Abstract:
An ultrasonic liquid-height detector employs conductive rods extending from a housing, and corresponding transducers, into a liquid, where elastic seals surround the rods at their exit point from the housing. Acoustic crosstalk between the rods through the seals and the housing (rather than through the liquid) is minimized by displacing the seals from each other as measured through the housing to increase a path length through the housing so that the crosstalk energy is delayed with respect to the direct energy indicating liquid height. A sampling window is positioned to sample transmitted sound across the rods at a time before the arrival of the crosstalk energy.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application 61/354,855 filed Jun. 15, 2010 and hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a fluid level sensor and, more particularly, to a fluid level sensor that provides improved operation over a wide temperature range. 
       BACKGROUND OF THE INVENTION 
       [0003]    One popular type of liquid level sensor employs an ultrasonic transmitter and receiver opposed across a gap. When a liquid level rises to fill the gap, the improved acoustic coupling between the transmitter and receiver can be detected. 
         [0004]    Some disadvantages to this design include the manufacturing complexity of accurately positioning ultrasonic transducers in proper alignment within the gap and limitations to the operating temperature of the sensor imposed by temperature sensitivity of the transducer materials. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides an ultrasonic liquid level sensor that places the ultrasonic transmitter and receiver within a protective housing and uses conductive rods to communicate the ultrasonic signals into the liquid for measurement. A problem of crosstalk between the conductive rods through the housing, inherent in a remote transducer design, is accommodated by construction of the housing so that the ultrasonic path length through the housing is different from that of the ultrasonic path length through the liquid. Special circuitry is used to sample the transmitted ultrasonic signal at a time period free from crosstalk as a result of these differences in ultrasonic path lengths. One embodiment of the invention also provides an isolating chamber for the conductive rods that reduces entrained air within the liquid being sensed. The chamber communicates with a larger reservoir of the liquid through a small port that minimizes the transfer of liquid between the isolating chamber and the larger reservoir and that may include a baffle selectively excluding air bubbles. 
         [0006]    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 
         [0007]      FIG. 1  is a simplified diagram of the liquid level sensor of the present invention and its associated detector circuitry showing a direct ultrasonic path through the sensed liquid and crosstalk ultrasonic path through a housing of the sensor as mitigated by a slot in the housing; 
           [0008]      FIG. 2  is a block diagram of the detector circuitry as may be implemented on a microcontroller or the like; 
           [0009]      FIG. 3  is a simplified plot of detected ultrasound versus time showing a sampling window used to eliminate crosstalk; 
           [0010]      FIG. 4  is a cross-sectional view of a baffle used to reduce the effects of entrained air; 
           [0011]      FIG. 5  is a figure similar to that of  FIG. 1  showing the reduction of crosstalk by unequal housing leg lengths; and 
           [0012]      FIG. 6  is a figure similar to that of  FIGS. 1 and 5  showing reduction of crosstalk by displacement of supporting seals away from the transducers. 
       
    
    
       [0013]    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 
       [0014]    Referring now to  FIG. 1 , an ultrasonic liquid level sensor  10  may include a sensing unit  12  positioned near a liquid  14  whose height is to be measured and sensing circuitry  16  for producing and processing signals transmitted to and received from the sensing unit  12 . 
         [0015]    The sensing unit  12  may include a housing  17  typically of an electrically insulating thermoplastic or the like, for example, glass-filled nylon, providing a protective enclosure for a transmitting transducer  18  and a receiving transducer  20  operating at ultrasonic frequencies. Such transducers may be, for example, piezoelectric materials operating to receive an electrical signal to produce an ultrasonic vibration or when stimulated by an ultrasonic vibration to produce a corresponding electrical signal. 
         [0016]    The transducers  18  and  20  may communicate via spring contacts  22  with a printed circuit board  25  in turn providing connecting leads  21  to the sensing circuitry  16 . The connecting leads  21  provide an electrical signal to the transmitting transducer  18  from the sensing circuitry  16  and communicate electrical signals generated by the receiving transducer  20  to the sensing circuitry  16 . Sensing circuitry  16  may in turn communicate a signal indicating a liquid height  29  via a serial data or a simple logic level over lead  23  with equipment (not shown) requiring information about liquid height  29 . 
         [0017]    Each of the transducers  18  and  20  may be coupled to a corresponding conductive rod  24  and  26 , for example, stainless steel, extending along an axis  30  into a liquid  14  whose height is to be measured. The conductive rods  24  and  26  are spaced apart across a gap  27  which will contain the liquid  14  when the liquid  14  is at a predetermined liquid height  29  and will be absent of liquid  14  when the liquid  14  is below the predetermined liquid height  29 . 
         [0018]    During operation of the ultrasonic liquid level sensor  10 , the conductive rod  24 , as driven by transmitting transducer  18 , will conduct extensional ultrasonic waves along the axis into the liquid  14 . It will be appreciated that alternative vibration modes may also be used including longitudinal, torsional and shear waveforms. Typically the waves will be implemented as periodic pulses having a predetermined pulse time. When liquid  14  is within the gap  27 , the ultrasonic waves will be conducted to conductive rod  26  and in turn to the receiving transducer  20  where it will be detected. 
         [0019]    This “direct” ultrasonic path  32  thus proceeds from transmitting transducer  18  through conductive rod  24 , through the gap  27  through the liquid  14  into conductive rod  26  and to receiving transducer  20 . As will be described, the presence of the liquid  14  above the predetermined liquid height  29  is deduced within the sensing circuitry  16  by comparison of the amplitude of the signal to a predetermined threshold at a predetermined time. 
         [0020]    In order to protect the transducers  18  and  20  from the liquid  14  as well as environmental contaminants, the housing  17  may fully enclose the transducers  18  and  20  and the conductive rods  24  and  26  may pass out of the housing  17  through apertures  34  sealed, for example, by O-ring seals  36  fitting tightly between inner walls of the apertures  34  and the outer diameters of the conductive rods  24  and  26 . The O-ring seals  36  may, for example, fluoroelastomer material which remains pliant at low temperatures. 
         [0021]    The present inventors have determined that such seals provide an alternative “crosstalk” ultrasonic path  37  from the first transducer  18  and first conductive rod  24  through seal  36  associated with conductive rod  24  through the housing  17  to the seal  36  associated with conductive rod  26  and then through conductive rod  26  to transducer  20 . Particularly at cold temperatures when the seals  36  become stiff and the strength of the ultrasonic signal along the direct ultrasonic path  32  between the rods  24  and  26  through the liquid  14  may decrease, the ultrasonic signal along the crosstalk ultrasonic path  37  can obscure the true value of the ultrasonic signal along the direct ultrasonic path  32  and thus adversely affect the functionality or sensitivity of the ultrasonic liquid level sensor  10 . 
         [0022]    Accordingly, the housing  17  and the location of the seals  36  are designed so that the direct ultrasonic path  32  is substantially different (preferably shorter) than the crosstalk ultrasonic path  37  where the path length is the physical length of the path weighted by the average speed of sound of the materials of the path, for example dividing the path length by the average sound speed. As a result, the signal received through the direct ultrasonic path  32  may be isolated in time from the signal received through the crosstalk ultrasonic path  37  by the sensing circuitry  16 . 
         [0023]    Specifically, in the embodiment of  FIG. 1 , the housing  17  provides sleeves  33  which extends along a proximal portion of the length of the rods  26 , where the sleeves  33  are separated by a slot  35 . The seals  36  are displaced to the ends of the sleeves  33  with the slot  35  ensuring that there is no direct path through the housing  17  between the seals  36  but rather only the circuitous path around the slot  35 . Generally, the wave group velocity of the material of the plastic housing  17  will be one third of that of the metal rod  24 . Negligible ultrasound energy is conducted from the plastic housing  17  into the liquid  14 . 
         [0024]    Referring now to  FIG. 2 , the sensing circuitry  16  may be implemented as discrete circuitry according to techniques known in the art or as software in a microcontroller providing analog to digital converter inputs and executing a stored program to implement the functions that will now be described. During operation, the sensing circuitry  16  may implement a pulse timer  40  to generate a periodic pulse signal  41 . This pulse signal  41  is received by a pulse generator  42  which applies a signal (for example a short pulse burst of the appropriate ultrasonic frequency) to transducer  18  to produce an extensional wave through rod  24  as has been described. Acoustic signals through crosstalk ultrasonic path  37  and direct ultrasonic path  32  (if liquid  14  is present) are received by transducer  20  through rod  26  and amplified by amplifier  48  which may also include standard filtering circuitry (e.g. band pass filtering) tuned to the frequency of the pulses generated by the pulse generator  42  to produce amplified and processed signal  46 . 
