Method for controlling airbag deployment

[0002] This is a Continuation-In-Part of U.S. patent application Ser. No. 09/697,852, filed on Oct. 27, 2000, abandoned.

[0003] The present invention relates generally to restraint systems and more particularly to a method for controlling airbag deployment in automobiles.

[0004] It is well known to provide occupant restraint assemblies for vehicles, such as passenger cars, to restrain the occupant during a crash. Typically, an occupant restraint assembly includes a seat belt, an airbag and an inflator for inflating the airbag. When a crash occurs, as sensed by a crash sensor, the airbag is deployed by inflating it with gas.

[0005] In order to control the firing of an airbag in a collision, it is important to know the mass of the seat occupant. Weight sensors, such as bladders and pressure pads disposed in or under the seat, are available for measuring the vertical force of the occupant on the seat cushion. However, the seat belt can add an unknown quantity to this vertical force. For an adult occupant, this force is likely to be small, since a tightly cinched seat belt would be uncomfortable. However, for a child in a car seat this force can be large, since the seat belt is cinched around the seat, not the child, and the belt should be deliberately drawn tight to secure the car seat.

[0006] Devices are available that measure tension in the seat belt webbing. However, these devices suffer from several drawbacks. Reliable, unobtrusive electrical connections must be made to them through the belt webbing. In measuring tension, these devices do not sense the desired parameter, which is the vertical component of tension. The seat belt may wrap around the occupant through a range of angles, depending upon seat fore-aft location, size of occupant and whether the occupant is in a car seat. As such, a reliable estimate of the vertical component of force may not be attainable by these devices. In addition, the seat belt may have frictional or clamp forces on it if it feeds through child seat attachment slots, so that the tension of the belt changes with location.

[0007] It is thus highly desirable to provide a simple, reliable means of determining the vertical component of total force applied by the seat belt, so that it may be subtracted from the total vertical force measured by a seat cushion weight sensor.

[0008] It is thus an object of the present invention to provide a simple, reliable method for determining the vertical component of total force applied by the seat belt, which is then used to properly control the firing of an airbag during a collision.

[0009] This object is accomplished by providing force sensors at two, or if necessary, all three of the attachment pieces of the standard three-point lap belt. These sensors (strain gauges) measure the resistance at these attachment points. By sizing the nominal resistance of the two floor attachment bolt strain gauges, the two resistors can be put in a series circuit such that only the sum of the two resistances need be measured. The measurements obtained are then used to obtain the vertical components of force due to the seat belt. This value may then be subtracted from the total vertical force measured by a weight sensor in the seat cushion to determine the mass of the vehicle seat occupant.

[0010] Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.

[0011] FIG. 1 is a schematic view looking down into a vehicle illustrating a portion of the seating and sensor assembly in accordance with the present invention;

[0012] FIG. 2 is a schematic side view illustrating a vehicle front passenger seat and sensor assembly with a child seat mounted on the seat, wherein the force contributions of the seat belt on the sensor assembly is illustrated;

[0013] FIG. 2A is a rearward looking view of FIG. 2 showing the force contributions of the seat belt on the sensors;

[0014] FIG. 3 is a schematic sideward looking view of the front passenger seat and sensor assembly showing the force contributions of the seat belt on the sensors;

[0015] FIG. 3A is a rearward looking view of FIG. 3 showing the force contributions of the seat belt on the sensors;

[0016] FIG. 4 is a side view of a seat belt attachment piece containing a seat belt tension measuring system according to a preferred embodiment of the present invention;

[0017] FIG. 5 is a cross-sectional and side view of FIG. 4;

[0018] FIG. 6A is a signal-processing diagram of the modules of FIG. 5;

[0019] FIG. 6B illustrates a signal-processing diagram for the seat belt tension module of FIG. 5; and

[0020] FIG. 6C illustrates an alternative signal-processing diagram of FIG. 5, wherein the seat weight sensor is also a strain gauge.

[0021] Referring now to FIGS. 1-3 a typical vehicle 10 having a back seat 12, a front driver seat 14 and a front passenger seat 16. Although shown as separate herein, the front driver seat 14 and front passenger seat 16 can also be formed as a single bench seat. The passenger seat 16 includes a seat pardon 18 and a back portion 20 extending up from the seat portion 18.

