METHOD FOR WARNING UNDER-INFLATION OF A TWIN WHEEL TIRE

Abstract

The invention relates to a method for detecting under-inflation of a tire of a twin wheel of a vehicle comprising the steps: - Moving the twin wheel at an identified speed V on the same ground equipped with sensors capable of detecting contact between the tire and the ground, two neighboring sensors being separated by a distance Di, - Determining for each sensor an instant at which the contact between the tire and the sensor changes state. - Determining a discrete metric line of contact for each tire of the twin wheel; - Determining a parabola arc that minimizes the difference with the points of the discrete line for each tire; - Algebraically comparing the second-order coefficients of the parabola arcs; - If the algebraic comparison exceeds a threshold S, alerting that one of the tires of the twin wheel is under-inflated. Fig. 5

Description

Title of the invention: Method for alerting against under-inflation on a twin wheel tire Field of the invention

[0001] The present invention relates to the field of vehicles transporting significant loads, which requires the use of twin wheels on certain axles of the vehicle in order to reduce ground pressure. Technological background

[0002] In the field of freight transport in particular, twin-wheel vehicles are frequently used for long-distance journeys between warehouses. The leitmotif of this industry is speed of delivery, which forces the vehicles to be used intensively. However, under-inflation of a tire, caused for example by a puncture, can lead to immobilization of the vehicle to repair the tires. However, if the puncture is not detected early, the damage caused to the tire can lead to the scrapping of the tire or even the destruction of the wheel of the assembled assembly. In the first case, this constitutes a financial loss for the carrier with a controlled immobilization of the vehicle.In the second case, this can cause an interruption in the use of the truck to best change the tire, or even repair the damage caused by the destruction of the tire on the vehicle, not to mention the possible damage generated on the road. It is common to retread worn tires in the road transport industry. A new tire that has been driven under pressure can thus lose its "second life" and the corresponding value.

[0003] The difficulty in inspecting twin wheel tires is the accessibility of the control on the tire located on the inside of the vehicle for instrumented checks such as inflation pressure but also visual checks on the entire tire. Indeed, these checks are generally carried out while the vehicle is stationary, the workshop check, for which accessibility is facilitated, only takes place at the vehicle's maintenance locations. As a result, the most efficient checks are planned and are generally too far apart from each other to detect under-inflation before it already becomes too worrying for the integrity of the tire.

[0004] The introduction of electronic pressure sensors such as TPMS (Tyre Pressure Monitoring System) in tires is a solution for providing early warning of under-inflation of a tire. However, this technological solution is expensive since it requires equipping each tire with a pressure sensor. These sensors generally operate on batteries. which requires specific maintenance of these. As a result, this solution is too expensive and intrusive compared to the current approach used in the transport industry.

[0005] Furthermore, patent FR3030742A1 discloses the principle of estimating the contact area of ​​a tire using proximity or force sensors. When a tire passes over it, the time signal from such a sensor successively shows an approach phase, a plateau phase corresponding to the rolling of the flattened tread in the contact area, and a moving away phase. Appropriate signal processing makes it possible to estimate the duration of the plateau, knowing the speed, and its longitudinal extent, i.e. the length of the contact area. Assuming that the contact width is constant and known, it is possible to estimate the surface area of ​​the contact area.

[0006] However, twin wheels have, by geometric construction, an axis of rotation positioned at the same distance from the ground. As a result, the surface of the contact patch is almost identical between the twin wheels, regardless of the inflation pressure. In other words, the sensitivity of the contact surface is low to the variation in inflation pressure. As a result, the contact surface between the ground and the tire is not a sensitive indicator of a drop in inflation pressure between the tires of the twin wheel.

