Abstract:
A product including a cryogenic container and an acoustic sensor positioned to sense the resonant frequency of the container and any liquid contents therein.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to fluid level monitoring and, more particularly, to an acoustic fluid level monitoring system for cryogenic containers. 
     BACKGROUND 
     Cryogenic containers have unique insulation requirements and are commonly used for very low temperature storage. Some vehicle fuel cells use cryogenic containers to store fuel in fluid form at very low temperatures. Measuring the fluid level inside of a cryogenic container can be difficult as both the containers and their contents pose special challenges. 
     SUMMARY 
     One embodiment includes a cryogenic container and an acoustic sensor positioned to sense the resonant frequency of the container and any liquid contents therein. 
     Another embodiment includes an inner container defining a storage area in which a fluid is stored, an outer container provided outside of the inner container, an insulation layer provided between the inner container and the outer container, and an acoustic sensor attached to the cryogenic container outside of the storage area. 
     Yet another embodiment includes a method of measuring the fluid level of a cryogenic container by measuring an acoustic resonant frequency of a cryogenic container, and correlating the acoustic resonant frequency of the cryogenic container to a fluid level inside the storage area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a product according to one embodiment of the invention; 
         FIG. 2  illustrates a product according to another embodiment of the invention; 
         FIG. 3  illustrates a product according to another embodiment of the invention; 
         FIG. 4  illustrates a product according to another embodiment of the invention; and 
         FIG. 5  illustrates a product according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description of embodiments is merely exemplary in nature and is not intended to limit the invention, its application, or uses. 
     Cryogenic containers are commonly used for low temperature storage, fore example, generally below −150° C., −238° F., or 123 K. Many include inner and outer containers separated by insulation. This design limits heat transfer to the storage area inside the inner container. Cryogenic containers are typically designed to have exceptionally efficient insulation to maintain low temperatures without requiring complex refrigeration equipment. This is partially accomplished by limiting the number of potential heat paths to the storage area. 
     A potential heat path can be any wire, pipe, tube, or the like that creates a path between the storage area and the outer container. Any such path can potentially allow heat to travel to the storage area and reduce the cryogenic container&#39;s efficiency. A cryogenic container may be more efficient by limiting the number of potential heat paths, so it can maintain low temperature storage for longer periods of time without refrigeration. 
     Cryogenic containers are often used for storing liquefied gases, such as hydrogen, nitrogen, helium, and others. Certain liquefied gasses can be used in fuel cells and require cryogenic containers for storage. Some fuel cells are used in automotive applications that require in-vehicle cryogenic containers for fuel storage. In such applications it may be necessary and challenging to monitor the fuel or fluid level inside the cryogenic container. 
       FIG. 1  illustrates one embodiment of an acoustic fluid level monitoring system  10 . System  10  may generally include a cryogenic container  20 , an acoustic sensor  40 , and a signal processor  50 . Cryogenic container  20  may include an inner container  22 , an outer container  24 , and an insulation layer  26  separating inner and outer containers  22 ,  24 . Inner container  22  generally defines storage area  28  that houses the stored material. Cryogenic container  20 , as shown, is generally known in the art so the following description simply provides a brief overview of one such cryogenic container. However, other containers not shown here could employ the disclosed system and method as well. 
     A substance, such as hydrogen, may be stored in storage area  28  in a fluid state. The substance is generally stored at very low temperatures and may also be pressurized. Generally, inner container  22  provides a barrier that prevents the stored substance from migrating from within storage area  28 , whether the substance is a fluid, gas, or mixture. Insulation layer  26  generally provides efficient thermal insulation between inner and outer containers  22 ,  24 . Insulation layer  26  may also provide structural support, as additional structural support may be required when storage area  28  is pressurized, for example. Outer container  24  generally provides additional structural support and protects insulation layer  26  and inner container  22  from external factors, such as the environment. 
     Generally, the substance stored in storage area  28  may be in both fluid and gaseous forms. The fluid is typically removed from storage area  28  through a suitable valve and pipe assembly (not shown). As the fluid is removed from storage area  28 , the remaining volume is occupied with the substance in gaseous form. For example, liquid hydrogen may be stored in storage area  28 . As the liquid hydrogen is removed from storage area  28 , gaseous hydrogen generally fills the remaining volume. 
     Monitoring the fluid level within storage area  28  becomes increasingly important as cryogenic containers are used in mobile applications, such as for vehicle fuel cells. Since the stored substance is used as a fuel for powering the vehicle, the substance must be periodically replaced. Monitoring the fluid level aids in the replacement process. 
     As shown in  FIG. 1 , system  10  utilizes acoustic sensor  40  to monitor the fluid level within storage area  28  by sensing the acoustic resonant frequency of cryogenic container  20 . To determine the fluid level, a first acoustic resonant frequency F 1  of cryogenic container  20  is measured while storage area  28  is empty. A fluid substance is added to storage area  28 , thereby changing the acoustic resonant frequency of cryogenic container  20 . A second acoustic resonance frequency F 2  can then be measured using acoustic sensor  40 . The difference between F 1  and F 2  can then be calculated and correlated to the fluid level within storage area  28 . As the fluid level changes within storage area  28 , the acoustic resonant frequency will also change. Stated another way, fluid within storage area  28  changes the frequency of vibration for cryogenic container  20 . 
