System and method for detecting an off-road travel surface condition for a vehicle

Background of the Invention

Field of the Invention The present invention relates to a system and method for detecting an off-road travel surface condition for a vehicle driving on a travel surface, wherein the vehicle comprises at least one vehicle wheel comprising a rim and a tire mounted onto the rim. WO 2006/054976 Al relates to a method for determining travel surface characteristics for a vehicle driving on a travel surface, wherein the vehicle has a vehicle wheel comprising a tire with one or more electronic signal producing sensors associated with the tire. The determining of a selected travel surface condition related information, e.g. related to loose gravel, ice, snow, water, pothole, protrusion or road hazard, is realized by monitoring the electronic signals and analyzing selected characteristics of the electronic signals. However, this publication contains only a vague description of possible methodologies suitable for a realization of said analyzing step. Furthermore, the disclosed approaches for such analysis may create problems in terms of reliability in case of electronic signals having a high noise level. Summary of the Invention

It is therefore an object of the present invention, to provide a system and a method for detecting an off-road travel surface condition which can provide high reliability and can be realized in a simple manner.

According to a first aspect of the present invention, a system for detecting an off-road travel surface condition for a vehicle driving on a travel surface is provided, wherein the vehicle comprises a vehicle wheel comprising a rim and a tire mounted onto the rim. The system comprises:

- a sensor configured to be mounted at a mounting position at a circumference of the tire for generating a radial acceleration signal indicating a time-dependent radial acceleration during a drive of the vehicle,

- an evaluation device for identifying an underlying si- nusoidal cycle in the radial acceleration signal caused by the influence of gravitation on the radial acceleration signal, and for analyzing deviations of the actual radial acceleration signal from the underlying sinusoidal cycle and for deciding whether an off-road travel surface condition is present based on a result of the analysis of said deviations.

Here and in the following, the term "off-road travel surface condition" shall mean a condition of the travel surface, which substantially differs from the condition of a normal road.

A "normal road", for example an asphalt road or a concrete road, has a substantially flat surface (e.g. without loose gravel, potholes etc.) and a relatively high hardness. At such a normal road, the weight of the vehicle does not cause a remarkable lowering of the road surface directly under the vehicle wheel (s) of the vehicle.

As will be explained in more detail in the following, the system for detecting an off-road travel surface condition according to the first aspect of the present invention can advantageously provide high reliability and can be realized in a simple manner. The system comprises a sensor configured to be mounted at the tire and for generating a radial acceleration signal. Advantageously, such sensors are often already foreseen to be mounted at tires (e.g. at an inner side of the running surface of a tire) of modern vehicles, e.g. as components of so-called electronic wheel units for measuring operation parameters of the respective wheel (e.g. tire pressure, tire temperature, rotational speed etc.). Such electronic wheel units often comprise an acceleration sensor for generating a radial acceleration signal for e.g. determining the rotational speed and/or rotational position of the vehicle wheel by detecting typical "signal features" in the radial acceleration signal associated with the more or less periodical passage of the mounting position at the circumference of the tire through the tire contact area, i.e. the area at which the outer circumference of the tire contacts the travel surface and which is also referred to as footprint of the tire.

Therefore, according to a preferred embodiment of the present invention, the sensor is a part of an electronic wheel unit configured to be mounted at a mounting position at the cir- cumference of a tire and further comprising at least one ad ^¬ ditional sensor for generating an additional operation parameter signal indicating an additional operation parameter as e.g. a tire pressure and/or a tire temperature. In an embodiment, the sensor is a part of an electronic wheel unit, and the electronic wheel unit is an electronic wheel unit used in a tire pressure monitoring system (TPMS) , i.e. having at least another sensor for generating a tire pressure signal (in addition to the sensor for providing the radial acceleration signal) . „

Such electronic wheel unit can further comprise an RF sender device for sending RF signals (e.g. digital data telegrams) containing information about the respective wheel operation parameter (s) to a vehicle-based RF receiver device. The RF receiver device of the vehicle can be configured to receive such RF signals from a plurality of electronic wheel units each installed at one of a corresponding plurality of vehicle wheels. As mentioned above, the system according to the first aspect of the present invention comprises the evaluation device for identifying an underlying sinusoidal cycle in the radial ac ^¬ celeration signal. Advantageously, electronic wheel units typically already comprise digital processing devices (e.g. microcontrollers) which can be modified (by modified software) in a simple manner to (additionally) realize the functionality according to the invention.

