Acceleration sensor

An acceleration sensor is formed of a housing, an inertia member located inside the housing so as to be freely movable in a longitudinal direction of the housing, a conductor provided on at least an end surface of the inertia member in the longitudinal direction of the housing, a pair of electrodes disposed at one end side of the longitudinal direction of the housing, and electrically connected together by a conductive bridging inertia member. An attractor is disposed at the other end side of the longitudinal direction of the housing and magnetically attracts the inertia member. A stopper is disposed at an opposite side of the inertia member with respect to the electrodes, and abuts against the tip surface of the inertia member when the inertia member moves forwardly. The stopper is disposed at a position which is located off-center with respect to or deviated from the axial center line of the housing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an acceleration sensor, and particularly to an 
acceleration sensor suitable for detecting variation of speed which occurs 
due to collision and so on, of a vehicle. 
2. Description of the Related Art 
As this type of an acceleration sensor, U.S. Pat. No. 4,827,091 discloses 
an acceleration sensor comprising a housing of conductive material, a 
magnetized inertia member which is mounted in the housing so as to be 
freely movable in a longitudinal direction of the housing, a conductor 
provided on at least one end surface of the magnetized inertia member in 
the longitudinal direction of the housing, a pair of electrodes which are 
disposed at one side of the longitudinal direction of the housing and are 
electrically connected together through the conductor when contacted with 
the conductor of the magnetized inertia member, and an attractor of 
magnetic material which is disposed at the other end side of the 
longitudinal direction of the housing and magnetically attracting the 
magnetized inertia member. 
In this acceleration sensor, the attractor attracts the inertia member, and 
thus the magnetized inertia member stands still at the other end side 
inside of the housing when no or little acceleration is applied to the 
acceleration sensor. 
When some large acceleration is applied to the acceleration sensor, the 
magnetized inertia member is moved against the attraction force acting 
between the magnetized inertia member and the attractor. During movement 
of the magnetized inertia member, an induced current flows in the housing, 
and the magnetized inertia member receives a magnetic force which urges 
the magnetized inertia member in an opposite direction to the moving 
direction. Therefore, the magnetized inertia member is kept braked, and 
its moving speed is reduced. 
When the acceleration is lower than a predetermined value (threshold 
value), the magnetized inertia member does not reach the end of the 
housing, and moves to a halfway position and stops there. Subsequently, 
the magnetized inertia member is pulled back to the other end side by the 
attraction force acting between the magnetized inertia member and the 
attractor. 
On the other hand, when the acceleration is greater than the predetermined 
value (threshold value) for example, when a vehicle equipped with this 
acceleration sensor collides against an object, the magnetized inertia 
member reaches the one end side of the housing. The conductive layer of 
the tip surface of tile magnetized inertia member contacts the pair of 
electrodes to conduct electricity through the electrodes. A voltage is 
beforehand applied across the electrodes, so that current flows across the 
electrodes at the time when the electrodes are short-circuited. The 
collision of the vehicle is detected on the basis of this current. 
A stopper is disposed at the opposite side to the magnetized inertia member 
with respect to the electrodes. When the magnetized inertia member with 
the acceleration greater than the above threshold value abuts against the 
electrodes and moves forwardly while pushing the electrodes, the 
magnetized inertia member finally abuts against the stopper. The 
magnetized inertia member keeps pushing against the stopper by the 
acceleration for a while, and for this period the conduction between the 
electrodes through the magnetized inertia member continues. As described 
above, the electrical conduction between the electrodes occurs for some 
long time, whereby the collision is electrically detected on the basis of 
this electrical conduction in a collision detection circuit. 
However, in the conventional acceleration sensor, the magnetized inertia 
member is repelled by the stopper when the magnetized inertia member abuts 
against the stopper, and thus there occurs a case where a time for the 
conduction between the electrodes is shortened. 
