WO2002052298A1

WO2002052298A1 - System and method for measuring kinematic parameters of a tyre during the running of a vehicle

Info

Publication number: WO2002052298A1
Application number: PCT/EP2001/014764
Authority: WO
Inventors: Alessandro Volpi; Gianmaurizio Falciola
Original assignee: Pirelli Pneumatici S.P.A.
Priority date: 2000-12-22
Filing date: 2001-12-14
Publication date: 2002-07-04

Abstract

System and method for measuring the velocity of a point on the surface of the wheel (1) of a vehicle, comprising a source (10; 10A) mounted on the vehicle and capable of sending a signal at a first frequency (f1) to a portion of the moving surface of the wheel (1), a receiver (10; 10B) mounted on the said vehicle in a position such that it can receive at least a fraction of the signal reflected from the said surface portion of the wheel, and at least one processing unit (15), for calculating, from the frequency difference (f2-f1) between the transmitted and the received signals, a quantity representative of the velocity of the portion of wheel with respect to the velocity of the vehicle. The source (10; 10A) and the receiver (10; 10B) are made in the form of ultrasonic transducers, which send the signal to a transverse groove portion of the tread pattern (4) which extends radially, and receive the reflected signal from this portion.

Description

SYSTEM AND METHOD FOR MEASURING KINEMATIC PARAMETERS OF A TYRE DURING THE RUNNING OF A VEHICLE

DESCRIPTION

The present invention relates to a system and a method for measuring kinematic parameters, such as velocity and acceleration, of a point on the tread of a tyre fitted to a vehicle during the rotation of the tyre, in other words during the running of the vehicle. In particular, the invention makes it possible to measure the actual velocity of one point on the tread, such as a point on the tread located diametrically opposite a point of contact of the tread with the ground, in other words the footprint, although this is not to be considered a limiting description.

A precise and real-time knowledge of the kinematic parameters of the tread can be applied in different ways in the control of a vehicle, for example in the braking system.

The proposed system makes it possible to obtain a measurement of the velocity of the tread from which it is possible to deduce, by an analog or digital derivation process, the instantaneous value of the acceleration of the tread. Additionally, the torsional deformation of the sidewall can be determined by calculating the difference between this value of acceleration (positive or negative) and that found by multiplying the angular acceleration of the wheel (measured, for example, by means of a phonic wheel or other known device) by the rolling radius of the tyre.

The invention can therefore be applied to the control of the behaviour of a vehicle based on the monitoring of its tyres, and for example to anti-lock braking control devices known as ABS (anti-block systems) . The optimal operation of the said ABS systems is impeded by the delay between the instant at which the tread starts to undergo a negative acceleration indicating an incipient locking of the tyre and the instant at which the hub starts to decelerate. A direct knowledge, without any delay, of the value of acceleration of the tread would enable the operation of ABS systems to be significantly improved.

The technical problem of determining the torsion of the tyre, defined as the angular difference between the position of the rim and the mean position of the elements constituting the tread which are fixed to the underlying structure, known as a belt, which as a first approximation is considered to be circumferentially inextensible, and the velocity of the wheels (and that of the vehicle) for the purpose of braking control has been tackled in numerous prior patents.

US Patent 5913240 describes a method for measuring the torsional deformation of a tyre during rotation, particularly for the purpose of improving the control of an ABS system, using two marks on the tyre located in radially separate positions, and two non-rotating sensors which are integral with the vehicle in positions radially corresponding to locations through which the marks pass during rotation, and which can recognize the (delay in the) passage of these marks, associated with calculation means for determining the torsional deformation of the tyre and/or parameters associated therewith. In one embodiment, the marks consist of two rings of magnetic material, magnetized with a succession of N and S poles, in such a way that two sensors, mounted integrally with the suspension near the sidewall of the tyre, can count the pulses produced by the said rings and thus determine the difference in velocity between the part of the tyre near the rim and the part nearer to the tread. The applicant has observed that the proposed method actually measures the velocity and acceleration of the upper part of the sidewall, which are always slightly delayed with respect to the same parameters of the tread. In another embodiment, the radially outer marks can coincide with the transverse grooves of the tread pattern illuminated by the light.

