GB2270167A

GB2270167A - Method and apparatus for the evaluation of wheel speed data in a motor vehicle

Info

Publication number: GB2270167A
Application number: GB9317289A
Authority: GB
Inventors: Bernhard Miller; Eberhardt Schunck
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1992-08-29
Filing date: 1993-08-19
Publication date: 1994-03-02
Also published as: GB9317289D0; DE4228894A1; JPH06227226A

Abstract

A method of evaluating wheel speed data in a two-axle motor vehicle comprises detecting first signals (nvl, nvr, nhl, nhr) representing the rotational speed of the wheels and determining, from the first signals, second signals (delta nv) and third signals (delta nh) representing the difference between the wheel speeds of the vehicle axles. Fourth signals (Zarvl, Zarvr, Zarhl, Zarhr) representing the relative movements between the vehicle chassis and the wheels (spring stroke travels) are also detected and are processed, preferably by means of characteristic value fields (1001v, 1001h) into correction values (Corv, Corh) for the second and third signals or for values (delta nv', delta nh') obtained by low-pass filtering thereof. By this method, different states of loading of the vehicle and non-linearities in the suspension systems can be taken into consideration in further processing of the wheel speed signals, for example for the determination of tyre state or steering angle. <IMAGE>

Description

2270167 METHOD AND EQUIPMENT FOR THE EVALUATION OF WHEEL SPEED DATA IN A

MOTOR VEHICLE The present invention relates to a method and equipment for the evaluation of wheel speed data in a motor vehicle.

To improve the travel comfort of passenger and/or commercial vehicles, there has been a change from the hitherto predominantly used passive suspensions to active suspensions. In the case of such active suspensions, the characteristic G.1c 4Che s:;--,pension systems between the vehicle chassis and the wheels can be influenced during use according to the state of travel, by way of control or regulation. Since the spring stroke movements, i.e. the suspension- induced relative movements between the vehicle chassis and the wheels, are of great significance in this case, these are usually detected in the case of active suspensions.

in addition, motor vehicles are nowadays equipped to an increasing degree with anti-lock systems or regulators for braking and/or traction slip. In these cases, the knowledge of the wheel speeds is of importance.

In EP-A 441 600, the rotational speeds of the diagonally opposite wheel pairs in a four-wheeled vehicle are subtracted from each other and the operational state of the tyres is determined from the result of this comparison.

2 Moreover, it is known from, for example, DE-A-25 18 816 to compare the rotational speeds of the wheels of a vehicle with a reference value and to determine tyre pressures from the deviation of the rotational speeds from the reference value. The reference value 5 in that case is formed from the rotational speeds of all wheels.

Also known are systems, in which the steering angle of a vehicle is determined from wheel speed differences, in particular of the front axle, in the case of curved travel.

According to a first aspect of the present invention there is provided a method for the evaluation of rotational wheel speed signals in a motor vehicle with at least two axles, comprising the steps of detecting first signals, which represent the rotational speeds of the wheels, determining at least second and third signals, which represent the differences between the rotational speeds of the wheels of the at least two vehicle axles, from the first signals, detecting fourth signals, which represent the relative movements between the vehicle chassis and the wheels, and determining correction values for the second and third signals or for the low pass filtered second and third signals from the fourth signals.

Such a method may have the advantage that different load states of the vehicle and non-linearities in the suspension systems (spring, shock absorber, stabilisers, tyre deformation) are taken into consideration in the evaluation of the wheel speed signals. This is effected by correction of the wheel speed signals by the detected suspension stroke travels. In addition, irregularities in the vehicle travel path can -be.t,.aken into consideration by suitable 3 filtering of the wheel speed signals. Such a preparation of wheel speed signals may be distinguished by a relatively high sensitivity to wheel speed differences.

Since the rear wheels and front wheels of a vehicle are exposed to loads of a different kind (tyre wear, wheel load, different.tyre types), the absolute speeds are preferably not compared between the front axle and the rear axle. Although this has the effect of excluding recognition of simultaneous fault occurrence of like kind at both wheels of an axle, the interference security of the entire system rises.

In an advantageous example, the corrected second and third signals and/or the corrected low-pass- filtered second and third signals, thus the corrected wheel speed differences of the two axles, are compared with each other. The operational state of the tyres can then be determined from the result of the comparison. Since a reduction in the pressure in one tyre generally leads to a reduction in its diameter, the rotational speed at this tyre increases in the case of constant longitudinal speed of the vehicle. Thus, by the axle-wise comparison of the wheel differences corrected in the manner described above, tyre pressure differences can be ascertained with high sensitivity.

In another advantageous embodiment, the corrected second and/or third signals and/or the corrected low-pass-filtered second and/or third signals, i.e. the load-corrected wheel speed differences and/or the load-corrected and surfaceirregularity-corrected wheel speed differences, are utilised for ascertaining steering angle signal. The steering angle of the vehicle can be ascertained with high sensitivity by this measure.

