Method and apparatus for maintaining a liquid metal switch in a ready-to-switch condition

A method and apparatus for maintaining a liquid metal switch in a state of readiness for switching. The liquid metal switch has a liquid metal volume contained in a cavity of a switch body. A signal path though the cavity is made or broken by energizing an actuator to move the liquid metal volume within the cavity in response to a switching signal. To maintain readiness, a signal generator supplies a vibratory signal to the actuator. The resulting vibrations in the liquid metal volume allow the liquid metal volume to be subsequently moved with reduced power.

FIELD OF THE INVENTION

The invention relates to the field of electrical and optical switching and, in particular, to switches that use liquid metal as part of the switching mechanism.

BACKGROUND OF THE INVENTION

Liquid metals, such as mercury, have been used in switches to provide an electrical path between two conductors. An example is a mercury thermostat switch, in which a bimetal strip coil reacts to temperature and alters the angle of an elongated cavity containing mercury. The mercury in the cavity forms a single droplet due to high surface tension. Gravity moves the mercury droplet to the end containing electrical contacts or to the other end, depending upon the angle of the cavity. In a manual switch, a permanent magnet is used to move a mercury droplet in a cavity and bring it into contact with electrical contacts.

Liquid metal is also used in relays. A liquid metal droplet can be moved by a variety of techniques, including electrostatic and electromagnetic forces, variable geometry due to thermal expansion/contraction, and magneto-hydrodynamic forces.

Rapid switching of high currents is used in a large variety of devices, but provides a problem for solid contact based relays because of arcing when current flow is disrupted. The arcing causes damage to the contacts and degrades their conductivity due to pitting of the electrode surfaces. Liquid metal switches can overcome this problem.

Micro-electromechanical (MEM) systems also utilize liquid metal switching. When the dimension of interest shrinks, the surface tension of the liquid metal becomes dominant force over other forces, such as body forces (inertia). Latching switches are described in the co-pending patent applications that use liquid metal as the part that causes the electrical or optical signal to be routed in one path, blocked or routed in another path. Sometimes the use characteristic of these switches requires them to be actuated with higher than normal switching energies to move the liquid metal from one location to another after the switch-state has remained unchanged for several minutes. This phenomenon is called “kick-starting”. Kick starting is undesirable because it requires the use of at least two different switch drive energies, as well as keeping track of the time since the switch-state was last changed. Alternatively, repeated attempts to change the state of the switch must be made until a signal is received that the switch-state has been changed successfully. Both of these processes add complexity and waste switching time.

SUMMARY

A method and apparatus for maintaining a liquid metal switch in a state of readiness for switching. The liquid metal switch has a liquid metal volume contained in a cavity of a switch body. A signal path though the cavity is made or broken by energizing an actuator to move the liquid metal volume within the cavity in response to a switching signal. To maintain readiness, a signal generator supplies a vibratory signal to the actuator. The resulting vibrations in the liquid metal volume allow the liquid metal volume to be subsequently moved with reduced power.

DETAILED DESCRIPTION

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail one or more specific embodiments, with the understanding that the present disclosure is to be considered as exemplary of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding parts in the several views of the drawings.

The present invention relates to liquid metal switches. A liquid metal switch uses an actuator, acting on a liquid metal volume, to change the switch-state of the switch. In the present invention, a constant or intermittent vibration is generated in the liquid metal. This causes the liquid metal to be agitated in a manner similar to that occurring when the switch-state is changed, thereby eliminating the need for “kick starting” or repeated attempts to switch.

The constant or intermittent vibration is generated in response to a vibratory drive signal that can be superposed with the normal drive signal. This requires little circuitry or logic compared with “kick-starting” or other approaches.

In the case where the drive elements are piezoelectric elements, the vibratory drive signal can consume very little power because of the capacitive nature of the piezoelectric elements.

FIGS. 1-6show exemplary liquid metal switches.FIG. 1is a side view of an exemplary optical switch. The switch100comprises a top cap layer20, a bottom cap layer22and a switching layer24. Optical waveguides206and208transmit optical signals to a cavity104in the switching layer. A liquid metal volume102is moveable within the cavity104to block the optical path from waveguide206or208. Motion of the liquid metal volume is resisted by surface tension bonds to wettable contacts106,108and110. The liquid metal volume may be moved by pressure in an actuation fluid, by electromagnetic forces or by direct mechanical forces.

