SEAT BELT BUCKLE SENSOR

Abstract: A seat belt buckle (20) incorporates a spring (61) mounted on the latch (38). When the latch engages with the tongue (60) of a seat belt, the tongue can move against the spring in response to tension loading of the seat belt while remaining securely latched. This motion due to belt tension is resisted by the spring, with a selected spring constant. A sensor (76) is used to monitor the presence and position of the tongue either directly or by measuring motion of the latch. The position information, which is related to spring compression, is used to determine loading on the tongue and thus seat belt tension.

Description: SEAT BELT BUCKLE SENSOR The present invention relates in general to sensors used to determine when and how an airbag should be deployed and in particular to sensors for monitoring seat belt latch status and belt tension. Originally airbags were viewed as an alternative to seat belts because they are passive devices which do not require the cooperation of the vehicle's occupants to be effective. However, experience has shown that airbags should be considered an adjunct to seat belts. An airbag system comprises two basic functional components: the airbag, including the means for deploying it; and the sensors and circuitry that determine whether the airbag should be deployed. A factor to be considered by airbag deployment logic is whether the occupant is properly wearing a seat belt. Sensors to determine if a seat belt is buckled were first widely used to remind the occupants of a motor vehicle to fasten their seat belts. The combination of seat belts and airbags can be optimized if it can be determined that the seat belt is actually in use. If the seat belt is fastened, then deployment logic can be based on knowledge of that condition. Accurate sensing information of seat-belt latch status is far more important when used as part of the decision logic and deployment strategy of an airbag system then when used simply as part of a seat belt fastening reminder system. The typical improved seat belt latch indicating system such as might be used in combination with an airbag deployment system, is a mechanical switch within the buckle of a seat belt combined with a resistor network. The simplest resistor network has a first resistor connected in series with the switch and a second resistor connected in parallel with the switch. Thus the resistance of the switch and the resistor network changes when the switch is closed. But the switch, because of the resistor connected in parallel across the open leads of the switch, can be positively monitored when open. Thus if resistance falls to zero or has a resistance value substantially different from the two designed states of the switch and resistor network, it is clear that the seat belt latch detector is broken. The environment in which seat belts are installed can be a hostile one. The switch is called upon to operate when dirty, wet, or even when coated with residue from spilled beverages. Further, the use of seat belt latch sensors to sound reminder chimes or a buzzer is sometimes perceived as annoying, and can lead those resistant to wearing safety belts to disable a seat belt latch indicator by causing the buckle to latch without engaging the hasps of the belt. Where seat belt latching information is being used in airbag deployment logic, the folly of disabling safety equipment can prevent the deployment logic from making the best possible decision by providing false information to the logic. It is an object of the invention to provide a seat belt sensor which can monitor latch status and belt tension, and which is both durable and better able to monitor latch status. 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 shows a child's car seat in the front passenger seat of a vehicle. FIG. 2 is an exploded isometric view of the seat belt buckle of this invention. FIG. 3 is a side elevation cross-sectional view of the buckle of FIG. 2 unlatched. FIG. 4 is a side elevation cross-sectional view of the buckle of FIG. 2 latched without tension on the seat belt. FIG. 5 is a side elevation cross-sectional view of the buckle of FIG. 2 latched with tension on the seat belt. FIG. 6 is a side elevation cross-sectional view of the buckle of FIG. 2 latched without a tongue. FIG. 7 is a top plan view of the buckle of FIG. 2 latched. FIG. 8 is a bottom plan view of an alternative buckle and tongue without tension on the seat belt. FIG. 9 is a bottom plan view of the buckle of FIG. 8 with tension on the seat