Pendulum mass acceleration sensor

Abstract: A shock sensor (20) having a housing (22) which mounts directly to a printed circuit board. The housing (22) contains a switch (44) or sensor (76) embedded in the housing and connected by leads (52) to the circuit board. A pendulum sensing mass (30) is suspended by two flexible pendulum arms (34). The arms (34) come together where they are attached to the housing and constrain the mass to swing along an arc (38) which lies in a single plane. A magnet (32) either forms the sensing mass or is mounted to the sensing mass. The reed switch or Giant Magneto-restrictive Sensor (GMR) type sensor is encapsulated within the base (23) of the housing (22). When a shock causes motion of the sensing mass (30) and the magnet (32), the motion is detected by the sensor or switch. A pair of mechanical contacts (64) on either side of the sensing mass can be substituted for the reed switch.

Description: BACKGROUND OF THE INVENTION The present invention relates to shock sensors used to detect crashes in automobiles in general, and to shock sensors employing a pendulum mass in particular. A typical automobile manufactured today has a number of safety systems which function to deploy air bags, and possibly to retract seat belts. The number of air bags provided in a single vehicle continues to increase. Systems now in development deploy multiple air bags to protect the passenger from front, rear, and side impacts, and to position the passenger's body to withstand acceleration. Deployment of safety systems requires sensors which can detect and characterize a crash as it occurs. Wide use of advanced active safety systems is dependent on producing systems which are sufficiently cost effective that they may be economically employed on a large number of vehicles. Typically the lowest cost sensors are those formed on an integrated circuit chip using the technology used to form electronic circuitry. This technology is used to fabricate micro devices or accelerometers which can detect accelerations indicative of a vehicle crash. These sensors are particularly cost effective when the sensor can be fabricated together with the deployment logic circuitry using the same technology which is cost effective for large scale integration. However the very small size of these devices makes them sensitive to electromagnetic interference and the like, which can result in false indications that a crash is taking place. Thus an important role remains for macro scale mechanical devices which are less prone to false readings. Such devices are used to verify the existence of an actual crash event. These macro scale devices employ a sensing mass mounted on a spring or pendulum. Motion of the mass is detected by actuation of a reed switch or a magnetic field sensor of the Hall type. The typical reed switch shock sensor is a bidirectional sensor, principally because of the advances made in low cost and small packaging for this type of sensor, see for example U.S. No.4,103,842 which has a pendulum-mounted magnet which is positioned over a Hall effect sensor; or U.S. Patent No.5,450,049 which uses a pendulum-mounted sensing mass and magnet positioned over a reed switch for detecting bidirectional acceleration. However, bidirectional or multidirectional shock sensors typically have proven too expensive or too bulky to satisfy the demand for a bidirectional shock sensor with the size and cost achieved by the unidirectional reed switch type sensors. Another sensor is shown in US Patent No.4039790 in which an inertia mass is supported in a housing for movement along a given path. Two pairs of spring blades are cantilever-mounted in the housing. Movement of the mass in one direction closes the first pair of blades and movement in a second direction closes the second pair of blades. Adjustment members are mounted in the housing and bear against one of the blades in each pair to adjust the switching sensitivity. The blades and a warning member, such as an electric light, are mounted in the base of a channel shaped member which snaps into an outer housing shell. A transparent cap is mounted on the housing shell making visible the warning light. Housing flanges are convexly formed and have a plurality of parallel ribs which may be adapted to a large variety of supporting flanges. In one embodiment, the mass is slidable on an elongate rod. Also, the mass may be suspended on two of the spring blades. What is needed is a bidirectional shock sensor having a small package size and the simplicity of design to achieve cost effectiveness. According to the invention there is provided a bidirectional shock sensor comprising: a housing, the housing having a base adapted for mounting on a circuit board, portions of the housing forming a pendulum cavity, the cavity having an upper portion and a lower portion; a pendulum mass supported from the cavity upper portion by two pendulum arms rigidly mounted at a mounting