Abstract:
A sensor comprising: a substrate consisting essentially of a non-conductive material; a first electrode, and a second electrode disposed on a first surface of the substrate, wherein the first electrode comprises a first major portion traversing a length of the substrate and a finger extending from the major portion, wherein the second electrode comprises a second major portion traversing the length of the substrate and a finger extending from the second major portion, wherein the first electrode finger extends toward the second electrode major portion and the second electrode finger extends toward the first electrode major portion and is substantially parallel to the first finger; and a third electrode connected to a ground, wherein the third electrode is interposed between and about the first and second electrodes.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Automotive systems continuously or periodically monitor numerous fluids to ensure that performance continues as expected. There are many fluid properties that can be monitored using techniques such as compositional analysis, quantitative analysis and contaminant concentration. Examples of these include monitoring for excess soot in lubricant oil, for the presence of water or methanol in gasoline, for the remaining quantity of lubricant, and the like.  
           [0002]    The measurement of capacitance or complex impedance between two parallel plate electrodes or between two coaxial tube electrodes can be used to quantify certain fluid properties. The fluid is typically passed through a gap maintained by the parallel plates and a dielectric constant of the fluid is determined as it is passed between the plates. Monitoring the fluid&#39;s dielectric constant can be used to detect changes in the fluid, indicating the presence of, for example, contaminants or additives. Alternatively, the measured capacitance may be used to determine the level of fluid in a container.  
           [0003]    Some previous capacitive sensors of fluid properties have used plastic components to fabricate the sensor. For sensors that include plastic components, the dielectric constant (i.e., complex impedance) is known to be nonlinear as a function of temperature. To compensate for this non-linear behavior, capacitive sensors fabricated with plastic components require additional data collection, which adds to the overall operating and manufacturing costs.  
           [0004]    Other types of capacitive sensors use rivets and spacer rings to separate opposing carrier plates. The spacer rings are positioned onto rivet shafts between the plates to form a gap. The rivets and spacer rings must be electrically insulated and are necessarily positioned outside the areas of the metal capacitor coatings or claddings, thereby adding to the overall costs to manufacture the sensor. Moreover, the structural stability of the sensor relies on the number of rivets and the spacing of the rivets from each other. The rivets cannot assure in all instances that the carrier plates will not bend or warp during use. Such warping is undesirable because it varies the spacing between the capacitor plates resulting in variability and error.  
           [0005]    To optimize the signal-to-noise ratio for these types of capacitive sensors, the gap (or distance) between the parallel plates needs to be minimized. However, if the gap becomes too small, the fluid flow within the gap is hindered and as a result, the response time of the sensor increases. Moreover, there is a propensity for the gap to trap material and further hinder the fluid flow.  
           [0006]    Even if the above noted problems are overcome, it is always a challenge to manufacture a device with a small gap economically and reproducibly.  
         SUMMARY OF THE INVENTION  
         [0007]    A sensor comprising a substrate consisting essentially of a non-conductive material; a first electrode, and a second electrode disposed on a first surface of the substrate, wherein the first electrode comprises a first major portion traversing a length of the substrate and a finger extending from the major portion, wherein the second electrode comprises a second major portion traversing the length of the substrate and a finger extending from the second major portion, wherein the first electrode finger extends toward the second electrode major portion and the second electrode finger extends toward the first electrode major portion and is substantially parallel to the first finger; and a third electrode connected to a ground, wherein the third electrode is interposed between and about the first and second electrodes.  
           [0008]    In another embodiment, a sensor for measuring a characteristic of a fluid comprises a first, a second and a third ceramic substrate. A first capacitive sensor is sandwiched between the first and second substrates. The first capacitive sensor comprises a first electrode, a second electrode and a third electrode, wherein a portion of the first and second electrodes form complementary parallel finger pairs and wherein the third electrode is grounded and is interposed between and about the first and second electrodes. A second capacitive sensor is sandwiched between the second and third substrates. The second capacitive sensor comprises a fourth electrode, a fifth electrode and a sixth electrode, wherein a portion of the fourth and fifth electrodes form complementary parallel finger pairs, and wherein the sixth electrode is grounded and is interposed between and about the fourth and fifth electrodes. Circuitry means are connected to the first and second capacitive sensors for producing an output signal based on an electrical field generated by the finger pairs.  
