GB2465836A

GB2465836A - A method, system and a computer program product for determining the vertical load on a suspended wheel

Info

Publication number: GB2465836A
Application number: GB0822197A
Authority: GB
Inventors: Matilda Hallnor; Nils Arne Sallstrom
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2008-12-05
Filing date: 2008-12-05
Publication date: 2010-06-09
Anticipated expiration: 2028-12-05
Also published as: GB2465836B; GB0822197D0

Abstract

The present invention provides a method, a system and a computer program product for determining the vertical load on a suspended wheel 10 of a vehicle. A static and dynamic vertical force F components acting on the wheel are determined. The dynamic vertical force component acting on the wheel 10 is further determined by making use of the dynamic behavior of the vehicle's suspension system (102 fig 3) during traction and/or braking of the vehicle. The dynamic behavior of the suspension may be determined from a longitudinal and a vertical suspension parameter. The parameters may be determined by measurement of the suspension reaction during braking and or traction control of the vehicle. The parameters may be derived or calculated from the suspension configuration.

Description

Method of determining vertical load on a wheel of a vehicle

Description

The present invention relates to a method as well as to a system and to a computer program product for determining or estimating vertical load acting on a road wheel of a vehicle, in particular a passenger car or a motor bike.

Background and prior art

Vehicle dynamics control systems, such as for instance anti-lock braking systems, traction control systems make use of numerous vehicle parameters in order to identify a vehicle state of motion.

A crucial parameter for several kinds of vehicle dynamics control systems is the vertical load or vertical force acting on a vehicle wheel.

Document DE 100 50 420 Al discloses for instance a driving dynamics control system of a vehicle comprising a vertical dynamic and/or cross dynamic control system in addition to a system controlling longitudinal forces acting on the vehicle wheels.

In this way power transmission and traction of the wheel relative to the ground can be calculated.

For determination of vertically oriented wheel load, at each wheel there is provided a vehicle body-height level sensor providing signals allowing for a determination of the vertically oriented wheel load.

Since vehicle wheels are mechanically coupled to a vehicle body via a suspension system, by determination of a deflection of the suspension system's spring, vertical wheel load can be estimated. In order to take into account dynamic forces, also a spring deflection velocity or a spring deflection acceleration of a suspended vehicle wheel can be considered for determination or estimation of vertical wheel load.

Problem In practice it has turned out, that vertical wheel load determination lacks accuracy. Also, vertical wheel load estimation or determination according to prior art solutions often requires a complex and costly arrangement of numerous sensors.

Object of the invention The present invention therefore aims to provide a method of determining the vertical load on a suspended road wheel of a vehicle as well as to provide a system and a computer program product for determination of vertical wheel load providing improved accuracy which may also allow for a flexible and less cost-intensive arrangement of sensors.

Summary of the invention

The present invention provides a method of S determining vertical load on a suspended wheel of a vehicle according to claim 1, a system for determining vertical wheel load according to claim 11 and a corresponding computer program product according to claim 12. Preferred and advantages embodiments of the invention or subject matter of the dependent claims.

The method of determining or estimating the vertical wheel load of a vehicle wheel beeing coupled to a vehicle's body via a suspension system, comprises the steps of determining a static vertical force component acting on the wheel, determining a dynamic vertical force component acting on the wheel at a time t and adding the dynamic and static vertical force components. The method according to the invention is characterized in that the dynamic vertical force component is determined by making use of the dynamic behavior of the vehicle suspension system and by making use of at least one longitudinal force component during traction and/or braking of the vehicle.

When for instance a vehicle, like a car or motorbike is subject to deceleration, e.g. due to braking, in an unsuspended vehicle there is a significant load or weight transfer off the rear wheels and onto the front wheels proportional to the centre of gravity height, the deceleration rate and inversely proportional to the wheelbase. In a suspended vehicle, due to this weight or load transfer, the vehicle may become subject to a pitch, where the vehicle's front tends to lower and the vehicle's back tends to raise. This phenomenon, also denoted as diving, can be counteracted by the suspension system, providing a kind of anti-dive functionality.

