RU2792711C2 - Method for determining soil hardness - Google Patents

Abstract

FIELD: agriculture.

SUBSTANCE: invention relates to a method for determining hardness of the soil on which a tire mounted on a vehicle moves. The tire is equipped with a sensor configured to receive a measurement signal representing the change in the curvature of the tire during its movement on the ground, and the method comprises the following steps: obtaining by the sensor a measurement signal representing the change in the curvature of the tire during movement, determining from the measurement signal measurement data containing: a) a first parameter (KL) representing the length of the contact patch in contact with the ground, and b) a second parameter (KS) representing the degree of flattening of the tire in contact with the ground, determining the hardness of the ground as a function of the first parameter (KL) and the second parameter (KS).

EFFECT: possibility of determining in real time the hardness of the soil along which the tire mounted on the vehicle moves.

11 cl, 6 dwg, 2 tbl

Description

GENERAL FIELD AND CONTEXT OF THE INVENTION

The present invention relates to determining tire rolling conditions on the ground. More specifically, the invention proposes to determine the state of the hardness of the ground using a measurement signal representing the circumferential curvature of the tire.

In fact, it has been found to be advantageous to know the tire rolling conditions on the ground at any time in order to interact with the driver or driving assistance systems to provide them with real-time information about changes in driving conditions and possibly respond to them. In particular, knowledge of the state of the soil allows you to adjust the conditions for using the vehicle. For example, in the presence of loose ground, tire pressure can be reduced to widen the contact patch area where the tire contacts the ground to limit ground compaction.

In addition, the local determination of soil hardness makes it possible to assess the feasibility of performing or not performing certain operations that depend on this hardness condition. For example, moving an implement over very loose ground can damage the ground or cause the implement to become stuck in the ground. The hardness of the soil can also affect the processing of the territory.

By combining hardness conditions with synchronous geolocation data, it is possible to create a map of soil hardness in an area, possibly in combination with other soil characteristics. Such a map can be useful in determining how to improve the soil on the site, for example by installing drainage or adding stone to it, or decompacting according to the needs of the soil.

FR 3015036 describes how to determine the meteorological condition of the ground, the type of surface coverage, the degree of tire wear or, alternatively, the type of tread pattern used, based on acoustic recording alone. Acoustic recordings are made using a microphone located at the front of the wheel arch at the rear of the vehicle. The spectral density of acoustic power is distributed over a given frequency interval. This spectrum varies depending on a set of parameters, such as meteorological conditions, road conditions, tire wear, type of tire tread pattern.

However, this method does not allow the determination of certain driving conditions that may be of interest for specific applications. In particular, this method does not allow determining the hardness of the soil.

DISCLOSURE OF THE INVENTION

The objective of the invention is to provide the possibility of determining in real time the hardness of the soil on which the tire is mounted on the vehicle.

To this end, the invention provides a method for determining the hardness of the ground on which a tire mounted on a vehicle travels, said tire being equipped with a sensor configured to obtain a measurement signal representing the change in curvature of the tire as it travels over the ground, the method comprising the following steps:

- receipt by the sensor of a measuring signal representing the change in the curvature of the tire during movement,

- determination from the measuring signal of the measurement data containing:

a) a first parameter representing the length of the contact patch in contact with the ground during one revolution of the wheel carrying the tire, and

b) a second parameter representing the degree of flattening of the tire in contact with the ground during one revolution of the wheel carrying the tire,

- determination of soil hardness as a function of the first parameter and the second parameter.

The method makes it possible to determine in real time the hardness of the ground on which a tire mounted on a vehicle travels, without taking into account the load or pressure of the vehicle, in a simple, accurate and reliable manner, based only on a measurement signal representing a change in tire curvature.

This method is preferably supplemented by the following features, considered individually or in any technically feasible combination thereof:

- during the movement for one revolution of the wheel, the curvature of the tire changes in accordance with the cycle, manifested in:

- parts not in contact with the ground,

- the part in contact with the ground, and the first parameter is determined from the part of the measurement signal corresponding to the part in contact with the ground, and the second parameter is determined from the part of the measuring signal corresponding to the transition in the curvature of the tire between the part not in contact with the ground and the part in contact with the ground ground;

- during the movement for one revolution of the wheel, the curvature of the tire changes in accordance with the cycle, manifested in:

- a part not in contact with the ground, characterized in the measuring signal by a stable curvature,

- the part in contact with the ground, characterized in the measuring signal by a jump in the change in the curvature of the contact,

- a transition, called the contact transition, between the part not in contact with the ground and the part in contact with the ground, characterized in the measuring signal by a jump in the change in the curvature of the entry into contact, which is opposite to the jump in the change in the curvature of the contact,

- a transition called the exit transition between the part in contact with the ground and the part not in contact with the ground, characterized in the measuring signal by a jump in the change in the curvature of the exit from the contact, which is opposite to the jump in the change in the curvature of the contact,

where the first parameter is determined from the distance between the jump in the change in the curvature of entry into the contact and the jump in the change in the curvature of the exit from the contact,

the second parameter is determined from the gradient between the contact curvature step and the contact curvature step;

- soil hardness is determined using a linear relationship linking the specified soil hardness, the first parameter and the second parameter;

- linear dependence has the form:

F=a+b × KL+c × KS,

where F is the hardness factor, KL is the first parameter, KS is the second parameter, a, b and c are predetermined fixed coefficients;

the hardness is determined by calculating the hardness factor from the first parameter and the second parameter and comparing said hardness factor with threshold values limiting the hardness categories.

