WO1987004124A1

WO1987004124A1 - Dynamic pressure sensor for vehicle tyres

Info

Publication number: WO1987004124A1
Application number: PCT/FR1987/000006
Authority: WO
Inventors: François Micheron
Original assignee: Thomson-Csf
Priority date: 1986-01-10
Filing date: 1987-01-09
Publication date: 1987-07-16
Also published as: EP0256039A1; FR2592954B1; FR2592954A1

Abstract

The sensor comprises two piezoelectric films (32, 33) having opposite polarization and mechanically integral with each other and associated with the wall (1) of the tyre. Said sensor is particularly applicable to vehicule tyres for detecting under-inflation conditions.

Description

DYNAMIC PRESSURE SENSOR FOR TIRE

The invention relates to a dynamic pressure sensor for tires. It is more particularly applicable to automobile tires.

The proposed sensor allows dynamic pressure measurement for tires, and more precisely detection of under inflation. This sensor has the advantage of being insensitive to normal stresses and temperature changes, and optimized for the detection of bending stresses.

It is known that the vast majority of tire bursts is due to under inflation; this under inflation depends on the initial cold pressure, the road temperature, the speed, the load, and the condition of the tire itself. The bursting is due to the combined effects of bending of the excessively accentuated flanks, which tend to destroy the texture, therefore to decrease its resistance, and to the joint temperature rise, due to internal mechanical losses, which accelerates the degradation process, and increases the pressure in the envelope.

This means that the pressure measurement is not completely significant of a possible burst, unless the pressure is at a very low level. A significant measure is that of the amplitude of the bending of the flanks, which incorporates all of the parameters mentioned above.

The invention therefore provides a solution to meet these needs. The invention therefore relates to a dynamic pressure sensor for a tire comprising a wheel provided with an inflatable tire having a deformable wall, characterized in that it comprises two piezoelectric films of opposite polarizations, mechanically integral with one another and associated with the tire wall.

The various objects and characteristics of the invention appear will appear more clearly in the description which follows made with reference to the appended figures which represent:

- Figure 1, an exemplary embodiment of the pressure sensor of the invention; - Figure 2, an alternative embodiment of the pressure sensor of the invention j

- Figure 3, an explanatory diagram of the operation of the pressure sensor of the invention;

- Figures 4 to 7, different circuits for transmitting reading information from the sensor according to the invention to user circuits.

Referring to Figure 1, we will first describe an embodiment of the pressure sensor of the invention.

According to the invention, the measurement of the bending of a relatively flexible envelope is greatly facilitated by the use of a sensor which is itself flexible and sufficiently thin to adhere to the internal face of the tire, or, in the case of a tire with a chamber, for sliding between tire and chamber without excessive deformation of the latter. This leads to proposing a piezoelectric polymer bending sensor, the subject of this patent application.

The measurement of the bending of the tire sidewall must be carried out in a very severe environment. This measurement must indeed be independent of the temperature conditions (typically - 0 to 80 ° CeIsius) and as little as possible dependent on the normal stresses applied to the tire.

The proposed sensor therefore comprises a bimorph of piezoelectric films, that is to say of two films integral with opposite polarizations. This bimorph, a few centimeters wide, a few hundred microns thick, is stuck inside the tire, from one edge to the other. In fact, the useful zones of this bimorph are those in contact with the sides; the length of this bimorph can be limited to one side. A bimorph can also be glued on each side, which brings the possibility new differential bending measurement.

In Figure 1, there is therefore shown in sectional view a tire consisting of an envelope 1 mounted on a rigid rim 2. This tire is inflated by means of a valve.

A bimor¬ phe 3 has been stuck on the inside wall of the envelope 1 comprising two piezoelectric films 32, 33 joined to one another and whose common face has been represented by a dotted line in FIG. 1. The bimorph is placed on the inner wall of the tire so as to connect the two ends 10 and 11 of the casing 1 as visible in section view. The ends 30 and 31 located at the ends 10 and 11 therefore do not support bending stress while the bimorph portions located at the sides 1 and l 'of the envelope undergo bending stresses due to the flattening of the tire.

