METHOD FOR DETERMINING AN AXLE LOAD AND SUSPENSION SYSTEM FOR A VEHICLE

Abstract

A method for determining an axle load and a suspension system are configured for a vehicle having at least one leaf spring connected at its ends in spring holders of a vehicle body and connected in its central region to a chassis of the vehicle. The following steps are performed: measuring a measurement distance of the vehicle body relative to the chassis; determining whether there is currently a loading or unloading process of the vehicle, determining a relevant hysteresis curve of a pre-stored hysteresis field depending on the determination of a loading or unloading process, and determining a current axle load projection value from the measurement distance and the relevant hysteresis curve. A loading process criterion and an unloading process criterion may be considered. The determined axle load projection value thus serves as a projected or estimated axle load. Furthermore, the hysteresis field can be updated.

Claims

1. A method for determining an axle load (AL) of a vehicle (1) having a suspension system (5) with at least one leaf spring (6) having ends (6a, 6b) connected to spring holders (7a, 7b) of a vehicle body (4) and having a central region (6c) connected to a chassis (3) of the vehicle (1), the method comprising the following steps: measuring a measurement distance (d, d_B1) of the vehicle body (4) relative to the chassis (3) (St2), determining whether the vehicle is currently undergoing a loading or unloading process (1) (St3), determining a relevant hysteresis curve (Hy1, Hy2) of a pre-stored hysteresis field (14) depending on the determination of the loading or unloading process (St4), determining a current axle load projection value (AL-RV) from the measurement distance (d) and the relevant hysteresis curve (Hy1, Hy2) (St5).

2. The method as claimed in claim 1, wherein the determination of whether there is currently a loading or unloading process of the vehicle (1) is carried out depending on a first criterion (K1), which includes one or more of the following conditions: a determination of a decrease or increase of the measured distance (d), an input of an activation signal, and a detection of a standstill of the vehicle (v=0) (St3).

3. The method as claimed in claim 1, further comprising the following steps: before determining whether the vehicle is currently undergoing a loading or unloading process, checking whether a second criterion (K2) is met, which evaluates a previous journey after the last loading or unloading process, (St1), and, upon determining that the second criterion (K2) is met, using an ideal characteristic curve (KL) as a starting point for determining the current axle load projection value (AL_B1) and the relevant hysteresis curve (Hy1, Hy2) (St2).

4. The method as claimed in claim 3, wherein the second criterion (K2) includes checking whether a previous journey, after the last loading or unloading process after a reset of the second criterion (K2) meets at least one of the following two criteria: the previous journey has a journey duration (Δ_t) greater than a minimum journey time (min:Δ_t), the previous journey has a travel distance (Δ_s) greater than a minimum travel distance (min: Δ_s) to form a sufficient number of vibrations or loadings and unloadings of the at least one leaf spring (6).

5. The method as claimed in claim 1, wherein the hysteresis field (14) has: an ideal spring characteristic curve (KL), which represents a reversible elastic expansion process, at least a first hysteresis curve (Hy1) indicating an inelastic loading process and a second hysteresis curve (Hy2) indicating an inelastic unloading process.

6. The method as claimed in claim 5, wherein a first top hysteresis curve (Hy1) starting from an empty or fully loaded vehicle (1) and a bottom second hysteresis curve (Hy2) starting from a fully loaded vehicle (1) define and/or limit the hysteresis field (14).

7. The method as claimed in claim 6, wherein the hysteresis curves (Hy1, Hy2) each contain value pairs of a measurement distance (d) and an axle load projection value (AL-RV).

8. The method as claimed in claim 6, wherein a first hysteresis curve (Hy1) for a loading process lies on the ideal spring characteristic curve (KL) in an initial loading point (B1) and for shorter distances (d) increasingly deviates from the ideal spring characteristic curve (KL), and a second hysteresis curve (Hy2) for an unloading process lies on the ideal spring characteristic curve (KL) in an initial unloading point (E1) and increasingly deviates from the ideal spring characteristic curve (KL) for increasing distances (d).