position to the housing, the pendulum arms being in spaced relation where they connect to the pendulum mass, so that the pendulum mass may swing along an arc within the cavity, the arc defined by the swing of the pendulum mass having a lowest point and defining a first direction and a second direction, the two pendulum arms being formed of flexible planar wires which have their greatest extent perpendicular to the plane defined by the arc along which the pendulum mass swings, and a means for detecting motion of the pendulum mass in the first direction and in the second direction opposite the first direction, characterised in that the wires are brought together adjacent the mounting position. When a shock or acceleration which has a direction which is at least partially aligned with the arc along which the sensing mass swings is imposed on the housing, the resulting motion of the sensing mass and the magnet forming a part thereof is detected by the sensor or switch. In the case of a reed switch acting as the sensor, and in which the reeds making up the switch are aligned in the plane in which the sensing mass pivots, the motion of the sensing mass and the magnet mounted thereto will cause the reed switch to change state from open to closed or from closed to open. One embodiment of the shock sensor may employ a Giant Magneto-restrictive Sensor (GMR) which may be of the digital type, thus providing a switch-like response to the analog type which produces a value corresponding to the displacement of the magnet mounted to the sensing mass. A further embodiment places a pair of mechanical contacts on either side of the sensing mass so that motion of the sensing mass along the arc through which it swings, causes the mass to engage one pair of mechanical contacts positioned on either side of the mass. It is a feature of the present invention to provide a sensor which provides an indication of acceleration from two opposed directions. It is another feature of the present invention to provide a shock sensor which can provide a signal proportional to the size of an acceleration experienced by the shock sensor. It is a further feature of the present invention to provide a shock sensor which incorporates both a binary switch and a sensor capable of providing a signal proportional to the displacement of a pendulum mass. 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, partially cut-away in section, of the shock sensor of this invention. FIG. 2 is a side elevational cross-sectional view of the shock sensor of FIG. 1. FIG. 3 is a cross-sectional view of the shock sensor of FIG. 2 taken along section line 3-3. FIG. 4 is a side elevational cross-sectional view of an alternative embodiment of the shock sensor of this invention employing mechanical contacts. FIG. 5 is a side elevational cross-sectional view of a further embodiment of the shock sensor of this invention employing a GMR sensor. FIG. 6 is a side elevational cross-sectional view of a still further embodiment of the shock sensor of this invention employing both a GMR sensor and a reed switch. FIG. 7 is a side elevational view of the shock sensor of FIG. 2 including an additional staple shunt. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly to FIGS. 1-7 wherein like numbers refer to similar parts, a shock sensor 20 is shown in FIG.1. The shock sensor 20 has a plastic housing 22 which composed of cover which is affixed to a body 26. A cavity 28 is defined within the housing 22 between the cover 24 and the body 26. The cover 24 defines an upper portion of the cavity 28, while the body 26 defines a lower portion of the cavity and a base 23 adapted for mounting to a circuit board (not shown). A pendulum mass 30 is supported from the cover 24 within the cavity 28. The pendulum mass 30 incorporates a permanent magnet 32 and is suspended from two planar wires 34 which connect to opposite sides 36 of the pendulum mass. As shown in FIG. 2, the pendulum mass 30 swings along an arc indicated by arrows 38 within the cavity 28. The low point 40 of the arc 38 defines the rest position of the pendulum mass 30 when it is not undergoing acceleration other than gravity. Feet 42 extend from the shock sensor housing 22 beneath the base 23, and are adapted to mount the shock sensor 20 to extend vertically from the horizontal surface of a circuit board (not shown). A reed switch 44 is positioned within portions 46 of the body 26 of the housing 22 by injection molding the plastic body 26 around the reed switch 44. The reed switch 44 has a glass capsule 48 and two ferromagnetic reeds 50 which have contacts 51 which overlap in the center of the reed switch 44. The reeds 50 extend through the ends of the glass capsule to form leads 52. The reed switch 44 is positioned with the contacts 51 beneath the low point 40 of the arc 38. The reed switch 44 represents a normally open reed switch and if the magnet 32 is formed with a single pair of vertically arranged poles, for