Abstract:
The invention relates to an anti-lock braking system as well as to a method of operating the anti-lock braking system. As a function of the motional state of at least one wheel control commands are generated for at least one actuator ( 12 ) of the anti-lock braking system. During generation of the control commands a quantity characteristic of the vertical tyre force is taken into account.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of International Application No. PCT/EP02/13239 filed Nov. 25, 2002, the disclosures of which are incorporated herin by refernce, and which claimed priority to German Patent Application No. 101 58 026.6 filed Nov. 27, 2001, the disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The invention relates to a method of operating an anti-lock braking system, in which as a function of the motional state of a wheel control commands are generated for an actuator. 
   With conventional ABS, the motional state of one or more wheels is monitored by means of wheel-speed sensors. In the event of incipient lock at one wheel, there is a sharp increase in wheel deceleration and wheel slip. As soon as these two parameters exceed specific critical values, a control unit of the ABS issues control commands to one or more actuators in the form of e.g. solenoid-valve units. The actuators then stop the build-up of the wheel brake pressure or reduce this pressure until the risk of locking has been eliminated. To prevent under-braking of the wheel, the brake pressure then has to be built up again. In the course of this brake regulation there has to be alternate detection of the stability and the instability of the wheel motion, and a cyclic sequence of pressure-rise, pressure-holding and pressure-reduction phases has to be used to regulate the wheel in the slip range with maximum braking force. 
   With conventional ABS, the motional state of one or more wheels is monitored by means of wheel-speed sensors. In the event of incipient lock at one wheel, there is a sharp increase in wheel deceleration and wheel slip. As soon as these two parameters overwrite specific critical values, a control unit of the ABS issues control commands to one or more actuators in the form of e.g. solenoid-valve units. The actuators then stop the build-up of the wheel brake pressure or reduce this pressure until the risk of locking has been eliminated. To prevent under-braking of the wheel, the brake pressure then has to be built up again. In the course of this brake regulation there has to be alternate detection of the stability and the instability of the wheel motion, and a cyclic sequence of pressure-rise, pressure-holding and pressure-reduction phases has to be used to regulate the wheel in the slip range with maximum braking force. 
   The ABS has to take a number of disturbance values into account. These include e.g. changes of the adhesion between tyre and road, uneven road surfaces that cause wheel- and axle vibrations, etc. 
   SUMMARY OF THE INVENTION 
   The invention relates to method of operating an anti-lock braking system that is notable for improved functionality, particularly given uneven road surfaces and pitching motions of the vehicle. The invention also relates to providing an improved anti-lock braking system. 
   In a method of operating an anti-lock braking system of the initially described type, this is achieved according to the invention in that a quantity characteristic of the vertical tyre (“tire ” in the American English spelling) force, also known as wheel load or wheel contact force, is determined and this quantity is taken into account when generating the control commands for the at least one actuator. This also includes the situation where a plurality of quantities characteristic of the vertical tyre force are used to generate the control commands. 
   Conventional anti-lock braking systems are unable to distinguish between a change of the adhesion between tyre and road, on the one hand, and a change of the vertical tyre force, on the other hand. For this reason, in specific situations, such as very uneven road surfaces or pitching motions of the vehicle that occur, for example, when pulling away or coming to a halt, there is sub-optimum brake regulation by the ABS. The fact, that the ABS according to the invention takes a quantity characteristic of the vertical tyre force into account as a separate parameter for the regulation of an ABS-assisted braking operation, means that in many situations the directional stability may be improved and the braking distance may be shortened. 
   The quantity characteristic of the vertical tyre force may be either the vertical tyre force itself or one or more other parameters that allow a conclusion to be drawn about the vertical tyre force. Such parameters may be derived from the suspension system and/or the damping system and/or from other subsystems of the vehicle. 
   For example, the spring travel, which may be measured, estimated or calculated, allows a conclusion to be drawn about the vertical tyre force. In addition to or instead of the spring travel, spring constants and/or damper constants may be taken into account when determining the quantity characteristic of the vertical tyre force. It is also conceivable to determine the quantity characteristic of the vertical tyre force with the aid of a detected spring force or a detected spring pressure (active suspension). 
