. Umted States Patent [19]
`Trost et a].
`
`US005705746A
`
`:
`
`[11] Patent Number:
`[45] Date of Patent:
`
`5,705,746
`Jan. 6, 1998
`
`[54] METHOD OF DETERMINING THE
`TRACTION OF MOTOR VEHICLE WHEELS
`‘
`_
`[75] Invent“: Dune ms‘; Jurgen mst- ‘39th °f
`gumgfmg’grg; Markus Rub‘ K‘rchard“
`“my
`[73] Assignee: Daimler-Benz AG. Stuttgart, Germany
`
`5,424,714
`5,431,455
`5,502,433
`5,532,673
`5,557,552
`5,612,879
`
`6/1995 Kin et a]. .............................. .. 130/271
`1/1996 Iwata et a]. .................... .. 364/426.018
`3/1996 Breuer et al. ........................... .. 73/146
`7/1996 Kin Ct 11].
`340/444
`9/1996 Nallo et a1. ........................... .. 340/433
`3/1997 Makino ................................. .. 340/433
`
`5,659,290
`
`3/1997 Haerl ..................................... .. 340/441
`
`[21] Appl. No.: 805,359
`[22] Filed:
`Feb. 24, 1997
`[30]
`Foreign Application Priority Data
`
`Primary Examiner—-George M. Dombroske
`Assistant Examiner—Eric S. McCall
`Attorney, Agent, or Firm—K1aus J. Bach
`
`Mar. 2, 1996 [DE]
`
`Germany ...................... .. 196 08 064.9
`
`[57]
`
`ABSTRACT
`
`[51] 1m. (31.‘5 ..................................................... .. B60Q 1/00
`[52] U.S. c1. ................................. .. 73/146; 73/3; 130/170;
`180/171; 130/271; 340/433; 340/441; 364/423.093;
`364/424.034; 364/426.01
`[53] Field of Search ................... .. 73/7. 3. 146; 130/170.
`130/171. 172. 271; 340/433. 441. 444;
`364/423.098. 424.034. 424.055. 426.01.
`426.015. 426.013. 426.025. 426.027
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`In a method of determining the traction of motor vehicle
`wheels on wet pavements, a surge force generated by a water
`?lm on the pavement in front of at least one of the vehicle
`wheels is measured and from the measured surge force value
`and the instantaneous vehicle speed a critical speed at which
`the vehicle wheel will aqua-plane is determined and this
`critical speed is displayed to the driver or it is used to control
`a vehicle drive unit to keep the vehicle speed below the
`critical speed.
`
`5,350,035
`
`9/1994 Bodier et a1. ......................... .. 180/271
`
`5 Claims, 2 Drawing Sheets
`
`23
`
`MOTOR
`
`Swagway_1007
`
`

`
`US. Patent
`
`Jan. 6, 1998
`
`Sheet 1 0f 2
`
`5,705,746
`
`Swagway_1007
`
`

`
`US. Patent
`Fig. 2
`
`Jan. 6, 1998
`
`Sheet 2 of 2
`
`5,705,746
`
`MOTOR
`
`Swagway_1007
`
`

`
`1
`METHOD OF DETERMINING THE
`TRACTION OF MOTOR VEHICLE WHEELS
`
`5,705,746
`
`2
`maintained at a level below the speed at which the vehicle
`wheels might lose traction. It is further possible in accor
`dance with the invention to use the signal not exclusively for
`the information of the driver but to employ it as a control
`signal for the vehicle drive unit. for example for the power
`output control device of an internal combustion engine so as
`to reduce the vehicle speed automatically in order to prevent
`aquaplaning. Since the surge forces act in the direction of the
`longitudinal vehicle axis. there is no need for a ?fth wheel
`rolling at an oblique angle to the driving direction.
`The invention will be described below in greater detail on
`the basis of particular embodiment shown in the accompa
`nying drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 shows in a diagram u=f(u) the relationship between
`the friction between the wheel and the pavement.
`FIG. 2 shows. in principle. an arrangement for performing
`the method according to the invention. and
`FIG. 3 is a side view of the wheel shown in FIG. 2
`indicating the forces acting on the wheel.
`
`DETAILED DESCRIPTION OF AN
`EMBODIMENT
`FIG. 1 shows diagrammatically the functional relation
`ship between the friction coe?icient it effective between the
`wheel of a motor vehicle on a wet pavement and the vehicle
`speed depending on the water ?lm thickness d. For clearer
`representation. only three curves 2. 3. and 4 are shown
`wherein the curve 4 is for a relatively small water ?lm
`thickness d3. the curve 3 is for a medium water ?lm
`thickness d2. and the curve 2 is for a relatively large water
`?lm thickness d1. It can be seen that. at low vehicle speeds.
