Patent Grant
8645038
This application claims the priority, under 35 U.S.C. §119, of German patent application DE 10 2010 004 113.0, filed Jan. 7, 2010; the prior application is herewith incorporated by reference in its entirety.
Field of the Invention
The invention relates to a method and a device for determining a maximum coefficient of friction μmax between a tire and an underlying surface. Knowing the maximum coefficient of friction means that an increased risk of an additional potential longitudinal or transverse slip of the tire may be taken into account in motor-vehicle electronics.
How to specify a maximum coefficient of friction as an estimated value on the basis of customary asphalt pavements and typical tires is already known. What is disadvantageous therein is that a maximum coefficient of friction specified through estimating is extremely imprecise and only seldom reflects the actual conditions with sufficient accuracy.
It is accordingly an object of the invention to provide a method and a device for determining a maximum coefficient of friction μmax between a tire and an underlying surface which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows for the maximum coefficient of friction μmax to be determined more accurately.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of determining a maximum coefficient of friction μmax between a tire and an underlying surface, the method which comprises:
empirically determining a reference curve μref(S) of a coefficient of friction μref as a function of a slip S between the tire and an underlying reference surface;
determining a first momentary slip S1 for the tire and a first momentary coefficient of friction μ1 between the tire and the underlying surface at a first instant t1;
determining a second momentary slip S2 for the tire and a second momentary coefficient of friction μ2 between the tire and the underlying surface at a second instant t2;
computing a slope of the momentary curve μ(S) of the coefficient of friction μ as a function of the slip S; and
computing the maximum coefficient of friction μmax between the tire and the underlying surface by transforming the reference curve μref(S) on the basis of the slope of the curve μ(S).
In other words, the objects of the invention are achieved with a novel method for determining a maximum coefficient of friction μmax between a tire and an underlying surface that entails empirically determining a reference curve μref(S) of a coefficient of friction μref as a function of a slip S between the tire and an underlying surface, determining a first momentary slip S1 for the tire and a first momentary coefficient of friction μ1 between the tire and the underlying surface at a first instant t1, determining a second momentary slip S2 for the tire and a second momentary coefficient of friction μ2 between the tire and the underlying surface at a second instant t2, computing a slope in the momentary curve μ(S) of the coefficient of friction μ as a function of the slip S, and computing the maximum coefficient of friction μmax between the tire and the underlying surface by transforming the reference curve μref(S) on the basis of the slope in the curve μ(S).
With the aid of the vehicle devices that are present in any event in modern motor vehicles it is possible to use the information provided by the vehicle devices such as, for example, by vehicle control devices such as ABS, ESP, ESC, and/or a tire control system to be able to compute the momentary slip S and the momentary coefficient of friction μ. By comparing the momentary curve μ(S) that can be determined from the measuring points (μ1; S1) and (μ2; S2) with the empirically determined reference curve μref(S) it is possible using a suitable mathematical method to very accurately estimate, in particular interpolate, the maximum coefficient of friction μmax for the momentary material pairing of the tire with the underlying surface. That enables prompt account to be taken of changing frictional characteristics of the underlying surface particularly when the underlying surface is changing. A clearly too-risky assumption for the maximum coefficient of friction μmax or a clearly too conservative assumption for the maximum coefficient of friction μmax will be avoided thereby. It will simultaneously be possible to take account automatically of frictional characteristics of the tire that change owing to, for example, different air pressures, different tire temperatures, and/or wear and tear. A motor vehicle's driving systems will be enabled thereby to allow for a non-ideal tire pressure and/or signs of ageing and/or signs of wear and tear in the tire when monitoring traveling situations of the motor vehicle. Prompt intervention by a drive-control system such as in the case of, for example, ABS, ESP, or ESC can be insured thereby and unnecessary intervention by a drive system avoided. Driving safety will be enhanced thereby with no adverse effect on driving convenience.