belt. FIG. 10 is a cross-sectional view of the buckle of FIG. 9 taken the section line 10-10. FIG. 11 is an isometric bottom view of a further alternative embodiment of the seat belt buckle. FIG. 12 shows the latch member of the buckle of FIG. 13 with the tongue engaging the tongue spring. FIG. 13 is a simplified isometric view of another buckle, similar to that shown in FIG. 8 unlatched. FIG. 14 is an isometric view of the buckle of FIG. 13 latched without tension on the tongue. FIG. 15 is an isometric view of the buckle of FIG. 13 with the tongue under tension. DETAILED DESCRIPTION OF THE INVENTION Referring to FIGS. 1-15 wherein like numbers refer to similar parts, a seat belt buckle 20 is shown in FIGS. 1 and 2. The seat belt buckle 20 is shown attached to a seat belt 21 that holds a child's car seat 22 positioned in a front passenger seat 24. A weight sensor 26 is connected to a microprocessor which deploys an airbag 28 when the deployment logic 30 calculates that deployment of the airbag 28 is required. The microprocessor shown as the deployment logic 30 receives input from various crash sensors. The deployment logic also receives a signal from the seat belt buckle 20 which indicates whether the seat belt buckle is latched and the tension on the seat belt 21 if the buckle is latched. The seat belt buckle 20, as shown in FIG. 2, has a base plate 32 with two upstanding tabs 34 that have vertical slots 36 which form guides for a latch 38. A structural housing plate 40 is engaged with the base plate 32 and is spaced from the base plate by spacers 42. The overall configuration of the belt buckle has similarities to a conventional seat belt buckle, with the following modification that allows the buckle to monitor latch status and seat belt tension. A bow shaped spring 44 moves with and is attached to the face 46 of the latch 38. The outer ends 48 of the spring bend away from the outwardly extending ends 50 of the latch 38. The outwardly extending ends 50 of the latch and the outer ends 48 of the spring ride in the guide slots 36 which are sufficiently wide to accommodate both the latch and the spring, as shown in FIG. 7. The latch 38 has a guide plate 52 that controls the motion of the latch so that the larger guide slots 36 do not result in undesired motion of the latch. The guide plate 52, as shown in FIG. 3, is held in engagement with the structural housing plate 40 by the latch following spring 61. This engagement between the guide plate and the structural housing plate 40 controls the motion of the latch. The latch 38, as shown in FIGS. 3-6, has an L-shaped cross-section, with the tongue engaging portion 56 of the latch forming the short leg of the L and the guide plate 52 forming the long leg of the L. The guide plate extends in a direction which is substantially perpendicular to the direction defined by the motion of the latch 38 as it engages the tongue 60 and the base plate 32. In FIG. 3 the seat belt buckle 20 is shown in the unlatched position with a spring 57 and an ejector 58 holding the latch 38 biased in an upward position opposing the downward biasing force of the latch following spring 61. FIG. 4 shows the seat belt buckle 20 latched to a seat belt tongue 60. The tongue has an opening 62 through which the latch tongue engaging portion 56 passes to thereby lock the tongue to the seat belt buckle. The opening 62 defines a latch engaging surface 64 which is held against the inside surface 66 of the tongue engaging portion 56 of the latch 38 by the spring 57 pushing against the ejector 58. The spring 44, as shown in FIG. 4, biases the latch 38 towards the ejector 58 by engaging the upstanding sides 68 of the vertical slots 36. Tension on the seat belt 21 compresses the spring 44 and moves the entire latch 38 in the direction of the applied tension, as shown in FIG. 5. If the seat belt buckle 20 is latched in the absence of the seat belt tongue 60, as shown in FIG. 6, the latch 38 can continue its downward rotation under the force the latch following spring 61. This is possible because the seat belt tongue is not present to engage the underside 70 of the tongue engaging portion 56 of the latch 38. Therefore the tongue engaging portion continues to move downwardly until the underside 72 of the guide plate 52 fully engages the structural housing plate 40, as shown in FIG. 6. The full range of motion of the latch 38, both in its up and down motion and in its lateral motion in response to seat belt tension, can be monitored by placing a small magnet 74 on the latch and