Abstract:
The present invention provides methods and apparatuses for determining a liquid level inside a container by using an effective capacitance associated with one or more sense electrodes that are located inside the container. Embodiments may support different types of liquids, including water, and support different electrical appliances, including electric kettles, coffee makers, and water treatment appliances having a non-transparency housing such as stainless steel and black color Lucite or glass that cannot directly indicate the water level. A value of capacitance characteristic associated with a sensing electrode is determined. The water level may be displayed to the user on any kind of electronic panel, e.g., liquid crystal display (LCD), light emitting diode (LED) display, or vacuum fluorescent display (VFD). Also, a correction factor may be applied to a determined capacitance associated with a sensing electrode to compensate for the operating temperature of the sensor electrode and the liquid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional patent application Ser. No. 61/021,948, filed Jan. 18, 2008, entitled “Liquid Level Determination by Capacitive Sensing,” hereby incorporated herein by reference as to its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Electrical appliances, e.g., electric kettles, coffee makers, and water treatment-appliances often use Lucite or glass tubing to indicate the water level or use a magnetic ball to sense the water level indirectly. However, with these approaches a stain or deposit inside the tube may result. The stain or deposit typically detrimentally affects the accuracy of the reading and is often difficult to clean. 
         [0003]    There is a real market need to provide apparatuses and methods that facilitate the reading of a liquid level inside a container. Moreover, it is desirable that the apparatuses and methods reduce the user&#39;s effort in maintaining the equipment in order to insure the accuracy of the reading. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides methods and apparatuses for determining a liquid level inside a container by using a variation of the capacitance between sense electrodes that are located inside the container. Embodiments of the invention support different types of liquids, including water, and support different electrical appliances, including electric kettles, coffee makers, and water treatment appliances having a non-transparency housing such as stainless steel and black color Lucite or glass that cannot directly indicate the water level. 
         [0005]    With an aspect of the invention, a value of capacitance characteristic associated with a sensing electrode is determined. The water level is determined from the determined capacitance value. The water level may be displayed to the user on any kind of electronic panel, e.g., liquid crystal display (LCD), light emitting diode (LED) display, or vacuum fluorescent display (VFD). 
         [0006]    With another aspect of the invention, a correction factor may be applied to a determined capacitance associated with a sensing electrode to compensate for the operating temperature of the sensor electrode and the liquid. The compensation may be provided by mathematical computation or by a lookup table 
         [0007]    With another aspect of the invention, a plurality of sensing electrodes may be situated inside a container. The liquid level is determined by the capacitance variance among the plurality of sensing electrodes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The foregoing summary of the invention, as well as the following detailed description of exemplary embodiments of the invention, is better understood when read in conjunction with the accompanying drawings, which are included by way of example, and not by way of limitation with regard to the claimed invention. 
           [0009]      FIG. 1  shows a container with a sense electrode for determining a liquid level in accordance with an embodiment of the invention. 
           [0010]      FIG. 2  shows a circuit with a sense electrode for providing a level sense voltage in accordance with an embodiment of the invention. 
           [0011]      FIG. 3  shows a circuit with a sense electrode for providing a level sense voltage in accordance with an embodiment of the invention. 
           [0012]      FIG. 4  shows experimental results of a resulting waveform corresponding to a low level of water in accordance with an embodiment of the invention. 
           [0013]      FIG. 5  shows experimental results of a resulting waveform corresponding to a high level of water in accordance with an embodiment of the invention. 
           [0014]      FIG. 6  shows experimental results of a resulting waveform corresponding to a low level of water in accordance with an embodiment of the invention. 
           [0015]      FIG. 7  shows experimental results of a resulting waveform corresponding to a high level of water in accordance with an embodiment of the invention. 
           [0016]      FIG. 8  shows an operational diagram of a system for determining a liquid level in accordance with an embodiment of the invention. 
           [0017]      FIG. 9  shows a system for determining a liquid level in accordance with an embodiment of the invention. 
           [0018]      FIG. 10  shows a flow diagram for determining a liquid level in accordance with an embodiment of the invention. 
           [0019]      FIG. 11  shows a container with a plurality of sense electrodes for determining a liquid level in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  shows container  101  with a sensor containing sense electrode  103  for determining liquid level  105  in accordance with an embodiment of the invention. As will be discussed, the value of the equivalent capacitance of sense electrode  103  is measured and consequently the liquid level can be determined. Even though the sensor shown in  FIG. 1  contains only one sense electrode, one or more sense electrodes may be contained in the sensor, in which metal components (where each metal component corresponds to a sense electrode) are molded in an innocuous non-metallic material. For example, a printed circuit board or wire may be molded in a plastic. If the sensor has only one electrode then the capacitor&#39;s other plate (electrode) is circuit ground (GND). With container  101  comprising a metallic material, container  101  can serve as one plate (electrode) and connect to ground or in series with a capacitor (e.g., 100 pf to 0.1 uF) to ground. While the following discussion refers to water, embodiments of the invention support different types of liquids. With an embodiment of the invention, the sensor mounts on the wall of container  101 . Different materials are characterized by different dielectric constants. The dielectric constant of the material affects the value of the equivalent capacitance. The following Table provides approximate dielectric constants for exemplary materials. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                 DIELECTRIC CONSTANTS OF VARIOUS MATERIALS 
               
