CA2272072A1 - Glass capsule enclosed shock sensor - Google Patents

Abstract

A shock sensor (20) having some structural similarities to a reed switch, particularly in the use of a glass capsule (22) which hermetically seals the components of the shock sensor. The shock sensor employs a sensing mass (31) mounted on a metallic planar spring (32). Under the influence of a crash-induced acceleration, the sensing mass is driven against a fixed contact to close an electrical circuit. To extend the closure duration and increase the reliability, the contact surface (40) which is formed on the sensing mass is oriented at an angle of 60 degrees out of the plane containing the spring.

Description

GLASS CAPSULE ENCLOSED SHOCK SENSOR
The present invention relates to shock sensors in general and to shock sensors used for engaging or deploying automobile safety devices in particular.
Shock sensors are used in motor vehicles, including cars and aircraft, to detect vehicle crashes. When such a crash occurs, the shock sensor triggers an electronic circuit for the actuation of one or more safety devices. One type of safety device, the inflatable airbag, has found widespread acceptance by consumers as improving the general safety of automobile operation. Reliable deployment of an airbag without unwanted deployments is facilitated by use of multiple sensors in combination with actuation logic which can assess the nature and direction of the crash as it is occurring and, based on preprogrammed logic, make the decision whether or not to deploy the airbag. However, solid state shock sensors are prone to losing touch with the real world and may occasionally indicate a crash is occurring due to radio frequency interference, electronic noise, cross-talk within the electronics, etc. Mechanical shock sensors when incorporated as an integral part of an airbag deployment system prevent unnecessary bag deployment due to the problems of microelectronics.
This insensitivity to electronic interference makes mechanical shock sensors an important part of airbag deployment systems. A number of types of shock sensors employing reed switches have been particularly advantageous in combining a mechanical shock sensor with an extremely reliable electronic switch which, through design, can be made to have the necessary dwell times required for reliable operation of vehicle safety equipment. The reed switch designs have also WO 98l27565 PCT/US97120641 been of a compact nature such that the switches may be readily mounted on particular portions of the vehicle.
Tests have shown that in a crash, particular portions of a vehicle will experience a representative shock which is indicative of the magnitude of the crash.
Thus shock sensors which have small packaging dimensions are critical to proper placement of a shock sensor.
The shock sensor of this invention has some structural similarities to a reed switch, particularly in the use of a glass capsule which forms a hermetic seal about the components of the shock sensor. But, whereas a reed switch, when functioning as part of a shock sensor, requires a moving magnetic mass, the shock sensor of this invention employs a sensing mass mounted on a metallic planar spring. Under the influence of a crash-induced acceleration, the mass mounted on the spring is driven against a fixed contact to close an electrical circuit.
The sensing mass is formed using injection molded powder-metallurgy technology or as a metal stamping.
The sensing mass is fabricated with the contact surface oriented 60 degrees out of the plane of the spring to extend the closure duration. The sensing mass also incorporates structural features which capture the sensing mass to prevent undesirable motion of the mass. The sensor is oriented such that the acceleration force due to a crash is approximately normal to the plane containing the spring. The orientation of the contact area on the sensing mass and a similarly oriented fixed contact dissipate contact energy sufficiently to eliminate most bouncing upon initial closure. The 60 degree contact angle provides a more reliable, less noisy closure signal in the presence of a crash-induced shock. Dwell time of initial contact closure because of the angled contacts is increased five to ten times on even marginal sensor closing events. The dwell time on higher force events is in some instances comparable to magnetically actuated crash-sensing devices. The fixed contact against which the sensing mass makes contact can alternatively be a smooth rod shaped contact which reduces sensitivity to manufacturing imperfections.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a side elevation view of the shock sensor of this invention.
Fig. 2 is an end-view of the shock sensor of Fig. 1 taken along line 2-2.
Fig. 3 is a cross-sectional view of the shock sensor of Fig. 1 taken along section line 3-3.
Fig. 4 is an isometric view of the acceleration sensing mass mounted to the shock sensor of Fig. 1 Fig. 5 is an isometric view of the shock sensor of Fig. 1 without the enclosed glass capsule.
Fig. 6 is a schematic view showing the comparative flexibility in tension of the U-shaped joint which supports the planar spring of the shock sensor of Fig. 1.
Fig. 7 is a schematic view showing the comparative flexibility in vertical shear of the U-shaped joint which supports the planar spring of the shock sensor of Fig. 1.
Fig. 8 is a schematic view showing the comparative flexibility in horizontal shear of the U-shaped joint which supports the planar spring of the shock sensor of Fig. 1.
Fig. 9 is an isometric view of an alternative embodiment of the shock sensor of this invention.
Fig. 10 is a side elevation view of a further embodiment of the shock sensor of this invention.
Fig. 11 is an isometric view of the acceleration sensing mass of the shock sensor of Fig. 10.
Fig. 12 is a cross-sectional view taken along section line 12-12 of the shock sensor of Fig. 10.
Fig. 13 is side elevation view of yet another embodiment of the shock sensor of this invention.