   When generating the control commands, tyre characteristics may simultaneously be taken into account. This taking account of tyre characteristics may be effected e.g. in that the tyre characteristics are already simultaneously involved in determining the vertical tyre force, or in that the quantity characteristic of the vertical tyre force and the tyre characteristics are taken separately into account when generating the control commands. 
   Determining the quantity characteristic of the vertical tyre force may be effected in various ways. For example, this quantity may be determined by estimation, measurement or calculation. According to a preferred variant of the invention, determination of the quantity characteristic of the vertical tyre force is a multi-stage process whereby, in a first step, one or more first parameters such as e.g. the spring travel are detected and, in a second step, on the basis of these parameters a conclusion is drawn about the vertical tyre force. The quantity, which is characteristic of the vertical tyre force and is taken into account when generating the control commands, may however also be determined directly, i.e. without such intermediate steps. 
   The control commands are expediently generated in such a way that upon a change of the vertical tyre force they are directed towards a change of a manipulated variable of the anti-locking braking system in the same direction. Thus, for example, upon a reduction of the vertical tyre force the brake pressure may be likewise reduced and upon an increase of the vertical tyre force the brake pressure is likewise increased. Preferably, determination of the vertical tyre force and regulation of the brake pressure based thereon is effected separately for the individual wheels of the vehicle. 
   Generation of the control commands may be effected in one or more stages. In the case of single-stage generation of the control commands, the quantity characteristic of the vertical tyre force is taken into account directly, e.g. as a separate input variable of an ABS control unit, when generating the control commands. It is however also conceivable, in a first stage, first to generate a compensation signal on the basis of the quantity characteristic of the vertical tyre force and by means of said compensation signal bring an influence to bear upon such control commands as have been generated uncompensated, i.e. without taking into account the quantity characteristic of the vertical tyre force. This may occur in such a way that firstly uncompensated control commands are generated while taking account of the motional state of at least one wheel and then the (compensated) control commands for the at least one actuator are generated on the basis of the uncompensated control commands while taking the compensation signal into account. 
   During generation of the control commands an electrical signal, which is characteristic of a braking request, may additionally be taken into account. Such a signal is generated e.g. in electrohydraulic braking systems. The method according to the invention may however also be used for conventional braking systems, in which the braking force summoned up by a driver is transmitted by means of leverage of the brake pedal mechanically to a vacuum brake booster and then, after boosting, onwards to a master cylinder. 
   The invention may be used both in the form of a control program, which is stored in a volatile or non-volatile memory or on a data carrier, or in the form of equipment. As far as the equipment development of the anti-lock braking system is concerned, means may be provided for determining the quantity characteristic of the vertical tyre force, wherein the determined quantity characteristic of the vertical tyre force is taken into account by the ABS when generating the control commands. 
   Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  a schematic block diagram of an ABS according to the invention; 
       FIG. 2  a schematic representation of the vertical tyre force; and 
       FIG. 3  a schematic representation of the spring travel and the spring force, which arise in a suspension system of a motor vehicle. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1  an embodiment of an ABS according to the invention for an electrohydraulic braking system is illustrated. The ABS in a known manner comprises a control unit  10  and one actuator  12  per wheel, each in the form of a wheel pressure modulator. The control unit  10  comprises a brake-by-wire (BBW) unit  14  as well as an ABS unit  16 . In the ABS unit  16  of the control unit  10  further functions such as the traction control system (TCS) and the electronic stability program (ESP) are programmed in. 
   The BBW unit  14  of the control unit  10  receives data regarding a braking request in the form of electrical signals, which have been generated e.g. by a pedal-travel sensor. The ABS unit  16  with ESP and TCS function of the control unit  10  receives from further sensors further data regarding the driving mode, such as velocity or cornering, and regarding the motional state of the wheels. From the braking-request data and the further data the control unit determines control commands for the actuators  12 . The actuators  12  convert the received control commands into the corresponding manipulated variable, i.e. into brake pressures for the wheel cylinders. The tyres/road controlled system and the sensors for detecting the further data in a known manner form a closed-loop control circuit. 
   The ABS illustrated in  FIG. 1  has an interface  18  enabling a higher-level control intervention from outside in the ABS regulation. The interface  18  therefore allows the vertical tyre force to be taken into account during generation of the control commands for the actuators  12 . 
