Method and apparatus for detecting shock absorber damage

In a method and apparatus for detecting shock absorber damage, features of a shock absorber are determined by analyzing a signal of an antilock braking system rotational wheel speed sensor.

DETAILED DESCRIPTION OF THE DRAWINGS The auto power density spectra or characteristic shock absorber damage values illustrated in FIGS. 1 to 4 represent results of test drives at the BMW test site by using a BMW 740iA with varying shock absorbing actions (parameters d, in the preceding equations). In this case, the “sporty” adjustment (high shock absorption) and the “comfortable” adjustment (low shock absorption) were used. The reciprocal values of the rotational speeds of the rim n from the ABS sensor signal were recorded as input data for shock absorber damage detection according to the invention. The temporal course of the radius change &Dgr;r was determined therefrom according to equation (1). As the analysis interval for the determination of the term &phgr; &Dgr;r (&ohgr; 1,i , d) in equation (5a), the range &lsqb;12-15&rsqb; Hz in the auto power density spectrum was selected; as the reference interval for the determination of the term &phgr; &Dgr;r (&ohgr; 2,i ), the range &lsqb;30-33&rsqb; Hz was selected. The quotients determined for these frequencies were added according equation (5a) and were emitted after low—pass—filtering. The auto power density spectra of the radius change &phgr; &Dgr;r of the left front wheel and of the left rear wheel of the data used to compute the characteristic shock absorber damage value are illustrated in FIGS. 1 and 2 . In the range of the analysis interval &lsqb;12-15&rsqb; Hz, the influence of the shock absorber condition (here “comfortable” or “sporty”) upon the course of the auto power density spectrum is clearly recognizable. In the range of the reference interval &lsqb;30-33&rsqb; Hz, such dependence cannot be detected. This means that the auto power density spectrum of the radius change is suitable for judging the shock absorber condition. Based on the illustrated auto power density spectra for the radius change, the courses of the characteristic shock absorber damage value for the left front wheel or the left rear wheel illustrated respectively in FIGS. 3 and 4 are obtained as a function of the shock absorber stage (that is, simulated shock absorber condition) “sporty” or “comfortable”. The temporal courses of the characteristic shock absorber damage values correspond to a constant speed of 80 km/h and an identical route profile. A comparison of FIGS. 3 and 4 shows that the difference in the characteristic shock absorber damage value as a function of the selected shock absorber stage is even more pronounced on the rear axle. Various advantages are connected with the method according to the invention and with the system according to the invention. To detect shock absorber damage according to the invention, no additional sensor system is required, so that a cost-effective solution is provided by integration into an existing control unit. Furthermore, according to the invention, shock absorber condition can be analyzed during actual driving operation. In addition, maintenance of the shock absorbers can be implemented which meets the requirements. For example, shock absorber condition data can be provided to the respective monitoring organization (such as the Technical Surveillance Group) during a general inspection so that the vehicle owner will have no additional cost for a shock absorber inspection which may be planned for the future. Using the structure illustrated in FIG. 6 , measurements were carried out at the BMW test site by means of a BMW 740iA. Different shock absorber actions were adjusted by the selection of the shock absorber position ( “sporty” corresponds to a high shock absorption and “comfortable” corresponds to a low shock absorption). The pertaining DSKW &Dgr;n,F courses are illustrated in FIG. 7 . The top diagram shows the DSKW &Dgr;n,F course for the left front wheel, while the bottom diagram of FIG. 7 shows the course for the left rear wheel, once again for the respective shock absorber positions “sporty” and “comfortable”. In partial areas, the algorithm is masked out, as described above. The illustration shows that the DSKW &Dgr;n,F or the DSKW &Dgr;r,F is suitable for drawing a conclusion with respect to the shock absorber condition. The above-mentioned characteristic damage value DSKW′ &Dgr;r,F or DSKW′ &Dgr;n,F is based on equation (8a) or equation (8b). For this purpose, FIG. 8 shows a simplified structure for determining these characteristic damage values according to the invention. The signal representing the rotational speed change or the radius change is first filtered by means of a corresponding band pass filter for an analysis interval or for first and second reference intervals. Subsequently, a value formation takes place of the determined maxima and minima, followed by filtering by a first low pass filter. This filtering is then followed by a quotient formation according to equations 8a and 8b. Finally, another filtering takes place by means of a second low pass filter. Tests using a BMW 740iA at the BMW test site were carried out also by means of this algorithm. Analogous to the above-explained approach, different shock absorber actions were set as a result of the two shock absorber stages “sporty” (high shock absorbing forces) and “comfortable” (low shock absorbing forces). Qualitatively, the courses are the same as those illustrated in FIG. 7 . Another additional processing of the DSKW x x,x is illustrated in FIG. 9 . Here, the DSKW x x,x can be corrected, for example, by way of a functional or analytical relationship or by an experimentally determined relationship (for example, characteristic diagram) with respect to driving speed. In addition to driving speed, corrections are also advantageous here by means of the throttle valve angle, torque, gear position, rotational engine speed and operating condition of the converter clutch (in vehicles with an automatic transmission) or of the clutch switch (in vehicles with a manual transmission). By selecting the corner frequency of the “low pass 3” filter, the characteristic shock absorber damage value DSKW x x,x can be averaged over a long term (for the useful life of the shock absorber). Shock absorber damage can be detected by querying the threshold value. As soon as the uncorrected or corrected DSKW x x,x is in the damage range ( FIG. 10 ), a conclusion is drawn that a shock absorber is defective and this information is processed. On the one hand, the information concerning a shock absorber defect can immediately be transmitted to the shop (defect memory with read-out in the shop or active shop notification); and on the other hand, it can be made accessible to the customer in the form of a display concept. The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.