[0007] The object of the invention which follows is to propose a solution for early and reliable detection of under-inflation of the twin wheel tires of a vehicle at low operating cost, even for small variations in inflation pressure. Description of the invention

[0008] The invention relates to a method for detecting under-inflation of a tire forming part of a twin wheel of a vehicle, comprising the following steps: • Moving the two tires of the twin wheel of the vehicle at an identified speed V on the same flat ground equipped with sensors, two neighboring sensors being distant from each other transversely to the direction of movement of the vehicle by a distance Di, so that each tire is situated vertically above at least three sensors during the movement, said sensors being capable of detecting contact between the tire and the ground; • Determine for each sensor at least one moment when the contact between the tire and the sensor changes state. • Determine a discrete metric line of contact of the tire with the ground for each tire of the twin wheel by means of the identified speed V, the transverse distances Di and the instants of change of state of the at least three sensors located vertically of the tire cor responding to the entry and / or exit of Contact Making; • Determine a parabola arc that minimizes the deviation from the points of the discrete line for each tire; • Algebraically compare the second-order coefficients of the arcs of parabolas, preferably perform an algebraic difference; and • If the algebraic comparison exceeds a threshold S, alert that one of the tires on the twin wheel is under-inflated.

[0009] The method is based on obtaining the entry and / or exit front of the contact area. Necessarily, several sensors are required per tire. Knowing that the sensitive parameter according to the invention is the curvature of this entry and / or exit front of the contact area, a minimum of 3 sensors are required per tire to identify the equation of the parabola and which will minimize the difference between the points of this front and the theoretical parabola. By nature, tires have entry or exit fronts that are similar to parabolas. Of course, the higher the number of measurement points, the greater the robustness of the result by minimizing measurement noise and measurement errors.

[0010] To obtain this theoretical curvature, it is first necessary to identify the instants of change of state of the sensor. The signal from the sensor is similar to a square wave consisting of a rising edge at the entrance to the contact area, a plateau when crossing the contact area and a falling edge when leaving the contact area.

[0011] The rising and falling edges reflect several phenomena simultaneously. First of all, the edge is an illustration of the spatial approach of the tire following its trajectory relative to the sensor. Indeed, whatever the technology of the sensor used, the tire gradually approaches the measuring sensor which quickly passes from a non-contact state to a progressive state of contact. The rising or falling edge reflects this evolution of the distance between the sensor and the tire. Then, the edge is also the translation of the length of the sensitive zone of the sensor following the direction of the trajectory of the tire. The sensitive zone of the sensor has a certain length, the contact is progressive following this length. A part of the sensitive zone is in contact while the other is still out of effective contact and the resultant of the sensor signal reflects these two phenomena by averaging them.Therefore, it is impossible to identify the exact moment of contact on the sensor signal since it is primarily spatial. The method therefore proposes a morphological approach to signal processing on the sensor signal to, on the one hand, identify the plateau which corresponds to complete contact over the entire active area of ​​the sensor and, on the other hand, identify the rising and falling edges which correspond to partial contact over the entire active area of ​​the sensor and to a distance between the sensor and the tire which varies greatly depending on the sensitivity of the sensor.

[0012] Therefore, it is necessary to define a single method for determining the start or end of the contact area, which is called the sensor state change. An approach using thresholds relative to a maxima that is similar to the sensor response at the plateau is a technical solution. More complex techniques can also be used. As long as the same technique is used on all the sensors for the entry and / or exit of the contact area, the under-inflation detection method is robust since the shape of the state change line remains identical. This is a strong point of the method since the length of the contact area and consequently the surface area evolve according to the detection method used.

[0013] Then, it is necessary to transform the time signals into metric signals. For this, the determination of the speed V and the transverse distances Di between the ground sensors is essential. Thus, a set of measurement points is obtained in a two-dimensional metric frame of reference. This makes it possible to obtain a method independent of the speed of passage of the tires on the ground equipped with sensors and to determine a threshold S which is independent of the passage speed V, which makes the method reliable.

[0014] The speed V corresponds to an instantaneous average speed at the moment the tire passes over the sensors. Since the time spent passing the sensors is short, the speed changes little during this period of time. This can be obtained by a specific measuring system located upstream or downstream of the ground sensors following the trajectory of the vehicle. This determination of the signals can also take place on the signal itself from the measuring sensor in connection with the sampling frequency of the measuring sensor and the shape of the output signal.

[0015] Once the discrete metric definition of the contact between the ground and the tire has been established, we can identify the equation of the parabola which minimizes the deviation, generally the quadratic deviation, between the discrete measurement points and the equation of the parabola.

