Abstract:
An apparatus for measuring and/or controlling the level of a liquid, specially fuel, contained in a reservoir. The apparatus comprises means supporting a pivot member inside the reservoir, the pivot member comprising a stationary body fixed relatively to the supporting means and a rotary body connected to a lever, the free end of which carries a float. The rotary body includes a permanent magnet and the stationary member houses a magnetic sensor located within the magnetic field of the permanent magnet. Angular movement of the lever produces rotation of the magnet and is detected by the magnetic sensor which emits signals to a micro processor which, in turn, transmits such signals to external processing means. The processing means correlate such signals, which are proportional to variations in the liquid level, to the volume of liquid contained in the reservoir.

Description:
FIELD OF THE INVENTION 
     This invention refers to an apparatus for measuring and/or controlling liquid levels. More particularly, this invention provides an apparatus for measuring and/or controlling liquid levels which is particularly suitable for use in underground fuel reservoirs in gasoline stations and which can be installed in existing operating reservoirs without requiring significant modifications. 
     BACKGROUND OF THE INVENTION 
     Gas stations normally have a number of underground fuel reservoirs which generally are installed about 3 feet below the ground level and include a threaded pipe at the top of the reservoir which permits access thereto for fuel level measurements. 
     Fuels have a high coefficient of volumetric expansion with temperature. This causes variations in the volume of the fuel contained therein as fuel temperature changes, even if there is no entry or exit of fluid in the reservoir. This variation could be easily mistaken as fuel input or discharge. 
     On the other hand, fuel leaks should be detected to protect the environment and personnel safety. International regulations (for instance, EPA Standard Evaluation) establish strict limits to minimum detectable leaks. These values could be as low as 0.38 liters per hour of leakage in a 30,000 liter reservoir. 
     Although fuel pumps can measure the quantity of fluid being dispended from a reservoir, pumps do not take into account the temperature of the fluid, nor the entry of fluid into the reservoir, nor possible leakage or pilfering that could take place. These requirements could be met only by high precision measuring systems enabling computerized processing of level and temperature readings carried out automatically and continuously. 
     OBJECTS OF THE INVENTION 
     An object of the invention is providing an apparatus for measuring and/or controlling the level of liquids contained in reservoirs or tanks which is highly precise, easy to install in existing reservoirs or tanks without requiring significant modifications, which consumes very little power, and which permits obtaining the detected information either through wired or wireless connections. 
     Another object of the invention is providing such apparatus having a digital electric output permitting computerized processing of the emitted signals. 
     A still further object of the invention is providing an apparatus of the above type which has few movable parts and a small volume enabling its installation in reduced spaces. 
     An additional object of the invention is providing an apparatus of the above type which in intrinsically safe for use in explosive environments and which resists corrosion and aggressive conditions of the environment. 
     BRIEF DISCUSSION OF THE PRIOR ART 
     The most primitive method for measuring fluid level employs a graduated rod having marks every 100, 200 or 500 liters which is introduced vertically in a fluid reservoir until it rests on the reservoir bottom. The rod is then extracted from the reservoir and the volume of liquid is estimated based on the height of the wet zone of the rod. This measuring method is very inexact and inconsistent in view of variations (ripples or waves) on the liquid surface. 
     Measuring methods using electric transductors are known in the art, the principal being those of capacitive or magneto-restrictive effect, and those employing pressure, ultrasound, radar or load cells. Of these, the only one that has been adapted to the requirements of a precise measuring method for underground fuel reservoirs in gas stations, is the level meter using the magneto-restrictive effect. 
     This system is very costly, requires considerable power to operate, and must be connected by means of special wiring to a remote control station having electric barriers of intrinsic safety in order that the whole system could operate in explosive environments. 
     Certain liquid level measuring apparatus employing magnetic means for detecting the position of a float are known. Amongst these, the following could be mentioned: 
     Published US Patent Application 2005/0247124 discloses a device for measuring the level of fuel contained in a vehicle tank. This device comprises a magnetic sensor of the linear type which detects changes in magnetic flow density, positioned between two polar pieces or stators, and through which magnetic flow generated by a rotatable magnet fixedly connected to a lever and a float circulates. This patent application describes a method of constructing this device for isolating the influence of external magnetic fields and contamination by magnetic particles suspended in the liquid, which are produced, inter alia by the vehicle fuel pump. The sensor is external to the magnet and positioned in a gap between the two polar pieces or stators. 
     U.S. Pat. No. 6,993,968 B2. This patent describes an arrangement similar to that of US 2005/0247124 and discloses means for preventing contamination of the magnetic device by magnetic particles suspended in the liquid. Such means comprise holes in a magnetic rotor arranged such that contaminating particles enter only through those holes and not between the rotor and the stator, since this would cause the system to lock and also alter the magnetic field. 
     European Patent EP 1450.142 A2 describes an arrangement very similar to that of US 2005/0247124. 
     U.S. Pat. No. 6,915,690 B2 also describes an arrangement very similar to that of US 2005/0247124. 
     U.S. Pat. No. 6,253,609 B1 discloses a level meter wherein a float and a lever drive a mechanical transmission which causes rotation of a magnet outside the reservoir. The magnet position is translated by undisclosed means to detect liquid level. 
     U.S. Pat. No. 6,453,741 B1 describes a level measuring apparatus comprising a magnetic coupling between a magnet immersed in the liquid and another magnet outside the liquid, the latter having an element converting rotation into electric signals (potentiometer, Hall-type sensor, etc.). 
     The above-mentioned patent documents coincide in using a magnetic field intensity sensor, such as a Hall-type sensor, and the magnitude they measure is proportional to the intensity of the magnetic field, which varies as the magnet rotates. 
     Besides, in the devices disclosed by the above-mentioned art, the sensor is positioned outside the magnet and the magnetic flux is guided by polar pieces (stators). 
     All the above arrangements depend on the intensity of the magnetic field and consequently are sensible to mechanical assembly, or aging of the magnet. 
     All of the above drawbacks make it impossible to use such arrangements for measuring the level of a liquid with high precision and through a long period of time. 
     U.S. Pat. No. 6,508,119 B2 describes an apparatus for measuring the level of fuel contained in a reservoir comprising a single magnetoresistive sensor positioned outside a magnet, close to the fuel reservoir wall and which changes its resistivity when the magnet connected to a lever and float moves. Since the single magnetoresistive sensor is positioned outside the magnet, has an electrical output which is not directly proportional to the rotation angle of the magnet. Besides, since a single sensor is used, compensation for temperature variations are very complicated. Consequently this device requires complex calibration for each case if precise measurements are to be made. On the other hand, this arrangement is not practical for existing underground reservoir installations because the external side of the reservoir wall is not accessible. 
     BRIEF SUMMARY OF THE INVENTION 
     In its broader aspect, the present invention provides an apparatus for measuring and/or controlling the level of a fluid, specially a fuel contained in a reservoir, comprising a support member positioned inside the reservoir and fixed with respect thereto, a lever having a first end connected to a float and a second end joined to a pivot member fixed to said support member such that the lever can move angularly with respect to said support member as the liquid level in the reservoir raises or lowers, said pivot member comprising a stationary body fixed to said support member and a rotatable body joined to the second end of said lever and which can rotate relatively to the stationary body around an axis; a permanent magnet mounted in said rotatable body, the magnetic field generated by said magnet being substantially perpendicular to said axis; a pair of magnetic sensors arranged in said stationary body and within the magnetic field generated by said magnet, said sensors reacting to said magnetic field and emitting signals depending on the angle of incidence of the magnetic field on the sensors, said signals being transmitted to processing means external to the reservoir, where readings could be made revealing the level of liquid in the reservoir and consequently the liquid volume of the liquid contained therein. 
     The level measuring apparatus of the invention utilizes two magnetoresistive sensors, spatially offset 90° one from the other and arranged coaxially with an annular magnet diametrically magnetized, the intensity of which is sufficient to saturate both magnetoresistive sensors. Thus, the electric output of the sensors depends on the angle of incidence of the magnetic field with respect to the sensors and is independent of the magnetization intensity of the magnet and its variations with time, and also insensitive to disturbances provoked by external magnetic fields. 
     In addition, since both sensors are integrated in the same encapsulation and have similar electric characteristics, the temperature effect is compensated because in order to calculate the angle of rotation of the magnet, the mathematical division between the two electric signals which are spatially generated at 90 degrees, is used, thus obtaining a result which is practically insensitive to changes of temperature in the sensors. 
     The invention will now be described with reference to the attached drawings which show, by way of non-limiting examples, preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an installation for controlling and/or measuring the level of fuels which employs the apparatus of the present invention. 
         FIG. 2  is a general, enlarged, partly sectional view showing in detail the metering assembly. 
         FIG. 3  is a cross section of the metering apparatus along line III-III of  FIG. 2  which shows in more detail the pivot member which connects the lever connected to the float with a tube housing a processor and its wiring. For clarity purposes, the lever is shown rotated 90°. 
         FIG. 4  is a perspective view of the float and the manner in which it is pivotally connected to the lever. 
         FIG. 5  represents, schematically, the interaction between the magnetic field generated by the magnet and the sensors. 
         FIG. 6  is a longitudinal section of the cover which supports the transceptor housing. 
         FIG. 6   a  shows an alternative embodiment where the cover is separated from the transceptor housing. 
         FIGS. 7-11  show alternative embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows schematically an installation for the control and/or measurements of the level of a liquid, particularly fuel contained in an underground reservoir which is generally cylindrical in shape and is arranged with its longitudinal axis positioned substantially horizontal (in practice, with a slight inclination to facilitate settlement of contaminants). A conduit  9  is fixed to the top of reservoir  6  and extends up to a chamber  12  covered by a removable cover  66  which can be entered from the floor of, for instance, a gas station. All these elements are part of a conventional installation. 
     The level measuring apparatus of the invention comprises a support member comprising a vertical pipe  1 , having a lower end resting on the wall of the underground reservoir  6 , and an upper end which extends into conduit  9 . The support member or pipe  1  has one or more spacers  7  (two are shown in the embodiment shown in  FIG. 1 ) which permit maintaining the pipe  1  vertically and prevent movements of the pipe which would cause it to depart from the vertical position. Pipe  1  could be of round, square or rectangular cross section. 
     A lever  2 , made of a light, rigid and strong material, for example aluminum or carbon fibers, is connected at one end to a pivot member  3  which permits the angular movement of the lever  2  with respect to a fixed point in pipe  1 . The other, free end of lever  2  is pivotally connected to a float  4  which can move angularly relative to the free end of the lever  2 . The combined weight of the lever  2  and the float  4  are selected such that the float  4  always floats, independently of the level and density of the liquid. 
     Spacers  7  comprise means (not shown) preventing rotation of pipe  1  about its longitudinal axis and maintain it in a position such that lever  2  and float  4  move in a vertical plane containing the horizontal axis of the reservoir  6 . 
     Pipe  1  is hermetically sealed to prevent liquid from entering therein; its lower end is closed with a plug  36  which rests on the reservoir wall, while its upper end terminates in an electrical connector  8  which permits transmitting the signals generated by the level meter to a wireless transceptor  47  positioned in chamber  12  by means of a cable  10 . The housing  11  of the wireless transceptor  47  is fluid-tight and hermetically sealed by a cap  15  which covers the upper end of conduit  9  and, therefore, isolates the reservoir from the outside. 
     Within the transceiver housing  11 , there is a battery  14  of the intrinsic safety type which supplies the power required for the operation of both the transceiver  47  and the measuring apparatus. Inside the housing  11  of transceptor  47  there is a suitable antenna  40  which enables the transceptor to communicate with a remote signal processing equipment  16  which has an outer antenna  13  receiving the signals emitted by the transceiver. The remote processing equipment equates  16  level readings to volume based on a calibration chart for the particular reservoir controlled by the apparatus of the invention. 
     It will be understood that upon varying the level of the liquid contained in the reservoir  6 , the float  4  will move up and down and cause rotation of lever  2  and, in turn, rotation in the pivot member  3 . 
     As can be seen in  FIGS. 2 and 3  which is a cross section of the metering apparatus along line III-III of  FIG. 2 , pivot member  3  comprises a stationary body  21  attached to pipe  1 , and a stub shaft  31  formed by a cylindrical projection extending from the stationary body  21  which rotatably supports a rotatable body  60  attached to lever  2 . Shaft  31  has, adjacent its free end, a groove  62  in which a key  22  is located to prevent disengagement of the rotatable body  60  from shaft  31 . 
     Rotatable body  60  has stepped cylindrical surfaces, coaxial with shaft  31 . A ring-shaped permanent magnet  20  is fixedly mounted at the end of rotatable body  60  proximal to stationary body  21 . Magnet  20  is surrounded by an annular yoke  200  made of magnetic material with a high magnetic saturation which concentrates the magnetic field generated by the magnet  20  and acts as a shield to prevent dispersion of the magnetic field outside the pivot member  3 . Magnet  20  is diametrically magnetized and, preferably, is made of a material having high magnetization strength and low demagnetizion with time, such as SmCo (samarium-cobalt). 
