Abstract:
A liquid level sensor employs a set of axially displaced magnetic sensing switches and a magnet on a float that may rise and fall on liquid level to activate and deactivate the switches. The switches and magnet are configured so that movement of the magnetic float activates a new switch before deactivation of an adjacent previously activated switch. An electronic circuit provides a signal based on the uppermost activated switch to provide a signal that is monotonic with liquid level. The design may be readily implemented using reed switches and a resistive ladder.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This Non-Provisional Application is national phase of International Application Number PCT/US2009/045220 filed May 27, 2009, and claims benefit to U.S. Provisional Application Ser. No. 61/057,073 filed May 29, 2008 hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to fluid level sensors, particularly sensors that may be suitable for use in monitoring urea levels in diesel engine emission control systems using “selective catalytic reduction” (SCR). 
     BACKGROUND OF THE INVENTION 
     A wide variety of liquid level sensing technologies exist including those using ultrasonic signals, changes in capacitance, and changes in electrical resistance. One type of liquid level sensor employs magnetic reed switches arranged along a column. The reed switches close in the proximity of a magnetic field which may be produced by a magnet held in a float rising and falling with the liquid level. In order to provide a continuous output signal that changes monotonically (that is, always increases and never decreases as the liquid level rises and always decreases without increasing as the liquid level falls), multiple floats are used, each constrained to move within a small range. As liquid rises, the lowest float is moved upward switching its associated reed switch and is held upward against a stop as the liquid rises pass it to a second float. The stops ensure that the lower floats never rise past the switching point of their associated reed switches such as would permit their associated reed switches to open and erroneously suggest a fall in liquid height when liquid is in fact rising. A drawback to this design is the cost and reliability problems associated with a multiple float system. 
     SUMMARY OF THE INVENTION 
     The present invention employs a single float moving past a series of reed switches. The reed switches are attached to a resistive ladder to provide an increasing voltage as the float rises and different reed switches are closed. An improved magnet design in the float and a staggering of parallel reed switches is used to obtain a “make before break” operation of the reed switches allowing an expanded operating range of a single float without the possibility of non-monotonic behavior in the output signal. 
     Specifically then, the present invention provides a liquid-level sensor having a guide that may extend along an axis in a vertical direction in a liquid holding tank. A float is positioned to move along the guide with changes in liquid height within the tank wherein the float includes at least one magnetic element applying a magnetic field to the guide, the magnetic field moving along the guide with movement of the float. The guide has a set of axially-spaced magnet sensors switching between an unactivated state and an activated state upon exposure to the magnetic field and the sensors are positioned along the guide so that movement of the magnetic float activates a new sensor before deactivation of an adjacent previously activated sensor. The sensor further includes an electrical circuit communicating with the magnet sensors to provide a signal being a function only of the state of an uppermost activated sensor. 
     It is a feature of at least one embodiment of the invention to provide a simple circuit that produces a monotonic signal with movement of the float. Monotonic in this sense means that the signal consistently increases or decreases as the height of the float increases or decreases and that the signal never both increases and decreases when the float is moving in one direction. 
     The sensors may be normally open magnetic reed switches. 
     It is therefore a feature of at least one embodiment of the invention to provide a low-cost and robust sensor system that works effectively with low-cost reed switches. 
     Each sensor may be a set of multiple magnetic reed switches wired in parallel and each reed switch may be displaced axially from other reed switches of the sensor. 
     It is therefore a feature of at least one embodiment of the invention to provide an effective reed switch having a larger range of activation than can be readily obtained by under the constraints of the reed switch physical design. 
     The axial spacing of the multiple magnetic reed switches of each sensor may be different from the axial spacing old magnetic reed switches between sensors. 
     It is therefore a feature of at least one embodiment of the invention to provide the ability to flexibly adjust the number of discrete levels that may be distinguished by the sensor and the axial separation among the levels for practical magnet design. 