In Figure 2, there is shown a sectional view of a pneuma¬ tick in which the sensor of the invention comprises two bimorphs 3 and placed there on the sidewalls 1 and the of the tire with their end 30 and 31 placed at level ends 10 and 11 of the rim. This arrangement allows, as indicated above, differential bending measurements.

Note that the charges generated in the bimorph are only due to bending; in other words, all pressure constraints perpendicular to the plane of the bimorph or all tensions in its plane have zero result, since they are exerted in the same way in the two opposite polarization films.

The same goes for the pyroelectric effect, that is to say that any temperature variation, which generates in the two films electric charges of opposite signs is seen to be self-compensated by the symmetrical construction of the bimorph. It is important, likewise, that the electrical quantity measured is not very dependent on the operating temperature; however, the transverse piezoelectric coefficient and the permittivity have substantially the same temperature dependence, so that their relationship is independent. From this done, it is the voltage generated rather than the current, which will be measured.

Referring to Figure 3, we will now describe the operation of a bimorph used in the context of the sensor of the invention.

In Figure 3, there is shown a bimorph 3 having two lamellae or films 32 and 33 of piezoelectric material and joined to one another. One end 35 of the bimorph 3 is immobilized in a jaw 7 in the same way as in FIGS. 1 and 2, the ends 30 and 31 are integral with the ends 10 and 11 of the tire casing at the level of the rim 2 .

The 3Ψ end of the bimorph is free and supports a bending force F which tends to bend the bimorph to bring it into the position indicated in dotted lines. The length of the bimorph is L.

Its width is b.

The thicknesses of the two films are identical and the total thickness of the bimorph is h.

Under the effect of the force F, it is assumed that the 3Ψ end of the bimorph is displaced by a distance D, generating in the bimor¬ phe a charge Q. Which creates between two electrical connections 5 and 6 connected to two electrodes placed on the two films 32 and 33, a voltage V between electrodes.

The piezoelectricity equations are in this case: Q _ CV + PF D = PV + SF with C: capacity of the bimorph = r—, ε: permittivity

3 L ² P: piezoelectric coefficient = ^~ - ~~ _> d ,. , where d-, is the transverse piezoelectric coefficient intrinsic to the piezoelectric material.

F: force applied to the end of the bimorph

D: displacement of the end of the bimorph 3 S: compliance of the bimorph = - ~ - where E is the Young's modulus of the polymer.

In the case of a measurement of tension, that is to say in open circuit, one obtains the relation tension V function of displacement D in the form

v α. _ s_e_) _ - _ _ s_e_

A numerical example applicable to a commercial tire leads to taking the following values: L = 8 cm b = 1 cm h = 0.5 mm ε = 10 ^'10 Fm ^' V hence C = 160 pF ^d 3I = 20. 10 ^"12 CN ^-1

hence P = 7.7. 10 ^"7 CN ^{" 1}

E = 3.10 ⁹ N m ^"2

hence S = 0.5 m N ^"1

and P - ² ₌ 7.4 10 -3 ³

^ = 9.6 10 ³ VM ^"1

A dangerous bending of D = 1 cm for example, will therefore lead to having a voltage between electrodes of V = 96 volts which will be easily measurable.

This calculation shows that the bimorph is capable of generating very high tensions at each bending, since in this case the peak-to-peak amplitude of the tension, at the frequency of rotation of the wheel reaches approximately 100 volts.

The energy generated at each bending is: = CV ² = 7. 1Û ^'7 Joule

For a speed of 3000 rpm, the electrical power supplied is:

7.10 ⁷ .50 = 35 μW. The piezoelectric polymer bimorphs are produced industrially, using for example the process which is the subject of patent 80 20213 filed on September 19, 1980 and published under No. 2 4 0 877.

