High-frequency, liquid metal, latching relay with face contact

An electrical relay using conducting liquid in the switching mechanism. The relay is amenable to manufacture by micro-machining techniques. In the relay, two electrical contacts are held a small distance apart. The facing surfaces of the contacts each support a droplet of a conducting liquid, such as a liquid metal. An actuator is energized to reduce the gap between the electrical contacts, causing the two liquid metal droplets to coalesce and form an electrical circuit. The actuator is then de-energized and the electrical contacts return to their starting positions. The liquid metal droplets remain coalesced because of surface tension. The electrical circuit is broken by energizing an actuator to increase the gap between the electrical contacts and break the surface tension bond between the liquid metal droplets. The droplets remain separated when the piezoelectric actuator is de-energized because there is insufficient liquid metal to bridge the gap between the contacts. Additional conductors are included in the assembly to provide a coaxial structure and allow for high frequency switching.

FIELD OF THE INVENTION

The invention relates to the field of micro-electromechanical systems (MEMS) for electrical switching, and in particular to a high frequency piezoelectrically actuated latching relay with liquid metal contacts.

BACKGROUND OF THE INVENTION

Liquid metals, such as mercury, have been used in electrical 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 of the cavity containing electrical contacts or to the other end, depending upon the angle of the cavity. In a manual liquid metal switch, a permanent magnet is used to move a mercury droplet in a cavity.

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

Conventional piezoelectric relays either do not latch or use residual charges in the piezoelectric material to latch or else activate a switch that contacts a latching mechanism.

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.

Micro-switches have been developed that use liquid metal as the switching element and the expansion of a gas when heated to move the liquid metal and actuate the switching function. Liquid metal has some advantages over other micro-machined technologies, such as the ability to switch relatively high powers (about 100 mW) using metal-to-metal contacts without micro-welding or overheating the switch mechanism. However, the use of heated gas has several disadvantages. It requires a relatively large amount of energy to change the state of the switch, and the heat generated by switching must be dissipated effectively if the switching duty cycle is high. In addition, the actuation rate is relatively slow, the maximum rate being limited to a few hundred Hertz.

SUMMARY

A high frequency electrical relay is disclosed that uses a conducting liquid in the switching mechanism. In the relay, two contacts are held a small distance apart. The facing surfaces of the contacts each support a droplet of a conducting liquid, such as a liquid metal. In an exemplary embodiment, a piezoelectric actuator is preferably energized to reduce the gap between the electrical contacts, causing the two conducting liquid droplets to coalesce and form an electrical circuit. The piezoelectric actuator is then de-energized and the electrical contacts return to their starting position. The liquid metal droplets remain coalesced because of surface tension. The electrical circuit is broken by energizing a piezoelectric actuator to increase the gap between the electrical contacts and break the surface tension bond between the conducting liquid droplets. The droplets remain separated when the piezoelectric actuator is de-energized because there is insufficient conducting liquid to bridge the gap between the contacts. Additional conductors are included in the assembly to provide a coaxial structure and allow for high frequency switching. The relay is amenable to manufacture by micro-machining techniques.

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 electrical relay of the present invention uses a conducting liquid, such as liquid metal, to bridge the gap between two electrical contacts and thereby complete an electrical circuit between the contacts. The two electrical contacts are held a small distance apart. Each of the facing surfaces of the contacts supports a droplet of a conducting liquid. In an exemplary embodiment, the conducting liquid is preferably a liquid metal, such as mercury, with high conductivity, low volatility and high surface tension. An actuator is coupled to the first electrical contact. In an exemplary embodiment the actuator is preferably a piezoelectric actuator, but other actuators such as magnetorestrictive actuators, may be used. When energized, the actuator moves the first electrical contact towards the second electrical contact, causing the two conducting liquid droplets to coalesce and complete an electrical circuit between the contacts. The piezoelectric actuator is then de-energized and the first electrical contact returns to its starting position. The conducting liquid droplets remain coalesced because of surface tension. In this manner, the relay is latched. The electrical circuit is broken by energizing a piezoelectric actuator to move the first electrical contact away from the second electrical contact to break the surface tension bond between the conducting liquid droplets. The droplets remain separated when the piezoelectric actuator is de-energized because there is insufficient liquid to bridge the gap between the contacts. The relay is amenable to manufacture by micro-machining techniques.

FIG. 1is a view of an embodiment of a latching relay of the present invention. Referring toFIG. 1, the relay100comprises three layers: a circuit layer102, a switching layer104and a cap layer106. The circuit layer102supports electrical connections to the elements in the switching layer and provides a lower cap to the switching layer. The circuit layer102also supports a ground trace118that forms parts of a ground conductor encircling the switching elements. The circuit layer102may be made of a ceramic or silicon, for example, and is amenable to manufacture by micro-machining techniques, such as those used in the manufacture of micro-electronic devices. The switching layer104may be made of ceramic or glass, for example, or may be made of metal coated with an insulating layer (such as a ceramic). A channel passes through the switching layer. At one end of the channel in the switching layer is a signal conductor134that is electrically coupled to one of the switch contacts of the relay. Further ground conductors130,132and120are electrically coupled to form a ground conductor or shield that is coaxial with the signal conductor134. The signal conductor134is electrically isolated from the ground trace by a dielectric layer122that surrounds the signal conductor. In an exemplary embodiment, the ground conductor120is preferably formed as a trace deposited on the under side of the cap layer106, while conductors130and132are fixed to the substrate of the switching layer. The cap layer106covers and seals the top of the switching layer104. The cap layer106may be made of ceramic, glass, metal or polymer, for example, or combinations of these materials. Glass, ceramic or metal is preferably used in an exemplary embodiment to provide a hermetic seal.

