THE PRESENT INVENTION relates to a sensor arrangement for detecting a crash in a vehicle and more particularly relates to a sensor arrangement adapted to sense a change in pressure within a sealed or substantially sealed chamber.

A sensor arrangement which senses the pressure within a sealed chamber may have various applications but, in particular, may be useful as a collision sensor or accelerometer in a motor vehicle. A sealed chamber may be located, for example, in the door of a motor vehicle, and in the event that a side impact should arise, the sealed chamber would be compressed, thus causing the pressure within the chamber to rise. Alternatively, a sealed chamber may have one flexible or deformable wall adapted to flex or be deformed as a consequence of movement of an inertia mass, thus causing the pressure in the chamber to vary. A pressure sensor could be provided to measure either the absolute pressure within the chamber or the pressure in the chamber relative to the pressure of the ambient air or the pressure in the chamber relative to the pressure in another chamber. The pressure sensor may be adapted to activate a safety device when the sensed pressure indicates that a side impact has occurred, for example, when the time derivative of the measured pressure exceeds a predetermined threshold. Alternatively, the

pressure sensor may activate the safety device when a deceleration in excess of a predetermined limit is detected.

In a sensor of this type it is preferable for means to be provided to test for the development of any leaks in the chamber. The pressure generated within the chamber in an accident situation will be very different if there is a an unintentional leak as compared to the situation if the chamber is completely air-tight, or only has a small, intentional, aperture.

The present invention relates to a sensor arrangement which incorporates means to test for the presence of an unintentional or unacceptably large leak in the chamber.

According to this invention there is provided a sensor apparatus for sensing a vehicle impact or deceleration of a vehicle, said apparatus comprising a sealed or substantially sealed chamber and a pressure sensor to sense the pressure of air or gas or other fluid within the chamber, means being provided to monitor the pressure within the chamber and to activate an alarm indicative of the fact that there is an unacceptably large leak in the chamber if the measured pressure within the chamber does not fulfil predetermined criteria.

Preferably the sensor is provided with means adapted to operate to change the pressure of air or gas, or other fluid within the chamber, the criteria including the criterion of the change of pressure being at least a predetermined change when the said means are activated.

Conveniently the pressure changing means comprise a heating coil adapted to heat the air or gas or other fluid within the chamber.

Alternatively the pressure changing means comprise means adapted to deform the chamber in a repeatable manner to vary the volume within the chamber.

Conveniently the said means to deform the chamber comprise a solenoid adapted to act to deform one wall of the chamber.

Preferably a core for the solenoid is connected to part of the chamber in order to deform that part of the chamber when the solenoid is activated.

In one embodiment the chamber is provided with a small venting hole.

Preferably the pressure sensor senses the difference between the pressure in the chamber and the pressure of the ambient air.

In one embodiment the predetermined criteria include the criterion that the pressure within the chamber should vary by at least a predetermined amount during a predetermined period of time.

The predetermined period of time may be one day.

Preferably the sensor apparatus is in the form of an accelerometer, said accelerometer comprising a housing defining said chamber, and an inertia mass adapted to move in response to acceleration or deceleration, movement of

the inertia mass causing a change in the volume of the said chamber.

Conveniently there is a pressure sensor in pressure communication with the fluid in the chamber, the pressure sensor being adapted to provide an electric output signal which is a continuous function of the acceleration and/or deceleration acting on the sensor.

Preferably the mass is connected to the housing by means of a deformable diaphragm. The mass and the diaphragm may be formed as a single integrated unit. The mass and the diaphragm may be formed of an elastomer or polymer material. The diaphragm may incorporate a metal with a diffusion barrier characteristic.

Preferably the pressure sensor means are contained within the housing.

Conveniently the diaphragm serves to separate two chambers defined by the housing, the pressure sensor being responsive to the differential pressure between the two chambers.

Advantageously the pressure sensor incorporates a thin partition, fluid from the two chambers being present on opposed respective sides of the partition, the partition being adapted to be deformed in response to differential pressure between the two chambers.

Preferably the partition is made of silicon with one or more Piezo-resistive or Piezo-capacitative elements formed integrally with the partition, said elements being connected to the electric output.

Conveniently the resonance frequency of the moving system constituted by the inertia mass, the diaphragm and the fluid within the two chambers is greater than 400 Hz.

Preferably the oscillation of the moving system comprising the inertia mass, the diaphragm and the fluid within the two chambers is damped by a damping factor greater than 0.2.

Advantageously at least one stopper means is provided to damp and limit oscillation of the said mass. The position of the stopper may be adjustable. The stopper may be in the form of a threaded screw. Two stoppers may be provided, one located on either side of the diaphragm.