The invention also relates to a tire containing a sensor sensitive to changes in tire curvature and configured to generate a measurement signal representing the change in the curvature of the tire when it moves on the ground, containing an active part and an electronic board, and the active part is configured to generate a measurement signal, and the electronic board configured to define measurement data containing:

a) a first parameter representing the length of the contact patch in contact with the ground during one revolution of the wheel carrying the tire, and

b) a second parameter representing the degree of flattening of the tire in contact with the ground during one revolution of the wheel carrying the tire,

wherein the sensor is configured to transmit measurement data outward from the bus.

The invention also relates to a data processing module configured to receive measurement data obtained from a measurement signal representing the change in curvature of a tire as it travels on the ground, said measurement data comprising:

a) a first parameter representing the length of the contact patch in contact with the ground during one revolution of the wheel carrying the tire, and

b) a second parameter representing the degree of flattening of the tire in contact with the ground during one revolution of the wheel carrying the tire,

wherein the data processing module is configured to determine the hardness of the soil as a function of the first parameter and the second parameter.

The invention also relates to a vehicle comprising:

- at least one tire,

- at least one sensor sensitive to changes in the curvature of the tire and configured to generate a measurement signal representing the change in the curvature of the tire when it runs on the ground, and the sensor is preferably located inside the tire,

- a data processing module configured to receive measurement data obtained from a measurement signal representing the change in curvature of the tire as it travels on the ground, and to determine the hardness of the ground as a function of the measurement data, the measurement data comprising:

a) a first parameter representing the length of the contact patch in contact with the ground during one revolution of the wheel carrying the tire, and

b) a second parameter representing the degree of flattening of the tire in contact with the ground during one revolution of the wheel carrying the tire, the vehicle being configured to carry out the method according to the invention.

Preferably, the sensor comprises an active part and an electronic board, the active part is configured to generate a measurement signal, the electronic board is configured to determine the measurement data, and the data processing module is located outside the bus.

The invention also relates to a computer program product containing program code instructions for carrying out the method according to the invention when said program is executed on a computer. The computer program product may take the form of a non-volatile computer-readable medium that stores code instructions for performing the method of the invention when said non-volatile computer-readable medium is readable by a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional features, objects and advantages of the invention will become more apparent from the following description, which is purely illustrative and non-limiting and should be read in conjunction with the accompanying drawings, in which:

- in Fig. 1 schematically illustrates a tire mounted on a vehicle rim;

- in Fig. 2 shows an example of a measurement signal recorded by a tire curvature sensor when the tire is rolling;

- in Fig. 3 shows a flowchart of steps in a method for assessing soil hardness according to one exemplary embodiment of the invention;

in fig. 4 shows an example of two-dimensional statistical analysis using two parameters obtained from the measurement signal for the front tire of a vehicle according to different ground hardness conditions;

- in Fig. 5 shows an example of univariate discriminant analysis for a vehicle front tire;

- in Fig. 6 shows an example of soil hardness classification.

IMPLEMENTATION OF THE INVENTION

In FIG. 1 shows a tire 1 mounted on a rim 2. Such a tire 1 comprises, on the one hand, a crown region 3 constituting a tread pattern and, on the other hand, sidewalls 4 terminating in the lower sidewall regions. The latter usually comprise a bead wire and a bead for fitting the tire 1 to the rim 2. The rim 2 is in turn connected to the vehicle 9 by an axle (not shown). The tire 1 thus forms a ground contact system providing communication between the vehicle 9 and the ground 7.

Thus, a tire is understood to be a resilient rigid body intended to be mounted on a wheel rim 2, usually in the form of a tire strip, to form a ground contact system providing a connection between the vehicle 9 and the ground 7, containing a tread that undergoes a change in its circumferential radius curvature under load. The tire 1 is usually made from an elastomer (eg rubber) and possibly other textile and/or metallic materials. The tire 1 may be airless, for example having flexible polyurethane spokes supporting the tread. However, in a preferred embodiment, the tire 1 comprises a flexible carcass containing a pressurized gaseous interior, typically air. Since this is the most common form of the tire 1, the following description is given without limitation with respect to such a pneumatic tire 1 having an internal pressurized gas.