DETAILED DESCRIPTION OF THE INVENTION
Referring more particularly to Figs. 1-13, , wherein like numbers refer to similar parts, a shock 5 sensor 20 is shown in Fig. 1. The shock sensor 20 is composed of a glass capsule 22 which defines an internal volume 24. The internal volume 24 may be filled with an inert gas or gas with a high dielectric breakdown strength. The glass capsule 22 has a first end 23 formed around a short lead 26 and a long lead 28, and a second end 25 formed around a mounting lead 30. Electrical contact is made between the short and long leads 26, 28 and 30 by an acceleration sensing mass 31 mounted to a planar spring 32.
The spring 32 has an attachment end 38 which is welded between a raised flange 36 formed in the end 39 of the long lead 28, and a planar tab 37 which is part of the mounting lead 30. The shock-sensing mass 31 is welded to the spring 32 adjacent the spring end 39 opposite the attachment end 38. The shock-sensing mass 31 has a contact surface 40 which is oriented 60 degrees from the plane of the spring 32.
Fig. 4 shows the orientation of the spring 32 with respect to the contact surface 90 on the mass 31. As shown in Fig. 2, the contact surface 40 engages against a fixed contact surface 42 formed on the end 94 of the short lead 26. The short lead 26 has a deformed portion 46 which defines the non-moving contact surface 42.
The spring 32, in a typical shock sensor 20, may have a thickness of about 0.05 mm and a width of about 0.76 mm. The overall length of the spring from end to end is about l0.7 mm. The dimensions of the WO 98l27565 PCT/US97/20641 spring thus render it substantially flexible only in a direction normal to the plane defined by the spring 32. The normal direction'of the spring 38 is aligned with the direction of acceleration which it is desired to sense. In use, the sensor 20 may be mounted by the leads 30, 26, 28 directly to a circuit board containing some or all of the electrical components used to actuate an airbag or similar device. The sensor may also be mounted in a package (not shown) to facilitate orienting and mounting the sensor on a particular part of a vehicle where, through tests and analysis, it has been determined the response of the structure provides reliable indication of the direction and severity of a crash.
The shock sensor 20 takes advantage of the manufacturing tools and techniques for making reed switches to fabricate a shock sensor. The reed switch manufacturing process has developed around the mass production of components such as leads, springs and contacts with high precision and low cost. The reed switch manufacturing process also facilitates the assembly of the leads and spring-mass, automatically positioning them with high tolerance and hermetically sealing a glass capsule about the switch components.
During manufacture the hermetic glass capsule 22 is positioned around the shock sensor leads 2~, 28 and 30 and heated until the ends of the capsule are sufficiently soft to seal to the leads. The connection between the long lead 28 and the mounting lead 30 serves to accurately position and hold the spring 32. However as the glass capsule 22 cools the ends, 23, 25 move towards each other as the glass contracts. This contraction of the glass capsule could result in sufficient strain to cause the capsule to fracture or to affect the overall reliability of the sensor 20. To overcome this shrinkage of the glass capsule a strain relief U-shaped member 48 is . incorporated in the structure of the mounting lead 30 between the planar tab 37 and the shank 50 of the mounting lead 30. The strain relief U-shaped member 48 is shown in Figs. 1 and 5-8. Fig. 6 shows greatly exaggerated the ability of the U-shaped member to respond to compression forces indicated by arrows 52. The direction of forces shown in Fig. 6 are produced by the shrinkage of the glass capsule 22.
Figs. 7 and 8 illustrate sheer forces indicated by arrows 59 in Fig. 7 and arrows 56 in Fig. 8. The sheer forces illustrated in Figs. 7 and 8 are exaggerated and in practice very little motion due to sheer can be produced in the U-shaped member. As will be appreciated by those skilled in the mechanical arts the U-shaped member is inflexible in sheer and relatively flexible in compression. This means the long lead 28 and the planar spring 32 which are mounted to the short mounting lead 30 are relatively rigidly supported while at the same time the U-shaped member 98 accommodates compressive forces produced by the cooling of the glass capsule 22.
The shock sensor 20, by utilizing the techniques of a reed switch manufacturer, transfers the advantages of low cost and high reliability inherent in reed switches to shock sensors which are suitable for use in automobile safety systems.
In operation, the shock sensor 20 is mounted in a vehicle with the plane defined by the spring 32 perpendicular to the expected line of action of a shock-inducing event or crash. The shock sensor 20 as shown in Figs. 2 and 3 is further oriented so the mass 31 is free to move towards the arrow 58, which shows the direction in which the crash load decelerates the vehicle and the shock sensor 20 mounted thereto. When the vehicle containing the shock sensor 20 experiences a shock-inducing crash, the vehicle rapidly decelerates, which, in turn, decelerates the glass capsule 22 of the shock sensor 20. The sensing mass 31, because it is relatively unconstrained by the spring 32, continues in accordance with Newton's First Law to move forward and thereby bring the contact surface 40 on the sensing mass 31 into contact with the fixed contact surface 92 which is rigidly formed from the short lead 26. The short lead 26 is held in position by the glass capsule 22.
