Current horn sensor designs have a large variability in force over the operating area of the sensor. At a given location on the horn sensor contact surface, the force required may also vary over operating conditions, such as temperature. The sensitivity level of the sensor cannot be adjusted easily without significant time and expense for retooling.

Current vehicles include an airbag for the driver as well as the front seat passenger. Manufacturers have sought to develop systems to disable the passenger side airbag if the weight on the passenger seat is below a given threshold.

US 5 739 757 teaches the use of a magnetostrictive sensor to measure strain on a wires under the cushion of the seat to determine the weight of a person or thing seated. However, this design may not always measure all of the weight on the seat, which may not be transmitted to the wires under the cushion, depending upon the position of the passenger.

The present invention provides an improved system and method for generating a signal representative of a force, such as the weight of a person or thing on a seat within a motor vehicle, or the force applied to a switch on a vehicle steering wheel.

The present invention provides a force measurement system utilizing active magnetostrictive sensors. The active magnetostrictive sensor includes an excitation coil and a detection coil. The excitation coil converts an electrical AC input signal into an acoustic wave by the magnetostrictive principle. The signal is AC to allow for a changing current through the coil, which when wrapped around a ferrite material creates an electromagnet. The acoustic wave travels through the ferromagnetic sensing material. The acoustic signal passing through the

ferrite core creates an electrical current in the detection coil. When stress is applied to the sensing material, the acoustic wave is affected and the detection coil monitors this change. The change can be measured by measuring voltage or current. These methods require a constant excitation voltage/current. This change in the current/voltage, as measured from the detection coil, has a linear relationship to the amount of force applied on the sensing material. The sensor operates in a linear fashion as long as some force or"preload"is used to get the operating point away from the non-linear region. Preferably a pre-load is used in the present invention.

The present invention provides a system for generating a signal representative of the weight of a person or thing seated within a motor vehicle includes at least one ferromagnetic element mechanically coupled between the structure of a seat and the vehicle floor, such that an elastic strain is induced therein responsive to all of the mechanical loading of the seat by the person or thing seated thereupon. Preferably, a ferromagnetic element is mounted directly between each seat bracket and the vehicle floor, in order to sense all of the weight on the seat. The strain in each element is detected and measured by the detection coil, as described above.

The present invention provides a system for activating a horn. The ferromagnetic element is coupled to a contact surface, which is activated by a user desiring to activate the horn. The force exerted by the user on the contact surface is measured by analyzing the signal (voltage and/or current) received by the detection coil. If a predetermined change is detected by the associated electronics, the system activates the horn.

This system provides a more constant force requirement across the area of the contact surface and under different operating conditions, such as temperature. Because the threshold can be adjusted in the electronics or software, the force required to activate the horn can be easily changed, such as for different vehicles or for user preference.

Brief Description of the Drawings Fig. 1 shows the force sensor system of the present invention implemented as a steering wheel switch and weight sensor.

Fig. 2 shows an active force sensor according to the present invention.

Fig. 3 is a graph illustrating the operation of the force sensor of Fig. 2.

Fig. 4Aa shows one possible arrangement of the force sensor of the present invention as a horn activation switch.

Fig. 4B shows a second arrangement of a horn activation switch, similar to Fig. 4A.

Fig. 4C shows a third arrangement of the horn activation switch of Fig. 4A.

Fig. 4D shows a fourth arrangement for the horn activation switch of Fig. 4A.

Fig. 4E shows a fifth arrangement for the horn activation switch of Fig. 4A.

Fig. 4F shows a sixth possible arrangement for the horn activation switch for Fig. 4A.

Fig. 5A shows an alternate embodiment of the magnetostrictive sensor of Fig. 2.

Fig. 5B shows an alternate embodiment of the magnetostrictive sensor of Fig. 5A.

Fig. 6 shows a more detailed view of the weight sensor of Fig. 1.

Fig. 7 shows an alternate embodiment of the force sensor of Fig. 6.

Detailed Description of the Invention Fig. 1 shows a force sensor system 20 of the present invention implemented here as a user activated switching system 22 and an occupant weight sensing system 24 both utilizing a controller 26. The controller 26 includes a microprocessor 28, including associated memory, storage, RAM, etc. and any necessary signal processor 30, such as amplifiers, filters, analog to digital converters, etc. Signal processing could be provided by the microprocessor 28 in a known manner. The user activated switching system 22 activates vehicle functions when selected by a user, while the occupant weight sensing system 24 determines the presence, weight and/or position of an occupant in the vehicle.

The user activated switching system 22 is shown installed on a vehicle steering wheel 32, although the switching system 22 could also be utilized in other locations in the vehicle. The steering wheel 32 includes a rim 34 connected to a hub 36, which is mounted on steering column 38 and is rotatable about its axis 40. A force sensor 42 is mounted on the hub 36, which is preferably for activation of a vehicle horn, but could be used for other vehicle functions as will be described below. The force sensor 42 is preferably mounted behind an airbag module 44 as shown, but may be mounted in front of an airbag module. The airbag module 44 is activated by an airbag actuator 46 as determined by the controller 26.