example North up and South down, as shown in FIG. 2, the reeds 50 will make contact if the magnet 32 swings sufficiently far to either side 54 of the housing 22. The shock sensor 20 is designed to provide a safing function as part of an automobile safety system which activates various safety devices such as air bags, and seat belt retractors. The magnitude and direction of a car crash is normally monitored by micro devices formed on an integrated circuit chip. However the size of these devices makes them and their monitoring electronics subject to electromagnetic interference which can produce a false indication of a crash. A mechanical sensor such as the shock sensor 20 provides a real world mechanical check on whether a crash is taking place. The safety device deployment logic will thus look to the safing sensor as a double check on the physical existence of a crash event before accepting and acting upon the results of electronic sensors. Most existing devices which fulfill this safing function are unidirectional for reasons of cost. The shock sensor 20 provides bidirectional sensing. Typically the sensor would be positioned to detect front and rear collisions in an automobile, and would return a single state, a closed circuit through the reed switch, in response to either a front collision or a rear collision. Low cost is achieved in the shock sensor by low part count, small size, and simplicity of manufacture and assembly. The body 26 of the housing 22 may be formed by injection molding which is performed with the reed switch positioned within the mold. This results in precise positioning of the reed switch 44 with respect to the body 26. The feet 42 are integrally formed with the body 26 and the leads 52 are soldered to the circuit board (not shown) thus providing a simple mounting system. The sensor 20 may be small in size, the illustrated device in FIG.1 is eighteen millimeters wide, approximately fifteen millimeters high, and six millimeters wide. The pendulum mass 30 is held by the two planar wires 34 which are welded or bonded to the sides 36 of the pendulum mass 30. The planar wires 34 are brought together and pass through the cover 24 and the portions 56 which extend through the cover 24 are bent over and heat staked to the top 57 of the cover. By bringing the wires together, the acceleration force needed to actuate the shock sensor 20 is brought into the range needed to fulfill the safing function. The pendulum assembly 58 may be cost-effectively fabricated by forming two covers 24 at the same time. The cover tops 57 are formed in a single mold facing each other. Two planar wires are bonded between two pendulum masses. The pendulum masses and the planar wires are inserted into the cover mold which causes the planar wires to be brought together and to run parallel for a space between the pendulum masses. The covers are formed with the planar wires in place. The wires are cut halfway between the two covers and bent downwardly to the cover top surfaces where they are heat staked in place. A short segment of plastic flashing 70 may be retained between the bent over planar wires 34. The cover with the depending pendulum mass 30 is then ultrasonically welded to the body 26 of the housing 22. Damping of the pendulum mass 30 is accomplished by interaction between the permanent magnet 32 and the ferromagnetic reeds 50 forming the reed switch 44. If desired, an additional ferromagnetic wire 53 forming a ferromagnetic shunt 49, as shown in the sensor 45 in FIG. 7, may be run over the top and across a reed switch 47 to form two additional leads 55 which extend to the circuit board (not shown). The shunt 49 has the general shape of a staple across the center of the reed switch 47. This additional ferromagnetic wire 53 would provide additional damping, and a circuit continuity check which can be used to determine whether the sensor 45 is present on the circuit board. The cavity is sealed and thus damping can be provided by gas or other fluid within the cavity. The pendulum mass 30 may be a magnet, thus combining the functions of a sensing mass with the magnet. The pendulum arms formed by the wires 34 will preferably be formed of flexible spring wire such as beryllium copper or stainless steel, approximately one to two millimeters wide and 0.75 millimeters thick. However, round wire could be used with some loss of the tendency of the pendulum to swing in a single plane. An alternative embodiment shock sensor 60 is shown in FIG. 4. The pendulum mass 62 is mounted to a housing 59 which defines a cavity 65 . The pendulum mass 62 moves against a first and second switches 61, 63 formed by upstanding reeds 64 which are pushed against fixed contacts 66. The upstanding reeds 64 are electrically connected to a depending lead 68 which extends from the bottom of the housing 59. Likewise the fixed contacts 66 are electrically connected to a second lead 72 which also extends from the bottom of the