Accordingly, in a preferred embodiment of the invention, the sensor and the evaluation device are integrated in an electronic wheel unit, e.g. in an electronic wheel unit forming a component of a TPMS. The electronic wheel unit can e.g. be configured to be arranged at an inner side of a running surface of a tire of the respective vehicle wheel.

The sinusoidal cycle in the radial acceleration signal is caused by the influence of gravitation on the radial acceleration signal. Assuming the simple case, in which the respective vehicle wheel rotates with a constant rotational speed, an acceleration of "1 g", i.e. of approximately 9.81 m/s ² , due to gravitation and thus directed downwards will contribute to the generated radial acceleration signal. As the wheel rotates, this gravitational contribution varies between +1 g and -1 g, depending on the wheel's rotational position, wherein the value "+1 g" corresponds to the situation, in which the gravitational acceleration is directed in the (positive) sensing direction of the sensor and the value "-1 g" corresponds to the situation, in which the gravitational acceleration is directed opposite to the (pos- itive) sensing direction of the sensor.

In this respect, if the (positive) sensing direction of the sensor is directed radially outwards (from the mounting position of the sensor) , the gravitational contribution "+1 g" will occur when the mounting position and thus the sensor is at the bottom position (close to the travel surface) , and the gravitational contribution "-1 g" will occur when the sensor is at the top position (opposite to the travel surface) .

If the vehicle wheel rotates very slowly, so that the centrifugal acceleration can be neglected, the sinusoidal cycle can be regarded as a more or less exact sinus function of the rotational position of the wheel (and in case of a constant speed equivalently as a sinus function of the time) , varying between -1 g and +1 g. However, a certain deviation from the exact sinus function occurs when the sensor passes through the contact area, at which the tire contacts the travel surface, because at the contact area the shape of the tire is deformed (due to the typical flexibility of the tire material) , so that the sensing direction of the sensor is rotated somewhat in regard of the radial direction of the vehicle wheel, when the sensor passes the contact area .

Considering also the centrifugal acceleration during the ro- tation of the vehicle wheel, the gravitational contribution will be superimposed by the speed-dependent centrifugal acceleration, which has e.g. a more or less constant (positive) value in the case of constant rotational speed of the vehicle wheel. However, a certain deviation from the constant value occurs when the sensor _r

passes through the contact area, because at the contact area the shape of the tire is deformed, so that the sensing direction of the sensor is rotated somewhat in regard of the radial direction of the vehicle wheel and in addition the effective radius of the vehicle wheel varies, when the sensor passes the contact area.

Due to the fact that the tire is deformed at the tire contact area, the radial acceleration signal shows characteristic signal features, when the sensor passes through the tire contact area. In electronic wheel units, these signal features e.g. allow for determining the length of the contact area.

Now, having the above explained contributions to the radial acceleration signal in mind, the basic idea of the present invention includes identifying the underlying sinusoidal cycle in the radial acceleration signal and analyzing deviations of the actual radial acceleration signal from the underlying sinusoidal cycle for deciding whether an off-road travel surface condition is present or not.

The term "identifying an underlying sinusoidal cycle" has a broad meaning in the sense of the invention. It shall comprise the determination of any feature characterizing the sinusoidal cycle and suitable for concluding that an off-road travel surface condition is present based on deviations of the actual radial acceleration signal from the expected signal, wherein the expectation relies on said determined characteristic of the sinusoidal cycle. Therefore, the identification of the sinusoidal cycle may comprise e.g. the determination of the mean value and the amplitude thereof. As an alternative, the identification may comprise e.g. the determination of only the minimum values of the sinusoidal cycle, or the determination of e.g. only the maximum values of the sinusoidal cycle. Such determination also allows for analyzing deviations and based thereon to decide whether an off-road travel surface condition is present. The identification of the underlying sinusoidal cycle can comprise e.g. a (first) step of determining a period of the sinusoidal cycle based on the result of an analysis of the above mentioned signal features (associated with the passage of the sensor through the contact area) and/or based on a frequency analysis (e.g. Fourier analysis of the radial acceleration signal), and a (second) step of determining a mean value and/or an amplitude and/or a minimum value and/or a maximum value of the sinusoidal cycle based on a mathematical fit (regression) providing one or more such parameters of the sinusoidal cycle.

