is used to carry out the braking interventions. Depending on whether an oversteering or understeering vehicle behavior has been detected, the braking intervention takes place at a front wheel or a rear wheel if the vehicle is a four-wheeled vehicle with two axles. 
     The described vehicle movement dynamics control system is relatively costly owing to the required sensor system. In addition, the expenditure on the adaptation of controller parameters such as, for example, of the yaw rate threshold value and the determination of the vehicle model of individual vehicles or vehicle types entails relatively high costs. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to make available a simpler vehicle movement dynamics control system with which a vehicle can be stabilized in critical driving situations, in particular in oversteering situations. 
     According to a first aspect of the invention, a system for stabilizing a motor vehicle is proposed. The system comprises
         at least one speed sensor for sensing a longitudinal speed and a transversal speed of the motor vehicle;   a calculation unit which is designed to calculate an attitude angle of the motor vehicle from the sensed longitudinal speed and the sensed transversal speed;   an evaluation unit which is coupled to the calculation unit and is designed to compare the attitude angle with a predefined threshold value and to detect an oversteering situation if the attitude angle exceeds the threshold value;   an actuation unit which is coupled to the evaluation unit, and an actuator which influences the driving behavior of the motor vehicle, wherein the actuation unit is designed to actuate the actuator in an oversteering situation.       

     According to a second aspect of the invention, a method is proposed which comprises the following steps:
         sensing of a longitudinal speed and of a transversal speed of the motor vehicle by means of at least one speed sensor;   calculation of an attitude angle of the motor vehicle from the sensed longitudinal speed and the sensed transversal speed;   observation of the attitude angle and detection of an oversteering situation if the attitude angle exceeds a threshold value; and,   if an oversteering situation has been detected, actuation of an actuator in order to influence the driving behavior of the motor vehicle.       

     One advantage of the invention is that the attitude angle of the motor vehicle is used as a control variable instead of the yaw rate. There is therefore no longer any need for a yaw rate sensor. Instead, the transversal speed of the vehicle is sensed, for which purpose a sensor which is of simple design and inexpensive can be used. In addition, it has been found that the attitude angle threshold value can be adapted at lower cost than the adaptation of the parameters of a yaw rate control system together with a vehicle model. 
     Furthermore, with the proposed vehicle movement dynamics control system it is not necessary to sense the wheel lock angle, as a result of which the corresponding sensor can be dispensed with. This provides further simplification and reduces the susceptibility to faults owing to the smaller number of components. In addition, the system is more cost-effective. 
     In one embodiment of the system and of the method, an optical speed sensor is provided for sensing the vehicle transversal speed and/or the vehicle longitudinal speed and is designed to sense and evaluate light which is reflected at a roadway surface on which the motor vehicle is moving during operation, in order to determine the vehicle transversal speed and/or the vehicle longitudinal speed. 
     The advantage of an optical sensor is, in particular, that no mechanical components which have to be mounted and adjusted in a specific way are required in order to permit a precise measurement. The manufacture of the sensor is therefore basically simpler and more cost-effective than in the case of a mechanical sensor. 
     The optical speed sensor can, for example, sense images of the roadway surface and acquire the longitudinal speed and/or transversal speed of the vehicle from a displacement of structures within successively recorded images. 
     The sensor can equally well be a sensor with a light source which emits a coherent measuring light beam, in particular a laser beam, in the direction of the roadway surface, which light beam is reflected by the roadway surface and interferes with the light in the light source. The interference can be detected by means of an interference detector, and the transversal speed or the longitudinal speed of the vehicle can be acquired on the basis of a measurement variable sensed by means of the interference detector, given suitable orientation of the measuring light beam. Using two measuring light beams permits the vehicle transversal speed and the vehicle longitudinal speed to be sensed. 
     The location of the interference can be in particular the light source whose operating state is changed on the basis of the interference, which is also referred to as a self-mixing effect. The changes in the operating state can be sensed with the interference detector. 
     In a further refinement of the system and of the method, the speed sensor comprises a position-determining system, and the vehicle transversal speed and/or the vehicle longitudinal speed are/is determined by tracking a position of the motor vehicle by means of the position-determining system. 
