Abstract:
Signals delta --  w representing the RPM difference of at least two wheels of at least one axle and signals V representing the vehicle speed are linked through a nonlinear characteristic diagram. Obtained thereby are signals which represent the transverse movements, specifically the transverse acceleration, of the vehicle. This makes it possible to allow for nonlinear effects such as the expansive deflection of the curve inside tires and the compressive deflection of the curve outside tires due to the roll moment backing, particularly through empirically determined parameters. The signal of the stationary transverse acceleration is dynamized by further processing, in a second order filter, of the signal of the stationary transverse acceleration (steering angle is constant) so obtained.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and system for generating a signal representative of the transverse movement of a vehicle, utilizing the RPM of at least two wheels of at least one axle. 
     The maximally accurate knowledge of the movements of a vehicle is of elementary significance for any type of control or regulation which counteracts these movements in the sense of increasing the travel comfort and road safety. 
     Such a counteraction can be accomplished, as described in DE-OS 35 29 178, by an active suspension and/or damping system whose suspension and/or damping performance is of a controllable or regulable configuration. 
     Furthermore, such influencing of the vehicle movements may be effected by controllable or regulable allocation of force to the individual drive wheels and/or by controllable or regulable steering operations. In exemplary fashion, reference is made in this context to a prior propulsion control system (PCT/EP89/00953) and to a curve recognition procedure in the context of an antilock control system (DE-OS 37 39 558). A process for steering control is proposed in DE-OS 39 30 445. 
     The causes of the vehicle movements are essentially 
     acceleration and/or braking operations, 
     steering operations and/or 
     road surface unevennesses. 
     Important measured quantities representing the travel dynamics, particularly the transverse dynamics of the vehicle, and representing essential input variables of the control or regulation systems described above are the steering angle, or the transverse acceleration of the vehicle causally associated with the steering angle. Meant by steering angle is here the swing angle of the steerable wheels. 
     The steering angle, respectively the transverse acceleration of the vehicle, can be measured either &#34;directly&#34; by suitable sensors (steering angle sensors or acceleration sensors) or can be derived indirectly from other sensor signals. In view of minimizing the number of sensor systems, the &#34;indirect&#34; measuring methods should be given preference over the &#34;direct&#34; methods. 
     EP-OS 0353 995 describes a steering angle detector system where the steering angle is determined from the wheel RPM, or wheel frequencies, which differ in steering operations. 
     Exactly the maximally accurate determination of the transverse movements of a vehicle, for instance of the transverse acceleration, is of major importance for an optimum design of a chassis system and/or steering system. 
     It is known to provide a system for evaluation of wheel speed signals where the wheel speed difference is determined at minimal components expense. Moreover, it is proposed in such a system to determine the transverse acceleration of the vehicle in proportion to the vehicle speed and to the wheel speed difference. This has been proved as not being optimal. 
     The problem underlying the present invention is to determine a signal based on the wheel speed differences, which signal optimally represents the transverse movement of the vehicle, especially the transverse acceleration. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and system for determining a first signal representing the transverse movement of the vehicle using a given characteristic diagram from second signals representing the RPM differences of at least two wheels, and from third signals V representing the vehicle speed. 
     The inventional process is based on signals delta --  w representing the wheel speed difference of at least two wheels of at least one axle, and on signals V representing the vehicle speed. Through linkage of these signals by a nonlinear characteristic diagram there are signals obtained which represent the transverse movements, specifically the transverse acceleration, of the vehicle. 
     The inventional process has the advantage that by allowing for the nonlinear correlations between the transverse acceleration and the wheel speed difference, for one, and the vehicle speed, for another, it is possible to make allowance, e.g., for the tire deflection attributable to the roll moment backing in curve travel. 
     Moreover, the inventional process has the advantage that the transverse acceleration remains steady at changing vehicle speed. Specifically, the amplification factor between the signal aq 00  representing the transverse acceleration and the signal delta --  w representing the wheel speed difference can be stored as a nonlinear characteristic curve over the speed. 
     The nonlinear effects, such as the expansive deflection of the curve inside tires and the compressive deflection of the curve outside tires, due to the roll moment backing, can be allowed for, specifically by empirically determined parameters. Furthermore, the process can be applied empirically to any vehicle. 
     A favorable embodiment of the inventional system provides for processing the signal aq 00 , captured as described above, in a second order filter. This offers the advantage that the signal aq 00 , which descr the transverse acceleration of the vehicle in stationary curve travel (steering angle is constant), is dynamized in the second order filter. Here, the speed dependence of the dynamics can be allowed for by filter coefficients which are switchable in contingence on the vehicle speed. 
     Advantageous is the use of the signals aq 00  and/or aq determined by the inventional process for chassis control and/or actuation of a steering system. 
     An object of the invention is also a device for the application of the inventional process, where first means I3 are provided with the aid of which from a stored nonlinear characteristic curve a first signal aq 00  is determined from second signals delta --  w representing the speed difference of at least two wheels and from third signals V representing the vehicle speed. 
     Furthermore, a further embodiment provides for second means 14 with the transfer performance of a second order filter, by means of which the first signals aq 00  are processed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the inventional process, or inventional device, will be described hereafter with the aid of FIG. 1, 2 and 3 in which: 
     FIGS. 1 and 2 illustrate block diagrams of the method and system of the present invention; and FIG. 3 illustrates general principles. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before addressing the embodiment with the aid of FIG. 1 and 2, several general explanations shall be given first with the aid of FIG. 3. 
     The speed difference of the wheels of an axle of a two-track vehicle 31 is in first approximation proportional to the yaw velocity psip, that is, to the speed of rotation about the vertical axis of the vehicle. The yaw velocity psip occurs in curve travel and correlates to the vehicle speed V and the curve radius R (FIG. 3) as follows: 
     
