Patent Publication Number: US-2006016256-A1

Title: Filling level sensor for a tank

Description:
List of References Symbols  
     
         
           2  Interior  
           4  Motor vehicle fuel tank  
           6  Filling level sensor  
           8  Base plate  
           10  Potentiometer  
           12  Lever arm  
           14  Float  
           16  Pivot pin  
           18  Arrow  
           20  Fuel  
           24  Wall  
           26  Filling level  
           28  Arrow  
           30  Free end  
           32  Permanent magnet  
           34  Front side  
           36  Particle contact  
           38  Field  
           40  Arrow  
           42  Resistor track  
           44  Contact track  
           46   a, b  Terminal  
           48  Housing cover  
           50  Cavity  
           51  Rear side  
           52  Bow  
           54  Free end  
           56  Permanent magnet  
           58  Inner side  
           60  Arrow  
           62  Guide member  
           64  Longitudinal center axis  
           66 ,  68  Flange  
           70  Rear side  
           72   a, b  Float  
           74  Center limb  
           76  Contact pin  
           78  Limit stop  
           80  Arrow  
       
    
      The invention pertains to a filling level sensor for a tank, particularly for the fuel tank of a motor vehicle.  
      Various systems are utilized for determining the filling level of a tank, particularly the fuel tank of a motor vehicle. Most of these systems are designed such that a float controls an electric potentiometer via a lever arm in dependence on the filling level in the fuel tank. This means that the slider of the potentiometer and consequently its electrical properties, e.g., the ohmic resistance between two potentiometer terminals, changes in dependence on the filling level. The change or adjustment of the potentiometer makes it possible to ascertain the filling level in the tank.  
      In comparison with older fuels, the fuels available on the market these days contain less sulfur or more aggressive additives, i.e., modem fuels are altogether more aggressive. This causes, for example, the resistor track or the contact track or the slider of the potentiometers to be attacked by the fuels, fuel vapors, etc. The electrical properties of the potentiometer change in an unpredictable fashion over the lifetime of a tank or a corresponding filling level sensor, respectively. This means that the filling level of the tank is no longer correctly indicated.  
      Some modem systems are also subject to wear phenomena, e.g., due to electromechanical sliders being displaced along resistor tracks or contact tracks. Contaminants may also be deposited on the track. This means that the contact between the slider and the track deteriorates or is even interrupted. This can lead to false readings of the filling level, malfunctions or even the complete failure of the filling level sensor.  
      Various measures are available in order to remedy this problem and to permanently obtain a flawless reading of the filling level. For example, it has been proposed to realize the sensitive electric components, e.g., the resistor track in the sensor, such that they are insensitive to fuel or the formation of a coating on the resistor track of the sensor or potentiometer is prevented. In publications DE 100 28 893 A1, U.S. Pat. No. 6,404,331 B1, DE 100 49 373 A1 or U.S. Ser. No. 09/679,425, it is proposed to utilize more precious materials, e.g., for the resistor track of the potentiometer in question. However, this increases the cost of a filling level sensor of this type due to the higher material expense and the more complicated manufacture. In addition, the accumulation of contaminants on the tracks or wear phenomena cannot be prevented with these measures.  
      It has also been proposed in various publications to completely encapsulate the filling level sensor such that its sensitive components can no longer come in direct contact with the fuel or fuel vapors. For example, DE 102 29 280 A1 proposes a pivoted toric magnet that is coupled to a Hall sensor. DE 197 01 246 A1 proposes a plurality of lamellae that can be moved magnetically. A filling level sensor with thermoelements is realized in DE 102 37 946 A1. The main disadvantages of these systems are their complexity, their complicated manufacture as well as the high costs and the susceptibility to errors resulting therefrom.  
      The invention is based on the objective of disclosing an improved filling level sensor.  