     Such a speed sensor also has the advantage that there is no need for a particular support or adjustment of mechanical components in order to be able to carry out precise measurements. 
     A development of the system and of the method comprises the actuator being designed to apply a braking force to a front wheel of the vehicle on the basis of the actuation by the actuation unit in an oversteering situation. As a result, the vehicle can be effectively stabilized in an unstable driving situation. 
     One embodiment of the system and of the method is defined by the fact that the vehicle transversal speed and the vehicle longitudinal speed are the only vehicle movement dynamics variables which can be sensed. 
     In particular, in this embodiment the yaw rate of the vehicle or the wheel lock of the steerable wheels is not sensed. The sensing of further vehicle movement dynamics variables, such as for example the transversal acceleration, is also dispensed with. As a result, only a small number of sensors is required, so that the proposed vehicle movement dynamics control system is particularly simple and cost-effective. 
     A vehicle movement dynamics variable is understood here to be a variable which characterizes the driving state of the vehicle, in particular a driving state variable. Pure operating data of the motor vehicle, such as for example a brake pressure which is set in a hydraulic brake system of the motor vehicle, are not considered to be vehicle movement dynamics variables in this sense. 
     In order to expand the field of use of the vehicle movement dynamics control system, in one embodiment of the system and of the method a steering angle sensor is provided for sensing a wheel lock angle which is set at steerable wheels of the vehicle, and an evaluation unit is provided which is designed to detect an understeering situation on the basis of the wheel lock angle and a simple vehicle model. In this embodiment, instabilities owing to understeering can also be detected as well as the instabilities which arise owing to oversteering. 
     A connected embodiment of the system and of the method is defined by the fact that the evaluation unit is designed to detect an understeering situation if the wheel lock angle is larger than the theoretically possible wheel lock angle (f(ν)). 
     In order to be able to stabilize the vehicle in understeering situations, one development of the system and of the method comprises the actuation unit being designed to act on a drive engine of the motor vehicle in an understeering situation in such a way that a speed and/or acceleration of the motor vehicle are/is reduced. 
     The vehicle may be, for example, a four-wheeled motor vehicle. In oversteering situations, such a vehicle is expediently stabilized by virtue of the fact that a front wheel, preferably the front wheel on the outside of a bend, is braked. In an understeering situation, such a vehicle can be stabilized by braking a rear wheel, in particular the rear wheel on the inside of a bend. One embodiment of the system and of the method therefore provides that the actuator is designed to apply a braking force to a rear wheel of the motor vehicle on the basis of actuation by the actuation unit in an understeering situation. 
     As previously mentioned, in an understeering situation the vehicle can also be stabilized by reducing the drive torque which is made available by the drive engine. Braking interventions are therefore not absolutely necessary at the rear wheels even if the vehicle movement dynamics control extends to understeering situations. If the vehicle movement dynamics control remains limited to oversteering situations, braking interventions at the rear wheels are not necessary either. 
     One development of the system and of the method therefore comprises the actuator being configured in such a way that it can apply a braking force exclusively to the front wheels of the motor vehicle. 
     As a result, the actuator can advantageously be configured more simply and cost-effectively than an actuator with which a braking force can also be built at the rear wheels. 
     The abovementioned advantages, particularities and expedient developments of the invention as well as further advantages, particularities and expedient developments of the invention will also become clear on the basis of the exemplary embodiments which are described below with reference to the Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Of the Figures: 
         FIG. 1  is a schematic illustration of a motor vehicle which comprises a system for carrying out vehicle movement dynamics control, 
         FIG. 2  is a schematic illustration of a subassembly of a hydraulic unit of the motor vehicle, 
         FIG. 3  is a schematic illustration of a laser unit for sensing the vehicle longitudinal speed and/or vehicle transversal speed, 
         FIG. 4  is a schematic illustration of a block diagram of the control unit of the vehicle movement dynamics control system in a first embodiment, and 
         FIG. 5  is a schematic illustration of a block diagram of the control unit of the vehicle movement dynamics control system in a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic illustration of a four-wheeled motor vehicle  101  with two front wheels  102   a ,  102   b  and two rear wheels  102   c ,  102   d.    