     The electrical circuit may be a resistor ladder of series-connected resistors defining connection nodes and wherein the sensors connect the nodes to ground when activated and wherein the signal is a voltage at a node. 
     It is therefore a feature of at least one embodiment of the invention to provide a simple circuit that may be sensitive only to the uppermost activated sensor. 
     Each reed switch may have a principle axis canted with respect to the axis of the guide. 
     It is therefore a feature of at least one embodiment of the invention to permit the use of multiple closely spaced reed switches in a narrow guide form facto with long magnetic reed switches having improved sensitivity. 
     The magnet element may be a pair of axially separated toroidal magnets having axially opposed magnetic fields, the ring magnets positioned around the guide and within the float. 
     It is therefore a feature of at least one embodiment of the invention to provide a magnet element producing a magnetic field having an extended axial activation length. 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified cross-sectional view of the present invention showing a magnet float having two opposed toroidal magnets positioned to move along a central guide holding staggered reed switches; 
         FIG. 2  is a detailed schematic representation of two banks of reed switches showing a “make before break” behavior between successive banks of reed switches; 
         FIG. 3  is a simplified schematic of the resistor ladder used to produce a voltage rising with rising liquid level as reed switches close; 
         FIG. 4  is a plot of voltage versus distance showing monotonic output provided by the present invention; and 
         FIG. 5  is a fragmentary cross-section through the shaft of  FIG. 1  showing a canting of the magnetic reed switches with respect to a shaft axis. 
     
    
    
     Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , a liquid level sensor  10  includes a mounting bracket  12  fixing a central shaft  14  in vertical orientation in a tank  16  holding a liquid  18  such as urea. 
     A float  20  rests in the liquid  18  to move freely up and down about the shaft  14  with respect to the mounting bracket  12 . The float  20  may be hollow to provide an inner air chamber sealed against the liquid  18  or maybe made of a buoyant material that resists the liquid  18 . In either case, the float  20  is designed so that its density is less than that of the liquid  18  so that the float  20  is sufficiently buoyant so that a portion rises above the surface of the liquid  18  and so that the entire float  20  rises and falls to track the surface of the liquid  18  and thus the height D of the liquid from an arbitrary reference point (typically a bottom of the tank  16 ). 
     The float  20  may incorporate two toroidal magnets  21  forming a magnet element  19  positioned about the shaft  14  and separated axially along the shaft  14 . The float  20  and the toroidal magnets  21  made thus present a central hole that receives the shaft  14  with the float  20  surrounding the shaft  14  to be guided thereby. 
     Referring also to  FIG. 5 , the shaft  14  may include a set of magnetically activated switches  24  comprised each of a set of reed switches  22 . The magnetically activated switches  24  may be protected from the liquid  18  by an outer non-magnetic tubular housing  44  sealed at its lower and upper ends to provide a cylindrical protected inner volume holding the magnetically activated switches  24 . 
     Referring now to  FIG. 2 , the reed switches  22  may be normally open reed switches which close under the influence of a magnetic field. The reed switches  22  may be collected into banks to form the magnetically activated switches  24 , for example, magnetically activated switch  24   a  and magnetically activated switch  24   b  separated actually along the shaft  14 . Within each magnetically activated switch  24 , the reed switches  22  (labeled A, B, and C) may be staggered axially along the shaft  14  to have an intra-switch separation distance  23  within each magnetically activated switch  24  and an inter-switch separation distance  25  between adjacent magnetically activated switches  24 . Each of the reed switches  22  within one magnetically activated switch  24  are connected in parallel so that any one of the reed switches  22  closing creates an electrical short across the magnetically activated switch  24 . This staggering increases the effective operating range of the magnetically activated switch  24  beyond the operating range of an individual reed switch  22 . 
