Abstract:
A target activated sensor including a magnet, a magnetic field sensing element located proximate the magnet and a single ferrous pole located proximate the sensing element. The sensing element senses a change in magnetic field caused by the presence of a target. The magnet, sensing element and single pole piece may be combined in a single package.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/555,787, filed Mar. 24, 2004, the teachings of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present application relates to position sensors and more particularly, to a target activated sensor that provides and output indicative of the position of a target relative thereto. 
     BACKGROUND 
     In a wide variety of applications it is advantageous or necessary to sense the position of a linearly or rotationally movable element. For example, in automobile seat applications the seat may be linearly movable, either manually or automatically via electro-mechanical means, on an associated track assembly. A sensor may provide a signal representative of the linear position of the seat on the track for a variety of purposes, e.g. to control deployment of an air bag, to control the electromechanical actuator that causes translation of the seat in connection with a seat position memory feature, etc. 
     For a seat position application, it is increasingly desirable for a sensor to provide multiple position outputs for purposes of ascertaining occupant position. For example, in applications where seat position is used to control air bag deployment early configurations involved only single stage air bag systems. A single stage air bag deploys with a known deployment force that may not be varied. In this application, seat position information was used only to determine when the airbag should be deployed. However, the advent of dual stage air bags, i.e. air bags that may be deployed with two distinct deployment forces, required increased resolution in position sensing. Also, the industry is now moving to variable stage airbags where the deployment force may be varied depending upon occupant position and classification. Variable stage airbag configurations require a sensor configuration that can detect multiple seat positions for use in determining the appropriate deployment force. 
     Another desirable feature of a position sensor, especially in the context of an automobile seat application, is that it be a non-contact sensor. A non-contact sensor includes a sensing element that does not physically contact the sensed object, allowing quiet operation of the sensor and minimizing wear. Preferably, the sensor operates with a relatively large air gap between the sensor and the sensed object to avoid inadvertent contact due to manufacturing or assembly variances. 
     Another issue associated with seat position sensors is that the seat track environment is very crowded with limited physical space for such sensors. Also the space available for the sensor may vary among vehicle types. As such, sensors which are compact in size are desirable. 
     Accordingly, there is a need for a seat position sensor that is compact in size and is configured to operate with a relatively large air gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of embodiments of the claimed subject matter will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, where like numerals depict like parts, and in which: 
         FIG. 1  is a schematic illustration in partial side-sectional view of an exemplary target activated position sensor consistent with the present invention; 
         FIG. 2  is a schematic diagram showing the magnetic fields associated with an exemplary target activated position sensor consistent with the present invention; 
         FIG. 3  includes plots of gauss vs. distance showing magnetic flux at various distances from an end of a single pole when a target is present and when a target is not present in an exemplary target activated position sensor consistent, with the present invention; 
         FIG. 4  is an end view of a portion of a vehicle seat assembly incorporating an exemplary target activated position sensor showing the target positioned in proximity to the sensor; 
         FIG. 5  is a side perspective view of the assembly shown in  FIG. 4  showing the target positioned at a distance form the sensor; 
         FIG. 6  is an exploded view of one exemplary housing and mounting bracket configuration useful in connection with a target activated position sensor consistent with the present invention; and 
         FIG. 7  includes plots of the difference in flux sensed by a magnetic sensor element when a target is present and not present vs. dimension showing the effect on the difference resulting from variation the dimension of eleven different component and tolerance variables associated with an exemplary target activated position sensor consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     For ease of explanation, sensor systems consistent with the invention will be described herein in connection with an automobile seat position sensing application. It will be recognized, however, that sensor systems consistent with the invention will be useful in other applications. In addition, exemplary embodiments described herein include use of Hall Effect sensors and a magnet. Those skilled in the art will recognize, however, that a variety of sensing means may be used. For example, optical, magneto-resistive, fluxgate sensors, etc. may be useful in connection with a sensor system consistent with the invention. It is to be understood, therefore, that illustrated exemplary embodiments described herein are provided only by way of illustration, and are not intended to be limiting. 
     Referring to  FIG. 1 , one exemplary target activated sensor system  100  consistent with the present invention is shown in detail. The illustrated exemplary sensor system includes a target or flag  102  and a sensor assembly  104  including a sensor housing  106  carrying a magnet  108 , a single pole  110  and a magnetic field sensing element  112 . In general, the sensor assembly  104  and target  102  may be mounted in a system, e.g. a seat position sensing system, in a manner allowing relative movement therebetween. For example, the target  102  may be affixed to a movable rail of a seat assembly and the sensor assembly  104  may be mounted to a fixed rail of a seat assembly, or vice versa. 
     When the target  102  is at a distance from the sensor assembly  104  a first level of magnetic flux from the magnet may be imparted to the magnetic sensor element  112 , resulting in a first output from the sensor  112 . When the target  102  is positioned proximate the sensor assembly  104 , as shown, for example in  FIG. 1 , the target  102  may cause an increased level of magnetic flux to be imparted to the magnetic sensor element  112 , compared to the first level of flux, resulting in a second output from the sensor  122 . The output level of the magnetic sensor element  112  may thus be indicative of the position of the target  102  relative to the sensor assembly  104 . When, for example, the target  102  is affixed to a movable rail of a seat assembly and the sensor assembly  104  is affixed to a stationary rail of the seat assembly, the output of the sensor  112  is indicative of the position of the movable rail and the seat affixed thereto, relative to the stationary rail. 
     With continued reference to  FIG. 1 , the target  102  may be constructed form a ferromagnetic material and may be configured to move relative to the sensor assembly in a direction indicated by arrow A. The magnet  108  may be disposed at least partially in the housing  106  and may be magnetized in a direction indicated by arrow A 1  which is substantially parallel, e.g. within manufacturing and assembly tolerances, to the direction of movement of the target  102 . In one embodiment, for example, the magnet  108  may be magnetized such that a first end  114  represents a north (N) pole of the magnet and a second end  116  represents a south (S) pole of the magnet. 
     The single pole  110  may be constructed from a ferromagnetic material and may be of unitary or multi-piece construction. The pole  110  may be disposed in the housing at a fixed distance from the end  116  of the magnet and within the magnetic field established by the magnet. In the illustrated exemplary embodiment, the magnetic sensor element  112  is affixed to a printed circuit board (PCB)  118 . The PCB  118  may carry conductive paths and/or electronics for communicating the sensor element outputs to a controller  120  for controlling a vehicle system  122 , e.g. a vehicle air bag, seat position controller, etc., in response, at least in part, to the outputs of the sensor element  112 . 
     In one exemplary embodiment, the magnetic sensor element  112  may be configured as a Hall Effect sensor positioned on the PCB  118  with a flux receiving surface  124  of the sensor spaced from and in opposing relationship to an end surface  126  of the single pole  110 . The output of the Hall Effect sensor may vary in response to the level of flux density imparted to the flux receiving surface  124 . The flux receiving surface  124  of the sensor may thus be substantially parallel, e.g. within manufacturing and assembly tolerances, to the direction of magnetization of the magnet  108 , as indicated by arrow A 1 . Placing the sensor element  112  in this position relative to the direction of the magnetization of the magnet  108  can, compared to other orientations, reduce the vector component of the magnetic field from the magnet that affects the sensor. 
     In one exemplary embodiment, the Hall Effect sensor may be a well-known and commercially available solid state, low current device with diagnostic capability. A two terminal Hall Effect sensor may be used to achieve operation over a wide voltage and temperature range and provide two current output levels, e.g. 5.5 mA and 15 mA. A programmable Hall Effect sensor may be used to allow selection of the Hall switch point. 
     The housing  106  may include a cavity  128  for receiving the end surface  126  of the pole piece and the magnetic sensor element  112 . The PCB  118  may extend across the cavity  128  to opposing sides thereof, and may be sealed within the housing by a housing cover  130 , e.g. as shown in  FIG. 3 . Sealing of the cover  130  over the PCB  118  may be accomplished by a variety of means including, but not limited to, a perimeter seal, grommet, o-ring, epoxy, ultrasonic welding, over-molding, etc. The cover  130  may extend over the magnet  108  to seal the magnet within the housing  106 , or may leave the magnet  108  exposed at an open side of the housing  106  to allow close positioning of the magnet to the target  102 . 
     Turning now to  FIG. 2 , the target  102  may be positioned relative to the sensor assembly to direct an increased magnetic flux through the magnetic sensor element  112 , with no intervening ferromagnetic pole between the sensor and the target, compared to when the target  102  is not in proximity to the sensor assembly.  FIG. 3  includes plots  300  and  302  of magnetic flux (gauss) at various distances from the end surface  126  of the pole  110 , i.e. moving in the direction of the target  102  shown in  FIG. 1 . Plot  300  is a plot of gauss vs. distance from the pole  110  when a target  102  is present at 6 mm from the pole, and plot  302  is a plot of gauss vs. distance from the pole  110  when no target  102  is present. 
     As shown, when the target  102  is present the flux is greater at distances from the end surface  126  of the pole  110 , i.e. in the location of the magnetic sensor  112 , than when no target  102  is present. In addition, the difference (delta) between the flux present at a particular distance from the pole when the target  102  is present and when the target  102  is not present is about 80 gauss or more over a range of distances from 1 mm to 3 mm. The relative positioning of the components  108 ,  110  and  112  may be chosen to achieve a desired delta (target present-not present) to accommodate a particular Hall Effect sensor selection, and/or a programmable Hall Effect sensor may be used to compensate for opening manufacturing tolerances. 
       FIGS. 4 and 5  show an exemplary application of the target activated sensor system consistent with the present invention configured for sensing the position of a vehicle seat (not shown) affixed to a vehicle seat assembly  400 . In the illustrated embodiment, the sensor assembly  104  is mounted on a movable rail  402  of the seat assembly  400  and moves relative to the target  102  (e.g., a flag) which is mounted to a fixed portion of the seat assembly  400 , e.g. via a fastener  404 . Alternatively, the sensor assembly  104  may be fixed and the target  102  may be movable. Also, one of the target or sensor assembly may be directly affixed to the stationary rail  406  of the seat assembly. 
     As shown, the target  102  may be positioned with a surface  408  thereof in opposed facing relationship to a bottom surface  410  of the sensor assembly  104  and the magnet to establish an air gap G between the magnet of the sensor assembly  104  and the target  102 . In one embodiment, the air gap G may be 3 mm±2 mm. Advantageously, however, a sensor system consistent with the invention may be configured with an air gap in excess of 5 mm. Use of a magnet  108  that is relatively long compared to its width contributes to the large air gap, while allowing use of a low grade magnet. In one embodiment, for example, the magnet  108  may have a length in its direction of magnetization of about 22–23 mm, a width of about 7–8 mm, and a height of about 9–10 mm. 
     With reference also to  FIG. 6 , the sensor assembly  104  may be mounted to the seat assembly  400  through a mounting bracket  600 . The mounting bracket  600  may be constructed from a ferrous or non-ferrous material and may be generally L-shaped including first  602  and second  604  arms. A first arm  602  of the bracket  600  may include openings  606  therein for receiving corresponding studs  608  ( FIG. 5 ) extending from a side surface of the movable rail  402  of the seat assembly  400 . The studs  608  may pass through the openings  606  and may be secured against removal from the openings, e.g. by welding or an appropriate clip or fastener. 
     The second arm  604  of the bracket may include first  610  and second  612  mounting openings for receiving first  614  and second  616  associated mounting flanges extending from the housing. Each of the mounting flanges may be generally L-shaped having a first arm  618  and a second arm  620 . To mount the sensor assembly to the bracket  600 , the flanges  614 ,  616  may be passed through the associated openings  610 ,  612  and the first arms  618  may be positioned to overlay the top surface  622  of the bracket adjacent the openings  610 ,  612 . Play between the mounting flanges  614 ,  6116  and the openings  610 ,  612  may be removed by a positive locking clip  624  inserted between the flanges and the rear surfaces of the openings and into the housing  106 . Electrical connections from the PCB  118  to, for example, the controller  120  may be established through a connector  626  configured to securely mate with an associated receptacle  628  extending from the housing  106 . 
     Advantageously, a sensor system with a single pole consistent with the present invention may be operated at a large air gap and may be provided in a compact package. In one embodiment for example, the sensor assembly  104 , as shown in  FIG. 1  may have dimensions of about 15 mm×15 mm×35 mm, excluding the mounting flanges  614 ,  616 . This small size is accommodating the limited space available in existing seat track assemblies. 
     Another advantage of the sensor assembly including a single pole consistent with the invention is robustness to manufacturing and assembly tolerances of the components.  FIG. 7 , for example, includes plots of the delta (target present-not present) in gauss vs. eleven different variable component and assembly dimensions. Plot  700 – 720  are plots of delta vs. air gap, magnet length, magnet width, magnet height, pole height, pole width, pole thickness, the distance from a Hall sensor sensing element to the magnet, and the hall sensor x, y and z position tolerances, respectively, for an exemplary embodiment of a sensor assembly  104  consistent with the present invention. As shown, the delta shows little change if the eleven variables are varied up to ten times greater than the variation would be in production. In addition, the variables with the greatest affect on delta are generally component tolerances not assembly tolerances. Thus, a sensor consistent with the present invention enables less precision and more efficiency in manufacturing. 
     Consistent with one aspect of the present invention, therefore, there is provided apparatus for sensing the position of a seat in a vehicle including: a permanent magnet establishing a magnetic field; a single pole spaced from the permanent magnet and disposed in the magnetic field; a magnetic sensor element disposed in the magnetic field and adjacent an end of the single pole; and a target comprising a ferromagnetic material. At least one of the target and the sensor assembly being configured for movement with the seat relative to the other of the target and the sensor assembly, whereby in at least one position of the at least one movable one of the target and the sensor assembly the target is disposed in the magnetic field and the magnetic sensor element is disposed between the end of the single pole and the target with no intervening ferromagnetic pole between the magnetic sensor element and the target. The sensor element is configured to provide a first output in response to a first level of magnetic flux from the magnet directed to the sensor element when the sensor element is disposed between the end of the single pole and the target, and a second output different from the first output in response to a second level of magnetic flux from the magnet directed to the sensor element when the sensor element is not disposed between the end of the single pole and the target. 
     Consistent with another aspect of the present invention, there is provided an apparatus for sensing the position of a seat in a vehicle including: a permanent magnet magnetized in a direction of magnetization for establishing a magnetic field; a single pole spaced from the permanent magnet and disposed in the magnetic field; a magnetic sensor element disposed in the magnetic field and adjacent an end of the single pole, the magnetic sensor element having a flux receiving surface and being responsive to first and second levels of flux imparted to the flux receiving surface for providing first and second outputs, respectively, the first output being different from the second output, the flux receiving surface being positioned substantially parallel to the direction of magnetization; a housing, the permanent magnet, the single pole and the magnetic sensor element being at least partially disposed in the housing; and a target including a ferromagnetic material. At least one of the target and the sensor assembly being configured for movement with the seat relative to the other of the target and the sensor assembly the movement being in a direction substantially parallel to the direction of magnetization, whereby in at least one position of the at least one movable one of the target and the sensor assembly the target is disposed in the magnetic field and the magnetic sensor element is disposed between the end of the single pole and the target with no intervening ferromagnetic pole between the magnetic sensor element and the target. The sensor element is configured to provide the first output in response to the first level of magnetic flux when the sensor element is disposed between the end of the single pole and the target and the second output when the sensor element is not disposed between the end of the single pole and the target. 
     Consistent with a further aspect of the present invention, there is provided an apparatus including a permanent magnet magnetized in a direction of magnetization for establishing a magnetic field; a single pole spaced from the permanent magnet and disposed in the magnetic field; and a Hall Effect device disposed in the magnetic field and adjacent an end of the single pole, the Hall Effect device having a flux receiving surface and being responsive to first and second levels of flux imparted to the flux receiving surface for providing first and second outputs, respectively, the first output being different from the second output, the flux receiving surface being positioned substantially parallel to the direction of magnetization. The Hall Effect device is configured to provide the first output when a target is disposed in the magnetic field and the Hall Effect device is disposed between the end of the single pole and the target with no intervening ferromagnetic pole between the Hall Effect device and the target. The Hall Effect device is further configured to provide the second output when Hall Effect device is not disposed between the end of the single pole and the target. 
     While the principles of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.