In this case, a portion of the load transfer is resisted by suspension arms of the suspension system. The suspension system's spring and the suspension arms are sharing the load transfer in some portion.

The method according to the present invention makes use of the suspension system's reaction in situations, where the vehicle is subject to acceleration or deceleration, in particular when the vehicle is subject to traction or braking.

The present invention not only considers and is not limited to the front vehicle suspension providing an anti-dive functionality at the front axle during braking.

It may also take into account for an anti-lift functionality typically provided by a rear suspension during braking. In the same way, the front and/or rear suspension may provide a respective functionality during traction.

In this way, the present invention provides an approach to consider the dynamic behavior and the reaction characteristics of a wheel suspension system during braking and traction conditions for the purpose of a more accurate and precise determination or estimation of vertical wheel load.

The longitudinal force component typically represents the force component acting at a vehicle axle or a vehicle wheel and which substantially points in longitudinal or horizontal direction.

According to a preferred embodiment of the invention, the dynamic behavior of the suspension is determined in terms of at least a longitudinal and a vertical suspension parameter, each of which being informative about the position of the suspension's instant center during a given state of motion of the vehicle. In this context, the expression instant center is defined as the effective geometric point at which the suspension force vectors are transmitted to the chassis of the vehicle. An alternative definition of the instant center is to imagine each suspension control arm mounted only at the vehicle frame. The axis, that the arm rotates around then creates an imaginary line running through the vehicle, typically along vehicle cross direction (y).

Forces, as far as suspension geometry is concerned, are typically transmitted in a plane perpendicular to this axis, i.e. the x-z-axis of the vehicle. 1hen elongated force lines of an upper and a lower control arm intersect, they cross in the instant center. Furthermore, the instant center can also be thought of as having the effect of converting multilink suspension into a single control arm, which pivots at the instant center. This is at least applicable for a given suspension deflection. Multilink suspension systems may have an instant center that moves as the suspension is deflected.

The longitudinal and vertical suspension parameters are indicative of the suspension systems dynamic behavior. By considering these parameters for determination and estimation of vertical wheel load, the dynamic vertical force component acting on the wheel can be determined with increased accuracy.

Depending on the suspension geometry, the instant center, hence the longitudinal and vertical suspension parameters may vary, e.g. when the vehicle is subject to traction or braking. By characterizing and investigating the dynamic behavior of the vehicle suspension system, each vehicle state of motion can be assigned with the appropriate longitudinal and/or vertical suspension parameter. In this way, the longitudinal and/or vertical suspension parameters can be determined in dependence of the vehicle's actual state of motion. Hence, a variation of the longitudinal and/or vertical suspension parameters can be considered for the determination or estimation of the respective vertical wheel load.

According to a further embodiment of the invention, for a given suspension configuration, the longitudinal and vertical suspension parameters and/or their ratio are assumed to be constant. Even though the suspension parameters may be subject to a variation, it has turned out, that this approximation still provides the required improved accuracy compared to prior art solutions.

According to a further preferred embodiment, the longitudinal and/or vertical suspension parameters are determined by a measurement of the suspension's reaction during traction and/or braking of the vehicle.

In this way, for a given suspension or suspension type, its characteristic parameters can be empirically determined.

Alternatively, according to another embodiment, the suspension parameters are derived or calculated from the geometry of the suspension system. This calculation considers the geometry and design as well as the fixing points of the suspension arms to the vehicle's frame or body.

Further, irrespective on whether the suspension parameters are empirically measured or theoretically calculated, the longitudinal suspension parameter is indicative of the longitudinal distance between the center of the wheel and the instant center. The vertical suspension paramter is further indicative of the vertical distance between the instant center and the center of the wheel, in particular, when the vehicle is subject to traction. Further, the vertical suspension parameter may be indicative of the vertical distance between the instant center and the ground, in particular, when the vehicle is subject to braking.

The dynamic vertical force component can be assumed to split into a suspension reaction force component and a dynamic spring force component.