     The front wheels  102   a ,  102  are steerable and the wheel lock angle can be set by the driver by means of a steering handle  112  which is connected to the front wheels  102   a ,  102   b  via a steering train  113 . 
     The vehicle  101  is driven by a drive engine  103 , which may be, for example, an internal combustion engine, an electric motor or a hybrid motor. The drive engine  103  generates an engine torque which is transmitted via a drive train (not illustrated in  FIG. 1 ) to two or four vehicle wheels  102   a ,  102   b ,  102   c ,  102   d , in order to drive these wheels  102   a ,  102   b ,  102   c ,  102   d.    
     In order to brake the vehicle  101 , a brake system is provided which is configured as a hydraulic brake system in the illustration. The brake system comprises an activation device  113  which is connected to a master brake cylinder  106  via a brake booster  105  and is preferably embodied as a brake pedal. The master brake cylinder  106  is connected in terms of flow to a reservoir vessel  107  for hydraulic fluid and is preferably embodied as what is referred to as a tandem master cylinder in which in each case one of two coupled pressure chambers is provided for supplying pressure to a brake circuit which comprises two of the four wheel brakes  108   a ,  108   b ,  108   c ,  108   d . The wheel brakes  108   a ,  108   b ,  108   c ,  108   d  are each assigned to a wheel  102   a ,  102   b ,  102   c ,  102   d . The brake circuits have, for example, an X distribution in which a pressure chamber of the master cylinder  106  supplies the front right-hand wheel brake  108   b  and the rear left-hand wheel brake  108   c , and the other pressure chamber supplies the front left-hand wheel brake  108   a  and the rear right-hand wheel brake  108   b.    
     The activation device  104  can be used to build up a brake pressure, via the brake booster  105 , in a hydraulic fluid which is located in the master brake cylinder  106 , which brake pressure can be transmitted via pressure lines to the wheel brakes  102   a ,  102   b ,  102   c ,  102   d . Owing to the application of pressure and a wheel brake  108   a ,  108   b ,  108   c ,  108   d , a braking torque is applied to the associated wheel  102   a ,  102   b ,  102   c ,  102   d , and the latter is therefore braked. 
     The master brake cylinder  106  is connected to the wheel brakes  108   a ,  108   b ,  108   c ,  108   d  via a hydraulic unit  109 . The hydraulic unit  109  serves as an actuator for influencing the driving behavior of the vehicle  101  and is actuated by means of a control unit (ECU)  110 . Vehicle movement dynamics control, and preferably also brake slip control (ABS—anti-lock brake system), are carried out by means of the control unit  110 . The vehicle movement dynamics control which is provided will be explained below in more detail. The brake slip control is carried out in a manner which is known per se to a person skilled in the art and will therefore not be described in more detail. 
     The control unit  110  is preferably a control device with a microprocessor for executing programs whose program code can be stored in the control unit  110 . The vehicle movement dynamics control system and the brake slip control system comprise, in this embodiment of the control device, software which is executed on the microprocessor of the control device. The algorithms which are provided are processed in a looped fashion here, with one loop being run through once in each clocking step in order to generate an output signal, if appropriate. 
     In one embodiment, the hydraulic unit  109  is divided into two subassemblies which are constructed in the same way and are each assigned to two wheel brakes  108   a ,  108   b ,  108   c ,  108   d . The wheel brakes  108   a ,  108   b ,  108   c ,  108   d , which are assigned to a subassembly, can be the wheel brakes  108   a ,  108   b ,  108   c ,  108   d  of a brake circuit, for example. 
     In  FIG. 2 , the hydraulic unit is illustrated in a schematic circuit diagram with one of the subassemblies which is assigned, by way of example, to the front right-hand wheel brake  108   b  and to the rear left-hand wheel brake  108   c . The wheel brakes  108   b ,  108   c  are connected inside the subassembly to the master brake cylinder  106  via an isolating valve  201  which is open in the currentless state. In addition, in each case an inlet valve  202   b ,  202   c , which is also opened in the currentless state, is assigned to the wheel brakes  108   b ,  108   c . During a braking process which is controlled exclusively by the driver, a brake pressure which is built up in the master brake cylinder  106 , which is also referred to as an admission pressure, is transmitted directly to the wheel brakes  108   b ,  108   c.    