Tire 1 is subjected to a force applied by the vehicle 9 through the axle and rim 2 towards the ground 7. This force results from the axle load due to the weight of the vehicle 9. Since the rim 2 does not deform, this force applied to the tire 1 deforms it when the tire 1 contacts the surface 8 of the ground 7: the part of the crown 3 under the rim 2 flattens out, increasing the area of the contact patch 6 in which the tire 1 contacts the ground, while the sidewalls 4 become stretched. This deformation is all the more pronounced when the pressure inside the tire is low. This deformation is also influenced by the nature of the ground 7, in particular the hardness state of this ground 7. More specifically, hard ground deforms little if any, while soft ground deforms under the action of the tire 1, so that the deformation of the tire 1 is less because part of it transferred to the ground 7.

The deformation of the tire 1 leads to a change in the circumferential curvature of the tire 1, namely the curvature of the crown region 3. As tire 1 rolls, this change in curvature follows the circumference of tire 1. Therefore, for a given point on tire 1, the curvature will change periodically with each revolution of the wheel.

Tire 1 is equipped with a sensor 10 configured to receive a measurement signal indicative of a change in tire curvature. This sensor 10 is located inside the carcass of the tire 1. The sensor 10 is preferably located opposite the crown region 3. The sensor 10 may be built into the structure of the tire body 10 or may be attached to it and, for example, held in place by a layer of adhesive. The sensor 10 comprises an active part 11 attached to the tire carcass 1 so that the deformation of the tire 1 results in a corresponding deformation of the active part 11 of the sensor 10 which generates a measurement signal which is a function of the deformation of its active part 11. Thus the measurement signal does indeed reflect the change tire curvature.

Preferably, sensor 10 is a piezoelectric sensor that generates a voltage proportional to the change in bending. More specifically, the sensor 10 may, for example, comprise an active part 11 consisting of a piezoelectric layer between two conductive layers. It is also possible for the sensor 10 to be a resistive sensor whose impedance is proportional to the curvature of the active part 11 of the sensor. It is also possible to use an accelerometer, although its use is much more complicated and requires more processing. Sensor 10 may also be suitable for measuring other parameters such as pressure. The sensor 10 may be embedded in other electronic equipment installed inside the tire 1, such as a pressure and/or temperature sensor of the TMS (tire monitoring system) type.

The sensor 10 also includes an electronic board 12 connected to the active part 11 of the sensor 10 and configured to receive a measurement signal from the active part 11. This electronic board 12 includes at least a processor and a memory and is suitable for processing data such as a measurement signal, to determine the measurement data from the measurement signal and transmit this measurement data. In a preferred embodiment, the sensor 10 is associated with a wireless transmitter, in particular of the RF type and for example of the Bluetooth low energy type or the 433 MHz low energy device type (LPD 433) to transmit the measurement signal to the automated a data processing module, preferably located outside the tire 1, for processing it. The wireless transmitter may be part of the sensor 10, such as a component of the electronic board 12, or may be located separately from the sensor 10. For example, the antenna could be placed inside the bus 1. In the case of wireless communication, an external receiver can receive signals sent by means of wireless communication associated with the sensor 10 and relay them to the automatic data processing module.

Of course, the sensor 10 may contain other elements involved in its correct operation, in particular a power supply module, for example consisting of a battery.

When the tire 1 rolls on the ground, the sensor 10 receives (step S1) a measurement signal representing the change in circumferential curvature of the tire. This measurement signal may be directly related to the curvature (and therefore be the measurement signal of the curvature) and therefore control its change, or may be indirectly related to the curvature. This is the case with a sensor 10 whose active part 11 is a piezoelectric sensor, since the measurement signal in this case corresponds to a change in curvature. It is this type of sensor that will be used in the following examples. The measurement signal generated by the active part 11 of the sensor 10 is then processed by the electronic board 12 to determine measurement data from the measurement signal. The purpose of processing the measurement signal is to extract useful information from this signal, which is then used later in the method.

In FIG. 2 shows a schematic example of a measurement signal recorded by sensor 10 sensitive to changes in tire curvature when tire 1 is rolling. The measurement signal is represented here by its voltage (in volts) and denoted as ∆curvature as a function of wheel rotation expressed in degrees.

During the movement of one revolution of the wheel, the curvature of the tire changes in accordance with the cycle, manifested in:

- parts not in contact with the ground,

- parts in contact with the ground.

The sequence illustrates two passes into the footprint in which the tire contacts the ground, the region of tire 1 in which the sensor 10 is located, the two passes being separated by the part of the cycle in which there is no contact with the ground. The part of the cycle in which there is no contact with the ground is characterized by a stable curvature, which leads to the stability of the measuring signal at a value close to zero. The part of the cycle in which there is contact with the ground is characterized in the measuring signal by a step change in the curvature 20, 30 of the contact. In FIG. 2, the jumps of change in curvature 20, 30 of the contact are directed downwards, which corresponds to negative voltage jumps. In particular, the jumps in the change in the curvature 20, 30 of the contact correspond to the flattening of the tire 1 in the patch 6 of the contact.