The use of the Doppler effect to measure the velocity of a vehicle in relation to braking control has been proposed in various patents, as outlined below.

US-A-5, 583, 800 describes a vehicle velocity sensor which uses the Doppler effect in a frequency reflected from the ground surface to determine the velocity of the emitting vehicle.

US-A-5, 428, 359 relates to a vehicle velocity sensor which uses the Doppler effect.

US-A-4, 414 , 548 describes a vehicle velocity sensor based on the Doppler effect, in which the spurious components of the received signal are eliminated.

US-A-4, 356, 489 proposes the measurement of the velocity of the vehicle by means of a Doppler radar unit, with elimination of the noise components in the received Doppler signal. US-A-3, 910, 647 relates to a slip control system which provides three Doppler radar sensors which operate in combination to calculate the forward velocity and the lateral velocity of the vehicle with respect to the road surface and to the lateral velocity of the rear part of the vehicle.

US-A-3, 889, 259 describes a vehicle velocity sensor for an anti-slip control system which uses a Doppler radar system to measure the actual velocity of the vehicle, the system being activated only when the brake pedal is pressed. US-A-3, 833, 906 describes a Doppler radar system for measuring the velocity of a vehicle, using two radar antennas, of which one is directed downwards and forwards and one is directed downwards and backwards. US-A-3, 701, 568 describes a motor vehicle braking system which uses radio-frequency devices which make use of the Doppler effect for measuring both the velocity of the vehicle with respect to the ground and the velocity of the wheels of the vehicle. This document describes a braking system which uses at least two radio-frequency transducers mounted on the vehicle, one of these transducers being aimed at the road surface while the other is aimed at a point on a wheel of the vehicle, in order to obtain two signals, proportional to the velocity of the vehicle and the velocity of the wheel respectively. These signals are then combined to monitor the braking torque.

The applicant has observed that the document indicates that the measurement of the velocity of rotation of the wheel can be determined by conventional wheel velocity sensors, such as magnetic sensors (col. 3, lines 57-64), rather than by sensors of the Doppler type.

US-A-3, 735, 200 describes the combined use of two ultrasonic transducers, one being aimed at a wheel and the other at the road surface, to determine losses of traction and a condition of imminent slipping in the wheel of a motor vehicle, by calculating and comparing with each other the two sets of return pulses which are frequency shifted by the Doppler effect.

The applicant has observed that the device is not intended to determine the peripheral velocity of the tread of a tyre fitted to a vehicle, and does not provide information which can be used to make a measurement of this kind by means of the device. The applicant has observed that the degree of slipping in a motor vehicle wheel can be determined simply by measuring kinematic parameters of a moving point on the surface of the tyre. For the purposes of the present invention, the kinematic parameters of a tyre are considered to be the kinematic parameters of at least one point on the surface of the tyre in contact with the road surface. These parameters may be, for example, the velocity, the variations of velocity, and the displacement.

The applicant has tackled the problem of providing reliable measurement of kinematic parameters of a moving point on the tread of a tyre fitted to a vehicle by exploiting the Doppler effect, with the purpose of using these parameters for controlling the movement of the vehicle. The reliability of the measurement is primarily dependent on the selection of the frequency and the selection of the point on the wheel whose velocity is to be measured, since the measurement is also affected by any irregularities in the wheel surface, as well as by unpredictable unevenness in the road surface, etc.

Additionally, the use of a radio-frequency signal for measuring the velocity is advantageously associated with the use of suitable filter circuits for detecting or discriminating" the reflected component at a frequency other than the transmitted frequency.

The applicant has found that the velocity of a moving point on the surface of a moving tyre can be measured by sending an ultrasonic signal towards a portion of a radially extending transverse groove on the tread of the said moving tyre.