     Referring still to  FIG. 2 , the toroidal magnets  21  of the magnet element  19  have opposed poles to increase the magnetic intensity for switching the reed switches  22  into a zone between the toroidal magnets  21 . In particular, the north faces of the toroidal magnets  21 , being a lower base of the upper toroidal magnets  21  and an upper face of the lower toroidal magnets  21  are opposed to each other along the axis of the shaft  14 . 
     The magnetically activated switches  24  are spaced apart by inter-switch separation distance  25  to define detection levels for a desired range of liquid heights D. The number of reed switches  22  in each magnetically activated switch  24  and the intra-switch separation distance  23  are set to ensure continuous activation of at least one reed switch  22  throughout the entire range of travel of the float  20 . In particular, the reed switches  22  are spaced apart by intra-switch separation distance  23  such that the magnet element  19  activates a reed switch  22  before deactivation of an adjacent previously reed switch  22 . In this way, for example, when the float  20  is falling, at least one reed switch  22 ′ of a lower magnetically activated switch  24   a  will be closed before, or at least simultaneously with, the opening of the last closed reed switch  22 ″ of adjacent upper magnetically activated switch  24   b  in a “make before break” operation. As will be seen, this provides for monotonic output of the liquid level sensor  10 . 
     Referring now to  FIG. 3 , a readout circuit for the liquid level sensor  10  of the present invention may produce a signal dependent only on the highest most magnetically activated switch  24  having one of its reed switches  22  activated. This may be done with the use of a resistor ladder  29  comprising a set of series connected resisters  30  defining interconnecting nodes  31  and having known voltage connected across the resistor ladder  29 , for example, having an upper end attached to a voltage (e.g. 12 V, 24 V, etc.) and a lower end attached to ground. 
     An upper node  31 ′ may provide an output voltage connected to a tab  36  that may connect to automotive instrumentation or the like such as a gauge or computer input. Each of the lower nodes  31  may be connected to one of the magnetically activated switches  24  such that when the magnetically activated switch  24  is activated they connect the given node  31  to ground. In this way, as the float  20  rises to activate successively higher magnetically activated switches  24 , resisters  30  are bypassed, generally decreasing the voltage at node  31 ′ as a function of float height. Generally the upper two resisters  30  will have different values from the remaining resisters  30 , these upper two resisters defining the lowest value of voltage obtained with the float  20  at its uppermost position and the range of voltages as the float  20  travels its full distance. Resistors  30  (beneath the upper two resisters  30 ) may be of uniform or different values so as to provide an arbitrary function relating float height to voltage. 
     Referring now to  FIG. 4 , the configuration of the magnetically activated switches  24  in  FIG. 3  provides for a stair stepped monotonic voltage output  38  at output tab  36  rising with the height D of the liquid  18 . The “make before break” feature prevents a voltage dip  40  such as would create spurious level artifacts suggesting a liquid level drop as the float  20  rises between magnetically activated switches  24 . The total staircase height and the height of each stair step may generally be adjusted by changing the values of the resisters  30 . The functional relationship between voltage and distance D may be varied by varying the resistance values of resistors  30  and/or by changing the spacing and number of the reed switches  22 . 
     Referring now to  FIG. 5 , reed switches  22  of arbitrary length may be fit within a compact shaft  14  defined by tubular housing  44  by canting axes  46  of the reed switches  22  with respect to the axis  50  of the shaft  14 . This allows overlap of the reed switches  22  in the axial direction without displacing their centers from the axis  50 . The reed switches  22  may be positioned and attached to a printed circuit board  42  (typically epoxy fiberglass or other nonmagnetic material) held within the tubular housing  44  according to techniques well known in the art. 
     It will be understood that other magnetic sensors such as Hall effect devices or magnostrictive devices may be used in lieu of the reed switches  22 . 
     Variations and modifications of the foregoing are within the scope of the present invention. It is understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention. The claims are to be construed to include alternative embodiments to the extent permitted by the prior art. 
     Various features of the invention are set forth in the following claims.