[0016] For each tire of the twin wheel, we obtain the equation of the parabola, this translates the convexity or the concavity of the contact line by positioning itself from the point of view of the contact area, respectively from the point of view of the exterior of the contact area, such as for example, the geometric barycenter of the contact area, respectively any point of the trajectory of the tire outside the contact area. The coefficient of order 2 of the equation of the parabola is the descriptor of the level of convexity or concavity of the parabola. If the coefficient of order 2 is positive, the parabola, seen for example from the contact area, is convex. If the coefficient of order 2 is negative, the parabola, for example for an observation from the contact area, is concave.

[0017] We then carry out the algebraic comparison of the second order coefficients, whether it is the ratio of the coefficients or their difference. In the case of a twin wheel having so the same distance between the wheel center and the ground for both tires, both mounted assemblies being inflated to the same inflation pressure and the tires being identical, as well as the wheels, the two coefficients are identical, the comparison tends towards a target value, 0 for the difference and 1 for the ratio. As soon as one tire is in a situation of under-inflation compared to the other, the two coefficients differ. For normal pressure and imposed load conditions, the parabolas are generally convex from the point of view of the contact area. The more the pressure of one of the mounted assemblies differs from the other, the more the second order coefficients move away from each other to the point where the under-inflated tire ends up with a concave parabola seen from the inside of the contact area, respectively convex seen from the outside of the contact patch. Thus, the comparison of the coefficients moves away from the target value.As initially, we do not know which tire is under-inflated compared to the other, the difference in coefficients is insensitive to the order of the coefficients, inversely to the ratio which will tend from the target value 1 towards the value 0 or infinity depending on whether the coefficient of the deflated tire is in the numerator or denominator.

[0018] When the distance from this target value is greater than a threshold S, under-inflation of one of the mounted sets of the twin wheels is confirmed and an alert is sent to the operator operating the measuring device on the ground.

[0019] The method is more precise than that used using the contact surface which is sensitive for twin wheels only when the inflation differences are significant between the two mounted assemblies, approximately a loss of inflation pressure of half, or approximately 4 bars for tires generally inflated to 7 or 9 bars. Here, a pressure difference of 2 bars, or approximately 20 percent, will be identified by the method which makes it possible to be earlier on the appearance of under-inflation and to act before a significant structural defect occurs on the structure of the tire itself.

[0020] Advantageously, the method comprises the complementary steps: • Identify the discrete line with the smallest second-order coefficient; • Identify the tire that generated the discrete line of smallest co efficient of order two, the identified tire is in an under-inflation condition.

[0021] If the detection of under-inflation is a minimum for the safety of the vehicle, the detection of the under-inflated individual makes it possible to accelerate the repair or the restoration of the vehicle, which constitutes a saving in time and therefore a monetary gain for the goods and people transport industry which generally uses twin wheels.

[0022] Having previously identified the two second-order coefficients of the parabolas, it You just need to evaluate the coefficients between them to identify the under-inflated tire compared to the other.

[0023] Preferably, each tire has a trajectory supported by at least five sensors during its movement on the ground.

[0024] A minimum number of three sensors is required to identify the equation of the parabola. But, by increasing the number of measurement points, the accuracy of the method is increased by minimizing measurement errors. In addition, an odd number of measurement points is preferable compared to a theoretical parabola whether convex or concave.

[0025] Specifically, the comparison of the second order coefficients being carried out by an algebraic difference, the threshold S corresponds to a curvature of value 5 m1.

[0026] The inventor has found that for heavy goods vehicle tires, having generally rectangular contact areas with contact lines at the entry or exit of the contact area that are rather rectilinear, a threshold value S of 5 makes it possible to state that one of the mounted assemblies of the vehicle is in an under-inflated condition compared to the other when the tires comply with the ETRTO standards (acronym in English for European Tires and Rim Technical Organization).

[0027] According to a first embodiment, the contact between the sensor and the ground being previously identified on the sensor signal by a constant amplitude A of the signal, the change of state of the sensor signal corresponds to the crossing of a threshold T by the signal, the threshold T corresponding to a percentage of the amplitude A of the signal.

