GB2436617A

GB2436617A - Acceleration sensor

Info

Publication number: GB2436617A
Application number: GB0606272A
Authority: GB
Inventors: Stephen Anthony George Ruff; Alan David Crozier
Original assignee: Martin Baker Aircraft Co Ltd
Current assignee: Martin Baker Aircraft Co Ltd
Priority date: 2006-03-29
Filing date: 2006-03-29
Publication date: 2007-10-03
Also published as: DE602007011516D1; GB2436706A; GB0705663D0; GB2436706B; ATE493663T1; GB0606272D0

Abstract

A vehicular acceleration sensor comprising an inertia weight 30. A holding mechanism may be comprised to hold the inertia weight 30 and which is movable by the inertia weight 30 in a generally linear direction. The holding mechanism may comprise a pair of lever arms 26, operable to move independently of one another, each lever arm 26 having a cup portion 28, which cup portions 28 face one another to hold the inertia weight 30 there between. The acceleration sensor further may comprise a trip mechanism including a circular trip plate 40 to convert linear movement of a part of the holding mechanism into a rotational movement of the trip mechanism, which rotational movement comprises an output of the sensor. In a preferred embodiment the rotational movement trips a trigger 47 to cause locking of a spool 3.

Description

I 2436617

AN ACCELERATION SENSOR

This invention relates to an acceleration sensor and more particularly to an acceleration sensor to sense vehicle acceleration and provide an output upon a vehicle experiencing an acceleration exceeding a threshold acceleration.

Vehicular acceleration sensors for providing an output when an acceleration threshold has been exceeded are known, especially for sensing an acceleration along a particular axis, i.e. in a particular direction. Some attempts have been made to provide so-called omni-drectional acceleration sensors which provide an output when an acceleration threshold is exceeded in any direction. Such acceleration sensors are mechanical devices including an inertia weight which somehow triggers an output, usually a mechanical output, when the inertia weight moves with respect to the remainder of the mechanics of the sensor causing the mechanical output to move in a particular direction.

EP077531 7 discloses a multi-directional acceleration sensor which provides a mechanical output along an axis, i.e. the mechanical output of the sensor is provided in a particular direction. Since the mechanical output comprises a mechanical element moving in a particular direction, along a particular axis, that element itself will experience acceleration or deceleration along that axis.

The acceleration experienced by the mechanical element effecting the mechanical output inevitably leads to the overall mechanism being more sensitive or less sensitive to accelerations or decelerations along the axis of the mechanical output. For example, if the mechanical output is along the x-axis and the sensor experiences a violent deceleration along the x-axis, then the mechanical element providing the mechanical output along the x-axis will experience that deceleration as well as the inertia weight thereby making the sensor more sensitive to decelerations along that x-axis. If an acceleration along the x-axis is provided in the opposite direction to which the mechanical element must move to provide its output, then it will be apparent that the sensor is less sensitive to such an acceleration. Such an acceleration sensor cannot therefore be regarded as truly omni-directional where the output is provided along a particular axis or in a particular direction.

This potential insensitivity or over-sensitivity to acceleration in a particular direction is called a "cross-talk" error for ease of reference. Cross-talk error occurs when an acceleration or a component of acceleration along a particular axis is aligned with a linear movement of a mechanical element in the trip mechanism or output mechanism of the acceleration sensor.

It is an object of the present invention to seek to provide an acceleration sensor which is omni-directional and which is not reliant on a mechanical output being provided along a particular axis.

One aspect of the invention provides a vehicular acceleration sensor comprising: an inertia weight; a holding mechanism to hold the inertia weight and which is movable by the inertia weight in a generally linear direction; a trip mechanism to convert linear movement of a part of the holding mechanism into a rotational movement of the trip mechanism, which rotational movement comprises an output of the sensor.

Preferably, the holding mechanism comprises a pair of lever arms operable to move independently of one another, movement of the inertia weight, held between the lever arms, when under acceleration, causing one or both arms to move in the plane.

Conveniently, the pair of lever arms each has a cup portion, which cup portions face one another hold the inertia weight therebetween.

Advantageously, the cup portions have substantially conical internal surfaces.

Enter Date & Draft No. Preferably, the trip mechanism has an axis of rotation and the holding mechanism is movable in a plane, the axis of rotation lying in the plane.