According to another embodiment by making use of the longitudinal and vertical suspension parameters, the suspension reaction force component can be precisely determined and/or estimated. The dynamic spring force component is preferably determined by means of a sensor arrangement being adapted to determine and/or to estimate a vertical spring load of the respective suspension system. The vertical spring load represents the vertical component of the spring load acting on the wheel center.

This sensor arrangement is typically designed and adapted for determination and/or estimation of the static and/or dynamic vertical spring force components acting on the wheel. This sensor arrangement may typically comprise at least one strain gauge being adapted to measure or to derive a vertical spring force component.

According to a further preferred embodiment, the suspension reaction force component is determined by multiplying the longitudinal force component with the vertical suspension parameter divided by the longitudinal suspension parameter. This longitudinal force at least represents the force component substantially acting on a vehicle axle in longitudinal direction (x). It is indicative whether the vehicle or the respective axle of the vehicle is subject to traction or braking.

Alternatively, also a force component directed at an angle with respect to the longitudinal axis (x) might be considered.

In a preferred embodiment, the longitudinal force is determined for each wheel individually, e.g. by making use of a separate sensor based on a strain gauge.

Depending on whether the vehicle is subject to traction or braking, the relevant vertical suspension parameter representing traction or braking is applied.

According to another independent aspect, the invention provides a system for determining the vertical load on a suspended wheel of a vehicle. This system comprises static sensor means for determining a static vertical force component acting on the wheel, dynamic sensor means for determining a dynamic vertical force component acting on the wheel and a processing unit being coupled to the dynamic sensor means and to the static sensor means. The processing unit is further adapted to determine the total vertical load acting on the wheel at a time t by making use of the dynamic behavior of the vehicle suspension system and by making use of at least one longitudinal force component during traction and/or braking of the vehicle.

This system for determining vertical wheel load is thus adapted to autonomously execute the above described method of determination of vertical wheel load.

According to another independent aspect, the invention provides a computer program product for determining vertical wheel load of a suspended vehicle wheel. The computer program product comprises computer program means being adapted to determine a static vertical force component acting on the wheel, to determine a dynamic vertical force component acting on the wheel at a time t and to add the dynamic and the static vertical force components. The computer program means are further adapted to determine the dynamic vertical force component by making use of the dynamic behavior of the vehicle suspension system and by making use of at least one longitudinal force component during traction and/or braking of the vehicle.

Additional features and advantages of the invention will be set forth in the following detailed description and in part will be readily apparent to those -10 -skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as in the appended drawings.

It is further to be understood, that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.

Brief description of the drawings

In the following, preferred embodiments of the invention will be described in detail by making reference to the drawings, in which Figure 1 schematically illustrates forces acting on a wheel during traction, Figure 2 depicts a schematic force diagram of forces acting on a wheel during braking, Figure 3 shows a simplified block diagram of a system for determining vertical wheel load, Figure 4 depicts a simulated diagram of vertical wheel load over time and -11 -Figure 5 illustrates a diagram of vertical wheel load over time as measured in experiments.

Detailed description

The simplified illustration of forces acting on a vehicle wheel according to figure 1 shows a vehicle wheel 10 on a ground surface 12. Altogether, there are three vertical force components acting on the wheel 10.

These are the vertical spring force component F5, which has a static contribution F5, static and a dynamic contribution 8F3. Additionally, there exists a vertical force component Fzreac originating from the reaction and dynamic behaviour of the suspension. This reaction force at least partly prevents the rear and/or front axle of the vehicle from vertically deflecting during traction or braking.

Consequently, a certain amount of anti-lift or anti-dive effect can be provided by the suspension. The suspension reaction forces can be precisely determined by knowing the force Fxa, Fb acting on a respective wheel 10 or axle in longitudinal direction (x) and by knowing the suspension design parameters Va, lvb and ih as they are depicted in figures 1 and 2. Here iVa, vb are the vertical suspension parameters, wherein va has to be applied during traction and wherein l is applicable during braking. Further, 1h represents the longitudinal suspension parameter.

