Method for Avoiding False Excitations of a Slip Control System of a Brake System of a Vehicle

Abstract

A method for avoiding false excitations of a slip control system comprises monitoring of a wheel speed signal for a predefined first monitoring time period for the start of a negative half-wave of an oscillation applied to a vehicle wheel, when a brake pressure gradient initiated by a brake start rises above a predetermined threshold. A ruck signal is derived from the wheel speed signal. An acceration signal is derived from the wheel speed signal during a predetermined second monitoring period. The wheel speed signal is monitored for a turning point of the ruck signal and a start of the acceleration signal. The oscillation imposed on the vehicle wheel is identified as a braking-induced vibration, and slip control is not carried out when the turning point of the ruck signal and the re-acceleration of the wheel are detected.

Claims

1. A method for avoiding false excitations of a slip control system of a brake system of a vehicle comprising: monitoring a wheel speed signal for at least one wheel for a predetermined first monitoring period for the start of a negative half-wave of an oscillation imposed on the vehicle wheel when a brake pressure gradient initiated by a brake application rises above a predetermined threshold; deriving a ruck signal from the wheel speed signal; deriving an acceration signal from the wheel speed signal during a predetermined second monitoring period following the point in time of the start of a detected negative half-wave of the speed signal; monitoring the wheel speed signal for a turning point of the ruck signal and a start of the acceleration signal indicating the re-acceleration; and identifying the oscillation imposed on the vehicle wheel as a braking-induced vibration, wherein slip control of the vehicle wheel is not carried out when the a turning point of the ruck signal and the re-acceleration of the wheel are detected.

2. The method of claim 1, wherein monitoring of the wheel speed signal further comprises determining a slip signal from the wheel speed signal and monitoring the speed of the vehicle, and identifying the oscillation imposed on the vehicle wheel as a braking-induced vibration when the slip signal does not reach a predetermined slip threshold.

3. The method of claim 1, further comprising increasing the control thresholds of the slip control system upon the detection of a negative half-wave.

4. The method of claim 1, wherein the oscillation imposed on the vehicle wheel is not identified as a braking-induced vibration when no turning point of the ruck signal derived from the wheel speed signal is detected during a predetermined second time segment of the second monitoring period.

5. The method of claim 1, wherein the oscillation imposed on the vehicle wheel is not identified as a braking-induced vibration if no re-acceleration of the acceleration signal derived from the wheel speed signal is detected during a predetermined first time segment of the second monitoring period.

6. The method of claim 5, wherein the control thresholds are reset when a braking-induced vibration is not identified.

7. The method of claim 5, wherein the second time segment is shorter than the first time segment.

8. The method of claim 5, wherein taking into account a signal delay, the first time segment corresponds to half of the second monitoring period.

9. The method of claim 5, wherein taking into account a signal delay, the second time segment corresponds to a quarter of the second monitoring period.

10. The method of claim 1, wherein predeterming the threshold further comprises determining the threshold as a function of one of: the wheel pressure and a filtered signal of the wheel pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:

[0021] FIG. 1 shows a t-v diagram for illustration of the method according to the invention;

[0022] FIG. 2 shows a t-p diagram for illustration of a second embodiment of the method according to the invention;

[0023] FIG. 3 shows a t-v/p diagram for illustration of a third embodiment of the method according to the invention; and

[0024] FIG. 4 shows a t-v/p diagram for illustration of a fourth embodiment of the method according to the invention.

DETAILED DESCRIPTION

[0025] A vehicle with a brake system comprising a slip control system is assumed below, wherein the vehicle wheels are slip-controlled by the slip control system. The vehicle wheels are suspended on a wheel support with vibration damping, which for its part can be displaced together with the vehicle wheel relative to the structure of the vehicle and is thereby able to vibrate.

[0026] Slip, which is determined by means of a difference between the peripheral wheel speed and the speed of the vehicle, can occur on such a vehicle wheel. As mentioned above, there are situations in which false excitations of the slip control system can occur if for example the oscillations in a wheel speed signal are falsely interpreted as slip and the slip control system responds thereto with slip control. In particular, owing to vibrations of the vehicle wheels in the longitudinal direction of the vehicle, which can be initiated by a rapid rise in brake pressure, there is a risk that the slip control system falsely detects said vibrations as slip.

[0027] Therefore, such vibrations must be recognized as braking-induced vibrations, so that false control is not carried out by the slip control system.

[0028] The wheel speed signal that is produced by means of a known wheel sensor system is used for the recognition of such brake pressure induced vibrations of a vehicle wheel. Such a wheel speed signal v is represented in FIG. 1 as graph K1.

[0029] The method begins with monitoring the profile of the brake pressure gradient p.sub.grad represented in FIG. 2 by the graph K3. With said brake pressure gradient p.sub.grad according to graph K3, in FIG. 2 the profile of the wheel brake pressure p is also represented as graph K4 and a profile of the filtered wheel brake pressure p.sub.f is also represented as graph K5.

[0030] If at the point in time t.sub.1 said brake pressure gradient p.sub.grad exceeds a threshold S1, which is determined as a function of the filtered brake pressure p.sub.f, a first monitoring period T.sub.1 of 30 ms starts at said point in time t.sub.1. During said first monitoring period T.sub.1, the wheel speed signal v is checked regarding the start of a negative half-wave of an oscillation imposed on the vehicle wheel. According to FIG. 1, such a half-wave of the wheel speed signal v begins after a period of time Δt′ at the point in time t.sub.2. For said period of time Δt′, Δt′≦T.sub.1 applies. If no start of a negative half-wave is detected during said first monitoring period T.sub.1, the slip control system begins to perform control using the systematic control thresholds, as no disruptive oscillation is expected.