[0022] The vehicle 10 also includes front airbag assemblies. Mounted within a steering wheel 40 is a driver's side front airbag assembly 42, and mounted within an instrument panel 44 is a passenger side front airbag assembly 46. The two front airbag assemblies 42, 46 are electrically connected to an electronic airbag control module 48, which controls the deployment system in typical fashion known to those skilled in the art and so not discussed further herein. Also electronically connected to the airbag control module 48 is a seat weight module 50. The seat weight module 50 can be housed separately, as shown, or can be formed as part of the airbag control module 48. Both of these modules 48, 50 can be software or hardware based, or a combination of the two. A seat weight sensor 52, such as a bladder or a pressure pad, is mounted within the seat portion 18 of the passenger seat 16 and is electrically connected to the seat weight module 50.

[0023] Referring now to FIGS. 2 and 2A, the passenger seat 16 is shown with a child seat 33 secured in place. In order to control the firing of the air bag, it is important to know the mass of the passenger seat occupant. As a child seat 33 is secured to the passenger seat 16 by fastening the three-point safety belt 30, a series of forces, depicted by arrows, are experienced within portions of the belt 30, back portion 20, and seat portion 18. As seen in FIG. 2, the three-point safety belt is mounted within vehicle 10 using anchor pieces 65, 65B. The upper anchor piece 658 may be mounted to the B-pillar 69 directly and may support a B-pillar mounted ring. The vertical component of these forces, along with the weight of the child seat 33 containing the child, are measured by a seat weight sensor 52 contained within the passenger seat 16. The vertical component of these forces must then be subtracted from the weight of the occupant as measured by the seat weight sensor 52 to determine the actual weight, hence mass, of the occupant.

[0024] Similarly, for a passenger 23 contained within the passenger seat 16, as depicted in FIGS. 3 and 3A, a series of forces are experienced within portions of the belt 30, back portion 20, and seat portion 18. The vertical component of these forces, along with the weight of the passenger, is measured by a seat weight sensor 52 contained within the passenger seat 16. The vertical component of the seat belt forces must be subtracted from the weight of the passenger 23 as measured by the seat weight sensor 52 to determine the actual mass of the passenger 23.

[0025] The present inventive means for determining the vertical component of force modifies the bolts used to attach the seat belt 30 to the vehicle 10 at two or three points. These points typically consist, as shown in FIGS. 1-3A, of an anchor piece 65 of one end of the seat belt 30 to the vehicle 10, an anchor piece 65 of the seat belt 30 tongue to either the seat or the floor, and an upper anchor piece 65B, where the belt 30 typically passes through a “B” pillar mounted ring on its path to the seat belt retractor mechanism 87. The vertical component of force can then be measured at these points and subtracted from the total force as measured by the seat weight sensor 52.

[0026] FIGS. 4 and 5 depict a seat belt mounting structure as used at either anchor piece 65 or the anchor piece 65B. The seat belt 30 is connected to a swivel plate 64. Each swivel plate 64 is held to the anchor piece 65, 65B of the vehicle 10 by amounting bolt 68 and a nut 71. The anchor piece 65, as shown in FIG. 5, may be a piece of the vehicle's structure, such as the floor, or a portion of the seat 14, 16. While not shown in FIG. 5, the upper anchor piece 65B may also be a piece of the B-pillar 69 of the vehicle. This is shown in FIGS. 2A and 3A. The swivel plate 64 is allowed to rotate around the mounting bolt 68, and is prevented from interfering with the vehicle anchor piece 65, 65B and surrounding components by both a shoulder 68A on the bolt 68 and a spacer 67. The mounting bolt 68 has a tangential flat 92 formed on one side thereof, and an off-center slit 70 extends inwardly from the head of the bolt and is parallel with the flat 92. The lower portion 72 of the bolt 68 formed by the slit 70 defines a cantilever that is relatively flexible in a direction normal to the plane of the slit 70. When the bolt 68 is secured to the anchor piece 65, 65B, the normal to the slit 70 (indicated by line L in FIG. 4) and the flat 92 forms an angle α (see FIG. 4) with the vertical.

[0027] In an alternative arrangement, the anchor piece 65, 65B may be threaded and a crushable washer (not shown) inserted between the anchor piece 65, 65B and the shoulder 68A of the bolt, which allows the bolt 68 to be tightened to a prespecified torque at the prescribed angle α. Many other possible configurations will be apparent to a person of skill in the art.

[0028] As best seen in FIG. 5, for a preferred embodiment having two anchor pieces 65, a strain gauge 90 is attached to the flat 92 on the bottom of the bolt 68. The strain gauge 90 has a pair of wires 89 that are coupled to a seat belt tension sensor module 34. The normal to the slit 70 and flat 92 is oriented with respect to vertical by some angle α which can be chosen to lie anywhere between 0 and a nominal angle β that the seat belt 30 makes with the vertical. During normal operation, the cantilever is bent slightly by the seat belt tension t, the effect of which is manifested in a change of resistance of the strain gauge 90 proportional to the strain on the bolt 68. This resistance change is converted by module 34 to a voltage, thus giving an estimate of the force exerted on the seat belt 30 at that anchor piece 65.