housing 59. Thus the shock sensor 60 again forms a bidirectional sensor which provides a single indication that a shock of a selected magnitude has occurred by completing a circuit. A new class of magnetic field sensors is known as Giant magneto-resistive sensors (GMR). An alternative embodiment shock sensor 74, shown in FIG. 5, has an integrated circuit chip 76 which contains a GMR sensor. The GMR sensor within the chip 76 is positioned below a magnet 78 mounted to a pendulum mass 80. A GMR type sensor utilizes a discovery made in 1988. The phenomenon discovered is that resistors built up of layers of thin magnetic film a few nanometers thick separated by equally thin nonmagnetic layers have resistance which depends on the strength of a magnetic field applied to the resistor. A decrease in resistance of between about ten and twenty percent in the built-up resistor is observed when a magnetic field is applied. The physical explanation for the decrease in resistance is the spin dependence of electron scattering and the spin polarization of conduction electrons in ferromagnetic metals. The extremely thin adjacent magnetic layers couple antiferromagnetically to each other so that the magnetic moments of each magnetic layer are aligned antiparallel to adjacent magnetic layers. Electrons, spin polarized in one magnetic layer, are likely to be scattered as they move between adjacent layers. Frequent scattering results in high resistance. An external magnetic field overcomes the antiferromagnetic coupling and produces parallel alignment of moments in adjacent ferromagnetic layers. This decreases scattering and thus device resistance. Within an integrated circuit package groups of four resistors based on the GMR technology are arranged in a Wheatstone bridge and two legs of the bridge are shielded from the applied magnetic fields. The other two legs are positioned between the magnetic shields. The magnetic shields act as flux concentrators to produce a device of tailored sensitivity to a magnetic flux of a selected intensity. A standard voltage or current is supplied to the solid state device, while a value relating to current or voltage is read out which is proportional to the magnetic field to which the device is exposed. The devices have an axis of sensitivity which is produced by the orientation of the magnetic flux shields. In the sensor contained in the chip 76 this axis is aligned in a direction of swing of the pendulum mass. GMR sensors are available from Nonvolatile Electronics Inc. of 11409 Valley View Rd., Eden Prairie, Minnesota (www.nve.com). GMR sensors are small, highly sensitive devices which have exceptional temperature stability, deliver high signal levels, require very little power, and cost less than many competitive devices. All these factors are important in devices used in automobile safety applications. Two types of GMR sensors are manufactured, the type just described which produces an analog output proportional to the displacement of the magnet 78. This type of output could be used not only for the safing function but also to provide the detailed data with respect to the timing and magnitude of the crash which is used by the deployment logic (not shown) to decide whether an air bag or other safety equipment should be operated. The second type of GMR is digital in nature, incorporating an electronic circuit which closes a switch based on the presence or absence of a magnetic field of a selected strength. This is very similar to the function provided by a reed switch with the advantage that the so-called "dwell time" which is a function of the difference between the magnetic field strength which causes the switch to close and the field strength which causes the switch to open, can be arbitrarily designed by adjusting the digital values used to clear latching and unlatching of a circuit. A GMR sensor requires a supply voltage, so the minimum number of leads is three for a digital device and four for a analog device. The shock sensor 74 shown in FIG. 5 has a GMR package in an eight pin chip 76. Eight leads 81 extend from the bottom of the housing 82 to a circuit board (not shown) for mounting the shock sensor 74. Another alternative embodiment shock sensor 84, which combines both a reed switch 86 and a GMR chip 88, is shown in FIG. 6. Such a sensor combines the safing function provided by the reed switch 86 with a characterizing function, which is provided by the GMR sensor 88. The characterizing function is a necessary input to the deployment logic (not shown) and is used by the deployment logic to analyze the nature and timing of the shock. Other combinations of sensors are possible, for example combining the mechanical switch illustrated in FIG. 4 with a GMR sensor, or combining two GMR sensors: one which provides a digital result for safing, and one which provides a displacement value as a deployment logic input. It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.