FIG. 2is a top view of the switching layer24of an exemplary liquid metal switch100in accordance with a first embodiment of the present invention. The switch includes a liquid metal volume102contained in a cavity104. The cavity is formed in the body of the switch. In addition to the liquid metal volume102, the cavity104contains an actuation fluid. Electrically conductive contacts106,108and110are located in the cavity104. In the figure, the liquid metal volume102provides an electrical signal path between contacts106and108, while the electrical path between contacts106and110is broken. The liquid metal volume can be moved within the cavity so that the electrical path between contacts106and110is completed, while the path between contacts106and108is broken. In this manner, signals attached to the contacts via conductors112,114and116can be blocked or routed through the switch. Other embodiments, using two or more contacts, will be apparent to those of ordinary skill in the art. A variety of actuation methods may be used to move the liquid metal volume. InFIG. 2the liquid metal volume is moved by creating a pressure difference across the liquid metal volume. Fluid reservoirs118and120contain actuators122and124. These may be heaters or volumetric actuators, such as piezoelectric elements. A heater may be used to heat a gas or to induce a phase change in a liquid, which increases the pressure in reservoir118. An increase in the volume of a piezoelectric element results in a decrease the volume of the reservoir, which increases the pressure. The increased pressure acts on the liquid metal volume and moves it towards contact110, breaking the electrical connection with contact108. In one embodiment, the contacts106,108and110have surfaces that are wettable by the liquid metal. The surface tension associated with the wetting action holds the liquid metal volume in the desired position, making the switch more robust against motion.

A similar arrangement may be used to switch optical signals.FIG. 3shows a liquid metal optical switch. The structure and operation of the switch as the same as described above, except that the electrical conductors are replaced by optical waveguides202,204,206and208. In the configuration shown inFIG. 3, an optical path is completed between waveguide204and waveguide208, since the fluid in the cavity104is transparent. The optical path between waveguide202and waveguide206is broken or incomplete, since the liquid metal102is opaque. In the one embodiment, the contacts210have surfaces that are wettable by the liquid metal. The surface tension associated with the wetting action holds the liquid metal volume in the desired position, making the switch more robust against motion.

A further embodiment of a liquid metal switch is shown in FIG.4. In this embodiment, electrical coils302and304may be energized to produce a magnetic field in the cavity104. A solid magnetic slug310is wetted by liquid metal102and is moveable within the cavity104. Energizing coil304will attract the solid magnetic slug310to contact116, while energizing coil302will attract the solid magnetic slug to contact114. The coils are actuated by electrical signals passed along conductors306and308. A similar arrangement may be used to construct an optical switch.

A top view of a further embodiment of a liquid metal switch100is shown in FIG.5. In this embodiment, piezoelectric actuators420and422in cavity418are energized to control the pressure of an actuation fluid in chambers423and425. The actuation fluid, in turn, moves the liquid metal volume.

FIG. 6is a sectional view through the section6—6of the relay shown in FIG.5. In this embodiment, the switch comprises a cover402, an actuator layer404, a diaphragm406, a cavity layer408and a base410. This layered structure is suitable for construction using micro-machining. The liquid metal volume in cavity104is separated into two smaller volumes102and103. The remainder of the cavity104is filled with an inert, electrically non-conducting fluid. The cavity also contains contacts412,414and416. The amount and location of the liquid metal is such that two contacts are connected at a time. Second cavity418houses piezoelectric actuators420and422. In operation, actuator422is extended in the direction of the arrow and deforms a flexible diaphragm406. At the same time, piezoelectric actuator420may be contracted. The resulting downward force on the liquid metal volume102, breaks the volume into two smaller volumes, one of which is moved towards liquid metal volume103and coalesces with it. As a result, the electrical path between contacts412and414is broken, while the electrical path between contacts414and416is completed. The operation may be reversed to return the switch to its original state. It will be apparent to those of ordinary skill in the art that a similar apparatus may be used for optical switching and that many other variations are possible.

A common aspect of the liquid metal switches described above is that a liquid metal volume is used to make or break one or more signal paths through the switch. The energy efficiency of such a switch depends, in large part, on the amount of force required to move the liquid metal volume. The properties of liquid metal are such that the liquid metal becomes more difficult to move if the metal is not agitated for a period of time. This may happen, for example, if the switch-state is not changed for several minutes. The present invention provides a method and apparatus for maintaining a liquid metal switch in a state of readiness to switch. An embodiment of the invention is shown in FIG.7. Referring toFIG. 7, a signal generator502includes both a switch signal generator504and a vibratory signal generator506. In a further embodiment, the signal generators504and506may be separate units, but generally efficiency is gained by incorporating both the switch signal generator and the vibratory signal generator in a single unit. The switch signal generator502generates a short-duration switching signal508whenever a change of switch-state is required. This signal is of a high enough level that the liquid metal volume is moved from one location to another. The interval between state changes may be longer than several minutes in some applications. The vibratory signal generator506generates a vibratory signal510. The vibratory signal may be a continuous signal, or an intermittent signal. If an intermittent signal is used, the pauses should be shorter than the time taken for the liquid metal to become more difficult to move. The switching signal and the vibratory signal are superposed at signal adder512to produce a combined signal514. The combined signal514is passed to the actuator in the piezoelectric switch100. In some switches multiple actuators are used, and so multiple signals may be required. For example, in an embodiment where two actuators are used, one signal may the inversion of the other. However, in a further embodiment, one actuator may be chosen to have the opposite polarity of the other so that a single signal may be used. The level of the vibratory signal should be sufficient to maintain the liquid metal in a ready-to-switch state. This level is generally much below the peak level of the switch signal.