Abstract:
An inductive liquid-level sensor employs a central shaft supporting multiple independent electrical loops that may be interrogated to detect the proximity of the conductive element in the float rising and falling with changes in liquid height outside of the shaft.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This Non-Provisional Application claims benefit to U.S. Provisional Application Ser. No. 61/085,266 filed Jul. 31, 2008 hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to liquid-level sensors, particularly sensors that may be suitable for use in monitoring urea levels in diesel engine emission control systems such as those using “selective catalytic reduction” (SCR). 
         [0003]    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 a loop, for example contained within a hollow shaft extending into the liquid. A float with a metal slug may slide up and down along the shaft with changes in the liquid height. The location of the metal slug along the length of the loop changes the inductance or mutual inductance of the loop which may be measured and converted into a voltage or digital signal indicating the height of the liquid. 
         [0004]    Such inductive systems can require careful calibration in order that a given inductance value of the loop match a predetermined float height under normal manufacturing variations. Differences in the measurement environment, wear, or damage that change the calibration, can result in measurement errors. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides an inductive liquid height sensor employing a set of electrically independent loops each of which may separately sense the proximity of a metallic ring in a float. By interrogating the loops, the location of the float and thus the liquid height may be readily determined without the need for precise measurements of inductance. By reducing the inductance measurement to a simple threshold detection, the sensitivity of the circuitry to manufacturing variations and tuning drift is greatly reduced. 
         [0006]    Specifically then, the present invention provides a liquid-level sensor having a shaft that may extend along an axis in a vertical direction in a liquid holding tank and a float positioned to move along the shaft with changes in liquid height within the tank. The float may include at least one conductive element for interaction with a fluctuating electrical field. The shaft has a set of axially-spaced, electrically independent conductive loops positioned so that different loops are proximate to the conductive element for different liquid heights, and an oscillator circuit may be connected to the electrically independent conductive loops to apply an oscillator signal to the loops producing a fluctuating electrical field proximate to the loops. A demodulator circuit connected to the electrically independent conductive loops detects a change in current flow caused by interaction of the conductive element with a fluctuating electrical field for a given loop and a decoder circuit communicating with the demodulator circuit provides a liquid-level output signal based on this detection and indicates a liquid-level height. 
         [0007]    It is thus a feature of at least one embodiment of the invention to provide a robust liquid-height sensor. The use of inductive sensing allows a complete encapsulation of the electrical circuit for protection against possibly caustic liquids while the use of multiple independent loops greatly reduces sensitivity of the sensor to manufacturing tolerances and loss of calibration. 
         [0008]    The set of axially-spaced, electrically independent conductive loops may be traces on the surface of a printed circuit board. 
         [0009]    It is thus a feature of at least one embodiment of the invention to provide a simple method of fabricating the complex loop structure. 
         [0010]    The oscillator circuit may sequentially apply the oscillator signal to the loops and/or the demodulator circuit may sequentially detect the current flow in the loops. 
         [0011]    It is thus a feature of at least one embodiment of the invention to provide a method of determining the location of the conductive element in the float by evaluating a time in a sequence at which the change in inductance occurs. 
         [0012]    The oscillator circuit and demodulator circuit may synchronously apply the oscillator signal to the loop and detect the current flow in the loop. 
         [0013]    It is thus a feature of at least one embodiment of the invention to reduce cross coupling between the loops permitting closer loop spacing and/or greater sensitivity in the detection of the conductive element. 
         [0014]    The liquid-level sensor may include an analog multiplexer/demultiplexer circuit selectively connecting the oscillator and demodulator to apply the oscillator signal to individual loops and to detect changing current flow in the same loops according to a sequencer circuit and wherein the sequencer circuit communicates with the demodulator circuit to provide the liquid level output. 
         [0015]    It is thus a feature of at least one embodiment of the invention to make use of standard integrated circuit components for the sequential interrogation of the independent loops. 
         [0016]    The liquid-level sensor may further include multiple multiplexer/demultiplexer circuits connected to a common multiplexer/demultiplexer, the multiple multiplexer/demultiplexer circuits connected to lower order bits of the sequencing circuit and the common multiplexer/demultiplexer connected to higher order bits of the sequencing circuit to provide a single output to the demodulator. 