     The hydraulic unit  109  also comprises a hydraulic pump  203  which is driven by an engine  204 , which can also be provided for driving the hydraulic pump of the subassembly (not illustrated) of the hydraulic unit  109 . By means of the hydraulic pump  203 , a brake pressure can be applied to the wheel brakes  108   b ,  108   c  independently of the driver. In order to build up pressure, the isolating valve  201  is closed and the switchover valve  205  which is closed in the currentless state is opened, with the result that a connection is produced between the master brake cylinder  106  and the intake side of the hydraulic pump  203 , and the master brake cylinder  106  is decoupled from the pressure side of the hydraulic pump  203 . The hydraulic pump  203  is therefore able to feed hydraulic fluid from the master brake cylinder  106  or the reservoir vessel  107  into the wheel brakes  108   a ,  108   b  and therefore increase the brake pressure in the wheel brakes  108   a ,  108   b . This serves in the vehicle movement dynamics control system provided to apply a braking force to one wheel  102   a ,  102   b ,  102   c ,  102   d  in each case, in order to stabilize the vehicle. 
     In order to be able to reduce the brake pressure in the wheel brakes  108   b ,  108   c  counter to an existing admission pressure, an outlet valve  206   b ,  206   c  which is closed in the currentless state is assigned to each wheel brake  108   b ,  108   c . If the outlet valve  206   b ,  206   c  is opened, hydraulic fluid can escape from the corresponding wheel brake  108   a ,  108   b  into a low pressure accumulator  207 . The hydraulic fluid can be fed back from the low pressure accumulator  207  into the master brake cylinder  106  by means of the hydraulic pump  203 , for which purpose the switchover valve  205  and the inlet valves  202   b ,  202   c  are closed and the isolating valve  201  is opened. The brake pressure in a wheel brake  108   a ,  108   b ,  108   c ,  108   d  is reduced, in particular within the scope of the brake slip control which is, if appropriate, provided, if the associated wheel  102   a ,  102   b ,  102   c ,  102   d  locks or risks locking during a braking process. 
     Furthermore, a brake pressure which is present in the wheel brakes  108   b ,  108   c  can be kept constant by closing the inlet valve  202   b ,  202   c  assigned to the wheel brake  108   b ,  108   c , and the associated outlet valve  206   b ,  206   c . The maintenance of the brake pressure in a wheel brake  108   a ,  108   b ,  108   c ,  108   d  is used in the vehicle movement dynamics control system provided and in the brake slip control system which is, if appropriate, provided, to maintain a brake pressure over a specific time—i.e. over one or more clocking steps of the control unit  110 . 
     The admission pressure which is set by the driver is sensed by means of a pressure sensor  208 , which is preferably contained in just one of the two subassemblies of the hydraulic unit  109 . 
     In the previously illustrated embodiment, the hydraulic unit  109  is enabled to increase and to reduce the brake pressure in all the wheel brakes  108   a ,  108   b ,  108   c ,  108   d  by means of the hydraulic pump  203  compared to the driver&#39;s specification. Such a hydraulic unit  109  is known per se to a person skilled in the art from conventional vehicle movement dynamics control systems. 
     In a further embodiment, the hydraulic unit  109  is simplified in such a way that the brake pressure in all the wheel brakes  108   a ,  108   b ,  108   c ,  108   d  can be reduced compared to the driver&#39;s specification, to be precise in such a way that an increase in the brake pressure compared to the driver&#39;s specification is possible, but only in the wheel brakes  108   a ,  108   b  of the front wheels  102   a ,  102   b.    
     For this purpose, the hydraulic unit  109  comprises a subassembly which is assigned to the front wheel brakes  108   a ,  108   b  and is configured in the previously described way. This subassembly serves to apply braking force to the front wheels  102   a ,  102   b  within the scope of the vehicle movement dynamics control system which is provided. In addition, the subassembly is used in the brake slip control system, which is, if appropriate, provided, to reduce the braking force at the front wheels  102   a ,  102   b.    