For determining the reference curve a multiplicity of tests can be conducted for a specific type of tire, particularly on an identical underlying surface, to obtain a multiplicity of measuring points that include both the region of static friction and the transition to sliding friction. The slip S can be determined for the respective tire with the aid of, for instance, a wheel-speed sensor by computing the rotational speed of the respective wheel ω and the rotational speed of a wheel ω0 that is entrained in an idling fashion, using the formula:
The momentarily acting coefficient of friction μ is computed from the ratio between the normal force FN acting in the respective tire and the transverse force FX acting transversally thereto. The normal force FN for the respective tire is a portion z of the absolute load at rest being the product of the vehicle's mass m and the force of Earth's gravity g. For the transverse force FX, what operates from the overall acceleration force as computed from the vehicle's mass m and its acceleration a is a relative acceleration component w so that
is produced as the coefficient of friction. The respective parameters for computing the coefficient of friction μ are either already known or can be determined from the customarily present driving systems and made available. For determining the momentary slip and the momentary coefficient of friction μ, the measuring instants t1 and t2 are sufficiently far apart in time to be able to determine the slope of the curve of a changing slip with sufficient accuracy. The measuring instants t1 and t2 are in particular sufficiently close together in time to have been assigned with high probability to the same acceleration process and/or the same braking process. It is furthermore possible to determine the momentary slip S and momentary coefficient of friction μ substantially continuously and use measured values that are sufficiently far apart for customary measurement inaccuracies not to be able to significantly adversely affect the slope's correct computation. One of the measuring instants t1 or t2 is particularly preferably an instant at which the tire is not revolving and the coefficient of friction μ and the slip S will by definition be zero. This zero point can also be specified as one of the measuring instants so that it is possible to compare the momentary measurement with the zero point in order to determine the slope of p(S). In the event that the momentary conditions are sufficiently close to the computed maximum coefficient of friction μmax, the zero point will additionally or alternatively no longer be taken into account but, instead, preferably a multiplicity of momentary measuring points so that account can be taken of a non-linear curve p(S) in the measured range.
What is particularly preferably determined is a maximum coefficient of friction μmax,x in the direction of the tire's travel and a maximum coefficient of friction μmax,y transverse to the tire's direction of travel. Account can thereby be taken of the fact that tires usually have different adhesion characteristics in the direction of travel and transversely thereto. It will in particular be possible to separately assess the risk of a tire's skidding or spinning while moving in a straight line on the one hand and round a bend on the other. Account can likewise be taken thereby of a motor vehicle's possible swerving or floating.
Affine mapping of the reference curve μref(S) preferably takes place while said curve is being transformed, with in particular exclusively mathematical rotating and mathematical scaling of the reference curve μref(S) taking place. That makes it possible to map the reference curve μref(S) onto the momentary actual curve using simple mathematical computations and to read the mapped maximum coefficient of friction μmax. The parameters for performing said affine mapping can in particular be determined on the basis of the determined slope of the actual curve of μ(S). In particular, if only the momentary maximum coefficient of friction μmax is to be computed, it will suffice to transform just the point for the maximum coefficient of friction μmax of the reference curve μref(S) to directly obtain the momentary actual maximum coefficient of friction μmax.
Preferably a plurality of reference curves μref(S) will be empirically determined as a function of tire wear and tear. The momentary tire wear and tear will be determined with the aid of a tire control system, which in particular determines a tire-profile depth of the tire, and the reference curve μref(S) corrected on the basis of the momentary tire wear and tear with the aid of the further reference curves μref(S). That will enable the tire wear and tear already to be taken into account in the reference curve μref(S). The maximum coefficient of friction μmax can consequently be determined with greater accuracy. More complex computations of the tire wear and tear in determining the momentary maximum coefficient of friction μmax will as a result be unnecessary. Computing the maximum coefficient of friction μmax will be simplified thereby and can be performed faster.
Preferably tire data, in particular tire dimensions, tire type, tire pressure, tire temperature, and/or tire-profile depth will be determined with the aid of a tire control system and the reference curve μref(S) corrected with the aid of the tire data to take account of tire data deviating from the momentary tire data when determining the reference curve μref(S). It is thereby possible to allow for the fact that the tire data used for empirically determining the reference curve μref(S) may differ from that used for the momentary measurement. For example the momentary tire pressure can be lower than the tire pressure applying when the reference curve μref(S) is determined empirically. Using suitable correction factors will enable thus differing conditions that influence the frictional characteristics of the tire/underlying surface material pairing to be taken into account. Suitable correction factors can be determined experimentally as a function of the parameter requiring to be taken into account.
