Abstract:
A bin or tank level monitoring system uses a capacitance-sensing device with at least one electrode vertically extending from near the top to near the bottom of the tank. Changes in the level of the material held in the tank causes the effective dielectric constant of the electrical capacitance between the electrode and for example, an adjacent conductive tank wall, to change continuously and proportionately. The change in the dielectric constant changes the actual capacitance between the electrode and the tank wall. Circuitry forming part of the system can measure this change in capacitance and use the measurement to provide an accurate indication of the level of material in the tank. A variety of configurations of the electrode or electrodes allows level detection for both conductive and non-conductive tanks and for different types of materials held in the tanks.

Description:
[0001]     CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0002]     This is a regular application filed under 35 U.S.C. §111(a) claiming priority, under 35 U.S.C. §119(e)(1), of provisional application Ser. No. 60/688,860, previously filed Jun. 8, 2005 under 35 U.S.C. §111(b). 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention relates generally to material level sensing and more particularly to a capacitive sensor for measuring the level of solids or liquids stored in containers.  
       BACKGROUND OF THE INVENTION  
       [0004]     Several means for measurement of the level of granular or liquid materials within a storage container or tank are known in the art. Some of the more common approaches in the industrial and agricultural industry include load cells to measure the entire weight of the container and contents and pressure sensitive switches that can detect the presence of the contained materials. The storage containers typically employed include steel or other metal containers such as a bin or tank.  
         [0005]     The load cell solution provides very accurate results but also tend to include costly transducers and require complex mounting solutions. This results in labor-intensive installation procedures, leading to costly maintenance expenses. A number of the pressure sensitive switches are mounted internally along the entire height of the container. Such an arrangement provides a very coarse indication of material level in the tank. The precision of the measurement depends on the number of switches utilized.  
         [0006]     Other methods of measuring the level of the container contents include an ultrasonic beam. These various other methods have had limited success.  
         [0007]     Currently, one successful approach is to use the changes in capacitance between a first electrode such as a conductive wire or strip within the container, and another conductive electrode within the container. The second electrode may be the tank wall. Air has a dielectric constant different from that of almost any type of material that a container might hold. As the level in the tank increases, the average dielectric constant between the first and second electrodes changes. This change in average dielectric constant changes the capacitance between the first and second electrodes. The capacitance value across the electrodes can be measured and correlated with the level of the material in the container.  
         [0008]     A need exists for a simple and inexpensive solution to measure the level of material stored in containers. The present invention provides a solution to these needs and other problems, and offers other advantages over the prior art.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention is generally directed to a bin or tank level monitoring solution. In a particular aspect of the present invention, there is provided a capacitance-sensing device utilizing as a first electrode, one or more cables, conductors, or probes extending substantially vertically from near the top to near the bottom of a container such as a metal tank. The measured capacity between the probes and the metal tank surface increases in a continuous and proportional manner as the level of the material in the container increases. This is due to the change in the dielectric constant between the tank wall and the probes.  
         [0010]     In one embodiment, the system comprises an indicating device and one to four sensing circuits to allow up to four containers to be monitored. In one embodiment, the system is suited to be located in an outdoor environment. In one preferred embodiment, the indicating device contains a microprocessor that converts the capacitance sensed signal from the sensor to a scaled output signal used to illuminate one or more banks of LEDs (light emitting diodes) that indicate the level of material within a tank and which of a group of tanks is currently being displayed. In one preferred embodiment, the operator can manually select the tank level to be displayed at a given time, or allow the indicating device to continuously scan all connected tanks and alternately display the level results on the shared bank of LEDs.  
         [0011]     Additional advantages and features of the invention will be set forth in part in the description which follows, and in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The following drawings in which like reference characters indicate like parts are illustrative of embodiments of the invention and are not intended to limit the invention as encompassed by the claims forming part of the application.  
         [0013]      FIG. 1  is a block diagram of the level sensor system.  
         [0014]      FIG. 2  is a detailed schematic of the sensor circuit shown in  FIG. 1 .  
         [0015]      FIG. 3  is a detailed schematic of the tank indicator circuit shown in  FIG. 1 .  
         [0016]      FIG. 4  illustrates a tank having one preferred electrode configuration.  
         [0017]      FIGS. 5 and 6  each show a tank having second and third preferred electrode configurations respectively.  
         [0018]      FIGS. 7 and 8  show two preferred configurations for the cross section shape of electrodes. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Numerous level sense systems exist, however, the current systems available fail to provide low cost, simple solutions such as those driven by smaller businesses. The present invention will be described in preferred embodiments and is not intended to be limited as described.  
         [0020]      FIG. 1  is a block diagram of one embodiment of electronic circuitry that measures capacitance across first and second input terminals. The sensor circuit  100  is used to generate a level sense signal on a path  115  based upon a sensed capacitance level between a pair of conductors  110  attached to electrodes within a tank  105 . The capacitance level between conductors  110  is determined by the level of material within the tank  105 .  
         [0021]     In one embodiment, as many as four tanks  105 , four conductor pairs  110 , four sensor circuits  100 , four paths  115 , and four low pass filters  120  may be present. The level sense signal on each path  115  generated by the sensor circuit  100  is a signal having periodic pulses. The frequency of these pulses changes inversely with the capacitance level across conductors  110 . That is, the time between leading edges of adjacent pulses increases with increasing capacitance.  
         [0022]     Each level sense signal is transmitted on a path  115  to an associated low pass filter  120  which provides a filtered level sense signal on an associated path  125 . Each low pass filter  120  removes the noise in the level sense signal on path  115  to make it more suitable for subsequent processing by a level computer  103 .  
         [0023]     Level computer  103  receives the filtered level sense signal on each path  125 , measures the period of the filtered level sense signal  125 , and generates a display drive signal on a path  135  based on the level sense signal  125 . Path  135  carries the display drive signal to a level display  140  that provides a visual indication based on the display drive signal on path  135 .  
         [0024]     The level computer  103  also generates a tank select signal on a path  145 . In one preferred embodiment, the tank select signal on path  145  comprises four bits, each associated with one of the four tanks  105 . A tank select display  150  receives the tank select signal on path  145 . The tank select display  150  illuminates selected display features to provide a visual indication based upon the tank select signal on path  145  of which tank  105  level is currently displayed. In one preferred embodiment, the tank select display  150  comprises four light emitting diodes (LEDs), each representing selection of one of four tanks  105 .  
         [0025]     Select switches  155  provide a switch control signal on paths  160  to the level computer  103  which controls selection of a particular tank  105  and calibration of the level sense signal for the selected tank  105 . In one preferred embodiment, the select switches  155  provide a switch control signal  160  to include selection of the one of the four tanks  105  providing the capacitance level. The identity of the selected tank  105  is displayed by the tank select display  140 .  
         [0026]     The level computer  103  generates a communication select signal which a transmit controller  170  receives on path  165 . The communication select signal on path  165  indicates whether the level computer  103  is transmitting information to or receiving data from a host device such as a barn monitoring system. The transmit controller  170  generates a communication signal on path  175  for wireless communication.  
         [0027]      FIG. 2  is a detailed schematic diagram of the sensor circuit  100  shown in the block diagram of  FIG. 1 . Sensor circuit  100  generates the level sense signal on path  115  based on the value of the capacitance sensed across paths  110  and  101 . Path  110  forms a first input terminal to circuit  100  and is connected to at least one electrode within tank  105 . Tank  105  is grounded to complete the connection to a second input terminal  101  of circuit  100 . Terminal  101  also serves as the ground bus for circuit  100 . Alternatively, a common conductor may connect a tank  105  and path  101 .  
         [0028]     A gas tube voltage limiter  102  removes any high voltage noise such as static electricity from the level sense signal voltage on path  115 . Capacitors  104  and  107  provide further protection between the level sense signal on path  115  and the tank  105  to prevent stray common-mode voltages from generating errors. The values of capacitors  104  and  107  are preferably large as compared to the capacitance level between paths  101  and  110 . In one preferred embodiment, each capacitor  104  and  107  has a value around 10 nanofarads (ten percent tolerance).  
         [0029]     Series resistor  108  provides current limiting and in one preferred embodiment may have a value of 1000 ohms (ten percent tolerance). To provide further static protection, diode  103  limits the sense signal to less than the 10 v. DC supply voltage at power terminal  111 , and diode  105  limits the sense signal to signal ground  101 . Thus, voltage spikes will not harm circuit  100 . In one preferred embodiment the BAV99 small diode manufactured by Fairchild Semiconductor Corporation, South Portland, Me., may serve as diodes  103  and  105 .  
         [0030]     An amplifier  118 ; capacitor  109 ; the capacitance from tank  105  across input terminals  110  and  101 ; and resistors  116 ,  114 ,  112  and  113  comprise an oscillator circuit  180 . Oscillator  180  design is tolerant of power voltage variations. The capacitance across input terminals  110  and  101  controls oscillator  180  frequency.  
