Abstract:
A diagnosable magnetic switch assembly and method for determining a seat occupant. A network in the assembly includes a resistor in series with a magnetic switch, and a resistor in parallel with the series combination. Such an arrangement provides distinct assembly resistance values when the magnetic switch is open or closed, and thereby provides a means for diagnosing the operational state of the diagnosable magnetic switch assembly. The diagnosable magnetic switch assembly may be included in a seat occupant indicator system that embeds the seat occupant indicator system and a magnet in a cushion of a seat assembly. When the seat assembly is occupied, the magnet is separated from the seat occupant indicator system by a sufficient distance so the seat occupant indicator system is in an open state. When an occupant sits in the seat assembly, the magnet and the seat occupant indicator system are brought closer together so that the magnetic field operate the seat occupant indicator system to a closed state. A controller measures the assembly resistance values to determine if the seat assembly is empty, occupied, or if an assembly fault state is occurring.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/148,500, filed Jan. 30, 2009, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF INVENTION 
       [0002]    The invention generally relates to magnetic switches, and more particularly relates to a seat occupant indicator system. 
       BACKGROUND OF INVENTION 
       [0003]    Many vehicles such as automobiles, tractors, and construction equipment are equipped with seat occupant detectors. Such detectors are often based on some kind of switch installed in the seat that is activated to one state when the seat assembly is empty, and another state when the seat is occupied by a person. Such switches may be used as part of a seat belt reminder (SBR) system for reminding an operator or a passenger to fasten a seat belt. Some occupant detectors use air bladders that sense pressure due to the weight of the person in the seat assembly. Other occupant detectors use a mechanical switch in the seat assembly or seat cushion that is actuated by the presence of a person occupying the seat assembly. However, occupant detectors using a bladder or a mechanical switch have undesirably high cost, and make noise when activating the mechanical switch. Furthermore, if the wiring harness connected to the bladder or mechanical switch is damaged resulting in either an open circuit or a short circuit, the fault in the wiring harness may go undiagnosed. What is needed is a simple occupant detector switch that can be easily diagnosed for wiring failures. 
       SUMMARY OF THE INVENTION 
       [0004]    In accordance with one embodiment of this invention, a diagnosable magnetic switch assembly is provided. The diagnosable magnetic switch assembly comprises a first terminal and a second terminal for making electrical contact with the diagnosable magnetic switch assembly. The diagnosable magnetic switch assembly further comprises a first network and a second network. The first network is connected between the first terminal and the second terminal, and includes a first resistor having a first resistor value. The second network is connected between the first terminal and the second terminal, and includes a series arrangement of a second resistor and a magnetic switch. The second resistor has a second resistor value. The magnetic switch is operable to an open state and a closed state in response to a magnetic field corresponding to a proximity of a magnet. By this arrangement the diagnosable magnetic switch assembly exhibits an assembly resistance value indicative of at least the open state, the closed state, or an assembly fault state. 
         [0005]    In another embodiment of the present invention, a seat occupant indicator system is provided. The seat occupant indicator system comprises a seat assembly, a diagnosable magnetic switch assembly, a magnet, and a controller. The seat assembly comprises a cushion formed of compressible material that includes a compressible region of the cushion that is compressed when an occupant is present on the seat assembly, The diagnosable magnetic switch assembly is arranged proximate to the compressible region, is operable to an open state and a closed state in response to a magnetic field, and is configured to exhibit an assembly resistance value indicative of at least an open state, a closed state, or an assembly fault state. The magnet provides the magnetic field and is arranged proximate to the diagnosable magnetic switch assembly such that when the compressible region is not compressed, the magnet has a position such that the magnetic field operates the diagnosable magnetic switch assembly to the open state, and when the compressible region is compressed, the magnet has a position such that the magnetic field operates the diagnosable magnetic switch assembly to the closed state. The controller is configured to determine the assembly resistance value and thereby indicate when a seat occupant is present or an assembly fault state exists. 