In particular a relative acceleration component is determined with the aid of a vehicle control device during a braking and/or an accelerating action. With the aid of a vehicle control device suitable therefor it can be determined which tire absorbs the occurring acceleration forces to what extent during an accelerating or braking action. Data of such kind can be provided by, for example, a vehicle control device whose function is to prevent the motor vehicle from skidding such as ESP, for instance.
Preferably a more relative wheel load is determined with the aid of a tire control system that in particular provides tire stand surface data (i.e., the surface of the tire that is momentarily supported on and touching the underlying surface) and/or pressure data and/or tire data. In particular an uneven distribution of the motor vehicle's weight over a plurality of tires can be taken into account thereby. For determining the relative wheel load for example the extent of deformation during a wheel rotation of the tire compared with other tires can be determined.
In particular the tire's slip is determined with the aid of a wheel-speed sensor. With the aid of the wheel-speed sensor it is possible to determine the tire's momentary rotational speed and the rotational speed of a tire that is entrained in an idling fashion.
The determined maximum coefficient of friction μmax is made available particularly preferably to a drive-regulating system, especially ABS, ESP, ESC, and/or—via a CAN bus—to motor-vehicle systems. This enables driving systems which for their operation require at least one estimated value for the maximum coefficient of friction to be supplied with very precise information about the momentary maximum coefficient of friction μmax. Due to the fact that the maximum coefficient of friction μmax is determined anew in an ongoing fashion, the driving systems are enabled to take account of changing frictional conditions without needing an additional separate sensor system therefor. The driving systems' operating mode is improved thereby. Desired safety reserves can furthermore be more precisely determined and maintained.
With the above and other objects in view there is also provided, in accordance with the invention, a device for determining a maximum coefficient of friction μmax between a tire and an underlying surface, in particular a device for carrying out the above-outlined method. The device comprises:
a non-volatile memory with an empirically determined reference curve μref(S) of a coefficient of friction μref as a function of a slip S between the tire and an underlying reference surface;
a measuring system for determining a first momentary slip S1 for the tire and a first momentary coefficient of friction μ1 between the tire and the underlying surface at a first instant t1 and for determining a second momentary slip S2 for the tire and a second momentary coefficient of friction μ2 between the tire and the underlying surface at a second instant t2; and
a computing unit for computing a slope of the curve μ(S) of the coefficient of friction μ as a function of a slip S and for computing the maximum coefficient of friction μmax between the tire and the underlying surface by transforming the reference curve μref(S) on the basis of the slope of the curve μ(S).
In other words, the novel device has a non-volatile memory having a stored empirically determined reference curve μref(S) of a coefficient of friction μref as a function of a slip S between the tire and an underlying reference surface. Further provided is a measuring system for determining a first momentary slip S1 for the tire and a first momentary coefficient of friction μ1 between the tire and the underlying surface at a first instant t1 and for determining a second momentary slip S2 for the tire and a second momentary coefficient of friction μ2 between the tire and the underlying surface at a second instant t2. The device further has a computing unit for computing a slope in the momentary curve μ(S) of the coefficient of friction μ as a function of the slip S and for computing the optimum coefficient of friction μmax between the tire and the underlying surface by transforming the reference curve μref(S) on the basis of the