Because the contact surface 40 on the sensing mass 31 and the fixed contact surface 42 on the short lead 26 engage at an angle a which is oriented 60 degrees from the plane of the spring 32 and the sensing mass 31, the closure between the contact surfaces 40, 92 is softer. The soft closure results from the contact 40 on the mass 31 sliding along the fixed contact surface 42 which, in turn, causes a limited deflection of the spring 32 in the plane of the spring. The sliding action between the contact surface 40 on the sensing mass 31 and the fixed contact surface 42 results in a frictional engagement between the contact surfaces 40, 42. The frictional engagement dissipates energy, helping to reduce bounce.
The spring 32 is much stiffer, in that is has greater resistance to bending, in the plane of the spring, than out of the plane of the spring. Closure of the switch formed by the shock sensor 20 results in WO 98l27565 PCTIUS97/20641 the angled contact surfaces 90, 42 causing some in-plane deflection of the spring 32. When the contact surface 40 on the sensing mass 31 begins to lift off from the contact surface 42 on the short lead 26, (due to elastic bounce), friction between the contact surfaces 40, 42 is reduced or eliminated. The reduction of the frictional forces between the contact surfaces 40, 42 allows the forces developed by the in-plane deflection of the spring 32 to move the shock sensing mass 31 back into engagement with the deformed portion 46 of the short lead 26. Thus, the tendency of the contacts of a switch to bounce open when subjected to a closing force is significantly decreased or eliminated by having the closing surfaces angled with respect to the direction of closing of the switch.
In practice, the exact analysis of the dynamics of the closure of the switch are complicated by cross-coupling between the spring constant of the spring 32 in and out of the plane of the spring, as well as by manufacturing tolerances which introduce imperfections in the alignment of the angled contact surfaces.
Experience with the construction of the shock sensors 20 has shown that manufacturing imperfections can actually enhance switch closure time by providing a softer, more gradual transition in the mating of contact surfaces from a weak point contact, as the contact surfaces wipe and twist towards a more rigid line or face contact.
The sensing mass 31 as best shown in Figs. 2-4 provides the ability to pre-load the spring in the un-actuated condition and control overtravel of the sensing mass 31. As seen in Fig. 4 the sensing mass 31 has a wrist 62 which has a slot 64 which receives the spring end 34. The wrist 62 is welded by a weld 66 which penetrates the wrist 62 and welds the wrist to the spring 32. The weld can be an electron 5 beam weld, a laser weld or a resistance weld. The sensing mass has a thumb 68 which extends from the body 70 of the sensing mass 31. The thumb 68 has an upper surface 72 parallel to the plane of the spring, which prevents over travel of the sensing mass 31 by 10 engaging the lead 26 if it slides too far along the contact surface 42. The thumb 68 has an inside surface 74 which engages a side 76 of the long lead 28. A similar surface 78 on a finger 80 extending from the body 70 engages the opposite side 82 of the long lead 28. A surface 84 on the underside of the body 70 supports the sensing mass 31 against the long lead 28. The surface 84 functions as a stop which allows the spring to be pre-loaded. Pre-loading the spring gives control over the actuator force needed to close the sensor. The entire shock sensor 20 is approximately the size of a reed switch being approximately 1.9 cm to 2.5 cm long along the glass capsule. Thus the sensing mass 31 is very small.
One method of fabricating the sensing mass 31 is as an injection molded powder metallurgy part. The technique of metal injection molding (MIMy allows the fabrication of highly detailed metal parts in a wide variety of alloys at reasonable cost. The sensing mass 31 shown in Figs. 1-5 is fabricated from an alloy obtainable from Ametek Inc. of Eighty Four, Pennsylvania U.S.A. One example was fabricated using 15 micrometer average particle size AM 388 powder, made by water atomizing the alloy. The alloy powder was mixed with a polyoleofinie binder to obtain a feed stock with 17 percent shrinkage. This feedstock was used for injection molding the desired part. In a process similar to the injection molding of plastic the heated mixture of metal powder and plastic binders are forced into a mold cavity under pressure where the mixture solidifies to form a green part. The part as molded was first debound in a solvent consisting of trichloroethylene. The debound part was then sintered in a vacuum furnace with an argon partial pressure of about 2000 micrometers of mercury. A slow ramp up was used to reach the sintering temperature of between about 870~ C and about 955~ C where the parts were held for between two and six hours. Other parts were made using P 729 powder also obtainable from Ametek Inc.