     In the wheel brakes  108   c ,  108   d  of the rear wheels  102   c ,  102   d , the brake pressure can only be reduced compared to the driver&#39;s specification by means of a further subassembly of the hydraulic unit  109 , in order to permit brake slip control. This subassembly differs from the previously described subassembly in that there is no isolating valve  201  or switchover valve  205  provided. In particular, the isolating valve  201  is removed from the pressure line, and the branch in the hydraulics which contains the switchover valve is omitted. As a result, the hydraulic unit  109  can be simplified compared to the conventional hydraulic unit of a vehicle movement dynamics control system. 
     If a brake slip control system is not provided, it is also possible to dispense with the second subunit of the hydraulic unit  109 , and a direct connection can be provided between the master brake cylinder  106  and the wheel brakes  108   c ,  108   d  of the rear wheels  102   c ,  102   d.    
     In one embodiment, input variables for the vehicle movement dynamics control system which is provided are only the longitudinal speed and the transversal speed of the vehicle. In particular, these are understood to be the longitudinal speed and transversal speed of the centre of gravity of the vehicle. 
     The transversal speed is sensed using a speed sensor  111 . The speed sensor  111  can also be used to sense the vehicle longitudinal speed. Alternatively, the vehicle longitudinal speed can be acquired, in a way known per se to a person skilled in the art, from the signals of wheel speed sensors which are arranged on the wheels  102   a ,  102   b ,  102   c ,  102   d  of the vehicle  101 . 
     The speed sensor  111  can basically be configured in any desired fashion. 
     One exemplary embodiment involves the optical speed sensor for sensing the longitudinal speed and/or transversal speed of the vehicle  101 , which is described in DE 10 2004 060 677 A1. The sensor comprises a camera which is mounted on the underfloor of the vehicle  101  and takes images of the roadway surface on which the vehicle  101  is moving. The sensor detects displacements in structures within successively taken images of the roadway surface and determines the longitudinal speed and/or transversal speed of the vehicle  101  from the displacements. The sensor can therefore be used to sense only the transversal speed or additionally also the longitudinal speed. 
     In a further embodiment, the speed sensor  111  comprises one or two laser units, one laser unit  301  of which is illustrated in  FIG. 3 . Each laser unit  301  is mounted on the underfloor of the vehicle  101  and emits a measuring light beam  302  at an angle with respect to the underfloor of the vehicle  101  in the direction of the roadway surface  303 . An optical element  304 —preferably an arrangement of optical lenses—concentrates the measuring light beam  302  and focuses it on a point of the roadway  303  or on a point in the vicinity of the roadway surface  303 . The measuring light beam  302  of a laser unit  301  is formed by coherent light and is generated, for example, in a semiconductor laser which can be configured in one embodiment as a vertical cavity surface emitting laser (VCSEL). The light is preferably in an invisible spectral range, in particular in the infrared spectral range, with the result that the measuring light beam  302  cannot be seen and road users are not distracted. However, alternatively it is also possible to use light in another spectral range. 
     Each laser unit  301  comprises a resonator  305  of the length L, which is limited by a front semitranslucent mirror  306  and a rear semitranslucent mirror  307 . The measuring light beam  302  is formed by light which emerges from the resonator  305  in the direction of the roadway surface  303  through the front mirror  306 . The length of the distance which the measuring light beam  302  travels between the resonator  305  or the front mirror  306  and the roadway surface  303  is denoted hereby L 0 . The measuring light beam  302  is scattered at the roadway surface  303 . Part of the scattered light is reflected in opposite directions to the measuring light beam  302  as a scattered light beam  308 . As a result of the optical device  304 , the scattered light beam  308  passes into the resonator  305  and interferes there with the light which is amplified in the resonator  305 . 
     If the resonator  305  and the roadway surface  303  move relative to one another with a speed component ν L  in the direction of the measuring light beam  103  owing to a movement of the vehicle  101 , the scattered light beam  308  experiences a Doppler shift. This is a change in the frequency of the light or of the wavelength as a function of the abovementioned speed component ν L . Owing to the feeding back of the scattered light beam  308  into the resonator  305 , what is referred to as a self-mixing effect occurs within the resonator  305 . That is to say modulation of the laser amplification occurs, the frequency of which depends on the Doppler shift of the scattered light beam  308  and therefore also on the described speed component. 