In one embodiment two heating elements are provided for heating the air, gas or other fluid, one heating element being located in each of the two said chambers.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIGURE 1 is a diagrammatic view of one embodiment of the invention,

FIGURE 2 is a diagrammatic view of another embodiment of the invention, and

FIGURE 3 is a diagrammatic view of a further embodiment of the invention, and

FIGURE 4 is a sectional view of another embodiment of the invention.

Referring to Figure 1 of the drawings, a chamber 1 is provided which is substantially air-tight. Contained within the chamber 1 is a pressure sensor 2 to determine the pressure within the pressure chamber 1 relative to the pressure of the ambient air, the pressure sensor 2 providing an output signal to a processor 3.

The processor 3 may be adapted to monitor the measured pressure within the chamber over a period of time. It would be anticipated that the pressure within the chamber will vary over a period of time due to various causes. For example, if the chamber is mounted on the door of a motor vehicle it would be expected that in the course of a normal day the temperature of the air would vary. For example, overnight, when the weather is generally cooler than during the day, the air temperature, and thus air pressure, within the chamber 1 would fall. During the day, especially if the vehicle is parked out of doors, the vehicle may be heated by sunshine falling on the vehicle, thus causing the pressure within the chamber to rise. This assumes that the chamber is substantially stiff. However, the chamber must be such that it is compressed in response to a side impact. In the case of a sealed substantially stiff chamber, variations in relative pressure between the interior of the chamber and ambient air pressure could also occur due to natural variations in atmospheric pressure.

Thus the processor 3 may be adapted to provide an appropriate alarm signal (for example, to illuminate a warning lamp on the dashboard of the motor vehicle) if the relative pressure within the chamber 1 remains constant over a relatively long period of time, such as a day.

The apparatus shown in Figure 2 includes a heating coil 4 present within the chamber 1. It is envisaged that in order to increase the reliability of the apparatus means such as the heating coil 4 are provided which, when operated, would tend to increase the pressure within the chamber if the chamber were substantially or completely sealed.

The heating coil, when activated, will serve to heat the gas within the chamber 1 thus causing the pressure of the gas to rise if the chamber 1 is substantially or completely sealed. The processor unit 3 may be adapted to activate the heating coil 4 at appropriate times, and the alarm will be activated if, when the heating coil is operational, the pressure sensor 2 does not sense a rise in pressure. In such a case, the anticipated rise in pressure would be greater and more rapid if the chamber is completely sealed, as compared with the situation if the chamber had a small venting hole. Even with a small venting hole, a discernible change of pressure should occur. The sensor in such a case could measure absolute pressure, i.e. pressure relative to a vacuum.

Figure 3 illustrates a modified embodiment of the invention which does not incorporate a heating coil 1. Instead, in this embodiment of the invention, means are provided to deform the chamber 1 in a repeatable manner, the deformation being such that the pressure within the chamber would be expected to change if the chamber is substantially or completely sealed. As illustrated diagrammatically, one wall of the chamber has a soft metal core 5 attached to it which is received within the winding of solenoid 6. On activation of the solenoid 6 the soft metal core 5 will move, thus deforming the wall of the chamber 1 and changing the volume of the chamber. This

should, if the chamber is substantially or completely sealed, give rise to a change of pressure. It is to be understood that the chamber may have a small venting hole 7.

Again the processor unit would be adapted to activate the solenoid at appropriate times, and to activate the alarm if no change in pressure is detected.

It is to be understood that the chamber 1 is shown in each of the drawings in a schematic manner, and the chamber 1 may have any appropriate configuration and may indeed comprise a plurality of separate chambers in communication through connecting conduits.

Referring now to Fig 4, a f rther form of sensor in accordance with the present invention comprises a housing 11 which defines a cavity 12 which is divided into separate chambers 12A,12B by means of a transversely extending flexible or resilient diaphragm 3. The diaphragm may be formed of an elastomer or of some other polymeric or plastic material. Gas may be able to diffuse through a diaphragm of this type. Thus the diaphragm may incorporate a thin metal foil having diffusion barrier characteristics, to prevent gas diffusion. The metal foil may be bonded to the elastomer or polymeric material, or may be deposited on that material. Formed integrally with the diaphragm, or mounted on the diaphragm, is a mass 14. Thus, in one embodiment, the mass and the diaphragm together form a single integrated unit made principally of an elastomer or polymeric material.

The mass 14, as will become clear from the following description, is intended to act as an inertia mass, the mass moving, under its own inertia, when a

vehicle in which the sensor is mounted accelerates or decelerates. The described accelerometer will, of course, be mounted in the motor vehicle so that when the vehicle accelerates or decelerates the mass 14 will move transversely of the diaphragm 13, thus tending either to reduce the volume of chamber 12A or tending to reduce the volume of chamber 12B.