[0028] If the detection of the plateau on the measurement signal which corresponds to the complete contact between the active zone of the sensor and the tire is easy by a morphological approach or threshold or optimum detection, a first definition of the contact line between the tire and the ground is given by the exceeding of a threshold T which is a function of the amplitude level A of the signal which corresponds to the plateau. This threshold T is a percentage of the amplitude A of the plateau. The percentage is a function of the sensor technology used to take into account the non-linearity of the sensor and the measurement noise associated with the technology. Thus, the contact line, at the entrance or exit of the contact area, corresponds to all the measurement points having crossed the threshold T by rising on the one hand or falling on the other hand.

[0029] According to a second embodiment, the contact being previously identified on the sensor signal by a constant amplitude A of the signal, the instants of change of state of the sensor correspond to the intersection between the tangent of the sensor signal at the abscissa point corresponding to the ordinate at mid-amplitude A / 2 and a straight line whose ordinate is equal to the value A.

[0030] If the detection of the plateau on the measurement signal which corresponds to the complete contact between the active zone of the sensor and the tire is easy by a mor phological or threshold or optimum detection, a second definition of the contact zone is given by the set of points between two particular points.

[0031] These particular points, which are not necessarily measurement points, are defined by the intersection between two straight lines. The first straight line corresponds to that defining the plateau of amplitude A previously identified. The second straight line corresponds to the tangent of the sensor signal at the point having as abscissa half of the amplitude A of the plateau.

[0032] Whatever the method used on the measurement signals to identify the contact area, as long as it is the same between the various measurement sensors, the method remains robust. Indeed, the parabolic shape of the contact line is similar whatever the definition of the identified state change points. As a result, the comparison gives an equivalent result by possibly adjusting the threshold S to the method used.

[0033] According to a specific embodiment, the identification of the speed of passage of the tire over the sensor is obtained by the estimated duration of the rising edge and / or the falling edge of the signal from the sensor as the tire passes, knowing that the distance of these edges is constant depending on the geometric characteristics of the tire, preferably its diameter.

[0034] It is entirely possible to use the sensor signal to evaluate a speed of passage of the mounted assembly when crossing the measurement sensor. Indeed, the inventor noted that the shape of the sensor signal, whatever the technology used, is always related to a trapezoid and that the inclined edges of the trapezoid have a constant spatial length according to the identity of the tire, whatever the inflation pressure, the static load applied to the mounted assembly. This length can be adjusted according to the method of defining the contact line used and then serve as a reference distance. Knowing the sampling frequency of the sensor and the number of measurement points included on the inclined edge of the trapezoid, it is possible to define an instantaneous speed of passage of the tire on the measurement sensor. In fact, two measurements can be carried out during the same passage, at the entry and exit of the contact area.By taking either of the values ​​or the median value, it is entirely possible to identify a realistic passage speed which is only a function of the discretization of the measurement signal.

[0035] Preferably, having previously identified the two instants at which the contact between the tire and the sensor changes state when the tire passes over the sensors, the signals from the sensors are realigned with each other by making the instants corresponding to the middle of the interval defining the contact state of each sensor coincide.

[0036] If the sensor signals are not phased with each other in time, it is important to temporarily recalibrate the signals of each sensor on the ground to identify the equation of each parabola associated with each mounted assembly. The method consists of taking advantage of a particularity of the assemblies mounted in free rolling conditions. Indeed, the geometric barycenter of the contact area is located at an equal distance from the contact lines located at the entrance to the contact area and at the exit of the contact area. Each sensor making it possible to identify a so-called contact state between the tire and the sensor on the signal, the middle of this time interval corresponds to this geometric barycenter of the contact area. Thus, we can define a point, potentially imaginary, temporal per measurement signal corresponding to this middle. It is then sufficient to make these different potentially imaginary points coincide between the various measurement signals to phase all the sensors together. Brief description of the drawings