By using an ultrasonic beam aimed at the sides of the tread blocks, provided that these have a depth of at least 2-3 mm, it is possible to obtain a reflected signal of suitable intensity and with a marked frequency shift which can be easily determined by conventional discrimination and detection circuitry. The use of ultrasound makes the signal more stable and free from spurious absorption and/or echoes, since the wavelength is comparable with the dimensions of the reflecting surfaces (blocks) , and therefore capable of operating acceptably in the presence of mud, water, snow or vibrations. The location of the device slightly above the position of the wheel enables the best use to be made of the reflection effect, and the selection of a piezoelectric transducer simplifies the discrimination of the frequency-shifted component of the reflected signal.

The use of a single transducer facilitates the installation and calibration operations, and makes the calibration more stable over a period of time.

A first aspect of the present invention relates to a system for measuring kinematic parameters of a vehicle wheel, the said wheel comprising a tyre, comprising: at least one source mounted on the said vehicle and capable of sending a signal at a first frequency to a portion of the moving surface of the said wheel; - at least one receiver mounted on the said vehicle in a position such that it can receive at least a fraction of the signal reflected from the said surface portion of the wheel; at least one processing unit, for calculating, from the ratio between the frequencies of the transmitted and the received signals, a quantity representative of the said kinematic parameters; characterized in that the said source and the said receiver comprise at least one ultrasonic transducer, and the said surface portion of the said wheel is a transverse groove portion of the tread pattern of the tyre extending essentially in the radial direction. Preferably, the said kinematic parameters comprise the peripheral velocity of the surface of the said wheel .

Preferably, the said kinematic parameters comprise the variations of peripheral velocity of the surface of the said wheel.

Preferably, the said at least one source and the said at least one receiver are positioned at a height slightly above the height of the wheel, in such a way that the said groove portion of =^• the tread is essentially in a position diametrically opposite the footprint of the tyre.

Preferably, the said at least one ultrasonic transducer is of the piezoelectric type.

Preferably, the said at least one source and the said at least one receiver are incorporated in a single component .

Preferably, the said at least one source and the said at least one receiver lie on a plane inclined with respect to the direction of advance of the vehicle, and are positioned on opposite sides of a vertical plane passing through the tangent to the tyre along the said portions of transverse groove.

Preferably, the said first frequency is in the range from 200 to 300 kHz. The said system additionally comprises a filter capable of discriminating, from the received reflected signal, the component at a second frequency which is different from the transmitted frequency.

A further aspect of the present invention relates to a method for measuring kinematic parameters of a vehicle wheel, the said wheel comprising a tyre, characterized in that it comprises the steps of: sending an ultrasonic signal at a first frequency to a portion of transverse groove of the tread pattern of the said tyre which extends essentially in a radial direction, receiving at least a fraction of the signal reflected from the said portion of transverse groove, - processing the said fraction of the reflected signal in such a way as to obtain a quantity representative of the said kinematic parameters of the said wheel.

Preferably, the said portion of transverse groove coincides with the side of a block of the tread pattern.

Preferably, the axes of propagation of the said transmitted signal and the said reflected signal are essentially coincident. Alternatively, the axes of propagation of the said transmitted signal and the said reflected signal lie on a plane inclined with respect to the direction of advance of the vehicle.

Preferably, the said axes of propagation lie at equal angles to, but on opposite sides of, a vertical plane passing through the tangent to the wheel.

The method additionally comprises the step of discriminating, from the received reflected signal, the component at a second frequency which is different from that transmitted and is produced by the Doppler effect. Preferably, the said step of calculating the said fraction of the reflected signal comprises a frequency analysis of the signal.

Further characteristics and advantages of the present invention can be discovered in greater detail from the following description, with reference to the attached drawings, provided solely for explanatory purposes and without any restrictive intent, in which

Fig. 1 shows a schematic partial and sectional lateral view of a pneumatic-tyred wheel which illustrates the system according to the invention;

Fig. 2 shows a partial view from above of the device of Fig. 1;

Fig. 3 shows a partial view from above of a variant embodiment of the system according to the invention;

Fig. 4 shows a view illustrating the angular relations of the device of Fig. 3.