     Stationary body  21  has a cylindrical cavity  30  coaxial with shaft  31  which communicates with the inside of pipe  1  through a hole  28  in the pipe, but such cavity  30  is hermetically isolated from the outside of pipe  1 . 
     Cavity  30  houses two magnetoresistive sensors  23  and  23 ′ positioned concentrically with magnet  20  and within its magnetic field. Sensors  23 ,  23 ′ are welded to a plate  27  which is firmly attached to the stationary body  21  and fixed in position with an encapsulating adhesive  25 . Plate  27  is coupled by means of a connector  29  to a main board  26  arranged inside pipe  1 . Sensors  23 ,  23 ′ have each an electrical output which can be read through connector  29  by a processor  32  positioned on board  26 . A temperature sensor  24  is also welded to plate  27  for detecting the temperature of sensors  23 ,  23 ′ and thus compensate thermal variations of the electric signal emitted by each sensor  23 ,  23 ′. The temperature sensor  24  is also connected to processor  32  via connector  29 . 
     The magnetic field generated by the permanent magnet is very intense and uniform in the proximity of sensors  23  and  23 ′ such that it exceeds the magnetic saturation value of sensors  23 ,  23 ′ and renders insignificant the possible interference of disturbing magnetic fields foreign to the level meter. 
     Sensors  23 ,  23 ′ are of the magnetoresistive type. Suitable sensors are, for instance, those manufactured by Philips under designation KM243T, in which two sensors are located in a single capsule and are spatially offset 90 degrees one with respect to the other. However, it would be possible to use two independent sensors, offset an angle different from 90° with similar results. The magnetoresistive effect of sensors  23 ,  23 ′ when subject to an intense magnetic field  201  (see  FIG. 5 ) which exceeds their saturation value results in an electric output which is proportional to the angle of incidence of the magnetic field  201  on each sensor and not depending on the intensity of magnetic field  201 . Since sensors  23  and  23 ′ are offset 90 degrees, one relative to the other, the respective electric signals will be proportional to components X and Y of the magnetic field, which correspond with the rotation angle of magnet  20 . 
     These two electric signals emitted by sensors  23 ,  23 ′ permit calculating with great precision the angle between the lever and the support member, and the liquid level can be readily calculated as a function of such angle and the length of lever  2 . 
     Knowing the rotation angle of lever  2  and based on the length of lever  2 , the processor can calculate the level of the liquid  5  contained in reservoir  6 . Other additional calculations permit taking into consideration variations of the flotation point of the float  4  depending on whether or not lever  2  is immersed in the liquid. 
       FIG. 2  is an enlarged and partial section of pipe  1  and pivot member  3 , in which the wiring  33  inside the pipe  1  can be observed. Said wiring transmit signals calculated by the processor  32  to connector  8  and supplies power from battery  14  to the level measuring apparatus of the invention. 
     Inside pipe  1  there is an inner tube  35  which serves as a support for board  26  and temperature sensors  34 , which are spaced along pipe  1  and connected by means of conductors  37  and  38  to processor  32 . Tube  35  permits and facilitates assembly of all electrical components prior to their introduction into pipe  1  during construction of the level measuring apparatus. 
     Temperature sensors  34  emit signals read by the processor  32  and permit knowing the temperature of liquid  5  at different levels thereof, and to effect corrections taking into consideration the volume expansion or contraction of the reservoir with temperature variations. 
       FIG. 4  shows schematically the manner in which the float  4  is connected to the free end of lever  2 . As it can be seen, two arms  50 , fixed to the free end of lever  2  are connected to respective side pivots  51  provided in the float  4  which, in the embodiment shown, is prismatic in shape. It is desirable to have a large floating surface to enhance resolution of the measurement and decrease the mechanical hysteresis of the lever movement. 
     The proposed design permits that the float always exposes a constant and maximum flotation surface, independently of liquid level. 
     The width of the float is limited by the diameter of conduit  9  but its length could be quite large, because, it could rotate to a vertical position and pass through conduit  9 . 
     Alternatively, the desired results in respect to flotation area and its constancy could be attained employing a spherical float fixed to the free end of lever  2 , but in that case, the diameter of the float would be limited by the inside diameter of the conduit  9  since it has to be introduced therethrough. Besides, a spherical float is more costly than a prismatic float and the latter permits modifying its width to facilitate insertion through conduit  9 . 