In an embodiment, the above mentioned signal features associated with the passage of the sensor through the contact area are taken into account when conducting a mathematical fit for determining one or more parameters of the sinusoidal cycle. For example, when a sine function is fitted to the actual radial acceleration signal, the portion of the actual radial acceleration signal showing these signal features may be hidden (not used) in the data basis used for the mathematical fit. In order to take into account a noise level being present in the radial acceleration signal, according to a preferred embodiment, the generated radial acceleration signal is smoothed before applying signal processing steps for the identification of the sinusoidal cycle as e.g. the two steps mentioned above.

In the system according to the first aspect of the present invention, the evaluation device is configured to analyze deviations of the actual radial acceleration signal from the underlying sinusoidal cycle. In an embodiment, the evaluation device is configured to search for signal peaks in the actual radial acceleration signal constituting such deviations. Such signal peaks, in particular when the sensor passes the contact area, can be used as an indication of an off-road condition. In this case, deciding whether an off-road travel surface condition is present, can be dependent e.g. on the further criterion, whether such signal peaks exceed a predetermined threshold amount and/or whether such signal peaks occur steadily (indication of off-road travel surface condition) and not only one-time (possibly caused by signal noise) .

In an embodiment, the evaluation device is configured to analyze whether the actual radial acceleration signal assumes values below a minimum value of the underlying sinusoidal cycle, and configured to decide that the off-road travel condition is present, if such a value of the radial acceleration signal below the minimum value occurs .

According to a further development of this embodiment, the evaluation device is configured to take into account only signal values (e.g. peaks) occurring when the sensor passes the tire contact area (in particular e.g. at moments at which the sensor enters the tire contact area) . Said embodiments provide a particularly simple and reliable system for detecting an off-road travel surface condition for a vehicle driving on a travel surface.

In particular, said embodiments may provide a travel surface softness detection, since, in the case of driving on a soft ground (e.g. very dirty road, earth road, road accessible only to all-terrain vehicles, mucky path, forest soil etc.), the actual radial acceleration signal typically has a characteristic "negative peak" when the mounting position of the sensor enters the tire contact area, wherein this signal peak reaches below the minimum value of the sinusoidal cycle.

According to a further aspect of the present invention, a method for detecting an off-road travel surface condition for a vehicle driving on a travel surface is provided, wherein the vehicle comprises a vehicle wheel comprising a rim and a tire mounted onto the rim, and wherein the method comprises:

- generating a radial acceleration signal indicating a time-dependent radial acceleration at a position at a circumference of the tire,

- identifying an underlying sinusoidal cycle in the radial acceleration signal caused by the influence of gravitation on the radial acceleration signal,

- analyzing deviations of the actual radial acceleration signal from the underlying sinusoidal cycle,

- deciding whether an off-road travel surface condition is present based on a result of the analysis of said de ^¬ viations .

The method for detecting an off-road travel surface condition according to the further aspect of the present invention can advantageously provide high reliability and can be realized in a simple manner.

The embodiments and further developments described herein with reference to the system according to the first aspect of the present invention can accordingly be provided as embodiments and further developments, respectively, of the method according to the further aspect of the present invention. According to still another aspect of the invention, a computer program product is provided, comprising executable instructions for performing a detection method as described herein when executed by a data processing device.

Such computer program product can be used for operation control of the above mentioned evaluation device (which is e.g. a component of an electronic wheel unit, which is installed in the respective vehicle wheel) .

Brief Description of the Drawings

The invention will now be described in more detail by way of example embodiments with reference to the accompanying drawings, in which

Fig. 1 illustrates a schematic view of a vehicle wheel comprising a rim and a tire mounted onto the rim and equipped with an electronic wheel unit,

Fig. 2 illustrates a block diagram of the electronic wheel unit illustrated in Fig. 1, Fig. 3 illustrates an example of a diagram illustrating a radial acceleration signal generated by a sensor in the electronic wheel unit in case of a driving of a vehicle on a hard ground, Fig. 4 illustrates an example of a diagram illustrating a radial acceleration signal generated by a sensor in the electronic wheel unit in case of a driving of a vehicle on a soft ground, and Fig. 5 illustrates a flowchart of a method according to an embodiment of the invention.

Description of the Preferred Embodiments

Fig. 1 illustrates a vehicle wheel 1 comprising a rim 2 and a tire 3 mounted onto the rim 2. The vehicle wheel 1 is one of a plurality (e.g. four) of vehicle wheels of a vehicle (not illustrated) driving on a travel surface 4, so that the wheel 1 rotates according to the speed of the vehicle. In Fig. 1 the rotation of the wheel 1 is symbolized by an arrow 5.

