Abstract:
An improved vehicle restraint system includes a seat belt tension sensor and an occupant detection control module for characterizing the occupant of a vehicle seat to determine whether to allow or suppress deployment of supplemental inflatable restraints for the occupant. The belt tension sensor includes on-board signal processing circuitry and is coupled to occupant detection control module via a two wire interface that both powers the sensor and its signal processing circuitry and supports communication of belt tension data to the occupant detection control module. The sensor produces an electrical signal responsive to seat belt tension, and the processing circuitry generates one of a specified number of messages pertaining to the range of the measured tension, and then modulates the current through the two wire interface to communicate the generated message to the occupant detection control module.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a motor vehicle inflatable restraint system including a seat belt tension sensor for characterizing the occupant of a vehicle seat to determine whether to allow or suppress deployment of restraints for the occupant, and more particularly to a system arrangement for processing and communicating information provided by the seat belt tension sensor.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various occupant-responsive sensing devices can be employed to characterize the occupant of a vehicle seat for purposes of determining whether deployment of air bags and other restraints should be allowed or suppressed. For example, it is generally desired to allow normal deployment for an adult, to reduce deployment force for a child, and to suppress deployment entirely for an infant seat secured to the vehicle seat with a seat belt. A particularly effective and yet inexpensive way of achieving this functionality is to sense both the seat belt tension and the occupant weight applied to the bottom cushion of the seat. In general, the measured occupant weight may be reduced in proportion to the measured seat belt tension to be more reflective of the actual occupant weight applied to the seat, and seat belt tension in excess of a calibrated value is indicative of a cinched down infant seat.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention is directed to an improved and cost effective mechanization of a vehicle restraint system including a seat belt tension sensor and an occupant detection module for characterizing the occupant of a vehicle seat to determine whether to allow or suppress deployment of supplemental inflatable restraints for the occupant. According to the invention, the belt tension sensor includes on-board signal processing circuitry and is coupled to occupant detection module via a two wire interface that both powers the sensor and its signal processing circuitry and supports communication of belt tension data to the occupant detection module. The sensor produces an electrical signal responsive to seat belt tension, and the processing circuitry generates one of a specified number of messages pertaining to the range of the measured tension, and then modulates a loop current in the two wire interface to communicate the generated message to the occupant detection module. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a system diagram of a vehicle restraint system, including a belt tension sensor and an occupant detection module according to this invention.  
         [0005]    [0005]FIG. 2 is a partial cross-sectional view of the belt tension sensor of FIG. 1  
         [0006]    [0006]FIG. 3 is a circuit diagram illustrating pertinent circuitry of the belt tension sensor and occupant detection module of FIG. 1.  
         [0007]    [0007]FIG. 4 is a timing diagram illustrating a communication protocol for sending messages from the belt tension sensor to the occupant detection module.  
         [0008]    [0008]FIGS. 5 and 6 depict flow diagrams representative of software routines executed by the occupant detection module of FIG. 1 according to this invention.  
         [0009]    [0009]FIG. 7 depicts a flow diagram representative of a software routine executed by the belt tension sensor circuit of FIG. 3 according to this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    The present invention is disclosed in the context of a vehicle restraint system  10  including an airbag control module (ACM)  12 , driver frontal and side air bags  14 ,  16 , and passenger frontal and side airbags  18 ,  20 . The ACM  12  determines whether and when to deploy the various airbags  14 ,  16 ,  18 ,  20  based on acceleration data obtained from a frontal crash sensor  22 , a driver side crash sensor  24 , a passenger side crash sensor  26 , and occupant status information obtained from occupant detection module (ODM)  28 . In general, the occupant status information may indicate simply whether to allow or suppress deployment, but in certain applications may provide additional occupant detail that enables ACM  12  to suitably control the deployment force of the respective air bags. The ODM  28  is responsive to the output Ws of a seat sensor  34  indicative of the occupant weight applied to a vehicle seat, a seat belt buckle switch  36  that indicates if a seat belt for the vehicle seat is buckled or unbuckled, and a belt tension sensor (BTS)  38  that indicates the amount of tension or force applied to the seat belt. As indicated in FIG. 1, the seat sensor  34  in the illustrated mechanization is a pressure sensor responsive to the fluid pressure in a seat cushion bladder  32 , although other information such as the temperature of the cushion or bladder is usually required to obtain reliable occupant weight data over a wide range of ambient conditions; see for example, the U.S. Pat. Nos. 5,987,370, 6,101,436, 6,138,067 and 6,246,936, which are assigned to the assignee of the present invention and incorporated herein by reference.  
