Abstract:
The concerns a measurement device and system utilizing the same for precise measuring the fluid level in a container, the measurement device ( 10 ) being located outside of the container ( 12 ) and comprising a predetermined number of basic blocks of a predetermined geometry, the basic blocks comprising at least one pair of capacitors with a predetermined relation between their capacitance a differential change in said relation along the device ( 10 ) being indicative the fluid level condition in the container ( 12 ).

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage application under 35 USC 371 of International Application No. PCT/IL2011/050085, filed Dec. 29, 2011, which claims the priority of Provisional Application No. 61/431,146, filed Jan. 10, 2011 and Provisional Application No. 61/545,767, filed Oct. 11, 2011, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention is generally in the field of fluid level measurements and relates to a measurement device and system utilizing the same for precise measuring the fluid level in a container. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fluid or liquid level monitoring is critical to a wide variety of applications. In general, fluid level applications include those requiring detection of the presence or absence of fluid at a selected point, and those requiring measurement of actual fluid level (depth or height) in a container. These techniques are based on the use of fluid level sensors to indicate the level of fluid within a tank or container. Fluid level sensors utilize mechanisms of various types, including inter alia acoustic, optical, electro-optical, resistance, capacitative mechanisms. Optical, electro-optical and acoustic type sensors are too expensive for some applications. 
         [0004]    The most commonly used fluid level sensors are the variable resistor sensor (utilizing a float to produce a resistance change in the variable resistor) and capacitive liquid level sensor (including a reference capacitor adapted to be fully submerged in the liquid and a measuring capacitor). These sensors however are too sensitive to environmental changes. 
       GENERAL DESCRIPTION 
       [0005]    The present invention provides a novel technique enabling precise measurement of a fluid level in a container using relatively simple and inexpensive equipment that can be easily used with various types of containers and environmental conditions. 
         [0006]    The invention is based on the general principles of capacitive sensors, namely a change in capacitance caused by a change in the medium in the vicinity of the capacitor plates. A change in the level of the fluid changes the medium in the capacitor&#39;s vicinity, and hence, causes a change in capacitance. 
         [0007]    However, contrary to the conventional approach for capacitive sensors, rather than measuring direct changes in capacitance, two or more functional capacitors are used, where each of which is differently affected by a change in the fluid level, and the fluid level is then determined by determining the differential change in capacitance between the two or more capacitors. 
         [0008]    A measurement device of the present invention thus includes a novel capacitance-based fluid level sensor of the invention, which has a predetermined number of basic blocks (one or more basic blocks) each defining at least one pair of capacitors with a predetermined relation of capacitance between them. Such a relation may be a predetermined profile of ratio between the capacitance along said basic block (i.e. a profile of differential change). In this case, the use of one basic block might be sufficient. Alternatively, or additionally, such a relation may be a difference (change) in the capacitance between the adjacent blocks, in which case at least two basic blocks are preferably used. 
         [0009]    If such a capacitance-based sensor of the invention is put in operation, i.e. is exposed to a fluid medium, e.g. is located in the vicinity of a fluid containing environment, the only factor affecting a change in said relation is a change in the fluid level in the vicinity of the basic block. 
         [0010]    A fluid level measurement system of the invention includes the measurement device and a control unit. The latter is actually an electronic circuit (chip) having, inter alia, a memory utility for storing certain reference data (calibration data), and processor utility which is preprogrammed for analyzing measured data from the measurement device (using the calibration data) and providing output data indicative of the fluid condition in the container. 
         [0011]    Generally, the fluid condition to be determined may be just presence or absence of the fluid in the vicinity of the specific basic block of the measurement device. Preferably, however, the measurement device is configured to determine the fluid condition indicative of the actual fluid level in the container. 
         [0012]    The measurement device of the present invention is configured for attaching or placing close to the outer surface of a container, and an electrical output of the device is connectable to the control unit via wires or wireless signal transmission (IR, RF, acoustic, etc.) as the case may be. The measurement device may actually be implemented as a label, flexible or rigid, to be adhered/attached to the outer surface of the container&#39;s wall. Preferably, the measurement device and the control unit are implemented as an integral structure. 
