The present invention relates to SAW based tyre pressure sensors, and in particular to adaptors which enable such sensors to be retro-fitted to existing tyre inflation valves.

Surface Acoustic Wave devices are known in the art to be useful as pressure sensors, and they have particularly application as remote or wireless pressure and temperature sensors in tyres. These SAW based pressure sensors must be placed in fluid communication with the pressurised atmosphere within the tyre, and this is typically done either by attaching the sensor to the inner wall of the tyre or integrating it into a tyre pressure valve. Applicant's own earlier Granted UK patent GB2352814 and UK patent application no. GB021163.0 disclose a particular example of a tyre inflation valve with an integrated SAW pressure sensor. However, this system requires the fitting of a new inflation valve when installing the system into an existing wheel, and also results in the SAW sensor being disposed of every time the valve is replaced, for example when a new tyre is fitted onto a rim, thereby increasing costs.

According to the present invention, there is provided a SAW based tyre pressure sensor assembly comprising a body having fastening means provided on an end thereof which, in use, is engageable with an inner end of a tyre inflation valve so to fasten the body thereto, a through passage extending from said fastening means through said body so as to provide, in use, a fluid coupling between an inflation duct of the inflation valve and the atmosphere surrounding the body, and a SAW based pressure sensor mounted within said body so as to be in fluid communication with the atmosphere surrounding the body. A pressure sensor assembly in accordance with the invention has the advantage that it may be retro-fitted to existing inflation valves quickly and easily, thereby avoiding the need to replace the entire valve assembly and hence reducing costs. Furthermore, when the valve needs replacement, the adaptor can be removed from the old valve and fitted onto the new one, thereby avoiding unnecessary disposal of fully working sensors.

The surface acoustic wave (SAW) devices referred to in this document are single port resonators, which are laid down as a group of three on to a single substrate mono-crystalline quartz die housed within a metal package. The package incorporates a diaphragm which, under the action of external pressure, presses on to the centre of the die causing it to bend. The resonant frequencies of the SAW resonators change with mechanical strain and temperature and by suitable interrogation and processing, independent pressure and temperature signals are obtained. The packaged sensor is interrogated by an RF signal and responds with a back-scattered RF signal in both cases via a local antenna

The fastening means is preferably formed so as to enable easy interfacing with existing valves without requiring excessive modification of the valve. In one embodiment, suitable for use with snap-in type inflation valves, the fastening means may be formed as a tube which pushes into the bulb end of the valve which, in use, is located within the pressurised chamber of the tyre. In particular, the tube may include circumferentially extending ridges or barbs to facilitate its insertion into but inhibit its removal from the inflation duct.

In another embodiment, the fastening means takes the form of a threaded tube, which may be parallel or tapered and which is engageable with a complementary female or male thread formed on the inner end of the inflation valve. In this arrangement, the inflation valve may require modification to accept the threaded tube by cutting a suitable thread thereon, or the threaded tube may be self tapping.

Instead of or in addition to the fastening means mentioned above, a suitable adhesive may be used to fasten the fastening means to the inflation valve.

In a further development of the invention, an antenna may be provided on the body which is connected to the SAW sensor for remote interface therewith. A grounding pate may also then be provided on the body which, in use, engages with the inner surface of the wheel so as to enhance the sensor antenna earth, either by a capacitive contact if the wheel is coated by a non-conductive material, e.g. paint, lacquer or anodising, or by a direct DC coupling if no such coating is present. In a particularly advantageous development, the grounding plate is formed as a leaf spring or spring loaded plate which is clamped between the assembly and the inflation valve and which has arms which are biased so as, in use, to bend away from the adaptor and hence firmly press against the inner surface of the wheel.

In order that the invention may be well understood, there will now be described some embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which:

Figure 1 is a perspective view of an assembly according to a first embodiment of the invention mounted onto a tyre inflation valve; Figure 2 is an exploded perspective view of the adaptor of figure 1; Figure 3 is a perspective view of an assembly according to a second embodiment of the invention mounted onto a tyre inflation valve; and Figure 4 is an exploded perspective view of the adaptor of figure 3. Referring to the drawings, there are shown two embodiments of a tyre pressure monitoring assembly suitable for converting standard "Snap In" 3 and "Clamp In" 6 tyre valves into SAW based tyre pressure monitoring sensors by retro¬ fitting. In each embodiment, the adaptor incorporates an enhancement to the earth of the sensor to the wheel via a capacitive / direct spring loaded contact 5.

With reference to Figures 1 and 2, the "Snap In" adaptor is a push fit into the rubber valve 3 secured by a barbed stud 1. A proprietary adhesive may also be used to increase the security of the fixing. The adaptor incorporates a through hole in its end which connects to side air holes 2 so as to allow air flow into and out of the tyre. In this way, the adaptor does not interfere with the functionality of the valve. An antenna 4 is also provided on the adaptor for remote signal coupling as is well known in the art.

Figures 3 and 4 show a "Clamp In" adaptor which is similar to the "Snap In" adaptor with the exception that it has a threaded stud 7. This necessitates that the "Clamp In" valve 6 be modified to accept the threaded stud 3. A proprietary adhesive may also be used to increase the security of the fixing. Again, an antenna 4 is provided for remote signal coupling to the SAW sensor.

As inflation valves typically have a rubber seal / interface with the wheel there is a need to enhance the sensor antenna earth. This is achieved in each embodiment by means of a spring loaded shim 5 sandwiched between the adaptor and the valve. The earth contact 5 is pressed against the inside surface of the wheel due to the spring loading of its leaf arms and thereby achieves a capacitive contact if the wheel is coated with a non-conductive material or a direct dc contact if no such coating is present.