measuring the magnetic field intensity at a suitable location fixed with respect to the seat belt buckle 20. A sensor 76 within an integrated circuit package 78 is mounted to a fixed portion of a latch follower spring support 73 that also encloses the guide plate 52 of the latch 38. The support incorporates a ridge 75 that strengthens the support. The sensor 76 is preferably of the Giant Magneto- Resistive (GMR) type and utilizes an effect discovered in 1988. The effect utilizes a phenomenon discovered in thin film devices that resistors built up 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 nonparallel 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 that is produced by the orientation of the magnetic flux shields. In the sensor 76 this axis is aligned in a direction towards the magnet 74. GMR sensors are available from Nonvolatile Electronics Inc. of 11409 Valley View Rd., Eden Prairie, Minnesota, U.S.A. (www.nve.com). GMR sensors are small, highly sensitive devices which have exceptional temperature stability, deliver high signal levels and require very little power and cost less than many competitive devices. All these factors are important in devices used in automobile safety applications. The GMR sensor detects magnetic field strength. By properly positioning the magnet 74 it has been found possible to produce field strength readings for situations in which the buckle is latched on a tongue, unlatched and latched without a tongue present. Motion of the tongue can be monitored because it overlaps the magnet and the sensor and affects the flux intensity transmitted from the magnet to the sensor. Thus each magnetic field strength measurement uniquely determines the status of the seat belt buckle 20 together with seat belt 21 tension. Although not absolutely necessary, the ability to obtain a unique signal for each condition of the seat belt buckle provides for simpler logic in as much as knowledge of the prior states of the sensor is not necessary to determine the current state of the sensor. FIG. 7 shows a top plan view of the seat belt buckle 20 wherein a sensor 80 has been positioned to one side of the spring support 73. A smaller shorter magnet 82 is positioned spaced above and mounted to the guide plate 52. An alternative embodiment seat belt buckle 84 is shown in FIG.11. A spring 86 is mounted to the latching portion 88 of a latch 90. The spring 86 is similar to the spring 44 shown in FIG. 2. The tongue 96 has an opening 98 through which the latching portion 88 passes. The opening defines an engagement side 100. Tension on the tongue causes the engagement side 100 to push against the latch, thereby compressing the spring and allowing the tongue to move towards the tongue receiving opening 102 of the buckle 84. A GMR sensor 104 and a magnet 106 are contained in a sensor housing 108 that is inset into a base plate. The GMR sensor detects the strength of the field produced by the magnet 106 as it varies due to the position of the latch 90, tongue 96 and the latch opening 98. The tongue is constructed of ferromagnetic material and the latch is also constructed of ferromagnetic material with the result that the magnetic field strength at the sensor 104 is dependent on the position of the latch and the tongue. The seat belt buckle 84 can detect latch status because the movement of the latch 90 towards and away from the sensor housing 108 will affect the magnetic field strength detected by the sensor 104. The presence of the tongue 96 can also be detected. Moreover, as greater levels of tension on the tongue will result in greater displacement of the latch, the sensor can also detect magnitude of tension on the belt. In general, high levels of tension may indicate the presence of a child's car seat, as a seat belt wearer would typically find such high levels of tension uncomfortable. A further embodiment seat belt buckle 110 is disclosed in FIGS. 8-10. A sensor 112 and magnet 114 are positioned on either side of the latch opening 116 in the base plate 117 of the buckle 110. A spring 118 positioned on the latch 120 similar to the one shown in FIG. 12 is employed. The spring is constructed of ferromagnetic material so that compression of the spring affects the transmission of magnetic field lines between the magnet and the sensor. Tension on the seat belt 122 pulls a tongue 124 against the spring 118. This displacement of the tongue 124, as shown in FIG. 9, changes the strength of the magnetic field that is carried through the ferromagnetic tongue 124. FIG. 10 shows the tongue 