Due to the vehicle's weight and the flexibility of the material of the tire 3 (e.g. rubber material), in a tire's contact area where the tire 3 contacts the travel surface 4, the shape of the tire 3 deviates from the ideal circular shape. In this contact area, which is also referred to as footprint of the tire 3, the outer circumference surface of the tire 3 is more or less flat. In Fig. 1, a length L of the contact area is marked.

The vehicle wheel 1 is equipped with an electronic wheel unit 12 mounted at a mounting position 18 at the circumference of the tire 3. Fig. 2 illustrates the structure of the electronic wheel unit 12 according to the illustrated embodiment. The electronic wheel unit 12 comprises:

- a pressure sensor 14 for generating a tire pressure signal "tps" indicating the air pressure in the tire 3,

- an acceleration sensor 16 for generating a radial acceleration signal "ras" indicating a radial acceleration during a drive of the vehicle, - an evaluation device 20 for processing the signals tps and ras and thereby generating data D, and - an RF sender device 22 for sending an RF signal 26 containing the data D.

In principle, the electronic wheel unit 12 as described so far has a structure and a function known from electronic wheel units in tire pressure monitoring systems (TPMS) . In fact, also the illustrated wheel unit 12 in this embodiment constitutes a component of such TPMS of the respective vehicle. This means that the data D, which are sent by means of RF signal 26 to a corresponding RF receiver device 28 of the vehicle (Fig. 1), contain at least information about the tire pressure of the respective tire 3. This information is created by the evaluation device 20 based on the tire pressure signal tps.

Referring again to Fig. 1, there is also illustrated the RF receiver device 28 of the vehicle, which is coupled to a decoder device 30 of the vehicle for decoding the information from the RF signal 26 (e.g. digital data telegrams, which are sent periodically or from time to time) . The decoded information is provided via a digital bus system 34 to a central electronic control unit (ECU) 32 of the vehicle.

Although not illustrated in Fig. 1, the RF receiver device 28 and the subordinated decoder device 30 are also used for receiving and decoding RF signals sent by other electronic wheel units installed at other wheels of the same vehicle (so that the tire pressure monitoring is accomplished for a plurality of the vehicle wheels, preferably for all of the vehicle wheels) . Insofar, a system 10 illustrated in Fig. 1 forms a part of a TPMS of the vehicle. However, the system 10 in the present embodiment constitutes also a system for detecting an off-road travel surface condition for the respective vehicle, when driving on a travel surface.

The system 10 for detecting an off-road travel surface condition according to the illustrated embodiment comprises: - the acceleration sensor 16 provided at the mounting position 18 at the circumference of the tire 3 for generating the radial acceleration signal ras indicating a time-dependent radial acceleration during a drive of the vehicle,

- the evaluation device 20 for identifying an underlying sinusoidal cycle in the radial acceleration signal ras caused by the influence of gravitation on the radial acceleration signal ras, and for analyzing deviations of the actual radial acceleration signal ras from the un ^¬ derlying sinusoidal cycle and for deciding whether an off-road travel surface condition is present based on a result of the analysis of said deviations. Hereinafter, the method accomplished by the system 10 for detecting an off-road travel surface condition will be described in more detail with reference to exemplary radial acceleration signals ras illustrated in Figs. 3 and 4. Fig. 3 illustrates an example of a radial acceleration signal "ras" depending on the time "t" in a case in which the vehicle is driving on a hard ground as e.g. an asphalt road. As can be seen from Fig. 3, the radial acceleration signal ras essentially follows a sinusoidal cycle having a period of approx. 1.65 s, a mean value of approx. 3 g and an amplitude of 1 g. The maximum values of 3 g + l g = 4 g correspond to situations, in which the sensor 16 is in a bottom position (in the middle of the contact area of the tire), here at t = 155.7 s, t = 157.4 s, t = 159.0 s, whereas the minimum values of 3 g - l g = 2 g correspond to situations, in which the sensor 16 is in a top position (opposite to the contact area), here at t = 156.6 s, t = 158.2 s, t = 159.9 s.

The portion of the run of the signal ras corresponding to the passage of the sensor 16 through the tire contact area strongly deviates from the underlying sinusoidal cycle, but this is expectable firstly due to enhancements of acceleration expe ^¬ rienced by the sensor 16 when it enters and when it leaves the contact area, and secondly due to a disappearance of the (centrifugal) acceleration when the sensor 16 is in the middle of the contact area.

Fig. 4 illustrates a representation of the signal ras corre ^¬ sponding to the representation of Fig. 3, but for a case, in which the vehicle is driving on a soft ground as e.g. a field path or a forest soil.