Expediently, the correction values are determined from the fourth signals by means of a characteristic values field. The correction values preferably represent different load states of the vehicle at an axle and/or non-linearities of the wheel suspension components 5 andlor nonlinearities of the tyres.

For preference, the difference between the second and third signals andlor the difference between the second and third signals corrected by the correction values is ascertained as a difference signal.

If the track displacement of the axles during curved travel is taken into consideration in the comparison of the wheel speed differences of the individual axles, an even more accurate result may be achieved.

If so desired, the absolute_ tyre pressure can be determined by means of a frequency analysis of the first andlor fourth signals.

According to a second aspect of the invention there is provided equipment for the evaluation of rotational wheel speed signals in a motor vehicle with at least two axles, comprising first means for the detection of first signals, which represent the rotational speeds of the wheels, first difference-forming means for determining at least second and third signals, which represent the differences between the rotational speeds of the wheels of the at least two axles, from the first signals, second means for the detection 'of fourth signals, which represent the relative movements between the vehicle chassis and the wheels, and third means for determining correction values for the second and third signals, or for the second and third signals filtered by means of low-pass filter means, from the fourth signals.

For preference the equipment comprises comparison means for the comparison of the corrected second and third signals and/or for the comparison of the corrected low-pass-filtered second and third signals and indicating means which are actuated in dependence on the 5 result of the comparison.

Preferably, the equipment comprises evaluating means for determining a signal, which represents the steering angle of the vehicle, from the corrected second and third signals and/or the corrected low-pass-filtered second and third signals.

Examples of the method and embodiments of the equipment of the present invention will now be more particularly described with reference to the accompanying drawings, in which:

Fig. 1 is a schematic block circuit diagram of first equipment embodying the invention; Fig. 2 is a schematic block circuit diagram of second equipment embodying the invention; Fig. 3 is a schematic block circuit diagram of third equipment embodying the invention; and Fig. 4 is a schematic block circuit diagram of an adjunct to the equipment of any one of Figs. 1 to 3.

Referring now to the accompanying drawings, there is shown in each of Figs. 1 to 3 a circuit diagram of a respective embodiment of vehicle equipment for processing wheel speed data. Those items with the same functional behaviour, are denoted by the same reference numerals.

In Figs. 1 to 3, the wheel units of a four-wheel, two-axle motor vehicle are denoted by 11 with suffices 'v', 'h', 'I' and 'r' to indicate position, wherein 'v' signifies front, 'h' signifies rear, '1' signifies left and Ir' signifies right. Thus, for example, the wheel unit 1M is that at the vehicle front axle on the lefthand side. The same suffices are used in the drawings to denote the position of other components or the association of components or signal values with the front or rear axle and lefthand or righthand side of the vehicle.

in Arranced at the wheel units are rotational wheel speed sensors 1 and spring stroke travel sensors 2. The sensors 1 detect rotational wheel speeds n in known manner, whilst the sensors 2 sense the spring stroke travels Zar in known manner. The sensors 2 can, in fact, be arranged to detect spring stroke speed, the thus obtained speed signal being processed by integration into a stroke travel signal.

The rotational wheel speed difference delta n is ascertained for each axle by a respective unit 1002. The thus obtained wheel speed difference per axle is dependent on varying irregularities in road surface (by 'road surface' is meant the surface of the path of the vehicle, irrespective of whether the path is a formed road, a track or unformed ground) contacted by the vehicle wheels. Thus, the wheel speed difference per axle will exhibit short-term fluctuations even in the case of straightahead travel with uniform loading of the axle and the same tyre air pressures, the fluctuations being due to the righthand tyre and the lefthand rolling over different surface irregularities of the road. These short-term fluctuations are eliminated by low-pass filtering in respective low-pass filters 1003.

There is thus obtained low-pass filtered wheel speed difference values delta nv' and delta nh' corrected for road surface irregularities.

A further undesired effect during the formation of the wheel speed differences per axle results from the fact that the vehicle can have different states of loading. The load acting on a suspension system mounted between the vehicle chassis and a wheel is, however, generally non-linearly associated with the spring stroke travel displacement resulting therefrom, since the springs, the shock absorbers and/or the stabilisers have non-linear effects at an axle. Consequently, different states of loading of the vehicle lead to nonlinear spring stroke travels. Thus, for example, a loading of the righthand axle half increased by one tenth by comparison with the lefthand axle half does not necessarily lead to a spring stroke travel lower by a tenth in the righthand suspension system. These non- linearities induced by different loadings are corrected in blocks 1001v and 1001h. For this purpose, the spring stroke travel Zar is processed for each axle by means of a characteristic field to produce a correction value Cor. This correction factor Cor is superimposed in a respective unit 1004 on the associated wheel speed difference signal delta nv or delta nh or on the rotational wheel speed difference signal delta nvl or delta W low-pass filtered in the manner described above. Thus, the wheel speed difference of each axle are present at the output of the respective unit 1004, wherein these differences are independent of axle load and road surface irregularity by virtue of the afore-described processing. Moreover, the characteristic field corrections emanating from the units 1001 also take into consideration differences in loading of the axles compared to each other.