       FIG. 5  is a cross section of pivot member  3  which shows schematically the interaction between the magnetic field  201  generated by the permanent magnet  20  and sensors  23 ,  23 ′. 
       FIG. 6  shows in greater detail cap  15  of conduit  9  which supports the housing  11  of transceiver  47 . The cap  15  is attached by screws  41  and nuts  42  to lugs radially projecting from an intermediate cover  43  having a central opening which is screwed to the top end of conduit  9 . The transceiver housing  11  in turn, is attached by screws  45  to cap  15 . An O-ring  44  between cap  15  and intermediate cover  43  ensures hermeticity of the assembly. 
       FIG. 6   a  shows an embodiment in which the transceiver housing  11  is separated from the conduit  9 . In this alternative embodiment, conduit  9  is closed by a cap  61  which is screwed to the top end of conduit  9  and the transceiver housing is attached to the cover  66  of chamber  12  by means of a threaded projection  69  which is screwed to a threaded cavity in a retainer member  68 . The transceiver  47  is connected to connector  8  by means of a cable  10  which passes through an opening in cap  61  which is provided with a stress-relieving member  62  to ensure fluid tightness of conduit  9 . 
     Construction of the pivot member  3  shown in  FIG. 3  also admits variations. Thus in  FIG. 7 , a stationary body  71  is fixedly joined to pipe  1  and a shaft  72  having an axial cavity  75  containing the magnetic sensors  23 ,  23 ′ is fixed to the stationary body  71  by screws  76 . An O-ring  70  provides an hermetic seal. A threaded fastener  73  permits maintaining connection between the rotatable body  74  and shaft  72 . 
       FIG. 8  shows another embodiment similar to that of  FIG. 7  wherein a stationary body  80  is joined to pipe  1 , and a shaft  81 , having an axial cavity  82  containing the magnetic sensors  23 ,  23 ′, is attached to stationary body  80  with a suitable adhesive. 
       FIG. 9  shows still another embodiment of the pivot member  3  in which a stationary body  90  is joined to pipe  1  and serves as the shaft for the rotatable body to which lever  2  is attached. The shaft is tubular and a cylindrical member  91 , having an axial cavity housing sensors  23 ,  23 ′ is inserted into the shaft bore and held in place by fastener  73 . 
       FIG. 10  shows yet another embodiment of pivot member  3  in which a stationary body  100  having an axial cavity housing sensors  23 ,  23 ′ is joined to pipe  1  and has an extension  104  in which an adjusting screw  102  is threaded. A rotatable body  103  which transmits the movement of lever  2  has a shaft  101  fixed thereto. The ends of shaft  101  are supported by respective pivots  105 ,  106  provided in the stationary body  100  and in screw  102 , respectively. 
       FIG. 11  shows an additional embodiment of pivot member  3  which is similar to that of  FIG. 7 . The stationary body  110  is attached to pipe  1  and has an integral projection defining a shaft on which a rotatable body  112  carrying the annular magnet  20  and yoke  200  is mounted. A fastener  73  prevents disengagement of the stationary and rotatable bodies. 
     It will be understood that other constructive variations are possible, which those skilled in the art could easily envision, provided such variations are within the spirit and scope of the invention, as defined in the attached claims. 
     The permanent magnet does not have to be annular. It would suffice that it creates a magnetic field perpendicular to the axis of rotation, and the magnetic sensors could react to perpendicular components of such magnetic field. 
     Although in the embodiment shown and described the signals emitted by the magnetic sensors are transmitted wirelessly to external processing means, such transmission could be wired. Wireless transmissions facilitates installation of the level meter of the invention in existing fuel reservoirs. 
     Occasionally, outside water could leak into fuel reservoirs. This water, being denser than fuel would tend to concentrate in the lower part of the reservoir. It is important to detect the existence of water and prevent it from being accidentally supplied to a vehicle together with the fuel since this could provoke serious damage to the engine. To prevent this situation, a level meter similar to that described above, but much smaller, could be installed in the lower part of pipe  1 . The float of such additional level meter would be arranged in the interface between water and fuel. 
     It will be understood that the present invention, although described and shown in respect of the measurement and/or control of fuel level in underground reservoirs, could be also employed for the measurement and/or control of other liquids in other environments, either above or below the ground level.