This application is related to the following U. S. patent applications: U. S. patent application serial number 09/884615 filed 06/19/2001 and titled,"Seat Belt Tension Sensor with Overload Protection".

U. S. patent application serial number 09/441,350, filed 11/15/1999 and titled,"Automobile Seat Having Seat Supporting Brackets with a Stepped Weight Sensor".

U. S. patent application serial number 09/374,874, filed 08/16/1999 and titled,"Automobile Seat Weight Sensor".

U. S. patent application serial number 09/374,870, filed 08/16/1999 and titled,"Vehicle Occupant Position Detector and Airbag Control System".

U. S. patent application serial number 09/422,382, filed 10/21/99 and titled, "Vehicle Seat Weight Sensor".

U. S. patent number 6,209, 915, issued 04/03/2001 and titled,"Seat Belt Tension Sensor".

The foregoing patents have the same assignee as the instant application and are herein incorporated by reference in their entirety for related and supportive teachings.

1. FIELD OF THE INVENTION This invention relates to an automobile sensor for detecting the magnitude of a tensile force in a seat belt used in a car seat, and in particular to a sensor that can detect the magnitude of tension in a seat belt and provide an electrical signal that is representative of the magnitude of tensile force.

2. DESCRIPTION OF THE RELATED ART Air bags have been heralded for their ability to reduce injuries and save lives. However, since their incorporation into automobiles, a problem has existed with people of smaller size and small children. Air bags are designed to cushion the impact of occupants and thus reduce the injuries suffered. However, the force needed to properly cushion the occupant varies based on the size and position of the person.

For example, a larger person requires the bag to inflate faster and thus with more force. A smaller person may be injured by a bag inflating at this higher inflation force. A smaller person is more likely to be sitting close to the dashboard and would therefore stand a higher chance of being injured by the impact of the inflating bag, as opposed to the passenger hitting the fully inflated bag to absorb the impact of the accident. An average-sized person can also be injured by an airbag inflation if they are leaning forward, as for example, if they are adjusting the radio.

Because of the concern over injury to passengers in these situations, the National Highway Transportation Safety Administration (or NHTSA), an

administrative agency of the United States, is instituting rules under FMVSS 208 requiring the air bag deployment system to identify the passenger size and position and inflate the air bag accordingly.

One way to accomplish this task is to use a seat belt tension sensor in conjunction with an occupant weight sensor. The weight sensor can provide an indication of the force placed by an occupant on the seat. However, if the seat belt is unduly tightened, it can place an additional downward force on the passenger, creating an erroneous weight reading. Similarly, it is common for infant car seats to be secured tightly to the seat. In this circumstance, it is critical for the system to recognize that the passenger does not warrant inflation of the air bag. By sensing the tension on the seat belt in addition to the weight reading from the seat, the actual weight of the occupant can be determined. This allows for the system to safely deploy the air bag.

SUMMARY It is a feature of the present invention to provide a seat belt tension sensor for attachment between a seat belt and a vehicle.

Another feature of the invention is to provide a seat belt tension sensor that includes a housing that has a cavity. An anchor plate has a first portion located in the cavity. The housing moves relative to the anchor plate between a first position and a second position. A sensor is mounted to the housing. The housing presses on the sensor as the housing moves from the first to the second position. The sensor generates an electrical signal in response to the housing moving between the first and second positions. The electrical signal changes as a function of the tension on the seat belt. A spring is located between the sensor and the anchor plate.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective exploded view of a seat belt tension sensor.

Figure 2 is a partial assembled view of figure 1.

Figure 3 is an assembled view of figure 1 with a portion of the housing removed.

Figure 4 is an assembled view of figure 1.

Figure 5 is an assembled view including the belt and fastener.

Figure 6 is a cross-sectional view of figure 5.

Figure 7 is a side view of the strain member.

Figure 8 is a schematic diagram of the wheatstone bridge circuit.

It is noted that the drawings of the invention are not to scale.

DETAILED DESCRIPTION The present invention is a seat belt tension sensor. Referring to figures 1-8, a seat belt tension sensor assembly 20 shown. Assembly 20 has a limit structure 22 and a sensor 24. Limit structure 22 is fastened between a seat belt webbing 30 and a structural part of the vehicle such as a floor (not shown). The belt webbing 30 has an end 31, an end 32, a belt loop 33 and stitching 34 that retains end 32.