         [0017]    Thus is thus a feature of at least one embodiment of the invention to permit scaling of the invention to an arbitrary number of independent loops. 
         [0018]    The conductive element may be a toroid fitting around the shaft to slide therealong. 
         [0019]    It is thus a feature of at least one embodiment of the invention to provide a system that minimally constrains the conductive element while ensuring consistent response. 
         [0020]    The float may be retained by the shaft to move therealong. 
         [0021]    It is thus a feature of at least one embodiment of the invention to make use of inductive coupling to allow a slide fitting between the float and shaft permitting a simplified construction of the sensor where the float is guided by the shaft. 
         [0022]    The conductive element may be sealed within the float material. 
         [0023]    It is thus a feature of at least one embodiment of the invention to provide a sensor system that may use a metallic conductive element in a corrosive environment by sealing it within, for example, a polymer float material resistant to the caustic substance. 
         [0024]    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 
         [0025]      FIG. 1  is a perspective view of the sensor system of the present invention showing a central shaft supporting a float which may move up and down on the shaft as a function of liquid height; 
           [0026]      FIG. 2  is a cutaway through the float and the central shaft to reveal an inductance ring within the float and a printed circuit board holding electrically independent loops within the central shaft; 
           [0027]      FIG. 3  is an elevational view of the printed circuit board showing the trace pattern; 
           [0028]      FIG. 4  is a fragmentary schematic showing the use of multiplexer units to separately excite and interrogate the loops of  FIGS. 2 and 3 ; and 
           [0029]      FIG. 5  is a plot of a signal from one of the independent loops demodulated to produce a location pulse. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    Referring now to  FIG. 1 , a sensor  10  according to the present invention may have a central cylindrical shaft  12  adapted to extend downward into a liquid  14  held in a tank  15 , the liquid having a liquid height  16 . The present invention contemplates that the liquid  14  may, for example, be a solution of urea forming part of an emission control system for diesel engines. 
         [0031]    Referring also to  FIG. 2 , a float  18  having a central cylindrical bore  19  may fit about the cylindrical shaft  12  to slide freely in a vertical direction  20  (as depicted) to rise and fall with changes in the liquid height  16 . The material of the float  18  may encapsulate a conductive material  26  embedded therein so that the conductive material  26  is shielded from the liquid  14 , the conductive material  26  preferably being in the shape of a ring with the opening of the ring  26  being concentric with the bore  19 . In this regard, the material of the float  18  may be, for example, a closed cell or encapsulated foam material having a density such that the overall density of the float  18  including the conductive material  26  is less than the liquid  14 . 
         [0032]    The conductive material  26  is preferably constructed of a ferromagnetic material such as steel; however, the conductive material  26  may be any material such as will inductively couple to electric fields from the shaft  12  as will be described. 
         [0033]    The shaft  12  may be constructed of a liquid-impermeable material, for example, an injection molded thermoplastic and may incorporate therein a printed circuit board  28  extending vertically along its length. The printed circuit board  28  supports multiple electrically independent conductive loops  30  on its front surface communicating with electronic circuitry  31  on an upper portion of the printed circuit board  28  with in a mounting head  22  of the sensor  10 . The electronic circuitry  31  of the mounting head  22  may communicate with other components by means of power and signal cabling  24 . 
         [0034]    Referring to  FIG. 3 , the electrically independent conductive loops  30  may be formed, for example, by conductive traces  32 . The traces  32  may form a set of rectangular conductive patterns  34  on the front surface of the board  28 , the patterns  34  open at an upper right-hand corner where they join by a conductive via to separate traces  38  on the rear side of the printed circuit board  28  that lead to the electronic circuitry  31  in the mounting head  22 . The patterns  34  each share a common return conductor  36  attached to the ends leading from the conductive via, the common return conductor  36  passing vertically along the edge of the printed circuit board  28 . 