`the curves are all relatively ?at. that is. an increase in the
`vehicle speed causes only a relatively small reduction of the
`friction coefficient u between the tires and the pavement. At
`speeds greater than certain limit values v(d1). v(d2). v(d3)
`respectively, the curves 2. 3 and 4 drop rapidly with small
`speed increases. The curve section which steeply declines
`with increasing vehicle speed indicates the speed range in
`which the wheel 5 that is its ?re no longer rolls in direct
`contact with the pavement but on a water ?lm forming
`between the tire and the pavement. In the areas of transition
`of the curves 2. 3. and 4 from the relatively ?at sections to
`the steeply declining sections that is at a limit friction
`coe?icient value p8,," aquaplaning of the wheel 5 starts. In
`other words the wheel 5 starts ?oating on the water ?lm at
`the speeds V at which the respective curves have the
`transition areas between their relatively ?at sections and the
`steeply inclined sections. This transition area or transition
`point new“: moves with decreasing water ?lm thiclmess d in
`the direction of higher vehicle speeds. Consequently. with
`dl>d2>d3. the limit speed at which a wheel starts
`aquaplaning-which will be called from here on. the “ lan
`ing speed” V A—is the highest for the curve 4 with the
`smallest water ?lm thickness d3 and the lowest for the curve
`2 with greatest water ?lm thickness d1(VA(d1)<VA(d2)<VA
`(do).
`FIG. 2 shows in a top view a motor vehicle front wheel
`5. which is rotatably supported on a wheel carrier 6 and is
`linked. with a control arm 7. by means of elastic bearings 8
`and 9 to a vehicle body which is not shown in the drawings.
`The wheel 5 is supported so as to pivot during spring action
`about a pivot axis 10 de?ned by the two bearing 8 and 9. If
`the wheel 5 rolls on wet pavement in the direction of the
`arrow 11. the wheel is subjected to a force e?ective in the
`
`10
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`25
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`35
`
`The invention relates to a method of determining the
`traction that is the ground adhesion of motor vehicle wheels
`on wet road surfaces wherein a force effective at least at one
`wheel is measured and from this measured force. a signal
`representative of the ground adhesion is generated.
`DE OS 34 09 040 discloses a method wherein the
`loxodromic forces acting on a wheel rolling at an oblique
`angle are used as a measure for the instantaneous road
`surface traction of the wheel. Such forces are measured for
`example at wheels with toe-in adjustment. However, since
`the toe-in adjustment of vehicle wheels is only very small
`accurate termination of the loxodromic forces is not easily
`possible. This is especially true if the loxodromic forces are
`only very small so that they are hardly measurable at all.
`However. the smaller the loxodromic forces are the closer
`the wheel is at the critical point where ground adhesion is
`lost. Consequently, the moment at which a vehicle loses
`ground adhesion. for example because of aquaplaning. can
`not be determined accurately. As a result. a vehicle may have
`lost ground adhesion before a corresponding signal can be
`given to the driver.
`DE US 34 09 04-0 proposes to provide a ?fth wheel
`speci?cally for measuring the loxodromic forces which is
`extended only when needed. the wheel being arranged so as
`to roll at an increased oblique angle so that it generates
`greater loxodromic forces. Such an additional retractable
`wheel however requires additional expenses and additional
`space. In addition, such a ?fth wheel would have to be
`constantly extended when driving on wet roads if the driver
`wishes to be forewarned of possible ground adhesion loss.
`Such a wheel rolling at a relatively large oblique angle and
`used for extended periods increases the vehicle rolling
`resistance and. consequently. the fuel consumption and tire
`wear.
`It is the object of the present invention to provide a
`method of determining the ground adhesion of vehicle
`wheels on wet road surfaces wherein a force effective at least
`at one of the vehicle wheels is permanently measured and
`from this force a signal representative of the wheel traction
`is generated
`SUMMARY OF THE INVENTION
`In a method of determining the traction of motor vehicle
`wheels on wet road surfaces. a surge force generated by a
`road surface water ?lm in front of at least one of the vehicle
`wheels is measured and from the measured surge force and
`the instantaneous vehicle speed a signal is generated corre
`sponding to a critical speed at which the vehicle wheel will
`aquaplane and this speed signal is displayed to the driver or
`used to control a vehicle drive unit to keep the vehicle speed
`below the critical speed.
`When driving on wet roads a water surge area is generated
`in front of the tires which has to be overcome by the vehicle
`wheels. This generates on the vehicle wheels a surge force
`which acts in the direction of the longitudinal vehicle axis
`and which increases with increasing vehicle speed. The
`surge force further increases with the thickness of the water
`?lm on the pavement From this surge force elfective on a
`wheel and the momentary vehicle speed a signal is generated
`in accordance with the invention which indicates the vehicle
`speed at which. with the instantaneous water ?lm thickness.