O
CHEMICAL COMPOSITION of AM388 N
(Weight o) a C: 0.010 Ni: 7.7l Cr: Fe: Mo: Si:

Mn: Co: Cu: BAL W: S: Cb:

B: 0: P: Sn: 5.70 n a POWDER PHYSICAL PROPERTIES Apparent Density: 4.17 g/cc (Size Distribution) N

N

MICROTRAC FLOW:

(Microns) PERCENT

OTHER: Tap Density - 5.00 g/cc N

-62 100.0 -44 100.0 MICROTRAC

-31 97.5 900 26.55 -22 82.3 50% 14.80 -16 56.3 100 6.86 b n -11 29.9 -5.5 5.7 -3.9 1.4 O
CHEMICAL COMPOSITION of P729 0~'0 N
(Weight o ) C: 0.008 Ni: 15.l4 Cr: Fe: Mo: Si:
Mn: Co: Cu: BAL W: S: Cb:
B: 0: 0.21 P: Sn: 8.17 N: 0.001 n N
N
J
POWDER PHYSICAL PROPERTIES Apparent Density: 3.68 g/cc o (Size Distribution) N
MICROTRAC FLOW: o (Microns) PERCENT w OTHER: Tap Density - 5.33 g/cc -62 100.0 -44 93.3 MICROTRAC

