Abstract:
A reed switch in a solid plastic body is positioned by a clip adjacent to a fluid container. A change in fluid level opposite the reed switch moves a magnet connected to a float, turning the reed switch on or off. Glass filled polyester is injected into a die which surrounds the reed switch supported on electrical plug blades. The reed switch has high stiffness leads positioned on the blades, and is oriented within the mold so the injected plastic flows along the glass tube of the reed switch, minimizing stresses on the reed switch, which could result in the reed switch becoming crack and/or broken. The molded plastic body incorporates an integrally formed clip arm that functions to position and hold the plastic body onto a larger structure. The magnet and float may move in a tube, may pivot, or slide along rails in response to a change in fluid level.

Description:
FIELD OF THE INVENTION 
     The present invention relates to liquid level sensors in general and to sensors employing reed switches in particular. 
     BACKGROUND OF THE INVENTION 
     Determining the amount or level of liquid in a tank is a problem of widespread importance. Applications range from fuel tanks, lubricating fluid tanks, and water level monitors within humidifiers, boilers, and dishwashers. Typically the concerns are the same: to reliably indicate when liquid rises to a preselected level or when a liquid falls below a preselected level. A common mechanical float sensor incorporates a buoyant float, which responds to a rise in the level of a liquid by pushing against a switch causing the switch to open or close. 
     Float sensors are typically a critical part of an apparatus, serving as part of the liquid control system in the apparatus. The flooding or running dry which results from the float sensor&#39;s failure can be costly or at least inconvenient. Float sensors employing electric switches face additional challenges. An electric switch submerged in or positioned near a liquid has inherent reliability problems. The liquid may be corrosive, may promote galvanic corrosion, or may form a varnish, which builds up on exposed surfaces and contacts resulting in the buildup of an insulating layer, which affects electrical parts. 
     Alternatively, the raising float may contain a magnet, which interacts with a reed switch causing it to open or close. Reed switches are widely used where an extremely reliable switch is required. Reed switches are reliable because the contacts that close the switch are located within a hermetically sealed glass envelope. A reed switch typically has two ferromagnetic reeds that extend from opposite ends of a sealed glass tube. The contacts are formed on opposed surfaces of the reeds which overlap and are spaced apart a small amount when no magnetic field is present. In the presence of a magnetic field the ferromagnetic reeds attract and are brought into engagement at the contact surfaces, thus closing a circuit between the ferromagnetic reeds. 
     Reed switches do have some limitations which flow from their advantages: namely the glass capsule which contains the ferromagnetic reeds is inherently subject to being broken, and a reed switch must be reliably positioned with respect to the actuation magnet in a cost affective manner. 
     What is needed are float sensors employing reed switches which are more reliable and more easily installed. 
     SUMMARY OF THE INVENTION 
     The float sensor of this invention employs a reed switch contained within a solid plastic body which is positioned by a clip adjacent to an impervious wall forming part of a fluid container. Addition or drainage of the fluid causes the fluid level within the fluid container to vary, opposite the reed switch, this in turn causes a float to move toward or away from the reed switch. This motion results in turning the reed switch on or off. 
     The plastic body is formed by injecting glass filled polyester into a die that surrounds a reed switch with two high stiffness leads that form part of a lead frame. The reed switch is oriented within the die so that when plastic is injected, it flows along the glass tube of the reed switch. This flow path minimizes the disturbance of the reed switch due to the plastic flow that could result in the reed switch becoming broken. The molded plastic body incorporates an integrally formed clip arm that functions to position and hold the plastic body onto a larger structure. 
     The reed switch may be actuated by a magnet mounted to a float, which is pivotally mounted by an arm on a structure. The arm has the magnet mounted between the float and a pivot. Motion of the float in response to fluid level changes results in movement of the magnet into and out of position with respect to a reed switch contained within a plastic body, to cause the reed switch to open and close with the motion of the magnet. The reed switch is mounted spaced from the float by an impervious wall that prevents moisture or liquids from contracting the plastic body. 
     An alternative means for reed switch actuation consists of a torpedo shape float contained within a cylindrical tube. The bottom of the tube has openings through which fluid can enter to raise the float and cause a magnet contained in the float to move relative to a reed switch within a plastic body, causing the reed switch to open or close. The float is separated from the plastic body by an impervious wall. 
     A further alternative reed switch actuation means consists of a float containing a magnet, which is constrained to move along opposed rails which capture the float. The float is again separated from a plastic body containing a reed switch by an impervious wall. Motion of the float due to liquid level changes causes the reed switch to change the activation state. 
     It is a feature of the present invention to provide an improved packaging for a reed switch. 
     It is a further feature of the present invention to provide float sensors that are more reliable, more easily assembled, and low cost. 
     It is another feature of the present invention to provide a method of encapsulating a reed switch. 
     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 an isometric view of a reed switch module, magnet, and float forming the float sensor of this invention, partially broken away in section. 
     FIG. 2 is an exploded isometric view of the components forming the reed switch module of FIG. 1 
     FIG. 3 is a cross-sectional view of an alternative float and magnet employed with the reed switch module of FIG.  1 . 
     FIG. 4 is a cross-sectional view of the reed switch module of FIG.  1 . 
     FIG. 5 is an isometric view of one-half of a fluid reservoir employing another alternative float sensor and magnet employed with the reed switch module of FIG.  1 . 
     FIG. 6 is a cross-sectional view of the reservoir and reed switch module of FIG. 5 taken along line  6 — 6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more particularly to FIGS. 1-6, wherein like numbers refer to similar parts, a float sensor  20  is shown in FIG.  1 . The float sensor  20  has a reed switch module  22  and a float assembly  23 . The float assembly  23  is located in a fluid reservoir (not shown) and is separated from the reed switch module  22  by an impermeable wall  24  shown schematically in FIG.  1 . The float assembly  23  has a float arm  26  which is pivotally mounted by pivots  30  at a first end  32  of the arm to a float support structure  28 . The opposite end  34  of the arm  26  supports a float bulb  36 . The entire arm  26  which is constructed of the same buoyant material as the float bulb  36 , together with the float bulb  36  responds to a rising liquid level by pivoting a magnet  38  away from the reed switch module  22 . The magnet  38  is insert molded into the material forming the float arm  26 . Portions of the float arm  26  form a cup structure  40  positioned on the arm  26  between the float bulb  36  and the pivot end  32  of the arm, the cup structure  40  partially surrounds the magnet  38 . The magnet  38  is moved toward and away from the reed switch module  22  by the buoyancy of the arm  26  and float bulb  36  causing the arm  26  to pivot. The reed switch  46  within the module is caused to close or form a short circuit when the magnet  38  is closest to the reed switch module  22 . 
     The float sensor  20  is typically employed in a salt brine reservoir formed by a structure (not shown) to which the float support structure  28  is mounted internal to the brine reservoir, and the reed switch module  22  is mounted external to the brine reservoir. The structure  28  performs the function of protecting the float  36  from salt that is poured down onto the structure  28  and into the brine reservoir (not shown). The support structure  28  has opposed rails  39  terminated by stops  41  which positions the support structure  28  on the reservoir structure (not shown). 
     The reed switch module  22  has a lead frame formed by a first blade  42  and a second blade  44 . The blades  42 ,  44  form a standard 6.3 mm male plug. The male plug readily mates with the electronics of various appliances (not shown) which employ the float sensor  20 . The reed switch module  22  contains a reed switch  46 . The reed switch  46  has a first ferromagnetic reed  48 , a second ferromagnetic reed  50 , and a hermetic sealed glass capsule  52  which encloses the ends  54  of the reeds  48 ,  50 . Contact of the reeds  48 ,  50  closes the reed switch  46 . The first reed  48  is spot welded, or laser welded, or soldered to a tab which extends from the first blade  42 . The second reed  50  is similarly welded or soldered to the end  56  of an arm  58  that extends from the second reed  50 . 
     The plastic housing  60  which surrounds and encapsulates the reed switch  46  has a plug base  62 , a reed switch surrounding extension  64 , and a retaining clip  66 . The clip  66  has a projection  68  mounted to a resilient clip body  70 . The projection has a retaining face  72  which extends vertically from the resilient clip body, and an inclined ramp face  74  which is inclined relative to the vertical retaining face  72 . The clip  66  is designed for assembly of the reed switch module  22  to a mounting structure  76  as shown in FIGS. 3 and 6. The ramp face  74  causes the clip body  70  to be depressed as the reed switch module is slid into a pocket  78 . The retaining face  72  engages a wall  80  formed by a hole in the mounting structure  76  as shown in FIGS. 3 and 6. 
     For resistance to water, the reed switch module  22  is fabricated as an insert molded part formed of glass filled polyester. Reed switches are typically mounted to an electrical assembly such as a circuit board. Where a reed switch is being used as a switch in a float sensor application, particularly where moisture may be present, encapsulation is desirable to prevent corrosion and to prevent breakage of the reed switch. But insert molding a reed switch within a module presents problems. 
     An insert-molded part must be rapidly formed if it is to be economical. The insert molding process involves a cycle whereby a mold is opened, molded-in-assemblies are positioned, the mold is shot with molten, e.g. hot flowable, plastic under high pressure, the mold is opened, and the part is trimmed and thus finished. Rapid injection is important so that the mold cavity will be completely filled before significant cooling takes place. The mold or die is formed of metal. Plastic rushing into the mold can slam the reed switch against the mold sides breaking it. Rapid cooling of the injected plastic is important so that the part may be removed from the mold quickly, keeping down cycle times and thus the cost of parts. The capital cost of the molds and molding equipment forms a significant part of the overall cost, and therefore machine productivity is important. 
     Thus the economics and process limitations present problems when a reed switch is positioned within a mold to be molded-in to a part. 
     The reed switch module  22  is designed to overcome problems by supporting the reed switch from the relatively wide flat blades  42 ,  44  and injecting the plastic along and above and below the blades  42 ,  44  from the plug face  82  of the module  22 . The inflowing plastic therefore first impacts the small end  84  of the reed switch  46  and, flowing parallel to the reed switch glass capsule  52 , progressively engulfs the reed switch capsule  52 , thereby progressively supporting the capsule  52  as the plastic begins to act on the inherently less rigid end  86 . To position and handle the reed switch and blades  42 ,  44  the blades may be formed as a lead-frame wherein the blades are connected to a strip of metal from which they are cut after the module  22  is formed. The blades  42 ,  44  extend from the mold cavity used to form the module  22  and thus are not enclosed within the injected plastic. 
     The reed switch module  22  can be used with a variety of float assemblies to form float sensors. An alternative embodiment float sensor  88  employs a reed switch module  22  as shown in FIG.  3 . The float  90  is a torpedo shaped insert molded part which incorporates a washer shaped magnet  92  positioned in a guide tube  94  which surrounds and constrains the motion of the float  90 . The float  90  is formed with low density foam forming part of the plastic injected. The float  90  has a conical base  100  which rests on support blocks  96  which position the float  90  with respect to the reed switch module  22  and allow fluid to flow around the float  90  assuring that the float  90  is free to float within the tube to the tube bottom wall  98  which underlies the conical base  100 . The tube bottom  98  has six holes  99  through which fluid such as salt brine flows as liquid in a reservoir  102  rises. An impervious wall  104  which forms a part of the fluid reservoir, separates the fluid reservoir from the reed switch module  22  and the structure to which it is mounted  76 . 
     It should be understood that a float sensor employing a tube containing a float with an attached magnet, which interacts with an adjacent reed switch is conventional. The tube  94  is from an existing design. The design of the float  90  and its relation to the reed switch module  22  constitute the improvement. 
     The magnet  92  is of the high intensity type which can reliably actuate the reed switch  46  when spaced  10  to 12 millimeters from the edge of the magnet  92 . The magnet  92 , shown in FIG. 3 as forming a circumferential ring, will typically be nickel coated to protect it from the environment. When the float  90  is seated on the support blocks  96  the reed switch is in the activated state. 
     A further embodiment float sensor  106  which incorporates the reed switch module  22  is shown in FIGS. 5 and 6. One half of a plastic reservoir body  108  used to hold a rinse aid for a dishwasher is illustrated. A rinse aid is a liquid which prevents spotting of dishes and glasses by reducing surface tension or otherwise modified water quality. A float  110  is positioned within the reservoir  108 . A simple cylindrical magnet  112  is molded in place within the float  110 . The float  110  has two opposed grooves  114 . The lower of the grooves  114  is shown in FIG.  5  and FIG. 6 riding on a lower guide rail  116  which is one of a pair of opposed rails. The other rail would be positioned on the other half of the reservoir body, not shown. The float  110  is thus constrained to float between a lower stop  118  and the upper wall  120  of the plastic reservoir  108 . 
     A portion  122  of the plastic reservoir body  108  separates the interior  124  on the reservoir formed by the plastic body  108  from the reed switch module  22 . The presence of sufficient rinse aid causes the magnet  112  to move up against the upper wall  120  which closes the reed switch, the output of which can be used to indicate the presence of rinse aid. 
     An alternative method of forming the reed switch module  22  is to form a shell having the exterior dimensions allowed the module as shown in FIG.  1  and to position the blades  42 ,  44  with the reed switch mounted therein within the shell. The shell is then filled with an epoxy, polyurethane or other moldable plastic. 
     It should be understood that each float employs a high intensity magnet which may be solid metal or may be particles embedded in plastic. And each magnet is thus capable of causing a reed switch placed 10-12 mm away to close. 
     It should be understood that where a float is described and illustrated, the float could be larger to achieve greater buoyancy force to move the magnet.