FR2856147A1 - Steering wheel absolute angular position determining process, involves discriminating two respective absolute angular positions of wheel corresponding to binary sequence by estimation if sequence is unique and not unique - Google Patents

Abstract

The process involves determining if binary sequence is unique. An absolute angular position of a coder where the sequence is unique on a sector is discriminated. A position theta 2 of a steering wheel corresponding to the unique sequence is discriminated by an estimation if the sequence is unique. A position theta 3 corresponding to the sequence is discriminated by the estimation if the sequence is not unique. The estimation is used as absolute angular position theta before the determination of the angular positions theta 2 and theta 3. The angular position is used as initial angular position theta 0 for determining variations of the angular position theta from the initial position by signals (A, B), when one of the angular positions theta 2 and theta 3 is available.

Description

The invention relates to a method for determining the angular position.

  absolute of the steering wheel of a motor vehicle compared to the chassis

of said vehicle.

  In many applications, in particular such as trajectory control systems or electric power steering systems, it is necessary to know the absolute angular position of the steering wheel relative to the chassis.

  The term absolute angular position is understood to mean the angle separating the position of the steering wheel at a given instant from a reference position, this reference position being fixed and given relative to the chassis.

  In contrast, the relative angular position is the angle separating the position of the steering wheel from any arbitrary and variable initial position relative to the chassis.

  To determine the absolute angular position of the steering wheel, it is known to use the measurement of the differential speed of the wheels of the same axle. Indeed, it is possible to establish a bijective relationship between this differential speed and the angular position since, when the vehicle is registered in a trajectory, straight or curvilinear, each of the wheels has a trajectory whose center of curvature is identical. One of the problems which arises is that the absolute angular position is then obtained with mediocre precision which depends on the driving conditions of the vehicle.

  Furthermore, devices are known for incremental measurement of the angular position of the steering wheel which make it possible to obtain the relative angular position of the steering wheel with great precision. However, to obtain the absolute angular position, it is then necessary to provide for the determination of at least one reference angular position. Such a strategy is for example described in document EP-1,167,927. One limitation of such devices is that the detection of the reference angular position is only possible once per revolution, which, under certain conditions of driving, can lead to a determination of the absolute angular position only after a time, and therefore a distance traveled by the vehicle, which is not negligible.

  Finally, document FR-0212013, which was not published on the date of filing of this application, is known from a system for determining the absolute angular position of the steering wheel which comprises: an encoder intended to be rotated jointly with the steering wheel, said encoder comprising a main multipole track and a so-called "top tour" multipolar track which are concentric, said top tour track comprising M singularities distributed angularly; - a fixed sensor placed opposite and at an air gap distance from the encoder, comprising an electronic circuit capable of delivering two digital signals of position A, B square in quadrature which are representative of the angular position of the encoder and a top-turn signal C under form of M pulses per turn of the encoder, in which the M singularities are distributed angularly so that the top turn signal C is arranged to, in combination with the signals A and B, define a binary pattern comprising unique binary sequences each representative 'at least one absolute angular position of the encoder; a device for processing signals A, B, C which comprises counting means capable of determining, from an initial position, the variations in the angular position of the encoder; and a device for analyzing the differential speed of the wheels of the same axle of the vehicle which is capable of determining an estimate of the absolute angular position of the steering wheel as a function of said differential speed.

  This document provides, upon detection of a single binary sequence, to readjust the relative angular position from signals A and B so as to obtain the absolute angular position.

  A limitation of this use of the system is that the registration is carried out after the detection of a complete single binary sequence, which requires a sufficiently large rotation of the flywheel, typically between 30 and 75 depending on the initial position. Consequently, there remain driving situations in which the registration is not carried out sufficiently quickly. In particular this is the case, for starting the vehicle in a straight line, during a micro-cut of the computer at high speed (100 - 130 km / h on the highway for example), or for starting the vehicle in a curve with very strong radius of curvature not requiring rotation of the steering wheel greater than +/- 20 for example.

