Abstract:
The present invention provides a fluid level sensing switch for use with a device having a fluid reservoir. The switch is float activated and closes a circuit so as to change an operating parameter in the event that the volume of fluid in the reservoir falls below a predetermined level. Once the volume of fluid is restored to a predetermined safe level, the switch automatically resets to an open position to permit normal operation of device.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to an electrical switch which includes a fluid level sensor, and more particularly to a switch that is adapted to automatically complete an electrical circuit when the volume of fluid being monitored is below a predetermined level. 
     In many devices that house fluids within a reservoir, it is desirable to monitor the fluid level. In certain devices it may be advantageous to change an operating parameter when the volume of fluid falls below a predetermined level. For instance, it would be desirable to automatically shut off an internal combustion engine if the level of lubricating oil in the engine crankcase becomes inadequate. It also would be desirable to shut off a pump such as that used in a bilge of a boat, once the water level is sufficiently reduced. Likewise, it would be desirable to provide a low fluid level warning to users of consumable fluid products stored in tanks, such as home heating oil. 
     Problems associated with ignoring low fluid conditions vary from matters of inconvenience, to safety, to major system damage. For instance, the result of a loss of oil or a low oil condition during the operation of relatively small spark ignition engines, such as those used in power generators or lawn mowers, are well known in the art and are described, in part, in U.S. Pat. No. 4,600,820. In general, if such engines experience operating conditions involving a rapid loss of oil, unintended extreme tilting of the engine, or simply insufficient maintenance, they may lack access to a sufficient supply of oil to adequately lubricate the internal components of the engine. Continued operation of an engine experiencing such a condition may lead to severe engine damage and could present a safety hazard. 
     The switch used with spark ignition engines that is disclosed in the aforementioned U.S. patent, exhibits certain disadvantages that are overcome by the present invention. In particular, the prior art device is designed to shut off an engine by grounding the ignition system in the event of a low oil level. The switch is mounted horizontally, through a side wall of an engine crankcase. Hence, the device is likely to be overly sensitive to engine tilting because the device will likely be mounted closely adjacent a side wall of the crankcase. Also, the entire switch assembly, including its balance beam float assembly and switching lever, are contained within a horizontal encapsulating shield having slots and holes. The slots and holes permit communication with the oil in the crankcase and the escape of foam and bubbles in the oil. 
     The horizontal mounting of the prior art assembly in an engine crankcase, and its need to be located in the vicinity of the top surface of the oil when the oil level is at a predetermined maximum recommended level, also subject the device to considerable turbulence and fluctuations in the oil due to splashing and aeration generated by the use of a common crankshaft mounted splasher device. These conditions may result in less reliable performance of the switch, and may lead to intermittent operation of the ignition system. In addition, the balance beam assembly utilized in the prior art device does not appear well suited to cope with more violent and higher amplitude vibrations, such as those encountered once the ignition has been interrupted in a spark ignition engine. Similar disadvantages would be experienced if the prior art device is used to automatically shut off a diesel engine by interrupting an electronically controlled fuel supply system under low oil level conditions. 
     Additional prior art designs used for low fluid level conditions, such as in automatic engine shut off applications, include a glass reed switch with a floating magnet. One problem with this technology is that the pair of glass reeds in such a switch are very thin and tend to become welded together if too much electrical current is applied to the switch. In the case of an ignition cut off switch which is designed to ground the ignition system of a relatively small spark ignition engine under low oil level conditions, the reeds may become permanently joined due to a current that may be in a range as low as approximately 2 to 5 amps. In the event of such a switch failure, even after adding oil to reach a safe level, the ignition system will remain grounded and the engine will not be able to be restarted until the switch is replaced. 
     Some manufacturers using glass reed switches have added electrical devices such as a Triac to accommodate the operating amperages. Such additional components add significantly to the cost of the switch, yet do not alleviate other problems with glass reed switches, such as sensitivity to high temperature and shock. 
     In light of the shortcomings of the above-mentioned exemplary prior art devices, it is desirable to have a switch which combines fluid level sensing and automatic opening and closing of an electrical circuit, and which permits an improved mounting position to reduce sensitivity to fluid reservoir tilting. It is further advantageous to provide a switch device that is less sensitive to turbulence and fluctuations in fluid level that are present in some operating environments, such as splashing and aeration. It also is advantageous that a fluid level sensing switch have rotational torque characteristics that are less susceptible to intermittent operation during large amplitude, varied and lower frequency movements, such as the vibrations commonly experienced during shut down of an internal combustion engine. The present invention overcomes the disadvantages of the prior art, while providing the above mentioned desirable features of a fluid level sensing switch. 
