GB2284482A

GB2284482A - Improvements in or relating to an accelerometer

Info

Publication number: GB2284482A
Application number: GB9324714A
Authority: GB
Inventors: Denis Bourcart
Original assignee: Autoliv Klippan SNC
Current assignee: Autoliv Klippan SNC
Priority date: 1993-12-02
Filing date: 1993-12-02
Publication date: 1995-06-07
Also published as: GB9324714D0

Abstract

A crash sensor comprises a housing (1) containing a mass (12) carrying a magnet (13), the mass being movable, by inertia, along a guide rod (6) against the biassing effect of a spring (9). A Hall effect sensor (15) is provided in the housing and is subjected to such a magnetic field, when the mass has moved to a predetermined position, that the output of the Hall effect transistor 15 is sufficient to activate a discriminator or Schmitt trigger (23) the output of which drives a push-pull output stage (25) to supply current to the squib of an air-bag or a trigger mechanism for a safety-belt pretensioner. Provision for checking the functions of the sensor device and circuitry is provided. <IMAGE>

Description

DESCRIPTION OF INVENTION "IMPROVEMENTS IN OR RELATING TO AN ACCELLERONETER" THE PRESENT INVENTION relates to an accelerometer and more particularly relates to an accelerometer adapted to detect a crash or accident in a motor vehicle.

It has been proposed to provide a crash sensor to detect a crash or accident in a motor vehicle, the sensor being adapted to activate a safety device such as an airbag or a seat-belt pretensioner.

Various types of accelerometer have been proposed before including accelerometers responsive to deformation of the vehicle and accelerometers responsive to a deceleration of a vehicle in excess of a predetermined limit.

Some prior proposed accelerometers have incorporated a mass held in an initial position by a spring, the mass being intended to move, against the bias provided by the spring, in the event that an accident should occur. The mass may be caused to activate a mechanical switch when it has moved, against the bias of the spring, to a predetermined position.

Two problems arise from an arrangement of this type. Firstly, the mass is to activate the mechanical switch at the limit of its movement. The mechanical switch may be rather "stiff" to activate, particularly when it is noted that the switch only operates when an accident occurs. Consequently the switch may not be operated during a period of several years. Thus, actuation of the switch may be unpredictable and erratic. This is clearly undesirable.

A further disadvantage that occurs when the switch is a mechanical switch, is that it is not possible to check the circuit in which the switch is located without closing the switch, and when the switch is closed the safety device incorporated in the circuit is activated.

The present invention seeks to provide an improved accelerometer.

According to this invention there is provided an accelerometer for use in a motor vehicle adapted to respond to a deceleration in excess of a predetermined deceleration, the accelerometer comprising a movable mass movable in response to said deceleration, a magnet movable in response to movement of said movable mass, and a Hall effect sensor located to be subjected to a magnetic field from said magnet.

Preferably the controlled current output path of the Hall effect sensor is connected to a signal evaluation or processing circuit.

Conveniently the signal evaluation or processing circuit comprises a discriminator, the output of which drives an output stage adapted to activate a safety device.

Conveniently the discriminator is a Schmitt trigger.

Advantageously the output of the discriminator is fed to an output stage comprising a push-pull amplifier.

Preferably means are provided to inhibit operation of the device in response to a signal from diagnostic circuitry.

Advantageously the inhibiting means comprise an amplifier connected in series with the discriminator.

Conveniently the power supply for the Hall effect sensor is provided with a temperature compensating circuit.

Preferably the mass is movable along a predetermined path against the bias of a spring.

Advantageously the mass is of annular form, and is guided on a guide rod, the spring being a helical spring surround the guide rod.

Conveniently the mass is of annular form, and the guide rod extends between two ends of a closed cylindrical housing, the mass being movable axially within the said housing, the housing defining a recess containing the Hall effect sensor.

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 cross-sectional view of an accelerometer comprising a housing containing a mass movable in response to a predetermined deceleration, and FIGURE 2 is a block circuit diagram of a circuit incorporating the accelerometer of Figure 1.

Referring initially to Figure 1 an accelerometer comprises a housing 1 constituted by a substantially cylindrical tubular housing element 2 having one end closed by a transverse wall 3, and having a cover 4 releasably mounted on the other end.

The closed end 3 of the housing defines an axial recess 5 which receives one end of a guide rod 6, the other end of the guide rod being received in a corresponding recess formed in the cover 4.