           [0009]    A system for detecting a change in fluid properties comprises a power supply, a source circuit and an output circuit. The source circuit includes a sensor, wherein the sensor comprises a substrate consisting essentially of a non-conductive material. A first electrode, and a second electrode are disposed on a first surface of the substrate, wherein the first electrode comprises a first major portion traversing a length of the substrate and a finger extending from the major portion. The second electrode comprises a second major portion traversing the length of the substrate and a finger extending from the second major portion, wherein the first electrode finger extends toward the second electrode major portion and the second electrode finger extends toward the first electrode major portion and is substantially parallel to the first finger. A third electrode is connected to a ground, wherein the third electrode is interposed between and about the first and second electrodes. The output circuit comprises amplification means for amplifying a differential signal to produce an output signal that is proportional to a change in an impedance property of a fluid.  
           [0010]    A process for measuring the capacitive properties of a fluid comprises attaching a sensor to a fluid container. The sensor comprises a substrate consisting essentially of a non-conductive material. A first electrode and a second electrode are disposed on a first surface of the substrate, wherein the first electrode comprises a first major portion traversing a length of the substrate and a finger extending from the major portion. The second electrode comprises a second major portion traversing the length of the substrate and a finger extending from the second major portion, wherein the first electrode finger extends toward the second electrode major portion and the second electrode finger extends toward the first electrode major portion and is substantially parallel to the first finger. A third electrode is connected to a ground, wherein the third electrode is interposed between and about the first and second electrodes. The process further includes applying an oscillating voltage source to the first electrode, generating an electrical field between the first electrode finger and the second electrode finger, wherein the electrical field extends into the fluid and monitoring a current passing to the ground from the second electrode.  
           [0011]    The above described and other features are exemplified by the following figures and detailed description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Referring now to the figures wherein the like elements are numbered alike:  
         [0013]    [0013]FIG. 1 shows an exploded perspective view of a capacitive sensor;  
         [0014]    [0014]FIG. 2 shows an exploded perspective view of a capacitive sensor in accordance of another embodiment;  
         [0015]    [0015]FIG. 3 shows an exploded perspective view of a capacitive sensor in accordance of another embodiment;  
         [0016]    [0016]FIG. 4 shows an exploded perspective view of a capacitive sensor in accordance of another embodiment;  
         [0017]    [0017]FIG. 5 shows cross sectional side elevation views of the capacitive sensors shown in FIGS. 1;  
         [0018]    [0018]FIG. 6 shows cross sectional side elevation views of the capacitive sensors shown in FIGS. 2; and  
         [0019]    [0019]FIG. 7 shows a block circuit diagram for converting capacitance to an output signal. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    [0020]FIG. 1 illustrates a sensor generally designated by reference numeral  10 . The sensor  10  includes three electrodes  12 ,  14 ,  16  deposited onto a major surface  18  of a substrate  20 . The electrodes  12 ,  16  are configured to-include complementary parallel finger pairs, e.g.,  12   a  and  16   a ,  12   b  and  16   b , etc. Each of the electrodes  12 ,  16  includes at least two fingers extending from a major portion  24 ,  26 , respectively, toward the opposing electrode main portion  26 ,  24 , respectively. The fingers  12   a ,  12   b ,  16   a ,  16   b , . . . , are preferably disposed substantially parallel to each other and more preferably, substantially perpendicular to the main portions  24 ,  26 . Electrode  14 , which is grounded, is interposed between and about electrodes  12 ,  16  without physically contacting either electrode. Electrode  14  functions as a guard electrode to intercept electrostatic flux passing near the sensor surface  18  from electrodes  12  and  16 . With electrode  14  grounded, the flux that passes from electrode  12  to electrode  16 , (or vice versa, depending on which electrode functions as the source electrode and which electrode functions as the detection electrode) samples a fluid of interest at a greater distance from surface  18  than those sensors without a guard electrode since electrostatic flux contributions at the sensor surface are minimal or substantially eliminated. A general discussion of capacitive sensors can be found in U.S. application Ser. No.: 09/643,236 entitled, “Capacitive Proximity Sensor” to Lambert, incorporated herein by reference in its entirety.  