Conveniently, the inertia weight comprises a ball.

Advantageously, the inertia weight comprises a non-spherical shaped ball, having one axis of rotational symmetry.

Preferably, the inertia weight comprises a ball with an equatorial bulge.

Conveniently, the inertia weight comprises a ball with a pair of opposed substantially conical protrusions.

Another aspect of the invention provides an acceleration sensor comprising a part of an inertia reel having a spool.

Preferably, at least a part of the sensor is housed in a part of the spool.

Advantageously, the majority of the sensor is housed in the spool.

Conveniently, the reel also includes a strap acceleration sensor and the vehicular acceleration sensor is at least partly housed in the strap acceleration sensor.

Preferably, the extent of rotational movement is indicative of an acceleration experienced by the inertia weight.

Advantageously, the rotational movement trips a trigger to cause locking of the spool.

Conveniently, the trip mechanism is rotatably mounted to a cam mechanism which holds the trigger.

Enter Date & Draft No. Preferably, the trip mechanism is biased into a position by an elastic member interacting between the cam mechanism and the trip mechanism, the biasing force exerted by the elastic member being overcome by rotation of the trip mechanism under vehicular acceleration above a predetermined threshold.

A further aspect of the invention provides an acceleration sensor comprising an inertia weight located toward one end of a lever arm which is movable in a generally linear direction in response to movement of the inertia weight, movement of the inertia weight or a part of the lever arm adjacent the inertia weight in a generally linear direction being converted to a rotational movement of a trip mechanism as an output of the sensor.

Another aspect of the invention provides an acceleration sensor having an inertia weight and a rotational output, wherein the extent of rotation is indicative of an acceleration experienced by the inertia weight.

A further aspect of the invention provides an inertia reel having a vehicular acceleration sensor according to any preceding claim.

Preferably, the reel further comprises a strap acceleration sensor.

Conveniently, there is a common trigger tripped by actuation of either the vehicular acceleration sensor or the strap acceleration sensor.

Another aspect of the invention provides a method of sensing a vehicular acceleration above a predetermined threshold comprising: holding an inertia weight movable under acceleration; translating movement of the inertia weight into a substantially linear movement; and converting the substantially linear movement into a rotational movement of a trip mechanism, wherein the extent of rotational movement is indicative of an acceleration experienced by the inertia weight.

Enter Date & Draft No. In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an inertia reel incorporating a sensor embodying the present invention; Figure 2 is a cross section through the inertia reel of Figure 1; Figure 3 is an exploded view of the inertia reel of Figure 1; Figure 4 is a partial perspective view of a strap acceleration sub-assembly for use with the reel of Figure 1; Figure 5 is a perspective view of a sub-assembly of an acceleration sensor embodying the present invention.

Figure 6 is a perspective view of the sub-assembly of Figure 3 together with a trip plate; Figures 7, 8 and 9 are perspective views showing the operation of the trip plate of Figure 6; Figures 10 and 11 show detail of a cam plate, pawl and trigger for use with the acceleration sensor embodying the present invention; Figures 12A and 12B show the cam plate, pawl and trigger of Figures 10 and 11 when reset, prior to triggering; Figures 13A and 13B show the cam plate, pawl and trigger of Figures 10 and 11 after triggering; Enter Date & Draft No. Figures 14 and 15 show manual operation of the cam plate of Figures 10 and 11.

Figure 16 is a cross-section through a sensor sub-assembly embodying the present invention experiencing an on-axis acceleration; Figure 17 is a cross-section through the sub-assembly of Figure 16 when experiencing an off-axis acceleration; Figure 18 is a sensor sub-assembly embodying another aspect of the present invention; and Figure 19 is the sensor sub-assembly of Figure 18 experiencing an off-axis acceleration.

Referring to the figures, an acceleration sensor embodying the present invention will be described in the context of an inertia reel for a seat, the acceleration sensor embodying the present invention sensing vehicle acceleration or deceleration and providing an output in response to the acceleration or deceleration exceeding a threshold acceleration/deceleration.

The inertia reel senses vehicle acceleration and the output triggers a mechanism to brake the reel and stop any further strap from being paid out by the inertia reel spool. The inertia reel also includes a separate strap acceleration sensor to brake the spool when the strap wound around the spool undergoes a strap acceleration above a pre-determined threshold.