         psip=V/R                                                   (1) 
    
     In the case of curve travel free of lateral force, that is, at disappearing king pin inclinations, the outside wheels roll on a circular arc with the radius R+b at the turning circle frequency 
     
         omega.sub.-- a=1/r * [psip*(R+b)]                          (2) 
    
     while the inside wheels run on a circular arc with the radius R-b at the turning circle frequency 
     
         omega.sub.-- i=1/r * [psip*(R-b)]                          (3) 
    
     where R is the roll radius of the wheels and 2*b signifies the track of the wheels of an axle. 
     Deriving as difference frequency delta --  w between the turning circle frequencies of the curve outside and curve inside wheels is 
     
         delta.sub.-- w=omega.sub.-- a--omega.sub.-- i=2/r * [psip*b] (4). 
    
     Thus, the yaw angle velocity psip is 
     
         psip=[r/(2*b)] * delta.sub.--w                             (5). 
    
     Known from the prior art (Zomotor, A.: Fahrwerktechnik: Fahrverhalten [Chassis engineering: Travel performance], Wurzburg, Vogel-Buchverlag, 1987) is the linear single-track model for the transverse dynamics of a vehicle. Deriving thereof for the transverse acceleration aq is 
     
         aq=(psip+betap) * V                                        (6), 
    
     where V is the vehicle speed and betap is the time derivation of the floating angle. This establishes the correlation between the steering angle delta and the yaw angle velocity psip, or the floating angle velocity betap, by the transfer functions 
     
         G.sub.psip (s)=Z.sub.psip (s)/N(s)=psip(s)/delta(s)        (7) 
    
     or 
     
         betap(s)=Z.sub.betap (s)/N(s)=betap(s)/delta(s)            (8) 
    
     where a signifies the Laplace variable. Both transfer functions G psip  (s) and G betap  (s) differ obviously only by their numerator polynomial Z psip  (s), respectively Z betap  (s), while the denominator polynomial N(s) is in both cases the characteristic polynomial of the single track model. 
     Inserting equations (7) and (8) in equation (6) returns for the transfer function between the transverse acceleration aq and the yaw angle velocity psip ##EQU1## 
     The denominator order of the transfer function being one but the numerator order being 2, an additional pole needs to be inserted as a filter for the mathematical realization of equation (9). The amplification and dynamics are dependent on vehicle parameters and on the vehicle speed. 
     Basing on the frequency difference between the curve inside and curve outside wheel, the yaw angle velocity psip can now be calculated using equation (5). Since in the case of a stationary curve travel, where the steering angle delta is constant, the floating angle velocity betap is zero [transfer function (8) has differentiating character], the static transverse acceleration aq 00  in stationary curve travel is according to equations (5) and (6) directly proportional to the vehicle speed and the RPM difference delta --  w 
     
         aq.sub.00 =[r/(2*b)]* delta.sub.-- w * V                   (10). 
    