The housing defines conduits 15,16 which lead to a pressure sensor arrangement 7 provided in the lower part of the housing 11. As can be seen, the lower part of the housing 11 defines two lower cavities 22A,22B, the lower cavity 22A being a communication within the conduit 15 and the lower cavity 22B being in communication with the conduit 16. The lower cavities 22A,22B are separated by a dividing wall 23 having an aperture 24 formed therein. A bridging element 25 incorporating a silicon sensor element is provide within the lower cavity 22A extending across the aperture 24. Part of the bridging element 25 is defined by a flexible thin partition 26 formed of silicon. One side of the partition is thus exposed to the pressure present in the chamber 12A, and the other side of the partition is exposed to the pressure present in the chamber 12B. The partition is deformed by a differential pressure across the partition. A piezo-resistive or piezo-capacitive element 27 is formed integrally with the silicon partition 26, preferably in a region which the silicon partition is of reduced thickness. The piezo-resistive or piezo-capacitive element may be formed within the silicon material by known diffusion and deposition techniques. The operative terminals of the piezo-resistive or piezo- capacitive element 17 are connected to electric output leads 18,19.

It will be understood that each chamber 12A,12B is filled with an appropriate fluid medium. The fluid medium may comprise gas or air or may comprise a hydraulic fluid medium.

In the illustrated embodiment, each chamber 12A,12B, is provided with an electric resistance heating element 20,21 through which electric current may be passed to heat the fluid medium within the respective cavity.

It is to be observed that the chambers 12A and 12B are substantially equal volume, and the illustrated embodiment is substantially symmetric about a substantially vertical line of symmetry.

It is preferred that the resonance frequency of the moving system comprised by the inertia mass 14, the diaphragm 13 and the fluid within the chambers 12A and 12B should be greater than 400 Hz, and preferably the oscillation of the moving system is damped by a damping factor greater than 0.2.

As the vehicle in which an accelerometer as described with reference to Fig. 4 is mounted accelerates or decelerates, the inertia mass 4 will either move towards the right or the left in Fig. 4 thus tending to increase the pressure within chamber 12A, whilst decreasing the pressure within chamber 12B or vice-a-versa, that is say increasing the pressure within chamber 12B whilst decreasing the pressure within chamber 12A. The changes in pressure are caused by the inertia mass moving the diaphragm which effectively changes the volume of each of the chambers. The pressure sensor unit 17 is adapted to sense the changes in pressure and provide an appropriate

continuous electric output signal on the electric leads 8 and 9. The output signal can be analyzed by appropriate circuitry, that circuitry being adapted to activate a safety device, such as a seat belt pre-tensioner or an air bag, should the output signal from the sensor unit have predetermined characteristics.

The heating elements 20 and 21 will raise the temperature of the fluid within the chambers 12A and 12B. If the pressure sensed by the pressure sensor 17 is not within predetermined criteria when the fluid in one or the other of the chambers is heated an alarm may be activated. By altering the power supplied to the two heating elements an oscillation of the mass 4 could be initiated, thus simulating a variable acceleration.

In many instances it may be preferable for the fluid within the chambers to be gas, such as air. Such a fluid has a very low mass and should the fluid leak no pollution problems will arise.

The pressure sensor unit 17 may provide a substantially continuous electric output which is related to the instantaneous acceleration of the vehicle in which the arrangement is mounted.

Each chamber 12A,12B is associated with a stop element 18,19 in the form of a grub screw passing through the wall of the housing 1 defining the respective chamber. Each stop element 28,29 may thus be selectively positioned. The stop members 28 and 29 are positioned to damp and limit the oscillating movement of the inertia mass 4. This can therefore prevent excessive oscillations of the mass 14, which could damage the silicon partition 26, and which could also damage the diaphragm 13.

Whilst the invention has been described with reference to a specific embodiment illustrated in Figure 4 in which the inertia mass is mounted on a diaphragm which separates two chambers, it is to be appreciated that in another embodiment of the invention there may be means defining a single chamber, an inertia mass being provided for movement relative to that chamber to apply pressure to a fluid medium within the chamber. Thus, the mass could be mounted on a diaphragm which is provided at one end of the chamber, the diaphragm effectively separating the interior of the chamber from the exterior of the chamber, or, alternatively, the mass could comprise a piston element located within a cylinder which defines the chamber.

In either event, pressure sensor means would be provided to sense the pressure of the gas or fluid within the chamber, the pressure sensor having an electric output signal which is a function of the pressure acting on the pressure sensor.

The provision of the diaphragm does enable the described accelerometer to operate by measuring differential pressure, thus avoiding any problems that might arise due to changes in absolute pressure, for example if the vehicle ascends to a significant height in a mountainous region.