slope of the curve μ(S). The device can in particular be embodied and developed as described above with reference to the inventive method. With the aid of the vehicle devices present in any event in modern motor vehicles it is possible to use the information provided by the vehicle devices such as, for example, by vehicle control devices such as ABS, ESP, ESC, and/or a tire control system to be able to compute the momentary slip S and the momentary coefficient of friction μ. By comparing the momentary curve μ(S) that can be determined from the measuring points (μ1; S1) and (μ2; S2) with the empirically determined reference curve μref(S) it is possible using a suitable mathematical method to very accurately estimate, in particular interpolate, the maximum coefficient of friction μmax for the momentary material pairing of the tire with the underlying surface.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for determining a maximum coefficient of friction μmax between a tire and an underlying surface, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Referring now to the figures of the drawing in detail and first, particularly, to
A slip S resulting between the top 14 of the underlying surface 10 and the outer side 16 of the tire 12 between the tire 12 and the underlying surface 14 is determined by the rotational speed of tire ω and the rotational speed of a tire ω0 entrained in an idling fashion according to the formula:
For the traveling situation shown in
The maximum coefficient of friction 26 can be computed by means of a device 32 shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 004 113 | Jan 2010 | DE | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4180223 | Amberg | Dec 1979 | A |
| 5163530 | Nakamura et al. | Nov 1992 | A |
| 5197008 | Itoh et al. | Mar 1993 | A |
| 5255192 | Ito et al. | Oct 1993 | A |
| 5307274 | Takata et al. | Apr 1994 | A |
| 5508924 | Yamashita | Apr 1996 | A |
| 5513907 | Kiencke et al. | May 1996 | A |
| 5526263 | Tanaka et al. | Jun 1996 | A |
| 5576959 | Hrovat et al. | Nov 1996 | A |
| 5581465 | Adler et al. | Dec 1996 | A |
| 5701248 | Wanke | Dec 1997 | A |
| 5710704 | Graber | Jan 1998 | A |
| 5711023 | Eckert et al. | Jan 1998 | A |
| 5711024 | Wanke | Jan 1998 | A |
| 5742507 | Eckert | Apr 1998 | A |
| 5826210 | Izumi et al. | Oct 1998 | A |
| 5869742 | Koster et al. | Feb 1999 | A |
| 6061642 | Nakajima | May 2000 | A |
| 6092014 | Okada | Jul 2000 | A |
| 6208927 | Mine et al. | Mar 2001 | B1 |
| 6216079 | Matsuda | Apr 2001 | B1 |
| 6278929 | Tozu et al. | Aug 2001 | B1 |
| 6401023 | Takahashi | Jun 2002 | B1 |
| 6415215 | Nishizaki et al. | Jul 2002 | B1 |
| 6456920 | Nishio et al. | Sep 2002 | B1 |
| 6463378 | Nishio | Oct 2002 | B2 |
| 6618663 | Aga | Sep 2003 | B2 |
| 6671604 | Frentz et al. | Dec 2003 | B1 |
| 6810317 | Sauter et al. | Oct 2004 | B2 |
| 6839615 | Yanase | Jan 2005 | B2 |
| 6868324 | Matsumoto et al. | Mar 2005 | B2 |
| 6892123 | Hac | May 2005 | B2 |
| 6904349 | Mori | Jun 2005 | B2 |
| 6904351 | Hac | Jun 2005 | B1 |
| 6922624 | Isaji et al. | Jul 2005 | B2 |
| 6971726 | Levy et al. | Dec 2005 | B2 |
| 7139650 | Lubischer | Nov 2006 | B2 |
| 7191047 | Chen et al. | Mar 2007 | B2 |
| 7206702 | Isono et al. | Apr 2007 | B2 |
| 7248958 | Watanabe et al. | Jul 2007 | B2 |
| 7315777 | Ono | Jan 2008 | B2 |
| 7451032 | Brown et al. | Nov 2008 | B2 |
| 7509204 | Phillips et al. | Mar 2009 | B2 |
| 7620504 | Ogawa | Nov 2009 | B2 |
| 7680577 | Mori | Mar 2010 | B2 |
| 7734406 | Oppenheimer et al. | Jun 2010 | B1 |
| 8027775 | Takenaka et al. | Sep 2011 | B2 |
| 8086382 | Dagenais et al. | Dec 2011 | B2 |
| 8155852 | Takenaka et al. | Apr 2012 | B2 |
| 8271175 | Takenaka et al. | Sep 2012 | B2 |
| 20010053953 | Gong et al. | Dec 2001 | A1 |
| 20030195690 | Batistic et al. | Oct 2003 | A1 |
| 20040186648 | Zheng et al. | Sep 2004 | A1 |
| 20050038588 | Shukla | Feb 2005 | A1 |
| 20110106458 | Shiozawa et al. | May 2011 | A1 |
| 20110166761 | Kammann et al. | Jul 2011 | A1 |
| Number | Date | Country |
|---|---|---|
| 10338965 | Mar 2005 | DE |
| 102004053880 | May 2006 | DE |
| 102007062203 | Jun 2009 | DE |
| 2009080416 | Jul 2009 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20110166761 A1 | Jul 2011 | US |