Referring now to Figure 1, the inertia reel 1, comprises a main body 2, for accommodating a spool 3 and the sensor arrangements 4. A webbing strap (not shown) is intended to be inserted in a slot 5 in the spool 3 and wound around the spool 3. Usually, more than one inertia reel will be used on each Enter Date & Draft No. seat so that the webbing straps combine to form a harness arrangement for the occupant of the seat.

Referring now to Figures 2 and 3, the spool 3 is inserted in the body 2 and journalled thereto by bearings 3A at either end. One end of the spool 3 engages a torsion spring 6 held within the body 2 in a cut-out formed at one end of the spool 3. The torsion spring 6 serves to bias the spool to retract the webbing strap wound around the spool 3. The torsion spring 6 is held within the body by a capping seal 7 and a cover 8.

Turning now to Figure 4, the other end of the spool 3 terminates in a cylindrical wall 9 in which are provided three equispaced holes 10, in each of which sits a respective ball bearing II, for engaging with a cylindrical sleeve sitting coaxially inside the cylindrical wall 9. The cylindrical wall 9 of the spool 3 has a free edge into which are machined a variety of circumferentially arranged outwardly projecting teeth having rounded edges which form a ratchet wheel 12.

A cylindrical inertia weight 13 (comprising the aforementioned cylindrical sleeve), sits within the cylindrical wall 9, and is machined with three equispaced helically shaped grooves 14 in its outer surface which each, together with a corresponding hole 10, hold the ball bearings 11. Another torsion spring 15 sits around the inertia weight 13 and biases the inertia weight 13 with respect to the cylindrical walt 9 of the spool 3 from relative movement so that the inertia weight 13 is urged axially inside the cylindrical wall 9. It is the torsion force of the torsion spring 15 which the inertia weight 13 must overcome when under strap acceleration and it is thus the torsion spring 15 which, inter a/ia, determines the threshold strap acceleration which will cause braking of the spool 3. Under excessive strap acceleration (significant enough to overcome the predetermined strength of the torsion spring 15), the inertial weight 13 dwells with respect to the spool 3. The inertia weight 13 rotates with respect to the spool 3 and the ball bearings 11 Enter Date & Draft No. (seated between the helical grooves 14 and the holes 10 in the cylindrical wall 9) drive the inertia weight 13 axially outwardly of the cylindrical wall 9 so as to close a gap between the inertia weight and a trigger which is to be described later. At the trigger, the axial movement of the inertia weight 13 with respect to the cylindrical wall 9 pushes the trigger thereby actuating a pawl to engage with the ratchet wheel 12.

The aforementioned elements comprise the spool sub-assembly 16 and are all located within the body 2 of the reel 1, with the exception of the central part of the spool 3 and the strap webbing which are necessarily exposed.

Referring to Figures 2, 3 and 5, a vehicle acceleration sensor sub-assembly 21 is provided which at least partially sits within the cylindrical inertia weight 13. The sensor sub-assembly 21 comprises a carriage 22 which has a first part 23 adapted to be inserted in the cylindrical inertia weight 13 and a second part 24 comprising a cylindrical shroud 24 of greater diameter than the first part to shroud partially a cam plate. The shroud 24 is provided with a cut-away to allow a trigger mechanism access to parts of the sensor sub-assembly 21.

The first part 23 of the carriage 22 comprises a pair of curved spaced-apart side walls 23 for insertion in the cylindrical inertia weight 13. The end of each of the spaced-apart side walls 23 carries a pair of locating holes 25.

The sensor sub-assembly 21 comprises a pair of independent lever arms 26 which are identical to one another and which are each fixed at one end by a pivot pin 27 within and to the carriage 22, the pins 27 sitting in the locating holes 25. Referring to Figures 3 and 5, each arm 26 is T-shaped and is provided with a hemispherical cup 28, at the base of the T. The cross-member or bar of each T-shaped arm 26 accommodates a respective one of the pivot pins in a through hole along its length. Thus, each lever arm 26 is pivotally mounted to the carriage 22 at one end and defines a cup-shaped Enter Date & Draft No. receptacle 28 at the opposite end, the cup end. The arms 26 are thus free to pivot in a common plane normal to the pins 27 by a small angular amount (between 3-10 ). At the cup end of each arm, the cup can prescribe a linear motion. Each cup 28, carries a small tongue 29, which projects from the convex surface of the cup 28 in a direction parallel to the bar of the T and the pivot pins 27.