         [0031]     When the voltage value on the non-inverting+input terminal  127  of amplifier  118  goes positive relative to the voltage at—terminal  128 , the output of amplifier  118  goes positive as well. A triangular or sawtooth waveform  128  is present on the inverting—input terminal of amplifier  118 , and which is based on a square wave clock signal  126  generated by a frequency divider  121 . One can consider that an internal jumper connects the CK 1  and CK 0  terminals of frequency divider  121 .  
         [0032]     Capacitor  109  is in parallel with the tank  105  capacitance. The voltage across tank  105  capacitance and capacitor  109  rises as current flows through resistors  112 ,  114 , and  116  into tank  105  capacitance and capacitor  109 . This capacitor voltage raises the terminal  128  voltage of amplifier  118  above the voltage at terminal  127 . Amplifier  118  then pulls output terminal  129  to near 0 v. Resistors  114 ,  112  and  113  form a voltage divider that generates a resulting square wave threshold signal on terminal  127 , which is sixty-one percent of the amplitude of the square wave clock signal at terminal  126 . This determines the peak-to-peak voltage of the triangular waveform  128 .  
         [0033]     When the output terminal  129  of the amplifier  118  is in a high state near 10 v., the square wave clock signal at terminal  126  is also high. When the triangular waveform on terminal  128  reaches the level of the square wave threshold signal voltage on terminal  127 , the output terminal  129  of amplifier  118  switches to a low state. This process of switching repeats which generates a signal at terminal  129  having a frequency based upon the sum of the tank  105  capacitance and the value of capacitor  109 .  
         [0034]     In one preferred embodiment, amplifier  118  is preferably a low power, low offset voltage comparator similar to the LM 193, manufactured by National Semiconductor Corporation, Santa Clara, Calif. In that preferred embodiment, capacitor  109  is selected to be 27 picofarads (ten percent tolerance), resistors  114  and  116  are selected to be 47 kilohms (one half percent tolerance), and resistors  112  and  113  are selected to be 150 kilohms (one half percent tolerance). The frequency divider  121  may be a fourteen stage ripple divider oscillator similar to the CD4060, manufactured by Texas Instruments, Dallas, Tex.  
         [0035]     The frequency divider  121  divides the frequency of the signal at the output of amplifier  118  by a factor of 256 and transmits the resulting low frequency signal to the gate of a transistor  123 . Each time the transistor  123  is switched on, current flows through transistor  123  and resistor  124 . Voltage regulator  171 , which may be similar to model LM317L available from Fairchild Semiconductor, provides a constant 1.25 v. to resistors  124  and  172 . The value of resistor  172  may be 412 ohms, allowing a bias current of 3.0 ma. The value of resistor  124  is 178 ohms. When transistor  123  switches on, the circuit draws an additional 7.0 ma. but voltage at path  115  remains essentially constant  
         [0036]     Thus, the edges of the waveform provided by frequency divider  121  cause changes in current flow only on path  115 . Level computer  103  will sense this change in current when determining the frequency of the signal output from frequency divider  121 . Converting voltage changes to current changes reduces noise on path  115  which may be located at some distance from the associated low pass filter  120 .  
         [0037]     In one preferred embodiment the transistor  123  is preferably a low on-resistance N-channel MOSFET similar to the IRLML2803, manufactured by International Rectifier, El Segundo, Calif. In one preferred embodiment, the resistor  124  value is 1.69 kilohms (one percent tolerance).  
         [0038]     Terminal  90   a  receives 12 v. DC from the low pass filters  120 . Diode network  122  drops the voltage at power terminal  111  to about 10 v. and capacitor  119  further filters ripple from the DC voltage at terminal  111 . The voltage at power terminal  111  provides power voltage for amplifier  118  and divider  121 . Resistor network  117  provides pull-up voltages for amplifier  118  and the NOT CKO terminal of voltage divider  121 .  
         [0039]     The design for sensor circuit  100  allows connection to the system with only two wires if ground is a reliable third connection: power connection at terminal  90   a  and signal connection at path  115 . If ground is not reliable, then a neutral or ground wire must connect to terminals  101 .  
         [0040]      FIG. 3  is a schematic diagram of one preferred embodiment for displaying the level of material stored in up to four tanks  105 .  FIG. 3  shows low pass filters  120 , the level computer  103 , and the display and control elements shown in the block diagram of  FIG. 1 . The level computer  103  senses the spacing between adjacent pulses in each level sense signal on a path  115  and passing through a low pass filter  120  from a sensor circuit  100 .  
         [0041]     This embodiment shows four low pass filters  120  in  FIG. 3 , each receiving a sensor signal on an associated signal path  115  from an associated sensor circuit  100 . A first low pass filter  120  comprises resistors  131  and  136 , one of the resistors in network  141 , and capacitor  142 . A second low pass filter  120  comprises resistors  132  and  137 , one of the resistors in network  141 , and capacitor  143 . A third low pass filter  120  comprises resistors  133  and  138 , one of the resistors in network  141 , and capacitor  144 . A fourth low pass filter  120  comprises resistors  134  and  139 , one of the resistors in network  141 , and capacitor  144 .  
         [0042]     Resistors  131 - 134  convert the current signal from the respective sensor circuit  100  to a filtered sensor voltage. A microprocessor  130  receives these sensor voltages and measures the time between similar edges of adjacent pulses. Resistors  136 - 139  provide current limiting if faults in connections arise.  
         [0043]     The combination of the resistors in network  141  and capacitors  142 ,  143 ,  144 , and  146  determine the cutoff frequency of each low pass filter  120 . Each low pass filter  120  generates a filtered level sense signal on one of the paths  125  that removes the noise of the level sense signal on the corresponding path  115  to make it more suitable for the level computer  103 .  
         [0044]     In one preferred embodiment, resistors  131 ,  132 ,  133 , and  134  each have values of 51.1 ohms (10% 0.5 W.), and resistors  136 - 139  each have values of 220 ohms (10% 0.5 W.). This preferred embodiment&#39;s resistor network  141  is selected to be 1000 ohms (10%) each and capacitors  142 ,  143 ,  144 , and  146  have 100 nanofarad values (10%).  
         [0045]     For convenience, a pair of conductors may carry both power and signal between the low pass filters  120  and the sensor circuits  100 . The power connection is between conductor  90   b  and each terminal  90   a  in a sensor circuit  100 . In some circumstances the installer may wish to provide a third, neutral connection between ground terminals  101  in the low pass filters  120  and the sensor circuits  100 .  
         [0046]     The microprocessor  130  forms a major part of level computer  103 . An important purpose for microprocessor  130  is to measure the period of the filtered level sense signals provided on paths  115 . A crystal oscillator  151  with capacitors  152  and  153  provides a precise time standard for measuring the time between similar adjacent voltage transitions in the sensor signal.  
         [0047]     The microprocessor  130  generates a display drive signal carried on paths  135 . The display drive signal is transmitted to the level display  140  which comprises a first bank of LEDs  168  driven by serial shift registers  154  and  156 . The first bank of LEDs  168  is representative of the level in a given tank (all LEDs lit represent a full tank) based upon the selection of a particular tank  105 .  
         [0048]     Microprocessor  130  generates a tank select signal carried on paths  145 . In one preferred embodiment, the tank select signal comprises four bits, each representative of the capacitance level available on one of four connections  110  to one of four tanks  105 . Each tank select signal is transmitted to a tank select display  150  comprising a second bank of four LEDs  169 . Each LED in the second bank of LEDs  169 , indicates selection of one of four tanks  105 .  
         [0049]     The select switches  155  provide control signals on paths  160  to the microprocessor  130  which controls the calibration of the level sense signal  115 , as described above. The select switch bank  155  consists of the tank select switch  147 , a low level calibration switch  148  and a high level calibration button  149 . If switch  147  is not depressed, the default control of level display  140  is timed to cycle among each tank  105  connected to the system for a period of two seconds each, as controlled by microprocessor  130 . During the time that display  150  indicates a particular tank  105  is temporarily selected, pressing switch  147  permanently selects that particular tank  105  until switch  147  is pressed again.  
         [0050]     Calibration of the system involves manipulation of switches  147  and  148  while each of the tanks  105  are empty, and manipulation of switches  147  and  149  while each of the tanks  105  are filled. For each tank  105  when it is empty, low level calibration occurs when operator depresses and holds the tank select button  147  to select the desired tank  105  while at the same time pressing the low level calibration button  148 . For each tank  105  when it is full, high level calibration occurs when the operator depresses and holds the tank select button  147  to select the desired tank  105  while at the same time pressing the high level calibration button  149 .  
         [0051]     In one preferred embodiment the microprocessor  130  is preferably a processor similar to P87LPC767N, the crystal oscillator  151  is selected to be 11.0592 megahertz, and capacitors  152  and  153  are selected to be 27 picofarads (ten percent tolerance). The level display  140  may comprise a first bank of sixteen LEDs  168 . Select switches  155  are preferably normally open push button switches.  
         [0052]     A communication select signal  165  is generated by the microprocessor  130  which is transmitted to the transmit controller  170 . The communication select signal  165  determines whether the microprocessor  130  is transmitting information to or receiving data from the host device.  