The controller also controls other vehicle functions, such as the horn 48, headlights 50 and cruise control 52, based on activation of the force sensor 42.

In the user activated switching system 22, the force sensor 42 senses force imparted by the user upon the force sensor 42, either directly or through the airbag module 44 positioned in front of force sensor. In response, the controller 26 activates the horn 48 or other vehicle functions, such as the headlight 50 or cruise control 52.

In a crash the controller 26 also selectively activates the airbag module 44 based upon inputs from force sensors 42 and 42c located below the vehicle seat 60, as well as other data inputs, which sense the weight of an occupant seated in the vehicle seat. Preferably, the force sensors located below the vehicle seat sense all of the weight between the seating surface 62 of the vehicle seat 60 and the vehicle floor 64. In this example, the force sensors 42 and 42c are installed between the vehicle seat brackets 72 and the vehicle floor 64. The vehicle seat 60 in this example includes four brackets, two are shown, and that sensors, two are shown, would be installed between all of the brackets and the vehicle floor 64. The force sensed by each of the sensors located below the seat can be summed to determine the total weight upon the seating surface 62. Alternatively, or additionally, the controller 26 can determine whether the occupant is seated toward the front of the seat, in the middle of the seat, toward the left or right of the seat 68, or whether there is no occupant in the seat.

Utilizing known algorithms and rules, the controller 26 determines the occurrence of a crash using a crash detection sensor 76, such as an accelerometer or ball and tube sensor. The controller 26 then determines whether to activate the airbag module 44 based upon the severity or type of crash and based upon information from the sensors located beneath the seat. If the controller 26 determines from the sensors located beneath the seat that there is no occupant in the seat 60, the controller 26 does not activate the airbag module 44. If the weight on the seating surface 62 is determined to be below a threshold level, such as when the seat occupant is a child, the controller 26 does not activate the airbag module 44. For a multistage airbag module 44, the controller 26 may determine which stages of airbag module to activate, for more or less force, based upon the information from the sensors.

Fig. 2 shows a force sensor 42 that could be utilized for any of the force sensors of the system shown in Fig. 1. In Fig. 2, the force sensor 42 includes a ferromagnetic sensing material 80, which receives the force to

be measured, such as the force from the user to activate the user activated switch or the force from the weight of the occupant. The force sensor 42 includes an excitation coil 82 and a detection coil 84 each preferably wrapped about a ferrite core 86,88. The ferrite cores 86,88 are preferably abutting, adjacent the ferromagnetic sensing material 80, or actually comprise the ferromagnetic sensing material, as described in other embodiments below. The function generator, preferably an AC sine wave generator 90 sends an excitation signal to the excitation coil 82. A bandpass filter/amplifier 92 is connected the detection coil 84. A function generator 90 and bandpass filter/amplifier 92 are preferably part of the signal processor 30 of Fig. 1.

The excitation coil 82 converts the electrical AC input signal into an acoustic wave by the magnetostrictive principal. The acoustic wave travels through the ferromagnetic sensing material 80 and the ferrite core 88 of the detection coil 84, which creates an electrical current in the detection coil 84. When force or stress is applied to the ferromagnetic sensing material 80, the acoustic wave is affected by the change in strain in the ferromagnetic sensing material, as is the electrical current in the detection coil 84. The current, or voltage, is measured to determine the change. The frequency and amplitude of the signals and the number of turns in the coils 82,84 will depend upon the particular application, as well the materials of the cores 86,88 and the thickness, texture and shape of the ferromagnetic sensing material 80. One of ordinary skilled in the art would be able to determine suitable parameters for a given design. Any of the pairs of excitation coil 82 and detection coil 84 could be replaced with a single coil that alternates as an excitation coil and detection coil.

Fig. 3 shows the RMS voltage output from the force sensor 42 in Fig. 2 for a given force. Preferably, the ferromagnetic sensing material 80 in Fig. 2 is mechanically reloaded so that the output versus force input is in the linear region. The threshold shown in Fig. 3 could be for activation of the horn or other vehicle functions for the user activated switching system 22 of Fig. 1. The threshold can be easily changed by simply

changing the software in the controller 26. Thus, the sensor 42 can used for various applications and in different vehicle and can be easily configured to activate at the proper threshold.

Figs. 4A-F show alternative configurations of the force sensor 42 on steering wheels 32a-f that each comprise a rim 34a-f and a hub 36a-f.

In Fig. 4A the excitation coil 82 and detection coil 84 are positioned at opposite ends of the ferromagnetic sensing material 80, that is to say on opposite sides of the axis of rotation 40 of the steering wheel 32a.