The two lever arms 26 are presented to one another with the concave surfaces of the cups 28 facing one another so as to define a substantially spherical cavity into which is placed a spherical inertia weight which is termed herein, for the sake of easy reference, a G weight 30 so as to distinguish it from the cylindrical inertia weight 13 responsible for strap acceleration sensing. The G weight 30 is responsible for vehicle acceleration sensing.

The cups 28 hold the G weight in the sensor sub-assembly 21.

When pivotally mounted in the carriage 22, the lever arms 26 are able to move independently of one another in their common plane constrained by the axes of the pivot pins 27 under influence of movement of the G weight 30, in response to vehicle acceleration or deceleration. The inertia weight of the G weight is intentionally substantially greater than that of the other component parts of the sensor sub assembly 21.

The carriage 22, constrains movement of the lever arms 26, such that they cannot move apart to the extent that the G weight 30 would fall out of the cups 28 -i.e. the distance between the cups 28 is never greater than the diameter of the G weight ball 30.

Referring now to Figure 6, with the lever arms 26 mounted in the carriage 22, the cup ends of the lever arms 26 are capped with a circular trip plate 40.

The trip plate 40 carries a pair of diametrically opposed tabs 41 which are each located to engage a respective tongue 29 of the cups 28. The trip plate also has ramped side walls 42 on opposite sides of its periphery and a Enter Date & Draft No. centrally mounted post 43 (best seen in Figures 2, 3 and 7) projecting in the opposite direction to the ramped side walls. The trip plate 40 is rotatably mounted with respect to a cam plate to be described later, and is thus rotatably mounted with respect to the body 2.

It will be appreciated from Figure 6 that movement of the G weight ball 30 in any direction will cause either one or both of the independent lever arms 26 to pivot about their axes and cause at least one of the lever arms 26 to move outwardly, thereby engaging the tongue 29 with the tab 41 and converting the substantially linear motion of the lever arm 26 at the cup end into a rotational motion of the trip plate about the central axis of the post 43 and the extent of rotation is indicative of the acceleration experienced by the inertia weight.

In Figure 6, the two straight direction arrows show the cup ends of the lever arms 26 both moving substantially in a linear direction outwardly with respect to one another so that the tongues 29 both engage with the tabs 41 and convert the linear motion into rotational motion of the trip plate 40 (shown by the curved rotation arrow). The function of the ramped side walls 42 will be explained later. It is, however, important to note that under vehicular acceleration, the G weight baIl 30 has substantially more inertia than the remainder of the sensor sub-assembly 21 and therefore dwells relative to the other components causing relative linear movement between the G-weight 30 and the levers 26. It is this linear relative movement which the combination of the tongues 29 and tabs 41 convert into rotational motion of the trip plate 40.

It should be noted that the tongues 29 are placed outboard of the cups 28 so as to create as large a moment arm as possible to act upon the tabs 41 of the trip plate 40.

The trip plate 40 offers a rotational mechanical output of the sensor sub-assembly 21, which rotational input is not affected in its sensitivity to the direction of the acceleration experienced by the G weight 30. Thus, the rotational output is not sensitive to the direction of acceleration and the Enter Date & Draft No. sensor sub-assembly 21 can be regarded as a truly omni-directional acceleration sensor. Importantly, the extent of rotation is indicative of the acceleration experienced by the inertia weight yet remains insensitive to the direction of acceleration.

Figures 7, 8 and 9 show how the mechanical rotational output of the trip plate is used to trigger braking or locking of the spool. A locking pawl 45 is hingedly mounted to the body 2 and a torsion spring 46 is fixed to the pawl 45 to bias the pawl 45 toward the adjacent ratchet wheel 12. The pawl 45 is held off from engaging with the ratchet wheel 12 by a trigger finger 47 which is pivotedly mounted to and extends from the pawl 45. A further torsion spring 48 biases the trigger finger 47 in one direction.

