WO1998023469A1

WO1998023469A1 - A safety arrangement

Info

Publication number: WO1998023469A1
Application number: PCT/SE1997/001985
Authority: WO
Inventors: Mark Willerton
Original assignee: Autoliv Development Ab
Priority date: 1996-11-27
Filing date: 1997-11-26
Publication date: 1998-06-04
Also published as: GB2319871A; GB2319871B; DE19782145T1; GB9624647D0

Abstract

In a safety arrangement for use in a motor vehicle, a capacitor (4, 5, 6, 7) of relatively large capacitance is provided associated with means (2, 3) to charge the capacitor. The capacitor is connected to a circuit (30, 31, 32) through which it may be discharged in order to activate a safety device. The circuit includes switch means (30, 32) adapted to complete the circuit. A further capacitor (8) of small capacitance is provided which can also be charged. The switch means (30) to close the said circuit can only be activated if the further capacitor (8) is charged.

Description

"A SAFETY ARRANGEMENT"

THE PRESENT INVENΗON relates to a safety arrangement for use in a motor vehicle.

Certain safety devices presently provided in motor vehicles are activated by discharging a charged capacitor through a predetermined circuit. The flow of current from the capacitor activates the safety device.

Often the capacitors used for this function have to have a substantial capacitance.

A problem may arise when use of a vehicle in which such an arrangement is provided is terminated. On termination of the use of the vehicle, the or each capacitor will be fully charged. The charge may stay on the or each capacitor for a substantial period of time, bearing in mind the substantial capacitance of each capacitor. It is not practicable to drain away a substantial charge very rapidly, since the rapid discharge of a capacitor of high capacitance through a resistor of low resistance may lead to a current flow which is sufficient to damage the capacitor. This may shorten the life of the capacitor. Consequently there is a risk that, subsequent to termination of the use of the vehicle, because a charge remains on the or each capacitor, the charge may inadvertently be caused to flow through the circuit, thus inadvertently deploying the safety device.

The present invention seeks to provide an improved safety device.

According to this invention there is provided a safety arrangement for use in a motor vehicle, the safety arrangement comprising at least one first capacitor of relatively large capacitance, means to charge the or each first capacitor, the or each first capacitor being connected in a respective circuit through which it can be discharged in order to activate a safety device, the said circuit incorporating means adapted to complete the circuit, the arrangement also comprising a further capacitor of relatively low capacitance, means to charge the further capacitor, the said means to complete the respective circuit only being actuable if the said further capacitor is charged, means different from said respective circuit being provided to discharge the or each first capacitor and means being provided to discharge the further capacitor, the arrangement being such that the further capacitor may be discharged more rapidly than the or each first capacitor.

It is to be understood that the further capacitor may be discharged very rapidly. Once the further capacitor has been discharged, it is no longer possible to actuate the means that are provided to complete the or each respective circuit. Consequently, the or each first capacitor cannot be discharged through the respective circuit. Of course, it is the discharge of the or each first capacitor through the respective circuit that serves to activate the safety device, and consequently, once the further capacitor has been discharged, the safety device cannot be inadvertently deployed. Preferably means are provided to sense the occurrence of an accident, and the said means to complete the or each said respective circuit comprises an activator adapted to activate the safety device and means rendered conductive in response to the accident sensing means.

Conveniently the means to complete the or each respective circuit comprises a transistor, the controlled current path of the transistor forming part of the circuit, the said accident sensor means being adapted to alter the condition of a switch to apply a potential from the further capacitor to the control terminal of the or each transistor, thus to render the transistor conductive.

Conveniently the means to complete the circuit comprise a further switch in series with said transistor switch and said activator.

Preferably the means to discharge the further capacitor are activated when said further switch is activated.

The controlled current path of a conventional transistor is the collector- emitter path, with the control terminal comprising the base. However, it is preferred that the or each transistor is a MOSFET, in which case the control current path is the source-drain path and the control terminal is the gate.

Conveniently the activator comprises a resistor to ignite a pyrotechnic squib. The squib may actuate a gas generator forming part of an air-bag, or may activate a pyrotechnic charge present within the pre-tensioner adapted to pretension a safety belt. Of course, the squib could activate other types of safety arrangement in a motor vehicle. Preferably the means to discharge the or each first capacitor comprises a resistor of relatively high resistance and the means to discharge the further capacitor comprises a resistor of relatively low resistance.