9. The method as claimed in claim 1, wherein after determination of a current axle load projection value (AL-RV) on the basis of a hysteresis curve (Hy1, Hy2) in an unloading or loading process the second criterion (K2) is subsequently reset, and upon subsequently meeting the second criterion (K2), checking whether the determined axle load projection value (AL-RV) and the current measurement distance (d) are on the ideal spring characteristic curve (KL), wherein deviations from the ideal spring characteristic curve (KL) are evaluated as errors in the stored hysteresis curve (Hy1, Hy2) and a corrected axle load projection value (AL_korr) is determined based on the ideal spring characteristic curve (KL).

10. The method as claimed in claim 9, wherein when a deviation of the corrected axle load projection value (AL_korr) is determined from the axle load projection value (AL_Bi) previously determined on the basis of the hysteresis curve, a new hysteresis curve (Hy1, Hy2) and/or a corrected hysteresis field (14) is determined and stored in the memory device (11)

11. A suspension system (5) for a vehicle (1) with a vehicle body and a chassis, the suspension system comprising: a leaf spring (6) having ends (6a, 6b) in spring holders (7a, 7b) of a vehicle body (4) and a central region (6c) connected to a chassis (3) of the vehicle (1), for suspension of the vehicle body (4) relative to the chassis (3), a distance measuring device (10) for measuring a measurement distance (d) of the vehicle body (4) relative to the chassis (3) and for outputting a measurement distance measurement signal (S1), a memory device (11) for storing a hysteresis field (14), a control and evaluation device (12) which is configured to record the measurement distance measurement signal (S1), to determine whether there is currently a loading or unloading process of the vehicle (1), depending on the determination of the loading process or unloading process, to determine a relevant hysteresis curve (Hy1, Hy2) of the hysteresis field (14), and to determine a current axle load projection value (AL-RV) from the measurement distance measurement signal (S1) and the relevant hysteresis curve (Hy1, Hy2).

12. A suspension system (5) as claimed in claim 11, wherein the leaf spring (6) is a stack spring or a trapezoidal spring with a stack of spring layers (9) or spring leaves, between which friction occurs during loading and unloading processes.

13. The suspension system (5) as claimed in claim 11, wherein the central region (6c) of the leaf spring (6) is configured to be mounted on or against a star axle of the chassis.

14. The suspension system (5) as claimed in claim 11, wherein the distance measuring device (10) measures the measurement distance (d) mechanically as a length spacing or contactlessly.

15. The suspension system (5) as claimed in claim 11, wherein the control and evaluation device (12) is configured to assign a point on an ideal spring characteristic curve (KL) to a currently measured measurement distance (d) upon determining that a second criterion (K2) indicating a previously completed journey of the vehicle (1) is met.

16. A vehicle comprising: a chassis with at least one rigid axle (3), a vehicle body (4), and the suspension system (5) as claimed in claim 11, disposed between the rigid axle (3) and the vehicle body (4).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the drawings:

[0029] FIG. 1 shows an axle suspension of a vehicle with a leaf spring;

[0030] FIG. 2 shows a characteristic field with a representation of spring compression processes (loading processes);

[0031] FIG. 3 shows a characteristic field with a representation of spring expansion processes (unloading processes);

[0032] FIG. 4 shows a characteristic field when determining a spring compression process in the case of a partially loaded vehicle;

[0033] FIGS. 5a, b, c show steps for the determination or recalculation of hysteresis curves of the characteristic field;

[0034] FIG. 6 shows a flow diagram of a method according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 shows a region of an axle suspension of a vehicle 1, in particular a commercial vehicle, with vehicle wheels 2, wherein the vehicle wheels 2 are mounted on a common rigid axle 3. A vehicle body 4 is suspended relative to the rigid axle 3 by means of a suspension system 5 with two lateral leaf springs 6. In the side view of FIG. 1 a vehicle wheel 2 and a leaf spring 6 are shown accordingly.

[0036] The leaf spring 6 is accommodated at its front and rear ends 6a, 6b in spring holders 7a, 7b of the vehicle body 4 and is accommodated on the rigid axle 3 in its central region 6c, i.e. it rests on the rigid axle 3 and is fixed to it. Thus, vertical movements of the body 4 lead to a compression and expansion of the leaf spring 6, i.e. a measurement distance d of the vehicle body 4 relative to the rigid axle 3 changes. During a spring compression process, for example when crossing a speed bump, the vehicle body 4 is displaced towards the rigid axle 3, i.e. the measurement distance d decreases under elastic deformation of the leaf spring 6, which is thus forced upwards in its central region 6c.