         [0013]    The capacitance-based sensor, and preferably the entire measurement device of the present invention, includes or is configured as an elongated ruler-like structure having a longitudinal axis, such that when the sensor is placed on the container&#39;s wall its longitudinal axis extends along an axis of a general change of the fluid level in the container. The ruler structure carries an electrodes&#39; arrangement extending along said axis and being formed by at least one electrode cell and an additional electrode. The electrodes of the electrode cell together with the additional electrode define at least one pair of capacitors forming the basic block of the measurement device. 
         [0014]    Preferably, an array of basic blocks is provided being arranged along said axis. Generally speaking, scaling a position along the measurement device (elongated structure) may be implemented by providing an array of basic blocks. The configuration is such that a relation between the electrode cell (capacitive cell) and the additional electrode (or a respective segment of the additional electrode common for all the cells, as will be described further below) is equal for all the basic blocks. More specifically, the basic blocks have identical configurations, e.g. have identical electrode pairs equally distanced from the additional electrode. 
         [0015]    In the description below, the at least one pair of electrodes is at times referred to as “capacitive cell”. It should, however, be understood that a capacitor is actually formed by one of the electrodes of such cell and the corresponding additional electrode or a corresponding segment/portion of the additional electrode, as will be clear from the description below. 
         [0016]    In some embodiments, the additional electrode extends along the capacitive cell, e.g. defining two portions extending at opposite sides of the cell. In case of an array of capacitive cells, the additional electrode is common for all the cells. This additional electrode may be configured as a closed-loop frame surrounding the array of capacitive cells or as a two-strip element enclosing the array of cells between the two strips. Thus, scaling a position along the measurement device is implemented by providing an array of cells and/or segmenting the additional electrode. 
         [0017]    In some other embodiments, the basic block includes a capacitive cell and its own additional electrode. Thus, in case of an array of basic blocks, they are arranged along the direction of change in the fluid level in the container, and a change in said predetermined relation within the cell is indicative of a change in the fluid condition at the location of said cell, while a difference in the relation values for adjacent cells is indicative of the actual fluid level. 
         [0018]    Thus, generally, according to the invention, each basic block comprises at least one pair of first and second capacitors, formed by first and second electrodes/plates and a common additional electrode. 
         [0019]    As indicated above, the additional electrode may also be common for all the blocks. In this case, the first and second plates of the basic block form respectively first and second capacitors with first and second segments of said additional electrode with which the plates are aligned. Moreover, the first and second electrode plates of the cell have different geometries, such that a surface area of the first plate increases while a surface area of the second plate decreases in a direction along said axis of the ruler. For example, the first and second plates of the cell may be two triangle-like or trapezoid-like parts of a rectangle at opposite sides of the rectangle&#39;s diagonal. A ratio between the surface areas of the first and second plates of the cell varies according to a certain known profile. With such asymmetric geometry of the first and second plates of the cell, for a given plane across the cell (constituting a fluid level in a container), a first area of overlap between the first plate and the corresponding segment of the additional electrode and a second area of overlap between the second plate and its corresponding segment of the additional electrode are different. When moving said plane through the cell (i.e. corresponding to a change in the liquid level) in a direction along the axis of the ruler, the first and second areas respectively change positively and negatively or vice versa, thus creating a profile of a ratio between the capacitance values varying along said axis due to the different geometries of the first and second plates of the cell. A reference profile (i.e. when the ruler is screened from fluid medium in the container) describing a change in the ratio between the first and second areas during such movement along the cell is known. Considering the use of an array of identical basic blocks, the reference profile repeats from block to block, and all the blocks are exposed to the same environmental conditions outside the container. 
         [0020]    As also indicated above, each of the identical basic blocks may have its own additional electrode which is a common electrode for the first and second capacitors. The first and second capacitors have different capacitance values of a known difference between them when the respective basic block is screened/shielded from the environment. This difference is common for all the basic blocks. Thus, when an array of such basic blocks is aligned along the direction of change of the fluid level, a difference between the “capacitance-difference” for the adjacent basic blocks corresponds to the fluid level in the container. 
