Method and apparatus for sensing seat occupant weight

[0002] This application claims priority to provisional application No. 60/120,653 filed on Feb. 24, 1999.

[0003] 1. Field of the Invention

[0004] This invention relates to a method and apparatus for measuring the weight of a seat occupant. Specifically, a simplified sensor arrangement is mounted within a vehicle seat to measure only vertical forces while canceling out lateral and longitudinal forces to provide an accurate weight measurement.

[0005] 2. Related Art

[0006] Most vehicles include airbags for the driver and passenger. It is important to control the deployment force of the airbags based on the size of the driver or the passenger. One way to control the deployment force is to monitor the weight of the seat occupant. If a smaller person such as a child or infant in a car seat is in the passenger seat, the weight on the seat will be less than if an adult occupies the seat.

[0007] Current systems for measuring the weight of a seat occupant are complex and expensive. Sensors are placed at a plurality of locations in the seat bottom and the combined output from the sensors is used to determine the weight of the seat occupant. Each sensor experiences a substantially vertical force, due to the weight of the seat occupant, but is also subject to longitudinal and lateral forces caused by acceleration, deceleration, or turning. The lateral and longitudinal forces picked up by the sensor incorporate an error component into the weight measurement. Current seat weight sensors are very sophisticated using multiple strain gages and complicated bending elements to provide high measurement sensitivity in the vertical direction and low sensitivity to lateral and longitudinal forces.

[0008] One weight measurement system corrects for this error by providing a double bending beam in each sensor. Thus, each sensor is made up from two (2) separate half-bridge strain gages that are diametrically opposed to cancel forces in a longitudinal or lateral direction. The two half-bridge strain gages are connected to form a full Wheatstone bridge. By doing this, the differential of the signal generated by each half-bridge strain gage is the output.

[0009] An error force in the direction of the strain gage grids appears on each half-breed strain gage equally, and the differential is zero (0). Forces occurring in other directions are generally not significant to the generation of the signal due to being perpendicular to the strain gage grid. Thus, a characteristic of the double bending beam sensor is to cancel the forces along the axis parallel to the gage grids. The forces along the perpendicular axis are not problematic because these forces are perpendicular to the gage grid. This allows error to be eliminated or canceled out at each sensor location in the seat. The output from the combined sensors is then added together to determine the weight of the seat occupant. The use of a double bending beam for each sensor location is very expensive.

[0010] Thus, it is desirable to have an improved seat occupant weight measurement system that is simplified and inexpensive yet provides accurate measurements by eliminating error caused by lateral and longitudinal forces.

[0011] In a disclosed embodiment of this invention, a system for measuring the weight of a seat occupant includes a seat bottom for receiving a substantially vertical seat occupant weight force and at least one pair of sensors. A first sensor is mounted within a first portion of the seat bottom and a second sensor is mounted within a second portion of the seat bottom. The first sensor generates a first weight signal comprised of a first vertical force component and a positive error component induced by application of non-vertical seat forces to the seat bottom. The second sensor generates a second weight signal comprised of a second vertical force component and a negative error component induced by application the non-vertical seat forces. A processor is used to determine seat occupant weight based on the first and second weight signals. Seat occupant weight is determined by summation of the first and second vertical force components and adding the positive and negative error components eliminates error induced by the non-vertical seat forces.

[0012] In a preferred embodiment, the seat bottom is divided into a first quadrant, a second quadrant, a third quadrant, and a fourth quadrant. The first sensor is mounted within the first quadrant for generating the first weight signal comprised of the first vertical force component and a first error component induced by application of non-vertical seat forces to the seat bottom and the second sensor is mounted within the second quadrant for generating the second weight signal comprised of the second vertical force component and a second error component induced by application of non-vertical seat forces. A third sensor is mounted within the third quadrant for generating a third weight signal comprised of a third vertical force component and a third error component induced by application the non-vertical seat forces. A fourth sensor is mounted within the fourth quadrant for generating a fourth weight signal comprised of a fourth vertical force component and a fourth error component induced by application the non-vertical seat forces. The first and third sensors are preferably orientated within the first and third quadrants, respectively, such that the first and third error components are generated as positive errors. The second and fourth sensors are orientated within the second and fourth quadrants, respectively, such that the second and fourth error components are generated as negative errors. The processor determines seat occupant weight based on the first, second, third, and fourth weight signals and adding the positive and negative errors eliminates the error induced by the non-vertical seat forces.