  The invention aims to overcome the limitations mentioned above by proposing in particular a method for determining the absolute angular position of the steering wheel which allows, under all driving conditions, to determine said position more quickly and with a optimal precision.

  To this end, the invention proposes a method for determining the absolute angular position 0 of the steering wheel of a motor vehicle relative to the chassis of said vehicle, by means of a system comprising: - an encoder set in rotation jointly at the wheel, said encoder comprising a main multipolar track and a so-called "top tour" (lb) multipolar track which are concentric, said top tour track comprising M singularities distributed angularly; - a fixed sensor placed opposite and at an air gap distance from the encoder, comprising an electronic circuit capable of delivering two digital signals of position A, B square in quadrature which are representative of the angular position of the encoder and a top-turn signal C under form of M pulses per turn of the encoder, in which the corresponding M singularities are distributed angularly so that the top turn signal C is arranged to, in combination with the signals A and B, define a binary pattern comprising unique binary sequences on a lathe or on a lathe sector which are each representative of an absolute angular position of the encoder on the lathe or on the sector; a device for processing signals A, B, C which comprises counting means capable of determining, from an initial position, the variations in the angular position of the encoder; a device for analyzing the differential speed of the wheels of the same axle of the vehicle which is capable of determining an estimate of the absolute angular position of the steering wheel as a function of said differential speed; Said method comprising the initial procedure providing for: - determining at least one estimate 0 'of the absolute angular position of the steering wheel by means of the analysis device; - construct the binary sequence corresponding to the signals A, B, C delivered; - determine if the binary sequence is unique; - if so, test whether an estimate 0 'makes it possible to discriminate the absolute angular position 10 of the encoder in the case where the binary sequence is unique on a sector and to discriminate the absolute angular position of the handwheel 02 which corresponds to the unique binary sequence; - if not, test whether an estimate 0 'makes it possible to discriminate the absolute angular position 03 of the steering wheel which corresponds to the binary sequence; In which an estimate O 'is used as the absolute angular position 0 before the determination of the angular positions 02 and 03, then, when one of the angular positions 02 or 03 is available, using said angular position as the initial angular position 0 to determine, from this initial position, the variations of the absolute angular position 0 by means of the signals 20 A, B. Other objects and advantages of the invention will become apparent during the description which follows, given with reference to attached drawings, in which: - Figure 1 is a front view of the encoder of a determination system usable according to the invention, said encoder comprising a main multipole track and a top lap multipole track; - Figure 2 is a schematic and partial view of a steering assembly 30 for a motor vehicle, which is equipped with a system for determining the absolute angular position of the steering wheel; - Figure 3 shows the algorithm of an embodiment of the initial procedure of a determination method according to the invention; - Figure 4 shows the algorithm of an embodiment of the calibration procedure usable in a method according to the invention.

  The invention relates to a method for determining the absolute angular position 0 of the steering wheel 2 of a motor vehicle relative to the chassis of said vehicle by means of a system comprising an encoder 1 rotated in conjunction with the steering wheel 2 and a fixed sensor 5 able to detect the pulses from the encoder 1. The method can be implemented in a host computer provided for this purpose, located in a dedicated computer of the vehicle or integrated into the sensor.

  In connection with FIG. 2, a steering assembly is described comprising a steering column 3 on which is mounted an encoder 1 such as that shown in FIG. 1, so as to secure the rotation in rotation of the column 3 and the encoder 1. In known manner, the column 3 is associated with the steering wheel 2 by means of which the driver applies a torque and therefore a steering angle 20. Furthermore, column 3 is arranged to transmit this steering angle to the vehicle's steering wheels. To this end, the wheels can be mechanically associated with column 3 by means of a rack pinion and a rack in order to transform the rotational movement of column 3 into angular displacement of the wheels, or be decoupled from column 25 3. In the latter case, the encoder 1 can be associated directly with part of the handwheel 2.