             
          
           
               
                   
                   
                   
                 Dielectric constant 
               
               
                   
                 Dielectric Material 
                 Thickness (mil) 
                 K 
               
               
                   
                   
               
             
          
           
               
                   
                 Acrylic 
                 84.5 
                 2.4-4.5 
               
               
                   
                 Glass 
                 74.5 
                 7.5 
               
               
                   
                 Nylon Plastic 
                 68 
                 3.0-5.0 
               
               
                   
                 Polyester Film 
                 10 
                 3.2 
               
               
                   
                 LIQUIFIED AIR 
                 — 
                 1.5 
               
               
                   
                 Air 
                 — 
                 1 
               
               
                   
                 Water 
                 — 
                 80 
               
               
                   
                 Ice 
                 — 
                 3.2 
               
               
                   
                 Automotive Oil 
                 — 
                 2.1 
               
               
                   
                 COFFEE REFUSE 
                 — 
                 2.4-2.6 
               
               
                   
                 ETHANOL 
                 — 
                 24.3 
               
               
                   
                 GASOLINE 
                 — 
                 2 
               
               
                   
                 GERBER OATMEAL 
                 — 
                 1.5 
               
               
                   
                 (IN BOX) 
               
               
                   
                 GLYCERIN, LIQUID 
                 — 
                 47-68 
               
               
                   
                 HEAVY OIL 
                 — 
                 3 
               
               
                   
                 LACTIC ACID 
                 — 
                 22 
               
               
                   
                 JET FUEL (JP4) 
                 — 
                 1.7 
               
               
                   
                 LPG 
                 — 
                 1.6-1.9 
               
               
                   
                 OIL, ALMOND 
                 — 
                 2.8 
               
               
                   
                   
               
             
          
         
       
     
         [0021]    Container  101  may assume different forms and include electric kettles, coffer makers, and water treatment appliances with a non-transparent housing such as stainless steel. 
         [0022]    The equivalent capacitance (Cw) of sense electrode  103  is characterized by the following relationships:
       Directly proportional to the area of sense electrode  103     Directly proportional to the dielectric constant of the material (liquid) surrounding sense electrode  103     Inversely proportional to the distance between the objects (between sense electrodes when there is a plurality of sense electrodes or between the sense electrode and the equivalent capacitor plate)—With a single-electrode-sensor, the equivalent capacitor corresponds to the electrode with GND or metallic container. With a two electrode sensor, the equivalent capacitor corresponds to two electrodes.       
 
         [0026]    The equivalent capacitance Cw may be determined by the following mathematical relationship: 
         [0000]    
       
         
           
             
               
                 
                   Cw 
                   = 
                   
                     
                       
                         ( 
                         
                           1 
                           + 
                           BT 
                         
                         ) 
                       
                        
                       AEK 
                     
                     D 
                   
                 
               
               
                 
                   EQ 
                   . 
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0000]    where A is the area of the plates in square meters (m 2 ), B is the coefficient of temperature variation (which may be determined by experiment and varied with different hardware and electronic design), Cw is the water equivalent capacitance of in Farads (F), D is the distance between the electrode plates in meters (m), K is the dielectric constant of the material separating the plates, E is the permittivity of free space (8.85×10 −12  F/m), and T is the dielectric and electrode temperature. 
         [0027]    Because the resulting voltage (corresponding to circuits  200  and  300  as shown in  FIGS. 2 and 3 ) is an inverse function of the capacitance, the resulting voltage V is given by: 
         [0000]    
       
         
           
             
               
                 
                   V 
                   = 
                   
                     k 
                     Cw 
                   
                 
               
               
                 
                   EQ 
                   . 
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0000]    where k is a constant based on the characteristics of apparatus  100 . Constant k may be determined experimentally. As will be discussed, V corresponds to a DC signal and is measured by a processor (e.g., a microcontroller) through an analog-to-digital (A/D) converter. From EQs. 1 and 2, the resulting voltage is given by: 
         [0000]    
       
         
           
             
               
                 
                   V 
                   = 
                   
                     kD 
                     
                       
                         ( 
                         
                           1 
                           + 
                           BT 
                         
                         ) 
                       
                        
                       AEK 
                     
                   
                 
               
               
                 
                   EQ 
                   . 
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
         [0028]    The dielectric constant K can then be determined from EQ. 3 by: 
         [0000]    
       
         
           
             
               
                 
                   K 
                   = 
                   
                     kD 
                     
                       
                         ( 
                         
                           1 
                           + 
                           BT 
                         
                         ) 
                       
                        
                       AEV 
                     
                   
                 
               
               
                 
                   EQ 
                   . 
                   