[0037] The measurement distance d is in particular also represented as a function of a load 8, which according to FIG. 1 is symbolically accommodated on the vehicle body 4. In the case of a multi-axle vehicle, the axle load distribution of the load 8 will generally also have to be taken into account.

[0038] The leaf spring 6 is formed according to the embodiment shown in FIG. 1 as a stack of spring layers 9, i.e. there are multiple spring layers 9 which are layered on each other. In the case of a design of the leaf spring 6 as a trapezoidal spring, the spring layers 9 are generally formed in different lengths and each has a central bore, through which a common so-called heart bolt is set; furthermore, the spring layers 9 are held together by clamps. Furthermore, the leaf spring 6 can also be formed in principle as a parabolic spring with one spring layer 9 or even a stack of spring layers.

[0039] The method according to the invention is in particular advantageously implemented when designing the leaf spring 6 as a trapezoidal spring with different length spring layers 9, since the hysteresis effects or damping properties described below come particularly to bear.

[0040] The measurement distance d is measured by a distance measuring device 10, which can be in the form of a mechanical distance sensor, which is in contact with the vehicle body 4 and the rigid axle 3, or also as a contactless measurement distance sensor, for example an ultrasonic sensor, such as can in principle also be provided with a pneumatic suspension system. The distance measuring device 10 provides the distance measurement signal S1 to a control and evaluation device 12, which stores the measurement data and data calculated from this in an internal or external memory 11.

[0041] The measurement distance d therefore depends on the axle load AL. However, in particular a trapezoidal spring 6 exhibits a hysteresis behavior, which is shown in FIGS. 2 to 3 and is in particular related to the internal friction between the spring layers 9. In FIG. 2, the measurement distance d is plotted against the axle load AL. In the ideal spring characteristic curve KL, the measurement distance d accordingly decreases with increasing axle load AL, wherein in general there is a non-linear spring characteristic curve, in particular a progressive spring characteristic curve, in particular for a stack of spring layers with different lengths. Due to the friction between the spring layers 9, however, the adjustment of the measurement distance d does not follow the ideal spring characteristic curve KL, but can be reproduced by a hysteresis field 14, which is represented as a set of hysteresis curves Hyi with i=1, 2, 3, . . . , i.e. thus Hy1, Hy2, Hy3, . . . : Here, FIG. 2 first shows a loading process, i.e. a spring compression process:

[0042] A loading or unloading process is present when a first criterion or loading process criterion K1 is met; this may provide in particular that the vehicle 1 is at a standstill, i.e. v=0. Furthermore, it is preferably provided as a criterion for a loading process that the axle load AL increases, i.e. a decrease in the measurement distance d is measured. A hysteresis field 14 is preferably determined from previous measurements and stored in the memory device 11, for example as a set of hysteresis curves Hy1, Hy2, . . . , wherein the hysteresis curves Hy1, Hy2, . . . can be stored as a determined mathematical function, for example polynomials, or can also be stored as value pairs (d, AL), wherein current corresponding interpolations are then formed between these pairs of values.

[0043] The loading process starts from an initial loading point B1, which is thus specified as a value pair (d_B1, AL_B1). In general, a second criterion or relaxation criterion K2 is provided, which indicates that a point on the ideal spring characteristic curve KL has currently been reached; the second criterion indicates that the vehicle 1 has previously travelled a sufficient distance. In a subsequent loading process, successive loading points B2, B3, B4 up to for example B7 along the first (upper) hysteresis curve Hy1 are recorded, wherein for example B7 indicates full loading, so that the loading process is terminated. The hysteresis curve Hy1 is defined by the friction forces between the spring layers 9 during the loading process. The frictional forces counteract the bending of the leaf spring 6, in which the spring leaves 9 shift relative to each other under pressure load, so that a smaller measurement distance d is formed, wherein a curved hysteresis curve Hy1 is formed, which can be measured accordingly as value pairs (d_B1, AL_B1), (d_B2, AL_B2), . . . .

