Method for determining a wheel tire ground print

Abstract

Method of determining the footprint of a tire on the ground, using devices already installed in standard production tires. Since the frequency of the transmitter clock present in a wheel unit is perturbed by mechanical vibrations when the tire contacts the ground, the frequency variations of the reference clock are representative of the value of the footprint. The method therefore includes determining the footprint on the ground of a tire of a wheel by measuring continuously, as a function of time, a signal representative of the variations in frequency of the reference clock supplied at the output of the time control device; determining a time interval during which the frequency variations of the reference clock occur; and, on the basis of the rotation speed of the tire, deducing therefrom an angular range of contact () corresponding to the footprint between the ends representing the ground contact length of the tire.

Claims

1. A method for determining a footprint (L) of a tire (3), corresponding to a length of contact of said tire (3) with the ground, of a vehicle equipped with a monitoring system that includes one wheel unit (4) for each tire (3), each wheel unit (4) comprising a microcontroller (14), a speed sensor (12), a transmitter (10), a reference clock (16) of the transmitter (10) configured to operate at a given frequency (F.sub.0), and at least one time control device for the system, where an actual frequency (F) output by said reference clock (16) is subject to variations (F) with respect to the given frequency (F.sub.0) when the wheel unit is within the footprint of the tire, the method comprising: continuously measuring, by way of said time control device, for the variations (F) of the output frequency (F) of the reference clock (16) with respect to the given frequency (F.sub.0), as a function of time; determining a time interval (T.sub.L) during which said variations (F) of the frequency of the reference clock (16) occur; and on the basis of the rotation speed of the tire (3), deducing therefrom an angular contact range () corresponding to the footprint (L) of the tire (3), wherein the time control device is a clock (13) of the microcontroller (14), which generates a signal representative of the variations (F) of the output frequency (F) of the reference clock (16) as a relative variation (F/F.sub.0) of the frequency (F) timed by the frequency of the clock (13) of the microcontroller (14), and wherein the method further comprises using the reference clock (16) of the transmitter to calibrate the clock (13) of the microcontroller (14) when the microcontroller (14) is in an angular range outside the contact angle () corresponding to the footprint (L).

2. A method for determining a footprint (L) of a tire (3) of a wheel (1), corresponding to a length of contact of said tire (3) with the ground, of a vehicle equipped with a monitoring system that includes one wheel unit (4) for each tire (3), each wheel unit (4) comprising a microcontroller (14), a speed sensor (12), a transmitter (10), a reference clock (16) of the transmitter (10) configured to operate at a given frequency (F.sub.0), and at least one time control device for the system, where an actual frequency (F) output by said reference clock (16) is subject to variations (F) with respect to the given frequency (F.sub.0) when the wheel unit is within the footprint of the tire, the method comprising the following successive steps: measuring continuously, and recording (30, 60, 60), as a function of time, the frequency (F) at the output of the reference clock (16) and the variations (F) of the output frequency (F) with respect to the given frequency (F.sub.0); determining and recording the time interval (T.sub.L) during which said variations (F) of the frequency of the reference clock (16) occur; recording a rotation speed of the wheel (1) as a function of time based on an output of the speed sensor; using the recorded time interval (T.sub.L) of the variations of frequency (F) of the reference clock (16) and the recorded rotation speed of the wheel (1) to calculate and record an angular contact range (0) of the tire (3); defining a detection threshold (F.sub.Min) below which said frequency variations (F) are disregarded; if the frequency variations (F) are above the frequency threshold (F.sub.Min), calculating the footprint (L) on the basis of the contact angle (), the time interval (T.sub.L) and the speed of the wheel (1); defining boundaries (L.sub.min, L.sub.max) of the value of the footprint (L) beyond which the calculated value of the footprint (L) is not accepted; defining a frequency of calculation for determining the value of the footprint (L) of the tire (3); and restarting the calculation at the frequency thus defined, wherein the time control device is a clock (13) of the microcontroller (14), which generates a signal representative of the variations (F) of the output frequency (F) of the reference clock (16) as a relative variation (F/F.sub.0) of the frequency (F) timed by the frequency of the clock (13) of the microcontroller (14), and wherein the method further comprises using the reference clock (16) of the transmitter to calibrate the clock (13) of the microcontroller (14) when the microcontroller (14) is located in an angular range outside the contact angle () corresponding to the footprint (L).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other data, characteristics and advantages of the present invention will become apparent in the light of the following non-limiting description, referring to the attached drawings, which show, respectively:

(2) in FIG. 1, a schematic front view of a wheel fitted with a tire, showing the deformation of the tire on the ground and the resulting footprint of this tire;

(3) in FIG. 2, a block diagram of an exemplary wheel unit capable of implementing the method according to the invention when the frequency of the microcontroller of the wheel unit is used as the clock reference frequency;

(4) in FIG. 3, an example of a diagram of the measurement of the relative variation of the reference clock frequency of the wheel unit for determining the footprint of the tire on the ground, after calibration with the frequency of the internal clock of the microcontroller;

(5) in FIG. 4, a block diagram of the wheel unit capable of implementing the method according to the invention when a phase lock detection function is used for measuring the perturbation of the reference clock;