The same references are used in the different figures to identify parts which are identical or essentially equivalent.

Figures 1 and 2 show a pneumatic-tyred wheel 1, fitted on the axle of a vehicle (not shown) and comprising a rim 2 and a tyre 3, provided with a tread

4. Generally, the tyre is of the radial type, although this description is not to be considered as limiting.

The tread pattern 4 includes at least one set of blocks 5 whose configuration can vary from one tyre to another, but which in all cases includes sides with a portion having an essentially radial extension. More generally, the tread pattern of the tyre includes a certain number of transverse grooves which extend radially.

In a preferred embodiment of the invention, an emitter or source of an acoustic signal, preferably ultrasonic, is fitted at a point T relatively near the top of the tyre, in other words near the point essentially diametrically opposite the footprint of the tyre, and this emitter is capable of emitting a relatively concentrated beam directed towards an area of the tread in the proximity of the point T. Additionally, a receiver, capable of detecting the ultrasonic signal reflected from the sides of the blocks or of the transverse grooves, and in particular of determining the frequency difference due to the Doppler effect between the frequency fl of the "illuminating" signal and the frequency f2 of the reflected signal, is positioned in the proximity of the said source. As is known, the frequency of the reflected signal f2 decreases as a function of the velocity of the reflecting surface when the latter moves away from the source, and increases when the reflecting surface moves towards the source. In the embodiment illustrated in Figures 1 and 2, in which the arrow A indicates the direction of advance of the vehicle, the tread surface has a velocity (tangential) in the direction away from the source.

Preferably, according to the present invention, both the source and the receiver are made in the form of transducers, for example piezoelectric transducers, capable, respectively, of generating the audio signal at ultrasonic frequency when supplied with a suitable alternating voltage, and of generating an (alternating) electrical voltage when struck by an acoustic signal. The two transducers, also known as sonar transducers, commonly used in underwater detection devices, can be fitted in the same container to form a single component, or a single transducer can be used, with time division, as both source and receiver. The sonar transducers of the present invention are transducers suitably designed to operate in air; those normally available on the market for underwater operation are not suitable for this application.

In the embodiment shown in Figures 1 and 2, the source and receiver are combined in a single transducer 10, positioned above the tyre and located in such a way that it illuminates the point T during a first predetermined time interval, and receives the reflected beam during a second predetermined time interval. The said time intervals can be superimposed or coincident, or the signals can be continuous. Alternatively, it is possible to use a single transducer which is made to operate in a pulsed way. For example, it is possible to operate with pulses of 100 cycles at 215 kHz with 1 pulse every 2 s . A reading of the velocity every 2 ms is sufficient to achieve the objective of improving slip control, by the ABS system for example. The arrangement shown in Figures 1 and 2 is to be considered as an example. The source and the receiver could, for example, both lie in a plane τ essentially perpendicular to the axis of the wheel and tyre.

As shown schematically in Fig. 2, the transducer 10 is preferably associated with a filter 16 capable of suppressing the components of the reflected signal having frequencies different from the frequency f2 of the received signal. A processing unit 15 calculates the peripheral velocity of the tread from the difference between the frequency of the emitted signal fl and the frequency of the received signal f2. The ultrasonic frequency of the source is, for example, of the order of 200-300 kHz. For example, it is possible to use known transducers of the ITC 9083 type, one of which is supplied in such a way as to generate an ultrasonic signal at 215 kHz.