Further, in the conventional acceleration sensor, when the magnetized 
inertia member abuts against the stopper, the magnetized inertia member 
repetitively contacts with and separates from the stopper, and there 
frequently occurs chattering in the electrical conduction between the 
electrodes. That is, the magnetized inertia member abuts against the 
stopper and slightly repelled back. Thereafter, the magnetized inertia 
member is accelerated, and abuts against the stopper again and repelled 
back again. Subsequently, the magnetized inertial member is accelerated 
again and abuts against the stopper again. Such contact with (abutting 
against) and separation from the stopper are repeated. Such repetitive 
motion of the magnetized inertia member in the forward and backward 
directions as described above causes the electrodes to be frequently 
electrically interrupted, and thus the chattering is induced. 
OBJECT AND SUMMARY OF THE INVENTION 
An object of this invention is to provide an acceleration sensor in which 
the conduction between electrodes through an inertial member is continued 
for a long time. 
Another object of this invention is to provide an acceleration sensor in 
which chattering is prevented. 
The acceleration sensor according to this invention includes a housing, an 
inertia member which is mounted inside of the housing so as to be freely 
movable in the longitudinal direction of the housing, a conductor provided 
on at least one end surface of the inertia member in the longitudinal 
direction of the housing, a pair of electrodes which are disposed at one 
side of the longitudinal direction of the housing and electrically coupled 
together through the conductor when engaged with the conductor of the 
inertia member, an attractor which is disposed at the other end side of 
the longitudinal direction of the housing and magnetically attracts the 
inertia member, and a stopper which is disposed at an opposite side to the 
inertia member with respect to the electrodes and with which the tip 
surface of the inertia member engages when the inertia member moves 
forwardly, the stopper being disposed at a position which is deviated from 
the axial center line. 
According to the acceleration sensor of this invention, when the 
greatly-accelerated inertia member moves forwardly and abuts against the 
stopper, the inertia member is inclined to such a direction that the 
direction of the axial center line of the inertia member intersects the 
axial center line of the housing. Through this motion, the inertia member 
is pushed against the inner peripheral surface of the housing. As a 
result, a relatively-large friction force occurs between the inertia 
member and the inner peripheral surface of the housing, and thus the 
inertia member is hardly moved. That is, even when the stopper repels back 
the inertia member, the inertia member is hardly moved backwardly, and the 
inertia member engages the stopper or stops in the neighborhood of the. 
stopper for a longer time, so that the conduction between the electrodes 
continues for a long time. 
Further, the repetitive reciprocative motion of the inertia member in the 
forward and backward directions is prevented, and the chattering of the 
electrodes is prevented.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A preferred embodiment according to this invention will be hereunder 
described with reference to the accompanying drawings. FIG. 1 is a 
cross-sectional view of an acceleration sensor in a longitudinal direction 
of a housing, according to an embodiment of this invention, and FIG. 2a is 
a cross-sectional view of the acceleration sensor which is taken along a 
line 2--2 of FIG. 1. 
In FIG. 1, a housing 12 of copper alloy is held inside of a cylindrical 
bobbin 10 which is formed of non-magnetic material such as synthetic 
resin, and a magnetized inertia member (magnet assembly) 14 is mounted 
inside of the housing 12. The magnet assembly 14 is equipped with a 
solid-cylindrical permanent magnet (magnet) 16, a cylindrical case 18 
containing the magnet 16 therein, which has a bottom and no lid and is 
formed of non-magnetic conductive material such as copper, and a synthetic 
resin packing 20 for holding the magnet 16 in the case 18. 
The magnet assembly 14 is inserted into the housing 12 so as to be freely 
movable in the longitudinal direction of the housing 12. The outer 
diameter of the magnet assembly 14 is set to be slightly smaller than the 
inner diameter of the housing 12, and a slight gap is formed between the 
outer peripheral surface of the magnet assembly 14 and the inner 
peripheral surface of the housing 12. 
The bobbin 10 has one end serving as an insertion portion 22 which extends 
into the housing 12, and an opening 24 is formed at the tip portion of the 
insertion portion 22. A pair of flanges 26 and 28 are projectingly 
provided to the bobbin 10 at a side portion of the insertion portion 22, 
and a ring-shaped attractor (return washer) 30 of magnetic material such 
as iron is provided so as to be sandwiched between the flanges 26 and 28. 
The bobbin 10 is provided with another flange 32, and a coil 34 is wound 
between the flange 28 and the flange 32. Another flange 36 is provided at 
the other end side of the bobbin 10, and a contact holder 38 is secured to 
the flange 36. 
The contact holder 38 is formed of synthetic resin, and a pair of 
electrodes 40 and 42 are embedded at rear ends into the contact holder 38 
at portions 38'. The tip end side of each of the electrodes 40 and 42 is 
projected into a vacant room 44 of a central portion of the contact holder 
38. The vacant room 44 extend at rear sides of the electrodes 40 and 42 so 
that the electrodes bend rearwardly. In addition, the tip end side of each 
of the electrodes 40 and 42 is bent in an arcuate form and disposed so 
that a part thereof is located on substantially the same plane as the tip 
surface of the housing 12. 
The contact holder 38 is provided with an opening 46 through which the 
inside of the vacant room 44 is intercommunicated to the outside. A 
stopper 48 is projectingly provided on a surface 44a of the vacant room 44 
confronting the tip surface of the magnet assembly 14. The stopper 48 is 
deviated from the axial center line of the housing 12. 
In this embodiment, as shown in FIG. 2, the electrodes 40 and 42 are 
disposed in a radial direction and extend toward the center of the vacant 
room 44. The stopper 48 is provided at the opposite side of the magnet 
assembly 14 with respect to the electrodes 40 and 42. The stopper 48 is 
disposed in the vacant room 44 so as to abut against the end portion of 
the tip surface of the magnet assembly 14. 
In the acceleration sensor thus constructed, the magnet assembly 14 is 
attracted by the return washer 30 in a state where no external force is 
applied, so that the magnet assembly 14 is located at a backward limited 
position where the rear end of the magnet assembly 14 abuts against the 
tip surface of the insertion portion 22. Upon exertion of the external 
force in a direction as indicated by an arrow A, the magnet assembly 14 is 
moved in the direction as indicated by the arrow A against the attraction 
force acting between the magnet assembly 14 and the return washer 30. 
Through this motion, induced current flows in the housing 12 of copper 
alloy, and a magnetic field which is caused by the induced current induces 
a magnetic force in the direction opposite to the moving direction, so 
that a braking force is applied to the magnet assembly 14. 
When an external force supplied to the acceleration sensor is small, the 
magnet assembly 14 is stopped at the time when it reaches a halfway 
position of the housing 12, and finally the magnet assembly 14 is returned 
to its backward limited position as shown in FIG. 1 by the attraction 
force between the magnet assembly 14 and the return washer 30. 
When a great external force occurring at the collision time of the vehicle 
or the like is applied in the direction as indicated by an arrow A, the 
magnet assembly 14 moves forwardly to the tip of the housing 12, and 
engages with the electrodes 40 and 42. It further moves forwardly while 
pushing and bending the electrodes 40 and 42, and finally abuts against 
the stopper 48. This condition is shown in dotted lines in FIG. 1 and FIG. 
2b. 
When the magnet assembly 14 engages with the electrodes 40 and 42, the 
electrodes 40 and 42 are short-circuited through the case 18 of the magnet 
assembly 14 which is formed of conductive material, so that current flows 
through the electrodes 40 and 42. Through this current flow, an 
acceleration variation which is greater than a predetermined threshold 
value is detected, and the vehicle collision is detected. 
When the magnet assembly 14 moves forwardly and abuts against the stopper 
48, since the stopper 48 is deviated from the center of the tip surface, 
the magnet assembly 14 is inclined to such a direction that the axial 
center line of the magnet assembly 14 intersects the axial center line of 
the housing 12. Through this inclination, the magnet assembly 14 is pushed 
against the inner peripheral surface of the housing 12, as shown in FIG. 
