Reed switches have found wide use in shock sensors, particularly as safing sensors in motor vehicles. Typically vehicle crash sensing is performed by integrated micro device sensors which are incorporated onto chips which assess the magnitude and direction of the crash and employ preprogrammed logic to decide whether and how to deploy or activate various safety systems. These systems include airbags and seatbelt retractors. Such micro sensors can be very cost-effectively incorporated into a safety system logic. However, such small scale devices are subject to electromagnetic interference and related phenomenon giving rise to possible false sensor outputs.

The need for a macro scale sensor arises to provide a safing sensor that provides the programmed logic with an indication that a crash of sufficient magnitude to warrant deployment of safety systems is in fact occurring. Shock sensors employing reed switches meet the need for a large scale device while at that the same time allowing a relatively small sized package which can be directly mounted onto a circuit board. A reed switch is resistant to electromagnetic interference and the hermetic seal formed by the glass capsule about the reeds results in a highly reliable switch which is sealed from the atmosphere. Thus, reed switch based shock sensors are

usually the preferred choice for safing sensors forming part of a vehicle safety system.

Reed switch based shock sensors have been designed with multiple axes of sensitivity, yet such devices are typically considerably more expensive than unidirectional shock sensors or are more sensitive to large scale vibration. A typical reed switch based shock sensor has an acceleration sensing magnetic mass which is held against a stop by a spring. The spring is typically pre-loaded so that no motion of the sensing mass takes place unless the acceleration loads exceed a selected value. Obtaining pre-loaded sensing masses in a bidirectional shock sensor has proven problematic. This problem is overcome by the shock sensor disclosed in claim 1.

Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.

Brief Description of the Drawings FIG. 1 is an exploded isometric view of the shock sensor of this invention.

FIG. 2 is a top plan view, part cut-away in section on multiple planes of the shock sensor of FIG.

1, showing the sensor in a shock-responsive position.

FIG. 3 is a reduced scale illustrative top plan view showing the reed switch connecting in the two housing halves of the shock sensor of FIG. 1 prior to the final assembly position shown in phantom view.

FIG. 4 is an isometric view, partially cut-away in section, of an alternative embodiment of the shock sensor of this invention.

Detailed Description of the Invention Referring more particularly to FIGS. 1-4, wherein like numbers refer to similar parts, a shock sensor 20 is shown in FIGS. 1 and 2. As shown in FIG. 1, the shock sensor has a single reed switch 22 which is disposed between two identical housings 34, each of which contain a magnetic shock sensing mass which is disposed for sliding along axes parallel to the reed switch. The reed switch has a first staple-formed lead 24 at one end, and a second staple formed lead 26 at the other end. The leads 24,26 are connected to, and are actually co-formed with, ferromagnetic reeds that are positioned within a glass capsule 30 which hermetically seals the reeds therein. The reeds 28 terminate at overlapping contact surfaces 32 which are spaced apart when the switch is in an not in an activated condition, and which are brought into engagement in the presence of a magnetic field which causes the reeds to attract.

The housings 34 are preferably identical injection molded plastic parts, one positioned on either side of the reed switch 22. Each housing defines a cylindrical cavity 36 having an axis that is generally parallel to the reed 28 in the assembled sensor 20. As shown in FIG. 2, each cavity 36 terminates at a blind end 38 opposite an open end 40.

A molded shaft 42 is positioned along the axis of each cylindrical cavity. The shaft preferably has semicylindrical portions that are joined by parallel planar segments. Each shaft 42 has a protruding terminal key 44 which engages within a protruding annular keyway 46 which extends from the center of the

surface 48 defining the blind end of the cavity 36.

The engagement of the shaft key within the keyway serves to position the shaft 42 along the axis of the cylindrical cavity.

Each shaft 42 extends from a generally cylindrical disk 50, the disk may be composed of an array of frustoconical barbs 52 which deform when inserted into the cylindrical cavity 36 to engage the cylindrical wall 54 of the cavity. A gripping extension 55 extends outwardly from each disk. The gripping extensions allow the shafts to be mechanically held and positioned. Although the disks and the attached shafts 42 may be held in place by potting around the gripping extensions 55, the frustoconical barbs 52 alone may be sufficient to lock the disks in place.

A generally annular or ring-shaped magnet 56 having a central opening 58 is positioned about each shaft 42. A spring 60 extends between each magnet 56 and an inner face 62 of the disk 50, thereby biasing the magnet against the surface 48 which forms the blind end 38 of the cylindrical cavity 36. A small protrusion 63 extends toward the blind end 38 from the inner face 62 of the disk 50. The small protrusion 63 serves to reduce bounce from the inner face 62 when the magnet 56 collides with the face due to a crash shock. By causing magnet to cock to one side, the protrusion 63 causes an engagement between the magnet and the shaft 42 which dissipates energy, thus reducing bounce and increasing the switch dwell time.

Each magnet 56 has an enlarged portion 64 of the central opening 58 that accommodates the spring 60 between the shaft and the magnet. The spring is

retained against the magnet by a radially extending surface 66 which connects the enlarged portion 64 of the opening with a narrower portion 68 of the central opening 58. Shaping of the magnet as described in US 5 212 357 can also increase the dwell time.

The shock sensor 20 achieves bidirectional shock sensing with two mechanically independent sensing masses 56. Each magnetic sensing mass is pre-loaded against a surface 48, and thus is not subject to vibration-induced motion which does not exceed the pre-load. The sensing masses 56 interact electro- mechanically through their action on the individual reeds 28. When the sensing magnetic masses are in their rest positions the magnetic fields they produce permeate the adjacent reeds 28. This reduces the size of the magnet needed to cause closure of the reed switch 22. By reducing the size of the magnets the entire package is reduced in size, thus lowering cost and improving packaging efficiency.