[0031] With the detection of a half-wave at the point in time t.sub.2, a second monitoring period T.sub.2 of 80 ms starts. Said second monitoring period T.sub.2 is used for checking whether the detected half-wave is induced by a brake pressure gradient, i.e. is caused by braking-induced vibration, and therefore no control may be performed by the slip control system because of a falsely identified slip.

[0032] Owing to the detection of the start of the negative half-wave, it is now initially assumed that it is a braking-induced vibration, and the control thresholds of the slip control system are therefore changed to “insensitive”, i.e. they are increased. A check is made using further criteria as to whether it is actually a braking-induced vibration, and—if this is not the case—the increased control thresholds are changed back to the normal values thereof again, i.e. back to “sensitive”.

[0033] This case is illustrated in the t-v/p diagram according to FIG. 3: because of the threshold value S1 being exceeded at the point in time t.sub.1 by the brake pressure gradient p.sub.grad (graph K3) owing to the steep rise thereof, the first monitoring period T.sub.1 begins. Within said monitoring period T.sub.1, the start of a negative half-wave of the wheel speed signal v (graph K1) is detected at the point in time t.sub.2. From said point in time t.sub.2, the onset of control of the slip control system is inhibited owing to the increased control thresholds. During the subsequent second monitoring period T.sub.2, which ends at the point in time t.sub.3, the further criteria are not detected. Thus a braking-induced vibration is not assumed, but an actually existing slip. The slip control thereby begins at the point in time t.sub.3 with the control thresholds again reduced, i.e. the sensitive control thresholds.

[0034] If there is a braking-induced vibration following the detection of a negative half-wave, a check is made as to whether during said second monitoring period T.sub.2 the wheel speed signal v indicates a turning point of a ruck signal derived from the wheel speed signal and the start of an acceleration signal indicating the re-acceleration and derived from the wheel speed signal. In addition, a slip signal K2 (cf. FIG. 1) determined from the wheel speed signal v and the speed of the vehicle is compared with the value of a maximum anticipated slip, i.e. with a slip threshold S2.

[0035] Said criteria for recognizing a braking-induced oscillation of the wheel speed signal v are met according to FIG. 1. Thus, at the point in time t.sub.21 a turning point P1 (corresponding to a turning point of a sinusoidal oscillation) of the ruck signal j derived from the wheel speed signal v is detected, and then at the point in time t.sub.22 a reversal point P2 (corresponding to a reversal point of a sinusoidal oscillation) is detected, wherein the reversal point P2 indicates the start of the re-acceleration of the acceleration signal derived from the wheel speed signal v. Finally, the slip K2 does not reach the predetermined slip threshold S2.

[0036] This ensures that the negative half-wave of the wheel speed signal v occurring in the first monitoring period T.sub.1 is braking-induced and therefore brake slip control of the slip control system is not carried out.

[0037] This case is also shown by the t-v/p diagram of FIG. 4: because the threshold value S1 is exceeded at the point in time t.sub.1 by the brake pressure gradient p.sub.grad (graph K3) owing to the steep rise thereof, the first monitoring period T.sub.1 begins. Within said monitoring period T.sub.1, the start of a negative half-wave of the wheel speed signal v is detected at the point in time t.sub.2. From said point in time t.sub.2, the onset of control by the slip control system is inhibited owing to the increased control thresholds. During the subsequent second monitoring period T.sub.2, which ends at the point in time t.sub.3, both a turning point and a reversal point are detected. Thus a braking-induced vibration is assumed. The normal slip control only starts in the next half-wave at the point in time t.sub.4. The profile of the speed of the vehicle v.sub.F is also plotted in this diagram of FIG. 4.

[0038] If one of said three criteria are not met during the second monitoring period T.sub.2, the slip control system assumes therefrom that there is slip. The half-wave of the wheel speed signal v represented in FIG. 1 changes at the point in time t.sub.22 into a positive half-wave, which ends at the point in time t.sub.3. The period of time Δt.sub.2 between the two points in time t.sub.2 and t.sub.3 does not exceed the second monitoring period T.sub.2.

[0039] Also, the oscillation imposed on the vehicle wheel is then not identified as a braking-induced vibration if no re-acceleration of the acceleration signal derived from the wheel speed signal is detected during a predetermined first time segment T.sub.21 of the second monitoring period T.sub.2. The duration of said first time segment T.sub.21 is preferably half of the second monitoring period T.sub.2 taking into account a signal delay. According to FIG. 1, said re-acceleration of the vehicle wheel occurs at the point P2, i.e. after a period of time Δt.sub.21=t.sub.22−t.sub.2, to which Δt.sub.21≦T.sub.21 applies, following the point in time t.sub.2.

[0040] Furthermore, the oscillation imposed on the vehicle wheel is also not identified as a braking-induced vibration if no turning point P1 of the ruck signal j derived from the wheel speed signal v is detected during a predetermined second time segment T.sub.22 of the second monitoring period T.sub.2. Said second time segment T.sub.22 is shorter than the first time segment T.sub.21 and its duration, taking into account a signal delay, is a quarter of the second monitoring period T.sub.2. According to FIG. 1, the time difference Δt.sub.22=t.sub.21−t.sub.2 between the turning point P1 and the point in time t.sub.2 meets the condition: Δt.sub.22≦T.sub.22.

[0041] The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the scope of the following claims.