[0037] The invention will be better understood on reading the following description given solely by way of non-limiting example and made with reference to the appended figures in which the same reference numbers designate identical parts throughout and in which: • [Fig.l] illustrates an example of application of the method according to the invention for a vehicle with twin wheels; • [Fig.2a] and 2b show two views of an evaluation system according to the invention; • [Fig.3] shows a signal measured at the output of a sensor of the system and a processing for determining the instants of change of state of the sensor according to the invention; • [Fig.4] shows the same signal measured at the output of a sensor of the system and another processing for determining the instants of change of state of the sensor according to the invention; • [Fig.5] shows a representation of the contact areas of the axles of a vehicle on a measuring system, one of the tires of a twin wheel being under-inflated; • [Fig.6] is a representation over time of the second order coefficients of the parabolas of the tires of a twin wheel, one of the tires being in the process of deflating from a given moment. Detailed description of the embodiments

[0038] [Fig.l] shows a convoy consisting of a tractor 12 and a semi-trailer 13 which passes over a measuring system 200 as described in [Fig.2a] and 2b.

[0039] In this example, the tractor 12 consists of a front axle 401 and a rear axle 501 and the semi-trailer 13 consists of a group of three axles 601. furthermore, the rear axle 501 of the tractor 12 is fitted with two “twin” tires at each of its ends.

[0040] Only the twin wheel 510 located on the right side of the axle 501 of the tractor 12 is in an under-pressure situation. In the case of twin tires, the increase in length or contact area surface area resulting from under-pressure on one of the twin tires will be distributed between the two tires on the side affected by the under-pressure. Thus, even if a single tire of the twin wheel 510 is in an under-pressure situation, it is indeed the length and the contact area surface area of ​​the two tires that will increase. Consequently, it is not possible to distinguish which tire of the twin wheel 510 is under-inflated.

[0041] The sensitivity to possible underpressure of a tire is therefore much lower than in the case where the axle is fitted with a single tire assembly.

[0042] The measuring system 200, given here as an example, appears in figures 2a and 2b and consists of: • A housing 10 consisting of two access ramps 15 and a measuring zone located between the two access ramps 15. • Two tire presence detection devices each consisting of three piezoelectric sensors 110, positioned along a line transverse to the direction of travel of a vehicle arriving at the housing. In this example, the piezoelectric sensors are buzzers glued to the structure of the housing 10. • A line of measuring sensors 100 positioned along a line transverse to the direction of travel of the vehicle arriving at the housing 10. These measuring sensors can be either variable reluctance sensors or eddy current sensors. Alternatively, this line of electromagnetic sensors can also be replaced by an optical measuring system applying the principle of laser triangulation. • Processing electronics 140 to which the measurement sensors 100 and the tire presence detection sensors 110 are connected. In this example, the processing electronics 140 also contains an RFID reader allowing the reading of RFID chips integrated into the tires or stuck on the tires whose under-inflation is evaluated.

[0043] When a tire 20 passes over the housing 10 of the measuring system, the presence of the tire is first detected by a first line of tire presence detection sensors, then, when the tire leaves the housing 10 of the measuring system, its presence is detected by a second line of tire presence detection sensors. The distance between the two tire presence detection devices being known, it is then possible to calculate the tire speed by a very simple formula: Average speed = d / tO.

[0044] In this formula, the distance d is the distance separating the two transverse lines of tire presence detection sensors 110, and the time t0 is the time elapsed between the detection of the presence of the tire by any one of the sensors belonging to the first line of tire presence detection sensors 110 and the detection of the presence of the tire by any one of the sensors belonging to the second line of tire presence detection sensors 110. The speed of the tire being known, it is then very easy to estimate a local length of the contact area of ​​the tire, vertical to the sensor considered by taking into account the sampling frequency of the sensor as we will see in [Fig.3].

[0045] [Fig. 3] is the temporal representation of the output signal of a sensor of a measuring system over which a tire has passed as well as a first method of detecting the instants of change of state of the sensor.

[0046] Curve 1001 is the mathematical representation of the various measurement points recorded by the sensor. This curve 1001 appears as a trapezoid as it passes through the tire. From a low level, which reflects the absence of contact between the sensor and the tire, we observe a first rising edge up to a plateau which lasts a few moments before a falling edge which returns to the initial level.

[0047] Here, the upper plate is almost constant and tangent to a straight line 1002 whose ordinate is equal to the value A. This value A is constant since when the sensor is under the tire, the detected value is constant during the crossing of the tire on the sensor.