The curvature also appears in a transition, called the contact transition, between the part not in contact with the ground and the part in contact with the ground, characterized in the measuring signal by a step 21, 31 of the change in the curvature of the contact, which is opposite to the step of change in the curvature of the contact 20, 30 , which in this case is downward. The change in curvature also appears in a transition, referred to as the transition out of contact, between the part in contact with the ground and the part in contact with the ground, characterized in the measuring signal by a step 22, 32 in the change in the curvature of the out of contact, which is opposite to the step in the change in the curvature of the contact, namely in this case upward. The step 21, 31 of the change in contact curvature and the step 22, 32 of change in the exit curvature correspond to sudden changes in the radius of curvature of tire 1 as it enters and exits the contact patch.

As the tire rotates, the same cycle repeats with a measurement signal that is stable when there is no ground contact, followed by a jump

21, 31 change in contact curvature, jump in change in curvature 20, 30 of contact, jump 22, 32 change in exit curvature, and finally another stable measurement signal in the absence of contact with the ground. This cycle corresponds to one revolution of the wheel and therefore 360°, represented by the length CT in FIG. 2.

For each cycle, the jump 22, 32 of the change in the exit curvature provides the main advantage that it is a sharp jump and, especially, essentially independent of the state of the ground and tire 1. In particular, the jump 22, 32 of the change in the curvature of the exit from the contact. contact corresponds to a change in the curvature of the tire 1 when leaving the contact patch, when the area of the tire 1 in which the sensor 10 is located abruptly changes from the characteristic of the flat state of the part in contact with the ground, to the characteristic of the bent state of the part not in contact with the ground. On loose ground, as tire 1 gradually rolls, it compacts the ground beneath it, forming a rut and therefore a fairly hard track bottom on which tire 1 rests, leaving the contact patch. In addition, the advancement of the vehicle 9 essentially transfers the load to the entry into the contact patch. Thus, the tire 1, upon exiting the contact patch, has an exit behavior in terms of curvature very similar to the behavior of the tire 1 on the road surface.

Thus, it is easy to identify each cycle corresponding to a revolution of the wheel by identifying each step 22, 32 of the change in the out-of-contact curvature. It is also possible to identify cycles with a special device such as a rev counter. On this basis, the data can be expressed as a function of the angle at each cycle. This means in particular that the cycles and their data can be compared independently of the speed of the vehicle 9. Only one cycle is required to carry out the steps of the method and thus they can be carried out for every cycle. However, in order to make the method more reliable against potentially isolated unpredictable incidents (eg, the presence of a stone), a combination of several measured cycles can be used, for example, using a moving average.

The state of soil hardness influences the characteristics of the measurement signal profile. The invention thus aims at extracting parameters from the measurement signal in order to deduce from it the state of soil hardness. Thus, the method comprises determining (step S2) from the measurement signal measurement data comprising at least a first parameter KL representing the length of the ground contact area 6 during one revolution of the wheel carrying tire 1 and a second parameter KS representing the flattening speed tires in contact with the ground during one revolution of the wheel carrying tire 1. The measurement data may contain other parameters or values derived from the measurement signal.

The first parameter KL is determined from the part of the measuring signal corresponding to the part in contact with the ground. More specifically, the first parameter KL is determined from the distance between the step 31 of the change in the curvature of the entry into contact and the step 32 of the change in the curvature of the exit from the contact. Since they represent two local maxima in a cycle, it is easy to determine the top of each jump and from this determine the distance between them, expressed in degrees. Since the jump 31 of the change in the curvature of contact and the jump 32 of the change of the curvature of the exit from the contact correspond respectively to the beginning and end of the part in contact with the ground, the first parameter KL is indeed a function of the area of contact of the tire 1 with the ground. In fact, KL corresponds to the length of the contact patch 6. In the example where the measurement signal is expressed in volts V as a function of degrees °, the first parameter KL can be expressed in degrees of angle.

The second parameter KS is determined from the portion of the measurement signal corresponding to the tire curvature transition between the non-ground contact portion and the ground contact portion. More specifically, the second parameter KS is determined from the gradient between the contact curvature step 31 and the contact curvature step 30 . More specifically, the second parameter KS may correspond to the maximum change in curvature change between the contact curvature step 31 and the contact curvature step 30, namely, it may correspond to the maximum gradient. In the example where the measurement signal is expressed in volts V as a function of angular degrees °, the second parameter KS may have units of V/°, namely as the second derivative of the tire curvature 1.