-31 92.9 l00 6.01 -22 75.2 500 l5.55 -16 5l.7 900 29.5l -11 32.6 ~o -2.8 17.0 -5.5 8.0 WO 98l27565 PCTIUS97J20641 An alternative embodiment shock sensor contact arrangement 88 is shown in Fig. 9. The contact arrangement 88 is identical to that illustrated in Figs. 1-5 except that the short lead 90 which corresponds to short lead 26 utilizes a contact lead which has a cylindrical contact surface 92. A
cylindrical contact surface 92 makes contact along a tangent line 94 with the contact surface 40 along a line 96 on the sensing mass 31. As described in reference to the shock sensor 20 closure properties can be sensitive to manufacturing imperfections.
While imperfections in achieving alignment of angled contact surfaces can in some cases improve the basic performance of the shock sensor 20, in other circumstances they may not be sufficiently controllable. A cylindrical surface 92 presents a line of tangent contact 94 which reduces contact geometry complexity. The result is a contact arrangement 88 which exhibits closure duration and bounce characteristics that are similar to the average characteristics of the shock sensor 20 but are more consistent and repeatable.
Another alternative embodiment shock sensor l20 is shown in Fig. 10 which has a glass capsule 122.
The capsule has a first end 123 formed around a contact lead 126 and a support lead i28. A second end 125 is formed about a third lead 130. The third lead 130 has a planar tab 137 to which is welded to an attachment end 138 of a planar spring l32. The spring 132 supports an acceleration sensing mass 13l.
The acceleration sensing mass 131 is shown enlarged in Fig. 11. The mass 131 is formed by stamping or coining. Stamping involves only bending and shearing whereas coining, in this context, involves more or less forging the part from a metal blank. The mass 131 is formed of a copper alloy similar or identical to the alloys used to form the powder 5 metallurgy formed sensing mass 31 of the shock sensor 20.
The sensing mass 131 has a contact surface 190 which terminates at a stop surface 172. The mass 131 has a wrist l62 which is welded to the free end 134 of 10 the planar spring 132. Extending from the wrist 162 are a first leg 168 and a second leg 180 which position the sensing mass 131 on the positioning lead 128 and prevent excessive movement of the mass 131 in the plane of the spring 132. The pre-load 15 position of the sensing mass is controlled by a flange 181 which is formed between the legs l68, l80.
The flange 181 has a tapered portion 183 which assures that a single line contact is made with the positioning lead 128. In construction of the shock sensor 120 the first end 123 of the glass capsule 122 is formed around the contact lead 126 and the positioning lead 128. The sensing mass 13l together with the spring l32 are mounted to the third lead l30 which is then positioned relative to the positioning lead 128 and the contact lead 126 to form the desired amount of pre-load against the positioning lead 128.
The second end l25 is then sealed about the third lead 130 fixing the amount of reload between the sensing mass l31 and the positioning lead 128 and the contact lead 126. The contact lead l26 extends past the contact surface 140 so that any burr formed when the lead is manufactured extends past the contact surface 140, thus assuring the contact between the sensing mass 131 and the lead l26 is not made by the burr.
As shown in Fig. 13, another alternative embodiment shock sensor 220 of this invention has a glass capsule 222 formed around a short lead 226 and a long lead 228, and a mounting lead 230. Electrical contact is made between the short and long leads by a spring 232.
The spring 232 has an attachment end 234 which is welded to a raised flange 236 at the free end 238 of the generally rigid long lead 228. The spring 232 has shock-sensing upper mass 240 and lower mass 242 which are welded to the spring 232 adjacent the contact end 244. The contact end 244 is comprised of a twisted portion 246 having a contact surface 248.
The spring 232 defines a plane and a centerline 249.
The twisted portion 246 is twisted about the center line 249 of the spring 232 to bring the contact flat 248 into a plane which is rotated by about 60 degrees with respect to the plane of the spring 232. The short lead 225 has a deformed portion 256 which defines a non-moving contact surface 258.
The contact surface 298 on the spring 232 and the fixed contact surface 258 on the short lead 226 engage at an angle a which is oriented 60 degrees from the direction of motion of the spring 232 and the sensing masses 290, 242, thus the closure between the contacts 248, 258 is softer. The soft closure results from the contact 248 on the spring sliding along the fixed contact surface 258 which, in turn, causes a limited deflection of the spring 232 in the plane of the spring 232. The sliding action between the spring contact surface 248 and the fixed contact surface 258 WO 98l27565 PCT/US97I20641 results in a frictional engagement between the surfaces 248, 258. The frictional engagement dissipates energy, helping to reduce bounce.
The shock sensors 20, 120 provide the ability to detect the presence of the shock sensor in a circuit even when the shock sensor is not in the actuated condition. This provides the ability to determine that the shock sensor has been properly incorporated in the crash detecting circuit. In the shock sensor 20 passing a test current between the long lead 28 and the mounting lead 30 provides the continuity check which indicates the presence of the shock sensor 20. The shock sensor l20 has sufficient pre-load between the sensing mass 131 and the I5 positioning lead 128 that a test current will pass between the support lead l28 and the third lead l30.
It should be understood that the coined acceleration sensing mass 131 could be used in the shock sensor 20 or the sensing mass 31 formed of powder metal could be used in the shock sensor l20.
It will also be understood that wherein a 60 degree angle is disclosed between the plane containing the spring 32 and the contact surfaces 40, 42, displacement of the contact surfaces by angles greater than or less than 60 degrees could be used.