         [0053]      FIG. 4  illustrates a tank system  200  wherein cables  205 ,  210 , and  215  collectively comprise a first electrode of a tank capacitance to be connected to terminal  110  of a sensor circuit  100 . The wall of tank  105  is conductive and forms the second electrode of the tank  105  capacitance. As the level of material filling tank  105  changes, the capacitance between cables  205 ,  210 , and  215  on the one hand and the wall of tank  105  also changes because the effective dielectric constant changes for the tank  105  capacitance.  
         [0054]     The cables  205 ,  210  and  215  are strung from the top surface  201  to the bottom surface  207  with strain relief. The cables may comprise aircraft quality cable with plastic insulation on the exterior. However, in certain installations, non-insulating cable may be adequate.  
         [0055]     One type of material commonly held in a tank  105  is animal feed. Experience shows that feed stored in tanks may drop significantly faster in the center of the tank as compared to the exterior surface, resulting in an upper surface of the material that is not level. This is known as tunneling and increasing the number of cables reduces this potential level error. As shown in  FIG. 4 , cables  205 ,  210 , and  215  form acute angles with at least a portion of the tank  105  wall. This angled configuration compensates to some extent for situations where the surface of the material is not level.  
         [0056]      FIGS. 5 and 6  show alternative embodiments for electrode configuration. Tank  105  has a bottom  255  above which the level or height of material can vary. In  FIGS. 5 and 6 , the level of the material held in tank  105  is shown at  265 .  
         [0057]     In  FIG. 5 , a pair of brackets  245  attached to a conducting wall of tank  105  extends over the top of tank  105 . The tank top  205  of  FIGS. 4   a  and  4   b  may be considered to comprise brackets  245 . Electrodes  205  and  270  extend vertically downwards from cantilevered ends of brackets  245  and dead end on the bottom  255 . Standoffs  275  hold electrodes  205  and  270  at a substantially constant spacing from the wall of tank  205 . One or both of electrodes  205  and  270  may have a tensioner  257 , which may be a spring or other elastic device that does not interfere with the electrical conductivity of electrodes  205  and  270 . Tensioner  257  keeps electrode  270  taut to further promote constant spacing of electrode  270  from the wall of tank  105 .  
         [0058]     Electrodes  270  are insulated from tank  105  at brackets  245  and bottom  255 , and from standoffs  275  as well if standoffs  275  are conductive. A jumper  252  electrically connects electrodes  205  and  270 . Conductors  240  connect electrodes  205  and  270  and tank  105  to a sensor circuit  100 . Standoffs  275  maintain electrodes  270  at a constant spacing from the wall of tank  105 , thereby providing more linear response to changes in level  265  by the capacitance between electrodes  205  and  270  as one plate of the capacitor and the wall of tank  105  as the other capacitor plate.  
         [0059]      FIG. 6  shows another embodiment for the electrode configuration within a tank  105 . Electrodes  205  and  206  are suspended from a bracket at the top of tank  105 . Electrodes  205  and  206  hang down and are maintained relatively taut by weights  280  and  281  attached to the bottom ends of electrodes  205  and  206 . Insulated spacers  285  maintain a constant spacing between electrodes  205  and  206 . Conductors  240  connect to input terminals  110  and  101 .  
         [0060]      FIGS. 7 and 8  show cross sections of electrodes with interior conductors  260  and  290  respectively, and exterior insulating jackets  268  and  295  respectively. It is also possible to suspend a single electrode  205  from bracket  245  or a tank top  201  and with or without a weight for tensioning, where the wall of tank  205  is conductive.  
         [0061]     One should understand that the numerous characteristics and advantages of various embodiments of the present invention as set forth above are illustrative. This is true especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the term in which the appended claims are expressed. For example, the particular components such as operational amplifiers and comparators may vary by manufacturer, having differing design tolerances, pin-out and packaging. Additionally, the discrete components such as resistors, diodes and capacitors may have a wide range of operating parameters which will affect the results in varying degrees. The particular components may be selected depending on the particular application for the level sense control circuit while maintaining substantially the same functionality without departing from the scope and spirit of the invention. For example, it can be appreciated by those familiar with the art, that the number of tanks to be monitored may vary from installation to installation. Therefore, alternative embodiments may include a different microprocessor or multiple microprocessors to manage the information.  
         [0062]     In addition, although the preferred embodiment described herein is directed to a level sense circuit for liquid or granular storage systems, it will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems, like gas storage systems in which the dielectric value changes measurably without departing from the scope and spirit of the present invention.  
         [0063]     As suggested in connection with  FIG. 6 , the system can be configured for use in a non-conductive container, bin or tank that utilizes the concepts described herein by adding additional probes or cables and connecting them to the other side of the bin sensing circuit  100  in place the connection to the tank  105 .