         [0006]    In yet another embodiment of the present invention, a method of detecting a seat occupant is provided. The method of detecting a seat occupant includes the step of configuring a diagnosable magnetic switch assembly to operate to an open state and a closed state in response to a magnetic field corresponding to a proximity of a magnet and to exhibit an assembly resistance value indicative of at least an open state, a closed state, or an assembly fault state. The method also includes the step of arranging the diagnosable magnetic switch assembly and a magnet in a seat cushion to vary the magnetic field effective to operate the diagnosable magnetic switch assembly to detect the presence of a seat occupant on the seat cushion. The method further includes the steps of determining the assembly resistance value, and outputting an indication when a seat occupant is present or an assembly fault state exists. 
         [0007]    Further features and advantages of the invention will appear more clearly on a reading of the following detail description of the preferred embodiment of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]    The present invention will now be described, by way of example with reference to the accompanying drawings, in which: 
           [0009]      FIG. 1  is a cross section view of an empty seat assembly with of a diagnosable magnetic switch assembly within seat cushion in accordance with one embodiment; 
           [0010]      FIG. 2  is a cross section view of an occupied seat assembly with of a diagnosable magnetic switch assembly within seat cushion in accordance with one embodiment; 
           [0011]      FIG. 3  is a schematic/block diagram of a diagnosable magnetic switch assembly in accordance with one embodiment; and 
           [0012]      FIG. 4  is a flow chart of a method to determine the presence of a seat occupant in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0013]    In accordance with an embodiment of a seat occupant indicator system  10 ,  FIGS. 1 and 2  illustrate a seat assembly  12  in a vehicle  11 . The system  10  may be used in a variety of vehicles such as an automobile, construction equipment, or aircraft. The indication of an occupant being present in the seat assembly  12  may be used by a seat belt reminder system or for controlling the deployment of a supplemental restraint such as an airbag. The seat assembly  12  includes a cushion  14  formed of compressible material such as foam or the like, it being understood that the intent is that there is something compressible, and that the invention is not limited to foam. The cushion  14  has a compressible region  16  that is substantially uncompressed when the seat assembly  12  is empty, and is compressed when an occupant  18  is present on the seat assembly  12 . The amount of compression is dependent on a variety of factors including, but not limited to, the durometer of the material forming the seat cushion  14  and the weight of the occupant  18 . 
         [0014]    The seat assembly  12  may also include a diagnosable magnetic switch assembly  20  arranged proximate to the compressible region  16 . The diagnosable magnetic switch assembly  20  is generally operable to an open state or a closed state in response to a magnetic field corresponding to a proximity of a magnet. In one embodiment, if the strength of the magnetic field impinging on the switch assembly  20  is less that a threshold, the switch assembly may operate to the open state. Similarly, if the strength of the magnetic field impinging on the switch assembly  20  is greater than a threshold, the switch assembly may operate to the closed state. In one embodiment, the diagnosable magnetic switch assembly  20  is configured to exhibit an assembly resistance value indicative of at least an open state, a closed state, or an assembly fault state. The arrangement and operation of the diagnosable magnetic switch assembly  20  is described in more detail below. 
         [0015]    The seat assembly  12  may also include a magnet  22  for providing a magnetic field to operate the diagnosable magnetic switch assembly  20 . The magnet  22  may be a permanent magnet or an electro-magnet. A permanent magnet is advantageous in that it is generally less expensive than an electro magnet and does not require a power supply. An electro-magnet is advantageous in that the magnetic field can be varied for calibration and/or diagnostic purposes. 
         [0016]    The magnet  22  may be arranged in the seat assembly  12  to be proximate to the diagnosable magnetic switch assembly  20 . As illustrated in  FIG. 1 , the arrangement is such that when the seat is empty, the compressible region  16  is not substantially compressed and the magnet  22  has a position such that the magnetic field operates the diagnosable magnetic switch assembly  20  to the open state. When the seat assembly  12  is empty or unoccupied, the compressible region  16  is uncompressed and the separation between the magnet  22  and the diagnosable magnetic switch assembly  20  is designated as distance D 1 . As illustrated in  FIG. 2 , when the seat assembly  12  is occupied by the occupant  18 , the compressible region  16  is generally compressed by the occupant  18  so the magnet  22  and the diagnosable magnetic switch assembly  20  are generally in closer proximity as compared to when the seat assembly  12  is empty. In general, as the magnet  22  moves closer to the diagnosable magnetic switch assembly  20 , the strength of the magnetic field impinging on the diagnosable magnetic switch assembly  20  increases. As such and the magnet  22  may be urged into such a position by the presence of an occupant  18  that the magnetic field operates the diagnosable magnetic switch assembly  20  to the closed state. The values of distances D 1  and D 2  may be empirically determined based on the durometer of the cushion  14 , the strength of the magnetic field generated by the magnet  22 , and the sensitivity of the diagnosable magnetic switch assembly  20 . 
         [0017]    It should be appreciated that the arrangement of the magnet  22  and the diagnosable magnetic switch assembly  20  shown in  FIGS. 1 and 2  is exemplary and non-limiting. As illustrated, the seat cushion  14  is depicted as being formed of a single material where the magnet  22  and the diagnosable magnetic switch assembly  20  are molded into the foam forming the seat cushion  14 . Alternately, the seat cushion  14  may be formed of various layers of material, each possibly having a different durometer. In such an embodiment the magnet  22  and the diagnosable magnetic switch assembly  20  may be placed as the layers are assembled to form the cushion  14 . Alternately, the diagnosable magnetic switch assembly  20  may be arranged to be between the occupant  18  and the magnet  22 . Also, alternate arrangements may have either the magnet  22  or the diagnosable magnetic switch assembly  20  at the upper or lower surface of the seat cushion  14 . 
         [0018]    The seat occupant indicator system  10  may also include a controller  24  configured to measure or determine the assembly resistance value and output a signal  40  indicating when a seat occupant is present, the seat is empty, or an assembly fault state is indicated based on the assembly resistance value. As used herein, an assembly fault state is indicated when the assembly resistance value is not a value indicative of either the seat being empty seat or a value indicative of the seat being occupied. The controller  24  may include a microprocessor or other control circuitry as should be evident to those skilled in the art. The controller may include memory, including non-volatile memory, such as electrically erasable programmable read-only memory (EEPROM) for storing one or more routines, thresholds and captured data. The one or more routines may be executed by the microprocessor to perform steps for determining the assembly resistance value and outputting an indication that a seat occupant is present or an assembly fault state exists. For one embodiment of the diagnosable magnetic switch assembly  20 , the controller  24  may be configured to indicate that the diagnosable magnetic switch assembly is in the open state when the assembly resistance value is substantially equal to a first resistor value, indicate that the diagnosable magnetic switch assembly  20  is in the closed state when the assembly resistance value is substantially equal to a parallel combination of the first resistor value and a second resistor value, and indicate a fault condition when neither the open state nor the closed state is indicated. 
         [0019]      FIG. 3  illustrates a schematic/block diagram of one embodiment of a seat occupant indicator system  10 . The diagnosable magnetic switch assembly  20  includes a first terminal  26  and a second terminal  28  for making electrical contact with the controller  24  and a network of electrical components within the diagnosable magnetic switch assembly  20 . The network of components within the diagnosable magnetic switch assembly  20  may be segregated into a first network  30  and a second network  32 . Separating the components into two networks is generally for the purpose of simplifying the explanation of the diagnosable magnetic switch assembly  20 . The first network  30  is connected between the first terminal and the second terminal and includes a first resistor RP having a first resistor value RPV. By this arrangement, the first network  30  provides a first electrical path having a resistance value that is generally independent of the state of the magnetic switch SW. The second network  32  is also connected between the first terminal  26  and the second terminal  28  and includes a series arrangement of a second resistor RS having a second resistor value RSV, and a magnetic switch SW operable to open and the close in response to a magnetic field. By this arrangement, the second network  32  provides a second electrical path having a resistance value that is generally dependent on the state of the magnetic switch  32 . Either the first network  30  or the second network  32  may include other components such as inductors or capacitors for various purposes such as reducing susceptibility to various forms of electromagnetic energy or reducing the radiation of various forms electromagnetic energy, for example electromagnetic energy in the form of radio waves. 
         [0020]    In one embodiment, the magnetic switch SW may be selected so that an open resistance of the magnetic switch is much greater than the first resistance value RPV. For this condition the assembly resistance value when the magnetic switch is open is substantially equal to the first resistor value and thereby indicates that the diagnosable magnetic switch assembly  20  is in the open state. Similarly, the magnetic switch SW may have a closed resistance of the magnetic switch SW that is much smaller than the second resistance value RPS. For this condition the assembly resistance value is substantially equal to the parallel combination of the first resistor value RVP and the second resistor value RVS when the magnetic switch SW is closed, and thereby indicates that the diagnosable magnetic switch assembly  20  is in the closed state. Alternately, the magnetic switch SW open resistance and closed resistance may be such that the open resistance value and/or closed resistance value needs to be included in the characterization of the diagnosable magnetic switch assembly  20 , particularly if the open and closed values of the magnetic switch SW vary due to changes in temperature or due to wear. 
         [0021]    As used herein, an assembly resistance value substantially equal to a specific resistor value includes a range of values greater than and/or less than the specific resistor value. The range may correspond to the tolerance of the resistor values plus any additional resistance present in the electrical path between the controller  24  and the diagnosable magnetic switch assembly  20 . For example, the controller  24  may be connected to the first terminal  26  and the second terminal by wires  34  and  36  through a connector  38  that may contribute to the assembly resistance value measured by the controller  24 . An exemplary range of values may be the specific resistor value +/−10%. 
         [0022]    The first resistor value RPV and the second resistor value RSV are preferably selected so that the assembly resistance values for the open state and the closed state are readily distinguished from potential fault conditions. Exemplary fault conditions may include, but are not limited to, a short circuit between wires  34  and  36 , a short circuit from either wires  34  or  36  to the vehicle chassis ground, an open circuit due to connector  38  being disconnected, or a damaged magnetic switch SW. 
         [0023]    By way of an example, suitable resistance values for RPV and RPS may be 2000 Ohms and 500 Ohms, respectively. In this example, the assembly resistance value range will be determined using an exemplary range of +/−10%. As such, the open state will be indicated when the assembly resistance value is about 2000 Ohms+/−10%, or about 1800 Ohms to about 2200 Ohms. Similarly, the closed state will be indicated when the assembly resistance value is about 400 Ohms+/−10%, or about 360 Ohms to about 440 Ohms. If the assembly resistance value is not between about 360 Ohms to about 440 Ohms and not between about 1800 Ohms to about 2200 Ohms, then an assembly fault state may be indicated. It should be evident to those skilled in the art that there are several means available to readily measure the assembly resistance value with the degree of accuracy necessary to determine the various states of the diagnosable magnetic switch assembly  20  described herein. 
         [0024]    If an assembly fault state is indicated, the assembly resistance value may be used to further diagnose various conditions such as an open circuit condition, a short circuit condition, and a magnetic switch fault condition. The open circuit condition may be indicated when the assembly resistance value is substantially greater than the first resistance value RPV. Continuing with the exemplary values for RPV and RSV given above, if the assembly resistance value is greater than about 2200 Ohms, then an open circuit condition may be indicated. In response, the controller  24  may output a diagnostic signal  40  directing a technician to check the wires  34  and  36  for damage, and confirm that connector  38  is properly connected. The short circuit condition may be indicated when the assembly resistance value is substantially less than a parallel resistor value equal to the first resistor value RPV and the second resistor value RVS in parallel. For this example, the parallel resistor value is equal to the parallel combination of 2000 Ohms and 500 Ohms, about 400 Ohms. Continuing with the exemplary +/−10% range given above, if the assembly resistance value is less than about 360 Ohms, then a short circuit condition may be indicated. In response, the controller  24  may output a diagnostic signal directing a technician to check the wires  34  and  36  for insulation damage that may be causing the short circuit condition. A magnetic switch fault condition may be indicated when the assembly resistance value is substantially less than the first resistance value RPV and substantially greater than the parallel resistor value of the first resistor value RPV and the second resistor value RSV in parallel. Continuing with the exemplary values for RPV and RSV given above, if the assembly resistance value is greater than about 440 Ohms and less than about 1800 Ohms, then a magnetic switch fault condition may be indicated. In response, the controller  24  may output a diagnostic signal directing a technician to replace the diagnosable magnetic switch assembly  20 . 
