Abstract:
A level sensing device is disclosed that provides a lightweight but robust design and incorporates the use of an LVDT to determine the level of a fluid in a container.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to apparatus, systems and methods for sensing the level of a fluid in a container. In particular, the present disclosure relates to apparatus, systems and methods for sensing the level of a fluid in a container through the use of a linear variable differential transformer (LVDT). 
       BACKGROUND 
       [0002]    Digital fluid level transducer/senders that are both rugged and lightweight could be used in fuel tanks for unmanned aircraft. Transducers capable of sustaining loads induced by launch, flight and landing are currently of an analog design, which imparts additional equipment needs on the system. 
         [0003]    Digital transducers currently on the market are long, fragile instruments best suited for the lab or static tank environment with large spaces available for the bulky converters/senders mounted to the end of the instrument and external to the tank. 
         [0004]    Linear variable differential transformers (LVDT), available with digital outputs, initially existed only in laboratories for positional measurement; however their robustness and capacity for surviving shock loads allowed their use in the field. 
         [0005]    LVDTs, unfortunately, have been designed around a static LVDT transducer with a translating core. This design results in a product whose overall extended length is at least twice that which is to be measured. The volume to accommodate this length is not a luxury available within the unmanned aircraft structure. 
         [0006]    Therefore, a digital level sensing apparatus is needed that can meet the size and weight constraints of an unmanned aircraft that is rugged enough to withstand this challenging environment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a sectional plan view showing an embodiment of a level sensing apparatus installed in a representative tank with fluid, and 
           [0008]      FIG. 2  is a sectional plan view showing an embodiment of a level sensing apparatus. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0009]    Embodiments in accordance with the present disclosure are set forth in the following text to provide a thorough understanding and enabling description of a number of particular embodiments. Numerous specific details of various embodiments are described below with reference to level sensing devices for fluids in a container, but embodiments can be used with other features. In some instances, well-known structures or operations are not shown, or are not described in detail to avoid obscuring aspects of the inventive subject matter associated with the accompanying disclosure. For example, an LVDT is a well known electrical component requiring specific electrical interfaces. A person skilled in the art will understand, however, that the invention may have additional embodiments, or that the invention may be practiced without one or more of the specific details of the embodiments as shown and described. 
         [0010]    Referring to  FIGS. 1 and 2 , a level sensing device  10  is disposed in a container  36  and an LVDT  20  is encased in a float casing  18  using materials compatible with and sized appropriately for the medium being measured such that the LVDT  20  resides at a relatively constant level with respect to a fluid level  44  whether a fluid  38  be ascending, descending or constant. 
         [0011]    The LVDT  20  translates slidably upon a core rod assembly  22 , consisting of a core  23  and connecting rods  21  fixedly attached to either side of the core rod  22 . The connecting rods  21  may be of any non-magnetic material, such as plastic, ceramic, composite or metal, though most often it is of a 300-series stainless steel. The core rod assembly  22  is fixedly attached to and aligned by a top cap  32  and a bottom cap  14 . The top cap  32  and the bottom cap  14  are aligned by and fixedly attached to an outer casing  12 . The top cap  32  and the bottom cap  14  possess mating features to fixedly attach the core rod assembly  22 , connector  34  and casing  12 . The envelope created by the fixing of caps to either end of the casing shall be referred to hereafter as a housing  15 . 
         [0012]    As the LVDT  20  generates signals associated with its position and translation, leads (wires)  24  extend from the LVDT for the purpose of connecting to an external measuring system  42 , thereby transmitting and communicating these signals. The non-limiting arrangement and shape of these leads  24  may include wires stretched in a linear, sine wave, braided or other manner. 
         [0013]    For this embodiment, the preferred, non-limiting, arrangement are wires  24 , flexibly attached to the LVDT  20 , coiled about the core rod assembly  22  in a helical (spring-like) manner to reduce stresses on the wires  24  in order to limit or eliminate fatigue and breakage of the wires  24  through continuous movement during operation. These wires  24  may extend from one or both ends of the LVDT  20 , each subsequently terminating flexibly at the top cap  32  or bottom cap  14  or both. 
         [0014]    An electrical connector  34  may be fixedly attached to the top cap  32  to receive the leads  24  from the LVDT  20 . This connector  34  allows electrical continuity and the conveyance of the electrical signals from the LVDT  20  to external components  42  through the top cap  32  while providing a means of disconnecting from said components for removal from the system as a whole for repair or replacement. 
         [0015]    Another, non-limiting implementation, would be a sealed pass through feature in the top cap  32 , allowing the LVDT leads  24  to exit the internal chamber without allowing the measured medium (fluid)  38  from escaping from the container  36 . These leads  24  are typically terminated to the external measuring system  42  in a permanent (i.e.: soldered) or temporary (i.e. screw terminal) manner. 