As shown in FIG. 2, when the shock sensor experiences a front end crash, the magnet 56 has apparent motion towards the inner face 62 of the disk 50 as indicated by arrow 57. In a front end crash, acceleration takes place in a direction from front to rear in the vehicle. As the magnet is unconstrained along an axis defined by the shaft 42, except by the spring 60, it is not accelerated to the same degree as the housing 34. This produces the apparent motion of the magnet, acting as an acceleration sensing mass, towards the site of the crash, thereby moving the magnet 56 against the abutment or stop formed by the surface 62. Motion of

the magnet relative to the reed switch 22 causes the reeds 28 to attract causing closure of the reed switch 22.

The housings 34 are arranged so that the magnets 56 contained in opposed housings are biased by the springs 60 against surfaces 48 or abutments on the housing which are diametrically opposed. Thus a forward crash as shown in FIG. 2 causes the right magnet to move towards the second surface 62. A rear end crash will cause the left magnet to move towards the second surface 62. Thus a shock sensor 20 having two directions of sensitivity one hundred and eighty degrees apart is provided.

Simplicity, and experience with similar actuation mechanisms, such as those taught in US 5 416 293 and US 5 194 706, provides assurance of functionality.

The motor vehicle industry typically looks for sensors with mechanical simplicity combined with low cost and ability to accommodate changes in design parameters.

Design parameters are readily adjusted in the sensor by varying the spring constant and the magnet size or type. Typically, the magnets for cost reasons will be formed of metal particles embedded in plastic.

Nevertheless, cast or powder metallurgy produced magnets may be used.

As shown in FIG. 1, each housing 34 has a ledge 69 which runs along a side 71 of the housing 34 opposite the side 73 in which the opening 75 to the cavity 36 is formed. The ledge has a first hole 70 therein that extends vertically through the ledge and the housing. The first hole is adjacent a reed switch accepting face 77 of the housing 34. A lower shelf 79 extends from the lower portion of the reed switch

accepting face 77. The lower shelf 79 adjoins and reinforces the side 71 of housing that contains the first hole 70. An upper shelf 81 extends from the upper portion of the face 77 in spaced parallel relation to the lower shelf 79. The upper shelf reinforces a portion 83 of the side 71 that is stepped back from the ledge 69.

As shown in FIG. 3, two identical housings 34 are joined to form the shock sensor 20 by placing the first lead 24 of the staple formed reed switch 22 through a first hole 70 on a first housing 85, and by placing the second lead 26 through a first hole 70 on a second housing 87. The identical housings 85,87 are brought into inverse mirror image engagement by pivoting the housings. As the reed switch 22 is brought into parallel engagement with the sides 77 of the housings 85,87, a portion 89 of each lead 24,26 which extends horizontally from the reed switch glass capsule 30 is captured by a slot 91 which has a lower surface 90 coplanar with the ledge 69 and an upper surface 93 spaced from the lower surface which positions the reed switch in the vertical plane thereby assuring repeatable positioning of the reed switch with respect to both housings 85,87.

A strap 95 having a first vertical lead 97 and a second vertical lead 99 extends across the reed switch 22. The first vertical lead 97 is positioned in a second hole 101 in the ledge 69 of the first housing 85. Similarly the second vertical lead 99 is positioned in a second hole 101 in the second housing 87. The strap has a horizontal section 103 that extends over the reed switch 22 along an interface formed where the upper shelves 81 terminate.

A notch 105 is formed in the portion 83 of the side 71 above the slot 91. The notch 105 receives the horizontal section 103 of the strap 95. Short transverse sections 107 connect the horizontal section 103 of the strap 95 to the vertical leads 97,99. Once pivoted into engagement with one another and connected with the strap 95, the two housings 34 are held together by a pressure-sensitive adhesive label, or piece of tape, 108 which overlies the opposed upper shelves 81. The label holds the strap 95 and the housings 34 in position.

To be cost effective, a shock sensor must be designed for automated assembly. The shock sensor 20 incorporates interlocking parts that can be assembled without bonding or potting. The label 108 does not require a narrowly controlled environment or time to cure, and is thus compatible with rapid automatic assembly. Tolerances are achieved through self- alignment of the reed switch and the housings.

An alternative embodiment shock sensor 109, shown in FIG. 4, employs a single housing 34 together with a housing closure 111. The housing closure 111 has an upper shelf 113 which is semicylindrical in shape, and which extends down to the body 115 of the closure 111.

The housing closure 111 has a ledge 117 similar to the ledge 69 formed on the housing 34. The ledge 117 has a first hole which receives the second lead 26 on the reed switch 22, the side 119 of the closure 111, has a portion 121 which forms a reed switch positioning slot 123 and a strap positioning notch 125 similar to those on the housing 34. Because only a single magnet is present it will typically need to produce a greater

magnetic field than the same magnet used in the shock sensor 20.

It should be understood that the springs 60 which are placed in the identical housings 34 could have differing spring constants which would allow tailoring of the sensitivity in one direction verses sensitivity in the opposite direction. If this technique is used, to prevent confusion two housings having different appearances and keying features should be employed.

It should be understood that the shock sensors described herein are not limited to the use of identical housings arranged in mirror image but includes shock sensors wherein the housing on which the acceleration sensing masses and the reed switch are mounted may be a unitary whole, or may be constructed from two or more separate housings which differ in various respects from each other.

It should be understood that wherein the reed switch is described normally open so that movement of the acceleration sensing magnetic mass causes the reed switch to close, movement of the acceleration sensing mass could be used to open a normally closed reed switch.