The trigger finger 47 terminates at its tip in a bifurcated portion, each portion comprising a contact pad 49, 50, each responsive to a respective trigger input. The first contact pad 49 rests upon a ramped side wall 42 of the trip plate 40 and is biased by the torsion spring 48 toward the ramped side wall 42. In the normal reset condition shown in Figure 7, the first contact pad 49 sits at the bottom of the ramped side wall 42. The contact pads 49, 50 can be best seen in Figures 11, 1 2A and 13A.

When vehicular acceleration exceeds the predetermined threshold, the trip plate 40 rotates causing the first contact pad 49 to rise up the ramped side wall 42 rotating underneath it as seen in Figure 8. The spring-loaded trigger finger 47 is also being held in engagement with a cam plate 52 to which the trip plate 40 is rotatably mounted by another torsion spring 53 which passes through a securing hole 51 in the trigger plate post 43.

Referring now to Figures 10 and 11, the cam plate 52 is a circular disc having a central hollow sleeve 54 projecting in one direction and an outer cylindrical side wall 55 projecting in the opposite direction. The side wall 55 has about a quarter thereof cut away and supports an annular flange 56 along its outer Enter Date & Draft No. edge remote from the disc. The annular flange lies parallel to the disc but presents a cam edge 57 having a radius from the central axis of the cam plate 52.

The trigger finger 47 has, toward the base of the finger, a small shoulder 58 sitting proud from the trigger finger, and it is this shoulder 58 which engages with a cam edge 57 of the cam plate 52 and which prevents the pawl 45 from engaging the ratchet wheel 12 (see Figures 7, 10, 11, 12A and 12B).

Movement of the first contact pad 49 of the trigger finger 47 up the ramped side wall 42 causes the shoulder 58 to lift off from the cam edge 57 and allows the pawl to be spring biased by the torsion spring 46 into engagement with the ratchet wheel 12.

The secondary trigger on the trigger finger 47 was mentioned earlier with respect to the strap acceleration sensor sub-assembly 16, in particular, the cylindrical dwell weight 13 which moves axially outwardly from the confines of the cylindrical wall 9 of the spool 3 upon excessive strap acceleration so as to close a gap between the dwell weight 13 and the trigger 47. It is this secondary part of the trigger, the second contact pad 50, which is normally spaced apart from the inertia weight 13 which, upon excessive strap acceleration, is approached as the dwell weight closes the gap (see Figure 4) and, at the predetermined strap acceleration threshold, contacts the second contact pad 50 (see Figure 13A) and urges the trigger finger 47 against the restoration force of the torsion spring 48 and lifts the shoulder 58 off from the cam edge 57 again causing engagement of the pawl with the ratchet wheel 12. This arrangement of the shoulder 58 initially resting on the cam edge 57 of the cam plate is best seen in Figures 10, 11, 1 2A and I 2B..

As previously mentioned, the trip plate 40 is rotatably spring loaded with respect to the cam plate 52, by a torsion spring 60 and it is the strength of the torsion spring linking the two items which determines the predetermined acceleration threshold.

Enter Date & Draft No. Referring to Figures 10, 11, 14 and 15, the disc of the cam plate 52 is provided with a series of equispaced radially extending slots 61 and the centrally mounted sleeve 53 is dimensioned to receive and journal the centrally mounted post of the trip plate 40. The torsion spring 60 is placed over and around the sleeve 53 with one end of the torsion spring 60 being fixed in a hole provided in the cam plate 52 and the other end of the torsion spring passing through the securing hole 51 (see Figures 7, 14 and 15) formed in one end of the centrally mounted post 43.

Referring to Figures 2 and 3, an end plate 64 fixes over all this sub-assembly to close that end of the body 2, but the radially extending slots 61 in the cam plate 52 are still exposed through the end plate 64. A slide housing 65 holding a cable slide 66 is fixed to the end plate 64 and an actuating pin 67 extends from the slide housing 65 into one of the radial slots 61 in the cam plate 52. The actuating pin 67 can be moved by activation of the cable slide 66 to rotate the cam plate through in the region of 90 by a stroke in the region of 0.7" -typically effected by a lockable lever. In the normal operating condition as shown in Figure 14, the widest part of the cam edge 57 is presented to the shoulder 58 on the trigger finger 47 thereby holding off the pawl 45 from engagement with the ratchet wheel 12. Referring to Figure 15, a counter clockwise rotation of 90 moves the cam edge 57 presented to the shoulder 58 from its thickest part to its thinnest part thereby allowing the shoulder 58 to move radially inwardly under the biasing of the torsion spring 46 and to eventually allow engagement of the pawl 45 in the ratchet wheel 12.