More generally, the source and receiver assembly can be positioned at a height below that of the point T, and the illuminated area can be any point of the tread under the point T. In these cases, the components moving closer or further away in the directions of illumination and reflection must be considered, as the following description will make clear to a person skilled in the art. In the embodiment shown schematically in Fig. 3, two separate piezoelectric transducers or components 10A and 10B are provided, these being the source and the receiver respectively, and are fitted in positions relatively close to each other, but not in positions that could be considered as coincident. The transducers are integral with the suspension of the vehicle and therefore their positions relative to the surface of the tyre and the angles indicated in the diagram do not vary with the travel of the suspension. In Fig. 3, the processing unit 15 and the filter 16 essentially have the same functions as the identical components in Fig. 2. The ultrasonic sensors suitable for the application are preferably of the high-frequency type. This is because millimetre-length waves must be used in order to obtain effective echoes from the vertical walls of the blocks of the tread or the sides of the blocks of the tread. Sensors designed to operate in air, although only over short distances, generally less than one metre, are easily available on the market. To enable the same transducer to be used as the emitter and receiver of the ultrasonic signal, the distance between the sensor and the object whose distance and/or velocity are to be measured (in the present case, the point on the tread surface indicated by T) must be at least 8 cm. The use of two transducers operating continuously enables these limitations to be overcome and enables the signal to noise ratio of the system to be improved.

With reference to Figures 3 and 4, if r is the tangent to the tyre along the line of blocks on which the transducer operates, and π is the plane in which the axes of the two transducers lie, the axis of the transducer 10A (direction of emission of the ultrasonic beam) forms an angle cti with the projection of the straight line r on the plane π. The axis of the transducer 10B (along which the reflected ultrasonic beam is received) forms an angle α₂ with the projection of the straight line r on the plane π. In the arrangement shown in Figure 3, it is specified that oti = α₂ = α, so that this angle is the angle between the projection of the straight line r on the plane π and the direction of emission or reception of the ultrasonic beam, corresponding to the geometrical axis of each of the two transducers. This arrangement is shown by way of example and is not in any way to be understood as having a limiting intention. With reference to Figure 4, β is the angle between the plane π on which the two transducers are fitted and the plane containing the straight line r, and θ is the angle between the directions of emission and reception α and the straight line r. Reference will be made to the arrangement in Figures 3 and 4 for the calculation of the most general expression of the velocity of slip of the tread, but clearly the description is also valid for the situation shown in Figures 1 and 2 when it is assumed that oi_l = α₂ = 0°.

With the arrangement shown in Figures 3 and 4, the velocity of slip of the tread can be calculated, for example, in the following way.

Let V_R be the velocity of the tread 4 relative to the transducer 10; it is considered to be positive if, as shown in Fig. 1, the tread moves in the same direction as the sound emitted by the transducer. Figure 1 shows the sonar transducer facing the direction of advance of the vehicle, and the part of the tread diametrically opposite the footprint. The latter moves in the same direction relative to the reference point of the vehicle. In normal running conditions of the vehicle, V_R, in absolute terms, is approximately equal to the velocity of the vehicle and has a positive sign if the vehicle is moving forwards. Let V be the velocity of sound and ω and Ω be, respectively, the detected frequency of the echo received from the target, which in the illustrated case is the walls of the tread blocks, and the base frequency of the ultrasonic emitter transducer in the component 10,

If the velocity V_R, which in absolute terms is approximately equal to that of the vehicle, is less than 60 m/s, the following approximate formula can be considered true: ω/Ω = (1-V_R cosθ / V)² . 1)

If a velocity of less than 60 m/s is assumed, and only the relative velocity is considered, that fact that the air between the transducer and the target (the tread) is not stationary is disregarded. This approximation is applicable only to velocities which are low with respect to the velocity of sound V. The velocity of the air between the sensor and the tyre would be approximately equal, in absolute terms, to that of the vehicle, if the system were not protected by the mudguard. In general, however, it can be assumed that the air velocity is actually less than that of the vehicle and that, therefore, the introduced error is negligible.