KS can be approximated in several ways. For example, the second parameter KS may correspond to the maximum (in terms of absolute value) of the derivative of the measuring signal between the jump 31 of the change in the curvature of the entry into contact and the jump of the change in the curvature of the contact 30, i.e. the derivative is estimated from the difference between two successive (or closely spaced) measurement points, obviously taking into account their angular separation. Since this is a decreasing gradient in the example, this maximum in terms of absolute value corresponds to the minimum of the derivative of the measuring signal between the step 31 of the change in the curvature of the entry into contact and the step of change in the curvature 30 of the contact. It is also possible, instead of searching for the extremum of the derivative, to select fixed measurement points, such as, for example, those located at an equal distance from the vertices of the jump 31 of the curvature change of the entry into contact and the jump of the change of curvature of the contact 30, and calculate the derivative using these points. It is possible to use measurement points corresponding to the value of the measurement signal, such as, for example, a signal passing through zero in the illustrated case. It is also possible to use more complex approaches, such as, for example, the Savitsky-Golay algorithm. However, the choice of a relatively low sampling rate, typically less than or equal to 500 Hz and preferably less than or equal to 400 Hz, such as the 300 Hz frequency in the example, constitutes a smoothing of the measurement signal and allows the selection of approaches such as those outlined above that are less computationally demanding. .

The first parameter KL and the second parameter KS have the advantage that they exhibit a wide degree of variability with soil hardness and are easy to obtain as shown above. More specifically, as the hardness of the soil decreases, the first parameter KL increases and the second parameter KS decreases. Thus, the looser the ground, the longer the length (represented by the first parameter KL) of the contact patch 6 in which the tire contacts the ground 7, while the flattening speed (represented by the second parameter KS) becomes smaller. Conversely, as the hardness of the soil 7 increases, the first parameter KL decreases, while the second parameter KS increases.

Considered individually, the first parameter KL and/or the second parameter KS may be dependent on load, pressure and/or speed. However, taking into account both the first parameter KL and the second parameter KS means that the hardness of the soil can only be determined from these parameters, without knowing the load, pressure, speed and slip of the tire 1 on the ground.

Preferably, it is the electronic board 12 of the sensor 10 that determines from the measurement signal the measurement data comprising the first parameter KL and the second parameter KS. This measurement data is then transmitted by the sensor 10 to the data processing module 15 which performs the next part of the method. This data processing module 15 is preferably located outside the tire 1, for example in the vehicle 9, but the processing module 15 may also be located remotely from the vehicle 9 and the data communication may then include intermediate transmission means. Then, transmission of the measurement data between the sensor 10 and the data processing unit 15 is performed wirelessly. The data processing unit 15 generally includes a processor and a memory, and is suitable for receiving and processing measurement data in carrying out the next part of the soil hardness determination method.

It is possible to transmit the measurement signal to the processing module 15 in order to carry out the next part of the method. However, determining the measurement data using the sensor 10 and transmitting only this measurement data to the data processing module 15 has the advantage of reducing the amount of data transmitted between the sensor 10 and the data processing module 15. Since data transmission requires a lot of power, the transmission of the measurement data rather than the measurement signal makes it possible to reduce the power consumption of the sensor 10, which has a limited power supply inside the bus 1.

It is also preferable not to use the electronic board 12 of the sensor 10 to carry out the next part of the method, but to use the data processing module 15 instead to process the measurement data. This then limits the calculations performed by the electronic board 12 of the sensor 10, which saves power and memory on the electronic board 12. In addition, it is easier to change the methods for carrying out the next part of the method on an easily accessible data processing module 15 than on the sensor 10, which is inside the bus 1 .

After the processing unit 15 has received the measurement data, the processing unit 15 may determine the ground hardness as a function of the first parameter KL and the second parameter KS contained in the measurement data, which vary depending on the ground hardness, as shown below.

In FIG. 4 shows an example of a two-dimensional statistical analysis using two parameters KL and KS obtained from the measurement signal for a tire 1 of a vehicle 9 running on a ground 7 of known hardness. In this example, the measurement data is obtained from the measurement signal received by the piezoelectric sensor 10 located in the front tire of an agricultural tractor while driving on the same ground with three different hardness configurations:

- C0 (very loose soil): newly plowed soil that has been harrowed,

- C2 (sufficiently loose soil): freshly plowed and harrowed soil, double compacted by the passage of tire 1 (front and rear wheels of the vehicle 9, which corresponds to the passage of the vehicle 9),

- road (hard ground): asphalt surface.

It should be noted that the measurement points group together various loads, pressures and speeds (below 20 km/h).

The measurement points are expressed as a function of the first parameter KL (along the abscissa and in degrees of angle) and the second parameter KS (along the ordinate and in V/°). These values obviously depend on the type of tire 1 and the sensor 10 used. The crosses correspond to movement on very loose topsoil C0 and are grouped together in the first, 95% confidence ellipsoid 41. The circles correspond to movement on fairly loose soil C2 and are grouped together into a second, 95% confidence ellipsoid 42. The points correspond to measurement points when driving on a road (very hard ground) and are grouped together into a third, 95% confidence ellipsoid 43. The examples in FIG. 4 show the values assumed for the parameters KL and KS. For example, on the road, the second parameter KS is in the range of -0.7 V/° to -1 V/°, and the first parameter KL is in the range of 35° to 43°.

