GB2238662A - Crash sensors - Google Patents

Abstract

A sensor for initiating air bag inflation during automobile crashes includes a hermetically sealed container 2 containing a fluid for controlling the movement of an inertia mass 6. If the crash is of sufficient severity and duration, the inertia mass 6 moves to close a pair of contacts 14 and 15 and thus cause air bag inflation. The hermetic sealing leaves the operating characteristics of the sensor unchanged despite changes in ambient atmospheric pressure. The fluid may be air or other gases such as helium of helium mixed with air to achieve a desirable operating characteristic. <IMAGE>

Description

Ps CE, "Improvements in and relating to hermetically sealed damped

sensors" The present invention relates to a method and apparatus for hermetically sealing a sensor, especially a sensor which is used to activate an air bag in an automobile, and to a hermetically sealed sensor.

Many sensors for activating air bags have been described in the prior art. However, even when these sensors have been sealed through the use of epoxy and O-ring seals, it has. been impossible to prevent the penetration of moisture into the sensors. Also, a non-hermetic seal will permit gas to flow into and out of the sensor.

It has also been possible for water to enter a sensor thorugh a leaking connector in the system or to enter the stranded wires that lead into the sensor if the insulation on one of those wires becomes cut. In that case, of course, the epoxy or O-ring seal would be bypassed.

Water entering a sensor, if it contains salt or other ions, may affect the resistance of any diagnostic resistor wihin, thus causing the possibility of a failure indication. on the other hand, if the water does not contain ions sufficient to short out the resistor, or if the failure indication is ignored, water may penetrate to the inside of the sensor where it may cause the sensor to malfunction.

A less obvious consequence of non-hermetic sealing of the sensor is that gases entering therein can have a very 1 significant effect on the response characteristics of the sensor. Even the ambient air pressure can have a significant effect on the response characteristics. If the vehicle is operated for prolonged periods of time at high altitudes, the sensor will become significantly more sensitive to pulses of acceleration of short duration, and thus more likely to cause air bag deployment in a crash where that deployment is not desired.

The invention provides a crash sensor comprising: a hermetically sealed container; a movable mass mounted within the container; a fluid disposed in the container for controlling the movement of the mass; and a pair of electrical contacts cooperating with the movable mass to indicate a crash.

The invention also provides a method of constructing a sensor, comprising the steps of: providing a container; mounting a gas-damped sensor within the container; providing a gas within the container; and sealing the container.

The invention further provides a method and apparatus directed to a hermetically sealed sensor for activating an air bag in the event of an automobile crash. The apparatus comprises a cylindrical housing, which can be hermetically sealed around the other components by such means as soldering, a diagnostic resistor for indicating whether the sensor is in operating condition, a movable ball contained within a cylinder and rubber seal, a pair of contacts which are normally spaced apart and contained within a magnet which itself is contained within a cylindrical housing which fits tightly within the aforementioned outer cylindrical housing, and a pair of terminal pins connected external to the outer cylindrical housing. The method of operation is such that a pressure differential on the ball causes it to impinge on one of the contacts forcing that contact to impinge in turn on the other contact, thereby establishing an electrical connection to the terminal pins which activate the automobile air bag.

One form of sensor constructed in accordance with the invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is an isometric view of a sensor; Figure 2 is an exploded perspective view of the components of the sensor shown in Figure 1; Figure 3 is a fragmentary cross-section taken along the line 3-3 of Figure 1; Figures 4 to 14 are graphs of the response characteristics of a sensor under various conditions.

Referring to the accompanying drawings, and initially to Figures 1 to 3, one form of hermetically sealed sensor 1 comprises a cylindrical can 2 which has fitted within it a cylindrical resistor holder 3. The resistor holder 3 has mounted therein a diagnostic resistor 4 which gives a warning indication when its resistance changes. A curved washer 5 is disposed adjacent to the resistor 4.

A cylinder 7 with an external rubber seal 8 fits within a 1 pair of axial projections 9 and 10 on the resistor holder 3. A ball 6 is disposed within the cylinder 7. The ball 6 and the cylinder 7 are dimensioned so that there is a very small space therebetween to create a dash-pot effect. The cylindrical resistor holder 3 is matingly connected to a cylinder housing 11. A magnet 10 has a central aperture 12 which fits matingly around an axial projection 13 of the cylinder housing 11. Also mounted within the cylinder housing 11 are a pair of contacts 14 and 15 that are spaced apart under normal, non-crash circumstances. A cylindrical head 16 adjacent to the magnet 10 has apertures 17 and 18 for holding terminal pins 19 and 20. The pins 19 and 20 are in electrical contact with the contacts 14 and 15, and are secured to the head 16 by a compression seal 21 made, for example, from glass. The components are assembled as shown in Figure 2 and the head 16 is secured to the can 2 by soldering or other similar means to form a hermetically sealed enclosure.

The operation of the sensor is as follows. When a crash occurs, inertia causes the ball 6 to move towards the contacts. The movement of the ball is impeded by the gas within the sensor. When the ball 6 impinges on one contact 14, that in turn impinges on the other contact 15. That completes an electrical circuit between the output terminal pins 19 and 20, causing a signal at the pins. If the electrical signal is of sufficient duration, air bag inflation will be initiated.