The sensor is cut to the length adapted to the tire, bending must occur perpendicular to the machine axis (axis along which the polymer was stretched during its manufacture).

The bimorph strips are generally metallized by vacuum deposition of aluminum; the contacts are made either by bonding using a conductive adhesive, or by pressure. The bimorph must be mechanically protected, which is simply achieved by gluing adhesive tapes on both sides; one of the tapes can be double-sided adhesive, so as to ensure bonding inside the tire.

It is desirable that the transmission of the indication of under inflation is transmitted to the driver without rotating contact. Optical or ultrasonic transmission will be provided using devices known in the art. It is also possible to carry out transmission by electromagnetic waves using a device as described below. Information can only be transmitted in the event of critical under inflation, that is to say a threshold must be placed. In this case, the best device consists in using, as shown in FIG. 4, a transmission circuit comprising a neon lamp 50 connected by a load 51 to the connections 5 and 6 of the bimorph 3. This circuit combines both the function threshold, and the shaping of the pulses.

The voltages generated are sufficient for the excitation of a neon lamp. As shown in FIG. 5, the charging circuit can be an HF oscillating circuit 52 coupled to an antenna 53. The load circuit can be an oscillating circuit, the choke 52 of which is made up of a loop secured to the wheel axle. The signals transmitted by the antenna 53 are received by a remote reception circuit, not shown in the figures.

As shown in FIG. 6, the charging circuit can also be a rotating loop 54 secured to the pneumatic wheel, while a fixed loop and a receiving circuit, not shown, are secured to the axis of the wheel.

Finally, the charging circuit can be a diode bridge 56 supplying an HF or BF transmitter referenced 57 in the figure and emitting either via an antenna 53, or via a mobile loop 54 .

Claims

1. Dynamic pressure sensor for a tire comprising a wheel fitted with an inflatable tire having a deformable wall (1), characterized in that it comprises two piezoelectric films (32, 33) of opposite polarizations, mechanically secured - Only one from the other and associated with the wall (1) of the tire.

2. Pressure sensor according to claim 1, characterized in that it is stuck to the wall of the tire (1).

3. Pressure sensor according to claim 1, characterized in that it is integrated into the wall of the tire (1).

4. Pressure sensor according to claim 1, in which the deformable wall (1) is mounted on a rim (2) by two inner edges (10, 11), characterized in that it is associated with said wall (1) and connects the two inner edges (10, 11).

5. Pressure sensor according to claim 1, in which the wall (1) has two lateral sides (I, l ') characterized in that it comprises two sensor elements (3, 3') each comprising two piezoelectric polarization films opposite, each sensor element being arranged on a lateral flank.

6. Pressure sensor according to claim 1, characterized in that the piezoelectric films are made of polymer material.

7. Pressure sensor according to claim 1, characterized in that the two films are bonded to each other.

8. Pressure sensor according to claim 1, characterized in that it comprises on two outer faces, two measuring electrodes (5, 6).

9. Pressure sensor according to claim 8, characterized in that it comprises a signal transmission circuit secured to the wheel and connected to the measurement electrodes, as well as a remote receiver receiving the signals transmitted by the circuit. program.

10. Pressure sensor according to claim 9, characterized in that it comprises a threshold circuit (50) and a ^" charging circuit (51, 52, 54, 56) connected to the two measurement electrodes.

11. Pressure sensor according to claim 10, characterized in that it comprises a transmitting antenna 53 connected to the charging circuit (52).

12. Pressure sensor according to claim 10, characterized in that the charging circuit 54 is a loop secured to the pneumatic wheel and that a fixed loop 55 is secured to the axis of the wheel.

13. Pressure sensor according to claim 10, characterized in that the charging circuit comprises a diode bridge 56 and an HF or BF transmitter (57) emitting via an antenna (53) or a loop mobile (54).