     SUMMARY OF THE INVENTION 
     The present invention is generally embodied in an improved fluid level sensing switch. The switch comprises a base, a float pivotally connected to the base, a counterweight pivotally connected to the base, a linkage connecting the float and the counterweight, a conductive damper, an electrical terminal and a conductive pin. 
     When the volume of fluid is at or above a predetermined sufficient level, the switch remains open. If the volume of fluid falls below the predetermined sufficient level, then the float pivots downward, forcing the counterweight into the conductive damper, which in turn contacts the terminal. In this condition, the switch is closed and completes the circuit. 
     In another aspect of the invention, the counterweight is formed of a material that is of greater density than the material of the float. 
     In still another aspect of the invention, the conductive damper, conductive pin and terminal may be of sufficient physical size to enable them to carry sufficient amperage reliably, so as not to require additional electronic devices, such as a Triac, to accommodate the operating amperage in the circuit. Indeed, the invention lends itself to use of components tailored to specific needs, and can be structured to carry relatively low amperages, or current in excess of 60 amps. 
     In a further aspect of the invention, the electrical terminal includes a magnet. The magnet provides a hysteresis effect to prevent unintended intermittent opening and closing of the switch, such as during the more violent vibrations encountered in shutting off an engine. The magnet helps maintain contact between the conductive damper and the terminal, thus keeping the switch closed, for instance to ground an engine ignition system, until a sufficient fluid level raises the float. As the float is raised by an increase in fluid level, the linked counterweight pivots which, in turn, forces the conductive damper to break its magnetic coupling to the terminal, opening the switch. 
     In another aspect of the invention, the switch. has a cover attached to the base. 
     In still another aspect of the invention, the cover is adapted to be snap fit over the base. 
     In a further aspect of the invention, the cover has at least one opening arranged at or near its top, and at least one opening, such as a hole or slot near its bottom. The upper opening or openings allow air to pass out of the fluid that is within the cover, while the lower opening or openings permit fluid near the bottom of the reservoir in which the switch is housed to enter and exit the device through the cover. Communication with fluid that is very low in the reservoir reduces the sensitivity of the switch to conditions that involve turbulence, splashing or aeration, because the switch is protected by the cover, and because fluid near the bottom of the reservoir is less likely to be turbulent or to contain air bubbles or foam. 
     In a further aspect of the invention, the base of the switch is adapted to be mounted vertically, to extend upward from the bottom of a reservoir. Therefore, sensitivity to tilting of a reservoir can be reduced by mounting the switch very close to the center of the reservoir, where the fluid level will generally be most stable. Moreover, in a particularly advantageous aspect of the invention, the switch may be adapted to be mounted through the bottom wall of a reservoir. This mounting configuration would permit convenient withdrawal of the switch from the bottom of the reservoir for service or replacement. 
     In yet a further aspect of the invention, the switch uses a four bar linkage to generate sufficient torque to operate the float and counterweight, and to overcome wet film sticking between components. The four bar linkage further permits a very compact switch structure. The switch enjoys damping a wide range of vibration amplitudes and frequencies due to the enhanced torque management characteristics obtained by use of a float and a counterweight that are each separately pivotally mounted and that are connected via a linkage, in further conjunction with a low mass conductive damper. 
     It is to be understood that both the foregoing general description and the following detailed description of a preferred embodiment of the present invention in the form of a low oil level ignition cut off switch are exemplary, provided for purposes of explanation only, and are not restrictive of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings depict a fluid level sensing switch of the present invention in the form of a low oil level ignition cut off switch for a spark ignition engine. In describing the preferred embodiment, reference is made to the accompanying drawings wherein like parts have like reference numerals, and wherein: 
     FIG. 1 is a side cross section view of a preferred embodiment of fluid level sensing switch in accordance with the present invention, except with a conductive damper shown in side view. The switch is shown in the open position, with the float in its highest position. 
     FIG. 2 is a side cross section view of the preferred embodiment similar that shown in FIG. 1, but with the switch shown in the closed position, and the float in its lowest position. 
     FIG. 3 is an end cross section view of the preferred embodiment through the center of the device. The switch is shown in the closed position, but with the float shown in an intermediate position. 
     FIG. 4 is an exploded view of the preferred embodiment, without the cover, mounting gasket and compression limiters, and terminal connecting spring and wire. 
     FIG. 5 is a perspective view of the assembled components of FIG. 4, further showing the cover, mounting gasket and compression limiters and terminal connecting spring and wire in an exploded view. 