It is to be appreciated that switch means may be provided to complete the circuit incorporating the means to discharge the capacitors in response to termination of operation of the vehicle. Thus, for example, when the ignition of the vehicle is switched off, switch means may be closed which complete the discharge path for the or each first capacitor through the respective resistor of high resistance and which closes the discharge path for the further capacitor through the resistor of low resistance. The further capacitor will thus be swiftly discharged.

Thus, it is to be understood that the said means to discharge the capacitors are activated when use of the vehicle is terminated.

Preferably there are a plurality of said first capacitors, multiplexing means being provided associated with the charging means, the first capacitors and the further capacitor being charged sequentially.

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 drawing in which:

FIGURE 1 is a block and circuit diagram of an arrangement in accordance with the invention, and

FIGURE 2 is a circuit diagram of a modified embodiment of the invention. When a safety device in a motor vehicle is to be activated by means of a resistive squib, it is often desired to store the electric charge necessary to activate the squib on a capacitor. The capacitor must be a capacitor of significant value if it is to store sufficient charge. Preferably, therefore, a catalytic capacitor will be used. When, following a period of operation, i e vehicle is de-activated, for example, by turning off the ignition, it is desired to discharge the capacitor, in order to minimise any risk of inadvertent activation of the safety device.

If a catalytic capacitor is discharged through a low value resistance, the level of current flow may damage the capacitor as a consequence of the heat generated within the capacitor. Consequently, a high value resistance, typically, is utilised to discharge the capacitor, meaning that the capacitor may remain charged for a long period of time. It is possible that the safety device may be inadvertently activated during this period of time.

In the present invention, however, a further capacitor is provided, which is a capacitor of a relatively small capacitance. This enables a non-electrolytic capacitor to be used. This capacitor is used to supply potential to control a transistor switch that serves to complete a circuit incorporating the large value capacitor and the resistor that forms part of the squib. The relatively small value capacitor can be discharged swiftly, when the vehicle is de-activated through a relatively low value resistor. When this capacitor has been discharged, it is no longer possible to activate the safety device.

Referring to the drawing, a DC source 1 supplies power to a DC/DC converter 2 which supplies current through a multiplexing switch 3 sequentially to a plurality of capacitors 4,5, 4 to 9. One terminal of each capacitor is connected to earth. The other terminal of each capacitor is connected to a capacitance measuring circuit 10. A separate capacitance measuring circuit may be provided to measure the capacitance of each capacitor, or a single capacitance measuring circuit may be utilised with an appropriate multiplexing switch.

The capacitance measuring circuit is connected to a reference voltage selector 11. The reference of voltage selector 11 controls a reference voltage generator 12. The reference voltage generator comprises four rails 13,14,15,16 which carry respectively the reference voltages 1.7 volts, 1.85 volts, 2.0 volts and 2.15 volts. The reference voltage generator 12 incorporate six transverse rails 17 to 22.

Switch means are provided which are selectively operable to connect the rails 17 to 20 to the rails 13 to 16. These switches are identified in the drawing by dotted lines 23. The reference voltage selector 11 will selectively close the switches 23, in response to ti e measured capacitance of the capacitors 4 to 7 so t .hat an appropriate reference voltage is applied to each of the rails 17 to 20. The rail 21 is connected to the 2.0 volt rail 15. The rail 22 is connected to the 1.7 volt rail 13.

The rails 17 to 22 are associated with a multiplexing switch 23, so that one rail can be selected. The multiplexing switches 3 and 23 are activated to operate in synchronism.

The multiplexing switch 23 is connected to one input of a comparator 24, the other input of which is connected to a node 25 between a 180k resistor 26 and a 20k resistor 27 which are connected between the multi-plexor switch 3 and earth.

When it is desired to charge capacitors, for example when the arrangement is initially activated, the capacitance measuring circuit 10 will initially determine the capacitance of the capacitor 4. The reference voltage selector will subsequently close a selected switch 23 so that the rail 17 is connected to a selected one of the reference voltage rails 13 to 16. Assuming that the capacitor 4 is relatively new, and thus has a high capacitance, the switch connecting the rail 17 to the rail 13 carrying the reference voltage 1.7 volts will be closed. Alternatively, if the capacitor 4 is very old, and only has a low capacitance, then the switch connecting the rail 17 to the reference voltage rails 16 carrying 2.15 volts would be closed.

Assuming, for the sake of the present description, that the capacitor is relatively new, and the switch connecting the rail 17 to the rail 13 is closed, 1.7 volts would be present on the rail 17. The multiplexing switches 23 and 3 are both located in their upper-most positions as illustrated.