         [0035]    Referring now to  FIG. 4 , each of these traces  38  may be received by terminals of both of a corresponding first and second terminal set of an analog multiplexer/demultiplexer  40 . The first terminal set is labeled  1 Y 0 - 1 Y 3  and the second terminal set is labeled  2 Y 0 - 2 Y 3 . These terminal sets may be selectively and independently connected to terminals  1 Z and  2 Z according to binary signals received at inputs S 0  and S 1 . In this way, the analog multiplexer/demultiplexer  40  forms a double pole, quadruple throw switch, one poll associated with each of the terminals  1 Z and  2 Z and a different respective terminal set  1 Y 0 - 1 Y 3  and  2 Y 0 - 2 Y 3 . 
         [0036]    The binary signals at inputs S 0  and S 1  may be received from a clock  41 , for example, being an oscillator and digital counter of the type well known in the art. In particular, the binary signals at inputs S 0  and S 1  may be received from two low order bits of the output C 0  and C 1  of the clock  41 . Thus for outputs of the clock  41  ranging in binary value from 0-4, different ones of terminals  1 Y 0 - 1 Y 3  will be connected to  1 Z and corresponding different ones of terminals  2 Y 0 - 2 Y 3  will be connected to  2 Z. An analog multiplexer/demultiplexer  40  suitable for use with the present invention is manufactured by Philips Semiconductors under the trade name 74HC4052 Dual Four-Channel Analog Multiplexer, Demultiplexer. 
         [0037]    Terminal  1 Z may be connected to an oscillator  42  through impedance  44  so that with each connection to a terminal  1 Y 0 - 1 Y 3 , a high-frequency electrical signal is attached to one of the loops  30  associated with the particular terminal  1 Y 0 - 1 Y 3 . This signal is likewise received at terminals  2 Y 0 - 2 Y 3  which sequentially connect to terminal  2 Z providing sensed oscillator signal  47  as modified by the loops  30 . This sensed oscillator signal  47  is in turn is attached to a terminal  1 Y 0  of a second analog multiplexer/demultiplexer  46  whose output terminal  1 Z is connected to a demodulator  48 . The output of the demodulator  48  provides a detection signal  50  indicating whether the loop receiving the oscillator signal is proximate to the conductive material  26 . 
         [0038]    Referring now to  FIG. 5 , generally the sensed oscillator signal  47  will have an envelope  52  exhibiting a dip  54  when the sensed oscillator signal  47  is received from a loop  30   d  (for example) proximate to the conductive material  26 . This dip  54  is caused by absorption of energy from the oscillating field by the conductive material  26 . 
         [0039]    This sensed oscillator signal  47  may be compared to a fixed threshold level  56  by the demodulator  48  which converts the dip  54  into a positive-going pulse  58  that forms the detection signal  50 . The detection signal  50  in conjunction with the values of S 0  and S 1  identify the particular loop  30   d  from the other loops  30  attached to the analog multiplexer/demultiplexer  40 . 
         [0040]    Generally, more than four loops  30  will be used and these additional loops  30  (not shown) may be incorporated by using additional multiplexer/demultiplexers  60  and  62 . These additional multiplexer/demultiplexers  60  and  62  are connected analogously to analog multiplexer/demultiplexer  40 , also receiving signals from clock bits C 0  and C 1  at corresponding inputs S 0  and S 1  and receiving the oscillator signal at their terminals  1 Z. The terminals  2 Z of the multiplexer/demultiplexers  60  and  62  connect to terminals  1 Y 1  and  1 Y 2  of analog multiplexer/demultiplexer  46 . 
         [0041]    The inputs S 0  and S 1  of the second analog multiplexer/demultiplexer  46  are connected to higher order bits C 2  and C 3  of the clock  41 . It will be understood therefore that under the steady cyclic counting of the clock  41 , each of the analog multiplexer/demultiplexers,  40 ,  60 , and  62 , cycles through a corresponding set of loops  30  and one of the signals from the analog multiplexer/demultiplexers,  40 ,  60 , and  62 , is connected via analog multiplexer/demultiplexer  46  with the demodulator  48 . In this way, the detection signal  50  and the outputs from the counter C 0 -C 3  uniquely identify the location of the conductive material  26  proximate to a given loop  30 . 
         [0042]    The detection signal  50  and the outputs from the clock  41  can be provided, for example, to a microprocessor that may be used to interpret this signal as desired. 
         [0043]    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. 
         [0044]    Various features of the invention are set forth in the following claims.