Patent Publication Number: US-8981793-B2

Title: Non-invasive level measurement for liquid or granular solids

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/342,303 filed Apr. 12, 2010 entitled “Non-invasive Level Measurement for Liquid or Granular Solids”, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     This invention relates generally to the field of sensors for the measurement of level of liquid or granular solid in an array of containers where it is physically not possible to insert a probe inside each container, surround the container with launching electrodes, or attach sensors to walls. More particularly, it relates to the field of level sensors where an electromagnetic wave is launched from the outside of the container and the reflected signal processed to extract the information about liquid level. 
     Level of liquid or granular solid inside containers e.g. bottles, vials etc. need to be monitored for hospitality, pharmaceutical, healthcare, industrial and other areas. A common example is monitoring the amount of drink inside beverage bottles in bars. Every year a significant amount of beverage is lost due to shrinkage that needs to be checked. Furthermore, multiple beverage containers, located in trays and shelves need to be monitored in an economic and timely fashion for inventory. 
     Prior art e.g. weighing of individual containers to determine content is an expensive proposition since as many force sensors are required as the number of containers. Capacitive or transmission line sensors are economic but need conducting electrodes around the containers. Invasive techniques e.g. inserting a probe in the container are not acceptable due to cost and inconvenience. 
     Prior art U.S. Pat. No. 6,564,658 teaches the use of slow-wave structures to measure liquid level and teaches the confinement of electromagnetic energy in a small volume. One embodiment of the invention teaches liquid level measurement by placing the electrodynamic element outside the container. However, the fields generated by the electrodynamic element can attain only partial penetration and not throughout the bulk of the liquid. As a result, this method is unlikely to provide the adequate sensitivity in many applications. 
     Thus, a better solution is needed to accurately measure level of liquid or granular solid inside containers where neither electrodes or sensors around the container cannot be used, nor a probe be inserted. Furthermore, the solution needs to amenable to measuring multiple containers in an array and yet be economic. All of the said features are provided by the following invention. 
     SUMMARY 
     Embodiments of the present technique provide a method and apparatus for non-invasive level measurement for liquid or granular Solids, where the said liquid or granular solid is stored in a container made from electrically non-conducting material such as glass, plastic, paper, wood etc. 
     In one embodiment, the present invention provides a system consisting of a intelligent tray that is capable of accommodating at least one container the level of contents inside which needs to be monitored. The containers rest on the intelligent tray and each container is served by a launcher for launching electromagnetic waves. The launchers are embedded in the tray, are electrically passive and constructed from electrically conducting and dielectric materials. When more than one container is present on the tray, a switching arrangement selectively connects each launcher (each serving a container) to the measurement system. 
     In one embodiment, the present invention provides a system for measuring complex reflection coefficient (magnitude and phase) of electromagnetic waves. The measurement system (reflectometer) consists of radio frequency generator, directional couplers, magnitude/phase detectors and processor. A radio frequency wave is launched into a particular container using a launcher, and the wave travels through the body of the material located inside the container. The mode of propagation is not Transverse-electric-magnetic (TEM) and bears resemblance to propagation inside dielectric waveguide or optical fiber. At the interface of the contents and air, the wave suffers a reflection and travels back again through the bulk of the liquid thereby creating standing waves inside the container. The reflected wave finally appears at the launcher input and therefore affects the complex reflection coefficient at that point. By measuring the complex reflection coefficient at the launcher input, it is therefore possible to determine the level of contents inside the container. 
     In one embodiment, the present invention provides a switching arrangement for selectively connecting each launcher to the reflectometer. 
     These and other embodiments of the invention are described in further detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a typical intelligent tray with multiple beverage containers 
         FIG. 2  depicts a intelligent tray that can installed on the top of an existing shelf 
         FIG. 3  depicts a typical beverage container on a intelligent tray with electromagnetic waves launched at the bottom of the container 
         FIG. 4  depicts simplistic representation (geometrical optics) of waves through a dielectric material suffering total internal reflection 
         FIG. 5  depicts one embodiment of the launcher for launching Transverse Magnetic waves 
         FIG. 6  depicts cross-section of intelligent tray with embedded launcher for launching Transverse Magnetic waves 
         FIG. 7  depicts simulation showing TM01 waves launched in a container (electric field) 
         FIG. 8  depicts simulation showing TM01 waves launched in a container (magnetic field) 
         FIG. 9  is a block diagram of the switching and calibration technique for a plurality of launchers in a intelligent tray 
         FIG. 10  depicts experimental data obtained by changing liquid level inside a container 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a typical construction of a intelligent tray  102  capable of accommodating multiple containers  103   a ,  103   b ,  103   c ,  103   d  and so on. The containers might be of different sizes and shapes. The measurement apparatus (reflectometer) may be integrated with the tray  102  or be separate from it. The goal of the apparatus is to measure liquid content in each container automatically and non-invasively. 