This is manual locking of the spool 3. It will be appreciated that manually locking the spool 3 does not involve knocking the shoulder 58 off of the cam edge 57 (as shown in Figures 13A and 13B).

It should be noted that if the trigger finger 47 had been triggered by either the vehicle acceleration sensor or the strap acceleration sensor, then the shoulder 58 would have been knocked off from engagement with the cam Enter Date & Draft No. edge 57 of the cam plate 52 and the pawl 45 engaged with the ratchet wheel 12. To reset the system, the cam plate 52 needs to be rotated by actuating the cable slide 66 so as to move the cam edge 57 from presenting its thickest part to the shoulder 58 to the point at which the cam edge 57 presents its thinnest part, which is radially inward of the thinner part and, importantly, also radially inward of the surface of the shoulder 58 presented to the cam edge 57 when the pawl 45 is engaged in the ratchet 12. The trigger finger 47 is spring-loaded and thus, when the thinnest part of the cam edge 57 is adjacent the shoulder 58, the trigger finger 47 is able to swing back towards the pawl 45 and the shoulder rests again adjacent the cam edge 57 (the situation in Figure 15). Counter rotation of the cam plate 52 in the clockwise direction (starting from the position shown in Figure 15) presents an ever thickening portion of the cam edge 57 to the shoulder 58 driving the shoulder 58 and thence the trigger finger 47 radially outwardly thereby pulling the pawl 45 back from engagement with the ratchet wheel until the normal operating condition is achieved (the system has been reset) as shown in Figure 14.

As well as providing the means for resetting the trigger mechanism, the ability to rotate the cam plate 52 by the cable slide 66 also provides for manual operation, i.e. manual locking, of the inertia reel as previously described.

Once again, it should be noted that the cam plate 52 only has rotary motion and is therefore insensitive to vehicle acceleration or vibration.

The above described inertia reel is omni-directional but advantages of the invention can still be appreciated where it is desired to make the vehicle acceleration sensor mechanism not reliant or not sensitive to a mechanical linear output in a particular direction. Thus, advantages of the invention can be conferred by physically attaching the G weight 30 to a lever arm 26 or making the G weight 30 part of a lever arm 26and converting the substantially linear motion of the G weight into a rotational output which is itself Enter Date & Draft No. independent of and not sensitive to vehicle acceleration but the extent of rotation is indicative of the acceleration experienced by the inertia weight.

In the above described embodiment the G weight comprises a spherical ball but it has been found that by modifying the shape of the ball and to some extent the conical surfaces making up the cups and the lever arms, an additional mechanical advantage can be obtained when the reaction force does not go through the centre of gravity of the G weight in the case of an off-axis acceleration.

In the normal case with a spherical G weight 30, the moment on the cup is less when under off-axis acceleration than when experiencing an on-axis acceleration. For an on-axis acceleration (see Figure 16), the moment on the cup is equal to mass of G weight * acceleration * LI, where LI is the moment arm from the pivot pin of the lever arm to the centre of gravity of the G weight 30. When the acceleration is off-axis (see Figure 17), then the moment arm is equal to the mass * acceleration * L2, where L2 is a component of the moment arm experienced under on-axis acceleration. Since the component of the moment arm L2 is less than the maximum moment arm LI, less moment is experienced by the cup, i.e. there is a reduced mechanical advantage.

Referring now to Figures 18 and 19, another embodiment of the present invention utilises a shaped G weight 80 comprising a spherical ball 81 with a pair of opposed conical lobes 82 projecting from opposite sides ofthe ball.

The conical lobes 82 are located and held in the cups and the modified G weight 80 works in exactly the same way as the spherical G weight 30 except when the sensor experiences an off-axis acceleration. Any off-axis acceleration creates a reaction force and the moment arm about the centre of gravity of the modified G weight 80 serves to rotate the G weight and cause the lobes 82 to press on the cups 28 and force the cups apart. This is contrasted with an off-axis acceleration experienced by a spherical G weight Enter Date & Draft No. in which the spherical G weight 30 would freely rotate (that rotation comprising wasted energy since the rotation would not impart any force in a direction to effect movement of the lever arms). Rotation of the modified G weight 81 engages the cups 28 and provides an additional mechanical advantage serving to improve significantly the performance of the sensor compared to the standard spherical C weight configuration whilst retaining the same level of performance for on-axis accelerations.