  The flywheel 2 is arranged to be able to perform a plurality of turns, typically two, on each side of the "straight line" position in which the wheels are straight.

  The steering assembly further comprises a fixed element 4 which is integral with the chassis of the motor vehicle, the sensor 5 being associated on said element so that the sensitive elements of said sensor are arranged opposite and at a gap distance from the encoder 1 .

  In order to be able to determine the absolute angular position of the encoder 1, and therefore of the steering wheel 2, with respect to the fixed element 4, and therefore to the chassis, the encoder 1 comprises a main multipolar track 1a and a so-called "top tour" multipolar track. lb which are concentric. The top lap track lb comprises M (with M> 1) singularities 1 bl distributed angularly.

  In a particular example, the encoder 1 is formed by a multipolar magnetic ring on which is magnetized a plurality of pairs of poles lc North and South equally distributed with a constant angular width so as to form the main tracks la and top tour lb, a magnetic singularity lbl of the top tour track lb being formed of two adjacent poles whose magnetic transition 15 is different from the others.

  According to the embodiment shown in Figure 1, the main tracks la, arranged towards the inside of the ring, and top tour lb, arranged towards the outside of the ring, comprise 24 pairs of poles lc, the pairs of poles lc of track 20 top tour lb being phase-lagged by a value ó compared to those of main track la.

  Each singularity lbl is formed of a pair of poles lc, the width of the poles being arranged so that a pole is out of phase with - relative to the corresponding pole 25 of the main track 1a. Thus, each pulse of signal C corresponds to the detection of a phase shift inversion between the main track 1a and the top tour track 1b. Furthermore, the sensor 5 comprises an electronic circuit provided with at least three sensitive elements, at least two of which are positioned facing the main track 1a and at least one is positioned facing the top tour track 1b. In a particular example, the sensitive elements are chosen from the group comprising Hall effect probes, magnetoresistors, giant magnetoresistors.

  The sensor 5 used is capable of delivering two periodic electrical signals S1, S2 in quadrature via the sensitive elements arranged opposite the main track 1a and an electrical signal S3 via the sensitive elements arranged opposite the track top tour 1 b.

  1o The principle of obtaining signals S1 and S2 from a plurality of aligned sensitive elements is for example described in the document FR-2 792 403 issued by the applicant.

  However, sensors 5 comprising two sensitive elements which are capable of delivering the signals S1 and S2 are also known.

  From the signals Sli, S2 and S3, the electronic circuit is capable of delivering digital signals of position A, B square in quadrature and a top-turn signal C in the form of M electrical pulses per turn of the encoder 1.

  A principle for obtaining digital signals A, B and C, as well as different embodiments of the magnetic singularities lbl, are described in documents FR-2 769 088 and EP-0 871 014.

  According to one embodiment, the electronic circuit further comprises an interpolator, for example of the type described in document FR-2 754 063 from the applicant, making it possible to increase the resolution of the output signals. In particular, a resolution of the angular position of the encoder 1 less than 1 can be obtained.

  The sensor 5 can be integrated on a silicon or equivalent substrate, for example AsGa, so as to form an integrated and personalized circuit for a specific application, a circuit sometimes designated by the term ASIC to refer to the integrated circuit designed partially or completely according to needs.

  Although the description is made in relation to an encoder / magnetic sensor assembly, it is also possible to implement the invention in a similar manner using an optical type technology. For example, the encoder 1 may be formed of a metal or glass target on which the main tracks la and top tour lb have been engraved so as to form an optical pattern similar to the multipolar magnetic pattern exposed above, the elements io sensitive then being formed of optical detectors.