     The change Δg over time in the laser amplification as a function of the speed component ν L  of the relative movement between the resonator  305  and the roadway surface  303  in the direction of the measuring light beam  302  is described by the following equation: 
                     Δ   ⁢           ⁢   g     =       -     κ   L       ·     cos   ⁡     (       4   ·   π   ·     f   0     ·     v   L     ·     t   /   c       +     4   ·   π   ·     L   0     ·       f   0     /   c         )                 (   1   )               
Here, κ denotes a laser-specific coupling coefficient which has a value between zero and one, f 0  denotes the frequency of the light emitted by the laser unit  108   i , c denotes the speed of light and t denotes the time. Equation (1) results, for example, from the theory of the self-mixing effect in M. H. Koelink et al., “Laser Doppler velocimeter based on the self-mixing effect in a fiber-coupled semiconductor laser: theory”, Applied Optics, Vol. 31, 1992, pages 3401-3408.
 
     The periodic modulation of the laser amplification leads to corresponding period modulation of the intensity of the light emitted by the resonator  305 . The speed component ν L  can be determined from the frequency with which the measured intensity changes periodically. In order to measure the intensity of the light emitted by the resonator  305 , the photodiode  309  is provided, which senses light which emerges from the resonator  305  through the rear mirror  307 . Such a diode  309  is usually used to keep the intensity of the laser light constant or regulated and is therefore generally already a component of the equipment of commercially available laser diodes. In the present application, the photodiode  309  is used in order acquires the frequency of the changes in intensity from the time profile of the measured intensity and determines the speed component ν L  on the basis of this frequency. 
     The transversal speed of the vehicle  101  can be sensed by means of a suitably equipped laser unit  301 . The laser unit  301  can, for example, be mounted vertically below the center of gravity of the vehicle on the underfloor of the vehicle, and the measuring light beam  302  can be oriented in the transversal direction of the vehicle. By means of the angle at which the measuring light beam  302  is directed onto the roadway surface  303  it is possible to determine the transversal speed of the vehicle  101  from the speed component ν L  measured by means of the laser unit  301 . The angle is obtained from the installation position of the laser unit  301  and can be stored as a parameter in the vehicle. The influence of changes in geometry on the measurement of speed due to rolling movements and pitching movements of the vehicle  101  can generally be ignored. 
     A further laser unit  301 , which is preferably also attached vertically below the center of gravity of the vehicle and whose measuring light beam  302  is oriented in the longitudinal direction of the vehicle, can be used to sense the vehicle longitudinal speed. 
     In a further embodiment, the speed sensor  111  can comprise a position-determining system. This is preferably a satellite-supported system which is based, for example, on the global positioning system (GPS) or the GALILEO system. The transversal speed and/or longitudinal speed of the vehicle  101  can be acquired here by tracking the position of the vehicle  101  by means of the position-determining system. 
     The components of the provided vehicle movement dynamics control system which are contained in the control unit are illustrated in  FIG. 4  in a schematic block diagram. 
     The input signals, i.e. the sensed longitudinal speed ν and transversal speed ν y , are fed to the calculation unit  401 . The latter determines the attitude angle β of the vehicle  101  from the input signals. This is the angle between the longitudinal axis of the vehicle and the direction of movement of the vehicle  101 . The attitude angle β is preferably determined in the calculation device  401  in accordance with the relation 
                   β   =     arc   ⁢           ⁢   tan   ⁢         v   y     v     .               (   2   )               
The calculated value is then transferred to the evaluation unit  402 . The control unit  110  preferably calculates a value for the attitude angle β in each clocking step.
 
     In the evaluation device  402 , the sign of the attitude angle β which gives the direction of a bend through which the vehicle  101  is traveling is acquired. In addition, the absolute value of the attitude angle β is compared with a threshold value. The threshold value characterizes the boundary between stable and unstable driving states. Said threshold value is acquired, for example, by means of driving trials for a specific type of vehicle and is stored as a parameter in the control unit  110 . A stable driving state is detected if the absolute value of the attitude angle β is smaller than the threshold value. In this case, no intervention is made into the driving behavior of the vehicle  101 . An unstable driving state, in particular an oversteering driving state, is detected if the absolute value of the attitude angle β is greater than the threshold value. In this case, the difference between the attitude angle β and the threshold value together with the sign of the attitude angle are transferred to the actuation unit  403 . 