Possible distortions in the rotational speed differences caused by angle of yaw, track displacement and so forth are corrected by a block 1012 with a characteristic field. This correction can be carried out, for example, in such a manner that the rotational speed difference of the rear axle is corrected by way of the field characteristic shortly before the subtraction of speed differences of the two axles, thus at a point in the sequence at which not.hing uncorrected is normally present apart from the afore-mentioned distortions of angle of yaw, track displacement etc. The characteristic field of the block 1012 can be of such a nature that, for curved travel, the wheel speed difference values of the rear axle, which are generally smaller, are adapted (raised) to the difference values of the front axle.

For correction of the angle of yaw, the longitudinal vehicle speed v, can be fed to the block 1012 together with the corrected wheel speed difference of the front axle. The longitudinal vehicle speed vi can be ascertained, for example, by the wheel speed of one wheel of an axle being additively superimposed (unfiltered) in a block 1016 on the wheel speed difference value for that axle.

The inclusion of the vehicle speed v 1 takes into account the fact that a different track displacement or yaw movement is generally present at higher vehicle speeds than at lower vehicles speeds. This is largely due to drift movements of the tyre. In a simpler form of signal processing, however, the application of the longitudinal vehicle speed to the block 1012 can be dispensed with.

Whilst the wheel speed differences of the rear axle can be assimilated to those of the front axle subject to consideration of the track displacement or of yaw movements in the manner described above, it can, of course, also be provided that the wheel speed differences of the front axle are corrected correspondingly or the differences of the two axles are corrected in such a manner that the axles are, for purposes of computation, theoretically disposed in the middle of the vehicle with respect to the track displacement or the yaw movements.

The wheel speed differences corrected for axle load,, road surface irregularity, track displacement and/or angle of yaw are not compared (by subtraction) in a unit 1011. If all four tyres have the envisaged air pressure, a signal delta n of the magnitude zero is ideally present at the output of the unit 1011. The output signal of the unit 1011 is low-pass filtered in a unit 1013 in order to blank out short-term fluctuations in the difference of the wheel speed differences per axle. The filtered signal is now compared with threshold values in a block 1014. If the difference derived from comparison of the individual axle differences lies above a threshold interrogated in the block 1014, a report of air pressure loss in a tyre is communicated to the driver, for example by way of an indicating device 11015.

- Since, as already mentioned, all influencing magnitudes which can cause different rotational wheel speeds are taken into consideration (load differences, irregularities of road surface, track displacement, angle of yaw), the rotational wheel speed difference between the axles (output signal delta n of the block 1011) is a reliable indication of a change in radius at one wheel. Accordingly, through the described procedure pressure deviations can be recognised between the individual wheels of a motor vehicle. A simple tyremonitoring system at economic cost is thus provided, which apart from the recognition of pressure drop or pressure rise of a tyre in consequence of air loss or tyre heating (for example, defects or brakes pulling strongly to one side) also recognises one-sided tyre wear, for example due to a wrongly set toe-in angle.

The system may be able to be realised on the basis of an already available sensor system (wheel speed sensors, spring stroke travel sensors for wheel suspension, etc).

A further development of the system illustrated in Fig. 1 is shown in Fig. 2. When the driver receives a report from the device 1015 that the operational state of at least one tyre is not in order, the embodiment of Fig. 1 does not indicate at which axle of the vehicle the defective tyre is present. If, however, the wheel speed differences, after correction for axle loading and road surface irregularities, for each axle are compared as shown in Fig. 2, optionally after smoothing in low-pass filters 10131, with threshold values in blocks 1014', information is obtained as to which axle has the wheel speed difference. This can then be indicated in the appropriate one of two indicating devices 10151.

A further embodiment, in which the steering angle is ascertained from the wheel speed differences, is illustrated in Fig. 3. For this purpose, the wheel speed difference, corrected for axle loading and road surface irregularities, of an axle, preferably the front axle, is applied to a unit 1020. Also applied to the unit 1020 i s the longitudinal vehicle speed, which is ascertained in the manner previously described. A signal proportional to the steering angle of the vehicle can now be formed from the wheel speed difference and the vehicle speed in the unit 1020 by means of a characteristic values field.

This procedure has the advantage that the steering angle of the vehicle is ascertainable from an available sensor system, thus without use of steering angle sensors with high sensitivity and independently of axle loading and road surface irregularity. The ascertaining of steering angle from the wheel speeds can be additional to the already described tyre monitoring or can be installed as a separate system without tyre monitoring.

