Abstract:
A method and a device for controlling the slippage at at least one vehicle wheel in a closed loop. In order to reduce an unacceptable slippage at at least one wheel, the braking force at this wheel is influenced and wheel vibrations are determined. Furthermore, the tendency of the characteristic of the slippage is ascertained during a vibration, and the braking force is influenced as a function of the ascertained tendency.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and a device for controlling the slippage at a vehicle wheel in closed loop. 
     BACKGROUND INFORMATION 
     U.S. Pat. No. 5,193,889 describes a known method and system in which measures are proposed for controlling vibrations in the speed of at least one wheel, e.g., during an anti-lock control or a traction control. In response to a detected wheel vibration, the brake pressure is influenced in order to avoid an amplitude increase of the vibration. However, in this situation, the closed-loop control itself is impaired in this manner, since during a vibration, the suppression of the vibration and not the closed-loop control itself is in the fore. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to control vibrations at at least one wheel during a braking intervention independent of the driver, and nevertheless not to restrict the braking control itself too much. 
     An active braking intervention, be it within the framework of a traction control, an anti-lock control, or an operating-dynamics control, is considerably improved, since wheel vibrations are damped and at the same time, the performance of the braking control itself is not substantially impaired. The same holds true for axle vibrations, in which both wheels of an axle vibrate. 
     It is particularly advantageous that the control comfort is improved during a traction control, accompanied by simultaneous, optimal traction of the vehicle. 
     Furthermore, it is of special importance that no additional hardware is necessary to carry out the measures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a survey diagram of a control unit for carrying out the active braking intervention. 
     FIG. 2 shows a flow chart which represents a program, implemented in the microcomputer of the control unit, for carrying out the present invention. 
     FIG. 3 a  shows a first timing diagram illustrating an operation of the present invention. 
     FIG. 3 b  shows a second timing diagram illustrating an operation of the present invention. 
     FIG. 3 c  shows a third timing diagram illustrating an operation of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a survey diagram of a control unit for carrying out the active braking intervention. The control system is used for carrying out a traction control; in other embodiments, alternatively or additionally, it is used for carrying out an anti-lock function and/or a driving-stability control. Provision is made for a control unit  10  which has at least one microcomputer. Performance quantities are fed to control unit  10  via input lines, the performance quantities being evaluated for carrying out the provided function(s). The wheel-speed signals of the individual vehicle wheels are supplied to control unit  10  from corresponding measuring devices  20 ,  22 ,  24  and  26 , via input lines  12 ,  14 ,  16 ,  18 . In addition, provision can be made for input lines  28  through  32  which supply further performance quantities such as brake-pressure signals, the vehicular velocity, etc., from corresponding measuring devices  34  through  38 . To influence the brake pressures in the individual wheel brakes, provision is made for output lines  40  through  44 , via which valve arrangements and pump(s)  46  through  50  influencing the brake pressure are actuated within the sense of the implemented function. Such arrangements are known both for hydraulic and for pneumatic braking systems. If a braking system having purely electrical brake application is used, instead of valves and pump(s)  46  through  50 , electric motors are provided which are activated via output lines  40  through  44  within the sense of the implemented function. 
     Particularly in the drive case, some vehicles tend to experience wheel and/or axle vibrations. In this case, the speed of an affected wheel (and thus its drive slip) shows an oscillating characteristic. This reduces the control performance of a traction control system (ASR). Above all under micron-split conditions, one has to expect a considerable influence. The following described procedure makes it possible to modulate the wheel pressure, even given an oscillating characteristic of the slippage or of a wheel speed, in such a way that the traction is increased and the control comfort is optimized simultaneously by selective build-ups and reductions in pressure. 
     As generally known, in response to the occurrence of drive slip, the wheel pressure is controlled as a function of the wheel slip and/or the wheel acceleration, along the lines of a reduction in the drive slip. In so doing, the extent of the pressure change follows the characteristic of at least one of these variables. In response to a sign change, preferably in response to the first sign change of the wheel-acceleration signal during a closed-loop control, a filter mark DRAFILT is set. This remains set until the wheel-acceleration signal is longer than a predefined filter time (e.g. 300 to 500 msec), greater or less than a threshold value (e.g. 0 g). In other words, the mark defines a time period while a check is being made for vibrations. 