         [0021]    To increase sensitivity, the substrate  20  may further include additional electrodes or interdigitated electrodes (not shown). For example, the substrate  20  may include additional electrodes on the same surface  18  or may include additional electrodes disposed on its other major surface  22 . In this manner, the capacitive sensor  10  comprises numerous individual sensors, wherein the spacing between each successive sensor may be progressively varied. For example, the separation of individual sensors may be small near one end of the sensor  10  and larger at the other end to provide a finer graduation of sensing of a liquid level as the liquid nears the bottom of a container. For fluid property monitoring, the sensor may include a plurality of tightly spaced electrodes that, in combination, provide an increase in signal to noise ratio and sensitivity.  
         [0022]    Another embodiment is shown in FIG. 2. Here, a capacitive sensor  30  comprises three electrodes  32 ,  34 , and  36 , sandwiched between two substrates  38 ,  40 . Similar to FIG. 1, each electrode  32  and  36  includes complementary pairs of parallel fingers, e.g.,  32   a  and  34   a ,  32   b  and  34   b , etc. The substrates  38 ,  40  provide support for the electrodes  32 ,  34  and  36  as well as protection from the fluid or operating environment. The sensors employing multi-layered substrates do not contain a gap that is in communication with the environment about the sensor. That is, there is no separation of the substrates at its joined interface apart from the presence of the electrodes sandwiched between the substrates. Electrode  34  is interposed between and about electrodes  32 ,  36 , and is grounded.  
         [0023]    [0023]FIG. 3 illustrates a sensor  50  configured for dielectric sensing and fluid level sensing. The capacitive sensor  50  includes substrates  52 ,  54 ,  56 , electrodes  58 ,  60 ,  62  sandwiched between substrates  52 ,  54  and electrodes  64 ,  66 ,  68  sandwiched between substrates  54 ,  56 . As will be discussed in further detail below, electrodes  58 ,  62  are configured for sensing the intrinsic property of the fluid from the measured impedance whereas electrodes  64 ,  68  are configured for monitoring the fluid level. Electrodes  60  and  66  are interposed between and about the corresponding electrodes as shown and are grounded. Electrodes  60  and  66  function as guard electrodes in the manner previously described.  
         [0024]    The electrodes  58 ,  62  include a plurality of complementary pairs of parallel fingers, e.g.,  58   a  and  62   a ,  58   b  and  62   b , etc, extending along a portion of the length of the substrate  52 . For sensing fluid properties, it is preferred that the fingers be positioned on the substrate to maintain continuous electrical field communication with the fluid of interest for detecting a dielectric change in the fluid properties, e.g., from the presence of contaminants, additives, degradation products or the like. For example, if the fluid to be sensed comprises oil disposed in an oil pan of an automotive vehicle, it is preferred that the electrodes be disposed such that during operation of the motor vehicle, the fingers of the sensor continuously maintain electrical field communication with the oil. In this manner, changes detected by the sensor will not result in false readings due to a failure to maintain constant electrical field communication.  
         [0025]    In contrast, the electrodes  64 ,  68  shown configured for sensing the level of fluid includes a plurality of complementary pairs of parallel fingers, e.g.,  64   a  and  68   a ,  64   b  and  68   b , etc, extending along a length of the substrate  56 . The exact length along the substrate length depends on a number of factors including, but not limited to, the fluid properties, the height of the tank or vessel that contains the fluid, and the like. In this manner, the electrodes can detect a change in the level of fluid, for example, by detecting a dielectric constant of the fluid in the sensor portion submerged in the fluid and a portion that is outside of the fluid, i.e., the space above the level of fluid (air, vapors from the liquid and the like). For example, a signal generated by this type of sensor informs the user of the level of fluid remaining in the tank.  
         [0026]    The sensor  70  of FIG. 4 permits simultaneous temperature measurement as well as fluid monitoring. In this particular example, the sensor  70  includes substrates  72 ,  74 ,  76 , electrodes  78 ,  80 ,  82  sandwiched between substrates  72 ,  74  and electrodes  84 ,  86  sandwiched between substrates  74 ,  76 . Electrodes  78 ,  82  are configured for sensing the intrinsic property of the fluid from the measured impedance whereas electrodes  84 ,  86  are configured for sensing the temperature of the fluid. Electrode  80 , which is grounded, is interposed between and about electrodes  78 ,  82 . As previously discussed, the electrodes  78 ,  82  for monitoring a dielectric constant change in the fluid include complementary pairs of parallel fingers, e.g.,  78   a  and  82   a ,  78   b  and  82   b , etc, extending along a portion of the length of the substrate  72 , whereas the temperature sensing circuitry comprises resistor circuitry formed by electrodes  84 ,  86  and disposed between insulating layers  74 ,  76 .  
         [0027]    Optionally, the above-noted sensors may include additional components such as heater circuitry, a lead gettering layer, and/or the like.  
         [0028]    The electrodes are preferably fabricated from a conductive material. More preferably, the electrodes are fabricated from metals such as platinum, palladium, gold, osmium, rhodium, iridium, ruthenium, zirconium, yttrium, cerium, calcium, aluminum, and the like, as well as alloys, oxides, and combinations comprising at least one of the foregoing metals.  
         [0029]    With respect to the size and geometry of the sensing electrodes, e.g.,  12 ,  16 ,  32 ,  36 ,  58 ,  62 ,  64 ,  68 ,  78 ,  82 , they are configured to provide the desired capacitance with an electric field that extends a desired distance away from the surface of the sensor. The distance between parallel pairs of fingers, (e.g.,  12   a  and  16   a ) together with the geometry of the interposed guard electrode (e.g.,  14 ), determines the distance that the electrical field penetrates into the fluid of interest. Preferably, the distance between pairs of electrode fingers is less than or equal to about 2 millimeters.  
         [0030]    Electrodes can be formed using conventional techniques such as sputtering, chemical vapor deposition, screen printing, and stenciling, among others, with screen printing electrodes onto appropriate green tapes generally preferred due to simplicity, economy, and compatibility with a subsequent co-fired process.  
         [0031]    The substrates, e.g.,  20 ,  38 ,  40 ,  52 ,  54 ,  56 ,  72 ,  74 ,  76 , serve to mechanically support the electrodes in a known relationship with respect to the fluid to be sensed. The non-conductive substrates preferably comprise a dielectric material such as a ceramic, glass, silica, or a similar material that is capable of inhibiting electrical communication and providing physical protection to the electrodes from the fluid to be sensed. In the event more than one substrate is employed, e.g., sensors  10 ,  30 ,  50 ,  70 , it is preferred that each substrate comprise a material having substantially similar coefficients of thermal expansion, shrinkage characteristics, and chemical compatibility in order to minimize, if not eliminate, delamination and other processing problems. In a preferred embodiment, each substrate is fabricated from high purity alumina, (e.g., greater than or equal to about 96 weight % (wt %) alumina) and preferably, flux material. The substrates may preferably comprise greater than or equal to about 80 wt % alumina and less than or equal to 20 wt % flux material, with greater than or equal to about 90 wt % alumina and less than or equal to about 10 wt % flux material more preferred, and greater than or equal to about 96 wt % alumina and less than or equal to about 4 wt % flux material even more preferred based upon the total weight of the substrate composition. The composition of the flux material can be one or more oxides such as silica, lanthanum oxide, alumina, boron oxide, yttria, and the like, as well as combinations comprising at least one of the foregoing flux materials. An exemplary flux material composition comprises, by weight, about 47.5% silica, about 22.5% lanthanum oxide, about 22.5% alumina, about 5% boron oxide and about 2% yttria, based upon the total weight of the flux material.  
         [0032]    The substrates shown in FIGS.  1 - 4  generally have an elongated rectangular shape and are designed for vertically mounting in a container or tank. However, depending on the desired application, other shapes, e.g., rounded, multi-sided and the like, and configurations, e.g., contoured surfaces, ribbon-like surfaces and the like, may be preferred. The thickness of the substrate should be sufficient to support the electrodes, preferably provide handling capabilities, and be environmentally stable for its end application. In this regard, the substrate should be able to tolerate vibrations, heat and the like.  
         [0033]    [0033]FIGS. 5 and 6 show a side cross-sectional view of sensor  10  and sensor  30 , respectively. In operation, a voltage applied to, for example, electrode  12  causes an electric field  90  which induces a charge on both the electrode  14 , which is grounded, and on the counter electrode  16 , which is maintained at approximately ground potential. The distance between complementary electrode finger pairs, e.g.,  12   a  and  16   a , as well as the dimensions of the guard electrode  14 , determines the distance that the electrical field  90  penetrates into the fluid of interest.  
         [0034]    [0034]FIG. 7 shows a block circuit diagram for converting capacitance detected by the sensor to an output signal for use in detecting a change in fluid properties. An oscillating voltage source  100  is applied to a source circuit  101 . The source circuit includes a sensor  103 , (e.g.,  10 ,  30 ,  50 ,  70 ) which produces an attenuated current signal upon application of the oscillating voltage  100 . A ground shield electrode  102  is disposed between the electrodes to minimize parasitic capacitance. The source circuit  101  further includes an analog inverter  106  and a balance impedance  108 . The oscillating voltage source  100  is fed through the analog inverter  106  and balance impedance  108  to produce a current having a known value. The balance impedance is set to null the output at a predetermined capacitance. For example, the balance impedance can be based on the known capacitance with pure gasoline in fluid communication with the sensor. The current signal produced in this portion of the circuit is combined with the attenuated circuit signal produced by the sensor at junction  104 . If the attenuated signal and current from the balance impedance  108  cancel, there is no change in the dielectric of the fluid from the predetermined condition. In contrast, a detectable signal is produced if there is a change in the dielectric constant of the fluid, e.g., the presence of contaminants, additives or the like. The signal is then fed through an output circuit  109 . The output circuit includes an operational amplifier  110  with current feedback impedance  112  and provides amplification to the signal. The amplified signal is then passed through an AC/DC converter  114  to a DC amplifier  116  to produce an output signal that is proportional to changes in the impedance of the fluid.  
         [0035]    Advantageously, the capacitive sensor does not require a gap between electrode substrates. Accordingly, the sensors do not exhibit the gap problems noted in the prior art, e.g., manufacturing difficulties, fluid flow effects, etc. Moreover, the use of metal oxide materials such as alumina for the structural components used in the fabrication of the sensor advantageously overcomes the non-linear behavior temperature effects noted with the use of plastic components. Sensors with the foregoing metal oxide structure have shown minimal temperature sensitivity. Use of a grounded guard electrode, e.g.,  14 ,  34 ,  60 ,  66 ,  80 , between the source and detection electrodes eliminates slow drifts in the output that would otherwise occur from high dielectric constant materials, such as water, that might be adsorbed into the substrate and cause a change in its dielectric constant, i.e., a source of error. The use of the guard electrode makes the sensor responsive primarily to bulk fluid properties and not to fluid at or near the sensor surface.  
       EXAMPLE 1  
       [0036]    In this example, a capacitive sensor was fabricated from a green ceramic tape comprising 96% by weight alumina powder and a 4% by weight mixture of SiO 2 , La 2 O 3 , B 2 O 5 , Y 2 O 3 . The tape was prepared by a doctor blade method from an alumina slurry. The sensor comprises (5) five layers of stacked alumina tape. The outer surfaces of the stacked alumina tape were screen-printed using platinum ink with parallel digital finger type of electrodes. Nineteen (19) pairs of digital electrode were formed on each surface. A temperature sensing circuit was screen-printed using platinum ink on one of the surfaces of the third layer. Four contact pads were also formed, i.e., two for the capacitance electrodes and two for the temperature sensing circuitry. The alumina tape was then thermally laminated, cut into shape and fired at 1,450° C. for 2 hours. The dimensions of the sensor include a thickness of 0.81 millimeters (mm), a length of 20 mm, and a width of 10 mm. The capacitance in air using the sensor measured at 10 kilohertz (KHz) was 13.58 picoFarads (pF). Upon immersion in gasoline the capacitance value increased to 15.54 pF. The capacitance after the addition of 10% ethanol in the gasoline increased the capacitance to 16.94 pF. As shown by this example, by monitoring the capacitance of a fluid the sensor can be used to distinguish changes in the fluid dielectric properties.  
       EXAMPLE  2   
       [0037]    In this example, a capacitive sensor was formed in accordance with Example 1. A 65 micrometer layer of alumina in the form of a green tape was laminated onto each surface containing the electrodes. The capacitance of air measured by the sensor at 10 KHz increased to 26.45 pF. The capacitance of gasoline measured by the sensor increased to 28.21 pF whereas capacitance after addition of 10% ethanol to the gasoline increased to 29.08 pF. Therefore, by monitoring the capacitance of a fluid, the sensor can be used to distinguish changes in the fluid dielectric properties.  
         [0038]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.