         [0016]    One or both caps may possess mounting features which allow the device as a whole to be attached to a bracket, tank wall or other non-limited object on one or both ends. The top cap  32  and the bottom cap  14  may contain a cavity feature  46   a ,  46   c  and  46   b  respectively sized in a manner to receive a partial length or the whole of the LVDT float  18  or leads  24  so as not to limit the total desired translation of the LVDT float  18 . 
         [0017]    Additionally, the top cap  32  may possess one or more non-limited sealing features, such as gaskets and o-rings  30 , so as to limit or eliminate the propagation of the medium being measured through the device to a volume exterior. 
         [0018]    The casing  12 , tubular in manner, encompasses the float  18  and core rod assembly  22  for durability, protection and stability of the LVDT  20 , core rod assembly  22  and internal electrical components. The float  18  may or may not slidably engage the casing  12 . 
         [0019]    The caps  32  and  14 , the casing  12  or both may possess openings  16  and  26  to allow the passage of fluid to and from the exterior to the interior of the housing  15 . These openings are non-limited in shape, size and position. A filter material (not shown) may be placed in the openings  16  and  26 . 
         [0020]    The geometric features of the openings  16  and  26  in combination with the density or porosity of the filter material, if present, may be altered to provide rapid fluctuation, damped response or other dynamic conditions to the fluid contained within the housing  15 , thereby affecting the response of the LVDT  20 . 
         [0021]    All attachments are to be considered non-limiting and may be temporary (i.e.: screw threads) or permanent (i.e.: welded). 
         [0022]    The caps  14  and  32 , casing  12  and connectors  34  may be of any suitable material, such as plastic, elastomeric, ceramic, composite or metal. The components are not restricted to any particular geometrical shape, though items of a circular cross section are typically chosen for their ease of manufacture. 
         [0023]    This invention may be utilized in any system requiring a digital interface with a vessel containing fluid whose level must be known at predetermined intervals or at all times. Fluid, as used herein, refers to a liquid and gaseous liquids are referred to as air or vapor. 
         [0024]    The level sensing device  10  may be attached to the vessel body through either the top cap  32  or the top and bottom cap  14 , depending upon whether or not a sump exists within the vessel and measurement of the fluid level within the sump is desired. The following description exemplifies a typical installation of this invention within an unmanned aerial vehicles&#39; fuel tank, inclusive of a sump whose level is not a required output. 
         [0025]    The level sensing device  10  is attached to a top surface  48  of the container or fuel tank  36  either through a series of bolts around a mounting flange feature on the top cap  32  or with a nut applied to threads of the top cap  32  from the inside of the tank  36  such that the tank&#39;s skin is sandwiched between the nut and the top cap  32  flange. In both instances, a seal will be present between the top surface  48  and the top cap  32  so as to prevent migration of fluid from inside of the tank to the outside environment. 
         [0026]    With this installation, the case  12 , whose length is determined by the range of fluid levels desired to be measured, with all internal features and components, is within the tank and in contact with the tank&#39;s fluid  38  when the fluid is within the range desired to be measured. The electrical connector  34  is external to the tank  36  in this same installation, such that the external system  42 , such as, but not limited to, a computer, avionics, simple relays or visual indicators, may be attached via a mating connector and harness/cable/wire arrangement. 
         [0027]    When installed, and the tank&#39;s fluid level  44  is below the desired measurement range, the fluid  38  is not in contact with the LVDT  20 , which now rests at the bottom of the case  12  on the bottom cap  14 , with the electrical leads  24  at their greatest extension. 
         [0028]    As the fluid level  44  within the tank  36  rises and approaches a level to be measured, the fluid enters the case  12  through the holes  16  in the bottom cap  14 . As the fluid level  44  rises, air or vapor present in the case  12  exits through the holes  26  in the case  12 . The fluid level at which time the float  18  achieves neutral buoyancy is considered the lowest point of the measurable fluid level range. 
         [0029]    As the float  18  has been designed to buoyantly support the LVDT  20  and electrical lead  24  weight upon the fluid being measured, the float  18  and LVDT  20  begins translating along the core rod assembly  22 . As the LVDT  20  approaches and subsequently passes the core  23 , the digital signal generated by the LVDT  20  varies. This signal, communicated by the electrical leads  24  to the electrical connector  34  external interface, is utilized by the external system  42  as needed. 
         [0030]    At the top end of the fluid level range, the float  18  is disposed within the cavity feature  46   c  of the top cap  32 , as do the compressed electrical leads  24 , such that the fluid within the tank may measured accurately to the upper surface of the tank. The electrical leads  24  may also compress into an additional pocket  46   a  in the connector if so designed. 
         [0031]    The rate at which the float  18  rises with the fluid level  44  may be regulated by the quantity and sizes of the holes  16  in the bottom cap  14 , and any vents  26  in the case  12  and the top cap  32  or a combination of all mentioned. This may be implemented such that fluid level may be measured more accurately or at a slower rate of change when the fluid level external to the case  12  changes abruptly, either local to the case  12  or within the tank  36 , such as caused by fluid sloshing or momentary flow from one side of the tank to another, which is typical during attitude changes of an aircraft.