       FIG. 2  depicts a portion of the intelligent tray  202  laid out over an existing shelf  201 . A container, e.g. a beverage bottle  203  rests on the intelligent tray  202 . 
       FIG. 3  depicts a container  301  resting on a intelligent tray  302 . A launcher  303  is embedded inside the intelligent tray  302  and is used to launch electromagnetic waves  304  inside the container  301 . The incident wave  304  gets reflected at the interface between contents and air  306  due to a mismatch of dielectric constants and creates a reflected wave  305 . 
       FIG. 4  depicts a simplified illustration of the mechanism of launching electromagnetic waves inside a container, whose walls are represented by  401   a  and  401   b , and the contents inside are designated by  402 . The dielectric constant of the contents is assumed to be higher than that of the walls of the container. A wave launched within a range of certain angles would propagate through  402  by total internal reflection as shown by ray segments  403   a ,  403   b ,  403   c  and  403   d . The reflected wave is not shown. This geometrical optics representation is valid only if the wavelength is small compared to the cross-sectional dimensions of the container. In practice, the said condition is usually not valid and a rigorous solution to Maxwell&#39;s equations is necessary to solve for fields in and out of the container. The relative values of the dielectric constants of the contents and container will determine how the electric and magnetic fields will be distributed inside the contents, within the container walls, and also the surrounding air. However, the geometrical optics representation of total internal reflection is used here just for a general understanding. 
       FIG. 5  depicts an embodiment of the launcher mentioned as  303  in  FIG. 3 . The launcher consists of a ground plane  503 , a monopole  502  and a top hat  501  mounted on the top of the monopole  502 . The monopole may be a straight element, but to conserve space and reduce the thickness of the intelligent tray, parts of it may be constructed from helical and/or spiral elements. The top hat  501  may be rectangular, circular or some other suitable shape. The radio frequency signal is launched between the ground plane  503  and monopole  502 , at the point designated  504 . This type of launcher can be used to launch Transverse Magnetic Waves. 
       FIG. 6  depicts the launcher of  FIG. 5  embedded in a intelligent tray  610  (side view). The ground plane, monopole and top hat are represented by  603 ,  602  and  601  respectively. A microstrip trace  604  is used for feeding the signal.  608  is the dielectric material between the ground plane  603  and microstrip trace  604 . There is a dielectric layer  606  above the top hat  601  for physical protection. 
       FIG. 7  shows the electrical field from a simulation exercise. The simulation used a container with radius 10 mm, negligible wall thickness and height 75 mm. Water (dielectric constant=81) was used as contents. Mode TM01 was launched at 1990 MHz inside the container. Due to reflection at air-water interface, standing waves were created, and we observe the maxima at  701   a  and  701   b  and a minimum at  702 . It is also observed that the wavelength is considerably larger than an unbounded TEM wave in the same material due to the non-TEM nature of propagation. 
       FIG. 8  depicts the magnetic field from same simulation as in  FIG. 7 . We observe the transverse nature of the magnetic field and also the maxima and minima at the same regions corresponding to the electric field. 
       FIG. 9  depicts a method for using a common measurement apparatus  903  to characterize multiple launchers  901   a  through  901   n  through a switching network  904 . The said apparatus (reflectometer)  903  is capable of measuring complex refection coefficient (i.e. magnitude and phase of reflected wave referenced to the transmitted wave) at different frequencies. To remove the effect of the switching network  904  and various lengths of transmission line, a calibration method is used. The said calibration can be performed by one or more reference devices  906   a  through  906   n  located in close proximity to the launchers  901   a  through  901   n . There may be more than one type of reference device if required. Switches  905   a  through  905   n  are used to switch between a launcher and its corresponding reference device(s). Based on the complex reflection data from a reference device(s), the complex reflection coefficient of the launcher alone—minus the effect of the switching network—can be computed as follows. 
     Let the reference circuit elements be ‘open’, ‘short’ and ‘matched termination’ and let o, s and l be the corresponding complex reflection coefficients measured by the measurement apparatus. Let us define 
     
       
         
           
             
               
                 
                   
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     Let be the measured complex reflection coefficient for the launcher. The calibrated (i.e. corrected for feed line and switching network) complex reflection coefficient con is given by 
     
       
         
           
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       FIG. 10  depicts experimental results by changing the water level in a plastic container of radius 10 mm. A launcher as in  FIG. 5  was used to launch TM01 wave at 1.42 GHz and complex reflection coefficient measured. The launcher used a rectangular top hat and a monopole with helical structure. A simple matching network was added at the input to the launcher to bring the magnitude of reflection coefficient and phase variation with change in water level at manageable values. The water level was changed between 1 ml to 11 ml in steps of 1 ml that translated to the maximum water level of 39 mm and steps of 3.6 mm. The reflection coefficient, plotted in polar co-ordinates for various values of water level is shown in  FIG. 10 . The points ‘1 ml’ and ‘11 ml’ correspond to the respective volumes and a clear monotonicity in the trajectory with respect to volume is observed. If the height of the liquid column exceeds the guide wavelength, information from more than one frequency is necessary to determine the level unambiguously.