         [0021]    When the measurement device is put in operation, i.e. is placed on the container&#39;s wall, the only factor that can affect a change in the predetermined relation of capacitance within the basic block or between the basic blocks of the array is that associated with a change in the fluid level in the container, namely a dielectric constant of a medium between the plates of a capacitor changes. Thus, the technique of the present invention advantageously allows for eliminating a need for determination of the fluid level from actual measurement of a capacitance value (which by itself is sensitive to changes in environmental conditions other than a change in the fluid level), bur rather allows for utilizing a change in the relation (e.g. ratio profile or difference) between the capacitance values for the first and second capacitors of the basic block and/or those of the adjacent blocks, while said relation is sensitive only to the change in the liquid level. 
         [0022]    According to one broad aspect of the invention, there is provided a measurement device for measuring a fluid level in a vicinity of the device, the measurement device comprising a capacitance-based fluid level sensor with a predetermined number of basic blocks of a predetermined geometry, the basic block comprising at least one pair of capacitors with a predetermined relation between their capacitance, a differential change in said relation along the device being indicative of the fluid level condition in the vicinity of the device. 
         [0023]    According to another broad aspect of the invention, there is provided a measurement device for measuring a fluid level in a vicinity of the device, the measurement device comprising an elongated structure having a longitudinal axis which when said structure is put in operation extends along a general direction of change of the fluid level, the measurement device comprising: an electrodes&#39; arrangement formed by a predetermined number of basic blocks of a predetermined geometry, the basic block comprising at least one electrode cell and an additional electrode, the electrode cell comprising first and second electrodes having different geometries such that a surface area of one of the first and second electrodes increases in a direction along said longitudinal axis while a surface area of the other of said first and second electrodes decreases in said direction, forming a certain profile of a ratio between said first and second surface areas, the first and second electrodes of the cell forming first and second capacitors with said additional electrode, a position of change in a ratio between capacitance values of the first and second capacitance along said axis being indicative of the fluid level. 
         [0024]    According to another broad aspect of the invention, there is provided a measurement device for measuring a fluid level in a vicinity of the device, the measurement device comprising an elongated structure having a longitudinal axis which when said structure is put in operation extends along a general direction of change of the fluid level, the measurement device comprising: an electrodes&#39; arrangement formed by at least two basic blocks of a predetermined identical geometry, the basic block comprising at least one pair of capacitors formed by first and second electrodes and an additional common electrode, the first and second capacitors having different capacitance values with a predetermined known difference between them, a position of change in said predetermined difference along said axis being indicative of the fluid level. 
         [0025]    The measurement device is configured and operable for providing measured data indicative of a change in the relation between the capacitance within the block and/or between the blocks in a direction along said axis, said change in the relation being indicative of a change in the fluid level in the container. 
         [0026]    Preferably, the measurement device comprises an array of said basic blocks arranged in a spaced-apart relationship along said longitudinal axis. In some embodiments utilizing a common additional electrode for multiple blocks, the first and second portions of the additional electrode extend along the opposite sides of the array. 
         [0027]    According to yet another broad aspect of the invention, there is provided a measurement system comprising the above-described measurement device and a control unit comprising an electronic circuit configured and operable for analyzing said measured data and generating output data indicative of the fluid level in the container. 
         [0028]    Preferably the measurement system is an integral substantially flat structure carrying the measurement device (electrodes printed on a substrate) and a chip-like control unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
           [0030]      FIG. 1  illustrates a fluid container equipped with a measurement system of the invention; 
           [0031]      FIG. 2  illustrates a specific but not limiting example of the configuration of the measurement unit of the invention suitable for use in the system of  FIG. 1 , configured for capacitance ratio profile measurement; 
           [0032]      FIG. 3  shows more specifically an example of the basic block for capacitance ratio profile measurement according to the invention for use in measurement unit. 
           [0033]      FIGS. 4A-4B  show another not limiting example of the configuration of the measurement unit ( FIG. 4A ) of the invention adapted for difference measurement and the configuration of the correlating basic block ( FIG. 4B ); and 
           [0034]      FIG. 5  shows an electrical diagram of the measurement unit of the invention for the difference measurement configuration. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0035]    Referring to  FIG. 1 , there is illustrated an example of a measurement system  10  of the invention for measuring a liquid level in a container  12 . The measurement system  10  is configured as a flat elongated structure (e.g. label) attachable to an outer surface of the container&#39;s wall  12 A. The flat structure  10  carries a measurement device  14  and a control unit (electronic circuit) which is not shown here. 
