Abstract:
An improved force-measurement system utilizes active magnetostrictive sensors 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 the horn sensor switch on a vehicle steering wheel. The active magnetostrictive sensor includes an excitation coil and a detection coil.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 60/271,617; filed on Feb. 27, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an improved system and method for activation of a vehicle horn or other vehicle functions and for detecting the weight of a person seated within a motor vehicle. 
     Current horn sensor designs have a large variability in force over the operating area of the sensor. In other words, the force needed to activate the horn may vary greatly between two different points on the horn sensor contact surface. Even at a given location on the horn sensor contact surface, the force required may also vary (e.g. two to seven pounds) over operating conditions, such as temperature, etc. Further, 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. The danger that the passenger side airbag poses to infants in car seats and small children as well as small adults has been well documented. Manufacturers have sought to develop systems that would disable the passenger side airbag if the weight on the passenger seat is below a given threshold, thereby indicating either the presence of an individual for whom the airbag would be dangerous or the absence of any passenger at all. 
     One known system, described and claimed in U.S. Pat. No. 5,739,757, entitled “Vehicle Passenger Weight Sensor,” the assignee of which is the assignee of the present invention, uses 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. 
     SUMMARY OF THE INVENTION 
     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 further 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. Further, 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 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a schematic of the force sensor system of the present invention implemented as a steering wheel switch and weight sensor; 
         FIG. 2  schematically illustrates 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. 4   a  illustrates one possible arrangement of the force sensor of the present invention as a horn activation switch; 
         FIG. 4   b  illustrates another arrangement of a horn activation switch, similar to  FIG. 4   a;    
         FIG. 4   c  is a third possible arrangement of the horn activation switch of  FIG. 4   a;    
         FIG. 4   d  is a fourth possible arrangement for the horn activation switch of  FIG. 4   a;    
         FIG. 4E  illustrates a fifth possible arrangement for the horn activation switch of  FIG. 4A ; 
         FIG. 4F  illustrates a sixth possible arrangement for the horn activation switch for  FIG. 4A ; 
         FIG. 5A  is an alternate embodiment of the magnetostrictive sensor of  FIG. 2 ; 
         FIG. 5B  is alternate embodiment of the magnetostrictive sensor of  FIG. 5A ; 
         FIG. 6  is a more detailed view of the weight sensor of  FIG. 1 ; and 
         FIG. 7  is an alternate embodiment of the force sensor of FIG.  6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a force sensing system  20  implemented here as a user activated switching system  22  and 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. It should be noted that signal processing could be provided by the microprocessor  28  in a known manner. Generally, the user activated switching system  22  activates vehicle functions when selected by a user, while 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 illustrated as installed on a vehicle steering wheel  32  because this location benefits greatly from the present invention, although it should be recognized that the switching system  22  could also be utilized in other locations in the vehicle. The steering wheel  32  generally includes the steering rim  34  connected to the base or hub  36 , which is mounted on steering column  38  and rotatable about its center line  40 . The present invention provides a force sensor  42   a  mounted on 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   a  is preferably mounted behind the air bag module  44  as shown, but may also be mounted in front of air bag module  44 . As is known, the air bag module  44  is selectively activated by air bag actuator  46 , as determined by the controller  26 . The controller  26  also controls other vehicle functions, such as the vehicle horn  48 , headlights  50  and cruise control  52 , based on activation of the force sensor  42   a.    
     In the user activated switching system  22 , the force sensor  42   a  senses force imparted by the user upon the force sensor  42   a , either directly or through airbag module  44  positioned in front of force sensor  42   a . In response, the controller  26  activates the horn  48  or other vehicle functions, such as headlight  50  or cruise control  52 . 
     In the event of a crash, the controller  26  also selectively activates the air bag module  44  based upon inputs from the force sensors  42   b  and  42   c  (among other data inputs), which sense the weight of an occupant seated in vehicle seat  60 . Preferably, force sensors  42   b,c  sense all of the weight between the seating surface  62  of vehicle seat  60  and the vehicle floor  64 . In this example, the force sensors  42   b,c  are installed between vehicle seat  60  brackets  72  and the vehicle floor  64 . It should be noted that the vehicle  60  in this example includes four brackets (two shown) and that sensors (two shown) would be installed between all of the brackets and the vehicle floor  64 . The force sensed by each of the sensors  42   b,c  can be summed to determine the total weight upon seating surface  62 . Alternatively, or addition, the controller  26  can determine whether 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  60 . 
     Utilizing known algorithm and rules, the controller  26  determines the occurrence of a crash, from crash detection sensor  76  (such as an accelerometer or ball and tube sensor). The controller  26  then determines whether to fire airbag module  44  based upon the severity or type of crash and based upon information from the sensors  42   b,c . For example, if the controller  26  determines from sensors  42   b,c  that there is no occupant in seat  60 , the controller  26  does not activate airbag module  44 . Further, if the weight on seating surface  62  is determined to be insufficient (the occupant may be a child) the controller  26  does not activate air bag module  44 . Further, for a multistage airbag module  44 , the controller  26  may determine which stages of air bag module  44  to activate, for more or less force, based upon the information from sensors  42   b,c.    
       FIG. 2  schematically illustrates a force sensor  42  which could be utilized as the force sensors  42   a,b  and  c  of FIG.  1 . As shown in  FIG. 2 , the force sensor  42  generally includes ferromagnetic sensing material  80 , which receives the force to be measured (e.g. the force from the user to activate the user activated switch or the force from the weight of the occupant). The force sensor  42  further includes an excitation coil  82  and detection coil  84  each preferably wrapped about a ferrite core  86 ,  88  respectively. The ferrite cores  86 ,  88  are preferably abutting, adjacent the ferromagnetic sensing material  80  (or actually comprise the ferromagnetic sensing material  80 , 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 . 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 onto an acoustic wave by the magnetostrictive principal. The acoustic wave travels through the ferromagnetic sensing material  80  and the ferrite core  88  of detection coil  84 , which in turn 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 material  80 , 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. 