     According to equation (10), as described in the prior art, a simple linear correlation is expected between the RPM difference delta --  w, the vehicle speed V and the transverse acceleration aq to be determined. 
     But the simple linear correlation reproduces the transverse acceleration only insufficiently well. One reason is, e.g., that the tire deflections due to roll moment backing in curve travel remain unallowed for. The effect in curve travel is that the curve inside tires deflect expansively while the curve outside tires deflect compressively. As a superimposition on the difference frequency caused by the curve travel there occurs thus a difference frequency caused by a radius reduction or radius enlargement of the wheels due to the roll moment backing described above. 
     The nonlinear correlation between the transverse acceleration, the wheel speed difference and the vehicle speed is allowed for by empirically determined parameters, for instance in the form of a nonlinear characteristic diagram or a nonlinear characteristic curve. This makes it possible to adapt the determination of the transverse acceleration to any vehicle. This can be accomplished for example by equipping a vehicle of a series to which the procedure is to be applied, in addition to means for determining the wheel differences and the wheel speed, with transverse acceleration sensors. The aforementioned parameters are arrived at by measuring the transverse acceleration in traveling various curves at various vehicle speeds. In vehicles of same type without transverse acceleration sensors, these parameters can then be used to determine the transverse acceleration from the difference frequency and the vehicle speed. 
     Determined as described above, this value of the transverse acceleration corresponds to the static transverse acceleration aq 00 , since the floating angle velocity betap [equations (6) and (8)] is zero only in stationary curve travel (steering angle delta is constant). The floating angle velocity betap differing from zero, in nonstationary curve travel (steering angle delta not constant), is to be allowed for according to equation (9). Deriving from equation (6) for the static value of the transverse acceleration aq 00 , in the case of stationary curve travel (floating angle velocity betap is zero) is 
     
         aq.sub.00 =psip * V                                        (6). 
    
     Inserting this in equation (9) gives 
     
         aq/aq.sub.00 =1/Z.sub.psip (s) * [Z.sub.psip (s)+Z.sub.betap (s)](9&#39;). 
    
     Allowance for the not disappearing floating angle velocity betap in nonstationary curve travel can thus be made, according to equation (9&#39;), by processing static acceleration aq 00  in a second order filter. Here, the speed dependence of the dynamics can be allowed for by filter coefficients switchable in contingence on travel speed. 
     FIG. 1 shows a block diagram of the embodiment. Referenced 11 and 12 are respective sensors &#34;S&#34; for capturing the wheel speed differences delta --  w and the vehicle speed V. First means 13 are provided for processing the output signals of sensors 11 and 12. The output signals aq 00  of the first means 13 prevail on the input side of the second means 14. The output signals V of the means 12 are optionally passed to the second means. Beyond that, additional variables, specifically vehicle parameters, can be processed in the second means 14. Referenced 15 is a chassis control and/or steering system to which the output signals aq of the second means 14 and/or the output signals aq 00  of the first means 13 are transmitted. 
     The mode of operation of the embodiment will be described as follows: 
     The second signals delta --  w representing the RPM differences of at least two wheels of at least one axle and the third signals V representing the vehicle speed are captured by means 11 and 12. Specifically, this can be accomplished by wheel speed sensors used with an antilock brake system (ABS). Known thereby are the RPM of the individual wheels as well as the mean RPM of several wheels, as vehicle speed. 
     Stored in the first means I3 are the parameters described above, which represent the nonlinear correlations between the transverse acceleration, the wheel speed difference and the vehicle speed. These may be stored in the form of a nonlinear characteristic diagram. 
     An embodiment of the first means 13 is illustrated in FIG. 2. Here, the second signals delta --  w representing the RPM difference are relayed to means 21. The means 21 have the transfer performance described by equation (5), so that on the output side of the means 21 a signal psip is present which represents the yaw angle velocity in the case of stationary curve travel with a constant roll radius of the tires. The signal psip is linked with the signal V representing the vehicle speed by a nonlinear characteristic curve, resulting in the first signal aq 00  of the static transverse acceleration. The assumption here is that in the stationary case (steering angle is constant) the static transverse acceleration aq 00  is according to equation (5) proportional to the yaw angle velocity. Therefore, it is sufficient to store the nonlinear performance in the form of a nonlinear characteristic curve. 
     The aforementioned dynamization of the stationary value of the transverse acceleration takes place by processing the appropriate first signal aq 00  in the second means (FIG. 1), the transfer performance of which is described by equation (9&#39;). Here, the speed dependence of the dynamics can be allowed for by filter coefficients switchable in contingence on travel speed, for which purpose the travel speed, in addition to other vehicle parameters, is optionally transmitted to the second means 14. 
     Present on the output side of the means 14 is thus the fourth signal aq representing the transverse acceleration of the vehicle. This signal aq can now be transmitted for processing to a system which influences the travel dynamics of the vehicle. Such systems are for instance chassis control or steering systems.