Alternatively, in a more accurate method, the full formula for the Doppler effect can be used, as follows: ω/Ω = [(1 + V_s cosθ / V) (l-v_τ cosθ / V) ] / [(1-Vs cosθ / V) (1 + V_τ cosθ / V) ] 2) In formula 2), V_Ξ is the velocity of the sonar transducer, V_τ is the velocity of the tread at the echo detection point, and V is the velocity of sound. All the velocities are relative to a system integral with the air with respect to which the system moves, and are considered to be positive in the direction of movement of the vehicle (arrow A) . For a sonar facing in the direction of movement of the vehicle, the component of the relative velocity of slip of the tread with respect to the sonar V_R, parallel to the direction of propagation of the sound wave, is given by V_R cos θ = V_τ cos θ - V_s cos θ, or V_τ cos θ = (V_R + V_s) cos θ Replacing V_τ cos θ with the expression cited above in formula 2), we obtain: ω/Ω = { (1 + V_s cosθ / V) [1-(V_R +V_S) cosθ / V] } / {(1-Vs cosθ / V) [1+(V_R+V_S) cosθ / V] } 3)

To solve equation 3) with respect to the unknown V_R, it is necessary to know the value of the velocity V_s by measuring it independently. The said velocity corresponds exactly to the velocity of the vehicle in ideal conditions only. This is because the aerodynamic field is perturbed by the movement of the vehicle and by the presence of aerodynamic screening (mudguards, etc.) and the presence of the tyre itself. Therefore, a device capable of measuring the mean velocity of the aerodynamic flow between the sensor and the tread is provided. To obtain this measurement, it is possible to install a second ultrasonic transducer, at a fixed distance from the emitting transducer 10A. If the emitter operates in pulsed mode, the sensor can measure the time elapsing between the emission of one pulse and its reception by the second sensor. If the emitter operates in continuous mode, the signal can, for example, be interrupted for approximately 1 ms and a measurement made of the time taken for the silent pulse to be received. The frequency of these interruptions can be very low, being preferably in the range from 100 to 1 Hz, since the aerodynamic velocity field undergoes only low-frequency variations.

Alternatively, it is possible and helpful to use other devices, such as dynamic pressure or hot wire anemometers, for measuring the velocity of the emitter, which is clearly equal to that of the receiver, with respect to the air between them. However, the use of a second ultrasonic sensor is preferable, since this measures the mean velocity of the aerodynamic field in the area in question, rather than at a single point. Moreover, it uses parts of the ultrasonic sensor which are present in all cases and is particularly immune to problems caused by dirt, which could compromise the operation of the alternative sensors mentioned above.

If Δt is the time interval required for the reception of a pulse, and d is the distance between the emitter and the receiver, then

V_s = (1/cosθ) * (V cosδ - d / Δt) 4) where the angle δ is: δ = arctan{V_s sinθ/ (d/Δt + V_s cosθ)] 5) The relation 4) is not an algebraic expression but a true equation, since the unknown Vs also appears on the right of the equals sign, within the definition of the angle δ.

It should be noted that the angle δ is always less than θ, if the ultrasonic emitter is aimed in the direction of advance of the vehicle.

If the velocity is markedly lower than the velocity of sound, the angle δ is virtually equal to θ and equation 4) is simplified to the following expression, which enables V_s to be calculated directly: V_s = V - d / (Δt * cosθ) 6)

Additionally, if the angle θ is sufficiently small for its cosine to be considered equal to 1, the relation becomes: V_s = V - d / Δt 7)

If it is desired to use the full equation for the calculation of the velocity V_s, this can easily be solved by an iterative method, using the value of V_s given by the expression 6) as a first approximation. Regardless of the choice of the measuring method, whether it is the approximate method for flow velocities much lower than the velocity of sound, or the correct method with direct measurement of the velocity of the aerodynamic field between the Doppler sonar and the tread, the system is capable of providing a real-time estimate of the velocity of slip of the tread with respect to the sensor.

This information can then be used for the calculation of data useful for the control of the vehicle, for example for improving the control dynamics of the ABS system.

For example, the derivative of the velocity V_R can be calculated, and this item of data used directly in the control computer of the ABS system. The calculated acceleration is that of the tread, and its use is an improvement over the alternative method, universally used at the present time, of evaluating the locking tendency of the wheel from the derivative of the angular velocity of the rim, corresponding to the derivative of the frequency of the encoder integral with the hub. The advantage consists in the elimination of the delay between the deceleration of the tread and that of the hub, due, as stated before, to the torsional deformation of the tyre sidewall. The method is also advantageous with respect to that of the described prior art, since it directly determines the acceleration of the tread.