[0013] A method for determining the weight of a seat occupant includes the following steps. A vertical occupant force is applied against a seat bottom. A first weight signal is generated that has a first vertical force component and a positive error component induced by application of a non-vertical force to the seat bottom. A second weight signal is generated that has a second vertical force component and a negative error component induced by the non-vertical force. The first and second weight signals are combined and the error induced by the non-vertical force is canceled by adding the positive and negative error components together. The first and second vertical force components are added together to determine seat occupant weight.

[0014] These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

[0015] FIG. 1 is a perspective view showing a vehicle with an airbag system and an occupant sitting in a seat with the airbag in an active state.

[0016] FIG. 2A is a view of one embodiment of a seat bottom incorporating the subject weight measurement system.

[0017] FIG. 2B is a view of an alternate embodiment of a seat bottom incorporating the subject weight measurement system.

[0018] FIG. 2C is a view of an alternate embodiment of a seat bottom incorporating the subject weight measurement system.

[0019] FIG. 2D is a view of an alternate embodiment of a seat bottom incorporating the subject weight measurement system.

[0020] FIG. 2E is a view of an alternate embodiment of a seat bottom incorporating the subject weight measurement system.

[0021] FIG. 2F is a view of an alternate embodiment of a seat bottom incorporating the subject weight measurement system.

[0022] FIG. 2G is a view of an alternate embodiment of a seat bottom incorporating the subject weight measurement system.

[0023] FIG. 2H is a view of an alternate embodiment of a seat bottom incorporating the subject weight measurement system.

[0024] FIG. 3 is a schematic view of a control system for the subject weight measurement system.

[0025] A vehicle includes a vehicle seat assembly, shown generally at 12 in FIG. 1, and an airbag system 14. The seat assembly 12 can be either a driver or passenger seat and includes a seat back 16 and a seat bottom 18. When a vehicle occupant 20 is seated on the seat 12 a vertical force Fv is exerted against the seat bottom 18. The vertical force Fv represents the weight of the seat occupant 20.

[0026] The airbag system 14 deploys an airbag 24 under certain collision conditions. The deployment force for the airbag 24, shown in dashed lines in FIG. 1, varies according to the weight of the occupant 20. The vehicle includes a unique system for measuring the weight of the seat occupant 20.

[0027] The system preferably includes two (2) pairs of sensors 26, i.e., four (4) sensors total 26, which are mounted within the seat bottom 18. It should be understood that while two pairs of sensors, i.e., four (4) sensors, are preferred, a single pair or more than two (2) pairs of sensors could also be used.

[0028] In the preferred embodiment shown in FIG. 2A, a first sensor 26ais mounted within a first portion 28 of the seat bottom 18, a second sensor 26bis mounted within a second portion 30 of the seat bottom 18, a third sensor 26cis mounted within a third portion 32 of the seat bottom 18, and a fourth sensor 26dis mounted within a fourth portion 34 of the seat bottom 18. Preferably, the seat bottom is divided into four (4) quadrants with the first portion 28 of the seat bottom 18 being the right front quadrant, the second portion 30 being the left front quadrant, the third portion 32 being the left rear quadrant, and the fourth portion 34 being the right rear quadrant. The first sensor 26ais mounted within the right front quadrant, the second sensor 26bis mounted within the left front quadrant, the third sensor 26cis mounted within the left rear quadrant, and the fourth sensor 26dis mounted within the right rear quadrant. A first weight signal S1 generated by the first sensor 26acorresponds in magnitude to a portion of seat occupant weight on the right front quadrant, the second weight signal S2 generated by the second sensor 26bcorresponds in magnitude to a portion of seat occupant weight on the left front quadrant, the third weight signal S3 generated by the third sensor 26ccorresponds in magnitude to a portion of seat occupant weight on the left rear quadrant, and the fourth weight signal S4 generated by the fourth sensor 26dcorresponds in magnitude to a portion of seat occupant weight on the right rear quadrant.

[0029] When the occupant sits on the seat, the first weight signal S1 is comprised of a first vertical force component Frf and a first error component E1_rfinduced by application of non-vertical seat forces to the seat bottom. Non-vertical forces, i.e., lateral Flt and longitudinal Flg forces, which are exerted against the seat bottom 18, can result from vehicle acceleration, deceleration, or turning, for example. These lateral Flt and longitudinal Flg forces induce error into the sensor measurement at each sensor location within the seat bottom 18.

[0030] The second weight signal S2 is comprised of a second vertical force component Flf and a second error component E2lfinduced by lateral Flt and longitudinal Flg forces. The third weight signal S3 is comprised of a third vertical force component Flr and a third error component E3lrinduced by lateral Flt and longitudinal Flg forces. The fourth weight signal S4 is comprised of a fourth vertical force component Frr and a fourth error component E4rrinduced by lateral Flt and longitudinal Flg forces. Thus, each sensor 26 generates a signal that includes some component of error.

[0031] The sensors 26 are preferably strain gages 40 comprised of longitudinally extending elements that are placed in tension or compression based on the direction of the applied force. The manufacture and operation of these strain gages is well known in the art. In order to provide a simplified and inexpensive weight sensing system, each pair of sensors 26 mounted within the seat bottom 18 are orientated such that one sensor 26awill generate a positive error (+) and the other sensor 26bwill generate a negative error (−). Lateral Flt and/or longitudinal Flg forces acting the sensors 26 are a result of such forces acting on the seat 12. Due to the seat frame being of rigid construction, the forces will generally exert the same force on each sensor 26. The lateral Flt and longitudinal Flg forces that generate the error will apply the same magnitude of force to both sensors, however the direction or “sign” of the force is dependent upon the orientation of the sensor within the seat bottom 18. Thus, when the output from each pair of sensors 26 is combined, the errors will cancel each other out. If two (2) pairs of sensors are used then two (2) sensors must be orientated within the seat bottom to generate positive errors (+) while the other two (2) sensors must be orientated to generate negative errors (−).

[0032] For example, as shown in FIG. 2A, the first 26aand third 26csensors are orientated within the first 28 and third 32 quadrants, respectively, such that the first E1_rfand third E3lrerror components caused by the lateral Flt and longitudinal Flg forces are generated as positive errors (+). The second 26band fourth 26dsensors are orientated within the second 30 and fourth 34 quadrants, respectively, such that the second E2_lfand fourth E4_rrerror components caused by the lateral Flt and longitudinal Flg forces are generated as negative errors (−). As the example shows in FIG. 2A, the first 26aand third 26csensors are in more forward positions than the second 26band fourth 26dsensors, however other sensor configurations could be used.

[0033] While the configuration disclosed in FIG. 2A is preferred, it should be understood that other configurations, such as those shown in FIGS. 2B through 2H could also be used. FIGS. 2A-2D show gages 40 that are oriented forward or rearward within the sensor 26 to be positive (+) or negative (−). In FIG. 2A, the first 40aand third 40cgages are orientated forwardly to generate positive (+) errors and the second 40band fourth 40dgages are orientated rearwardly to negative (−) errors. In FIG. 2B, the first 40aand second 40bgages are orientated forwardly to generate positive (+) errors and the third 40cand fourth 40dgages are orientated rearwardly 46 to generate negative (−) errors. In FIG. 2C, the second 40band fourth 40dgages are orientated forwardly to generate positive (+) errors and the first 40aand third 40cgages are orientated rearwardly to generate negative (−) errors. In FIG. 2D, the first 40aand second 40bgages are orientated rearwardly to generate negative (−) errors and the third 40cand fourth 40dgages are orientated forwardly to generate positive (+) errors. FIGS. 2E-2H show gages 40 that are orientated left to right within the sensor 26 to be positive (+) or negative (−). In FIG. 2E, the first 40aand fourth 40dgages are orientated to the right to generate positive (+) errors and the second 40band third 40cgages are orientated to the left to generate negative (−) errors. In FIG. 2F, the second 40band third 40cgages are orientated to the right to generate positive errors (+) and the first 40aand fourth 40dgages are orientated to the left to generate negative (−) errors. In FIG. 2G, the second 40band fourth 40dgages are orientated to the right to generate positive errors (+) and the first 40aand third 40dgages are orientated to the left to generate negative errors (−). In FIG. 2H, the first 40aand third 40cgages are orientated to the right to generate positive (+) errors and the second 40band fourth 40dgages are orientated to the left to generate negative (−) errors.