         [0036]    The measurement device  14  extends along the structure  10  defining a longitudinal axis  16 . When the structure  10  is attached to the container&#39;s wall  12 A, the axis  16  is substantially parallel to a general direction of change in the liquid level in the container  12 . The measurement device  14  is configured as a capacitance-based fluid level sensor and includes a predetermined number of basic blocks, e.g. at least one basic block, or preferably, as exemplified in the figure, includes an array of a certain number of blocks, generally designated B i , four such blocks being shown in the present example. Each basic block is formed by an electrode cell C and an additional electrode  18 , which as shown in the present example is common for all the basic blocks/cells and is thus a segmented electrode. 
         [0037]    Thus, in the present example, four electrode cells C 1 -C 4  are shown associated with one common electrode  18 . The cells are arranged in a spaced-apart relationship along the axis  16 . This arrangement, with one or more cells/basic blocks, actually presents a ruler extending along the axis of change of the liquid level in the container. The common segmented electrode  18  may be in the form of a frame surrounding the cells&#39; array or in the form of two-strip element enclosing the array of cells between the two parallel strips extending along the axis  16 . 
         [0038]    Reference is made to  FIG. 2  showing more specifically an example of the measurement system  10  of the present invention for ratio profile capacitance measurement. To facilitate understanding, the same reference numerals are used for identifying common components in all the figures. The system  10  is an integral flat structure carrying the measurement device  14  and an electronic circuit  20 . In the present example, the measurement device  14  includes an array of six basic blocks comprising electrode cells C 1 -C 6  respectively and a common electrode  18  defining two electrode parts  18 A and  18 B at opposite sides of the cells&#39; array. The cells C 1 -C 6  have identical configurations and are equally distanced from the common electrode  18 . Thus, a relation between the electrode  18  and the cell C is equal for all the blocks. As shown, each cell is aligned with corresponding first and second segments of the first and second electrodes  18 A and  18 B at opposite sides of the cell. Hence, each block includes first and second capacitors formed by the first and second electrodes of the cell and the corresponding first and second segments of the electrodes  18 A and  18 B. 
         [0039]      FIG. 3  illustrates more specifically the configuration of a basic block B i  configured for ratio profile capacitance measurement. The block includes an electrode cell C comprising first and second electrically conductive plates P 1  and P 2  which have different geometries, designed such that a surface area S 1  of the first plate P 1  increases while a surface area S 2  of the second plate P 2  decreases in a direction D along the axis  16  of the ruler, or vice versa. As shown in this example, the first and second plates P 1  and P 2  are two triangle-like elements of a rectangle-like cell C at opposite sides of the rectangle&#39;s diagonal  22 . 
         [0040]    This asymmetric arrangement of the first and second plates P 1  and P 2  of the cell provides that for a certain plane L across the cell, a first area of overlap between the first plate P 1  and a corresponding segment of the electrode  18 A and a second area of overlap between the second plate P 2  and the corresponding segment of the electrode  18 B are different. Accordingly, capacitance values for the first and second capacitors formed by respectively the plate P 1  and electrode  18 A and the plate P 2  and electrode  18 B are different. As a result, a ratio between the corresponding first and second capacitance values changes according to a certain profile. A reference profile, corresponding to the steady state of the ruler (screened from fluid medium in the container), i.e. corresponding to a profile of the ratio between the surface areas of the plates P 1  and P 2 , is known. As the blocks are identical and a relation between the common electrode  18  is the same for all the blocks, the reference profile is also the same for all the blocks. The reference profile within the block repeats from block to block. 
         [0041]    When a liquid level in the container moves from position L to L′ through the block in a direction D, the ratio between the first and second areas changes according to the known profile. Thus, the factor that affects a change in a ratio between the first and second capacitance values for one position of the plane L (liquid level) and the first and second capacitance values for another position of the plane L′ is associated with a change in the liquid level in the container, i.e. a change in the dielectric constant of the medium between the capacitor elements. Hence, a position corresponding to a detected change in the ratio corresponds to the liquid level in the container. 
         [0042]    The reference profile is stored in a memory utility of the electronic circuit  20 , together with data corresponding to the arrangement of the cells and segmented electrode. The measurement device continuously or periodically measures voltages on all the electrodes and generates measured data indicative thereof. This data is received and analyzed by a processor utility of the electronic circuit  20  and the liquid level is calculated. The calculation may be as follows: 
         [0000]    
       