(6) in FIGS. 5a and 5b, examples of diagrams of the measurement of the phase lock detection function for determining the footprint of the tire, and

(7) in FIG. 6, a flow diagram of an example of the sequence of steps in accordance with the method according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) FIG. 1 shows schematically, in a front view, a wheel 1 of a motor vehicle (not shown), fitted with a tire 3 around a rim 7, and with a wheel unit 4 which, in this example, is placed on the tread, inside the tire 3. This tire 3 is partially deflated, and its flattening on the ground 5 is represented by an area of length L located between the two contact ends A and B, called the footprint L. This footprint L is also defined by the contact angle formed by the two points A and B and the vertex of the angle in the center C of the wheel 1. When the wheel revolves, the wheel unit 4 is located between the ends A and B of the footprint once in every wheel revolution. The wheel unit 4 comprises, notably (FIG. 2), a transmitter 10 having a clock 16 timed to a frequency F. The frequency of the clock 16 of the transmitter 10 of the wheel unit 4 is perturbed when this wheel unit 4 is located in the footprint L.

(9) The length of the footprint L is then determined on the basis of: the frequency variations of the clock 16 of the transmitter 10 of the wheel unit 4 which is periodically located in this footprint area L, the rotation speed of the wheel 1, and the radius of this wheel 1. Examples of wheel units implementing the measurement of the frequency variations of the clock 16 of the transmitter 10 are detailed below with reference to FIGS. 2 and 4.

(10) The view of FIG. 2 shows a block diagram of an example of a wheel unit 4 equipped with a time control device formed by an internal clock 13, having a frequency f, of a microcontroller 14. This wheel unit 4 comprises, in addition to the microcontroller 14 and its internal clock 13, a pressure sensor 12 and a speed sensor 12 connected to the transmitter 10 via a link 15 between the microcontroller 14 and the transmitter 10. This transmitter 10 transmits in the radio frequency domain by means of its antenna 24, and comprises a phase lock loop (abbreviated to PLL) 20, a frequency divider 19, a voltage-controlled oscillator 18, (abbreviated to VCO), a power amplifier 22 and a reference clock 16.

(11) Provided with these devices, the wheel unit 4 operates as follows: the microcontroller 14 receives and processes the measurements made by the sensors 12 and 12, then transmits these measurements in the form of digitized data to a central unit placed in the vehicle (not shown), via the link 15 and the antenna 24 of said transmitter 10.

(12) The data are initially digitized by the microcontroller 14 on the basis of the frequency f of its internal clock 13. The frequency of the data transmission is then timed by the frequency F of the reference clock 16, whose oscillations are locked to a quartz crystal 17.

(13) This reference clock 16 is therefore precise, but its frequency F is perturbed when the wheel unit 4 is located, once in every wheel revolution, between the ends A and B of the footprint L of the tire's ground contact (see FIG. 1). In the illustrated example, the frequency F of the reference clock 16 is about 20 MHz (more generally, between 10 and 30 MHz), and it undergoes variations that may reach 50 kHz when the wheel unit 4 is located within the footprint L.

(14) The internal clock 13 of the microprocessor 14 (having a frequency f of about 30 MHz) is relatively less precise than the reference clock 16, but has a frequency which is not perturbed when the wheel unit 4 is in the footprint L, unlike that of the reference clock 16. The internal clock 13 is then advantageously used as a benchmark for the measurement of the perturbations of the frequency F of the reference clock 16.

(15) The detection of these perturbations by the variations of the frequency F of the reference clock 16 therefore indicates that the wheel unit 4 is in the footprint L of the tire on the ground, and the measurement of the duration of this detection can then be used to calculate the length of the footprint L.

(16) FIG. 3 shows this detection of the perturbations of the frequency F of the reference clock 16, in a diagram of the measurement of the relative variations F/F.sub.0, as a function of time t, of the frequency F with respect to F.sub.0, F.sub.0 being the non-perturbed frequency of the reference clock 16.

(17) The measurement of the frequency F of the clock 16 of the transmitter 10 is timed by the frequency f of the internal clock 13 of the microprocessor 14 via the frequency divider 19. Advantageously, in order to increase the precision of the measurement, the internal clock 13 of the microcontroller 14 is calibrated in advance by the reference clock 16, which is more precise, during the phases in which the reference clock 16 is not perturbed by the contact of the tire with the ground (in other words, outside the footprint L in FIG. 1).

(18) The measurement of the relative variations F/F.sub.0 takes the form of a curve 30. In a time interval T.sub.L, the measurement F/F.sub.0 undergoes significant changes in valuedefined by F above a threshold value F.sub.Min (see FIG. 6)on either side of the zero value, between the two instants t.sub.A and t.sub.B. These instants t.sub.A and t.sub.B define the boundaries of the time interval T.sub.L which therefore correspond to the period during which the frequency F of the reference clock 16 is perturbed and during which the wheel unit 4 is located in the ground contact area. The length of the footprint L corresponding to this area is then deduced, given the contact angle calculated on the basis of the time interval T.sub.L and the wheel speedand the radius of the wheel. Outside the time interval T.sub.L, the relative variation F/F.sub.0 is zero, because the frequency F of the reference clock 16 is no longer perturbed when the wheel unit 4 is located outside the footprint.