[0034] In the occupant weight sensing application, the individual force measurement from a single sensor 26 is not used. It is a combination of the plurality of sensors 26a, b, c, dthat is used to determine the occupant's weight and weight distribution in the seat 12. A central processing unit (CPU) 36, shown in FIG. 3, determines seat occupant weight based on the summation of the first, second, third, and fourth weight signals S1, S2, S3, S4. Adding the positive and negative error components together eliminates the error induced by the lateral Flt and longitudinal Flg forces.

[0035] The total weight is equal to S1+S2+S3+S4=(Frf+E1rf)+(Flf−E2lf)+(Flr+E3lr)+(Frr−E4rr)=Frf+Flf+Flr+Frr. In addition to determining the total weight of the seat occupant, it is sometimes necessary to determine the front to rear and/or left to right weight distribution. The front to rear weight distribution can also be determined by adding the third S3 and fourth S4 weight signals or the first S1 and second S2 weight signals and dividing by the total weight: [(Frf+E1rf)+(Flf−E2lf)]/(Frf+Flf+Flr+Frr)=(Frf+Flf)/(Frf+Flf+Flr+Frr). The left to right distribution is determined by adding the first S1 and S4 or second S2 and third S3 weight signals together and dividing by the total weight: [(Frf+E1rf)+(Frr−E4rr)]/(Frf+Flf+Flr+Frr)=(Frf+Frr)/(Frf+Flf+Flr+Frr).

[0036] Once the CPU 36 has determined the seat occupant weight, the information is used to control the airbag system 14. An airbag control module 38 is in communication with the CPU 36 such that when the predetermined collision characteristics are achieved, the deployment force of the airbag 24 is controlled by the air bag control module 38 based on seat occupant weight. It should be understood that the airbag control module 38 and the CPU 36 can be separate processing units or the airbag control module 38 can be incorporated into the CPU 36.

[0037] The method for determining the weight of the seat occupant begins with providing a vertical occupant force Fv against the seat bottom 18. A first weight signal S1 is generated that has a first vertical force component Frf and a first error component E1_rfinduced by application of a non-vertical force to the seat bottom 18. A second weight signal S2 is generated that has a second vertical force component Flf and a second error component E2_lfinduced by application of the non-vertical force to the seat bottom 18. A third weight signal S3 is generated that has a third vertical force component Flr and a third error component E3_lrinduced by application of the non-vertical force to the seat bottom 18. A fourth weight signal S4 is generated that has a fourth vertical force component Frr and a fourth error component E4_rrinduced by application of the non-vertical force to the seat bottom 18. The first E1_rfand third E3_lrerror components are preferably generated as positive error (+) while the second E2_lfand fourth E4_rrerror components are preferably generated as negative error. The first S1, second S2, third S3, and fourth S4 weight signals are combined and the error induced by the non-vertical forces is cancelled by adding the positive and negative error components together. The first Frf, second Flf, third Flr, and fourth Frr vertical force components are combined to determine seat occupant weight.

[0038] The first weight signal S1 is generated to correspond to a portion of seat occupant weight on the first quadrant 28, the second weight signal S2 is generated to correspond to a portion of seat occupant weight on the second quadrant 30, the third weight signal S3 is generated to correspond to a portion of seat occupant weight on the third quadrant 32, and the fourth weight signal S4 is generated to correspond to a portion of seat occupant weight on the fourth quadrant 34.

[0039] As discussed above, a first strain gage 40ahas a first orientation within the first sensor 26ato produce a positive error in response to application of the lateral and longitudinal forces to the seat bottom 18 and a second strain gage 40bhas a second orientation, opposite from the first orientation, within the second sensor 26bto produce a negative error in response to application of the non-vertical force to the seat bottom 18. The third 26cand fourth 26dsensors include strain gages 40c, 40dhaving similar orientation.

[0040] Additional steps include providing a system controller 38 for controlling deployment of an airbag 24 wherein a seat occupant weight signal is generated based on the addition of the first, second, third, and fourth vertical force components. The signal is to the controller 38, which controls the deployment force of the airbag 24 based on the seat occupant weight.

[0041] The subject seat occupant weight sensing system uses the output from a plurality of simplified sensor elements to measure the weight of a seat occupant and further provides an accurate weight measurement by eliminating error caused by lateral and longitudinal forces. Although a preferred embodiment of this invention has been disclosed, it should be understood that a worker of ordinary skill in the art would recognize many modifications come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.