With the apparatus in this condition, the reference voltage of 1.7 volts is passed, by the multiplexing switch 3 to one input of the comparator 24. Initially this is higher than the potential reference at the node 25. The comparator 24 t .hus activates the control circuit 28 which in turn activates the DC DC converter 2.

The DC/DC convenor 2 steps-up the voltage supplied by the DC source 1. The DC source 1 may only be a 12 volt source, but the output of the DC/DC converter 2 may be relatively high, for example, in excess of 20 volts. Current from the DC/DC converter 2 passes to the capacitor 4 and charges up the capacitor. The potential present on the capacitor passes to the resistive bridge 25,27 and the potential present at the node 25 is one-tenth of the voltage present on the capacitor. When the potential present at the node 25 is 1.7 volts, that is to say when the capacitor has been charged to of 17 volts, the comparator provides an output signal to the control 28 which ceases operation of die DC/DC converter 2. The procedure may be repeated with respective capacitors 5, 6 and 7, with the multiplexing switches being moved one position downwards following the charging of each capacitor. The capacitors 8 and 9 may finally be charged, but it is to be appreciated here that the capacitor 8 will be charged to a predetermined potential of 20 volts and the capacitor 9 will be charged to a predetermined potential of 17 volts because the rail 21 is permanently connected to the reference rail 15 and because the rail 22 is permanently connected to the reference rail 13.

The arrangement may be such that the multiplexing operation is carried out with a predetermined rhythm. For example, the capacitance measuring circuit may measure the capacitance of a different capacitor every ten seconds. Once the capacitance has been measured, which takes a very brief period of time, appropriate reference voltages are selected by the selector 11, and the appropriate reference voltage is thus applied to the comparator 24. If the capacitor needs charging, the DC/DC converter will supply current to charge the capacitance.

When all the capacitors have been charged, the cycle of operation is repeated, thus frequently topping up any charge that may have leaked away. Thus, the capacitors are continually recharged.

It is to be appreciated that the capacitors 4 to 7 are relatively large capacitors, whereas the capacitors 8 and 9 are relatively small capacitors.

The capacitors 4 to 7 are each connected in series with a respective field effect transistor 30, a resistance 31 adapted to ignite a pyrotechnic charge forming part of a safety device such as an air-bag, and a controllable current regulator 32 and earth. As will be described hereinafter, should an accident arise, the field effect transistors will be rendered conductive. The current regulator 32 will also be activated so that a predetermined current flows from each capacitor through the respective field effect t .ransistor 30 and resistor 31 and current regulator 32 to earth. It is preferred to utilise a controllable current regulator 32, so that the current can be controlled. If a further field effect transistor were utihsed instead of the current regulator 32, with both the field effect transistors being made conductive in response to an accident arising, the current discharge from the capacitance would be such that the field effect transistors would be damaged. By utilising the arrangement described, it is to be appreciated that the field effect transistor 30 will not be damaged and may be re-used, since the level of current flowing will be controlled by the controllable current regulator.

It is to be observed that the capacitor 8 is connected by means of a switch 35 to the gates of the field effect transistors 30. In a manner that will be described hereinafter, when an accident is detected the switch 35 is closed, and the potential present on the capacitor 8 is applied to the gates of the field effect transistors 30, thus rendering those transistors conductive.

At this stage, it is pointed out that the leads connecting each of the capacitors 4 to 9 to the multiplexing switch 3 are also each individually connected to a respective series connection as resistor 40 and a switch 41, to earth. It is to be appreciated that when the switches 41 are closed, any charge present on the capacitors 4 to 9 will be dissipated to earth through the resistors 40.

Since the charge present on the capacitors 4 to 7 inclusive is a relatively high charge, the resistors 40 through which these capacitors are discharged to earth are selected to have a relatively high value, so that the capacitors 4 to 7 can be discharged to earth relatively slowly, thus not damaging the capacitors. Since the capacitors 8 and 9 are of a relatively low value, the resistors associated with the capacitors 8 and 9 may be of a relatively low value, enabling the capacitors 8 and 9 to be discharged relatively swiftly.

When a vehicle in which t .he safety device is fitted is not in use, the switches 41 will be closed. Thus, at the end of a journey, when the driver removes the ignition key from the vehicle, the switches 41 will be closed. The capacitor 8, being of a relatively low value and being connected to earth through a resistor of relatively low value will be totally discharged in a very brief period of time. Consequently, once the capacitor 8 has been discharged, it will not be possible to apply a potential to the gates of the field effect transistors 30 even if the switch 35 should inadvertently be closed. This means that the risk of accidental deployment of the safety device is minimised even though there will be a substantial charge remaining on the capacitors 4 to 7 inclusive, since it may take several seconds for the charge on these capacitors to drain away to earth.