    
    
     It should be understood that the drawings are not to scale and that certain aspects may be illustrated in phantom views. While considerable mechanical details of a fluid level sensing switch, including details of fastening means and other plan and section views of the preferred embodiment, depicting the invention in the form of a low oil level cut off switch, have been omitted, such detail is not per se part of the present invention and is considered well within the comprehension of those skilled in the art in light of the present disclosure. It should also be understood that the present invention is not limited to the preferred embodiment illustrated. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the above discussion it will be appreciated that the present invention generally may be embodied in numerous configurations of a fluid level sensing switch that is adapted to automatically complete an electrical circuit when the volume of fluid being monitored is below a predetermined level. Referring to FIGS. 1-5, a preferred embodiment of the present invention is shown as it is generally embodied in a low oil level cut off switch. The fluid level sensing switch  10  includes a base  12 , a float  14  pivotally connected to the base  12 , a counterweight  16  pivotally connected to the base l 2 , and a linkage  18  connecting the float  14  and the counterweight  16 . The switch  10  further includes a conductive damper  20 , an electrical terminal  22  and a conductive pin  24 . 
     The material requirements for components of the fluid level sensing switch will depend upon the particular switch structure and its anticipated operating environment. In an operating environment such as that which could be expected for this exemplary embodiment of a low oil level ignition cut off switch, the base  12  would be required to be rigid and to withstand a significant range of temperature, such as from approximately −40° C. to 177° C., as well as vibrations up to approximately 30 g&#39;s. The base  12  would also be required to be resistant to body impact, as well as submersion in petroleum products, such as lubricants or fuels, and solvent sprays, such as are commonly used in degreasing. Accordingly, the base  12  of this exemplary embodiment would preferably be formed of a plastic, such as of the Nylon 6-6® type, or of other suitable materials, such as PPA, PCT, PEI, or PEK. 
     The base  12  of the preferred embodiment includes a generally horizontal portion  26  and an upstanding, two-tiered pedestal portion  28 . Referring to FIGS. 3-5, the base  12  also may incorporate upstanding tab portions  30  to provide fastening means. The base  12  further includes mounting holes  32  and a slot  34  for conductive pin  24 . As shown in FIG. 5, pin  24  preferably projects upward from the base  12 . Thus, when switch  10  is installed on the outer bottom wall of an electrically conductive engine crankcase (not shown), the conductive pin  24  will contact the crankcase and accordingly be capable of sinking a sufficient amount of current and voltage to ground the ignition system and stop the engine without damage to the switch. 
     As is best shown in FIGS. 1,  2  and  4 , the float  14  is preferably pivotally mounted at its rear to the base  12  proximate the upper tier  36  of the pedestal portion  28 , via press fit pin  38 . The counterweight  16  and conductive damper  20  are both preferably pivotally mounted along their respective lengths to the base  12  proximate the lower tier  40  of the pedestal portion  28 , via conductive pin  24 , although one will appreciate that they may utilize separate pivot axes. The linkage  18  is pivotally connected at a first end  42  to the float  14  forward of pin  38 , via press fit pin  44 . The linkage  18  is further pivotally connected at a second end  46  to the counterweight  16  at its rear, via press fit pin  48 . While press fit pins, preferably made of 1010 cold rolled steel, are employed in the exemplary embodiment, one skilled in the art will appreciate that other fasteners and structures for pivoting may be employed, as well as alternative suitable materials. 
     The float  14  is preferably solid, has a density greater than the fluid, and has similar material requirements to the base  12 . Despite having a density greater than the fluid in which the float  14  is located, the float  14  experiences two forces which tend to provide lift. First, the buoyant force produced by the volume of the fluid that is displaced by the float  14  itself. Second, the counterweight  16  is configured to preferably bias the rear of the counterweight  16  upward to assist in forcing the float  14  to pivot upward through linkage  18 . The bias of the counterweight  16  and a similar configured bias of the conductive damper  20  tend to cause the two to pivot away from the terminal  22  at the rear, holding the switch  10  in the open position. The material of counterweight  16  of the preferred embodiment has a density greater than the material of float  14 , but otherwise has similar environmental requirements. Accordingly, the counterweight  16  of the switch  10  may be constructed of a material such as Zamac®, a pot metal, sintered powdered metal, or any other suitable alternative material. 
     The damper  20  is rigid, of low overall mass, and has similar environmental requirements to those of float  14  and counterweight  16 . It may be constructed of suitable conductive material, such as 1010 cold rolled steel. The conductive damper  20  pivots on conductive pin  24 , which is made of suitable electrically conductive material, such as spring steel. The terminal  22  also is made of a suitable electrically conductive material, such as 1010 cold rolled steel, and is fitted within the base  12 . Terminal  22  further houses a press fit magnetic insert  50  which may be made of Alnico®, or other suitable magnetic material. This construction allows the damper  20  to be attracted to the magnetically charged terminal  22 . Thus, if switch  10  experiences a closed position, the damper  20  is adapted to remain in magnetic contact with the terminal  22  until the float  14  pivots upward and the forward end of the counterweight  16  rotates downward, contacting the damper  20  at its end opposite and forcing it to break the magnetic bond to the terminal  22 . The low mass of damper  20  reduces the likelihood that damper  20  will become magnetically decoupled from terminal  22  due to the vibration experienced during engine shut down. 
     In this embodiment, a seal  52 , such as of the Viton® rubber O-ring type, is used to seal the terminal  22  to the base  12 . The terminal  22  further has a hole  54  therethrough to accept a first end  56  of a wire  58  which is to be connected at its second end  60  to the low voltage side of an engine&#39;s spark ignition coil (not shown). A spring  62  is used to hold the wire  58  in conductive engagement with the terminal  22 , which also serves to hold the terminal  22  within the base  12 . The spring  62  is preferably constructed of suitable material, such as spring steel. 
     It is preferable that the switch of the present invention include a protective cover  64  attached to the base  12 . The cover  64  of the preferred embodiment has material requirements similar to those of float  14 , and may be constructed of like material. In the preferred embodiment, as shown in FIG. 3, the cover  64  has opposed recesses  66  in its inner wall, which engage locking detents  68  on tab portions  30 , so as to hold the cover  64  within a groove  70  of the base  12 . 
     The cover  64  has at least one opening  72  arranged at or near its top. The upper opening  72  of the preferred embodiment allows air to pass out of the oil that is within the cover  64 . The cover  64  further has at least one opening, such as a hole or slot  74  near its bottom. The lower opening or openings  74  permit oil near the bottom of the crankcase to enter and exit the device through the cover  64 . Communication with oil that is very low in the crankcase reduces the sensitivity of the switch  10  to turbulence, splashing and aeration, because the switch  10  is protected by the cover, and because oil near the bottom of the crankcase is less likely to be turbulent or to contain air bubbles or foam. Also, the openings  72  and  74  may be relatively small to help limit the exposure of the switch  10  to metal shavings which may contaminate the oil. 
     Referring to FIG. 5, to seal the switch  10  to the bottom of an engine (not shown), the preferred embodiment employs a gasket  76 , such as of common compressed paper or other suitable type. The switch  10  further may use compression limiters  78 , preferably constructed of aluminum or other suitable material, to prevent crushing of the non conductive base  12  when installing the switch  10  to the engine with fasteners such as bolts (not shown). 
     In operation, as shown in FIG. 1, if the fluid level in the reservoir is at or above a predetermined minimum level, the float  14  will achieve its upward most pivotal position, while the end of counterweight  16  which is forward and of greater mass will achieve its lowest position. In such coordinated positions, the counterweight  16  will not permit the electrically conductive damper  20  to contact terminal  22 . Rather, conductive damper  20  will rotate to a position leaving the switch  10  in an open position. 
     If the fluid level is reduced to a point below a predetermined minimum safe operating level, then as seen in FIG. 2, the float  14  will pivot downward, driving the rear end of the counterweight  16  into the conductive damper  20  and, in turn forcing the damper  20  to achieve a magnetic coupling to terminal  22 . When this occurs in the exemplary embodiment, the ignition system of the engine is linked through the switch  10 , with its conductive pin  24 , to the electrically conductive engine crankcase. This closing of the switch  10  and resulting closing of the ground circuit, causes the ignition system to ground out, shutting off the engine. 
     Once the engine has reached a low oil condition and experience automatic shut off via the fluid level sensing switch  10 , the conductive damper  20  will remain magnetically coupled to terminal  22  until the fluid level in the crankcase is increased sufficiently to raise the float  14 . In this instance, the counterweight  16  will be forced to pivot about the conductive pin  24 , driving its forward end downward. As the forward end of the counterweight  16  moves downward, it contacts the forward end of the damper  20 , forcing the damper  20  to pull away from its magnetic coupling to terminal  22 . When the damper  20  is decoupled from terminal  22 , the switch  10  is open. With the switch  10  in the open position, the ground circuit is open, permitting normal operation of the engine ignition system. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the design and construction of a fluid level sensing switch without departing from the scope or spirit of the invention. It also should be understood that any of a variety of fastening means and suitable materials of construction and dimensions may be used to satisfy the particular needs and requirements of the end user. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein.