Abstract:
A method identifies a level of each of a plurality of liquids in a reservoir. The method includes using a sensing assembly having first and second electrodes extending along a side of the reservoir, wherein the first and second electrodes combine to form a plurality of capacitors each having a unique capacitance. The method includes the steps of applying an input AC signal voltage to the first electrode. An output AC signal voltage is then measured across the second electrode. The method continues by locating clusters of frequency variations. The method then associates a level identification for each of the plurality of liquids with each of the clusters of frequency variations.

Description:
BACKGROUND ART 
   1. Field of the Invention 
   The invention relates to a method for measuring fluid levels in a reservoir. More particularly, the invention relates to a method for measuring fluid levels in a reservoir having multiple fluids being stored therein by analyzing the resonance of the output signal of the fluid level sensing assembly. 
   2. Description of the Related Art 
   The storage and delivery of liquids is an important feature of many mechanisms. In an automotive environment, proper delivery of liquids is essential for the functioning and maintenance of a motor vehicle. By way of example, a motor vehicle will not function without fuel, typically liquid gasoline. That same motor vehicle will not function properly without the proper amount of oil stored in the internal combustion engine allowing it to lubricate and cool itself. These are just two fluids in a particular environment that require close observation to make sure its host mechanism, i.e., the motor vehicle, can operate properly. 
   Currently, there are a number of ways in which a fluid level may be measured. The mechanisms used to measure the fluid level help determine if more fluid is required in order to continue the proper maintenance and operation of the host mechanism. Fluid level measuring mechanisms include floating arm mechanisms, pressure sensors, capacitive sensors, and ultrasonic sensors. The most commonly used fluid level measuring system is the floating arm mechanism. 
   The floating arm mechanism is an imperfect mechanism for several reasons. First, the floating arm mechanism requires moving parts inside a liquid-filled container. This requires increased time to install the floating arm mechanism and seal it and the container or reservoir. Resistive strips used by the floating arm mechanism are susceptible to contamination and can develop contact problems. The contamination and contact problems result in erroneous measurements. Looking forward, the floating arm fluid measuring mechanism will not be able to differentiate between different types of liquids within the same reservoir. This problem will increase in the automotive environment as different types of fuels will be accepted by each motor vehicle resulting in the stratification of the fluids within a particular reservoir. 
   Another reason the floating arm mechanisms are inferior is that they measure liquid levels inefficiently when the reservoir holding the liquid is unusually shaped. Oftentimes when a motor vehicle is an all-wheel drive vehicle, an extra drive shaft is required to extend along the underbody of the motor vehicle. The extra shaft typically extends through the space used by the fuel tank. Therefore, the fuel tank must be modified resulting in an unusually shaped fuel tank. Multiple floating arm mechanisms are required to get accurate readings from these unusually shaped fuel tanks. This adds considerable costs to the fuel tank construction. 
   Compounding the problem of measuring liquids in a reservoir is the fact that liquids can separate into multiple phases wherein the various phases do not mix and physically separate. This typically occurs when the liquid in the reservoir is contaminated. Readings taken from level sensors will be inaccurate if they cannot adequately differentiate between the differing phases. 
   In some situations, the liquids may be compound liquids by design. More specifically, new forms of gasoline include gasoline/alcohol mixtures. These types of fuels can easily separate or stratify because they are more susceptible to absorbing water. 
   SUMMARY OF THE INVENTION 
   A method identifies a level of each of a plurality of liquids in a reservoir. The method includes using a sensing assembly having first and second electrodes extending along a side of the reservoir, wherein the first and second electrodes combine to form a plurality of capacitors each having a unique capacitance. The method includes the steps of applying an input voltage to the first electrode. An output voltage is then measured across the second electrode. The method continues by locating clusters of frequency variations. The method then associates a level identification for each of the plurality of liquids with each of the clusters of frequency variations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a side view of one embodiment of the sensing assembly in a reservoir, shown in cross section, that is partially filled with two liquids; 
       FIG. 2  is an electrical schematic of one embodiment of the sensing assembly; 
       FIG. 3  is a graphic representation of an output of the sensing assembly when the reservoir is empty; 
       FIG. 4  is a graph representation of an output of the sensing assembly when the reservoir has two liquids therein; and 
       FIG. 5  is a logic chart of one method of operation for the sensing assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a sensing assembly is generally indicated at  10 . The sensing assembly  10  senses liquid levels in a reservoir  12 . The sensing assembly  10  creates a sensing signal  54  (shown in  FIGS. 3 and 4 ) to identify levels  11 ,  14  that liquids  16 ,  17  are at within the reservoir  12 . The sensing assembly  10  is designed such that it is presumed that air  18  fills the portion of the reservoir  12  that the liquids  16 ,  17  do not. While it is contemplated that the embodiment shown in the Figures illustrates a sensing assembly  10  for a fuel tank  12  of a motor vehicle, it should be appreciated that the reservoir  12  may be any reservoir designed to hold liquids  16 ,  17  therein and that there may be any number of liquids stored within the reservoir  12 . 