FIG. 2is a sectional view of an embodiment of a latching relay100of the present invention. The section is denoted by2—2in FIG.1. Referring toFIG. 2, the switching layer incorporates a switching cavity108. The cavity may be filled with an inert gas. First and second electrical contacts,110and112, are situated within the cavity108. A first actuator114is attached to the signal conductor134at one end and supports the first electrical contact110at the other end. In operation, the length of the actuator114is increased or decreased to move the first electrical contact110towards or away from the second electrical contact112. In an exemplary embodiment, the actuator is preferably a piezoelectric actuator. The second electrical contact112is positioned facing the first electrical contact110. The second electrical contact112may be attached directly to the signal conductor136or, as shown in the figure, it may be attached to a second actuator116that operates in opposition to the first actuator. The facing surfaces of the first and second electrical contacts are wettable by a conducting liquid. In operation, these surfaces support droplets of conducting liquid, held in place by the surface tension of the fluid. Due to the small size of the droplets, the surface tension dominates any body forces on the droplets and so the droplets are held in place. In an exemplary embodiment, the electrical contacts110and112preferably have a stepped surface. This increases the surface area and provides a reservoir for the conducting liquid. The actuators114and116are coated with non-wetting, conducting coatings126and128, respectively. The coatings126and128electrically couple the contacts110and112to the signal conductors134and136, respectively, and prevent migration of the conducting liquid along the actuators. Signal conductor136is electrically insulated from the ground traces by dielectric layer122.

FIG. 3is a sectional view through section3—3of the latching relay shown in FIG.4. The view shows the three layers: the circuit layer102, the switching layer104and the cap layer106. Referring toFIG. 3, the first actuator114is positioned within the switching cavity108. The switching cavity108is sealed below by the circuit layer102and sealed above by the cap layer106. The ground conductors120,122,130and132surround the actuator114and its non-wetting, conducting coating126. This facilitates high frequency switching of the relay.

FIG. 4is a view of the relay from above (relative toFIGS. 1,2and3) with the cap layer removed, that is, the section4—4in FIG.1. The switching layer104incorporates the switching cavity108, formed in the channel between the two signal conductors134and136. Within the switching cavity108are the first and second electrical contacts,110and112, and the actuators to which they are attached. The first actuator, with coating126, is attached to the first signal conductor134at one end and supports the first electrical contact110at the other end. The second electrical contact112is positioned facing the first electrical contact110. The second electrical contact112may be attached directly to the second signal conductor136or, as shown in the figure, it may be attached to the second actuator, with coating128, that operates in opposition to the first actuator. Ground conductors130and132line the channel in the switching layer.

In operation, the electrical contacts110and112support droplets of a conducting liquid, such as liquid mercury.FIG. 5is a further view of the relay from above with the top layer removed. Referring toFIG. 5, the conducting liquid droplets140and142cover the electrical contacts. The volume of the conducting liquid and the spacing between the contacts is such that there is insufficient liquid to bridge the gap between the contacts. When the liquid droplets are separated, as inFIG. 5, the electrical circuit between the contacts is open.

To complete the electrical circuit between the contacts, the contacts are moved together so that the two liquid droplets coalesce. This may be achieved by energizing one or both of the actuators. When the droplets have coalesced, the electrical circuit is completed. When the actuators are de-energized, the contacts return to their original positions. However, the volume of conducting liquid and the spacing of the contacts is such that the liquid droplets remain coalesced due to surface tension in liquid. This is shown in FIG.6. Referring toFIG. 6, the two droplets remain coalesced as the single liquid volume144. In this manner the relay is latched and the electrical circuit remains completed when the relay actuators are de-energized. When the electrical circuit is closed, the signal path is from the first signal conductor, through the first conductive coating, the first contact, the conducting liquid, the second contact and the second conductive coating, and finally through the second signal conductor. The ground conductor provides a shield surrounding the signal path. The use of mercury or other liquid metal with high surface tension to form a flexible, non-contacting electrical connection results in a relay with high current capacity that avoids pitting and oxide buildup caused by local heating. To break the electrical circuit again, the distance between the two electrical contacts is increased until the surface tension bond between the two liquid droplets is broken.

FIG. 7is a view of the inside surface of the cap layer106. The cap layer106provides a seal for the channel in the switching layer. A ground trace120is deposited on the surface of the cap layer, and forms one side of the ground conductor that is coaxial with the signal conductors and switching mechanism. A similar ground trace is deposited on the inner surface of the circuit layer.

While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.