         
           
             LeftCap 
             = 
             
               LeftCapVal 
               - 
               LeftAmbientCap 
             
           
         
       
       
         
           
             RightCap 
             = 
             
               RightCapVal 
               - 
               RightAmbientCap 
             
           
         
       
       
         
           
             LeftLevel 
             = 
             
               1 
               - 
               
                 LeftCap 
                 
                   LeftCap 
                   + 
                   RightCap 
                 
               
             
           
         
       
       
         
           
             RightLevel 
             = 
             
               RightCap 
               
                 LeftCap 
                 + 
                 RightCap 
               
             
           
         
       
       
         
           
             
               Level 
                
               
                 [ 
                 % 
                 ] 
               
             
             = 
             
               
                 ( 
                 
                   RightLevel 
                   + 
                   LeftLevel 
                 
                 ) 
               
               · 
               100 
             
           
         
       
     
         [0043]    Here, LeftCap is the measured capacitance of the first capacitor formed by the plate P 1  and corresponding segment of electrode  18 A, LeftCapVal is the actual value of the corresponding capacitance, LeftAnbientCap is the effect of environment. Similarly, RightCap is the measured capacitance of the second capacitor formed by the plate P 2  and corresponding segment of electrode  18 B, RightCapVal is the actual value of the corresponding capacitance, RightAnbientCap is the effect of environment. 
         [0044]    As indicated above, the measurement device may be in the form of a flat structure such as a label, flexible or not. The electrical circuit formed by one or more basic blocks, each including at least one electrode cell C and an additional electrode  18  (e.g. common for all the cells or not), may be printed on the label. Thus, the system is simple and can be easily used with any container, irrespective of the environment where the container might be used. 
         [0045]    Turning now to  FIGS. 4A-4B , another specific but non-limiting example of the measurement system  10  of the present invention for differential capacitance measurement is shown. In this example, the measurement device  14 , being a capacitance based sensor, includes an array of multiple (generally at least two) basic blocks—five basic blocks B 1A -B 5A  being shown in the figure, each comprising a relatively small electrode  112  (designated Rxs), a relatively large electrode  114  (designated Rxb), a grounding electrode  116  (designated Grd) and an additional electrode or so-called transmission electrode  118  (designated Tx. The smaller electrode and the large electrode of the block form an electrode cell, where each of the electrodes  112  and  114  defines a capacitor cell with the electrode  118 . The blocks B 1A -B 5A  have identical configurations and are equally distanced from each other, providing a predetermined equal incremental indication of the fluid level within the container corresponding to a difference (relation) in the capacitance between the two adjacent blocks. 
         [0046]      FIG. 4B  illustrates more specifically the configuration of the basic block B i  for differential capacitance measurement. The block B i  includes a capacitive cell C formed by large and small measurement electrodes, designed such that a surface area of electrode  114  is significantly larger than that of electrode  112 . The difference in capacitance (C Rx ) is calculated between the capacitors  112 - 118  and  114 - 118  according to the following formula: 
         [0000]    
       
      
       C 
       Rx 
       =C 
       Rxb 
       −C 
       Rxs  
      
     
         [0047]    The values of C Rxb  and C Rxs  are measured vs. the transmission electrode  118 , and then subtracted from each other to receive a capacitance differential value. As the capacitance changes significantly with the environmental conditions, such as temperature changes, while both capacitors of the block are exposed to the same environment, measurement of the differential capacitance enables eliminating the changes in the capacitance due to environmental conditions. 
         [0048]      FIG. 5  is an example of a schematic electronic diagram of the block suitable to be used in the above-described measurement device. As explained above, the small electrode  112  and the large electrode  114  of the basic block, as well as the common transmission electrode, are connected to a CPM core (control unit), that translates the measured differential capacitance into incremental values, for instance 20%, 40%, 60% etc., indicating the level of the fluid in the container. The CPM core may transmit, e.g. digitally, the calculated incremental value of the fluid level to a display unit (not shown) of the device or an external unit, enabling indication of the fluid level in the container to a user. Alternatively or additionally, the value is transmitted to a control console of the device (not shown), in which filling or drainage of fluid in the container is initiated according to the level of fluid measured by the measurement device.