   The reservoir  12  includes a bottom surface  13  that defines a normal  15  extending up therefrom. In the instance when the bottom surface  13  does not extend through a single plane parallel to the horizon, the normal  15  will be treated as though it is extending normal to the horizon. Therefore, the surface of the liquid  16  is always perpendicular to the normal  15 . 
   The sensing assembly includes a base  20 . The base  20  is a rectangular piece of material that is capable of having electronics fixedly secured thereto. In addition, the base  20  is fabricated of a material that does not corrode with exposure to either air  18  or any liquids  16 ,  17  that are contemplated to be stored within the reservoir  12 . The base  20  is mountable to a side  21  of the reservoir  12  allowing it to be secured in a predetermined position within the reservoir  12 . 
   The sensing assembly  10  also includes a first electrical conductor  22  and a second electrical conductor  24 . The first  22  and second  24  electrical conductors extend into the reservoir  12 . These electrical conductors  22 ,  24  allow the sensing assembly  10  to be connected to a control circuit (not shown) that creates a signal which is modified by the sensing assembly  10 . The second electrical conductor  24  is disposed from the first electrical conductor  22  a predetermined distance. It should be appreciated by those skilled in the art that the lead configuration for the electrical conductors  22 ,  24  may change at some point within the reservoir  12  based on design requirements. 
   Extending down along the base  20  is a first electrode  28 . The first electrode  28  is electrically connected to the first electrical conductor  22 . The first electrode  28  extends between a first conductor end  30  and a first distal end  32 . The first electrode  28  defines a first electrode length  34  extending therebetween. As is shown in  FIG. 1 , the first electrode  28  is an elongated, continuous plate of conductive material wherein the first electrode length  34  is greater than its width. 
   The sensing assembly  10  also includes a second electrode  36 . The second electrode  36  is electrically connected to the second electrical conductor  24 . The second electrode  36  is an elongated conductor that extends between a second conductor end  38  and a second distal end  40 . The second electrode  36  is spaced apart from the first electrode  28 . The second electrode  36  defines a second electrode length  42  that extends between the second conductor end  38  and the second distal end  40 . The second electrode  36  also is fixedly secured to the base  20  and extends therealong. 
   A plurality of plates  44  extend along the base  20  and are fixedly secured thereto. The plurality of plates  44  are positioned between the first electrode  28  and the second electrode  36 . Each of the plurality of plates  44  are equal in size and are coplanar. Each of the plurality of plates  44  is operatively connected to the second electrode  36  such that each of the plurality of plates  44  is electrically connected to the second electrical conductor  24 . This electrical connection includes an inductor  46 . The inductors  46  are connected between each of the plurality of plates  44  and the second electrode  36  such that the inductors  46  and the plurality of plates  44  are connected in series with respect to the second electrode  36 . 
   In operation, the sensing assembly  10  uses each of the plurality of plates  44  and the first electrode  28  to create a plurality of capacitances by having the first electrode  28  act as a plurality of first plates  48  ( FIG. 2 ) and each of the plurality of plates  44  acting as second plates  49  for capacitors  50 . A signal is generated by a signal generator  52  and the control circuit identifies the level  14  of the liquid  16  by how the signal generated by the signal generator  52  is modified by the sensing assembly  10 . This operation will be discussed in greater detail subsequently. 
   Referring again to  FIG. 1 , the sensing assembly  10  has the first electrode  28  extending down the base  20  at an angle with respect to the normal  15  of the reservoir  12 . The first electrode  28  extends at an acute angle with respect to the normal  15 , which allows the portion of the first electrode  28  disposed adjacent the second distal end  40  of the second electrode  36  to be closer to the plates  44 . Continuing with this, the first electrode  28  is further from the plates  44  disposed adjacent the second conductor end  38  of the second electrode  36 . By positioning the second electrode  36  in such a manner, the capacitances for the capacitors  50  that are closer to the distal ends  32 ,  40  of the electrodes  28 ,  36  are greater than those disposed the first  30  and second  38  conductor ends. 
   Referring to  FIG. 3 , a graph representing the sensing signal  54  as seen by the control unit is shown. In  FIGS. 1 and 2 , there are 32 capacitors  50  created between the first electrode  28  and each of the 32 plurality of plates  44 . It should be appreciated that any number of capacitors  50  may be created based on the design specifications and the accuracy requirements for a particular reservoir  12 . 
   Returning attention to  FIG. 3 , there are 32 spikes  56  in the sensing signal  54  which represents an output from each of the 32 capacitors  50 . Each spike  56  represents a frequency change and, for purposes of this disclosure, will be termed as a rapid frequency change. Because the first electrode  28  extends along the base  20  at an angle with respect to each of the plurality of plates  44 , each capacitor  50  has its own resonant frequency. In the example shown in  FIGS. 1 and 2 , each of the inductors  46  has an equal inductance of 100 micro henries. With the total capacitance of the capacitors  50  being 1.6 pico farads, each capacitor  50  has an approximate value of 50 femto farads. 
   Based on these values, a frequency range for the spikes  56  extends between 50 MHz and 75 MHz when there is no liquid  16  in the reservoir  12 . This frequency range is used as a calibration for the sensing assembly  10 . More specifically, when the reservoir  12  is filled with air  18  and void of any liquid, the output of each of the capacitor/inductor pairs resides within the 50–75 MHz range. It should be appreciated that the distribution of the spikes  56  is not even and is dependent on the liquid that is poured into the reservoir  12 . 
   Referring to  FIG. 4 , it can be seen that several of the spikes  56  have shifted outside the calibration range of 50–75 MHz. Of the spikes that have moved outside the calibration range, several have shifted to form a first shifted set of spikes  58 . In addition, a second shifted set of spikes  60  have shifted to a frequency differing from the first shifted set  58 . In terms of frequency, the two shifted sets of spikes  58 ,  60  are distinct from each other and are separated by ranges of frequencies that have absolutely no spikes therein. These are variation free ranges  61 . The two shifted sets  58 ,  60  are distinct because they each represent one of the two liquids  16 ,  17  and each liquid  16 ,  17  has a unique dielectric constant. The different dielectric constants change the frequency outputs at each of the individual capacitors  50  that are surrounded by a particular liquid  16 ,  17 . The first shifted set of spikes  58  identify a first liquid that is found within the reservoir  12  and the second set of spikes  60  identify a second liquid  17  within the reservoir  12 . And the number of spikes  58 ,  60  in each of the shifted sets or clusters outside the calibration range identify the level of each of the liquids  16 ,  17  within the reservoir  12 . By counting each of the spikes within each of the clusters  58 ,  60 , and knowing the distance between the plurality of plates  44 , the levels  11 ,  14  of the liquid  16 ,  17  in the reservoir  12  are calculated. 
   In addition, by knowing the capacitive values and the inductive values, it can be determined as to which types of liquid  16 ,  17  are in the reservoir  12 . This can be done by matching values with these stored in a memory device (not shown) that have been measured through testing. 
   In operation, and with reference to  FIG. 5 , the method begins at  70 . A signal is applied to the first electrode  28  at  72 . The output from the second electrode  36  is measured at  74 . Frequency variations in the output are identified at  76 . Clusters of spikes  58 ,  60  are identified at  78 . Each frequency variation is associated with a capacitor  50 . This capacitor location is associated with a frequency at  80 . The amount of frequency variations in each cluster  58 ,  60  is measured at  82  and the volume of each liquid  16 ,  17  in the reservoir  12  is determined at  84 . Each liquid  16 ,  17  is identified at  86 . And the method returns at  88 . 
   The invention has been described in an illustrative manner. It is to be understood that the terminology, which has been used, is intended to be in the nature of words of description-rather than of limitation. 
   Many modifications and variations of the invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described.