         [0025]    In one embodiment of the diagnosable magnetic switch assembly  20  the magnetic switch SW may include a reed switch. Reed switches are economical and readily available devices that do not require a power supply to operate to an open state or a closed state in response to a magnetic field. In another embodiment of the diagnosable magnetic switch assembly  20  the magnetic switch SW may include a Hall effect switch. Hall effect switches require a power supply to operate to an open state and a closed state in response to a magnetic field, and so the system  10  incurs the additional cost of wiring power to the diagnosable magnetic switch assembly  20 . However, programmable Hall effect switches that can be calibrated after being installed into the cushion  14  to switch to open or closed in response to a specific magnetic field strength are readily available from such companies as Micronas and Allegro. Using such a Hall effect switch may allow the seat occupant indicator system  10  to be calibrated after assembly and thereby compensate for variations in the strength of magnetic field generated by magnet  22  and for variations of dimension D 1  and D 2  for different occupants  18 . 
         [0026]      FIG. 4  illustrates an embodiment of a method or routine  400  for detecting a seat occupant  18  residing in a seat assembly  12  having a seat occupant indicator system  10 . The method  400  may include a step of configuring a diagnosable magnetic switch assembly  20  to operate to an open state and a closed state in response to a magnetic field, and to exhibit an assembly resistance value indicative of at least the open state, a closed state, or an assembly fault state. At step  410 , the diagnosable magnetic switch assembly  20  and a magnet  22  are arranged in a seat cushion  14  so as to vary the distance between the diagnosable magnetic switch assembly  20  and the magnet  22  in response to the presence or absence of an occupant  18 . Changing the distance generally changes the strength of the magnetic field impinging on the diagnosable magnetic switch assembly  20  in response to the presence of the occupant  18 . The magnetic field varies to operate the diagnosable magnetic switch assembly  20  to vary the assembly resistance value and thereby indicate the presence of the seat occupant  18  on the seat cushion  14 . At step  420  the assembly resistance value is measured or determined by a controller  24 . At step  430  the assembly resistance value is compared to a range of expected resistance values to see if the resistance value is substantially equal to the first resistance value RPV. If the assembly resistance value is substantially equal to the first resistance value RPV, then the method  400  proceeds to step  440  where a signal  40  is output by the controller  24  indicating that the seat assembly  24  is not occupied by a person  18 , e.g. the seat assembly  12  is empty. If the assembly resistance value is not substantially equal to the first resistance value RPV, then the method  400  proceeds to step  450  where the assembly resistance value is compared to a parallel resistance value about equal to the parallel combination of the first resistance value RPV and the second resistance value RSV. If the assembly resistance value is substantially equal to the parallel resistance value, then the method  400  proceeds to step  450  where a signal  40  is output by the controller  24  indicating that the seat assembly is occupied by a person  18 . If the assembly resistance value is not substantially equal to the parallel resistance value, then the method  400  proceeds to step  470  where a signal  40  is output by controller  24  indicating an assembly fault state and thereby indicating that there may be a problem with the seat occupant indicator system  10 . The step  450  of outputting a signal  40  indicating that the assembly fault state is present may include indicating that the assembly fault state is caused by an open circuit condition, a short circuit condition, or a magnetic switch fault condition. As described above, these conditions are determined by comparing the assembly resistance value to the expected resistance values. 
         [0027]    Accordingly, a system  10  and method  400  for determining if a seat assembly  12  is occupied by an occupant  18  are provided. The system  10  includes a diagnosable magnetic switch assembly  20  that exhibits an assembly resistance value dependent on the state of the diagnosable magnetic switch assembly  20 . In one embodiment, when the system  10  is operating normally, the assembly resistance value is substantially equal to either a first resistance value RPV, thereby indicating an open state, or a parallel resistance value substantially equal to the parallel combination of the first resistance value RPV and a second resistance value RSV, thereby indicating a closed state. If the assembly resistance value is not substantially equal to either the first resistance value RPV or the parallel resistance value, then an assembly fault state may be indicated and a signal may be output indicating that the system  10  should be serviced. By having such an arrangement, the system  10  may more reliably indicate that an occupant  18  is present, or that the seat assembly  12  is empty as compared to systems that do not include electrical components such as RP and RS to provide a switch assembly resistance other than a simple open circuit or short circuit. Furthermore, the system may be embedded or molded within a seat cushion  14  and thereby reduce manufacturing costs associate with installing a switch assembly to a seat frame or other mechanical support. 
         [0028]    While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.