`aqua planing of the vehicle could occur. Consequently. the
`driver can be reliably forewarned at which speed aqua
`planing may be expected. The vehicle speed can therefore be
`
`45
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`50
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`55
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`65
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`Swagway_1007
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`

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`5,705,746
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`3
`direction of the longitudinal vehicle axis which force is
`called a surge force F, which increases with the vehicle
`speed V and with the Water ?lm thickness d. With the surge
`force F,. the wheel 5 is further subjected to a vertical force
`F., effective on the wheel Sin a direction normal to the road
`surface 13. The vertical force F., tends to lift the wheel 5 oh‘
`the road surface 13 (see FIG. 3). The larger the vertical force
`component F,, is. the smaller are the wheel adhesion to the
`pavement and the friction coe?icient. The surge force FI
`corresponds to the vertical force component F... that is the
`larger the surge force F, is the larger is also the vertical force
`component Fv.
`The surge force F, effective in the longitudinal direction
`is transmitted, by way of the control arm 7, to the elastic
`bearings 8 and 9 and causes a deformation of these bearings.
`This deformation is measured by means of sensors 14 and 15
`arranged at the bearings 8 and 9 and the measured values are
`transmitted as deformation distances at. and x2 in the form
`of electrical signals to an eledronic control unit 18. The
`electronic control unit 18 further receives a signal indicating
`the actual vehicle speed value (arrow 19). On the basis of the
`instantaneous water ?lm thickness dis determined (see also
`FIG. 3).
`Depending on the instantaneous water ?lm thickness d the
`planing speed V A is read from the respective curve in the
`diagram 1u=f(v) (see FIGS. 1 and 2) and is transmitted to the
`driver by way of a control line 20 and a display 21 arranged
`in the vehicle cabin. The functional relationships between
`surge force F, vehicle speed V and water ?lm thiclmess d as
`well as the functional relationships between the friction
`coe?icient u and the vehicle speed V (FIG. 1) are experi
`mentally determined and stored in the memory of the control
`unit 18 as characteristic graphs.
`With the method according to the invention, the driver can
`therefore obtain the information at which speed he may
`expect a loss of wheel traction by aqua planing long before
`the vehicle reaches the planing speed V A. The loss of wheel
`adhesion by aqua planing can therefore be safely avoided by
`the driver by taking appropriate measures.
`In another embodiment according to the invention. the
`control signal generated by the electronic control unit 18 is
`transmitted. by way of a control line 22 (shown by dashed
`lines) to a drive unit 23 (for example. an internal combustion
`engine) of the motor vehicle. The arrangement may be such
`that the power output control device of this drive unit 23
`reduces the power output when a predetermined vehicle
`speed V2=VA-AV below the planing speed V A is reached.
`
`10
`
`20
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`25
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`30
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`35
`
`4
`Instead of using the distance sensors 14. 15 for determin
`ing the surge force F_,. two sensors may be utilized which
`sense the acceleration of the wheel carrier 6. One of the two
`sensors senses the acceleration in the longitudinal direction
`of the vehicle. and the other senses the acceleration of the
`wheel carrier in vertical direction.
`It is of course not necessary to limit the determination of
`the surge force to one wheel. It could be determined just as
`well for two wheels. for example. the two front wheels of a
`vehicle. Then the planing speeds for the two front wheels
`could be compared and the planing speed VA of the wheel
`in greater danger of losing traction could be utilized for
`transmission to the display 21 or for the control of the drive
`unit 23.
`What is claimed is:
`1. A method of determining traction of motor vehicle
`wheels on a wet road surface. on which a surge force is
`generated by a water ?lm on the road surface when said
`motor vehicle is rolling over said road surface in the
`longitudinal direction of said motor vehicle. comprising the
`steps of: measuring said surge force at least on one wheel of
`said motor vehicle. and generating from the measured surge
`force value and the instantaneous motor vehicle speed a
`signal corresponding to a critical speed at which. with a
`instantaneous water film thickness. the motor vehicle will
`plane (aqua planing speed).
`2. A method according to claim 1. wherein said critical
`speed at which said motor vehicle will aqua-plane is indi
`cated to the driver.
`3. A method according to claim 1. wherein said critical
`speed at which said motor vehicle will aqua-plane is taken
`from a characteristic performance graph which has been
`determined experimentally.
`4. A method according to claim 1. wherein said surge
`force is determined by at least one sensor sensing deforma
`tions in a motor vehicle wheel support structure.
`5. A method according to claim 1. wherein said instanta
`neous vehicle speed signal and at least one other signal are
`supplied to an electronic control unit. said at least one other
`control signal being generated by sensors arranged at the
`jointnre of a wheel control arm to a vehicle body so as to
`sense relative movement between said control arm and said
`vehicle body. and wherein said control unit generates a
`signal corresponding to an actual planing speed which signal
`is supplied to a display device arranged in the motor vehicle
`in view of the driver.
`
`* * *
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`* *
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`Swagway_1007