   There now follows a detailed description with reference to  FIG. 2  of the effective direction of the vertical tyre force F 2 .  FIG. 2  shows a wheel  22  running on a road  20  and having an instantaneous running radius R. As  FIG. 2  reveals, the wheel has a specific inclination γ relative to the road  20 . The point of application of the tyre force on the road  20  is denoted by CTC (centre of tyre contact). With reference to the road plane, the tyre force may be split into three components of force F X , F Y , F Z . The vertical tyre force F Z  denotes the component of the tyre force that acts normally relative to the road plane. 
   As already explained, in the ABS according to the embodiment illustrated in  FIG. 1  the vertical tyre force F Z  is taken into account by means of an intervention in the ABS closed-loop control circuit. To put it more precisely, the interface  18  makes it possible for a tyre force compensation signal to be taken into account for the generation of compensated control commands. In the embodiment illustrated in  FIG. 1 , compensation of the uncompensated control commands generated by the control unit  10  is effected by means of the interface  18 . For this purpose, the interface  18  has a signal-processing functionality, which allows the interface  18  to generate compensated control commands for the actuators  12  on the basis of the (uncompensated) control signals of the control unit  10 , on the one hand, and the compensation signal, on the other hand. This may be effected e.g. by adding the compensation signal as an additive component to the uncompensated control commands. 
   Other developments for the injection of the compensation signal into the ABS closed-loop control circuit are conceivable. It would, for example, be possible to use the compensation signal as a further input signal for the control unit  10  so that the control unit  10  generates the compensated control commands for the actuators  12  directly, i.e. without the intermediate step of generating uncompensated control commands. 
   There now follows a detailed description with reference to  FIG. 1  of the generation of the compensation signal. The compensation signal is generated in a separate open loop, which is connected by means of the interface  18  to the ABS closed-loop control circuit. To generate the compensation signal, there is recourse to parameters of suspension/damping systems  24  of the vehicle. To put it more precisely, a unit  26  for determining the vertical tyre force has recourse to the suspension/damping systems  24 . During this recourse of the unit  26  to the suspension/damping systems  24  the spring travel as well as the gradient of the spring travel are determined. 
   Some parameters of relevance to the determination of the vertical tyre force are graphically explained in detail in the suspension/damping systems  24  illustrated in  FIG. 3 . In  FIG. 3  the spring force F S  and the spring travel Δ are indicated in the form of arrows. The spring force F S  is the force that gives rise to compression of the spring  28 . The spring travel Δ denotes the relative displacement connected therewith at the wheel. 
   The unit  26  illustrated in  FIG. 1  for determining the vertical tyre force receives the measured, estimated or otherwise determined parameters, spring travel Δ and time gradient of the spring travel Δ, and from these and from further data determines the vertical tyre force F Z . The vertical tyre force F Z  is a function of the spring force F S  and the damper force F D , F Z =f(F S ,F D ). The spring force F S  may be derived from the spring travel Δ, with F S =f(Δ). The damper force F D  may be determined from the first time derivative of the spring travel Δ as well as from the damper characteristic curve K, F D =f(dΔ/dt,K). The damper characteristic curve K is filed in a data bank  32 , to which the unit  26  has recourse for determining the vertical tyre force. 
   The vertical tyre force determined by the unit  26  is supplied to a compensation-calculating circuit  30 . The compensation-calculating circuit  30  calculates a suitable compensation signal, which is injected by means of the interface  18  into the ABS closed-loop control circuit. For this calculation of the compensation signal, the compensation-calculating circuit  30  has recourse to the data bank  32 . The data bank  32 , in addition to the data regarding the damper characteristic curve, also contains data regarding the tyre characteristics, such as tyre diameter, tyre width, tyre pressure etc. The taking of such tyre characteristics into account when calculating the compensation signal allows even better regulation of the ABS. 
   In a departure from the embodiment illustrated in  FIG. 1 , the tyre characteristics may already be taken into account during determination of the vertical tyre force. In said case, the unit  26  has direct recourse to the data bank  32 . 
   The compensation-calculating circuit  30  calculates the compensation signal as a function of a change of the vertical tyre force in such a way that the control commands compensated by means of the compensation signal and supplied to the actuators  12  give rise to a change of the brake pressure in the same direction. This leads, in the case of a reduction of the vertical tyre force, to a reduction of the brake pressure, and vice versa. 
   In accordance with the provisions of the patent statues, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.