         [0011]    The BTS  38  may take various forms, as set forth, for example, in the Research Disclosure No. 41402, October, 1998, Page 1304, incorporated herein by reference. However, FIG. 2 illustrates a particularly advantageous mechanization in which BTS  38  is fastened to the vehicle floor outboard of the seat, and the seat belt  50  passes through a sensor slot  48 . The slot  48  is formed in the main body of the sensor case  40 , and the case  40  further includes a tang  42  with an opening  44  for fastening BTS  38  to the vehicle floor. When the seat belt  50  is in use, it engages an armature  46  supported within the case  40 , and tension in the seat belt  50  biases armature  46  rightward as viewed in FIG. 2 against the bias force of springs  52  and  54 . The rest position of armature  46  is defined by the stop  56 , and rightward displacement of armature  46  is measured by a Hall Effect sensor  62  positioned between a magnet  58  affixed to the armature  46  and a magnet  60  affixed to the case  40 . The sensor  62  produces an Output signal that is indicative of magnetic field strength, and such signal is applied to a circuit board  66  via conductor  64 . The circuit board  66  supports circuitry as described below in reference to FIG. 3 for processing the sensor output signal and communicating a belt tension message to ODM  28  via a conductor pair sheathed in the cable  68 .  
         [0012]    [0012]FIG. 3 depicts the circuits of BTS  38  and ODM  28  in block diagram format. The two wire interface comprising the conductors  70  and  72  is used both for supplying power from ODM  28  to BTS  38  and for communicating belt tension messages from BTS  38  to ODM  28 . Within ODM  28 , the conductor  70  is coupled to the vehicle ignition voltage Vign (typically 12 VDC) through a current limiting resistor  74  and a switch  76  that opens or closes in response to the signal on control line  78 . The control line  78  is selectively activated by a microprocessor-based ECU  80 , which also activates a transistor  82  via resistor  84  whenever the control line  78  is activated to close the switch  76 . Within BTS  38 , the sensor output signal on line  64  is applied as an input to the analog-to-digital converter (ADC) input port of a microprocessor-based ECU  86 , and the conductor  70  is supplied as an input to a voltage regulator (VR)  88  that supplies a regulated output voltage VDD (such as 5 VDC) to Hall Effect Sensor  62  and ECU  86  via line  90 . A ground or reference voltage is supplied from ODM  28  to BTS  38  via conductor  72 , the transistor  82  and the resistors  92  and  94 . Thus, operating power is supplied to BTS  38  from ODM  28  when ECU  80  activates the control line  78 .  
         [0013]    As more fully described below in reference to FIG. 7, the ECU  86  creates a digital representation of the sensor output signal, and determines a corresponding belt tension range. In the illustrated embodiment, for example, there are eleven legitimate belt tension ranges (0-3 lbs., 3-6 lbs., 6-9 lbs., 9-12 lbs., 12-15 lbs., 15-18 lbs., 18-21 lbs., 21-24 lbs., 24-27 lbs., 27-30 lbs., and over-30 lbs.), a failed low range (ERRlow) and a failed high range (ERRhigh). A message indicative of the determined belt tension range is then communicated to ODM  28  by modulating the conduction of a transistor  96  via resistor  98  in accordance with the timing diagram of FIG. 4. Each modulation pattern has a fixed period such as 100 msec, with a different number of pulses and/or pulse-widths occurring within the period. For example, a series of five two-msec pulses is used to represent the belt tension range of 12-15 lbs. The emitter of transistor  96  is coupled to the conductor  72 , while the collector of transistor  96  is coupled to conductor  70  via the parallel connected resistor  100  and capacitor  102 . When the transistor  96  is modulated to a conductive state, the voltage on conductor  70  falls to a value determined by the resistors  74 ,  100 ,  92  and  94  (which is still higher than the VDD output of voltage regulator  88 ), and the loop current in conductors  70  and  72  increases to a higher-than-normal value. This change in the loop current is detected by the comparator  104  in ODM  28 , which compares the voltage across resistor  94  to a threshold Vthr to produce a digital output signal on line  106  that is supplied as an input to the ECU  80 .  