     Acoustic sensor  40  measures the acoustic resonant frequency of cryogenic container  20  by sensing vibrations. Acoustic sensor  40  may be a piezo vibration sensor, a piezoelectric diaphragm, a laser vibrometer, an electromagnetic converter, or a semiconductor. Generally, signal processor  50  receives electrical or electromagnetic signals from acoustic sensor  40 , and process those signals to determine the fluid level within storage area  28 . Acoustic sensor  40  may use only one device for sensing the vibration of cryogenic container  20 , or may use several devices located in different areas. 
     Turning now in more detail to  FIGS. 1-4 , acoustic sensor  40  may be placed in various locations. As shown, inner container  22  includes interior surface  30  and exterior surface  32 , and outer container  24  includes interior surface  34  and exterior surface  36 . In one embodiment shown in  FIG. 1 , acoustic sensor  40  is attached to exterior surface  36  of outer container  24 .  FIGS. 2-4  are sectional views taken along line  3 - 3  of  FIG. 1 .  FIG. 2  illustrates another embodiment where acoustic sensor  40  may be located on interior surface  34  of outer container  24 .  FIG. 3  illustrates another embodiment where acoustic sensor  40  may be located within insulation layer  26 . And  FIG. 4  illustrates yet another embodiment where acoustic sensor  40  may be located on exterior surface  32  of inner container  22 . Other embodiments are also envisioned, such as locating acoustic sensor  40  on interior surface  30  of inner container  22 , thereby locating acoustic sensor  40  within storage area  28 . Regardless of its location, acoustic sensor  40  communicates with signal processor  50 . 
     Signal processor  50  may be any suitable device for receiving and processing signals from acoustic sensor  40 . And signal processor  50  may be connected to acoustic sensor  40  by wire  52 . They may also communicate by various wireless means using technologies such as radio frequency (RF), infrared (IR), or electromagnetism (EM), just to name a few. Signal processor  50  may be a digital computer with a digital signal processor (DSP) for receiving and analyzing signals from acoustic sensor  40 . Signal processor  50  may also have electronic memory and software for calculating the fluid level within storage area  28 . In one embodiment, signal processor  50  calculates a fluid level within storage area  28  after receiving a signal from acoustic sensor  40 . The fluid level may be calculated by way of a lookup table, calculation, or other methods known to those skilled in the art. The fluid level can be calculated using an initial acoustic resonant frequency of cryogenic container F 1  taken when storage area  28  is empty, and comparing F 1  to the current acoustic resonant frequency F 2 . Signal processor  50  may also receive other data, such as temperature and pressure of storage area  28 , and use such data to further refine the fluid level calculation based on the change in acoustic resonant frequency. 
     To measure the acoustic resonant frequency, an impulse may be generated to stimulate cryogenic container  20 . An impulse generally may be anything that stimulates oscillation or vibration of cryogenic container  20 . An impulse can be generated by impulse generator  42  or by natural phenomenon. For example, in an automotive fuel cell application when a fluid substance is stored within storage area  28 , the impulse may result from fluid sloshing, a natural phenomenon. The stored fluid sloshes as the vehicle accelerates, decelerates, or turns. The sloshing fluid within storage area  28  causes vibrations, allowing acoustic sensor  40  to then measure the acoustic resonant frequency of cryogenic container  20 . 
     In another embodiment, impulse generator  42  stimulates cryogenic container  20 . Impulse generator  42  may be an actuator, a piezoelectric device, an electromagnetic converter, a semiconductor, or mechanical sound spring. In one embodiment, acoustic sensor  40  and impulse generator  42  are one device serving both functions. For example, a piezoelectric device can be driven by an external power source to produce vibrations, causing cryogenic container  20  to vibrate. The same piezoelectric device can then be used in a passive mode to measure the acoustic resonant frequency of cryogenic container  20 . Alternatively, acoustic sensor  40  and impulse generator  42  may be separate devices located in various locations throughout cryogenic container  20 . 
     One embodiment may include a vehicle  100 , such as an automobile, truck, bus, boat, military vehicle, etc. Vehicle  100 , as shown in  FIG. 5 , may include a fuel cell  102  and a cryogenic container  20  for supplying liquid hydrogen to fuel cell  102 . An acoustic sensor  40  may be provided to sense the resonant frequency of container  20 . Acoustic sensor  40  is capable of communicating the sensed resonant frequency to signal processor  50 . Signal processor  50  processes the signal received from acoustic sensor  40  and communicates with a tank level communication means  104 , which then communicates the level of liquid hydrogen in container  20 . Embodiments of tank level communication means  104  include, but are not limited to, a gauge, a digital display, a speaker, an audiovisual device, or another sensor that communicates with a vehicle computer or other vehicle hardware component. Tank level communication means  104  may communicate to a vehicle occupant, a vehicle system, or to a remote system via a wireless communication system. 
     The above description of certain embodiments of the invention is merely exemplary in nature and, thus, variations, modifications and/or substitutions thereof are not to be regarded as a departure from the spirit and scope of the invention. Tank assemblies embodying the present invention may have none, some, or all of the noted features and/or advantages. That certain features are shared among the presently preferred embodiments set forth herein should not be constructed to mean that all embodiments of the present invention must have such features.