      This objective is attained by providing a filling level sensor for a tank, particularly for the fuel tank of a motor vehicle, with a bridge that can be displaced along at least two electric sliding contacts by an actuator in dependence on the filling level in the tank, wherein said filling level sensor is characterized in that the actuator consists of an actuator that generates a magnetic field, and in that the bridge consists of a conglomerate of electrically conductive particles that can be moved by the magnetic field.  
      In other words, the conglomerate forms a particle contact in the form of a bridge that locally bridges the electric sliding contacts at a certain location similar to a conventional slider. The position of the bridging point, namely the instantaneous position of the particle contact, depends on the filling level in the tank because the conglomerate can be displaced on or along the sliding contacts by the actuator such that the bridging respectively takes place at different locations depending on the filling level. The sliding contacts are realized in such a way that they have different electrical properties depending on the position at which they are bridged by the conglomerate. For example, different sliding contacts can be electrically interconnected in dependence on the position of the bridge such that different current paths are closed in dependence on the filling level in the tank or different resistors are realized due to the composition of the track.  
      The evaluation, e.g., of the current paths with the aid of a suitable electric circuit therefore provides information on the filling level in the tank. The sliding contacts deliver an electric signal that depends on the filling level in the tank.  
      The conglomerate is adjusted on the sliding contacts by the magnetic field generated by the actuator, i.e., the filling level sensor operates in a contactless fashion. A mechanical or movable connection between the actuator and the sliding contacts or the particle contact is not required.  
      Since the particle contact consists of a conglomerate in the form of a drop of electrically conductive particles that can be moved by the magnetic field of the actuator, the friction within the drop as well as the friction between the drop and the electric sliding contacts is reduced to a minimum. The adjusting forces required for moving the particles are also very low. Since the contact bridge consists of a coherent conglomerate of particles, this conglomerate only produces a local connection between the sliding contacts, namely also when concussions occur. This is the reason why the electrical properties of the bridged sliding contacts change in dependence on the filling level in a sufficiently accurate fashion.  
      The particles may consist of magnetic or magnetizable powder particles or hollow balls that are coated with a protective layer. Magnetic or magnetizable particles can be easily and inexpensively manufactured in the form of a powder or hollow balls. Due to their magnetic properties, they can be moved by the magnetic field of the actuator such that the bridge, i.e., the particle contact or powder drop, moves along the sliding contacts under the influence of the magnetic field. The additional protective layer, e.g., a protective lacquer or a precious metal coating, protects the particles or hollow balls from fuel and fuel vapors. Consequently, the magnetic properties of the conglomerate, i.e., the particle contact, are not altered and the filling level sensor does not deteriorate over time due to the effects of the fuel.  
      The protective layer may also reduce the internal friction within the conglomerate, i.e., the friction between the particles, as well as the friction between the conglomerate and the sliding contacts, and thusly contribute to a reduction in the force required for moving the bridge. For example, magnetic particles may be provided with a layer of gold or another precious metal that protects the particles, in particular, from oxidation or a chemical reaction with sulfur or other aggressive constituents of the fuel. It would also be conceivable to utilize a mixed-particle conglomerate, in which some of the particles are conductive and some of the particles are magnetic. The particles may also be bound in a carrier fluid, e.g., a viscous oil, and form the conglomerate together with this fluid.  
      The particles may consist of nanoparticles. Nanoparticles are able to form a conglomerate with particularly favorable properties. Due to the minute size of the nanoparticles, the conglomerate can be realized, for example, similar to an oil drop that adequately adheres to the sliding contacts and can be displaced thereon without disintegrating or excessively wetting the surface.  
      A series of discrete sliding contacts that are or are not bridged in pairs by the bridge, e.g., in dependence on the position thereof, need to be contacted at various locations in order to obtain an electric output signal that also takes into account slight fluctuations of the filling level. Consequently, the sliding contacts may consist of a resistor track and a contact track that form a potentiometer together with the conglomerate that connects and can be moved along the resistor track and the contact track. A potentiometer usually has only two or three terminals. The corresponding resistor track is frequently realized such that it has a continuous resistance characteristic, i.e., even the slightest movements of the bridge result in a continuous change in the electrical properties of the potentiometer. This means that the accuracy of reading of a corresponding filling level sensor or the accuracy of indication of the sliding contacts and consequently of the filling level sensor is improved.  
