Abstract:
A fuel level sensor system is provided that passively protects fuel sensors and fuel tanks from receiving voltages from fuel level sensing electronics that exceed safety threshold values. The fuel level sensor system includes a safety barrier device that is located outside of a fuel tank. The safety barrier device includes a housing and a transformer located within the housing. The transformer is electrically coupled between fuel level sensing electronics and a fuel level sensor within the fuel tank. The transformer protects the fuel tank from voltages that exceed a predefined threshold value.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to fuel systems and, more specifically, to fuel sensing systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    The Federal Aviation Administration (FAA) provides directives to the commercial aircraft industry based on the continuous study of failures and losses of aircraft. One such directive, titled SFAR 88, sets out that fuel level sensing systems must be intrinsically safe against worst-case voltage applications.  
           [0003]    Some examples of currently known suppression devices that are used to meet this directive include zener diodes, fuses, or crowbar circuits. However, these devices do not provide systems that can meet the requirements of the directive in the most cost-effective manner.  
           [0004]    Therefore, there is an unmet need to provide a low-cost, easy to install device for measuring fuel levels that adheres to regulatory standards.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides a low cost, intrinsically safe fuel level sensor system. The present invention provides passive protection with an easy-to-manufacture transformer unit that does not require modification of present fuel level sensor electronics.  
           [0006]    One embodiment of the present invention provides a fuel level sensor system that includes a safety barrier device that is located outside of a fuel tank. The safety barrier device includes a housing and a transformer located within the housing. The transformer is electrically coupled between fuel level sensing electronics and a fuel level sensor within the fuel tank. The transformer protects the fuel tank from voltages that exceed a predefined threshold value.  
           [0007]    In one aspect of the invention, the transformer includes a toroid, and first and second coils wrapped at least once around the toroid. The safety barrier device includes first and second shields located within the housing. The first shield is positioned around the first coil and the second shield is positioned around the second coil. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0009]    [0009]FIG. 1 is a block diagram of the present invention; and  
         [0010]    [0010]FIGS. 2 and 3 are diagrams of safety barriers formed in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    As shown in block diagram form in FIG. 1, the present invention provides a system  20  for sensing fuel level in a fuel tank, such as without limitation a fuel tank in an aircraft, and protecting a fuel level sensor  24  from any surges in voltage coming from aircraft circuits. The system  20  includes the fuel level sensor  24 , fuel level sensing electronics  26 , a drive safety barrier device  28 , and a receive safety barrier device  30 . The drive safety barrier device  28  is connected to the fuel level sensor  24  and driving circuitry of the fuel level sensing electronics  26 . The receive safety barrier device  30  is connected to the fuel level sensor  24  and sensing circuitry of the fuel level sensing electronics  26 .  
         [0012]    The drive safety barrier device  28  and the receive safety barrier device  30  limit the amount of voltage applied to the fuel level sensor  24  even when a large voltage (e.g., 760 VAC) is applied to either of the safety barrier devices  28  or  30 . The safety barrier devices  28  and  30  shield the fuel level sensor  24  from unwanted electrostatic and magnetic signals.  
         [0013]    As shown in FIG. 2, the drive safety barrier device  28  is designed to saturate at voltage, or current levels below those which pose a safety hazard as predicated in SFAR  88 . An exemplary drive safety barrier device  28  includes an energy-limited transformer  50  and a highly permeable to a magnetic field shield  52 . The shield  52  is suitably box-shaped that surrounds the transformer  50 . The shield  52  is suitably a highly permeable shield for protecting components within the shield  52  from magnetic interference. The shield is made from a material similar to Carpenter49™ material. The shield  52  includes a center wall  58  that separates the device  28  into two halves  60  and  62 . The transformer  50  includes a toroid  56 , an airplane-side coil  68 , and a sensor-side coil  70 . The toroid  56  is suitably a doughnut-shaped piece of magnetic material that passes through openings in the center wall  58  to allow the toroid  56  to occupy space with the two halves  60  and  62 .  
         [0014]    The airplane-side coil  68  is wrapped around the toroid  56  in the first half  60  and the sensor-side coil  70  is wrapped around the toroid  56  in the second half  62 . The airplane-side coil  68  includes a positive lead  74  and a negative lead  76  that are connected to the drive circuitry of the fuel level sensing electronics  26 . The negative lead  76  is connected to ground and the positive lead  74  receives a drive signal from the drive circuitry. It will be appreciated that the characteristics of the transformer  50  can be altered by changing the number of times a coil is wrapped around the toroid. It will also be appreciated that other magnetic devices such as can be used in place of the toroid  56 .  
         [0015]    The sensor-side coil  70  includes a positive lead  80  and a negative lead  82 . The positive lead  80  is connected to the fuel level sensor  24  and the negative lead  82  is connected to a negative lead on the receive safety barrier device  30 .  
         [0016]    The two halves  60  and  62  include shields  64  and  66  that surround the coils  68  and  70 . The first shield  64  is connected to the negative lead  76  and the second shield  66  is connected to the negative lead  82 . The shields  64  and  66  are suitably formed of Aluminum, but can be any other material that performs electrostatic or magnetic protection.  
         [0017]    The coils  68  and  70  are both isolated from the toroid by insulation (not shown). Non-limiting examples of the insulation is Capton™ or Delran™. The coils  68  and  70  are physically isolated from the insulation by an epoxy cover (not shown).  
         [0018]    The transformers  50  are suitably designed to saturate at a very low voltage in order to provide an energy-limited intrinsic safety barrier between the fuel level sensor  24  and the fuel level sensing electronics  26 . In one embodiment, the transformer  50  is designed to saturate just above operating voltage for the fuel level sensor  24 .  
         [0019]    As shown in FIG. 3, the receive safety barrier device  30  is constructed similarly to the drive safety barrier device  28 . The receive safety barrier device  30  includes a sensor-side coil  100  that includes a positive lead  102  that is coupled to the fuel level sensor  24  and a negative lead  104  that is connected to the negative lead  82  on the drive safety barrier device  28 . The receive safety barrier device  30  also includes an airplane-side coil  108  that includes a positive lead  110  that is coupled to the fuel level sensing electronics  26  and a negative lead  112  that is connected to ground.  
         [0020]    As will be appreciated the number of times the coils  68  and  70  wrap around the toroid  56  can vary depending upon the power level at which saturation is desired.  
         [0021]    The present invention operates as follows. A first drive signal is sent to the drive safety barrier device  28  that converts the first drive signal into a magnetic field signal. The magnetic field signal is converted into a second drive signal that is sent to the fuel level sensor  24  located within the fuel tank. The drive safety barrier device  28  protects the fuel tank from voltages that exceed a predefined threshold value.  
         [0022]    The fuel level sensor  24  generates a first sensed signal and sends the generated first sensed signal to the receive safety barrier device  30 . The receive safety barrier device  30  converts the first sensed signal into a second magnetic field signal and converts the second magnetic field signal into a second sensed signal. The second sensed signal is sent to the fuel level sensing electronics  26 .  
         [0023]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.