In connection with the corrections for axle loading (units 1001) and track displacement or yaw movement (unit 1012), it should be noted that for low vehicle speeds V 1 and large steering angles,the track displacement correction supplies a large component. Thereagainst, the effect of the loading correction is greater for high longitudinal vehicle speeds V 1 and small steering angles.

The embodiments illustrated in Figs. 1 and 2 recognise a relatively small pressure deviation between the individual wheels.

However, the absolute tyre pressure values can be deduced from the natural frequency of a wheel in the embodiment illustrated in Fig. 4.

In Fig. 4, the rotational wheel speed nij (nvl, nvr, nhl,nhr) and/or the spring stroke travel Zarij (ZarvI, Zarvr, Zarh], Zarhr) of a wheel are fed to a unit 41. In the unit 41, the frequency of the input signal nij or Zarij is analysed. The frequency analysis of several periods of decaying oscillation, for example after an excitation by the road surface. leads to the freauency vij (natural frequency behaviour) typical for that wheel. This frequency vij is dependent on the air pressure in the tyre pij of the wheel. The dependency of the frequency on the tyre air pressure is filed in a unit 42. By this means there is obtained the absolute tyre pressure of a wheel. The procedure illustrated in Fig. 4 considered on its own may, however, have the limitation that such a frequency analysis is possible only for certain travel path excitations. For example, such a system may not operate in the course of long motorway journeys, as in that case no oscillations may occur or only a few capable of the desired evaluation. In combination with the embodiments illustrated in Figs. 1 and 2, in which the tyre states can be monitored continuously, the absolute tyre pressure values of all wheels can, however, be deduced from the relative pressure deviations between the individual wheels through the measurement of the absolute tyre pressure of, for example, one wheel by the means illustrated in Fig. 4.

13 -

Claims

1. A method of evaluating wheel speed data in a motor vehicle with at least two axles, comprising the steps of detecting wheel speed signals having values representing the rotational speed of the wheels of the vehicle at said two axles and wheel movement signals having values representing relative movement of those wheels and the vehicle body due to suspension travel, determining the value of the difference between the wheel rotational speeds for each of the two axles from the associated wheel speed signal values, determining correction values from the wheel movement signal values, and correcting the difference values or filtered values thereof by the correction values.

2. A method as claimed in claim 1, comprising the steps of comparing the corrected values and determining a parameter of the operational state of the vj'ggeel tyres from the comparison result.

3. A method as claimed in claim 1 or claim 2, comprising the step of determining the steering angle of the vehicle from at least one of the corrected values.

4. A method as claimed in any one of the preceding claims, wherein the correction values are determined by means of at least one characteristic values field.

5. A method as claimed in any one of the preceding claims, wherein the correction values represent at least one of the vehicle load - state, a parameter of the suspension system and a parameter of the tyres.

6. A method as claimed in any one of the preceding claims, comprising the step of determining the value of the difference between said difference'values or between said corrected values.

7. A method as claimed in claim 2, wherein said ooerational state parameter is determined in dependence on track displacement of the 10 axles during curved travel of the vehicle.

8. A method as claimed in any one of the preceding claims, comprising the step of determining the absolute pressure of at least one of the wheel tyres by frequency analysis of at least one of the wheel speed and wheel movement signal values.

9. A method as claimed in claim 1 and substantially as hereinbefore described with reference to any one of Figs. 1 to 3 of the accompanying drawings.

10. A method as claimed in claim 9 and modified substantially as hereinbefore described with reference to Fig. 4 of the accompanying 20 drawing.

11. Equipment for evaluating wheel speed data in a motor vehicle with at least two axles, comprising means for detecting wheel speed signals having values representing the rotational speed of the wheels of the vehicle at said two axles and wheel movement signals having values representing relative movement of those wheels and the vehicle body due to suspension travel, means for determining the value of the difference between the wheel rotational speeds for each of the two axles, from the associated wheel speed signal values, means for determining correction values from the wheel movement signal values, and means for correcting the difference values or filtered values thereof by the correction values.

12. Equipment as claimed in claim 11, comprising low-pass filter means for filtering the difference-values.

13. Equipment as claimed in claim 11 or claim 12, comprising comparison means for comparing the corrected values and indicating means for providing an indication in dependence on the comparison result.

14. Equipment as claimed in any one of claims 10 to 13, comprising means for determining the steering angle of the vehicle from at least one of the corrected values.

15. Equipment substantially as hereinbefore described with reference to any one of the accompanying drawings.

16. Equipment as claimed in claim 15 and modified substantially as hereinbefore described with reference to Fig. 4 of the accompanying drawings.

17. A motor vehicle provided with equipment as claimed in any one of 5 cl aims 11 to 14.

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