It can be seen that the third confidence ellipsoid 43 is markedly separated from the other two. Thus, the two parameters KL and KS make it easy to identify very hard ground, such as a roadbed. It can also be seen that although the first confidence ellipsoid 41 and the second confidence ellipsoid 42 partially overlap, they are nevertheless sufficiently separated to distinguish the ground measurement point C0 from the ground measurement point C2. Thus, the joint consideration of the parameters KL and KS makes it possible to determine the state of soil hardness, despite the loads, pressures and speeds that can change.

More specifically, the measurement data containing the KL and KS parameters are used to analyze the measurement signal in order to determine the hardness category to which the ground on which the tire 1 travels belongs. The use of the categories means that any feedback of information to the driver or the use of an automated system may be lightened and simplified, while eliminating measurement inaccuracies and fluctuations. The use of hardness categories is not mandatory since hardness can be expressed in terms of a numerical value, such as a percentage, for example, although this is the preferred embodiment as set forth below.

The following categories may be used by way of non-limiting illustration:

In this table, C0 soil is newly plowed soil that has been harrowed, C1 soil is C0 soil that has been compacted by passing a wheel (front wheel only), C2 soil is C0 soil that has been compacted by passing two wheels (front wheel and rear wheel), soil C3 is soil C0 that has been compacted by passing three wheels (front wheel, rear wheel and then front wheel again). It should be noted that soils C2 and C3 (and possibly the next one) are grouped into the same category because the soil very soon reaches its maximum degree of compaction when hit by a vehicle 9. The advantage of these categories is that they take into account the effects, exerted on the hardness of the ground, by driving on it with the front wheels and rear wheels. Of course, other categories can be used as well. For example, even looser categories can be used, such as for very loose silty soil, or even for soil with insufficient bearing capacity.

Therefore, the soil hardness is determined using a relation relating the soil hardness, the first parameter KL and the second parameter KS. Thus, once determined, the first parameter KL and the second parameter KS are then used to determine the hardness factor (step 3) using this relationship. Thus, if the hardness factor is denoted by F, and the function corresponding to the ratio and related to the first parameter KL and the second parameter KS is denoted by f, then the following can be written:

F=f(KL,KS)

Typically, this relationship is linear. More specifically, a linear relationship may take the form:

F=a+b×KL+c×KS,

where F is the hardness factor, KL is the first parameter, KS is the second parameter, a, b and c are predetermined non-zero fixed real factors. The connection can be bilinear and therefore have the type:

F=a+b×KL+c×KS+m×KL×KS,

where a, b, c and m are predetermined non-zero fixed real coefficients.

It should be noted that the relationship relating the ground hardness, the first parameter KL and the second parameter KS may, obviously, take into account other parameters and, in particular, may take into account tire pressure, speed, or, otherwise, the length CT (wheel revolution).

The fixed coefficients a, b, c and m are preferably chosen to maximize discrimination between the categories of soil hardness. For example, one-dimensional discriminant analysis can be used. This discriminant analysis aims to maximize the distance between the centers of gravity of each of the hardness categories while minimizing the variation within the category.

In FIG. 5 shows an example of a one-dimensional discriminant analysis for the front tire 1 of the vehicle 9 for the same data as in FIG. 4. Samples (exactly 1388) are grouped together, their configuration is along the x-axis, and the y-axis corresponds to the discriminant axis. The measurement points were recorded with three different hardness configurations. The first set of measurement points 51 groups together the measurement points recorded on the road, the second set of measurement points 52 groups together the measurement points recorded on very loose soil corresponding to the C0 soil configuration, the third set of measurement points 53 groups together the measurement points recorded on semi-loose soil, corresponding soil configuration C2. The discriminant analysis of the parameters KL and KS consists in combining these two parameters in order to derive a third one from them, which corresponds to the stiffness factor F _AD , which corresponds to the ordinate value of each measurement point. Then the ratio is:

F _AD =0.012516 × KL+0.57576 × KS

Thus, with reference to the general formula given earlier, it was found that: a=0, b=0.012516 and c=0.57576. The values of the fixed coefficients obviously depend on the type of tire, on the sensor 10 used, on its location on the tire, front or rear, and on the ground configuration used for data collection.

Discriminant analysis allows you to determine the classification thresholds that separate different categories. In the example of FIG. 4, the resulting threshold values are shown as dotted horizontal lines. Thus, the first line 55 separates the category "road" from the category "soil C2" with a classification threshold of 0.088066. The second line 56 separates the C2 soil category from the C0 soil category with a classification threshold of 0.44572.