The cover 4 defines an annular recess 8 on its inner surface surrounding the bore 7, that recess receiving one end of a helical spring 9. The other end of the helical spring is received within a corresponding recess 10 formed in an annular element 12 which surrounds the guide rod 6. The annular element is made of metal or some other material having a significant mass. The annular element 12 carries an annular magnet 13.

The annular element 12 may move axially of the housing against the bias of the spring 9, being guided, by the guide rod 6, to move along a predetermined path.

The cylindrical tubular housing element 2 defines a recess 14 at a position remote from the closed end 3 thereof, that recess containing a Hall effect transistor 15. The Hall effect transistor is thus close to the space within the housing 1.

It is to be appreciated that the housing 1 is intended to be mounted in a motor vehicle with the guide rod 6 aligned with the axis of the motor vehicle, and with the cover 4 located towards the front of the vehicle. The spring 9 initially retains the annular element 12 adjacent the closed end 3 of the housing 1.

Should the vehicle be subjected to deceleration, the annular element 12 will tend to move, by its own inertia, and against the bias of the spring 9, towards the cover 4. When the vehicle is subjected to a deceleration in excess of a predetermined limit, the annular element 12 be moved to such a position that the magnet 13 will be located immediately adjacent the Hall effect transistor 15.

It is to be understood that a Hall effect transistor has the property that the current flowing through the controlled current path of the transistor is related, amongst other things, to the strength of the magnetic field to which the transistor is subjected. Thus, if the transistor is only subjected to a very weak magnetic field, such as the field that would exist in the vicinity of the recess 14 when the annular member 12 and the associated magnet 13 is pressed against the closed end 3 of the tubular housing element 2, the current that can flow through the transistor has a relatively low value.

However, as the mass 12 moves towards the cover 4, the intensity of the magnetic field present in the recess 14 will increase, and consequently the current flowing through the Hall effect transistor 15 will increase.

Figure 2 is a block diagram illustrating an electric circuit which incorporates the Hall effect transistor 15.

A power supply on terminals 18,19 is connected, through a thermal compensation circuit 20 to the Hall effect transistor 15. The controlled current output 21 of the Hall effect transistor 15 is fed to an amplifier 22 adapted to amplify the signal. The output of the amplifier 22 is fed to an input of a discrimination in the form of a Schmitt trigger 23. The Schmitt trigger 23 only provides an output when the input exceeds a predetermined limit.

The output of the Schmitt trigger 23 is fed through an output amplifier 24 to a push-pull output drive 25 having two outputs 26,27 which can be connected to supply power to an appropriate safety device. Thus the outputs 26,27 may be connected to an igniting squib of a gas generator forming part of a an air-bag or an igniting squib of a pyrotechnic device forming part of a safety-belt pretensioner, for example.

A further diagnostic amplifier 28 is provided having an input connected to the input of the Schmitt trigger 23 and an output connected to the output of the Schmitt trigger 23. A control input 29 of the diagnostic amplifier 28 is connected, by means of a terminal 30, to diagnostic circuitry.

It is to be appreciated that the power supply, through the terminals 18 and 19 may be temperature compensated by the temperature compensating circuits 20.

This compensation may counteract any variation of performance of the Hall effect transistor due to temperature and may also compensate for any variation in operation of the amplifiers caused by changes in temperature.

It is to be appreciated that when the mass 12 and the magnet 13 are in the initial position illustrated in Figure 1, whilst a certain limited current flows through the controlled current output 21 of the Hall effect transistor 15, that current, even when amplified by the amplifier 22, is not sufficient to trigger the Schmitt trigger 23. However, the current is sufficient to enable the circuit to be checked.

When the mass and the magnet 13 have moved by a predetermined distance towards the cover 4, in response to a predetermined deceleration (bearing in mind that the mass moves against the bias provided by the spring 9) the effect of the magnetic field from the magnet 13 on the Hall effect transistor 15 is such that the output current present on the controlled current output 21 of the Hall effect transistor is, when amplified by the amplifier 22, sufficient to trigger the Schmitt trigger 23. When the Schmitt trigger 23 has been triggered the output thereof is amplified by the output amplifier 24, and this amplified output is fed to the push-pull output drive, and as a consequence a current flows between the terminals 26 and 27, that current being sufficient to ignite the associated squib or pyrotechnic device.