During traction conditions and with the respect to figure 1, the wheel center moment equilibrium yields: -12 -

F --F

zreac -xrrac h (Eq. 1) (Eq. 2) Combining Eq. 1 and Eq. 2 then gives F=!F (Eq. 3) Vertical force equilibrium gives (Eq. 4) Hence, the total vertical wheel force can be expressed as F; +öF; +�F (Eq. 5) with Fxa as the wheel's or the respective axle's force component acting in longitudinal direction.

In a braking condition, as illustrated in figure 2, the vertical suspension design parameter lvb equals the vertical distance of the instant center 14 to the ground surface 12. This deviation compared to the traction mode according to figure 1 is due to the fact, that in braking conditions the tire to road patch moment equilibrium has to be considered instead of the wheel center moment equilibrium.

Hence, the tire to road patch momentum equilibrium yields: F2---F;,, (Eq. 6) (Eq. 7) -13 -Combining Eq. 6 and Eq. 7 then gives: FZ=--FXb (Eq. 8) Hence, the total vertical wheel force can be expressed as +5F = F111 +öF (Eq. 9) with Fb as the force component acting in longitudinal direction on the respective wheel or axle.

All forces indexed z are wheel center forces normal to the ground surface.

In particular, measurment of longitudinal forces Fxa or Fxb can typically be provided by means of strain gauges embedded in the suspension system.

Figure 3 illustrates a simplified block diagram of a system for determining vertical wheel load considering suspension reaction forces. A vehicle wheel is coupled to a vehicle body 104 by means of a suspension system 102. Static sensor arrangements 106 and dynamic sensor arrangements 108 are adapted to monitor the actual configuration and the state of the suspension system 102. Dynamic and static sensors 108, 106 may each include a sensor arrangement for measuring a spring force or spring load of the suspension system 102.

Alternatively, it is also conceivable, that the functionality of static and dynamic sensors 106, 108 is combined in a single sensor or sensor arrangement.

-14 -Further, the dynamic sensor 108, which may also be designed as longitudinal acceleration or longitudinal force sensor, provides the magnitude of the forces Fxa or Fib. The signals generated by the sensors 106, 108 are provided to a processing unit 110, which is adapted to determine or to estimate the actual vertical wheel load by making use of the above-mentioned suspension design parameters Va, lvb and lh. These parameters can even be assumed to be constant. In this case they may be stored in a memory.

Alternatively, a variation of these parameters may also be measured by an additional sensor arrangement.

A temporal variation of these suspension design parameters may also be estimated based on the actual vehicle's state of motion.

Finally, the vertical wheel load that has been determined by the processing unit 110 is provided to a vehicle dynamics control system 112, such as a traction control system or an anti-lock control system, where the information of vertical wheel load is further processed.

Figure 4 illustrates a simulated diagram 200 of vertical wheel load in kN versus time in seconds during a braking maneuver of 100 to 0 km/h with a deceleration of 0.7 g. As can be seen in the diagram 200, the total wheel load F 202 drastically deviates from the vertical spring forces F5 204. Further, in the time interval between 4 and 5.5 seconds, reflecting a quasi static braking condition, the contribution of the vertical reaction force component Fzreac to the total vertical force F 202 may amount to 30 % of the deviation of total vertical -15 -wheel force during braking conditions compared to a uniform motion.

Figure 5 finally illustrates a comparable diagram 300 obtained by a measurement during a braking maneuver from 100 to 0 kin/h by applying a deceleration of 1 g.

The graph 302 represents a reference measurement and reflects the total vertical wheel load Fz,ret during a braking maneuver. At least in the time interval between 1.5 and 2.5 s, the graph 306 represents the quasi static vertical spring wheel force component and the graph 304 depicts the measured quasi static vertical spring wheel force component F9 corrected by the vertical force component arising due to suspension reaction Fzreac. As can be seen from the graphs 302, 304, 306, the measurement according to the present invention taking into account suspension geometry and dynamic suspension behavior 304 is fairly close to the reference measurement 302, whereas the quasi static vertical spring wheel force F5 306 deviates of up to 25 % from the reference measurement 302.