[0044] Since the hysteresis field 14 is pre-stored, thus when a subsequent spring compression process occurs a currently measured measurement distance d starting from B1, for example the measurement distance value d_B2, can be determined and a current axle load projection value AL_B2 can be determined directly from d_B2 on the basis of the first hysteresis curve Hy1. Thus, an error can be avoided or minimized by using the ideal spring characteristic curve KL.

[0045] Thus, the axle load, which is not directly measurable, is projected and axle load projection values are determined, i.e. according to the graphical representation by means of the hysteresis curves as values on the abscissa.

[0046] FIG. 3 shows an unloading process in which the second (lower) hysteresis curve Hy2 is followed for the unloading process. The vehicle 1 has again previously met the second criterion K2, so that the internal friction forces and tensions of the leaf spring 6 are largely reduced. Thus, an initial unloading point E1 lies with high accuracy on the ideal spring characteristic curve KL. In the subsequent unloading process, the ideal spring characteristic curve KL is not followed again, but the lower hysteresis curve Hy2 determined through E1 with points E1, E2, E3 to E7, which runs below the spring characteristic curve KL, is followed.

[0047] FIGS. 2 and 3 thus show the characteristic field 14 with hysteresis curves Hy1 and Hy2, which in particular can represent a loading of the empty vehicle 1 and an unloading of the fully loaded vehicle 1. The top first hysteresis curve Hy1, which is based on a fully loaded vehicle 1, and the bottom second hysteresis curve Hy2 define and/or limit the hysteresis field 14 as enveloping curves and thus form three envelope curves with the ideal spring characteristic curve.

[0048] The respective hysteresis curve is thus characterized by the axle load value AL of the initial loading point B1, which can however also change in principle. Thus, changes can be made to the vehicle that change the total mass of the load 8. In particular, however, vehicles can also be partially unloaded and partially loaded, so that different initial loading points B1 and different initial unloading points E1 are to be selected, which subsequently each form different upper and lower hysteresis curves Hy1, Hy2. This is shown in FIG. 4, where the first criterion K1 is used for example after a longer journey, i.e. after meeting the second criterion K2, and thus in particular the change of the measurement distance dx is checked. Thus, a decrease of the measurement distance dx is determined here, which thus corresponds to an increase of the axle load, i.e. a loading process. Thus, the initial loading point Bx with the axle load projection value Ax can be selected, which lies on the spring characteristic curve KL based on dx, and the hysteresis curve Hy3 starting from Bx can be selected, for example by interpolation of measured values and/or by a polynomial of the n.sup.th degree.

[0049] Thus, a reliable determination of the axle load is already possible by defining a relevant hysteresis curve Hy1, Hy2, Hyx, . . . from a hysteresis field 14 by selecting the relevant starting loading point B1 or unloading point E1. Thus, in this determination of an axle load AL as an axle load projection value, no recalculation to the ideal spring characteristic curve KL is required at first, but the current axle load projection value AL-RV or Ax can be determined directly on the basis of the hysteresis field 14.

[0050] Since the hysteresis field 14 changes dynamically over time, especially due to aging or material fatigue, corrosion, changes of the interfaces and thus the frictional forces between the spring layers 9, according to a preferred design the hysteresis field 14 is continuously updated. This is shown by way of example in FIGS. 5a to 5c:

[0051] At the initial time, the hysteresis field 14 is stored. The vehicle is loaded again after a journey that meets the criterion K2, so that an initial loading point B1 can be set, which can be determined directly from the currently measured measurement distance value d_B1 and the spring characteristic curve KL. The loading actually follows the dotted line Hy_neu due to a changed hysteresis behavior until the second loading point B2 is reached during the loading process, for example. Due to the outdated or faulty hysteresis curve Hy1, the measured measurement distance value d_B2 is incorrectly assigned to the point B2_old and thus an axle load projection value AL_B2_old is determined.