     If DRAFILT is set, in response to an existing vibration, the identical vibrational state is ascertained. In the preferred exemplary embodiment, the vibration maximum or the vibration minimum is determined by an acceleration threshold value. In other embodiments, the identical vibrational state is determined with the aid of other variables characterizing a vibration, such as zero crossings, etc. 
     In response to the first recognition of this state, the prevailing slippage value SLIP is stored as slippage value SLIP_OLD. If the next identical vibrational state or one of the next states is recognized, the slippage value SLIP prevailing then is compared to a stored reference slippage value. Derived from this is a tendency for the characteristic of the slip during the vibration, i.e., it is determined whether the slip shows a tendency to a reduction or to an increase. If a tendency is shown h to a reduction, pressure is reduced in a defined manner in the affected wheel brake; in the reverse case, pressure is built up in a defined manner. 
     In the preferred exemplary embodiment, a hysteresis is provided for determining the tendency. For example, a reduction tendency is recognized when the deviation between the prevailing and the stored slippage value is less than a predetermined value (e.g. 2 km/h) or an appropriate percentage, while an increase tendency is recognized when the deviation between the prevailing and the stored slippage value is greater then a predetermined value (e.g. 2 km/h) or an appropriate percentage. 
     In the preferred exemplary embodiment, the pressure influence during the vibration is realized by the output of a suitable pulse with subsequent pause time, the pulse length being definitively predefined, or being a function of the size of the deviation between the prevailing and the stored slippage value. 
     If a pressure influence has been carried out at set mark DRAFILT, prevailing slippage value SLIP is stored as comparison value SLIP_OLD. 
     If mark DRAFILT is not set, the pressure is modulated as known according to the wheel characteristic. 
     Besides the use of the procedure in the case of traction control, it is also used, with correspondingly reversed conclusion, in the case of anti-lock controllers. A further application area is the use during an active braking intervention of a driving-stability controller. 
     If a braking system having electrical brake application is used, the braking force or the braking torque is controlled instead of the brake pressure. Accordingly, a motion of the servomotor(s) is brought about as a function of the ascertained tendency. In this context, braking force is understood as the generalization of the technical variables of brake pressure, braking force, braking torque, etc. 
     In light of a flow chart, FIG. 2 shows a preferred implementation of the described procedure as a program of the microcomputer of control unit  10 . 
     The sketched program is initiated at the start of a traction control, when a spin tendency has been detected at a drive wheel for the first time. Further program runs or programs are provided for the other drive wheel(s). In first step  100 , slippage SLIP for one drive wheel is ascertained on the basis of the wheel speed of the corresponding drive wheel and of at least one further wheel speed, for example, by comparison. In addition, acceleration DRA of this wheel is determined, e.g., by subtraction or differentiation. Thereupon, according to step  102 , the pressure build-up or the pressure reduction is controlled as known for the affected wheel as a function of the slippage SLIP and/or the wheel acceleration DRA. After that, in step  104 , it is checked whether the closed-loop control has ended, i.e., whether no unacceptable slippage exists any longer at the affected wheel and the pressure is completely reduced. If this is the case, the program is ended, at least for the affected wheel. 
     Otherwise, on the basis of the prevailing signal and a previous signal, it is checked in step  106  whether a sign change of acceleration signal DRA has taken place. If this is not the case, the program is repeated with step  100 . In the event of a yes response, mark DRAFILT is set in step  108 . 
     In the following step  110 , it is queried whether acceleration signal DRA is longer than predefined filter time TFILT (e.g. 300-500 msec), lies above or below a limiting value, e.g., zero. If this is the case, there is no vibration, so that according to step  112 , mark DRAFILT is reset. Thereupon, the program is repeated with step  100 . 
     If the condition in step  110  is not met, in step  114 , as in step  100 , wheel slip SLIP and wheel acceleration DRA are determined. Thereupon, in step  116 , it is checked whether the predefined vibrational state exists. For instance, the maximum or the minimum of the acceleration signal is ascertained. 
     If such a state does not exist, the program is repeated with step  110 . Otherwise, in light of a mark, not shown, it is determined in step  118  whether the state was ascertained for the first time. If this is the case, in step  120 , the prevailing slippage value SLIP is stored as SLIP_OLD. The program is thereupon repeated with step  110 . 