[0048] The rising and falling edges reflect on the one hand the progressive approach of the tire in rotary motion towards the sensor and on the other hand the progressive covering of the tire on the active part of the sensor. As these phenomena take place both during the attack movement on the sensor and during the escape movement carried out by the tire on the sensor, it is normal for the rising and falling edges to be symmetrical.

[0049] According to a first embodiment, the detection of the change of state of the sensor is carried out using a threshold value T which is proportional to the value A of the plateau. In this case, we are at mid-amplitude of A for the value of T. We can therefore define a straight line 1003 corresponding to this value which intercepts the curve 1001 at the rising edge by a point 1011 and at the falling edge by a second point 1012.

[0050] The interval of the sensor measurement points between these two reference points defines the contact zone of the tire with the sensor. The points outside this zone are referred to as out of contact. Thus, points 1011 and 1012 define the instants of change of state of the sensor. The duration of the zone between 1011 and 1012 represents the temporal length of the contact between the two structures, which will make it possible to determine the metric distance of this contact by multiplying this duration by the instantaneous speed of passage of the tire on the sensor.

[0051] Finally, the symmetry of this signal 1001 makes it possible to determine an axis of symmetry passing through the point 1015 which therefore corresponds to a given instant which is defined by the abscissa of this point 1015. By construction, this abscissa of the point 1015 corresponds to the median plane of the contact plane of the tire which is common to the entire transverse width of the tire, which makes it possible to phase the signals 1001 from the various sensors measuring the passage of the tire on the measuring system.

[0052] [Fig.4] is the same time representation of the output signal of a sensor of a measuring system over which a tire has passed as well as a second method of detecting the instants of change of state of the sensor.

[0053] Curve 1001 is the mathematical representation of the various measurement points recorded by the sensor. This curve 1001 appears as a trapezoid as it passes through the tire. From a low level, which reflects the absence of contact between the sensor and the tire, we observe a first rising edge up to a plateau which lasts a few moments before a falling edge which returns to the initial level.

[0054] Here, the upper plate is almost constant and tangent to a straight line 1002 whose ordinate is equal to the value A. This value A is constant since when the sensor is under the tire, the value detected is constant during the crossing of the tire on the sensor. Of course, depending on the technology of the sensor and the dimensions of the tire, the value can change.

[0055] The rising and falling edges reflect on the one hand the progressive approach of the tire in rotary motion towards the sensor and on the other hand the progressive covering of the tire on the active part of the sensor. As these phenomena take place both during the attack movement on the sensor and during the escape movement carried out by the tire on the sensor, it is normal for the rising and falling edges to be symmetrical.

[0056] According to a second embodiment, the detection of the change of state of the sensor is carried out using a property of the curve 1001. Generally at mid-amplitude A / 2 of the plateau, the rising and falling edges have inflection points. It is then possible to pass a tangent 1005 on the rising edge of the signal 1001 passing through the inflection point of the rising edge. Similarly, the tangent 1006 can be constructed on the falling edge. Here, the inflection points have been imposed by the intersection of the line 1003 with the signal 1001. The equation of the line 1003 is that the abscissa is equal to half the amplitude A of the plateau of the signal 1001.

[0057] The intersection of the tangent 1005, respectively 1006, with the curve 1002 re presenting the signal plateau, defines a single point which is called the sensor state change point whose time abscissa is referenced 1011, respectively 1012.

[0058] The interval of the sensor measurement points between these two reference points defines the contact zone of the tire with the sensor. The points outside this zone are referred to as out of contact. Thus, points 1011 and 1012 define the instants of change of state of the sensor. The duration of the zone between 1011 and 1012 represents the temporal length of the contact between the two structures, which will make it possible to determine the metric distance of this contact by multiplying this duration by the instantaneous speed of passage of the tire over the sensor.

[0059] It is noted here that the contact times are not identical depending on the method used. However, this does not affect the metric parabola which will be based on the time of change of state 1011 of the sensor, respectively the time 1012.