  The determination system further comprises a device 6 for processing signals A, B, C which comprises counting means capable of determining, from an initial position, the variations in the angular position 15 of the encoder 1. In a exemplary embodiment, the counting means comprise a register in which the value of the angular position is incremented or decremented by an angular value corresponding to the number of edges of the signals A and B which are detected, the initial value being for example fixed at zero during system commissioning. Thus, the processing device 20 makes it possible to know the relative position of the encoder 1 with respect to the initial position.

  The determination system also comprises a device for analyzing the differential speed of the wheels of the same axle of the vehicle which is capable of determining an estimate of the absolute angular position of the steering wheel 2 as a function of said differential speed.

  To obtain the absolute angular position of the steering wheel 2, provision is made to use an encoder 1 whose distribution of the singularities lbl of the top lap track lb is specific.

  In the embodiment shown in FIG. 1, the angular distribution of the ten singularities 1 bl of the top lap track lb can be represented by the binary pattern 000001000110100111001011 obtained by processing the signal C and the signals A and B during a rotation a turn, where the number 1 corresponds to the detection of a top turn pulse on the pair of poles corresponding to the singularity 1bl and the number O to the absence of detection of such a pulse.

  With this binary pattern, it is possible to establish, as a function of the initial position of the encoder 1 and of the direction of rotation, the number of states O or 1 to be read in order to unequivocally determine the absolute position of the encoder I on a tower. In the following description, this succession of O or 1 which makes it possible to determine an absolute position of the encoder 1 on a revolution, is called binary sequence 10.

  Consequently, the M singularities lbl are distributed angularly on the encoder 1 so that the signal C is arranged to, in combination with the signals A and B, define unique binary sequences which are each representative of an absolute angular position of the encoder 1 on one revolution. In particular, this absolute angular position can be defined relative to the "straight line" position of the encoder (arrow 8) which corresponds to an angular position equal to 0.

  In a variant not shown, provision can be made for the binary pattern to include tower sectors each provided with unique binary sequences as defined above. Consequently, these unique binary sequences are each representative of an absolute angular position of the coder I on the sector considered.

  The determination method according to the invention provides an initial procedure in which at least one estimate 0 ′ of the absolute angular position of the steering wheel 2 is determined by means of the analysis device.

  To do this, assuming that the slip between the ground and the wheels is negligible, there is a bijective relationship between the angular position 0 'and the differential speed of the wheels. This slip is particularly negligible when the measurement of the differential speed is carried out on the non-driving wheels, but also on the driving wheels when the grip is normal. According to one embodiment, the relationship is identified using measurements carried out on the vehicle under optimal conditions which may include: evolution of the vehicle on a flat surface; - vehicle speed stabilized; - slow rotation of the steering wheel; - nominal tire pressure; dry soil.

  Under these conditions, it is possible to establish the polynomial relation, for example of order three, which makes it possible to estimate the angular position O ′ as a function of the differential speed. By using this relationship in the analysis device, it is therefore possible to obtain at each instant the estimate O ′ of the angular position 0 as a function of the measured differential speed. To this end, the analysis device 15 is supplied with the speeds of the left wheels Vg and right wheels Vd of the same axle respectively and comprises calculation means arranged to supply the differential speed.

  In the algorithm represented in FIG. 3, it is planned to determine two estimates: a rough estimate 0 * 2 and a fine estimate 0 * 3 which are obtained respectively when certain driving conditions R2, R3 are met. The rough estimate 0 2 is typically used to determine the lap or tour sector in which the steering wheel is located and the fine estimate 0 * 3 to determine the absolute angular position of the steering wheel 25 before a single binary sequence is completely built.

  However, the method can be implemented using a single estimate 0 ′ whose precision is sufficient to determine the angular positions 02 and 03 as described below.

  The initial procedure also provides, by counting variations in the angular position of the encoder 1 (step E) and detection of the top turns (step F), to construct the binary sequence corresponding to the signals A, B, C delivered (step G). For example, starting from the position indicated by the arrow 7 in FIG. 1, the constructed sequence is 1 then 10 then 100 then 1001 then 10011, the latter being unique in the binary pattern. The angular position represented by the arrow 8 in FIG. 1 is the absolute angular position of the coder corresponding to this single binary sequence.