                       
                   
                    
                   4 
                 
               
             
           
         
       
     
         [0029]    From the known effect of the water level (which can obtained through experiment) on the dielectric constant K, water level  105  can be determined from EQ. 4 through calculations or from a lookup table. The following example utilizes the above equations: 
         [0000]    A=0.01 area of the plates in square meters
 
B=0.01 coefficient of temperature variation
 
K=7.5 dielectric constant of the material separating the plates, e.g., glass
 
E=8.85 10 −12  permittivity of free space
 
D=0.01 distance between the electrode plates in meters
 
T=300 dielectric and electrode temperature
 
k=7·10 −9  characteristic of apparatus which is a experimental value
 
V=kD/((1+B×T)×A×E×K)
 
V=2.637 volts where V is the output signal without water
 
         [0030]    With the present of water, the equivalent of permittivity (Eeq) is changed 
         [0000]    E 1 =1·10 −12  as an example 
       Eeq=E+E 1   
     Eeq=9.85 10 −12    
       [0031]    V 1 =kD/((1+B×T)×A×Eeq×K)
 
V 1 =2.369 volts where V 1  is the output signal with certain level of water
 
         [0032]    When water level  105  has been determined, a level indicator may be displayed on any kind of electronic panel e.g., liquid crystal display (LCD), light emitting diode (LED) display, or vacuum fluorescent display (VFD). Also, an associated processor (not shown) may use the determined water level to control the heating of the water. For example, if the water is too low and damage to container  101  may consequently occur, the processor may terminate heating the water. On the other hand, if the water level is too high, the processor may terminate heating the water so that the water does not overflow when heating the water. 
         [0033]      FIG. 2  shows equivalent circuit  200  with sense electrode  103  for providing level sense voltage  253  in accordance with an embodiment of the invention. Capacitance (Cw)  201  is affected by a change of the dielectric constant resulting from water level  105 . Excitation signal  251  (point A) comprises a 500-5000 KHz sinusoidal or square wave waveform having a zero DC component. (Embodiments of the invention may use a higher frequency range if the electromagnetic compatibility is not adversely impacted.) From level sense voltage (V)  253 , the effective dielectric constant is determined (based on EQ. 4) and consequently the water level can be obtained. 
         [0034]      FIG. 3  shows circuit  300  with sense electrode  103  for providing a level sense voltage  353  in accordance with an embodiment of the invention. As with circuit  200 , the water level is determined from level sense voltage  353  in order to determine equivalent capacitance  301 . 
         [0035]      FIG. 4  shows experimental results of resulting waveform  400  corresponding to a low level of water (where no water is present in container  101 ) in accordance with an embodiment of the invention. Waveforms  400 ,  500  (as shown in  FIG. 5 ),  600  (as shown in  FIG. 6 ), and  700  (as shown in  FIG. 7 ) are obtained from circuit  200 ; however, similar results are obtained from circuit  300 . Waveform  400  is obtained at point B  355  (circuit  200 ) or point B  355  (circuit  300 ). The amplitude of waveform  400  is affected by the permittivity of the liquid (water) in proximity to sense electrode  103 . A virtual capacitor effect (equivalent to capacitance (Cw)) occurs between sense electrode  103  and the liquid, in which a charge is held on sense electrode  103 . 
         [0036]      FIG. 5  shows experimental results of resulting waveform  500  corresponding to a high level of water (where electrode  103  is covered with water in container  101 ) in accordance with an embodiment of the invention. Waveform  500  is obtained at point  355  (point B in circuit  200 ) or point  355  (circuit  300 ). Comparing waveforms  400  and  500 , one observes that the amplitude of waveform  500  is less (when the water level is high) relative to waveform  400  (when the water level is low) in accordance with EQ. 2. (In the example shown in  FIGS. 4 and 5 , the amplitude of waveform  400  is approximately 2.359 volts and the amplitude of waveform  500  is approximately 2.094 volts.) The amplitude change of waveform  400  and  500  results from different permittivity characteristics (water and air) surrounding sense electrode  103 . 
         [0037]      FIG. 6  shows experimental results of resulting waveform  600  corresponding to a low level of water (when no water is present in container  101 ) in accordance with an embodiment of the invention. Waveform  600  is obtained at point C (circuit  200 ) or point C (circuit  300 ). The DC value of waveform  600  is affected by the permittivity of the liquid (water) in proximity to sense electrode  103 . 