Enter Date & Draft No.

Claims

CLAIMS: 1. A vehicular acceleration sensor comprising: an inertia

weight; a holding mechanism to hold the inertia weight and which is movable by the inertia weight in a generally linear direction; a trip mechanism to convert linear movement of a part of the holding mechanism into a rotational movement of the trip mechanism, which rotational movement comprises an output of the sensor.

2. An acceleration sensor according to Claim 1, wherein the holding mechanism comprises a pair of lever arms operable to move independently of one another, movement of the inertia weight, held between the lever arms, when under acceleration, causing one or both arms to move in the plane.

3. An acceleration sensor according to Claim 2, wherein the pair of lever arms each has a cup portion, which cup portions face one another hold the inertia weight therebetween.

4. An acceleration sensor according to Claim 3, wherein the cup portions have substantially conical internal surfaces.

5. An acceleration sensor according to Claim 2 or 3, wherein the trip mechanism has an axis of rotation and the holding mechanism is movable in a plane, the axis of rotation lying in the plane.

6. An acceleration sensor according to any preceding claim, wherein the inertia weight comprises a ball.

Enter Date & Draft No. 7. An acceleration sensor according to any one of Claims I to 5, wherein the inertia weight comprises a non-spherical shaped ball, having one axis of rotational symmetry.

8. An acceleration sensor according to Claim 7, wherein the inertia weight comprises a ball with an equatorial bulge.

9. An acceleration sensor according to Claim 7, wherein the inertia weight comprises a ball with a pair of opposed substantially conical protrusions.

10. An acceleration sensor according to any preceding claim comprising a part of an inertia reel having a spool.

11. An acceleration sensor according to Claim 10, wherein at least a part of the sensor is housed in a part of the spool.

12. An acceleration sensor according to Claim 11, wherein the majority of the sensor is housed in the spool.

13. An acceleration sensor according to any one of Claims 10 to 12, wherein the reel also includes a strap acceleration sensor and the vehicular acceleration sensor is at least partly housed in the strap acceleration sensor.

14. An acceleration sensor according to any preceding claim, wherein the extent of rotational movement is indicative of an acceleration experienced by the inertia weight.

15. An acceleration sensor according to any preceding claim, wherein the rotational movement trips a trigger to cause locking of the spool.

16. An acceleration sensor according to Claim 15, wherein the trip mechanism is rotatably mounted to a cam mechanism which holds the trigger.

Enter Date & Draft No. 17. An acceleration sensor according to Claim 16, wherein the trip mechanism is biased into a position by an elastic member interacting between the cam mechanism and the trip mechanism, the biasing force exerted by the elastic member being overcome by rotation of the trip mechanism under vehicular acceleration above a predetermined threshold.

18. An acceleration sensor comprising an inertia weight located toward one end of a lever arm which is movable in a generally linear direction in response to movement of the inertia weight, movement of the inertia weight or a part of the lever arm adjacent the inertia weight in a generally linear direction being converted to a rotational movement of a trip mechanism as an output of the sensor.

19. An acceleration sensor having an inertia weight and a rotational output, wherein the extent of rotation is indicative of an acceleration experienced by the inertia weight.

20. An inertia reel having a vehicular acceleration sensor according to any preceding claim.

21. An inertia reel according to Claim 20 further comprising a strap acceleration sensor.

22. An inertia reel according to Claim 21 having a common trigger tripped by actuation of either the vehicular acceleration sensor or the strap acceleration sensor.

23. A method of sensing a vehicular acceleration above a predetermined threshold comprising: holding an inertia weight movable under acceleration; Enter Date & Draft No. translating movement of the inertia weight into a substantially linear movement; and converting the substantially linear movement into a rotational movement of a trip mechanism, wherein the extent of rotational movement is indicative of an acceleration experienced by the inertia weight.

24. An inertia reel or acceleration sensor substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

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

Enter Date & Draft No.