  The method provides for determining whether the binary sequence constructed is unique (H test).

  When the constructed sequence is unique, the absolute angular position of the 0o coder is known (step 1) and the angular position of the handwheel 02 can be known (step K2) thanks to the estimation 0 * 2 (step M2) as soon as it Ci to a sufficient precision to allow to discriminate the lap or possibly the tower sector on which the sequence is unique. In the example considered above, the binary sequence 10011 makes it possible to determine the position "straight line" as an absolute position on the lap where the measurement is made, and as soon as the precision of the estimate 0 * 2 is less at +/- 180 you can discriminate the position between -720, -360, 0, 360 or 720 (in the case where the handwheel 2 is arranged to turn +/- 2 full turns). The driving conditions R2 for the determination of 0 2 are therefore provided to achieve this precision, for example a vehicle speed greater than 2 km / h and a travel time greater than 400 ms make it possible to obtain a typical precision of l '' +/- 50 order.

  In the case where the sequence constructed is not unique, the initial procedure 25 provides for testing (test J) if the estimate 0E3 makes it possible to discriminate the absolute angular position 03 of the steering wheel which corresponds to the binary sequence. In the case where the binary sequence constructed is 001 which has four occurrences in the pattern (-105, -15, 60, 165), one of the occurrences is validated (step K1) as soon as the precision of the estimate 0 * 3 allows, for example when 0 * 3 = 520 +/- 15, 30 the occurrence 165 is validated and 03 = 515.

  In one embodiment, the fine estimate 0 * 3 is obtained by iterative determination of the average difference between the angular positions measured from signals A, B (step E) and the angular positions calculated from the differential speed of the wheels, and adding said difference to the angular position measured from the signals (A, B) (step M3). Indeed, this point-to-point moving average allows in driving conditions R3 such as 5 vehicle speed greater than 5 km / h and steering wheel speed less than 20 / s, to obtain 03 with an accuracy less than +/- 15 after 2 seconds. This method for determining 0 * 3 is described in a French patent application filed by the applicant on the same day as the present patent application.

  On the basis of this initial procedure, the determination method provides for using an estimate 0, in particular 0'3, as the absolute angular position O before the determination of the angular positions 02 and 03. This information, although less precise, with the advantage of being available very quickly. In addition, the driving conditions R2 being less severe than those provided in R3, the estimate 0 2 will be available before the estimate 0 3. Then, when one of the angular positions 02 OR 03 is available, said position is used angular as initial angular position 0o. It is thus determined, from this initial position, the variations of the absolute angular position 0 20 by means of the signals A, B so as to know, continuously, said position thanks to the counting means.

  The method therefore provides for using the first information available from 02 or 03, which makes it possible, under all driving conditions, to quickly obtain an absolute angular position 0 which is precise. In particular, the absolute angular position of the steering wheel is available before the 15 km / h threshold beyond which it is necessary for the trajectory control system. In addition, it should be noted that the precision of the estimates 0 * 2 and 0 3 improves with driving time and that they make it possible to overcome a large part of the influence 30 of the road profile ( pothole, hump) on the speed of the wheels.

  Schematically, we can highlight two classic scenarios: the vehicle starts, drives and the driver turns the steering wheel enough: 02 is then available before 03; - the vehicle starts, drives and the driver turns the steering wheel a little (around +/- 7.5 for example): 93 is then available before 02.

  As a variant, the method also provides, when the angular position 00 is based on the angular position 03, to readjust the angular positions 0 determined subsequently as a function of the angular position 02 when this is available so as to make the angular positions obtained reliable.