124 pulling again the spring 118 and the position of the sensor 112 and the magnet 114. A seat belt slider 126 is also shown in FIGS. 8-10. A similar slider may be employed with the seat belt buckle 20 in FIG. 2. The slider 126 employs cam surfaces 128 which engages the outer ends of the latch, and when depressed retracts the latch 120 releasing the seat belt tongue. A simplified belt buckle 130 of this invention is shown in FIGS. 12, 13, 14 and 15, in which, for illustrative purposes, conventional portions of the buckle assembly have not been shown. FIG. 12 shows the interaction of a seat belt tongue 136 with a ferromagnetic latch spring 134. For clarity, in FIGS. 13, 14, and 15 only a buckle body 132, a latch 134, a latch spring 135, and a seat belt tongue 136 are shown together with a magnet 138 and a sensor 140 positioned on either side of the latch opening 142 of the buckle body 132. FIGS. 14 and 15 show the displacement of the tongue as seat belt tension pulls the seat belt against a person occupying the passenger seat. Seat belt usage becomes extremely uncomfortable with belt tension in excess of about thirty-five newtons force. A child's car seat, on the other hand, can be fairly easily tensioned to levels in excess of about eighty-nine newtons force. Therefore, typically, the sensing spring will bottom out at approximately eighty-nine newtons tension. Thus seat belt tension is a good indicator of whether a person or a child's car seat occupies the front passenger seat. This information concerning seat belt tension can also be used in conjunction with a seat weight sensor to determine the true weight of the seat occupant. The true weight of the seat occupant, in combination with seat belt tension measurements, can be used by deployment logic to decide how and if to deploy an airbag. It should be understood that magnetic field sensors employing other technologies including, but not limited to, silicon-based Hall effect devices, GaAs based Hall effect devices, magneto-resistive devices, including anisotropic magneto-resistive (AMR) and other sensors capable of detecting static or dynamic magnetic fields could be employed. It should further be understood that the magnet may be linked or attached to various portion of the latch or directly mounted to the seat belt buckle. The magnetic field sensor may be directly responsive to motion of the magnet or responsive to moment of a flux concentrator or shunt.

Claims: CLAIMS : 1. A seat belt buckle (20) comprising: a buckle having a base; a latch (38) mounted for movement on the base, the latch having a tongue engagement portion, the tongue engagement portion extensible through an opening defined by portions of the base; a tongue (60) having portions defining a latch engaging opening, and a portion of the tongue which is in load supporting engagement with the latch, the tongue engaging the latch to lock the tongue to the buckle; a spring (61) mounted to the latch so that when the latch tongue engagement portion is engaged by the tongue, the spring is positioned between the portion of the tongue which is in load supporting engagement with the latch and the base; a magnetic field sensor (76) mounted on the buckle; and a magnet (74) mounted on the buckle so as to cause a magnetic field at the sensor, wherein motion of the tongue away from the buckle to elastically deform the spring changes the intensity of the magnetic field at the sensor. 2. The seat belt buckle (20) of Claim 1 wherein the magnetic field sensor (76) is of the GMR type. 3. The seat belt buckle (20) of Claim 1 wherein the magnet is mounted to the latch and the sensor is mounted to a non-moving portion of the buckle, so that motion of the latch (38) in response to motion of the tongue (60) causes a change in the magnetic field at the magnetic field sensor (76) . 4. The seat belt buckle (20) of Claim 1 wherein the spring is positioned to be compressed between the latch (38) and a portion of the base. 5. The seat belt buckle (20) of Claim 1 further comprising portions of the latch (38) which extend substantially perpendicular to the direction defined by the motion of the latch which results in the latch engaging the tongue (60), said portions positioned for sliding engagement with a fixed portion of the buckle.

Inventors: HUSBY HARALD SNORRE, PATEL ASHOK F, PATEN VIHANG C, HUSBY, HARALD, SNORRE, PATEL, ASHOK, F., PATEN, VIHANG, C.
Applicants: BREED AUTOMOTIVE TECH, BREED AUTOMOTIVE TECHNOLOGY, INC.
Application year: 1999
Main classification: A44B11
IPC codes: B60R21, B60R22