       FIG. 3  illustrates the RMS voltage output from the force sensor  42  in  FIG. 2  for a given force. Preferably, the ferromagnetic sensing material  80  ( FIG. 2 ) is mechanically preloaded so that the output verses 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 . It should also be noted that the threshold can be easily changed by simply changing a 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. 4   a-f  illustrate possible configurations of the force sensor  42   a  on a steering wheel  32   a-f  each comprising a steering rim  34   a-f  and hub  36   a-f , respectively. In  FIG. 4   a , the excitation coil  82  and detection coil  84  are positioned at opposite ends of ferromagnetic sensing material  80  on opposite sides of the center line  40  of the steering wheel  32   a . Alternatively, as shown in  FIG. 4   b , the excitation coil  82  and detection coil  84  can both be on the same side of the center line  40  of the ferromagnetic sensing material  80  in order to simplify the connection to the controller  26  (FIG.  1 ). Of course, the excitation and detection coils  82 ,  84  could also be installed on different shaped hubs, as is shown in  FIG. 4   c.    
       FIG. 4   d  illustrates an embodiment where multiple strips  80   a-d  are formed in the ferromagnetic sensing material (by cutout sections between the strips  80   a-d ). In this embodiment, the force on the strip  80   a  can be distinguished from force on strip  80   b, c  and/or  d . Force on any one of the strips  80   a-d  will cause different changes in the signal received at detection coil  84 , which can be discerned by the controller  26  (FIG.  1 ). Varying the distances, widths, thicknesses, stiffnesses, textures, or other changes among the strips  80   a-d  will make it easier to discern activation of the different strips  80   a-d . In this manner, each of the strips  80   a-d  can be programmed to operate a different vehicle function, for example the vehicle horn  48  could be activated by force on strip  80   a , head lights  50  by force on strip  80   b  and cruise control  52  by forces on strips  80   c-d . (FIG.  1 ). It should be noted that the embodiment of  FIG. 4   d  would preferably be installed in front of an air bag module. 
       FIG. 4   e  illustrates an alternate embodiment of a steering wheel  32   e  wherein the excitation coil  80   e  and the detection coil  84   e  are coiled about portions of the ferromagnetic sensing material  80   e  which constitute the ferrite cores  86   e  and  88   e , respectively. In other words, in this embodiment, a separate ferrite core is eliminated and a portion of the ferromagnetic sensing material  80   e  is used as the ferrite core. It should be recognized that this could be implemented in any of the arrangements shown in  FIGS. 4   a-d , as well. This reduces the complexity and cost of the design. 
       FIG. 4   f  illustrates an alternate design for a sensor  40   f  including ferromagnetic sensing material  80   f  mounted on hub portion  36   f  of the steering wheel (not shown). Again, a portion of the ferromagnetic sensing material  80   f  is used to form ferrite cores  86   f ,  88   f  about which the excitation coil  82   f  and detection coil  84   f , respectively, are coiled. The sensor  40   f  could be used in the designs of  FIGS. 4   a-e.    
       FIG. 5A  illustrates an alternate magnetostrictive sensor  98  including an alternate ferrite core  86   a , which could be utilized for the excitation coil  82   a  and detection coil  84   a . The alternative ferrite core  86   a  generally comprises a cross having four posts  99  extending perpendicularly from ends of the cross. The coils  82   a,b  are coiled about the perpendicular posts  99 . This sensor  98  could be used in the sensor designs of  FIGS. 4   a-d.    
       FIG. 5B  illustrates an alternate magnetostrictive sensor  120  of the sensor  98  of FIG.  5 A. The ferrite core  126  comprises two u-shaped members  126 , each having a cross-bar  128  and two perpendicular posts  130 . The coils  82   a, b  are coiled about the cross-bars  128 . The cross-bar  128  of one u-shaped member  126  is positioned adjacent the cross-bar  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. 4   a-d.    
       FIG. 6  is an enlarged view of the weight sensor  42   b  of  FIG. 1  (weight sensor  42   c  would be identical). As shown 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 its ferrite core  86  may be positioned on one side of bolt  100 , while detection coil  84  and its ferrite core  88  are on the opposite side. Alternatively, different arrangements could be utilized. For example,  FIG. 7  illustrates an alternate sensor  42   g  which could be utilized in place of the sensor  42   b  of FIG.  6 . In  FIG. 7 , the ferromagnetic sensing material  80 F (including aperture  102   g ) has portions  86   g ,  88   g , which form the ferrite cores for excitation coil  82   g  and detection coil  84   g , respectfully. Again, integrating the ferrite core in the ferromagnetic sensing material  80   f  reduces the number of parts and simplifies the design. This sensor design could also be used in the horn switch applications by designing the armature to fit into the sensor. 
     All of the embodiments of the user activate switching system  22  ( FIG. 1 ) of the present invention offers 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. Further, different locations on the steering wheel will also have a relatively constant force threshold, unlike current horn activation switches. Further, the sensitivity level can be easily adjusted by a change in software and controller (or change in 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  42   b,c  are installed between the seat  60  and 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, controller  26 , is shared among the sensors  40   b,c.    
     In accordance with the provisions of the patent statutes and jurisprudence, exemplary configurations described above are considered to represent a preferred embodiment of the invention. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope. For example, 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.