  The initial procedure described above is in particular intended to be used during the initialization or the reinitialization of the determination system so as to readjust the relative angular position resulting from the signals A, B. In addition, this procedure can be used iteratively after readjustment to make the determination process more reliable. Furthermore, provision can also be made for the method to use other dynamic estimators of the angular position of the steering wheel, such as a yaw sensor, an accelerometer or a gyroscope, to accelerate, control and / or make the calculations performed more reliable.

  According to one embodiment, the method also comprises a calibration procedure (see FIG. 4) in which, before using the determination system, the angular position of the encoder is indexed electronically with respect to the angular position of the steering wheel. In particular, the offset of the "straight line" position 8 of the encoder relative to the "straight line" position of the wheels of the vehicle can be determined. This procedure makes it possible to cancel the angular positioning errors of the encoder when it is mounted on the vehicle, and therefore to dispense with precise mechanical indexing of the encoder relative to the angular position of the wheels.

  The calibration procedure provides for determining a 0 * 4 estimate of the absolute angular position of the steering wheel by means of the analysis device under particular driving conditions R4 which are more severe in terms of time and speeds than those used to determine 0 * 2 and 0 * 3. For example, driving conditions may impose a range of angular position of the steering wheel around the "straight line" position (for example +/- 45 around the straight line). Under these conditions, it is possible to obtain, for example by using the same calculation method (step M4) as that described above to determine 0 3, the angular position 04 with an accuracy of +/- 2. Consequently, the estimate 0 * 4 is available less quickly than the other estimates. As a variant, depending on the calibration precision that one wishes to obtain, the estimate 0 ′ 3 can be used in place of the estimate 0 * 4.

  0o Next, the estimate 0 * 4 is compared (step L) with the absolute angular position 0 determined by the method described above, so as to deduce therefrom the angular offset M0 between the encoder and the handwheel. Indeed, the estimate 0 4 is independent of the mounting of the encoder and is a function of the heading of the vehicle whereas the absolute angle 0 determined as a function of 02 OR 03 depends on the mounting of the encoder. Consequently, imprecise mounting of the encoder which induces a shift between the straight line position of the encoder and the heading of the corresponding vehicle, typically but not restrictively between +/- 15, can be corrected so as to cancel this shift. This indexing can be carried out at the end of the chain or during a

  maintenance operation, the value Mo being able to be memorized so as to be used during the determination of the initial angular position Oo to correct the estimates 0 * 2 and 0'3 obtained. As a variant, the calibration procedure can be carried out several times so that, by averaging the obtained Mo values, the reliability of the indexing carried out is reliable.

  According to one embodiment, the calibration procedure can be carried out iteratively so as to obtain angular offsets Mi which are used as they are obtained for the determination of the initial angular position 30 0o in an updated manner with respect to for the driving conditions and the characteristics of the vehicle. Thus, even in the event of a fault linked to a wheel or a wheel set (such as variation in tire pressure, train adjustment), it is possible to determine angles 02 and 03 reliably.

  According to one embodiment, the method according to the invention also provides for determining the difference between M0 and Mi, and, if the difference is greater than a threshold, for deducing therefrom a fault linked to a wheel. Indeed, if one of the tires is punctured, deflated or if a wheel of different diameter has been fitted, this results in a drift of the Mi values and the passage of the difference IMo - Mil above a threshold which allows the detection of these events. This determination of a fault related to a wheel can be refined if necessary by filtering the Mi values, detection of slow or fast drift, calculation at the start of the vehicle or in a stationary driving phase.

  As a variant, the method provides for determining the sign of the difference between M0 and Mi so as to deduce therefrom the wheel concerned by the fault. In particular, in the event of a puncture, if M0 - Mi> O the right wheel is affected. The left wheel is concerned in the opposite case.