If the monitored tyre is of the radial type, the system operates particularly well, since the belt of a radial tyre is essentially inextensible, and the torsional yielding of the tyre is concentrated in the sidewall of the tyre. In a tyre with a conventional casing, the incipient locking of the portion of the tyre in contact with the road is transmitted to the diametrically opposite position in a much less efficient way. Moreover, by means of the present invention, it is possible to determine the torque of the sidewall of each tyre in an ABS system, by means of the measurement in time of the velocity of the tread obtained from the ultrasonic sensors, and the measurement of the velocity of the hub by means of the phonic wheel or encoder. By correlating the value of the sidewall torsion of each tyre with the slip calculated from the measurement of the velocity of the tread surface in all the tyres of the vehicle, it is possible to calibrate the ABS according to the road surface which is to be travelled on. For example, in the case of a high torque value and a low slip value, it can be deduced that the road surface being travelled over is very abrasive. Conversely, a low torque value with a high slip indicates a smooth and/or wet surface with a low coefficient of friction. The objective value of slip which the ABS system is designed to determine can be optimized to allow for the road surface detected as described above. The sidewall torsion can be calculated by integration of the difference between the value of angular velocity of the hub and the signal of tread velocity measured by the Doppler sensor, obviously divided by 2πR. The angular acceleration of the hub can also be calculated on the basis of the encoder signal. Thus, if the signal to noise ratio of the ultrasonic sensor becomes degraded, for example because the sensor is near a source of ultrasonic disturbance, the ABS system does not cease to operate, even if it cannot benefit from the installed device for the duration of the disturbance.

If the ABS system is designed to operate by directly using the signal proportional to the velocity at the crown, this item of data can be introduced into the system in the requisite form, in other words in analog or digital form or as a pulse train, synthesized with a frequency proportional to the calculated velocity.

On the other hand, if the ABS system requires a signal relative to the angular position of the tread with respect to the rim, this can be used by calculating the position by integration of the difference between the ultrasonic crown velocity signal and the signal obtained by the encoder on the hub. The difference has to be filtered by a high-pass filter with a cut-off frequency preferably in the range from 0.1 to 10 Hz. The high-pass filter is required because of the need to determine the integration constant on the basis of the fact that the said difference is zero over the long term, since there is no slip between the tyre and the rim and no permanent torsional deformation of the sidewall. As an alternative to the system with a filter, whether analog or digital, it is possible to use an analog integrator of the operational amplifier type, with a loss resistor, whose relaxation time is in the range from 0.1 to 10 seconds.

The efficiency of a braking operation is affected by a vast number of parameters, the main one being the coefficient of friction between the tyre and the contact surface on which it is moving.

However, the coefficient of friction cannot be measured instantaneously and cannot be defined in advance, since it varies continually from point to point of the said surface and with the state (dry, wet, snow- or ice-covered) of the said surface.

The invention has now provided a way of identifying the limit condition of adhesion of the tyre.

The signal obtained from the sensor has a harmonic content proportional to the velocity of rotation. At the limit of adhesion, at least some of the projections of the tread (blocks and/or bars) start to slip; it has been found that the slipping of the said projections on the road generates vibrations in the frequency range from 300 Hz to 1000 Hz, independently of the rolling velocity.

These frequencies are contained in the reflected signal according to the invention. It is therefore possible to detect a condition of limit of adhesion, in the longitudinal direction (braking or acceleration) , in the lateral direction (the condition of side-slip) , and in a condition of combined stressing in the two aforesaid conditions, by checking for the presence in the signal of frequencies lying in the aforesaid range.