     If the vibrational state was not determined for the first time, in step  122 , the deviation delta between prevailing slippage value SLIP ascertained in step  114  and stored slippage value SLIP_OLD is formed, e.g., by subtraction. Thereupon, in step  124 , the deviation delta is compared to a limiting value delta  1  (e.g. 2 km/h). If the deviation is greater than the limiting value, the slippage on average is becoming greater, so that according to step  126 , pressure is built up, e.g., by output of a predefined, or slip-dependent and/or acceleration-dependent build-up pulse. After step  126 , in step  128  the prevailing slippage value SLIP is stored as reference value SLIP_OLD, and the program is repeated with step  110 . 
     If the response in step  124  is “no,” in step  130 , it is checked whether the deviation is less than a limiting value delta  2  (e.g., −2 km/h). If the deviation is less than the limiting value, the slippage on average is becoming less, so that according to step  132 , pressure is reduced, e.g., by output of a predefined or slip-dependent and/or acceleration-dependent reduction pulse. After step  132 , in step  128 , the prevailing slippage value SLIP is stored as reference value SLIP_OLD, and the program is repeated with step  110 . 
     If the deviation is not less than the limiting value, the slippage on average is remaining constant, so than no intervention is carried out. The program is repeated with step  110 . 
     The operating mode of this procedure is elucidated in the timing diagrams of FIGS. 3 a-   3   c . FIG. 3 a  shows the characteristic curve of velocity VRAD of a drive wheel over time, as well as the characteristic curve of vehicle velocity VFZ over time. FIG. 3 b  shows the characteristic curve of brake pressure PRAD at this wheel over time. FIG. 3 c  shows the time characteristic of mark DRAFILT. 
     As shown in FIG. 3 a , at instant T 0 , a spin tendency of a drive wheel is detected. This leads to a pressure build-up in this wheel according to FIG. 3 b . At instant T 1 , the wheel-acceleration signal changes its operational sign, i.e., a maximum of the speed signal is passed through (compare FIG. 3 a ). In the exemplary embodiment shown, this leads to a termination of the pressure build-up and to the setting of mark DRAFILT (compare FIGS. 3 b  and  c ). After the instant, wheel vibrations are indicated in FIG. 3 a . The above-described procedure is run through. In the exemplary embodiment shown, the minima of the speed signal, i.e., the zero crossings of the acceleration signal, are drawn upon to ascertain the identical vibrational state. In other embodiments, different characteristic variables such as the maxima or minima of the acceleration signal are monitored. Nothing changes in the mode of operation because of this. At instant T 2 , the vibrational state is recognized for the first time. The slippage value (e.g. the difference with respect to the traveling speed) is stored as the reference value. At instant T 3 , the vibrational state is ascertained a second time. A comparison of the prevailing slippage value to the stored value yields that a tendency to the decrease of slippage exists. Therefore, at instant T 3  according to FIG. 3 b , pressure is reduced. The corresponding is done at instant T 4 , as well, the stored slippage value at instant T 3  being taken as a basis for the comparison. According to that, the vibration has died out. At instant T 5 , the maximum filter time is exceeded, without the acceleration signal having exceeded the limiting values. According to FIG. 3 c , this leads to a resetting of mark DRAFILT, whereupon the pressure modulation is again carried out as a function of slippage and/or wheel acceleration. In the case shown, this leads to a pressure reduction (compare FIG. 3 b ). At instant T 6 , the sign of the acceleration signal changes once more (compare FIG. 3 a ). The mark is set as shown in FIG. 3 c . The pressure remains constant (compare FIG. 3 b ). This time, no vibration occurs. After the expiration of the filter time at instant T 7 , the mark is reset, and the pressure modulation commences again as a function of slippage and/or wheel acceleration (compare FIG. 3 c  and pressure build-up FIG. 3 b ). In so doing, the pressure change follows the characteristic of at least one of these variables. 
     The described procedure is used not only in connection with a traction control system, but also in the case of braking interventions of an anti-lock control system or a stability control, during which wheel vibrations or axle vibrations can occur. The tendency of the slippage during the vibration is determined here as well, and appropriate measures are carried out for the braking-force control.