The diagnostic amplifier 28 may be activated to inhibit operation of the output amplifier 24 should an appropriate signal be present at the terminals 30 connected to the diagnostic circuitry. Thus, the diagnostic circuitry may be designed to detect any problem in the squib line, connected between the terminals 26 and 27, and to inhibit operation of the output if a malfunction is detected.

It is to be appreciated that using a device as described above, there is no mechanical contact between the mass 12 and the switch. Thus there is no risk of any problem arising because of "stiffness" in a mechanical switch.

A Hall effect transistor is a robust element and will thus remain operational for the entire life of a motor vehicle.

It is possible to check the operation of the entire circuit, using appropriate diagnostic terminals, since a small current will flow through the Hall effect transistor even when the mass 12 and the associated magnet 13 are located immediately adjacent the closed end 3 of the housing element 2.

It is to be appreciated that if sensors have to be provided for different motor vehicle with different sensitivities, this can easily be accomplished simply by changing the operating parameters of the Schmitt trigger 23. Thus different devices may easily be manufactured, to the same basic design, which trigger at very different decelerations. Of course, if necessary, a mass of a different weight may be used and/or a spring of a different strength may be used within the housing 1 to provide alternative products with different operational sensitivities.

Various modifications may be effected without departing from the scope of the invention. For example, circuitry may be incorporated, in association with the Schmitt trigger, to determine the length of the output pulse from the amplifier 22. If the pulse is only a very short pulse, such as can arise from heavy breaking, the Schmitt trigger would not be triggered, whereas if the pulse had the longer duration, such as would be experienced in an accident situation, then the Schmitt trigger would be triggered.

It is to be appreciated that in a more complex arrangement the output from the amplifier 22 may be fed to different Schmitt triggers, with different sensitivities, each Schmitt trigger being adapted to drive an output stage controlling the actuation of the different safety device.

Thus, if a deceleration in excess of a first predetermined deceleration is detected, one air-bag may be inflated, or an air-bag may be half inflated, whereas if a greater deceleration is detected, a second air-bag may be inflated, or the first air-bag may be fully inflated.

It is to be appreciated that many different types of discrimination circuit may be used with the Hall effect sensor, and indeed one type of discriminator may provide an output which is directly indicative of acceleration or deceleration. An evaluation or signal processing device could evaluate or process the signal, for example by integrating the signal or differentiating the signal.

In this Specification the term Hall effect sensor is used to cover any sensor having a controlled current path such that the permissible current flow is related to the strength of a magnetic field to which the sensor is subjected.

Claims

CLAIMS:

1. An accelerometer for use in a motor vehicle adapted to respond to a deceleration in excess of a predetermined deceleration, the accelerometer comprising a movable mass movable in response to said deceleration, a magnet movable in response to movement of said movable mass, and a Hall effect sensor located to be subjected to a magnetic field from said magnet.

2. An accelerometer according to Claim 1 wherein the controlled current output path of the Hall effect sensor is connected to a signal evaluation or processing circuit, the output of which drives an output stage adapted to activate a safety device.

3. An accelerometer according to Claim 2 wherein the signal evaluation or processing circuit comprises a discriminator.

4. An accelerometer according to Claim 3 wherein the discriminator is a Schmitt trigger.

5. An accelerometer according to Claim 3 or 4 wherein the output of the discriminator is fed to an output stage comprising a push-pull amplifier.

6. An accelerometer according to any one of the preceding Claims wherein means are provided to inhibit operation of the device in response to a signal from diagnostic circuitry.

7. An accelerometer according to Claim 6 as dependent upon Claim 3 wherein the inhibiting means comprise an amplifier connected in series with the discriminator.

8. An accelerometer according to any one of the preceding Claims wherein the power supply for the Hall effect sensor is provided with a temperature compensating circuit.

9. An accelerometer according to any one of the preceding Claims wherein the mass is movable along a predetermined path against the bias of a spring.

10. An accelerometer according to Claim 9 wherein the mass is of annular form, and is guided on a guide rod, the spring being a helical spring surrounding the guide rod.

11. An accelerometer according to Claim 10 wherein the mass is of annular form, and the guide rod extends between two ends of a closed cylindrical housing, the mass being movable axially within the said housing, the housing defining a recess containing the Hall effect sensor.

12. An accelerometer substantially as herein described with reference to and as shown in the accompanying drawings.

13. Any novel feature or combination of features disclosed herein.