Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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
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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.
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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.
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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
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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
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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
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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
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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
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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
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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
0
POWDER PHYSICAL PROPERTIES Apparent Density: 4.17 g/cc
(Size Distribution) N
0
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
0
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
0
-5.5 8.0
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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
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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
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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
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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.