Thus, once determined, the first parameter KL and the second parameter KS are used to determine the hardness factor, and the hardness category is determined in situ (step S4) by comparing this hardness factor with the classification thresholds. In this example, the average probability of correctly detecting the three categories (road, C2 soil, and C0 soil) rises to 97% and is distributed as follows:

- 100% for road classification,

- 94% for soil classification C2,

- 96% for soil classification C0.

In this way an excellent correspondence is achieved between the hardness categories determined by the method and the actual hardness of the soil configurations at the measurement points.

It should be noted that the behavior of the tire 1 mounted on the front of the vehicle 9 is different from the behavior of the tire 1 mounted on the rear of the vehicle 9, mainly due to the different respective sizes of the front tire 1 and the rear tire 1, but also due to by the soil compaction by the front tire 1, which increases the hardness of the ground on which the rear tire 1 travels. As a result, the relationship between the ground hardness, the first parameter KL and the second parameter KS for the front tire 1 and for the rear tire may be different. In the preferred embodiment, the sensor 10 is located inside the front tire 1, which makes it possible to better take into account the effective hardness of the soil as it is before compaction due to the movement of the tire 1 over it. Thus, using the linear relationship expressed above, fixed coefficients a, b and c may differ depending on the position of the sensor 10 in the front tire 1 or in the rear tire 1. As an additional preference, the sensor 10 is placed in the front tire 1, and the other sensor 10 is placed in the rear tire that follows the specified front tire (namely, usually on the same side of the vehicle 9). Thus, two linear relationships can be obtained, one for the front tire and the other for the rear:

where F _AV is the hardness determined from the measurement data of the front tire, KL _AV is the first parameter for the front tire, KS _AV is the second parameter for the front tire, and _AV , b _AV , c _AV are predetermined fixed coefficients for the front tire, and where F _AR is the hardness determined from the measurement data of the rear tire, KL _AR is the first parameter for the rear tire, KS _AR is the second parameter for the rear tire, and _AR ., b _AR ., c _AR are predetermined fixed coefficients for rear tire. For example, the following two relationships are possible:

Since the hardness experienced by the front tire and the rear tire is determined by different discriminant analyses, F _AV and F _AR are not directly on the same scale. However, it is preferable to be able to compare and share the hardness values determined by the front tire and the rear tire by finding a common scale for expressing the two hardness values. The category "road" can be chosen as a general reference because it can be seen to contain a small amount of variation, allowing conversion factors to be determined to adjust the scale. It is a simple matter of changing the respective fixed coefficients a, b and c of the two ratios.

These hardness values can also be expressed as a function of the measurement spread: the highest hardness of the measurement points may then correspond to one end of the scale, while the lowest hardness of the measurement points may correspond to the other end of the scale. For example, percentage values can be used to express hardness: 100% for the highest hardness factor and 0% for the lowest hardness factor. Once again, this simply entails changing the respective fixed coefficients a, b and c of the two ratios.

Returning to the example of the two ratios above, these two ratios can express the stability factor as a percentage on the same scale:

As an example, FIG. 6 shows the percentage classification of soil hardness for the six categories of Table 1, with the classification thresholds represented by vertical dotted lines, and the probability density values for each category are modeled using a normal law.

Thus, table 1 can be supplemented with appropriate thresholds to obtain the following table:

These categories are predetermined and stored in the memory of the processing unit 15, which also stores the relation relating the ground hardness, the first parameter KL and the second parameter KS. These categories can be used by the processing module 15 to process the measurement data for the tire 1 and the corresponding sensor 10 to determine the hardness category of the soil on which the tire 1 travels. For example, if the hardness factor (in percent) F (%) calculated for a pair consisting of the first parameter KL and the second parameter KS, which is obtained from the measurement signal, is 92%, which means that tire 1 is driving on very hard ground, such as a road classified as D5.

Then, appropriate steps can be carried out according to the hardness thus determined. For example, in the case of an agricultural tractor with integrated transmission and no center differential, the front axle can be disengaged to avoid wear on hard ground. Conversely, it is also possible, for example, to lower the tire pressure when the ground is very loose in order to limit marks on the ground or to reduce the risk of wheel slippage.

These steps can be controlled by the data processing unit 15 which has determined the ground hardness, especially if it is the central control unit of the vehicle 9. It is also possible that the data processing unit 15 which has determined the ground hardness simply sends information consisting of the ground hardness (expressed as a value or synthesized in the form of a category indication), to another control and/or display device to inform the user.

The invention is not limited to the embodiment described and shown in the accompanying drawings. Modifications remain possible, in particular with regard to the composition of the various elements or by substitution with technical equivalents, without in any way departing from the scope of the invention.