     In the case of oversteering driving behavior of the vehicle, the actuation unit  403  determines an actuating request which is transferred to the hydraulic unit  203  and implemented by the hydraulic unit  203 . According to the actuating request, the front wheel  102   a ,  102   b , which is on the outside of the bend, of the vehicle  101  is braked in order to stabilize the vehicle  101 . The actuating request is preferably determined in accordance with the difference between the attitude angle and the threshold value, preferably embodied as a PID controller. The wheel on the outside of the bend is determined on the basis of the direction of the bend, which can be ascertained from the sign of the attitude angle. 
     A further embodiment of the vehicle movement dynamics control system differs from the previously described embodiment in that a further evaluation unit  501  is provided which is configured so as to detect unstable driving situations which are due to understeering of the vehicle  101 . The components of the vehicle movement dynamics control system which are contained in the control unit  110  in this embodiment are illustrated in  FIG. 5  in a schematic block diagram. 
     An input signal apart from the vehicle longitudinal speed ν and the vehicle transversal speed ν y  is the wheel lock angle δ of the front wheels  102   a ,  102   b  of the vehicle  101 . The latter can be sensed, for example, in a way known per se to a person skilled in the art by means of a steering angle sensor  114 . The steering angle sensor  114  can be arranged inside a steering train which connects the front wheels  102   a ,  102   b  to the steering handle which is operated by the driver. 
     The wheel lock angle δ and the attitude angle β which is acquired in the calculation device from the longitudinal speed ν and the transversal speed ν y  are fed to the calculation unit  501 . The latter acquires, on the basis of the longitudinal speed ν, a maximum possible theoretical wheel lock angle δ theor. . If the of the wheel lock angle δ is above the theoretical wheel lock angle δ theor . and if the acquired attitude angle β is below a threshold value, an unstable understeering situation is detected. 
     If an instability of the vehicle  101  has been detected in the evaluation unit  501  owing to understeering, the actuation unit  403  generates actuating requests for influencing the driving behavior. Compared to the embodiment of the control unit  110  which was described above with reference to  FIG. 4 , the actuation unit in the present embodiment is modified in such a way that it can generate such actuating requests. 
     In one embodiment, the actuation unit  403  influences the drive engine  103  or an engine control device in an understeering situation in such a way that the driving torque which is made available by the engine is reduced in order to stabilize the vehicle  101 . As a result, the vehicle  101  can be stabilized in understeering situations even if the vehicle  101  has only one simplified hydraulic unit  109  which permits a buildup of brake pressure only in the wheel brakes  108   a ,  108   b  of the front wheels  102   a ,  102   b.    
     If the hydraulic unit  109  permits a buildup of brake pressure in the wheel brakes  108   c ,  108   d  of the rear wheels  102   c ,  102   d , a braking force can be applied to the rear wheel  102   c ,  102   d  on the inside of the bend as an alternative to or in addition to the intervention into the engine control. For this purpose, a corresponding actuating request is generated in the actuation unit  403  and is transmitted to the hydraulic unit  109 . The rear wheel  108   c ,  108   d  on the inside of the bend is determined on the basis of the direction of the bend which is being traveled through and which can be determined from the sign of the sensed wheel lock angle δ. 
     Although the invention has been described in detail in the drawings and the preceding presentation, the presentations are to be understood as illustrative or exemplary and not restrictive; in particular, the invention is not restricted to the explained exemplary embodiments. Further variants of the invention and their implementation emerge from the preceding disclosure, the Figures and the patent claims in a way which is obvious to a person skilled in the art. 
     Terms such as “comprise”, “have”, “include”, “contain” and the like which are used in the patent claims do not exclude further elements or steps. The use of the indefinite article does not exclude a plurality. An individual device can carry out the functions of a plurality of units or devices specified in the patent claims.