-16 -List of re ference numerals wheel 12 Ground surface 14 Instant center 16 Wheel center Wheel 102 Suspension system 104 Vehicle body 106 Sensor 108 Sensor Processing unit 112 Vehicle dynamics control system 200 Simulated graph 202 Total vertical wheel load 204 Quasi static vertical spring wheel load 300 Measured graph vertical load versus time 302 Reference measurement 304 Measured and estimated vertical wheel load according to the invention 306 Measured quasi static vertical spring wheel load

Claims

-17 -Claims 1. Method of determining the vertical load on a suspended wheel (10; 100) of a vehicle, comprising the steps of -determining a static vertical force component (Fzs,static) acting on the wheel (10; 100), -determining a dynamic vertical force component (8F) acting on the wheel (10; 100) at a time t and adding the static and dynamic vertical force components (F5 static, wherein the dynamic vertical force component (F) is determined by making use of the dynamic behavior of the vehicle suspension system (102) and by making use of at least one longitudinal force component (Fxa, Fb) during traction and/or braking of the vehicle.
2. The method according to claim 1, wherein the dynamic behavior of the suspension (102) is determined in terms of at least a longitudinal (ib) and a vertical (lv) suspension parameter being informative about the position of the suspension's instant center (14) under a given state of motion of the vehicle.
3. The method according to any one of the preceding claims, wherein the longitudinal (ib) and/or vertical suspension parameters (lv) are determined -18 -in dependence of the vehicle's actual state of motion.
4. The method according to any one of the preceding claims, wherein, for a given suspension (102), the suspension parameters (l,, lh) and/or their ratio are assumed to be constant.
5. The method according to any one of the preceding claims, wherein the suspension parameters (lu, lh) are determined by measurement of the suspension's (102) reaction during traction and/or braking of the vehicle.
6. The method according to any one of the preceding claims 1 to 3, wherein the suspension parameters (lv, lh) are derived or calculated from the geometric configuration of the suspension system (102)
7. The method according to any one of the preceding claims, wherein the longitudinal suspension parameter (lh) is indicative of the longitudinal distance between the center of the wheel (16) and the instant center (14) and wherein the vertical suspension parameter (lv) is indicative of the vertical distance between the instant center (14) and the center of the wheel (16) or between the instant center (14) and the ground surface (12)
8. The method according to any one of the preceding claims, wherein the dynamic vertical force component (F) splits into a suspension reaction force -19 -component (Fzreac) and a dynamic spring force component (F3).
9. The method according to claim 8, wherein the suspension reaction force component (Fzreac) is determined by multiplication of the longitudinal force component (Fxa, Fb) acting on a wheel or an axle of the vehicle and the vertical suspension parameter (lv) divided by the longitudinal suspension parameter (lh).
10. The method according to any one of the preceding claims, wherein the longitudinal force component (F'xa, FXb) is individually determined, estimated or measured for each axle and/or for each wheel (10) of the vehicle.
11. A system for determining the vertical load on a suspended wheel (10; 100) of a vehicle, comprising: -static sensor means (106) for determining a static vertical force component (Fzs,statjc) acting on the wheel (10; 100), -dynamic sensor means (108) for determining a dynamic vertical force component (öF) acting on the wheel (10; 100), -a processing unit (110) coupled to the dynamic sensor means (108) and to the static sensor means (106) and being adapted to determine a total vertical load acting on the wheel (10) at a time t by making use of the dynamic behavior of the vehicle -20 -suspension system (102) and by making use of at least one longitudinal force component (Fxa, Fxb) during traction and/or braking of the vehicle.
12. A computer program product for determining the vertical load on a suspended wheel (10) of a vehicle, comprising computer program means being adapted: -to determine a static vertical force (Fz3,tatjc) component acting on the wheel (10), -to determine a dynamic vertical force component (dFz) acting on the wheel (10) at a time t and -to add the dynamic and static vertical force components, wherein the computer program means are further adapted to determine the dynamic vertical force component by making use of the dynamic behavior of the vehicle suspension system (102) and by making use of at least one longitudinal force component (Fxa, Fxb) during traction and/or braking of the vehicle.