It is also possible to distinguish between slips due to braking and those due to side-slip forces. By installing two systems of the type described on the two shoulders of the tyre, the torsion of the belt structure can be measured and consequently a measurement of the lateral forces can be obtained. The double system also makes it possible to detect a phenomenon of incipient aquaplaning, as one manifestation of this is an asymmetry of the longitudinal slip. To measure the said asymmetry, a plurality of signals can advantageously be sent, in each tyre, to different points on the same tread. Additionally, the signals can be sent by a plurality of transducers located on the vehicle. A plurality of reflected signals received from the same tyre can be processed in combination to determine the said condition of limit of adhesion.

Claims

1. System for measuring kinematic parameters of a vehicle wheel (1), the said wheel comprising a tyre, comprising: at least one source (10; 10A) mounted on the said vehicle and capable of sending a signal at a first frequency (fl) to a portion of the moving surface of the said wheel (1) ; - at least one receiver (10; 10B) mounted on the said vehicle in a position such that it can receive at least a fraction of the signal reflected from the said surface portion of the wheel; - at least one processing unit (15), for calculating, from the ratio between the frequencies of the transmitted and the received signals, a quantity representative of the said kinematic parameters; characterized in that the said at least one source (10; 10A) and the said at least one receiver (10; 10B) comprise at least one ultrasonic transducer, and the said surface portion of the said wheel is a transverse groove portion of the tread pattern (4) of the tyre extending essentially in the radial direction.

2. System according to Claim 1, characterized in that the said kinematic parameters comprise the peripheral velocity of the surface of the said wheel.

3. System according to Claim 1, characterized in that the said kinematic parameters comprise the variations of peripheral velocity of the surface of the said wheel.

4. System according to Claim 1, characterized in that the said at least one source (10; 10A) and the said at least one receiver (10; 10B) are positioned at a height slightly above the height of the wheel (1), in such a way that the said groove portion of the tread is essentially in a position diametrically opposite the footprint of the tyre.

5. System according to Claim 1, characterized in that the said at least one ultrasonic transducer (10; 10A, 10B) is of the piezoelectric type.

6. System according to Claim 5, characterized in that the said at least one source and the said at least one receiver are incorporated in a single component (10) .

7. System according to Claim 5, characterized in that the said at least one source (10A) and the said at least one receiver (10B) lie on a plane (π) inclined with respect to the direction of advance of the vehicle, and are positioned on opposite sides of a vertical plane passing through the tangent (r) to the tyre along the said portions of transverse groove.

8. System according to Claim 1, characterized in that the said first frequency (fl) is in the range from 200 to 300 kHz.

9. System according to Claim 1, characterized in that it comprises a filter (16) capable of discriminating, from the received reflected signal, the component at a second frequency which is different from the transmitted frequency.

10. Method for measuring kinematic parameters of a vehicle wheel (1), the said wheel comprising a tyre, characterized in that it comprises the steps of: - sending an ultrasonic signal at a first frequency (fl) to a portion of transverse groove of the tread pattern (4) of the said tyre which extends essentially in a radial direction, receiving at least a fraction of the signal reflected from the said portion of transverse groove, processing the said fraction of the reflected signal in such a way as to obtain a quantity representative of the said kinematic parameters of the said wheel.

11. Method according to Claim 10, characterized in that the said portion of transverse groove coincides with the side of a block of the tread pattern.

12. Method according to Claim 11, characterized in that the axes of propagation of the said transmitted signal and the said reflected signal are essentially coincident .

13. Method according to Claims 10 to 12, characterized in that the axes of propagation of the said transmitted signal and the said reflected signal lie on a plane inclined with respect to the direction of advance of the vehicle.

14. Method according to Claim 13, characterized in that the said axes of propagation lie at equal angles to, but on opposite sides of, a vertical plane passing through the tangent (r) to the wheel.

15. Method according to Claim 10, characterized in that it comprises the step of discriminating, from the received reflected signal, the component at a second frequency (f2) which is different from that transmitted and is produced by the Doppler effect.

16. Method according to Claim 10, characterized in that the said step of calculating the said fraction of the reflected signal comprises a frequency analysis of the signal.