The capacitor 9 is present to supply an energy back-up for the managing control arrangement 42 that will now be described.

In order to ensure that the safety devices controlled by the squibs 31 are only deployed at an appropriate instant, in the described arrangement, two sensors are provided. One sensor is an accelerometer 43 which senses acceleration of the vehicle in which the safety arrangement is provided, and which generates an output signal when an acceleration in excess of a predetermined acceleration is sensed. The second sensor is a so-called arming sensor which is illustrated as switch 44. The switch 44 is only closed when the arrangement is to be armed. The switch 44 may thus be closed when the ignition of the vehicle is activated. Alternatively, the switch 44 may be closed in response to any movement of the vehicle and may respond, therefore, to rotation of one of the wheels of the vehicle, or to a very sensitive motion sensor.

The accelerometer 43 is connected to a control arrangement 45. When an acceleration in excess of a predetermined minimum is sensed by the accelerometer, the control device 45 closes the switch 35, thus rendering the field effect transistors 30 conductive. The current generators are controlled by a signal from an OR-gate 46. The OR-gate 46 provides an appropriate signal to activate the current regulator 32 in response to one of the inputs of the OR-gate having a "low" signal thereon.

One input 47 to the OR-gate is connected by means of a resistor 48 to a reference voltage 49, and is also connected, by means of the switch 44 to earth. It is thus to be appreciated that when the safing sensor closes the switch 44, the potential that would otherwise be applied to the input 47 of the OR-gate 46 will be brought "low" thus activating the current regulator 32.

The other input 50 to the OR-gate 46 is connected to the output of an AND-gate 51, having two inputs. One input to the AND-gate 51 is an interface 52 connected to die control device 45. The other input to the AND-gate 51 is the output of a comparator 53, the inputs of which comprise a reference voltage 54 and an input 55 which is connected to the capacitOR 8. This part of the circuit becomes operative when it is desired to test the current regulators 32. The current regulator 32 can only be tested when die capacitor 8 has been totally discharged, since otiierwise there is a risk that the safety devices activated by the squib 31 would be inadvertently deployed.

Consequently, when it is desired to test the current regulators 32, initially the capacitor 8 is discharged. When the capacitor 8 has been discharged, die comparator 53 provides an appropriate output which is supplied to the AND- gate 51. Also, when the current regulators are to be tested, an appropriate signal from the control device 55 is passed through the inter-face to the AND-gate 51, and the AND-gate 51 thus provides a low signal on the output 50 which is, of course, an input to the OR-gate 46. The current regulators can, in this situation, be activated.

Figure 2 illustrates a modified embodiment of the invention.

In this embodiment of the invention, for the sake of simplicity of illustration, only one resistor is illustrated which is adapted to ignite a pyrotechnic charge.

Referring to Figure 2 a DC/DC converter 60 is provided, the output of which is connected sequentially, by means of a multiplexing switch 61, to a first capacitor 62, which has a relatively large capacitance, and a further capacitor 63 which has a relatively low capacitance.

The first capacitor 62 is connected in a series connection comprising a field effect transistor switch 62 and a resistor 65 which comprises the activator of a pyrotechnic squib. The source/drain path of the MOSFET 64 is connected in the circuit.

The further capacitor 63 is connected in a series circuit comprising a resistor 66 which has a high resistance and the controlled current path of a further MOSFET switch 67. The source/drain path of the MOSFET 67 is connected in the circuit. The gate of the MOSFET 64 is connected to a node between the resistor 66 and the MOSFET 67. The gate of the MOSFET 67 is connected to a control device 68 which is associated witii an accelerometer 69.

A node between the first capacitor 62 and the MOSFET 64 is connected to earth by means of a resistor 70 which has a relatively high resistance and a switch 71.

A node between the capacitor 63 and the resistor 66 is connected to earth by die series connection of a relatively low value resistor 72 and a switch 73.

When a vehicle in which the arrangement illustrated in Figure 2 is fitted is activated, current from die DC DC converter 60 is passed, by die multiplexing switch 61, sequentially to die first capacitor 62 and die further capacitor 63. If further capacitors, which are not shown in the illustration, are present in die arrangement, then those further capacitors will also be charged sequentially, by appropriate connections to d e multiplexing switch 61. It is to be appreciated tiiat the capacitors 62 and 63 are continually re-charged during subsequent charging cycles following the initial charging cycle.