Claims (11)

  1. Method for determining the absolute angular position 0 of the steering wheel (2) of a motor vehicle relative to the chassis of said vehicle, by means of a system comprising: - an encoder (1) set in rotation jointly with the steering wheel (2), said encoder comprising a main multipolar track (la) and a so-called "top tour" multipolar track (lb) which are concentric, said top tour track comprising 10 M singularities (lbl) distributed angularly; - a fixed sensor (5) disposed opposite and at an air gap distance from the encoder (1), comprising an electronic circuit capable of delivering two square digital quadrature position signals (A, B) which are representative of the angular position of the encoder (1) and a top turn signal (C) in the form of M pulses 15 per turn of the encoder (1), in which the M corresponding singularities (1 bl) are angularly distributed so that the top turn signal (C) is arranged to, in combination with signals A and B, define a binary pattern comprising unique binary sequences on a lathe or on a lathe sector which are each representative of an absolute angular position of the encoder (1) on the lathe or on the sector; - a signal processing device (6) (A, B, C) which comprises counting means capable of determining, from an initial position, the variations in the angular position of the encoder (1); a device for analyzing the differential speed of the wheels of the same axle of the vehicle which is capable of determining an estimate of the absolute angular position of the steering wheel (2) as a function of said differential speed; Said method comprising the initial procedure providing for: - determining at least one estimate 0 * of the absolute angular position of the steering wheel (2) by means of the analysis device; - construct the binary sequence corresponding to the signals (A, B, C) delivered; - determine if the binary sequence is unique; - if yes, test whether an estimate 0 * makes it possible to discriminate the absolute angular position of the encoder (1) in the case where the binary sequence is unique on a sector and to discriminate the absolute angular position of the handwheel (2) 02 which corresponds to the unique binary sequence - if not, test whether a 0 'estimate makes it possible to discriminate the absolute angular position 03 of the handwheel (2) which corresponds to the binary sequence; In which an estimate O is used as the absolute angular position 0 before the determination of the angular positions 02 and 03, then, when one of the angular positions 02 OR 03 is available, use said angular position as the initial angular position o00 for determine, from this initial position, the variations of the absolute angular position O by means of the signals io (A, B).   2. Method according to claim 1, characterized in that it further provides, when the angular position 0o is based on the angular position 03, to readjust the angular positions 0 determined subsequently as a function of the angular position 02 when this is available.   3. Method according to claim 1 or 2, characterized in that the determination of the estimate 0 ′ is carried out under determined driving conditions.   4. Method according to claim 3, characterized in that it provides for determining two estimates 0 2 and 083 as a function respectively of determined driving conditions, the rough estimate 0'2 being used to discriminate the angular position 02 and the fine estimate 0 * 3 being used to discriminate the angular position 03.   5. Method according to claim 4, characterized in that the fine estimate 0 * 3 is obtained by iterative determination of the average difference between the angular positions measured from the signals (A, B) and the angular positions calculated from of the differential speed of the wheels, and addition of said difference to the angular position measured from the signals (A, B).   6. Method according to any one of claims 1 to 5, characterized in that the measurement of the differential speed is carried out on the non-driving wheels.   s 7. Method according to any one of claims 1 to 6, characterized in that it comprises a calibration procedure in which, under particular running conditions: - an estimate 8 4 of the absolute angular position of the steering wheel is determined ( 2) by means of the analysis device; o0 - the estimate 0 4 is compared with the absolute angular position 0 determined so as to deduce therefrom the angular offset (Mo) between the encoder (1) and the steering wheel (2).   8. Method according to claim 7, characterized in that the calibration procedure is carried out iteratively so as to obtain angular offsets (Mi).   9. Method according to claim 7 or 8, characterized in that the offset (Mo) or the offsets (Mi) are used when determining the initial angular position o0 o0.   10. Method according to claim 7 or 8, characterized in that it provides for determining the difference between M0 and Mi, and, if the difference is greater than a threshold, to deduce therefrom a defect related to a wheel.   11. Method according to claim 10, characterized in that it provides for determining the sign of the difference between M0 and Mi so as to deduce therefrom the wheel concerned by the fault.