[0029] Assuming the cantilever is vertically oriented (where α is 0), the force exerted by the belt 30 on the cantilever section of the bolt 68 bends the cantilever slightly, thus stretching the strain gauge 90. However, since the cantilever bends only in the vertical direction, only the vertical component of the seat belt tension, F_v=t cos (β) is measured, as desired. In operation, F_vis calculated from the strain λ measured by strain gauge 90 as (F_v=kε), where k is the effective spring constant of the cantilever (force per Unit strain).

[0030] More generally, the cantilever may be oriented at some angle α other than vertical. This might be desirable in order to reduce the component of force along the stiff axis of the cantilever to ensure that it is not forced in this direction, and/or to ensure that the swivel plate 64 is always in contact with the cantilever even at its furthest orientation from vertical. In such a case, the vertical component of force when the seat belt 30 is oriented along its nominal angle β is related to measured strain by (F_v=k¹ε), where k¹=[k(cos(β))/(cos(β−α))] is the effective spring constant.

[0031] The change in resistance of the strain gauges 90 is algebraically summed by module 34 and an output (i.e. load signal) sent to sensor module 50 reflecting the net downward force produced by the seat belt 30 on the mass-sensing seat weight sensor 52, or bladder as shown in FIG. 6A. The seat weight module 50 processes the load signal to determine the vertical component of force and then subtracts the vertical force component from the weight as measured by the seat weight sensor 52 to accurately determine the mass of the occupant in the passenger seat 16. A signal is then sent from the seat weight module 50 to the airbag control module 48 regarding this mass. This signal is then processed by the airbag control module 48 to control the deployment system of the passenger side, front airbag assembly 46 in crash situations as a function of the vehicle mass and vehicle speed in typical fashion known to those skilled in the art.

[0032] In practice, the upward force exerted by the upper seat belt anchor piece 65B is likely to be small compared to the downward forces exerted by the two lower anchor pieces 65, and a strain gauge 90 may not be needed on the upper anchor piece 65B. However, in the following description, all three strain gauges 90 will be assumed. The change in resistance of each strain gauge 90 is proportional to the vertical component of force on its respective anchor piece 65, 65B. If all three strain gauges 90 have the same unstrained resistance R_Sand the same strain sensitivity, then the net downward force is proportional to the algebraic sum of the resistance deviations, ΔR_B1+ΔR_B2−ΔR_T, where the symbols refer to the change of resistance of the strain gauge 90 at the two lower anchor pieces 65 and the upper anchor piece 65B respectively.

[0033] A preferred embodiment for the seat belt tension sensor module 34 is shown in FIG. 6B. It consists of a power supply 100, here a voltage source, producing a voltage V across a Wheatstone bridge circuit consisting of two equal-value fixed resistors R in the upper arms, the two lower seat belt anchor strain gauge resistances R_S+ΔR_B1and R_S+ΔR_B2in the lower left arm, and the upper anchor point strain gauge resistance R_S+ΔR_Tplus a fixed resistance R_Sin the lower right arm. It is readily shown that when the changes in resistance ΔR of the strain gauges 90 are small compared to their nominal values R, the output voltage of the bridge is: $V_S=\frac{1}{{\left(2R_S+R\right)}^2}\left(\Delta R_{\mathrm{B1}}+\Delta R_{\mathrm{B2}}-\Delta R_T\right)V$

[0034] That is, the output voltage is directly proportional to the algebraic sum of the forces. The seat weight module 50 multiplies V_Sby the appropriate proportionality constant to obtain the net force of the seat belt 30 on the occupant and subtracts this from the force indicated by the seat weight sensor 52 to determine the weight, and hence mass, of the occupant.

[0035] If the seat weight sensor 52 was also a strain gauge with the same sensitivity as the strain gauges 90, it could be included in the Wheatstone bridge circuit, replacing the constant resistance R₆. This is shown in FIG. 6C. The output of the Wheatstone bridge circuit would be:

[0036] V_Sα(ΔR_B1+ΔR_B2−ΔR_T−ΔR_SWS)

[0037] where ΔR_SWSis the resistance change in the seat weight sensor. In this case, the output voltage V_Swould be directly proportional to the mass of the occupant, and the module 50 could be eliminated.

[0038] While the invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings.