Tilt switch employing graphite

The invention relates to a form of a tilt switch that solves the problem of poor electrical contact, excessive/unpredictable hysteresis, high contact resistance, short life and/or electrical bounce. The tilt switch uses conventional ball-in-tube construction and adds a graphite powder film to all electrically conductive surfaces in the switch. This non-mercury tilt switch provides additional features such as enhanced electrical contact, reduced or eliminated hysteresis, lowered contact resistance, increased contact life and eliminates electrical bounce.

FIELD

This invention relates generally to tilt switches in particular those that eliminate or minimize actuation electrical power, reduce operating hysteresis, provide positive electrical function, eliminate the need for switches containing mercury or other toxic liquid metals and reduce manufacturing costs.

DESCRIPTION OF THE RELATED ART

Various methods have been devised to provide tilt switches in prior art. These switches may be classified as electrically actuated and self actuated.

The first class, electrically actuated, utilizes some form of electrically powered sensor or inertial stabilized element to sense the angle between the local gravity vertical and the reference plane of the switch (i.e. the “tilt angle”). Examples of such devices are servo pendulum accelerometers (U.S. Pat. No. 3,111,036, Kistler, and U.S. Pat. No. 5,006,487, Stokes), vibratory accelerometers (U.S. Pat. No. 2,928,668, Blasingame and U.S. Pat. No. 4,306,456, Maerfeld), convective accelerometers (U.S. Pat. No. 2,455,394, Webber and U.S. Pat. No. 6,182,509, Leung), and gyroscopic stabilized platforms (U.S. Pat. No. 1,563,934, Sperry) to name a few. These instruments generally have very low “operating hysteresis” (i.e. the angular difference between actuation during increasing tilt angle and deactivation during decreasing tilt angle or vice versa). The cited instruments suffer since they require electrical power to maintain any angle measuring capability either before or after the desired tilt angle is achieved. This generally prevents the extended duration use of such electrical switches in portable, battery powered devices.

These instruments have an advantage over powered sensors since they require no power to maintain angle measuring capability either before or after the desired tilt angles are achieved. All of these instruments suffer from the fact that they generally have very large operating hysteresis or require heavy masses (e.g. heavy pendulum bob) or large dimensions (e.g. long pendulum) to reduce operating hysteresis. The liquid electrical switches suffer from the use of toxic metals (e.g. mercury switches) or decomposable electrolytes (e.g. electrolytic switches). Rolling ball electrical switches generally suffer from poor electrical contact, excessive/unpredictable hysteresis, high contact resistance, short life and/or electrical bounce.

Other examples of metal-to-metal electrical switches are ball contact magnetic relays where a ferromagnetic ball is attracted (or repelled) by a solenoid or magnet to make or break an electrical circuit using the ferromagnetic ball as an electrical bridge between two electrical contacts. These devices generally suffer from poor electrical contact characteristics due to the nature of the metal ball surface electrical properties. As such, a large magnetic force is required to hold the metal ball in place between the contacts ensuring a good electrical connection between the contact terminals of the switch.

Prior art methods of increasing the electrical conductive properties of the ball have been attempted. For example, U.S. Pat. No. 6,180,873, Bitko, utilizes the discovery that certain liquids have varying dielectric properties depending upon the thickness of the liquid layer. These liquids are called mesoscopically conductive liquids or mesoscopic conductors or mesoscopic liquids. Thick layers of these mesoscopic liquids are insulators; whereas thin layers are conductors. One embodiment of the Bitko device involves a use of mesoscopic conductors in a current carrying device wherein a conductor moves relative to a conducting surface, which it engages. The Bitko device has the disadvantage of requiring containment of a (sometimes toxic) liquid substance which increases production costs and adds leakage risk.

German Patents DE9007264 and DE4021055, Gillert, utilize a cylindrical housing of electrically conductive material that encloses a space having a downwardly tapered conical portion in which a ball rides in a partial filling of conductive powder or granular material, preferably graphite or metallic dust. In the rest position the ball is seated in the taper making no contact between the switch terminals. At a predetermined angle of tilt, the ball rolling is damped by the bed of powder. Thus, the switch contact is substantially free from bounce even when the apparatus is jolted due to the dampening effect of the graphite powder. The Gillert device uses a large amount of graphite powder to dampen the motion of the ball and is at a serious disadvantage since the pile of graphite can also short between the contacts thus increasing the hysteresis of the tilt switch.

SUMMARY

The present invention is directed to tilt switches and other devices exploiting conductive graphite films. Graphite films operate as an insulator and as a conductor as a function of the thickness of a layer of the graphite film.

In one embodiment, the graphite film is applied to a charge carrying device as an interface between electrodes. In long distances across a film surface, the graphite film has high resistivity, acting as an insulator and thereby preventing or substantially eliminating charge transfer between electrodes. The graphite film conductor separating the electrodes transfers charge or current when the current carrying members touch each other. In such an embodiment, the electrodes might be movable into and out of engagement or be permanently engageable. The relative movement of electrodes might involve rolling, rotating, sliding, or the like, or any combination thereof.

DETAILED DESCRIPTION

The present invention involves the use of graphite film conductors in devices wherein current is conducted, and particularly wherein the current is to be modified, e.g., insulated, reduced, amplified, or otherwise regulated. For example, the invention includes the use of graphite film conductors in devices wherein a current carrying element is insulated under certain circumstances but permitted to conduct under other predetermined circumstances, e.g., a switch.

Graphite film conductors are characterized by their ability to adhere to metal surfaces. This property produces a highly conductive, non-corroding surface on the metal surface. The natural self adhering graphite layer can be used on any conductive metal surface to enhance the electrical conductivity between two metal surfaces. This is particularly useful to enhance a point contact (e.g. a bearing resting on a flat surface) or a line contact (e.g. a cylinder resting on a flat surface) electrical connection as typically found in tilt switches.

A graphite film conductor can be applied in any manner for use in the invention. For example, dusting, wiping, brush application, rolling, solvent application, spraying, etc. can all be used in the invention. This disclosure contemplates that there will always be at least a minimal continuous layer (i.e., at least one molecule thick) of graphite film conductor between electrodes. A significant characteristic of graphite film conductors is that these films possess high resistivity in the transverse (i.e. parallel to the largest area dimension) direction across thin films but low resistivity in the normal direction through the graphite thin film.

The unique and advantageous properties of graphite film conductors ensure that such conductors will prove to be useful in a wide variety of applications. For example, graphite film conductors will be useful in the fabrication of various types of switches, magnetically operated relays, thermocouples, thermostats, pressure sensors, accelerometers, adjustable capacitors (i.e., electronically adjustable), and other such devices that will readily suggest themselves to the skilled worker in this art in view of the present disclosure.

The present invention provides, among other things, a current carrying device including a pair of electrodes and a mobile or variably positioned conductive or charge carrying element (or shorting element or member) surrounded by, or separated from an electrode by, a layer of graphite film. In one embodiment, the mobile shorting element is perpetually in electrically conductive proximity (or graphite film proximity) to at least one electrode. As such, the mobile shorting element functions as a variably positioned extension of at least one electrode. Alternatively, the current carrying device comprises a pair of electrodes coated with a graphite film, the coated electrodes separated by a layer of graphite film coated on a suitable shorting element.

Known tilt switches may experience dramatic electrical hysteresis in operation. For a typical tilt switch wherein the circuit closes at 42° the circuit may only leave contact at 30°. The application of a graphite film reduces this electrical hysteresis to 1° or less, a reduction of approximately 90%.

In one embodiment the electrodes and mobile current carrying element are configured so that at least one electrode and the mobile current carrying element are substantially in perpetual graphite film proximity; under specified conditions, the mobile current carrying element moves into graphite film proximity, and thus electrically connects, the remaining electrode. The action of the mobile current carrying element is such that the electrodes are functionally isolated from each other only by the orientation of the mobile current carrying element and the graphite film. When the distance between the mobile current carrying element and the remaining electrode is great, i.e., a super-graphite film distance, there is no electrical connection; when the distance is small, i.e., a sub-graphite film distance or within graphite film proximity, an electrical connection is effected.

The present invention provides a method for regulating or controlling current flow through a current carrying device including separating electrodes by a layer of graphite film, and regulating the current flow between the electrodes by varying the current carrying distance of the graphite film conductor separating the electrodes. In such a method, the current flow is either facilitated or prevented as a function of the contact with the graphite film separating the electrodes.

Such a device will be recognized by one of ordinary skill in the art as a useful substitute for a tilt switch, particularly a mercury switch.

More particularly, an embodiment of tilt switch10is depicted inFIGS. 1 and 2. This embodiment comprises case12and ball-shaped, i.e., spherical, shorting member14displaceably mounted within chamber18formed by the casing. Inner surface16of the casing, which includes cylindrical portion17and circular portion20, is symmetrically configured about longitudinal axis B of the chamber, and is formed of an electrically conductive material such as a metal. The diameter of the cylindrical portion is larger than the diameter D of shorting member14.

At an end of the casing opposite circular surface portion20, electrically conductive terminal30is sealed by insulator32within conductive shell26, which shell has extended flange24welded to extended flange22of case12. Conductive shell26has tab28which provides for electrical termination of the case. An end of terminal30projects into chamber18and includes terminal face51desirably, but not necessarily, shaped as a spherical segment of the same radius as sphere14, i.e., one half diameter D. Other surface shapes could be used as well.

Terminal30extends along axis A, which axis A is offset relative to axis B so that when shorting member14rolls into contact with terminal30, the axis A will pass through the geometrical center of shorting member14for alignment of that member in terminal face51. The mutually contacting faces of terminal30and sphere14define electrically conductive interface52(seeFIG. 4a) which is desirably, but not necessarily, shaped to maximize the contact area between terminal30and shorting member14. In similar manner, the diameter of shorting member14is preferably selected to maximize the contact area with inner surface17of the casing at interface50established therebetween (seeFIG. 4b). The contacting faces can be formed of any suitably conductive material such as steel, iron, copper, silver, gold, etc. Inner surface16and ball shaped shorting member14are coated with a film of graphite to provide the previously cited benefits of this invention.

Insofar as embodiments of the present invention are contemplated as substitutes for mercury tilt switches, graphite film coated conductors have the advantage of an increased temperature operating range. Thus, graphite film coated conductors will operate well outside of the typical mercury operating range of −40° C. to about 150° C. In addition, unlike most mercury tilt switches, the inventive tilt switch is made of non-frangible components (i.e. metals or plastics versus glass).

Generally, inner surface16of the casing, shorting element14and face51are all coated by the graphite film. It will be appreciated that inner surface16, shorting element14and face51are not perfectly smooth, and as shown inFIGS. 4aand4b, produce between one another, spacings of various gaps as a function of the force exerted by shorting element14toward face51. That force is a function of gravity and the roughness of the opposing materials. It is desirable for the geometry of those components to maximize the contact area which will provide the maximum number of sites where the interfacial gap is minimized.

To enhance the number of such sites, it is also desirable to highly polish or smoothly finish the surfaces which define interfaces50,52, thereby minimizing the number of large projections which, by virtue of their presence, tend to separate the surfaces in a manner creating large gaps instead of the desired small gaps.

The graphite film must possess a relatively high electrical resistivity in the transverse direction (so as to avoid conducting current directly between terminal30and casing12), and yet possess a relatively low electrical resistivity across a thin film (i.e., when disposed in interfaces50,52) so as to be highly electrically conductive in the direction normal to the film thickness.

FIG. 3depicts a device similar toFIG. 1except that the spherical shorting element has been replaced by cylindrical shorting element14′ of circular cross section, and shoulders60have been provided on a floor of casing12′ to keep the cylinder properly centered. Also, face51′ of insulated terminal30′ has been shaped as a segment of a cylinder to conform to the outer periphery of cylinder14′.

In operation, it is obvious that if the left end of insulated terminal30or30′ is tilted so that it is above the right-hand end, shorting element14or14′ will roll away from face51or51′, thereby providing an open circuit. The bulk resistance of the graphite film conductor is so large that no shorting can occur between terminals30and12, or30′ and12′. Tilting the left end of terminal30to a level below the right-hand end will cause shorting element14or14′ to contact the casing and face51or51′ simultaneously, thereby closing the circuit. Connection to the switch is made via the external terminal portion of terminal30, and to the casing via shell tab28. The graphite film conductor contacts interfaces50and52thereby closing the circuit. Electrical resistance tests carried out in similar devices have indicated the presence of a contact resistance comparable to those found in prior art mercury switches of approximately the same size.

In another embodiment, shown inFIGS. 5aand5b, the electrodes are in the form of a pair of semi-circular segments60,62extending through insulator32. The segments are horizontally spaced and include surfaces shaped complementarily to that of shorting member14, i.e., either spherical or cylindrical. The shorting member contacts both electrodes simultaneously during tilting of the casing to close the circuit. All internal metal surfaces and components are coated with a graphite film.

In another embodiment, shown inFIGS. 6a-6c, semi-circular electrode segments70,72are vertically spaced apart. Thus, shorting member14initially makes contact only with lower electrode72during tilting of the case (seeFIG. 6a). Thereafter, in response to further tilting of the casing, shorting element14also contacts upper electrode70to close the circuit (seeFIG. 6b). In that way, control is maintained over the extent to which the casing must tilt in order to cause the circuit to be closed. All internal metal surfaces and components are coated with a graphite film.

In still another embodiment of the invention, shown inFIG. 7, shorting elements14are disposed between two relatively rotatable cylindrical surfaces80,82. Surfaces80,82constitute electrodes, and shorting elements14roll and slide while conducting current between those electrodes. All internal metal surfaces and components are coated with a graphite film.

In yet another embodiment of the invention, shown inFIG. 8, the electrodes comprise surface90, and moveable member92variably positioned across surface90. All metal surfaces and components are coated with a graphite film.

Depicted inFIG. 9a,9bis a preferred embodiment of an omni-directional tilt switch which is normally open and is closed by being tilted in any direction by a predetermined angle. As a result of such tilting, electrically conductive ball100is displaced from a position seated on a spherical surface of terminal114(FIG. 9a) to a position engaging both terminal114and a wall of conductive casing112(FIG. 9b). All internal metal surfaces and components are coated with a graphite film.

InFIGS. 10aand10bthere is shown an embodiment of a tilt switch which is normally closed. That is, electrically conductive ball120normally engages head122of terminal124(FIG. 10a) and edge126of casing128. When casing128is tilted beyond a predetermined angle (FIG. 10b) ball120rolls into recess130of casing128and out of contact with terminal124to open the circuit. The surface of head122can be of any suitable shape, such as spherical to conform to the shape of ball120. All internal metal surfaces and components are coated with a graphite film.

InFIGS. 11aand11b, there is shown an embodiment of a dual angle tilt switch which is normally closed. That is, electrically conductive ball120normally contacts terminal124and terminal124′ when the orientation of the tilt switch is less than +/− angle θ. When insulated casing32is tilted more than +/− angle θ (FIG. 11b) ball120rolls into recess130of casing32and out of contact with terminal124′ to open the circuit. All internal metal surfaces and components are coated with a graphite film.

InFIGS. 12aand12b, there is shown an embodiment of a dual angle tilt switch which is normally open. That is, electrically conductive ball120normally contacts terminals124when the orientation of the tilt switch is less than +/− angle θ. Since terminals124are in common electrical communication with each other, no circuit is completed. When insulated casing32is tilted more than +/− angle θ (FIG. 12b) ball120rolls into recess130of casing32and shorts terminal124′ to terminal124to close the circuit. All internal metal surfaces and components are coated with a graphite film.

InFIGS. 13aand13b, there is shown an embodiment of a dual angle tilt switch which is normally closed. That is, electrically conductive ball120normally contacts terminal124and terminal124′ when the orientation of the tilt switch is less than +/− angle θ. When insulated casing32is tilted more than +/− angle θ (FIG. 13b) ball120rolls out of hole130of casing32and out of contact with terminal124to open the circuit. All internal metal surfaces and components are coated with a graphite film.

In all of the above embodiments ofFIGS. 1 through 13b, a graphite film functions to significantly reduce the electrical resistivity at the terminal interfaces in the manner explained earlier herein.

InFIGS. 14aand14b, there is shown an embodiment of a dual angle tilt indicator enhancement. That is, ball120normally rests at the apex of internal cavity130when the orientation of the tilt switch is less than +/− angle θ. In this orientation ball130completely fills aperture132thus indicating state A. When casing32is tilted more than +/− angle θ (FIG. 14b) ball120rolls into a recess of internal cavity130of casing32and is hidden from view in aperture132thus indicating state B. Of course, modifications of this indicator can be made to function as both a passive indicator and an electrical tilt switch in a common unit utilizing the advantages of the aforementioned graphite film.

Although the invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, modifications, substitutions and deletions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims.