Abstract:
A float sensor that is modular has a mounting bracket which incorporates an O-ring seal and bayonet style resilient mounting fingers which lock the mounting bracket to a tank flange. A tube is ultrasonically welded to the mounting bracket and positions a reed switch housing which is ultrasonically welded to the end of the tube opposite the mounting bracket. The reed switch housing incorporates a stop positioned above a descending cone. The cone terminates in radially outwardly extending fingers that lock a float to the reed switch housing. The float is free to float upwardly along the cone to engage the stop. The float has a ring shaped magnet foamed in place within a low density plastic body. The presence of fluid buoys the float upwardly until the float engages the stop.

Description:
FIELD OF THE INVENTION 
     The present invention relates to float sensors in general, and to float sensors employing a reed switch to detect the movement of a float in particular. 
     BACKGROUND OF THE INVENTION 
     Reed switches have long found use in float sensors where the inherent advantages of reed switches bring reliability and low cost to float sensor designs. The reed switch is a low-cost component and if properly designed can have a service life of millions of cycles. The reed switch, moreover, is hermetically sealed which is an advantage any time a switch is employed near liquids. A reed switch is actuated by a change in magnetic flux with respect to the reed switch. This means that the switch can be opened or closed by the movement of a magnet, which is some distance from the reed switch. The ability of the reed switch to respond to the movement of a magnet allows a float incorporating a magnet to actuate a switch in response to change in liquid levels, usually to close an electrical circuit. 
     To limit the current, which flows through a reed switch, a current limiting resistor will often be installed in series with the reed switch. By limiting the switch current, the life of the reed switch may be increased almost arbitrarily to more than one hundred million cycles. 
     Float sensors are typically employed to return a single digital bit upon a single condition. The condition often indicates a-fluid container is nearly full or nearly empty. In these days of broad band sensors, the continuing utility of a sensor that can reliably indicate a single condition has not decreased. In fact, reed switches are often employed with more sophisticated solid state sensors to provide greater reliability in detecting particular system conditions. 
     Float sensors employing reed switches are generally divided into two types: those where the float containing the activation magnet is mounted in a hinged manner and those where the float is constrained to movement along an axis by a central shaft, or by a surrounding tube. 
     DISCUSSION OF THE PRIOR ART 
     U.S. Pat. No. 5,239,285 shows a typical float sensor of the type where a float containing a ring-shaped magnet is constrained to move along a vertical axis by a central shaft. A reed switch contained within the shaft detects the movement of the magnet in the float as liquid causes the float to rise along the length of the central shaft. The shaft extends vertically downwardly from a bracket to which the central shaft is mounted. The shaft has a lower stop and an upper stop between which the float is moved by the rise and fall of fluid within a container. A reed switch is positioned within the shaft between the upper and lower stops so that motion of the float caused by a rise or fall of fluid level causes the reed switch to actuate. 
     Although reed switches have long been used by Original Equipment Manufacturer (OEM) suppliers to incorporate float sensors into their equipment, the new economy means that equipment suppliers want complete assemblies which provide the float sensor function which have a simple design interface. In this way the OEM supplier can decrease internal engineering while obtaining flexibility in outside design support, resulting in a faster, lower cost product cycle. 
     SUMMARY OF THE INVENTION 
     The float sensor of this invention provides a modular, adaptable float sensing module which can interface with a tank having a simple circular flange, wherein the upper and lower surfaces of the flange are parallel. The float has a mounting bracket which incorporates an O-ring seal and bayonet style resilient mounting fingers that lock the mounting bracket to the horizontal flange. The mounting bracket incorporates an electrical connector that forms the electrical interface. A tube of a selected length is bonded or ultrasonically welded to the mounting bracket and positions a reed switch housing a fixed distance downwardly into the tank from the horizontal flange. The reed switch housing is ultrasonic welded or bonded to the end of the tube opposite the mounting bracket to seal the lower end of the tube. The reed switch housing incorporates a stop positioned above a depending cone. The cone terminates in a plurality of radially outwardly extending fingers which lock a float to the reed switch housing, the float being free to float upwardly along the cone to engage the stop. The float is constructed of a ring shaped magnet, foamed in place within a low density plastic foam body that hangs beneath the stop when not supported by fluid within the tank. When fluid is present, the fluid buoys the float and the magnet contained therein upwardly until the float engages the stop. The magnet and reed switch are selected such that the reed switch is closed when the float engages the stop. 
     The present invention provides a modular float sensor that can be readily sized for a variety of different applications. 
     The present invention also provides a float sensor that can be rapidly assembled by hand or by machine. 
     The present invention further provides a float sensor that can be rapidly mounted to a standard interface without tools. 
     The present invention still further provides a float sensor that is cantilevered downwardly from a single support flange. 
     The present invention even further provides a float sensor that isolates the reed switch from the monitoring liquid. 
     Further features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side isometric view, partly exploded, of the float sensor of this invention installed in a tank, shown partially cut away in section. 
     FIG. 2 is an enlarged cross-sectional view of the reed switch housing and float of FIG.  1 . 
     FIG. 3 is an enlarged cross-sectional view of an alternative embodiment reed switch housing and float. 
     FIG. 4 is a side elevation pictorial view of several alternative embodiment float sensors. 
     FIG. 5 is an exploded isometric view of the float sensor of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more particularly to FIGS. 1 to  5 , wherein like numbers refer to similar parts, a float sensor  20  is shown in FIG.  1 . The float sensor, as shown in FIG. 5, has four major components: a tank mounting bracket  22 , a float positioning tube  24 , a reed switch housing  26 , and a float  28  incorporating a magnet  30 . A reed switch  31  is positioned within the reed switch housing  26  and is connected by wires  33  to an electrical connector  35 . 
     The float sensor  20  is readily adaptable to a variety of applications by combining an electrical connector which may be unique to each end user, with a positioning tube  24  of a selected length to position the float to detect a specific change in fluid level as specified by the end user. The interface between the float sensor, which may be supplied as a complete unit, and the tank, is both simple and minimal. The electrical connection employs a selected commercial or custom connector as specified. This simplifies and minimizes the interface control between the float sensor  20  and the system in which it is installed. 
     The mounting bracket  22  is designed to interface with a simple flat circular opening  32  formed in a side  34  of a tank  36  containing the fluid  38 , the level of which it is desired to monitor. The mounting bracket  22  has a circular flange  40  which overlies the circular opening  32 , a barrel section  42  which extends down into the tank  36  and a plurality of resilient bayonet fingers  44  which lock with the inside surface  46  of the tank  36 . An elastic O-ring  45  is positioned between the circular flange  40  and a portion  48  of the flat exterior surface  34  which surrounds the circular opening  32 . The mounting bracket  22  is installed in the tank  36  by pressing the bracket  22  downwardly through the circular opening  32 . The downward pressure causes the inclined surfaces  50  on the bayonet fingers  44  to force the fingers radially inwardly allowing the fingers  44  to pass through the circular opening  32 . The downward pressure also compresses the O-ring  45  until the locking surfaces  52  of the bayonet fingers  44  engage the tank  36  inside surface  46 . 
     The float positioning tube  24  nests coaxially with a cylindrical downwardly extending portion  56  of the mounting bracket  22  of the float positioning tube  24 . The float positioning tube  24  is ultrasonically welded or chemically bonded to the downwardly extending portion  56  of the mounting bracket  22  to form a strong hermetic seal. The length, wall thickness, and material choice and shape of the positioning tube  24  can be selected to meet particular requirements of an OEM tank manufacturer. In those cases where merely the position of the float  28  needs to be controlled, standard tubing can be cut to size, minimizing the number of parts that must be stocked. Control of inventory is axiomatic of the just-in-time manufacturing system where a close relationship with suppliers is used to minimize inventory and maximize quality control by providing a short supply chain between manufacturing and production. 
     Where the float sensor  20  is mounted in a vehicle or other high vibration environment, the rigidity of the positioning tube  24  can be controlled to detune the characteristic frequency of the float sensor system. Rigidity of the positioning tube  24  may be controlled by increasing the tube wall thickness or by selecting a more rigid material for fabricating the tube such as fiber-reinforced plastic or both. 
     The reed switch housing  26  incorporates a cylindrical upwardly extending flange  58  that coaxially slides over and nests with the lower end  60  of the positioning tube  24 . The reed switch housing  26  is sealed to the positioning tube  24  by ultrasonic welding or chemical bonding the cylindrical flange  58  to the lower end  60  of the positioning tube  24  to form a hermetic and durable seal. A circular flange  62  extends radially outwardly from the housing  26  and defines a downwardly facing surface  64  which acts as a stop  66  which limits upward movement of the float  28 . Concentric with the circular flange  62  and extending downwardly from the flange is a conical extension  68 , the apex of which terminates with four equally spaced resilient barbs  72 . 
     Coaxial with the cylindrical flange  58 , the circular flange  62  and the conical extension  68 , is a cavity  74 . The cavity  74  has a narrow cylindrical portion  76  which extends downwardly into the conical extension  68  which holds the magnet sensing reed switch  31  which is connected to wires  33  which in turn interface with the electrical connector  35 . The cavity  74  has a wide cylindrical portion  78  that is connected to the narrow cylindrical portion  76  by a tapered section  80 . Potting compound  81 , such as hot melt polyamide, can be used to fix the reed switch  31  in position within the reed switch housing  26 . 
     The float  28  has a float body  82  and a foamed-in-place magnet  30 . The magnet  30  is positioned near the top of the float body  82  and is of a generally annular or toroidal shape. The float  28  has a central opening  84  which passes through the magnet and into which the conical extension  68  fits as the float is buoyed upwardly against the stop  66 . The float  28  is captured by the resilient barbs  72  that extend into four equally spaced slots  86 . The motion of the float  28  is minimal, approximately 6 to 25 mm (0.25 to 1.0 inches). By minimizing float motion, the likelihood that the float will become jammed is minimized. The float may be sized to generate substantial buoyant and gravitation forces to maximize operational reliability. Motion of the float  28  carries the magnet  30  against the stop  66 . The reed switch  31  is positioned so that the switch changes state, generally closes, when the float is positioned against the stop  66 . Downward motion of the float  28  due to a fall in fluid level causes the magnet  30  to move away from the stop and the reed switch, resulting in the reed switch opening. 
     An alternative embodiment reed switch housing  88  and float  90  are shown in FIG.  3 . The reed switch housing  88  and float  90  are constructed to provide lateral support against sloshing to the float  90 . The reed switch housing  88  is identical to the housing  26 , except that a conical extension  92  does not immediately terminate in a plurality of barbs but rather forms a downwardly extending shaft  93  which extends through the float  90  and terminates with resilient barbs  94 . The float  90  supports an annular magnet  96  and has a central cylindrical opening  98  that receives the shaft  93 . The cylindrical opening  98  expands to a larger diameter opening  99  to form a radially extending surface  100  near the lower end  102  of the float  90 . The barbs  94  expand within a larger diameter opening  99  to lock the float between the barbs  94  and the stop  66 . 
     Three alternative embodiment float sensors within a tank  104  are shown in FIG.  4 . The uppermost float sensor  106  is similar to the float sensor  20  except the mounting bracket  108  interfaces with a cylindrical boss  110  which extends from the exterior surface  112  of the tank  104  downwardly into the tank. An O-ring  114  is seated between a conical surface  116  leading into the cylindrical boss  110  and a circumferential groove  118  formed between a planar flange  120  and a cylindrical portion  122  of the mounting bracket  108 . 
     A second alternative embodiment float sensor  124  has an approximately sixty degree bend in a float positioning tube  126 . The bend in the float positioning tube allows the float sensor  124  to be mounted from a surface  128  which is not horizontal. This capability may have utility in certain tank configurations where access to the top is not possible. The float sensor  124  also has an alternative embodiment mounting bracket  130  that incorporates a first O-ring  132  and a second O-ring  134 . The first O-ring is positioned between a flange  136  and the exterior surface  112  of the tank  104 , the second O-ring  134  is positioned between a cylindrical portion  137  of the mounting bracket  130  and a cylindrical sleeve  138  which extends downwardly into the tank  104 . 
     A third alternative embodiment float sensor  140  has a mounting bracket  142 , similar to mounting bracket  22 , wherein the float positioning tube  144  allows mounting of a float sensor  140  from a vertical wall  146 . 
     It should be understood that the float sensor mounting bracket and reed switch housing may be constructed as injection molded parts. The float may be constructed as a molded structural foam part, and the float positioning tube may be constructed as an extrusion, or an injection molded part. 
     It should also be understood that the float sensor may be assembled by ultrasonic welding, other types of plastic welding, or various bonding techniques as are suitable for the materials from which the components of the float sensor are fabricated. 
     It should be understood that reed switches that are normally closed or normally open could be used as the float sensing reed switch. Further a so-called FORM C configuration reed switch may be employed. A FORM C reed switch has one movable reed and two stationery reed contacts, all three reeds are ferromagnetic, however the contact area of one fixed reed, against which the movable reed is biased, is formed of a nonmagnetic metal which has been welded to the ferromagnetic lead. When exposed to a magnetic field, both fixed reeds assume the same polarity, which is opposite that of the movable reed, due to the presence of the nonmagnetic metal, which forms a flux interrupter, the only attractive force of sufficient magnitude is between the moveable reed and the normally open reed so the moveable reed transfers from the contact on the normally closed reed to the contact on the normally open reed. 
     It should be understood that the wires  33  shown in FIGS. 2 and 3 may be connected with leads extending from the connector  35  through the bracket  22  into the tank  36  by crimping or by soldering. Alternatively the wires may be run through the bracket  22  and extend from the connector  35  to a length specified by the end user. 
     It should be understood that depending on the thickness of the tank wall the tank may be formed with a localized thickening about the opening for the float sensor mounting bracket. 
     It will also be understood that a current limiting resistor will typically be employed either within the reed switch housing or as part of the electronics that interfaces with the float sensor through the electrical connector. 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.