         [0038]      FIG. 7  shows experimental results of resulting waveform  700  corresponding to a high level of water (where electrode  103  is covered with water in container  101 ) in accordance with an embodiment of the invention. Waveform  700  is obtained at point C (circuit  200 ) or point C (circuit  300 ). Comparing waveforms  600  and  700 , one observes that the DC value of waveform  700  is less (when the water level is high) relative to waveform  600  (when the water level is low) in accordance with EQ. 2. (In the example shown in  FIGS. 6 and 7 , the DC value of waveform  600  is approximately 4.313 volts and the DC value of waveform  500  is approximately 3.313 volts.) The DC change of waveform  600  and  700  results from different permittivity characteristics (water and air) surrounding sense electrode  103 . 
         [0039]      FIG. 8  shows operational diagram  800  of system  900  (as shown in  FIG. 9 ) for determining a liquid level in accordance with an embodiment of the invention. Signal driver  801  provides an excitation signal  251  (point A) at sensor electrode  803 . (Excitation signal  251  is injected at D 1 /R 1  (point A) with circuit  200  and at C 1  (point A) with circuit  300 .) Level sense voltage  253  is converted into a digital format by A/D converter  805  and read by processor  807 . Processor  807  processes level sense voltage  253  to obtain the water level as discussed previously. (With other embodiments of the invention, a comparator may be used in lieu of A/D converter  805  and processor  807 . The comparator may be used to sense one level.) The determined water level may be further compensated by the operating temperature as provided by temperature sensor  811 . Processor  807  subsequently displays a water level indication on display  809 . 
         [0040]      FIG. 9  shows system  900  for determining a liquid level in accordance with an embodiment of the invention. Processor  901  provides an excitation signal (e.g., 1 KHz square wave signal) to detection circuit  905 . Sense electrode(s)  903  in conjunction with detection circuit  905  (corresponding to circuit  200  or circuit  300 ) provides a level sense voltage to processor  901  through A/D converter  909 . (A/D converter  909  is connected to point C of circuit  200  or  300 .) With an embodiment of the invention, circuit  200  (or circuit  300 ) is placed on the same printed circuit board as A/D converter  909  and processor  901 . The printed circuit board may be placed on the handle, lid, or bottom of a kettle (container). The Processor  901  determines the water level from the level sense voltage from EQ. 4 or from a lookup table. The determined water level may be compensated by the operating temperature provided by temperature sensor  911  and displayed on display  907 . 
         [0041]      FIG. 10  shows flow diagram  1000  for determining a liquid level as performed by system  900  in accordance with an embodiment of the invention. In step  1001 , excitation signal  251  is injected into circuit  200 . Resulting level sense voltage  253  is measured by A/D converter  909  and provided to processor  901  in step  1003 . In step  1005 , the measured temperature of sense electrode  903  and the liquid are provided to processor  901  by temperature sensor  911 . Processor  901  compensates for the temperature when determining the water level using EQ. 4 in step  1007 . Processor  901  then displays the water level in step  1009 . 
         [0042]      FIG. 11  shows container  1101  with a sensor having a plurality of sense electrodes (L 1 -L 5 )  1103   a - 1103   e  for determining a liquid level in accordance with an embodiment of the invention. The sensor may have one or more electrodes that can be molded in PC plastic. The sensor may be mounted on the wall of container  1101 . Detection circuitry (circuit  200  or  300 ) is applied to each sense electrode. The detection circuitry may be assigned to each sense electrode or may be shared by the sense electrodes by switching the detection circuitry to a specific sense electrode when needed. Rather than using one sensor electrode as shown in  FIG. 1 , system  1100  incorporates five sense electrodes to determine water level  1105 . System 1100 determines the capacitance variance among electrodes  1103   a - 1103   e  for the indication of the water level corresponding to L 1 , L 2 , L 3 , L 4 , and L 5 . However, additional sense electrodes may be incorporated in order to obtain a greater accuracy of the water level. The equivalent capacitance (Cw) is determined for each sense electrode  1103   a - 1103   e . Because of the different dielectric characteristics of water relative to air, the equivalent capacitance of sensor electrodes below water are significantly different from the equivalent capacitance of sensor electrodes above water. In the exemplary embodiment shown in  FIG. 11 , the equivalent capacitances of L 1 , L 2 , and L 3  is larger than the equivalent capacitances of L 4  and L 5  by applying EQ. 1. Consequently, a processor (not shown) determines that water level  11   05  is near the bottom surface of sense electrode  1103   b . The processor may subsequently display an indication “L 3 ” on a display. 
         [0043]    As can be appreciated by one skilled in the art, a computer system with an associated computer-readable medium containing instructions for controlling the computer system can be utilized to implement the exemplary embodiments that are disclosed herein. The computer system may include at least one computer such as a microprocessor, digital signal processor, and associated peripheral electronic circuitry. 
         [0044]    Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.