      Many indicating instruments, e.g., fuel gauges, are designed for direct connection to a potentiometer. Consequently, indicating instruments of this type can still be used in connection with the filling level sensor according to the invention.  
      The resistor track design also makes it possible to adapt the resistor track to the tank geometry. In this case, it is advantageous if the resistor track has a non-linear resistance characteristic in the moving direction of the bridge, i.e., the particle contact or the particle bridge. For example, this makes it possible to eliminate a costly evaluation circuit that serves for converting the output values of a linear potentiometer into the filling level in the tank and that is usually arranged between the filling level sensor and a gauge.  
      The contactless coupling between the actuator and the conglomerate by means of the magnetic field makes it possible to suitably shield the sliding contacts relative to aggressive fuel or fuel vapors by means of an encapsulation. This means that the sliding contacts and the conglomerate can be arranged in a housing that is tightly sealed relative to harmful substances and that is pervious to the magnetic field. The sliding contacts and the conglomerate consequently are hermetically sealed relative to harmful substances and protected from oxidation or dirt layers, namely without being subjected to any wear or chemical influences. The actuator is situated outside the housing and acts upon the conglomerate in a contactless fashion through the housing wall. Movable parts do not have to be provided with a lead-through from the actuator to the bridge such that on the other hand the quality of the encapsulation can be significantly improved and the costs are drastically lowered. An abundance of suitable materials that are resistant to harmful substances as well as pervious to a magnetic field are commercially available.  
      The housing may comprise two housing halves of plastic that accommodate the conglomerate between one another, wherein the resistor track and the contact track are respectively impressed in one housing part or applied thereon in the form of a coating or fixed therein in the form of an insert. Additional components can be eliminated by impressing or applying a coating of the resistor track or the contact track on a housing half or a housing part of plastic, namely because it is no longer necessary to manufacture a separate resistor track on a substrate and to connect the resistor track to the housing part. The plastic should have, for example, a high mechanical strength and a low swelling tendency in fuel, e.g., polyphthalamide. The sliding contacts, in contrast, may consist of a material that can be suitably impressed on the housing parts. The housing may be reinforced with fiberglass and is particularly well suited, for example, for applying the sliding contacts by means of screen printing.  
      In addition, the assembly of the filling level sensor is significantly simplified because both housing parts can be manufactured separately and the conglomerate in the form of contact material, e.g., powder or hollow balls, can be easily and quickly introduced therein during the final assembly of the housing. The shape of the housing may be realized in such a way that a hollow guide channel for the particle contact is formed. This hollow guide channel provides the particle contact with a certain clearance for its movement along the resistor track and the contact track, but restricts other degrees of freedom transverse to the desired moving direction. This means that the particle contact cannot separate from the contact track or the resistor track and interrupt the electric connection between the tracks.  
      The actuator may consist of a permanent magnet. Nowadays, permanent magnets are inexpensively available in large quantities and generate sufficiently strong magnetic fields, wherein these permanent magnets can also be easily handled and mounted, for example, on a lever arm in the form of an actuator. This ensures that the corresponding magnetic field for adjusting the bridge is reliably generated over the lifetime of the filling level sensor. It is no longer necessary to provide an additional means for generating the magnetic field, e.g., an electric current flowing through a coil.  
      The actuator may contain two permanent magnets that enclose the bridge and the sliding contacts between one another. Two such permanent magnets, the polarities of which are suitably aligned or adapted relative to one another, generate a particularly strong magnetic field that guides and holds the particle contact in position between the two permanent magnets. If the sliding contacts are also arranged between the particle contact and a permanent magnet, the particle contact is subjected to a particularly strong attraction by both sliding contacts that respectively face one permanent magnet. This ensures that a reliable electric contact is produced between the bridge and the sliding contacts. Therefore, the uninterrupted contacting of the sliding contacts is also ensured when the filling level sensor is subjected to concussions. In addition, the conglomerate is prevented from dividing and forming two separate particle contacts.  
      The actuator may be motively coupled to a float, the position of which is dependent on the filling level in the tank. A float is a very simple and dependable element that follows the filling level of the liquid situated in the tank in a particularly reliable fashion. The motive coupling between the float and the actuator therefore ensures that the actuator also follows the filling level in the tank very well. Consequently, the filling level sensor delivers a very accurate signal at its sliding contacts in dependence on the filling level in the tank.  
      The housing may be realized on the form of an axial guide for the float, and the actuator may be rigidly fixed on the float. The float can be very easily guided along the housing in this fashion. A mechanical reversing mechanism, a special axial bearing, etc., is not required between the actuator and the float. This additionally reduces the number of mechanical parts such that the filling level sensor can be realized in a particularly simple and inexpensive fashion.  
      The risk of movable parts getting caught on the tank or tank installations is lowered. The overall installation volume of the filling level sensor is reduced and an improved flexibility is achieved with respect to the positioning of the filling level sensor within the tank. 
    
    
      The invention is described in greater detail below with reference to the embodiments illustrated in the figures. The schematic figures respectively show:  
       FIG. 1 , a front view of a filling level sensor with potentiometer and particle contact, wherein the housing cover is open in this figure;  
       FIG. 2 , the filling level sensor according to  FIG. 1  with attached housing cover viewed in the direction of the arrow II;  
       FIG. 3 , a representation analogous to  FIG. 2  of an alternative double-magnet variation of the filling level sensor according to  FIGS. 1 and 2 ;  
       FIG. 4 , an alternative filling level sensor without reversing mechanism, namely in the form of a) a top view sectioned along the line IVa-IVa and b) a side view;  
       FIG. 5 , a perspective representation of an alternative variation of the filling level sensor according to  FIG. 4 , and  
       FIG. 6 , a section through the filling level sensor according to  FIG. 5  along the line VI-VI. 
    
    
       FIG. 1  shows a filling level sensor  6  that is situated in the interior  2  of the fuel tank  4  of a motor vehicle. The filling level sensor  6  comprises a base plate  8  with a potentiometer  10  arranged thereon, as well as a float  14  that is arranged on a lever arm  12 . The float  14  floats on the surface of the fuel  20  situated in the interior  2  of the fuel tank. The lever arm  12  is supported on the base plate  8  with the aid of a pivot pin  16 , namely such that it can be pivoted in or opposite to the direction of the arrow  18 . The filling level sensor  6  is mounted with its base plate  8  on the wall  24  of the motor vehicle tank  4  or on a not-shown holder within the tank, e.g., a flow indicator unit or a module holder or the like.  
      When the filling level  26  of the fuel  20  in the fuel tank  4  rises in the direction of the arrow  28 , the float  14  follows the filling level  26  approximately in the direction of the arrow  28 , namely along a circular path around the pivot pin  16 . This causes the lever arm  12  to also move around the axis  16  in the direction of the arrow  18 .  
       FIG. 2  shows a permanent magnet  32  that is fixed on the free end  30  of the lever arm  12  that lies opposite the float  14 , wherein this permanent magnet is covered by the base plate  8  in  FIG. 1 . A particle contact  36  is situated on the front side  34  of the base plate  8  that lies opposite the permanent magnet  32 , wherein said particle contact is attracted toward the base plate  8  in the direction of the arrow  40  by the magnetic field  38  generated by the permanent magnet  32 .  
      The particle contact  36  subjected to the attraction of the magnetic field  38  is realized in an electrically conductive fashion and electrically bridges a contact track  44  and a resistor track  42 .  
      The particle contact  36  consists of a conglomerate of small powder particles or nanoparticles or hollow balls that are realized in an electrically conductive fashion and can be attracted by the magnetic field  38 . In this case, the magnetic particles are coated with a thin gold layer. The particles attract one another such that a coherent drop is formed.  
      The potentiometer  10  is composed of the resistor track  42 , the contact track  44  and the particle contact  36  that forms the bridge connecting said tracks. The contact track  44  and the resistor track  42  are arranged on the base plate  8  in such a way that the permanent magnet  32  always lies diametrically opposite the contact track  44  and the resistor track  42  referred to the base plate  8  when the permanent magnet  32  is pivoted together with the lever arm  12 . Consequently, the particle contact  36  moved by the permanent magnet  32  constantly bridges the contact track  44  and the resistor track  42  at a certain location that is dependent on the filling level  26  due to the motive coupling formed by the float  14 , the lever arm  12 , the magnet  32  and the particle contact  36 . An ohmic resistance that is dependent on the filling level  26  consequently can be tapped at the electric terminals  46   a  and  46   b  of the contact track  44  and the resistor track  42 . Although not illustrated in the figures, the terminals  46   a, b  are respectively connected to an evaluation circuit and an electric fuel gauge.  
      The housing cover  48  illustrated with broken lines in  FIG. 1  is connected to the base plate  8  in a hermetically sealed fashion, e.g., clipped, bonded, cast or welded. The base plate  8  and the housing cover  48  consequently enclose a cavity  50  that is hermetically sealed relative to the interior  2  of the vehicle fuel tank  4 . The contact track  44 , the resistor track  42  and the particle contact  36  therefore cannot come in contact with and be attacked by the fuel  20  or other harmful substances situated in the fuel tank, e.g., fuel vapors or the like.  
      The particle contact  36  is moved in a contactless fashion, namely by the magnetic field  38  of the permanent magnet  32 . The particle contact  36  moves in a nearly frictionless and wear-free fashion on the base plate  8 , as well as on the contact track  44  and the resistor track  42 . The cavity  50  guides the particle contact similar to a channel.  
      The respective lead-through of the connecting lines  46   a, b  through the housing cover  48  and the base plate  8  needs to be sealed accordingly.  
      The cavity  50  also forms a guide channel for the particle contact  36 . Consequently, the particle contact cannot separate from the contact track  44  and the resistor track  42  or disintegrate into several individual drops, namely even when the filling level sensor  6  is subjected to strong concussions.  
       FIG. 3  shows a modified embodiment of the arrangement according to  FIGS. 1 and 2 . In the region of the base plate  8 , the lever arm  12  is extended in a U-shaped fashion by attaching an additional bow  52 . This means that the lever arm  12  not only encompasses the base plate  8  on the rear side  51  as shown in  FIGS. 1 and 2 , but also on its front side  34 . The bow  52  is supported on the pivot pin  16  that is realized longer than in  FIGS. 1 and 2  analogous to the lever arm  12 . A second permanent magnet  56  is arranged on the free end  54  of the bow  52 .  
      The resistor track  42  on the base plate  8  is radially shifted toward the pivot pin  16  in comparison with the  FIGS. 1 and 2 . The contact track  44  is now situated on the inner side  58  of the housing cover  48 , namely diametrically opposite the resistor track  42 . This means that the particle contact  36  is situated between the resistor track  42  and the contact track  44 .  
      With respect to their magnetic dipole moments, both permanent magnets  32  and  56  are polarized such that they respectively attract the particle contact  36  toward the resistor track  42  and the contact track  44  in the direction of the arrows  40  and  60  and press the particle contact against said tracks. This results in a very good electric contact between the particle contact  36  and the resistor track  42  and the contact track  44 . In contrast to  FIGS. 1 and 2 , the magnetic field  38  generated by two permanent magnets  32  and  56  is stronger and more homogenous in the region of the particle contact  36 .  
       FIG. 4  shows an alternative filling level sensor  6  that does not require a lever arm  12 .  FIG. 4   b  shows a side view of the filling level sensor, and  FIG. 4   a  shows a section along the line IVa-IVa. The base plate  8  and the housing cover  48  form a cylindrical guide member  62  with a longitudinal center axis  64 . The float  14  following the filling level  26  of the fuel  20  has the shape of a hollow cylinder that encompasses the guide member  62  and can be axially displaced along its longitudinal center axis  64 . A not-shown axial guide prevents the float  14  from being turned relative to the guide member  62  in the circumferential direction. Both permanent magnets  32  and  56  are respectively fixed or installed, cast, foam-encased or the like directly in the float  14 . These two permanent magnets are displaced relative to the guide member  62  in the axial direction as they follow the filling level  26  together with the float.  
      A straight, axially extending cavity  50  of cuboid shape is formed in the interior of the guide member  62  on the boundary surface between the base plate  8  and the housing cover  48 . Corresponding to  FIG. 3 , the resistor track  42  is applied on the housing cover  48  and the contact track  44  is applied on the base plate  8 , e.g., by means of a screen printing method. In this case, both tracks extend straight in the axial direction of the guide member  62  and lead to the terminals  46   a, b . The particle contact  36  is arranged in the cavity between the tracks and forms the potentiometer  10  together with said tracks. The magnets  32 ,  56  fix the particle contact  36  between themselves such that it follows the magnets during the axial movement of the float  14 . This causes the particle contact to change its axial position between the contact track  44  and the resistor track  42  such that the electric resistance tapped at the terminals  46   a, b  is changed in dependence on the filling level  26 .  
      Since the cavity  50  is hermetically sealed relative to the fuel  20 , the electric potentiometer  10  is also protected from fuel  20 , fuel vapors and other harmful substances although the guide member  62  is partially immersed in the fuel  22 .  
       FIGS. 5 and 6  show an alternative embodiment of a filling level sensor  6  that operates in accordance with the principle shown in  FIG. 4 . The guide member  62  has an H-shaped cross section in this case and is encompassed in a U-shaped fashion by a float  14  that is adapted to this cross section. Two flanges  66  and  68  are arranged on the guide member  62 , namely on the open side of the U, i.e., on the rear side  70  of the guide member  62 . These flanges serve for solidly and securely mounting the fuel level sensor  6  on the wall  24  of the vehicle fuel tank  4  or on other not-shown internal brackets.  
      The float  14  essentially consists of two float members  72   a, b  that are arranged diametrically opposite referred to the guide member  62 , wherein a clamp  74  is used for holding together and for guiding the float members on the guide member  62  in the axial direction thereof, i.e., along the longitudinal center axis  64 .  
       FIG. 6  shows that the guide member  62  is again essentially composed of the base plate  8  and the housing cover  48 . The approximately cuboid or plate-shaped housing cover  48  is connected to the center limb  74  of the H-shaped base plate  8  that carries the contact track  44 , e.g., by means of welding. Analogous to  FIGS. 3 and 4 , these two parts form the cavity  50  for accommodating the particle contact  36 .  
      The housing cover  48  carries the resistor track  42 , the terminal  46   a  of which leads outward via a hermetically sealed contact pin  76 . Both permanent magnets  32  and  56  are embedded in the float members  72   a, b  diametrically opposite one another analogous to  FIG. 4   a . The particle contact  36  is situated in the cavity  50  between the permanent magnets  32  and  56  and is axially displaced along the longitudinal center axis  64  together with the magnets and the float  14  in dependence on the filling level  26 .  
      Naturally, the float  14  in the embodiment shown in  FIGS. 5 and 6  may also be realized in one piece. In this case, the clamp  74  is not required and can be eliminated.  
      Limit stops  78  are integrally formed onto the base plate and prevent the float  14  from disengaging from the guide member  62  in the direction of the arrow  80 . The mobility of the float  14  in the other direction is limited by the wall  24  of the fuel tank  4 .