[0052] Subsequently, the second criterion K2 is reset in the case of a start of a journey, in which it can therefore generally be assumed that the load 8 no longer changes. After a journey in which the second criterion K2 is again met, the ideal spring characteristic curve KL is again reached and thus the point B3 is reached again according to FIG. 5b—with the axle load AL unchanged. Thus, the measurement distance value d_B3 is subsequently measured and not the previously incorrectly determined measurement distance value d_B2_old. As a result the error can be detected qualitatively on the one hand and the current axle load projection value AL-RV can be corrected based on the currently measured measurement distance value d_B3 and the known ideal spring characteristic curve KL. Furthermore, according to FIG. 5c the hysteresis field 14 can be adjusted and the hysteresis curve Hy1 which is relevant for the loading process can be corrected, i.e. according to FIG. 5c it can be offset clearly upwards to the determined values.

[0053] Here, the new, corrected hysteresis curve Hy1 can be determined from one or more determined loading points B2. In principle, it can be determined mathematically by a suitable polynomial of the n.sup.th degree from the initial loading point B1 and another loading point B2.

[0054] FIG. 6 thus shows a flow diagram of a method for determining an axle load as an axle load projection value according to one embodiment:

[0055] After the start in step St0, it is then checked whether the second criterion K2 is met in step St1, i.e. K2=1, for example by comparing the journeys made since the last loading or unloading process. In particular, it is possible to check whether a journey time (Δ_t) is greater than a minimum journey time (min_Δ_t) and/or whether a journey distance (Δ_s) exceeds a minimum distance (min:Δ_s).

[0056] Here it may be provided that the journey time or distance has been consistently met in a single journey, or multiple consecutive journeys may also be permitted.

[0057] Where appropriate, it may also be checked, for example, whether a minimum speed has been maintained during the journeys.

[0058] If K2 is not met, the method is reset to before step St1; if K2 is met, it is recognized that the ideal spring characteristic curve KL can be used as a starting point for a subsequent change of the axle load.

[0059] In step St2, the measurement distance value d_B1 is then measured and from this the point B1 is determined as the current value pair (d_B1, AL_B1) by means of the ideal spring characteristic curve KL, whereby the current axle load projection value AL_B1 is determined.

[0060] It is subsequently checked in accordance with step St3 whether the first criterion K1 or loading process criterion K1 is met, i.e. there is currently a loading or unloading process, for which purpose it can be checked

[0061] whether the speed v=0, for example,

[0062] whether there is a change in the measurement distance d greater than a minimum value d_min, wherein d_min allows small fluctuations,

[0063] and/or, where appropriate, an input signal from the driver is also required.

[0064] Furthermore, as part of the first criterion K1, the distinction between a loading process and an unloading process may already be provided, i.e. whether d increases or decreases.

[0065] In the present case, a decrease of d is measured, so that a loading process can be concluded in step St3. Thus, in step St4 the upper hysteresis curve Hy1 can be used, which is determined by B1 and the characteristic field 14. If, on the other hand, an unloading process is determined, i.e. d increases, thus the lower hysteresis curve Hy2 is used.

[0066] Subsequently, the new measurement distance d2 is then measured in step St5 at the end of the loading process, from which the current axle load projection value AL_B2 is determined on the basis of the hysteresis curve Hy1. This current axle load projection value AL_B2 can subsequently be indicated or also used for driving dynamics controls in which the axle load AL is incorporated, i.e. in particular for control of the axle load distribution by controlling the pneumatic springs, for braking processes and also for stabilization processes. Since a loading process is carried out while forming hysteresis behavior of the leaf spring 6, the second criterion K2 is no longer met, so that in step St6 K2 can be reset, i.e. K2=0.

[0067] Subsequently, in step St7 after for example a short journey the second criterion K2 is met again as K2=1, so that it can be assumed from this that the behavior of the leaf spring 6 again follows the ideal spring characteristic curve KL. The current measurement distance d is measured and, on the one hand, a corrected axle load projection value AL-RV is determined on the basis of the spring characteristic curve KL. Furthermore, in step St8 the hysteresis field 14 is then corrected according to FIG. 5c), a new hysteresis curve Hy1 is determined and stored in the memory device 11, whereupon the method is reset to before the step St1.