         [0014]    As described below, the ECU  80  decodes the belt tension message transmitted by BTS  38 , and uses the corresponding belt tension range to determine if airbag deployment should be allowed or suppressed. If the belt tension range is indicative of a cinched infant seat (i.e., above a calibrated value such as 30 lbs.), the occupant status is set to SUPPRESS to suppress airbag deployment. Otherwise the belt tension range is used to compensate the measured seat weight Ws, or to inform ACM  12  of a failure of BTS  38 .  
         [0015]    The flow diagrams of FIGS.  5 - 6  represent software routines executed by the ECU  80  of ODM  28  according to this invention. The routine of FIG. 5 is continuously executed during vehicle operation to update the suppression status based on the various inputs depicted in FIG. 1, and to send the suppression status to ACM  12 , whereas the routine of FIG. 6 is an interrupt service routine executed in response to a logic level transition on the comparator output line  106  for receiving and decoding the belt tension message sent by BTS  38 .  
         [0016]    Referring to FIG. 5, the block  110  is initially executed to initialize system variables including the suppression status. Then the blocks  112  and  114  are executed to read various input signals such as the measured seat weight Ws and the seat belt buckle switch state, and to read the current belt tension range. The diagnostics block  116  determines if the input signals are consistent and within normal ranges, and the block  118  then determines the suppression status based on the measured seat weight, the seat belt buckle state and the seat belt tension range. For example, the belt tension range can be used to detect the presence of a cinched down infant seat, or to compensate the measured seat weight Ws so that it more accurately represents occupant weight. If an infant seat is detected, the suppression status is set to SUPPRESS; otherwise the suppression status is set to either SUPPRESS or ALLOW depending on the magnitude of the adjusted seat weight and the seat belt buckle switch state. The block  120  then sends the suppression status to ACM  12 , and possibly also to a driver display (not shown). As indicated at block  122 , the blocks  112 - 120  are repeatedly executed during vehicle operation, and when the vehicle ignition switch is turned off, the block  114  performs shutdown tasks to complete the routine.  
         [0017]    As indicated above, the interrupt service routine of FIG. 6 is executed in response to a logic level transition on the comparator output line  106 . The blocks  126  and  128  determine the pulse time and level, and the block  130  sets a timer to measure the duration of the next pulse. This process is repeated at each interrupt until block  132  determines that the message pulse train is complete. At Such point, the block  134  is executed to decode the belt tension message based on the number of pulses and their duration.  
         [0018]    The flow diagram of FIG. 7 represents a software routine executed during vehicle operation by the ECU  86  of BTS  38  for processing the Output signal of Hall Effect sensor  62 , and sending a corresponding belt tension range message to ODM  28 . The block  136  is initially executed to initialize system variables including the belt tension range. Then the blocks  138  and  140  are executed to read and filter the digital version of the Hall Effect output signal, and the block  142  determines if the signal is within a normal range of values. If the diagnostics indicate that the signal is above the normal range, the blocks  144  and  146  set the belt tension range signal (SIGNAL) to HIGH FAULT; if the signal is below the normal range, the blocks  148  and  150  set SIGNAL to LOW FAULT. Otherwise, the block  152  sets SIGNAL to a value corresponding to the measured belt tension, either by table look-up or successive comparison of the filtered signal to a series of calibrated thresholds. Then the block  154  is executed to send the determined SIGNAL to ODM  28  by modulating the conduction of a transistor  96  as described above in reference to the block diagram of FIG. 3 and the timing diagram of FIG. 4. As indicated at block  156 , the blocks  138 - 154  are repeatedly executed during vehicle operation, and when the vehicle ignition switch is turned off, the block  158  performs shutdown tasks to complete the routine.  
         [0019]    In summary, the present invention provides a cost effective arrangement for interfacing a remote belt tension sensor to an occupant detection module in a vehicle restraint system. The two wire interface not only supplies power to from the occupant detection module to the sensor and its associated signal processing circuitry, but also supports communication of belt tension messages to the occupant detection module through modulation of the loop current in the two wire interlace. Functions customarily performed in the occupant detection module, such as signal processing and diagnostics, are instead performed by the sensor circuitry, and the belt tension data is transferred to the occupant detection module in a digital format to reduce susceptibility to error from spurious electromagnetic interference.  
         [0020]    While this invention has been described in reference to the illustrated embodiment, it will of course be recognized that various modifications in addition to those mentioned above will occur to those skilled in the art. For example, the message protocol of FIG. 4 may be modified to include more or fewer ranges, the belt tension sensor  38  may be different than shown, and so on. Accordingly, it should be understood that restraint systems incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.