Claims (10)

CLAIMS:

1. A shock sensor (20) comprising:
(a) a hermetically sealed glass capsule (22);
(b) a first substantially rigid conductive lead (26) extending into the capsule;
(c) a second conductive lead (28) extending into the capsule;
(e) a planar spring (32) defining a plane having an end mounted to the second conductive lead and having a movable end (34);
(f) a sensing mass (31) mounted adjacent to the movable end of the spring:
(g) wherein the movable end is movable towards and away from the first conductive lead; and (h) a contact surface (40) having a substantially planar portion which is oriented about 60 degrees from the plane defined by the spring, wherein the contact surface is formed at the movable end of the spring and engages with the first conductive lead to close an electrical circuit when the spring movable end moves towards the first conductive lead.

2. The shock sensor (20) of Claim 1 wherein the sensing mass (31) forms the movable end (34) of the spring and the contact surface (90) is formed on the sensing mass.

3. The shock sensor (20) of Claim 1 wherein the contact surface (40) is formed by a twisted portion of the spring (32).

4. The shock sensor (20) of Claim 1 further comprising a third conductive lead (30) extending into the capsule, the third lead joining with the second conductive lead (28).

5. The shock sensor (20) of Claim 4 wherein the second conductive lead (28) incorporates a means for accommodating compression forces between the second lead and the third lead (30).

6. The shock sensor (20) of Claim 5 wherein the means for accommodating compression forces is a U-shaped portion (48) of the second lead (28).

7. The shock sensor (20) of Claim 2 further comprising a third lead (30) extending into the capsule (22) in spaced parallel relation to the first lead (26), wherein the sensing mass (31) is positioned between the first lead and the third lead, the sensing mass being movable between a first position in engagement with the third lead when the shock sensor is not undergoing shock and a second position where the contact surface on the sensing mass is engaged with the first lead.

8. The shock sensor (20) of Claim 1 wherein the first lead (26) has a contact surface (42) in spaced parallel relation to the contact surface (40) on the movable end (39) of the spring (32).

9. The shock sensor (20) of Claim 1 wherein the first lead (26) has a cylindrical contact surface (42) which forms a line of contact with the contact surface (40) on the movable end (34) of the spring (32).

10. The shock sensor (20) of Claim 7 wherein the sensing mass (31) has portions which extend towards the third lead (30) and bracket the third lead so as to constrain the movable end (34) of the spring (32) by engaging the third lead if the sensing mass moves substantially in the plane of the spring.