Claims: A bidirectional shock sensor (20,45,60,74,84) comprising: a housing (22) having a base (23) adapted for mounting on a circuit board, portions of the housing (22) forming a pendulum cavity (28,65), the cavity (28,65) having an upper portion (24) and a lower portion (26); a pendulum mass (30,62,80) supported from the cavity upper portion (24) by two pendulum arms (34) rigidly mounted at a mounting position to the housing (22), the pendulum arms (34) being in spaced relation where they connect to the pendulum mass (30,62,80), so that the pendulum mass (30,62,80) may swing along an arc (38) within the cavity (28,65), the arc (38) defined by the swing of the pendulum mass (30,62,80) having a lowest point (40) and defining a first direction and a second direction, the two pendulum arms (34) being formed of flexible planar wires which have their greatest extent perpendicular to the plane defined by the arc along which the pendulum mass (30,62,80) swings, and a means (44,61,63,76) for detecting motion of the pendulum mass (30,62,80) in the first direction and in the second direction opposite the first direction, characterised in that the wires (34) are brought together adjacent the mounting position. The shock sensor (20) of claim 1 wherein the means for detecting motion of the pendulum mass (62) comprises: a first switch (61) positioned within the cavity (65) and spaced from the lowest point so that motion of the mass (62) in the first direction engages the switch causing it to change states; and a second switch (63) positioned within the cavity (65) and spaced so that motion of the mass (62) in the second direction engages the switch causing it to change states. The shock sensor (20,45,84) of claim 1 wherein the pendulum mass incorporates a means for generating a permanent magnetic field (32), and wherein the means for detecting motion of the pendulum mass is a reed switch (44,47,86) positioned below the lowest point (40) of the pendulum mass arc (38). The shock sensor (74) of claim 1 wherein the pendulum mass (80) incorporates a means for generating a permanent magnetic field (78), and wherein the means for detecting motion includes a GMR sensor (76) mounted to the housing below the pendulum mass (80). The shock sensor (84) of claim 3 wherein the means for detecting motion of the pendulum also includes a GMR sensor (88) positioned above the reed switch (86) and below the pendulum mass (80). The shock sensor (45) of claims 3 to 5 further incorporating a ferromagnetic shunt (49) positioned below the sensing mass (30) to provide magnetic damping. The shock sensor (20,45,60,74,84) of claims 1 to 5 wherein the pendulum arms (34) are brought together above the pendulum mass (30,62,80) before being fixedly mounted to the housing (22). The shock sensor (20,45,60,74,84) of claims 1 to 5 including a plurality of leads (52,68,72,81) extending from the bottom of the housing, and wherein the means for detecting motion of the pendulum mass are connected to said leads (52,68,72,81). The shock sensor (20,45,60,74,84) of claims 1 to 5 wherein the lower portion (26) of the cavity (28,65) contains the means (44,61,63,76) for detecting motion of the pendulum mass (30,62,80), and the upper portion (24) of the cavity (28,65) is sealed to the lower portion (26), the two flexible planar pendulum arms (34) being rigidly mounted to the upper portion (24). The shock sensor of claims 3 to 5 wherein the means (44,61,63,76) for detecting motion of the pendulum mass (30,62,80) is encapsulated within material forming the housing (22).

Inventors: EDWARDS PAUL K, SIMPSON RICHARD G, EDWARDS, PAUL K., SIMPSON, RICHARD G.
Applicants: BREED AUTOMOTIVE TECH, BREED AUTOMOTIVE TECHNOLOGY, INC.
Application year: 1999
Main classification: G01P15
IPC codes: G01P15, H01H35