(19) FIG. 4 shows a block diagram using another time control device, in this case a phase lock detector 32, for measuring the perturbation of the reference clock 16 of the transmitter 10. This detector 32 belongs to a phase lock loop 20 which comprises the following components linked in sequence, in addition to this detector 32: a phase comparator 29, a feed pump 28 and a low-pass filter 26. The other referenced elements are described in the passage of the present description relating to the description of FIG. 2.

(20) The phase lock detector 32 communicates synchronization measurements between the input and output voltages of said loop 20 to the microcontroller 14 via a wire link 34. These voltage measurements are communicated in the form of logical values, a 0 indicating phase synchronization and a 1 indicating phase opposition.

(21) When the wheel unit 4 is in contact with the ground 5 (see FIG. 1), the operation of the clock 16 of the transmitter 10 is perturbed, and its frequency F varies, thereby unlocking the phase of the phase lock loop 20 (in other words, the input and output voltages of the loop are in phase opposition). The phase lock detector 32 then switches its signal from the 0 value to the 1 value. The period of time during which the signal supplied by this phase lock detector 32 remains equal to 1 corresponds to the time interval during which the wheel unit 4 is in the footprint L. The value of this footprint L can then be accessed by a simple calculation, as explained below with reference to FIGS. 5a and 5b.

(22) FIGS. 5a and 5b show examples of measurement diagrams, as a function of time t, of the signals S and S respectively, sent by the phase lock detector 32 and used to calculate the footprint L. In FIG. 5a, the signal S of said detector 32 is shown as a function of time t by the curve 60 as a direct output from this detector. The logical values output from the detector 32 are of the pseudo-digital type.

(23) A critical threshold S is then defined by a horizontal line of constant value, above which the values of the signal S are given the value 1 and below which the values of the signal S are given the value 0. The two instants t.sub.A and t.sub.B at which the curve 60 cuts the straight line S correspond, respectively, to the start and end of the time interval T.sub.L for which the wheel unit 4 is in the footprint area L. Given the value of the time interval T.sub.L, the footprint L can then be calculated as explained above with reference to FIG. 3.

(24) If the variations of the signal S do not allow a critical threshold S to be defined in a sufficiently usable way, an alternative solution shown in FIG. 5b uses a signal S supplied directly at the output of the phase lock loop 20. For this purpose, a phase detection function defines two phase states 0 and 1, which are, respectively, in phase and in phase opposition.

(25) These phase states are reproduced on the curve 60 of FIG. 5b where they indicate the measurement of the time interval T.sub.L for which the phase state is 1. This time interval T.sub.L corresponds, as described above, to the period for which the wheel unit 4 is in the footprint area L.

(26) To provide a more precise illustration of the successive steps of the method for determining the footprint L, FIG. 6 shows a flow diagram of the progress of these steps 100 to 135. A first step 100, called Start, serves to start the method and reinitialize it at a predetermined rate.

(27) The recordings made in step 110 are thoseas a function of timeof the rotation speed V of the wheel and the frequency F of the reference clock 16 (see FIGS. 2 and 4). The variations F of the frequency F of the reference clock as a function of time t are determined on the basis of the variations of the signal transmitted by a time control device of the wheel unit, for example the clock of the microcontroller or the phase lock detector of the transmitter as described above.

(28) In the test of step 120, the variations of frequency F are compared with a minimum threshold F.sub.Min below which the variations F are considered to be noise and are therefore disregarded. This threshold F.sub.Min is advantageously defined in the range from 1 to 10 kHz. If the variations F are below the threshold F.sub.Min, the process returns to the Start point of step 100. Otherwise, the process continues by calculating (step 122) the footprint L on the basis of the time interval T.sub.L as defined above.

(29) The test of step 130 then checks whether the value of the footprint L is within a range of values defined between the boundaries L.sub.min and L.sub.max, corresponding for example to contact angles between 10 and 30. If this condition is not met, the method returns to the start point of step 100. If the value of the footprint L is within the range of values L.sub.min-L.sub.max, step 135 validates the calculated value of the footprint L. After validation, the method is reinitialized.

(30) The invention is not limited to the exemplary embodiments described and represented.

(31) Thus, if the microcontroller has a frequency divider, if the transmitter/microcontroller has a frequency demodulator coupled to a frequency mixer/reducer, or alternatively if the microcontroller has a digital signal processor (DSP) linked to a CAN (Control Area Network) network bus, then the frequency divider, the frequency mixer/reducer or the CAN may be used as control devices according to the invention.

(32) Additionally, the signal of the reference clock of the transmitter, which is usually a quartz oscillator, may be frequency modulated to improve the precision of the measurements, and, in particular, the variations of frequency and the value of the footprint according to the method of the invention.

(33) Furthermore, the detection of the phase of the phase lock loop may be implemented by any known means, for example by using the application known as Lock Detector in English.

(34) The invention is also applicable to any method based on elements including a phase lock loop whose temporary perturbations are to be determined.