Abstract:
A level sensor for determining the presence or absence of a liquid in contact with the sensor includes an elongate probe, a transducer operably connected to the probe and configured to produce compressional waves in the probe, and circuitry for detecting acoustic energy emitted into the liquid when liquid is in contact with the probe. A mount for the releasably holding the sensor includes a base have a receiving region formed in part by a plurality of flexible securing fingers. The fingers have locking projections extending therefrom. A contact is mounted to the base and extends into the receiving region. A cartridge supports the level sensor and is received in the receiving region. The cartridge includes a circumferential recess for receiving the securing fingers. When the level sensor is positioned in the cartridge and the cartridge is inserted into the base, the level sensor is operably connected to the contact and the cartridge is resiliently secured in the base.

Description:
CROSS-REFERENCE TO RELATED APPLICATION DATA 
     The present application is based on International Application Number PCT/US2008/051118 filed Jan. 16, 2008, and claims priority from U.S. application No. 60/880,822 filed Jan. 17, 2007, the disclosures of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a fluid level sensor and mount. More particularly, the present invention relates to a fluid level sensor that is insensitive to mineral deposits on the surface of the sensor, and a mount for the sensor. 
     One popular type of level sensor is a capacitive type of sensor. This sensor is used, for example, in automated machines, such as automated ice making machine. In a typical capacitive type sensor, a metal electrode in the shape of a rod is mounted vertically over a water reservoir. The reservoir is filled and emptied each machine cycle to reduce the build up of dissolved solids which can give the ice a cloudy appearance. 
     A small high frequency voltage is applied to the metal electrode, and when the water level in the reservoir makes contact with the rod, the capacitance of the rod to ground changes. This change is detected in a signal processing controller, and shuts off the pump filling the reservoir. 
     This type of sensor has at least one fundamental problem. It has been observed that when the electrode or rod becomes coated with a non-conducting material, such as a calcium carbonate mineral deposit, it acts as a dielectric, adding capacitance in series with the electrode. This additional capacitance is inversely proportional to the coating thickness. As such, as the coating builds up, the additional capacitance dominates the electrode capacitance to ground, at which point sensitivity to liquid level is lost. 
     In many systems, due to the monitoring and control systems, the loss of sensitivity to liquid level is not a fail safe event. For example in many automated ice making machines, if the water level cannot be detected, water will pump into the reservoir until an overload condition—based on timing—is detected and the pump is shut off. This can render the machine inoperable until service personnel remove the deposits or replace the rod. However, it has been found that cleaning can accelerate the rate of build up on rods. 
     Accordingly, there is a need for a fluid level sensor that is insensitive to mineral deposits on the surface of the sensor. Desirably, such a sensor is an acoustic-type sensor. More desirably, such a sensor can take various shapes and configurations and can be formed from different materials to suit a desired application. Most desirably, such a sensor is supported within a holder or support that readily accepts the sensor and that precludes the need to directly hard-wire any of the sensor components. 
     BRIEF SUMMARY OF THE INVENTION 
     A level sensor for determining the presence or absence of a liquid in contact with the sensor includes an elongate probe, a transducer operably connected to the probe and configured to produce compressional waves in the probe and circuitry for detecting acoustic energy emitted into the liquid, when liquid is in contact with the probe. The probe can be formed as a rod, a strip, a tube or other appropriate shape, and can be formed from metal, polymer, ceramic or other appropriate material. 
     The probe has a wet end for contact with the liquid and a dry end for electrical contact. The transducer is mounted to the dry end. When formed as a rod, the wet end of the rod can be rounded and a collar can be mounted to the rod at about the dry end. When formed as a strip, the strip has a bend therein defining a wet leg and a dry leg such that the transducer is mounted to the dry leg. 
     A mount for the level sensor permits mounting the sensor within a system without hard-wiring the sensor to the system. The mount includes a base having a receiving region formed in part by a plurality of depending flexible securing fingers. The fingers have locking projections that project inwardly. One or more contacts are mounted inside of and to the base and extend into the receiving region. Preferably, the contacts are spring mounted to provide positive engagement between the contacts and the transducer. 
     A cartridge holds the level sensor and is configured for receipt in the receiving region. The cartridge includes a circumferential recess for receiving the securing fingers. The mount is formed from a non-electrically conductive material, such as nylon or the like. 
     A stop wall is positioned at the about the recess to prevent over-insertion of the cartridge in the base. The cartridge includes a central longitudinal opening for receiving the level sensor and a shoulder at an end thereof the cartridge opposite the recess. A seal is present at about a juncture of the shoulder and the central opening. 
     When the level sensor is disposed in the cartridge and the cartridge is inserted (snapped) into the base, the level sensor is operably connected to the contact and the cartridge is releasably locked in the base. 
     These and other features and advantages of the present invention will be apparent from the following detailed description, in conjunction with the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The benefits and advantages of the present invention will become more readily apparent to those of ordinary skill in the relevant art after reviewing the following detailed description and accompanying drawings and photographs, wherein: 
         FIG. 1  is schematic illustration of a known capacitance-type level sensor; 
         FIG. 2  shows the equivalent circuit of the capacitance probe; 
         FIG. 3  illustrates one embodiment of a level sensor (probe) in accordance with the present invention; 
         FIG. 4  illustrates a cartridge-type mount for the sensor in accordance with the present invention, showing the sensor cartridge prior to insertion into the mount; 
         FIG. 5  is a partially exploded view of the mount of  FIG. 4 ; 
         FIG. 6  is a cross-sectional view of the exploded illustration of  FIG. 5 ; 
         FIG. 7  is a cross-sectional view of the assembled mount and sensor of  FIG. 4 ; 
         FIG. 8  is a perspective illustration of an alternate embodiment of the sensor in a strip form; 
         FIGS. 9A-9C  are snap shots of spectra of a strip-formed sensor, showing the strip in a dry condition ( FIG. 9A ); with the strip positioned in oil ( FIG. 9B ), and with the strip having a drip of oil at the end thereof ( FIG. 9C ); and 
         FIGS. 10A-10B  are snap shots of spectra of a ceramic rod sensor, showing the rod in a dry condition ( FIG. 10A ) and with the rod positioned in water ( FIG. 10B ). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present invention is susceptible of embodiment in various forms, there is shown in the figures and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiment illustrated. 
     Referring briefly first to  FIG. 3  there is shown schematically, a discrete level sensor  10  embodying the principles of the present invention. The sensor  10  includes a probe  12  formed as an elongate element that can be fabricated from a variety of different materials and in a variety of different shapes. 
     The present sensor  10  is positioned in a system so as to detect the presence or absence of a liquid in contact with the probe  12 . In one exemplary application in use in an ice making machine (not shown), the sensor  10  is position to detect the present or absence of water (and thus the level of water) in a water reservoir. In this situation, the sensor  10  is continuously subjected to cycle of wetting and drying. As such, build up of, for example, mineral deposits can occur. 
     It has been observed that sensors that use thickness shear and torsional modes, can, in principle, be used to sense the presence of water. A preferred embodiment of the present sensor  10  operates on the principle that compressional waves (as indicated at  14 ), unlike shear waves, propagate in water. The present sensor  10 , as seen in  FIG. 3 , includes a probe that is formed as a rod in which compressional, flexural or rod modes are generated with a shear or compressional transducer  16  attached to an end  18  of the probe  12  opposite of the water sensing or wet end  20 . 
     These modes are trapped in the metal, ceramic or plastic (of the probe  12 ) until a fluid F touches a surface  22  of the probe  12 , at which point the out-of-plane component (as indicated at  24 ) of the wave motion converts to compressional waves. These waves then radiate into the water (as indicated at  26 ) where they dissipate. 
     In effect, the probe  12  acts as an antenna that radiates acoustic energy into the water. In that the energy loss can be substantial, the sensor is quite sensitive to water and other fluids contacting the bottom or sensing surface of the probe  12 , but is insensitive to mineral deposits on the sensor surface  22 . This is due to the nature of the compressional waves propagating in these deposits even more readily than shear. 
     Referring briefly to  FIGS. 1 and 2 , as set forth above, in prior capacitance-type level sensing systems  110  a small high frequency voltage is applied to the metal electrode  112 , and when the water level in the reservoir makes contact with the rod (shown by the lines  114  in phantom), the capacitance of the rod  112  to ground (as indicated at  118 ) changes. This change is detected in a signal processing controller  116 . 
     This type of sensor  110  has at least one fundamental problem. It has been observed that when the electrode or rod  112  becomes coated with a non-conducting material (as at  120 ), such as a calcium carbonate mineral deposit, it acts as a dielectric, adding capacitance in series with the electrode (as indicated at  122 ). This additional capacitance is inversely proportional to the coating thickness. As such, as the coating builds up, the additional capacitance dominates the electrode capacitance to ground  124 , at which point sensitivity to liquid level is lost. 
     Referring again to  FIG. 3 , unlike known sensors, the present sensor  10  can use either out-of-plane  24  to compressional mode conversion to create an acoustic antenna in the presence of fluids F or it can use a shear mode transducer  16  polarized along the axis A 12  of the sensor probe  12 , to create the out-of plane modes. 
     For example, when using a shear mode transducer  16 , the transducer  16  transmits energy into the probe  12 . The change in modes does not occur when water hits the end  20  of the probe  12 . Rather, the waves generated by (induced in) the electrode or probe  12  have nowhere to go so they bounce around in the probe. When water reaches the end  20  of the probe  12 , the waves have somewhere to go because the mode in the rod converts to compressional waves  26  in the water F. At that point, the acoustic energy starts leaking into the water, which is detected using circuitry commercially available from ITW ActiveTouch of Buffalo Grove, Ill. 
     In a present sensor system  10 , the transducer  16  is a standard shear mode transducer that is resonant at 1 megahertz. The burst frequency is 350 kilohertz which is determined by the dimensions of the rod and it&#39;s acoustic characteristics. A 1 MHz transducer is used because of it&#39;s ready availability, cost effectiveness and functionality. It is anticipated that compressional mode transducers can also be used. Shear is side to side motion, whereas compressional is more like a pressure wave. 
     The type of wave that is desirable for the level sensor  10  must be sensitive to fluid (e.g., water) to generate waves that are not sensitive to the fluid. The waves that propagate in, for example, water are compressional type waves. Compressional transducers operate on the principle that waves are formed when two surfaces move toward and away from one another in a repetitive motion. 
     It has been found that several advantages of the present sensor  10  over known sensing systems include: (1) a liquid level sensor impervious to mineral, grease and detergent build up on the probe; (2) insensitive to e.m.i, no radiation (creation) of e.m.i. and the ability to electrically ground the system; (3) a wide variety of metals, ceramics, PPS plastics and glass can be used for the probe  12 , so long as the material selected has the proper acoustic properties; and (4) present resonant decay processing schemes can be used to provide diagnostic information on demand. 
     It is anticipated that the probe  12  can be virtually any shape. In addition to the rod shape shown, other know suitable shapes include strips  40  and strips with a leg or bend  42  (as seen in  FIG. 8 ), to, for example, permit mounting the transducer  16 . When shaped as a strip with a bend  42 , the bend  42  can be at any angle α provided that the radius of curvature is less than the wavelength. 
     Signal processing schemes can include active metal resonant decay, as well as analog to digital conversion systems and the like. It will be further appreciated that multiple parallel probes can easily be assembled in single or multiple housings to provide for multiple discrete levels by staggering the ends of the probes and that signal processing schemes can be implemented that allow for a continuous level sensor for some applications. 
     Other configurations and materials of sensors embodying the principles of the present invention were tested to determine the sensitivity of the sensors. In one such example, a sensor was formed as a strip  40  of stainless steel in an L-shape, having a width w 40  of 0.25 inches and a thickness t 40  of 50 mils (50 thousandths of an inch). A transducer  16  was mounted to the sensor at the short leg (dry leg)  42  of the sensor  10 . 
     A 360 kHz signal was generated in the transducer  16 . Snapshots of three response spectra were taken on an oscilloscope, a first ( FIG. 9A ) with the strip dry, which is the induced signal or wave, a second ( FIG. 9B ) with the strip (at the tip) positioned in oil, and a third ( FIG. 9C ) with a drip of oil coming off of the tip of the strip. 
     As can be seen from the spectra, there is a clear distinction in the response spectra of the dry strip and the in-oil strip. There is also a significant difference between the dry strip and the oil drip strip and between the in-oil strip and the oil drip strip. Thus, the sensor can detect the presence or absence of liquid and, importantly, the sensor can distinguish between being submerged (within a “pool” of liquid) and merely the presence of remnants of liquid on the sensor. 
     A sensor formed from a ceramic rod was also examined. Here, a 403 kHz signal was generated in the transducer and induced in the rod. Snapshots of two response spectra were taken on an oscilloscope. A first spectra ( FIG. 10A ) shows the ceramic rod dry and a second spectra ( FIG. 10B ) shows the ceramic rod in liquid (in this case, water). Again, there is a clear distinction in the response spectra of the dry ceramic rod and the in-liquid rod; thus, the ceramic rod sensor can detect the presence or absence of liquid. Advantageously, such a rod can be used in extremely harsh environments, such as caustic or acidic environments. It is also anticipated that the rod can be elongated to extend into areas that otherwise may be difficult to access. 
     In a present rod shaped probe  12 , it has been found that the tip  20  can be formed having a rounded (e.g., hemispherical) shape to prevent the accumulation of liquid at the tip and to stimulate the formation and release of any droplets from the rod. In a present probe, the radius of the hemisphere is approximately equal to the diameter of the probe  12 . It has also been found that rounding the end  20  increases the sensitivity and signal level. Without being bound to theory, it is thought that this reduction in impact and increase in sensitivity is related to acoustic mode conversion. 
     A novel quick-install, quick-release mount  50  for the sensor  10  is illustrated in  FIGS. 4-7 . The mount  50  includes a base  52  that is configured to mount to an object, such as a wall W, near the location that is to be monitored. The base  52  has an inverted cup shape that defines a well  56 . The cup has channels  58  extending upwardly from the edge  60  to form multiple flexible fingers  62 . The fingers  62  can include a retaining lip or detent  64  at about the end of each finger  62  (at about the edge  60 ). 
     The base  52  is configured to house the electrical connections  66  for the sensor  10 . Accordingly, circuitry is provided on a board  68  or other carrier in the base  52 . Contacts  70 , preferably biased, e.g., spring-loaded, are positioned at an end  72  of the well  56 . Electrical conductors  66  (e.g., wires) are connected to the board  68  and extend out of the base  52  to, for example, a terminal box (not shown) for connection to a control system  88 . A cover  76  can be fitted over the board end of the base  52  to permit access to the board  68  or other components. 
     The probe  12  is carried by a cartridge  78  that fits into the base  52 . The probe  12 , as illustrated has a collar  28  at an upper end that surrounds the dry end  18  of the rod  12 . The transducer  16  is mounted to the dry end  18 , about central of the collar  28 . 
     The cartridge  78  is configured as a sleeve that fits over the rod  12 , with the rod  12  residing such that the cartridge  78  abuts the collar  28 . An isolation seal  80 , for example, an O-ring is positioned in the cartridge  78  to abut the rod  12  and collar  28 , such that the seal  80  isolates the rod  12  from the cartridge  78  and provides an environmental seal for the components (e.g., board  68  and contacts  70 ) within the base  52 . A retaining clip  82  is positioned at a lower end of the cartridge  78  to maintain the cartridge  78  positioned about the rod  12 . In a current mount  50 , the materials of construction are non-conducting, polymeric materials, such as nylon and the like. Other suitable materials will be appreciated by those skilled in the art. 
     In the present mount  50 , the cartridge  78  includes a circumferential shoulder  84  and a recess  86  adjacent to the shoulder  84 . The recess  86  is configured to receive the detents  64  (on the fingers  62 ) when the cartridge  78  is properly inserted in the base  52 . When inserted, as seen in  FIG. 6 , the cartridge  78  snaps into the base  52 , the contacts  70  are in contact with the transducer  16  and the probe  12  is securely held in place in the mount  50 . Because the cartridge  78  snaps into place in the base  52  and there is no hard-wired connection between the sensor (transducer  16 ) and the electrical control system  88 , this arrangement provides a readily managed and maintained level sensor system  10 . Probes  12  can be easily changed by snapping cartridges  78  in and out of the base  52  to, for example, change the level at which the sensor  10  is to generate a signal (e.g., to change the liquid level to be monitored), change the material of the probe  12 , or to replace a probe  12 , without undue time and labor. In that the mount  50  uses a circumferential shoulder  84 , detents  64  and fingers  62 , the probe  78  can be inserted and/or reinserted into the mount  50  in any angular orientation and function properly. 
     The shoulder  84 , detents  64  and fingers  62  can be formed having shapes other than those illustrated (e.g., square or hexagonal). In addition, rather than plastic fingers, the detent function can be accomplished by other structures, such as spring, balls fitted into channels, and the like, in which case the mount can be formed of a metal. 
     It will be appreciated that although certain materials are disclosed and described, various other suitable material could likewise be used, for example, in fabricating the various components of the invention. 
     All patents referred to herein, are hereby incorporated herein by reference, whether or not specifically done so within the text of this disclosure. 
     In the present disclosure, the words “a” or “an” are to be taken to include both the singular and the plural. Conversely, any reference to plural items shall, where appropriate, include the singular. 
     From the foregoing it will be observed that numerous modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present invention. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred. The disclosure is intended to cover all such modifications as fall within the scope of the claims.