         [0025]    The amplified and processed signal  46  may be received by a sampler  50  which is activated by a delay timer  44  in turn triggered by the pulse signal  41  so as to sample the processed signal  46  within a sampling window beginning as determined by the delay timer  44  and continuing a predetermined time thereafter. The length of time between triggering of the delay timer  44  and activation of the sampler  50  is such as to provide a sample  51  of the signal received along direct ultrasonic path  32  before the signal along crosstalk ultrasonic path  37  is received, the latter having a longer acoustic path length. 
         [0026]    This sample  51  of processed signal  46  is analyzed by analyzer  52  which may compare the sample  51  to a predetermined threshold  53  having amplitude between the amplitude of sample  51 , when there is no liquid  14  within the gap  27 , and the amplitude of the sample  51 , when there is liquid  14  in the gap  27 , to provide a signal  55  indicating whether the liquid  14  is at the predetermined liquid height  29 . The signal  55  may be digitized for serial transmission on lead  23  by serial transmitter  54 . 
         [0027]    Referring now to  FIG. 3 , the processed signal  46  may be sampled at time  56  after time  58  coincident with the generation of the acoustic pulse triggered by pulse signal  41 . This time  56  is set to be before a time  59  at which crosstalk will be received by transducer  20  along crosstalk ultrasonic path  37  so as to substantially eliminate the effect of crosstalk on the measurement. 
         [0028]    Referring now to  FIG. 4 , the conductive rods  24  and  26  of the sensing unit  12  may be placed within a chamber  60  held within a larger liquid containing reservoir  62 . Particularly when the liquid  14  is oil, the liquid  14 ′ within the reservoir  62  outside of the chamber  60  may include significant volumes of entrained air in the form of bubbles  64 . These bubbles  64  can reduce the acoustic coupling between the rods  24  and  26  when liquid  14  fills the gap  27  and thus are desirably reduced. This reduction may be done by limiting the communication between the chamber  60  and the reservoir  62  by means of an aperture  70  located near the bottom of the chamber  60 . The amount of liquid flowing through the aperture  70  is only that necessary to equalize the liquid levels within the chamber  60  and the reservoir  62  and thus, when the liquid level is substantially constant, even though the liquid  14 ′ is being rapidly exchanged in the reservoir  62  relatively little liquid will flow through aperture  70 . This allows the liquid  14  within the chamber  60  to eliminate entrained air through natural percolation and reduces the introduction of new air-entrained fluid from reservoir  62 . In addition, the aperture  70  may be part of a baffle port  66 . Specifically the baffle port  66  may include a plate  68  in front of aperture  70  and attached to a lower portion of the wall of the reservoir  62  requiring that liquid  14 ′ must pass upward over a lip of the plate  68  then downward through the aperture  70 . This circuitous path tends to eliminate bubbles  64 . The plate  68  may be spaced from the aperture  70  by approximately the diameter of the aperture  70 . The result is that the liquid  14  provides the same liquid height as the liquid  14 ′ but with substantially reduced air entrainment. 
         [0029]    Referring now to  FIG. 5 , the displacement in time of the signal measuring acoustic energy along the crosstalk ultrasound path  37  from the signal measuring acoustic energy along the direct path (not shown in  FIG. 5 ) may be produced not by a slot  35  but by axial displacements of the seals  36  from each other. In  FIG. 5 , this displacement is provided through equal lengths of the sleeves  33   a  and  33   b,  like those in  FIG. 1 , but by sleeves  33   a  and  33   b  extending unequal lengths along a portion of the length of the rods  24  and  26 . The seals  36  are displaced to the distal ends of the sleeves  33   a  and  33   b.  Here the difference in length of the sleeves  33   a  and  33   b  increases the crosstalk ultrasound path  37 . 
         [0030]    Referring now to  FIG. 6 , a housing  17 ′ constructed of metal may also be used with the drawback of higher sound velocity through the metal of housing  17 ′ than would be obtained through the plastic of the previously described housings  17 . This greater sound velocity may be accommodated by substantially increasing the lengths of the sleeves  33  in the design of  FIG. 1  to extend nearly the entire length of the rods  26  as sleeves  33 ′ thus effectively increasing the size of the slot  35  (and thus the crosstalk ultrasonic path  37 ) to accommodate this higher sound velocity. This increase in the crosstalk ultrasonic path  37  may be augmented by a decrease in the direct ultrasonic path  32  by substantially narrowing the gap  27  through the liquid by a curvature inward toward each other of the rods  26  at their distal ends. 
         [0031]    Various features of the invention are set forth in the following claims. It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being 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.