Alternatively, as shown in Fig. 4B, the excitation coil 82 and detection coil 84 are positioned at the same end of the ferromagnetic sensing material 80, on the same side of the axis of rotation 40 of the steering wheel 32b, to simplify the connection to the controller 26, as shown in Fig. 1. The excitation and detection coils 82,84 could also be installed on different shaped steering wheel hubs as shown in Fig. 4C.

Fig. 4D shows an embodiment having multiple strips 80a-d formed of the ferromagnetic sensing material with cutout sections between the strips 80a-d. In this embodiment, the force on each of the strips can be distinguished from force on any of the other strips. Force on any one of the strips 80a-d will cause different changes in the signal received at the detection coil 84 which can be discerned by the controller 26, as shown in Fig. 1. Varying the distances, widths, thicknesses, stiffnesses, textures, or other characteristics among the strips 80a-d will make it easier to discern activation of the different strips 80a-d. In this manner, each of the strips 80a-d can be programmed to operate a different vehicle function, for example the vehicle horn 48 could be activated by force on the strip 80a, head lights 50 by force on the strip 80b and cruise control 52 by forces on the strips 80c-d, as shown in Fig. 1. It should be noted that the embodiment of Fig. 4D would preferably be installed in front of an airbag module.

Fig. 4E shows an alternate embodiment of a steering wheel 32e wherein the excitation coil 82e and the detection coil 84e are coiled about portions of the ferromagnetic sensing material 80e which constitute the

ferrite cores 86e and 88e. In this embodiment, a separate ferrite core is eliminated and a portion of the ferromagnetic sensing material 80e is used as the ferrite core. This arrangement could be implemented in any of the arrangements shown in Figs. 4A-D to reduce the complexity and cost of the design.

Fig. 4F shows an alternate design for a sensor 42f including ferromagnetic sensing material 80f mounted on hub 36f of the steering wheel, which is not shown. Again, a portion of the ferromagnetic sensing material is used to the form ferrite cores 86f, 88f about which the excitation coil 82f and detection coil 84f, respectively, are coiled. The sensor 40f could be used in the designs of Figs. 4A-E.

Fig. 5A shows an alternate magnetostrictive sensor 98 including an alternate ferrite core 86a, which could be utilized for the excitation coils 82a and detection coils 84a. The alternative ferrite core 86a comprises a cross having four posts 99 extending perpendicularly from ends of the cross. The excitation and detection coils 82a, b are coiled about the perpendicular posts 99. This sensor could be used in the sensor designs of Figs. 4A-D.

Fig. 5B shows a magnetostrictive sensor 120 that is an alternative to the sensor 98 of Fig. 5A. The ferrite core 126 comprises two u-shaped members 126, each having a crossbar 128 and two perpendicular posts 130. The excitation and detection coils 82 a, b are coiled around the crossbars 128. The crossbar 128 of one u-shaped member 126 is positioned adjacent the crossbar 128 of the other u-shaped member. This sensor 120 is easier to manufacture and could also be used in the sensor designs of Figs. 4A-D.

Fig. 6 is an enlarged view of the weight sensor 42b located below the vehicle seat in Fig. 1. Any other weight sensors located below the seat would be identical. In Fig. 6 the ferromagnetic sensing material 88 is mounted between the bracket 70 of the vehicle seat 60 and the vehicle floor 64, such that all of the weight on the seat 60 passing through the bracket to the floor 64 passes through the ferromagnetic sensing material

88. A fastener, such as a bolt 100, connects the bracket 70 to the vehicle floor 64, passing through an aperture 102 in the ferromagnetic sensing material 88. The excitation coil 82 and the associated ferrite core 86 may be positioned on one side of the bolt 100, while the detection coil 84 and the associated ferrite core 88 are on the opposite side of the bolt.

Alternatively, different arrangements could be utilized. For example, Fig.

7 shows an alternate sensor 42g that could be utilized in place of the sensor 42b of Fig. 6. In Fig. 7 the ferromagnetic sensing material 80F, including the aperture 102g, has portions 86g, 88g, which form the ferrite cores for the excitation coil 82g and the detection coil 84g. Integrating the ferrite core in the ferromagnetic sensing material 80f reduces the number of parts and simplifies the design. This sensor design could also be used in the horn switch applications by designing the steering wheel to accommodate into the sensor.

All of the embodiments of the user activate switching system 22 of the present invention offer three main benefits over current switch designs. The switches of the present invention will have relatively constant force thresholds over changes in operating conditions, such as temperature. Different locations on the steering wheel will also have a relatively constant force threshold, unlike current horn activation switches.

The sensitivity level can be easily adjusted by a change in software and controller, or a change in the circuitry in signal processor 30.

The vehicle occupant weight sensing system 24 of the present invention measures all of the weight upon the seating surface 62, because sensors 42b, c are installed between the seat 60 and the vehicle floor 64. The change in height of the seat is minimal, since it only the height of the ferromagnetic sensing material 80, which can be minimal.

The electronics for this system, and the controller 26, is shared among the sensors 40b, c.