The limit structure 22 includes a housing 200, an anchor plate 220 and a strain sensor or member 240. Housing 200 has a bottom portion 201, top portion 202, flange 203, hole 204, spring channel 205 and posts 206. A cavity 210 is located within housing 200. Posts 206 have slots 207 that hold strain member 240.

Housing 200 has a notch 211, pylons 208, pins 209 and an upwardly extending lip

212. Housing 200 has a narrow portion 214 on an end of the housing.

Anchor plate 220 is loosely fitted within housing 200 resting on pylons 208.

Anchor plate 220 includes ends 221 and 222, a step section 223, a cutout 224, aperture 226 and an aperture 228. Arm 227 extends between aperture 226 and cutout 224. A projection 230 extends from arm 227 into cutout 224. A fin 232 extends into cutout 224. The anchor plate 220 is located in cavity 210. Aperture 226 goes over and surrounds flange 203. A gap 236 is formed between flange 203 and edge 234 Seat belt webbing 30 is attached through hole 204 and aperture 226. The end 32 of webbing 30 is routed through hole 204 and aperture 226, wrapped back onto itself forming loop 33 and sewn with stitching 34 to secure the seat belt webbing to assembly 20.

A spring 260 is mounted in spring channel 205. One end of spring 260 is mounted over projection 230. Sensor 24 has a strain member 240 that is mounted in slots 207. A support 262 fits into one end of spring 260. Support 262 rests adjacent a surface of strain member 240 and serves to focus the forces from spring 260 onto strain member 240. Fin 232 is in contact with the back surface of strain member 240.

A wire harness 280 has several wires 282. Wires 282 are pressed or soldered into circuit board holes 256 in printed circuit board 252.

The top portion 202 of the housing 200 is attached to the bottom portion 201 by ultrasonic welding along lip 212.

Seat belt tension sensor 20 is attached to a vehicle floor or seat or other member (not shown) by a fastener 40 such as a bolt, rivet or screw. Fastener 40 goes through aperture 228 and is attached to a vehicle structure or seat. The fastener shown is threaded; however, other types of fasteners would work such as a rivet.

Strain member 240, shown in figures 7 and 8, is formed of a material capable of supporting the tension applied by spring 260 and actuator 262 when the seat belt is tightened. Preferably, the strain member 240 is formed of 430 stainless steel.

The strain member 240 includes strain sensitive resistors 242 a, b, c, d formed thereon. These are formed by first depositing a dielectric layer 244 onto a substrate 241. Substrate 241 is preferably steel. The strain member 240 is then kiln fired at 850°C. Next, electrically conductive traces 245 and connector pads 246 a, b, c, d are similarly deposited onto the strain member 240. The strain member 240 is again kiln fired at 850°C. The strain sensitive resistors 242 a, b, c, d are next screened onto the strain member 240 in positions defined by the electrically conductive traces 245.

The strain member 240 is again kiln fired at 850°C. At this point, a final coating of a covercoat or epoxy (not shown) can be applied to protect the electrical components of strain member 240. This coating is not required, but may be desirable in circumstances where high abrasion or contaminants are expected. It should be noted that the strain sensitive resistors 242 a, b, c, d and connector pads 246 a, b, c, d together form the Wheatstone bridge circuit of Figure 8.

Details of the construction and operation of resistors 242 are shown in U. S. patent application serial number 09/441,350, filed 11/15/1999 and titled,"Automobile Seat Having Seat Supporting Brackets with a Stepped Weight Sensor".

Terminals 250 connect to strain member 240 and are soldered to pads 246 a, b, c, d. Terminals 250 have one end that are soldered in holes 258 of printed circuit board 252. Electronic circuitry 254, such as an integrated circuit is attached to printed circuit board 252 to amplify and filter the signal from the strain gage resistors 242. Printed circuit board 252 has holes that fit over pins 209 in housing 200.

Circuit board 252 is held in position in cavity 210 by pins 209.