A primary advantage of hermetic sealing which is particular -1 to an air-damped sensor such as is shown in Figures 1 to 3 is the fact that the pressure, as a function of time and temperature, will be known. In an air damped sensor the response characteristics of the sensor depend on the pressure of the gas in the sensor, as illustrated in Figures 4 to 14. An unexpected advantage of a hermetically sealed sensor is that the static pressue in the sensor can be increased so as to permit the use of shorter ball travel and thus small sensors for some applications. Thus, for an application such as the one shown in Figures 8 and 9 wherein the sensor is required to operate when the velocity change exceeds 17 miles per hour (7.5 m/s) during a crash, a pressure of 15 pounds per square inch (100kPa) absolute, as shown in Figure 8, yields a firing of the air bag for a 2 millisecond pulse at 12 miles per hour (5.5 m/s) even though the sensor was designed to fire only for 17 miles per hour (7.5 m/s) velocity change for a 25 millisecond pulse. By increasing the static pressure in the sensor, the response curve can be raised at lower speeds as shown in Figure 9, where the pressure inside the sensor is 20 psi (133 kPa) absolute. Still another unexpected advantage of a hermetically sealed sensor is that, when the pressure in the sensor is not known as would be the case for a non-hermetically sealed sensor operating at different altitudes, it is not possible to compensate for the performance of the sensor over varying temperature ranges. Therefore, a further advantage of the present invention is that it permits the compensation of 1 temperature effects to be optimized since the pressure is known.

Yet another unexpected significant advantage of hermetically sealed sensors is that they permit the control of the density of the gas inside the sensor to achieve particular desired effects. Figures 4 to 7 show a nominal 14 kilometers per hour sensor (that is to say, a sensor designed to sense a crash only at a speed of 14 km/h or more) in the crush zone of a motor vehicle with different internal pressures. The fluid used in this sensor is air. The pressure inside the sensor is 10 psi (67kPa) absolute in Figure 4, 20psi (133kPa) in Figure 5, 30psi (200kPa) in Figure 6, and 60psi (400 kPa) in Figure 7. As the pressure in the sensor varies, the lefthand portion of the curve (corresponding to lower crash speeds) changes significantly while the right end of the curve stays approximately the same. That is because for short duration pulses inertial or turbulent effects begin to arise in the clearance between the ball and the cylinder. When the flow between the ball and cylinder is controlled by the viscosity of the gas, the curve will be approximately a straight line indicating that the sensor is operating an an integrator for the acceleration. As the velocity of gas increases, or more precisely as the Reynolds number of the flow of the gas past the ball increases, the main resistance to fluid flow comes from accelerating the gas. This is a phenomemon dependent on the square of the velocity, as opposed to the viscous or laminal case in which the resis- f 1 1 tance is proportional to the velocity. Turbulent, nonlaminal flow causes the curve to rise for short duration pulses. The curve eventually turns down again for very short pulses because the gas behind the ball expands and acts like a spring.

The effect described above can be increased or decreased by changing the density of the gas in the cylinder. The density, in turn, can be changed by changing the type of gas in the sensor or by changing the pressure. Figures 10 to 14 show graphs for the response characteristics of a nominal 14 km/h sensor identical to that of Figures 1 to 3 except that helium has been substaituted for air. Since helium has approximately 1/7 the density of air, the inertial effects are much less significant and, in fact, are only marginally in evidence when the static pressure is 30 psi (200 kPa) absolute, as shown in Figure 13, and show a significant effect when the pressure is 60 psi (400 kPa) as shown in Figure 14. The pressures in Figures 10 to 12 are 10, 14.7, and 20 psi (67, 100 and 133 kPa) absolute, respectively. Thus, using helium alone or mixing helium with air or the use of other gases provides a valuable design parameter to permit the tailoring of the sensor response curb to meet particular requirements. Such a desirable requirement might, for example, be having the response curve turn up for short duration pulses to give immunity to hammer blows.

Claims (13)

1. A crash sensor comprising: a hermetically sealed container; a movable mass mounted within the container; a fluid disposed in the container for controlling the movement of the mass; and a pair of electrical contacts cooperating with the movable mass to indicate a crash.

2. A sensor as claimed in claim 1, comprising a diagnostic resistor mounted within the container and functioning when its resistance changes to give an indication of sensor malfunction.

3. A sensor as claimed in claim I or claim 2, comprising a cylinder mounted within the container to provide a path for movement of the movable mass during a crash.

4. A method of constructing a sensor, comprising the steps of: providing a container; mounting a gas-damped sensor within the container; providing a gas within the container; and sealing the container.

5. A method as claimed in claim 4, wherein the said ggas consists essentially of air, helium or a mixture thereof.

6. A method as claimed in claim 4 or claim 5, wherein the gas is provided a preselected pressure.

7. A method as claimed in claim 6, wherein the said gas is at a pressure of from 67 to 400 kPa.

8. A sensor made by a method as claimed in any one of claims 4 to 7.

9. A sensor as claimed both in claim 8 and in any one of claims 1 to 3.

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10. A sensor as claimed in any one of claims 1 to 3, or claim 8, or claim 9, arranged in use to be disposed in an automobile to initiate air-bag inflation in a crash.

11. A crash sensor substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

12. An airbag system for a motor vehicle, including a sensor as claimed in any one of claims 1 to 3 or in any one of claims 8 to 11.

13. A vehicle equipped with an air-bag system as claimed in claim 12.

Published 1991 at The Patent Office. State House. 66/71 High Holborn. London WCIR47P. Further copies rnay be obtained frorn Sales Branch, Unit 6. Nine Mile Point. Cwrillelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray. Kent.

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