The MOSFET 67 is maintained in a normally conductive condition. Consequentiy, die node between the MOSFET 67 and d e resistor 66 is maintained at a "low" potential. The charge present on the capacitor 63 will tend to drain away slowly to earth d rough the resistor 66, but since the resistor 66 has a high value, d e charge on the capacitor 63 only drains away slowly. Since the capacitor 63 is recharged regularly, by die operation of die DC/DC converter and die multiplexing switch 61, tiiere is always a substantial charge present on the capacitor 63. Since the node between die resistor 66 and d e MOSFET 67 is kept "low", only a "low" potential is applied to d e gate of die MOSFET 64, meaning tiiat the MOSFET 64 is maintained in a non-conductive condition.

Should an accident arise, die accelerometer 69 will generate a signal. The signal is fed to the control arrangement 68. The control arrangement 68 may comprise an appropriately programmed microprocessor, which analyses d e signal from the accelerometer and responds when die signal from die accelerometer falls within particular predetermined parameters which indicate tiiat an accident is occurring. When an accident is sensed in tiiis way, the MOSFET 67 is rendered non-conductive. The potential of die node between the MOSFET 67 and d e resistor 66 tiius rises rapidly to a potential which is substantially equal to the potential stored on the capacitor 63. This is sufficient to render die MOSFET 64 conductive, and consequentiy the charge present on die capacitor 62 is discharged dirough the resistor 65, tiius activating die squib associated witii d e resistor 65.

When operation of die vehicle in which the safety arrangement described above is terminated, the switches 71 and 73 are both closed. The resistor 72 is of a low resistance, and consequentiy, when d e switch 73 is closed, die capacitor 63 is relatively swiftly discharged. When die capacitor 63 has been discharged, it is no longer possible to render the MOSFET 64 conductive.

When die switch 71 is closed, d e capacitor 62 is discharged through the relatively high resistance resistor 70. Thus the capacitor 62 is discharged at a relatively slow rate, tiius minimising d e risk of any damage being effected to die capacitor 62.

Claims

CLAIMS:

1. A safety arrangement for use in a motor vehicle, the safety arrangement comprising at least one first capacitor of relatively large capacitance, means to charge the or each first capacitor, the or each first capacitor being connected in a respective circuit dirough which it can be discharged in order to activate a safety device, die said circuit incorporating means adapted to complete die circuit, die arrangement also comprising a further capacitor of relatively low capacitance, means to charge the further capacitor, the said means to complete the respective circuit only being actuable if the said further capacitor is charged, means different from said respective circuit being provided to discharge die or each first capacitor and means being provided to discharge d e further capacitor, the arrangement being such that die further capacitor may be discharged more rapidly tiian d e or each first capacitor.

2. An arrangement according to Claim 1 wherein means are provided to sense d e occurrence of an accident, and die said means to complete the or each said respective circuit comprises an activator adapted to activate die safety device and means rendered conductive in response to die accident sensing means.

3. An arrangement according to Claim 2 wherein the means to complete die or each respective circuit comprises a transistor, the controlled current patii of die transistor forming part of the circuit, die said accident sensor means being adapted to alter die condition of a switch to apply a potential from the further capacitor to die control terminal of the or each transistor, thus to render d e transistor conductive.

4. An arrangement according to Claim 3 wherein the means to complete said circuit comprises a further switch in series with said transistor switch and said activator.

5. An arrangement according to Claim 4 wherein the means to discharge die further capacitor are activated when said further switch is activated.

6. An arrangement according to Claim 3, 4 or 5 in which the or each transistor is a MOSFET.

7. An arrangement according to Claim 2, or any Claim dependent tiiereon, wherein the activator comprises a resistor to ignite a pyrotechnic squib.

8. An arrangement according to any one of die preceding Claims wherein the means to discharge die or each first capacitor comprises a resistor of relatively high resistance and d e means to discharge d e further capacitor comprises a resistor of relatively low resistance.

9. An arrangement according to any one of die preceding Claims wherein the said means to discharge die capacitors are activated when use of die vehicle is terminated.

10. An arrangement according to any one of die preceding Claims wherein tiiere are a plurality of said first capacitors, multiplexing means being provided associated witii die said charging means, the first capacitors and die further capacitors being charged sequentially.