Claims (33)

1. Method for determining the hardness of the soil on which the tire (1) is moving, mounted on the vehicle (9), and the specified tire (1) is equipped with a sensor (10) configured to receive a measuring signal representing the change in the curvature of the tire when it moves on the ground (7), the method comprising the following steps: receiving (S1) by the sensor (10) a measurement signal representing the change in the curvature of the tire (1) during the movement, determination (S2) from the measurement signal of the measurement data containing: a) a first parameter (KL) representing the length of the contact patch (6) in contact with the ground (7) during one revolution of the wheel carrying the tire, and b) a second parameter (KS) representing the degree of flattening of the tire (1) in contact with the ground (7) during one revolution of the wheel carrying the tire, determining (S4) the hardness of the soil as a function of the first parameter (KL) and the second parameter (KS). 2. The method according to claim 1, in which during the movement for one revolution of the wheel, the curvature of the tire (1) changes in accordance with the cycle, manifested in parts not in contact with the ground, the part in contact with the ground, wherein the first parameter (KL) is determined from the part of the measuring signal corresponding to the part in contact with the ground, and the second parameter is determined from the part of the measuring signal corresponding to the transition in the curvature of the tire (1) between the part not in contact with the ground, and the part in contact with the ground. 3. The method according to claim 1 or 2, in which during the movement for one revolution of the wheel, the curvature of the tire (1) changes in accordance with the cycle manifested in part not in contact with the ground, characterized in the measuring signal by a stable curvature, the part in contact with the ground, characterized in the measuring signal by a jump in the change in the curvature (20, 30) of the contact, transition, called the contact transition, between the part not in contact with the ground and the part in contact with the ground, characterized in the measuring signal by a step (21, 22) of the change in the curvature of the contact, which is opposite to the step of change in the curvature (20, 30) of the contact , transition, called the contact exit transition, between the part in contact with the ground and the part not in contact with the ground, characterized in the measuring signal by a jump (22, 32) in the change in the curvature of the contact exit, which is opposite to the jump in the change in the curvature (20, 30) of the contact , wherein the first parameter (KL) is determined from the distance between the jump (31) of the change in the curvature of contact entry and the jump (32) of the change in the curvature of the exit from the contact, and the second parameter (KS) is determined from the gradient between the jump (31) of the change in the curvature of entry into contact (31) and a jump in the change in curvature (30) of the contact. 4. The method according to one of paragraphs. 1-3, in which the soil hardness (7) is determined using a linear relationship relating the indicated soil hardness, the first parameter (KL) and the second parameter (KS). 5. The method according to claim 4, in which the linear dependence has the form F=a+b×KL+c×KS, where F is the hardness factor, KL is the first parameter, KS is the second parameter, a, b and c are predetermined fixed factors. 6. The method according to one of the preceding claims, wherein the hardness is determined by calculating a hardness factor (F) from the first parameter (KL) and the second parameter (KS) and comparing said hardness factor with threshold values limiting hardness categories. 7. Tire (1) containing a sensor (10) sensitive to changes in the curvature of the tire and configured to generate a measuring signal representing the change in the curvature of the tire when it moves on the ground, containing the active part (11) and the electronic board (12), and the active part (11) is configured to generate a measurement signal, and the electronic board (12) is configured to determine the measurement data containing a) a first parameter (KL) representing the length of the contact patch (6) in contact with the ground (7) during one revolution of the wheel carrying the tire, and b) a second parameter (KS) representing the degree of flattening of the tire (1) in contact with the ground (7) during one revolution of the wheel carrying the tire, the sensor (10) being configured to transmit measurement data outward from the tire (1). 8. A data processing module (15) configured to receive measurement data obtained from a measurement signal representing a change in tire curvature as it travels on the ground, said measurement data comprising a) a first parameter (KL) representing the length of the contact patch (6) in contact with the ground (7) during one revolution of the wheel carrying the tire, b) a second parameter (KS) representing the degree of flattening of the tire (1) in contact with the ground (7) during one revolution of the wheel carrying the tire, wherein the data processing module (15) is configured to determine the hardness of the ground as a function of the first parameter (KL) and the second parameter (KS). 9. Vehicle (9) containing at least one tire (1), at least one sensor (10) sensitive to changes in tire curvature and configured to generate a measuring signal representing the change in tire curvature as it moves on the ground, a data processing module (15) configured to receive measurement data obtained from a measurement signal representing a change in tire curvature as it travels over ground, and to determine soil hardness as a function of measurement data, the measurement data comprising a) a first parameter (KL) representing the length of the contact patch (6) in contact with the ground (7) during one revolution of the wheel carrying the tire, b) a second parameter (KS) representing the degree of flattening of the tire (1) in contact with the ground (7) during one revolution of the wheel carrying the tire, the vehicle being configured to carry out the method according to any one of paragraphs. 1-6. 10. The vehicle according to claim 9, in which the sensor (10) is located inside the tire (1). 11. The vehicle (9) according to claim 10, in which the sensor (10) contains an active part (11) and an electronic board (12), and the active part (11) is configured to generate a measuring signal, the electronic board (12) is configured to determining the measurement data, and the data processing module (15) is located outside the tire.

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