[0060] Finally, the symmetry of this signal 1001 makes it possible to determine an axis of symmetry passing through the point 1015 which therefore corresponds to a given instant which is defined by the abscissa of this point 1015. By construction, this abscissa of the point 1015 corresponds to the median plane of the contact plane of the tire which is common to the entire transverse width of the tire, which makes it possible to phase the signals 1001 from the various sensors measuring the passage of the tire on the measuring system.

[0061] [Fig.5] is a representation of the contact areas of the tires of a vehicle across their entire width for various axles of a vehicle.

[0062] Here, at the top of [Fig.5] is the right side of the vehicle when you are the driver of the vehicle. Knowing that the convoy moves in a straight line according to the solid arrow shown in the middle of [Fig.5].

[0063] The prints located on the left of the figure represent the front of the convoy. The prints located on the right of the figure correspond to the rear of the convoy. The first axle of the vehicle consists of a single wheel for which the method does not apply. However, the implementation of the method on this steering axle shows a symmetry of behavior between the right and left sides of the vehicle, which is expected by the method due to the trajectory of the vehicle expressed by the solid arrow.

[0064] The second and third axles of the vehicle are made up of twin wheels of different dimensions. The second axle, which traditionally represents the drive axle, only carries as load the weight of the engine which rests vertically on this axle. The third axle, which corresponds to the loading area of ​​the vehicle, is equipped with twin wheels whose dimensions are larger to support a load much greater than the load carried by the first two axles. Here, deliberately, a tire of a twin wheel of this third axle has been deflated. This is the twin wheel on the right side of the vehicle and the tire located in outside the vehicle. Generally, inspection of this envelope is difficult due to its location on the vehicle.

[0065] The vehicle passed over the straight-line measuring system at a driving speed of 30 km / h. The measuring signals were recorded by each sensor. Here, it can be seen, for the third axle, that each tire passed over approximately 7 sensors during the passage over the system.

[0066] After processing the signals as illustrated in [Fig.3] or [Fig.4], the contact time between the tire and the sensor was transformed by measuring the vehicle's passing speed into a metric length. In addition, since the system, comprising the sensors, is placed on the ground, the transverse distances Di between two neighboring sensors of the system are known, whether the sensors are aligned transversely to the direction of passage of the vehicle or staggered. All of this information makes it possible to construct a metric contact length for each sensor and to phase the sensors with each other spatially and temporally using the imaginary time points representing the center of contact according to the direction of movement of the tire.

[0067] The set of contact lines from each sensor and the spacing of these lines using the distances Di make it possible to obtain a representation of the contact area of ​​each tire. Each contact area measurement line begins and ends using the previously identified instants of change of state of the sensor.

[0068] It is then possible to determine for the entry line, respectively the exit line, of the contact area of ​​each tire, a parabola which minimizes the deviation from the points of change of state at the entry of the contact area, respectively the points of change of state at the exit of the contact area.

[0069] Here, in the figure, we have identified a parabola for the input line and the output line. Then we have extracted from the equation of the parabola, the second order coefficient of the parabola which is similar to the curvature of the parabola. And we have taken the average "a" of these coefficients which are displayed for each tire in [Fig.5].

[0070] We note that the average “a” between the right and left tires of the vehicle are almost identical for the first two axles, which is expected.

[0071] On the other hand, for the third axle, on the right side, there is a significant disparity in curvature between the two casings of the twin wheel, unlike what happens on the left side. This is normal since the inner casing of the right twin wheel of the third axle is deliberately under-inflated. Here, there is a difference of 7 units on the right side for a difference of just one unit on the left side.

[0072] [Fig.6] is a representation in time of the second order coefficients of the parabolas of the tires of a twin wheel, one of the tires being in the process of to deflate from an instant.

[0073] [Fig.6] is a graph whose abscissa is time with periodic measurements of the second order coefficient of the parabola best describing the line of contact of the tire with the ground at the entrance to the contact area following the direction of passage of the vehicle on the measuring system.