As can be seen from Fig. 4, in the region of each entry of the sensor 16 in the contact area, there is a large "negative peak" in the signal ras. By analyzing deviations of the actual radial acceleration signal ras from the underlying sinusoidal cycle and thus detecting such negative peaks, it is decided that an off-road travel surface condition as e.g. "soft ground" is present. In the illustrated example, it is analyzed whether the signal ras assumes values well below the minimum value of the underlying sinusoidal cycle, which in the illustrated example is 2 g. If this is the case, it is decided that the off-road travel condition is present.

For avoiding erroneous detections caused by a noisy signal ras, it is preferred that an evaluation algorithm conducted by the evaluation device 20 takes this decision only, if the respective criterion, here the occurrence of signal values e.g. a pre ^¬ determined amount (e.g. 1 g) below the above mentioned minimum value (2 g) is detected at least m times in n consecutive sinusoidal cycles, wherein n and m are integers with m ≤ n. In an embodiment, n is chosen in the range of 2 to 10, in particular 3 to 6.

In an embodiment, m is chosen in the range of n/4 to n, in particular n/3 to n/1.5.

For example, the integers n and m can be chosen as n = 3, m = 2, or n = 4, m = 2, or n = 5, m = 3.

The predetermined amount, by which the signal ras has to fall below the minimum value of the underlying sinusoidal cycle to fulfill the off-road detection criterion, can e.g. be fixedly predetermined, e.g. in a range of 0.5 to 2 g.

In an alternative embodiment, this amount is variably set depending on a previously determined noise amplitude of the signal ras. In an embodiment, the amount is set to a value which is at least 2 times the noise amplitude. For example, in the example of Figs. 3 and 4, such noise amplitude amounts to approx. 0.3 g, so that the amount e.g. may be set as 0.6 g. In this case, only signal peaks falling below 2 g - 0.6 g = 1.4 g would be considered as an indication of the presence of an off-road travel surface condition. According to the invention, alternatively or in addition to the above mentioned provision of an extra amount by which the signal ras has to fall below the minimum value of the underlying sinusoidal cycle to fulfill the off-road detection criterion, it can be provided to take into account only those signal peaks (preferably negative signal peaks) , which occur when the sensor passes the tire contact area (in particular e.g. at moments at which the sensor enters the tire contact area) .

Thus, a method for detecting an off-road travel surface condition according to an embodiment of the invention comprises steps as illustrated in Fig. 5.

In a step SI, a radial acceleration signal indicating a time-dependent radial acceleration at a position at a cir- cumference of the tire is generated.

In a step S2, an underlying sinusoidal cycle in the radial acceleration signal caused by the influence of gravitation is identified .

In a step S3, deviations of the actual radial acceleration signal from the underlying sinusoidal cycle are analyzed.

In a step S4, it is decided whether an off-road travel surface condition is present, based on a result of the analysis of said deviations .

The invention provides a simple and reliable methodology for detecting an off-road travel surface condition, for example a detection of a soft ground. When the vehicle is driven in an off-road condition such as gravel, sand, farm soil etc. certain vehicle systems (e.g. active damping systems) can be advan ^¬ tageously adjusted based on the result of the detection according to the invention. Such adjustments are advantageous for several reasons such as lifetime, driving safety, comfort, performance etc .

Moreover, based on the result of the detection, the driver of the vehicle can be informed about this detection result.

The invention can be realized e.g. using a TPMS wheel unit, if this wheel unit has a sensor for providing a radial acceleration signal .

A preferred use of the invention is the detection of an off-road travel surface condition for an agricultural vehicle (e.g. tractor) or a worksite craft (e.g. bulldozer or excavator) . In an embodiment of the invention, the respective vehicle wheel or tire, respectively, has an outer diameter of at least 0.7 m, e.g. in the range of 0.7 to 2 m. However, the invention may also be used for detecting an off-road travel surface condition for passenger cars or trucks.

List of reference signs

1 vehicle wheel

2 rim

3 tire

4 travel surface

5 rotation of vehicle wheel

L length of tire contact area

10 detection system

12 electronic wheel unit

14 pressure sensor

tps pressure signal

16 acceleration sensor

ras radial acceleration signal

18 mounting position

20 evaluation device

D data

22 RF sender device

26 RF signal

28 RF receiver device

30 decoder device

32 electronic control unit

34 bus system

SI generating radial acceleration signal

S2 identifying underlying sinusoidal cycle

53 analyzing deviations

54 deciding whether off-road travel surface con ^¬ dition is present