When a tension is applied to seat belt 30, housing 200 transfers force to posts 206 which applies pressure to the ends of strain member 240. The spring force of spring 260 resists this force in the center of strain member 240 causing strain in member 240. As the tension increases, the strain sensitive resistors 242 will change resistance resulting in an electrical output signal that changes in proportion to the amount of tension in seat belt 30. This electrical signal is processed by electronic circuitry 254 and provided to an external electrical circuit by wire harness 280.

In a collision situation, a large force is applied to the tension sensor. The force applied to the seat belt overcomes the spring resistance of spring 260 moving housing 200 and flange 203 into contact with anchor plate 220. In this case, edge 234 is in contact with flange 203. The large force from the seat belt is transferred through the anchor plate 220 to fastener 40, which is attached to the vehicle structure or seat. Thus, in a collision, the large seat belt tension force is transferred from the seat belt to the vehicle structure. In this way, no further tension is applied to the strain member 240 and the strain member 240 is thus protected from excessive damaging forces by limit structure 22.

An electrical output signal is generated by the resistors 242 that is proportional to the magnitude of the tension in the seat belt and is transmitted over a wire harness 280 to a conventional air bag controller or occupant classification module (not shown). The air bag controller can then use the seat belt tension information to compute a more accurate profile of the seat occupant and use that information to control deployment of the airbag. This is the normal operational state of the seat belt tension sensor in which all of the seat belt tension is carried through the sensor 20.

In a situation where the vehicle is involved in a crash, the seat belt tension sensor operates in a different mode called a high load or crash state. In the high load state, the limit structure 22 carries the majority of tension placed on the seat belt. The amount of tension in the seat belt in a crash situation is much larger than in normal operation. If the strain member 240 was designed to carry all of this tension, it would not flex enough to properly function as a strain gage sensor.

Therefore, in a crash situation, the limit structure 22 carries the tension through the much stronger limit structure 22.

Remarks The seat belt tension sensor has several advantages. It allows accurate sensing of seat belt tension, while at the same time providing the structural strength needed for occupant restraint in a crash situation. The seat belt tension sensor allows an airbag controller to make better decisions as to when and how to deploy and airbag based upon more accurate seat occupant information. In the case of a child's car seat being strapped into a car seat, the seat belt tension sensor in

conjunction with a seat weight sensor allows the airbag controller to properly compute that the seat occupant has a low weight and to prevent deployment of the airbag.

The gap between the anchor plate and the housing flange is the travel range of the sensor as it is actuated. This design solves several problems.

1) Maintaining sensitivity at low loads without damage at higher loads.

When the gap between the edge and the flange is closed the load applied to the strain sensor elements reaches its limit. After this, the load is transferred to the fastener. Limiting the maximum load applied to the strain sensor is necessary since the working range of the sensor is generally below 100-lbs. but the sensor must withstand large (often greater than 1000-lb.) loads without damage and must not compromise the integrity of the passenger restraint system.

2) Maintaining restraint system integrity.

The present design allows the use of the same or very similar mounting bolts and anchors and mounting technique as do current seatbelt attachment methods.

Thus, safety engineers are very familiar with the requirements of the attachment method and installation procedures are changed as little as possible.

3) Integration into multiple restraint systems.

This present invention allows the sensor to be attached at the most common point of a wide variety of belt systems. It is useable even with very short bolt to seat belt buckle distances.

Variations The sensor shown had several strain gage resistors, one skilled in the art will realize that the preferred embodiment would work with other types of sensors. For example, discrete chip resistors could be attached or foil type strain gages could be used.

Another variation of the seat belt tension sensor would be to utilize other electrical connections other than a wire harness. For example, a integral connector or terminals could be added.

The seat belt tension sensor shown was mounted between a seat belt and a vehicle structure. One skilled in the art will realize that the preferred embodiment could be mounted to various locations on the seat or vehicle interior. For example, the seat belt tension sensor could be attached to the upper or lower B pillar or at the seat frame.

The illustrated embodiment showed the use of the seat belt tension sensor in an automobile seat. It is contemplated to utilize the seat belt tension sensor in other occupant sensing applications such as chairs, sofas, scales, beds and mattresses, hospital equipment, cribs, airplane seats, train seats, boat seats, amusement rides, and theater seats.

While the invention has been taught with specific reference to these embodiments, someone skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.