[0074] The 2002 series corresponds to the measurements of the outer casing of the twin wheel on the vehicle and the 2003 series corresponds to the measurements of the inner casing of the same twin wheel on the vehicle. At the beginning, although a difference remains between the two values ​​of the second order coefficient of the parabola, it is stable over time and of low level. Thus, we can define a 2001 master curve of theoretical average evolution of the coefficient. From a moment, the inner tire began to gradually deflate. Looking at the 2003 series, we observe that the coefficient began to drift compared to the 2001 master curve. The same level of drift is observable on the 2002 series corresponding to the measurements on the outer casing of the twin wheel on the vehicle.Thus, the difference between the two Aa values, shown by the double vertical arrow, has increased until it exceeds a threshold S meaning that one of the two tires of the twin wheel is under-inflated. To identify the offending wheel, it is necessary to identify the one whose coefficient is decreasing instead of increasing.

[0075] The proposed method thus makes it possible to detect under-inflation of a tire of a twin wheel in an early and robust manner. In addition, the method makes it possible to identify which of the two tires is under-inflated relative to the other.

Claims

Claims

1. Method for detecting under-inflation of a tire forming part of a twin wheel of a vehicle comprising the following steps: - Moving at an identified speed V the two tires of the twin wheel of the vehicle on the same flat ground equipped with sensors, two neighboring sensors being distant from each other transversely to the direction of movement of the vehicle by a distance Di, so that each tire is situated vertically above at least three sensors during the movement, said sensors being capable of detecting the contact between the tire and the ground; - Determining for each sensor at least one instant when the contact between the tire and the sensor changes state.- Determine a discrete metric line of contact of the tire with the ground for each tire of the twin wheel by means of the identified speed V, the transverse distances Di and the instants of change of state of the at least three sensors located vertically of the tire corresponding to the entry or exit of the contact patch; - Determine an arc of parabola which minimizes the difference with the points of the discrete line for each tire; - Algebraically compare the second order coefficients of the arcs of parabolas, preferably carry out an algebraic difference; and - If the algebraic comparison exceeds a threshold S, alert that one of the tires of the twin wheel is under-inflated.

2. Method for detecting under-inflation of a tire forming part of a twin wheel of a vehicle according to claim 1 in which the method comprises the complementary steps: - Identifying the discrete line having the smallest coefficient of order two; - Identifying the tire having generated the discrete line of smallest coefficient of order two, the identified tire is in an under-inflation condition.

3. Method for detecting under-inflation of a tire forming part of a twin wheel of a vehicle according to one of claims 1 to 2 in which each tire has a trajectory bearing on at least five sensors simultaneously during its movement on the ground.

4. Method for detecting under-inflation of a tire forming part of a twin wheel of a vehicle according to one of claims 1 to 3 in which the comparison of the second order coefficients is carried out by an algebraic difference, the threshold S corresponds to a curvature of value 5 m1.

5. Method for detecting under-inflation of a tire forming part of a twin wheel of a vehicle according to one of claims 1 to 4 in which, the contact between the sensor and the ground being previously identified on the sensor signal by a constant amplitude A of the signal, the change in state of the sensor signal corresponds to the signal crossing a threshold T, the threshold T corresponding to a percentage of the amplitude A of the signal.

6. Method for detecting under-inflation of a tire forming part of a twin wheel of a vehicle according to one of claims 1 to 4 in which, the contact being previously identified on the sensor signal by a constant amplitude A of the signal, the instants of change of state of the sensor correspond to the intersection between the tangent of the sensor signal at the abscissa point corresponding to the ordinate at mid-amplitude A / 2 and a straight line whose ordinate is equal to the value A.

7. Method for detecting under-inflation of a tire forming part of a twin wheel of a vehicle according to one of claims 1 to 6 in which the identification of the speed of passage of the tire over the sensor is obtained by the estimated duration of the rising edge and / or falling edge of the signal from the sensor as the tire passes, knowing that the distance of these edges is constant depending on the geometric characteristics of the tire, preferably its diameter.

8. Method for detecting under-inflation of a tire forming part of a twin wheel of a vehicle according to one of claims 1 to 7 in which, having previously identified the two moments when the contact between the tire and the sensor changes state when crossing the pneumatic on the sensor, the signals from the sensors are time-recalibrated between them by making the instants corresponding to the middle of the interval defining the contact state of each sensor coincide.

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