2b. As a result, a relatively-large friction force occurs between the 
magnet assembly 14 and the inner peripheral surface of the housing 12, and 
the magnet assembly 14 is hardly moved. That is, the magnet assembly 14 is 
not easily backwardly moved even when the stopper 48 acts to repel the 
magnet assembly 14, and the magnet assembly 14 continuously engages the 
stopper 48 or stands substantially still in the neighborhood of the 
stopper 48 for a long time, and thus the conduction between the electrodes 
40 and 42 continues for a long time. 
The repetitive vibratory motion of the magnet assembly 14 is prevented, and 
thus the chattering of the electrodes 40 and 42 is prevented. 
The coil 34 is used to check the operation of the acceleration sensor as 
described above. That is, upon supply of current to the coil 34, a 
magnetic field urging the magnet assembly 14 in the direction as indicated 
by the arrow A is generated by the coil 34, and the magnet assembly 14 
moves forwardly to the tip of the housing 12 to short-circuit the 
electrodes 40 and 42. By forcedly moving the magnet assembly 14 on the 
basis of the current supply to the coil 34, it can be checked whether the 
magnet assembly 14 can be moved, and whether the electrodes 40 and can be 
short-circuited. 
Next, experimental results will be described. 
(Example of this Embodiment) 
The following experimental conditions were set for the acceleration sensor 
as shown in FIGS. 1 and 2. 
Inner diameter of housing 12 7.0 mm 
Outer diameter of housing 12 8.7 mm 
Length of housing 12 19.2 mm 
Diameter of magnet assembly 14 6.7 mm 
Length of magnet assembly 14 12.0 mm 
Projection length of stopper 48 3.0 mm 
Diameter of cylindrical stopper 48 1.6 mm 
Stroke until magnet assembly 14 abuts against electrodes 40 and 42 5.5 mm 
Stroke until magnet assembly 14 abuts against stopper 48 after it abuts 
against electrodes 40 and 42 4.0 mm 
In the acceleration sensor, a conduction time between the electrodes 40 and 
42 when the magnet assembly 14 engaged the stopper 48 by applying maximum 
acceleration (peak G) as shown by Nos. 1 and 2 in a table 1 was measured. 
The result is also shown in the table 1. 
FIGS. 3a and 3b are voltage waveform diagrams of an electrode output and an 
output waveform diagram of the collision detection circuit for maximum 
acceleration of 200 G in this invention and the comparative example as 
described below. 
(Comparative Example) 
The same measurement was made except that the stopper 48 was disposed along 
the axial center line of the housing 12 and the position of the opening 46 
was deviated. The result is shown in the table 1 and in FIGS. 3a and 3b. 
From the table 1, according to the example of this invention, the 
conduction time between the electrodes 40 and 42 is remarkably prolonged 
in comparison with the comparative example. 
From FIG. 3b, an intensive chattering occurred in the comparative example 
whereas no chattering occurred in this invention (FIG. 3a). 
TABLE 1 
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CONDUCTION 
NO. ACCEL. (G) TIME (ms) 
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THIS INVENTION 
1 200 10.00 
2 300 9.23 
COMP. EXAMP. 3 200 5.26 
4 300 3.94 
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In the above embodiment, the inertia member 14 is magnetized, however, a 
non-magnetized inertia member may be used. In this case, a magnetized 
return washer is used as the return washer 30. In this case, the housing 
12 may be formed of non-conductive material. 
As described above, according to the acceleration sensor of this invention, 
the position of the stopper against which the inertia member abuts is 
deviated from the axial center line of the housing, so that during a 
collision the inertia member engages the stopper for a long time or stands 
substantially still in the neighborhood of the stopper. Therefore, the 
conduction state between the electrodes is continued for a long time, and 
the chattering is prevented during the conduction state. As a result, the 
detection precision of the vehicle collision is remarkably improved.