Liquid metal, latching relay with face contact

Abstract

An electrical relay using conducting liquid in the switching mechanism. 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. A piezoelectric 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 piezoelectric 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 a piezoelectric 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. The relay is amenable to manufacture by micro-machining techniques.

Description

FIELD OF THE INVENTION

The invention relates to the field of micro-electromechanical systems (MEMS) for electrical switching, and in particular to a 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

An electrical relay is disclosed that uses a conducting liquid in the switching mechanism. 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. In an exemplary embodiment, a piezoelectric actuator, coupled to first electrical contact, is preferably energized close 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 returns to their starting positions. The liquid metal droplets remain coalesced because of surface tension. The electrical circuit is broken by energizing a piezoelectric actuator to move the electrical contacts farther apart 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 conducting liquid to bridge the gap between the contacts. The relay is amenable to manufacture by micro-machining techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however, both as to organization and method of operation, together with objects and advantages thereof, may be best understood by reference to the following detailed description of the invention, which describes certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a top view of a latching relay in accordance with certain embodiments of the present invention.

FIG. 2 is a sectional view of a latching relay in accordance with certain embodiments of the present invention.

FIG. 3 is a further sectional view of a latching relay in accordance with certain embodiments of the present invention.

FIG. 4 is a view of a switching layer of a latching relay in accordance with certain embodiments of the present invention.

FIG. 5 is a view of a switching layer of a latching relay in an open switch state in accordance with certain embodiments of the present invention.

FIG. 6 is a view of a switching layer of a latching relay in a closed switch state in accordance with certain embodiments of the present invention.

FIG. 7 is a view of a switching layer of a latching relay using unidirectional actuators in accordance with certain embodiments of the present invention.

FIG. 8 is a further sectional view of a latching relay showing an exemplary circuit routing, in accordance with certain embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

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 fluid, 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. In the sequel, piezoelectric and magnetorestrictive will be collectively referred to as “piezoelectric”. 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. 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. 1 is a top view of an embodiment of a latching relay 100 of the present invention. The section 2 — 2 is shown in FIG. 2 and the section 3 — 3 is shown in FIG. 3 .

FIG. 2 is a sectional view through the section 2 — 2 of the relay shown in FIG. 1 . Referring to FIG. 2 , the relay 100 comprises three layers; a circuit layer 102 , a switching layer 104 and a cap layer 106 . The circuit layer 102 supports electrical connections to the elements in the switching layer and provides a lower cap to the switching layer. The circuit layer 102 may 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 layer 104 may be made of ceramic or glass, for example, or may be made of metal coated with an insulating layer (such as a ceramic). The switching layer 104 incorporates a switching cavity 108 . The cavity may be filled with an inert gas. A first electrical contact 110 and a second electrical contact 112 are situated within the cavity 108 . A first actuator 114 is attached to the substrate of the switching layer at one end and supports the first electrical contact 110 at the other end. In operation, the length of the actuator is increased or decreased to move the first electrical contact 110 towards or away from the second electrical contact 112 . In an exemplary embodiment, the actuator is preferably a piezoelectric actuator. The second electrical contact 112 is positioned facing the first electrical contact 110 . The second electrical contact 112 may be attached directly to the substrate of the switching layer 104 or, as shown in the figure, it may be attached to a second actuator 116 that operates in opposition to the first actuator. The facing surfaces of the first and second electrical contacts are wettable by a conducting fluid. In operation, these surfaces support droplets of conducting fluid, 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. The cap layer 106 covers the top of the switching layer 108 , and seals the switching cavity 108 . The cap layer 106 may 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. In an exemplary embodiment, the electrical contacts preferably have a stepped surface. This increases the surface area and provides a reservoir for the conducting fluid. In an exemplary embodiment, the gap between the electrical contacts is 16 mils and the contacts are circular with a diameter of 30 mils. The step on the face of the contact extends 7 mils and has a diameter of 16 mils.

FIG. 3 is a sectional view through section 3 — 3 of the latching relay shown in FIG. 1 . The view shows the three layers: the circuit layer 102 , the switching layer 104 and the cap layer 106 . Referring to FIG. 3 , the first electrical contact 110 is positioned within the switching cavity 108 . The switching cavity 108 is sealed below by the circuit layer 102 and sealed above by the cap layer 106 .

FIG. 4 is a view of the relay from above (relative to FIG. 2 and FIG. 3 ) with the cap layer removed. The switching layer 104 incorporates the switching cavity 108 . The first and second electrical contacts 110 , 112 are situated within the cavity 108 . The first actuator 114 is attached to the substrate of the switching layer at one end and supports the first electrical contact 110 at the other end. The second electrical contact 112 is positioned facing the first electrical contact 110 . The second electrical contact 112 may be attached directly to the substrate of the switching layer 104 or, as shown in the figure, it may be attached to a second actuator 116 that operates in opposition to the first actuator.

In operation, the electrical contacts 110 and 112 support droplets of a conducting fluid, such as liquid mercury. FIG. 5 is a further view of the relay from above. Referring to FIG. 5 , the conducting fluid droplets 130 and 132 cover the electrical contacts. The volume of the conducting fluid and the spacing between the contacts is such that there is insufficient liquid to bridge the gap between the contacts. As shown in FIG. 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 are such that the liquid droplets remain coalesced due to surface tension in liquid. This is shown in FIG. 6 . Referring to FIG. 6 , the two droplets remain coalesced as the single liquid volume 140 . In this manner the relay is latched and the electrical circuit remains completed when the relay actuators are de-energized. 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. The first actuator may be bi-directional, in which case the length of the actuator is decreased to break the bond. Alternatively, if the first actuator is unidirectional, a second actuator may be used, as shown on FIG. 7 . Referring to FIG. 7 , if the actuator length is increased when the actuators are energized, the first actuator 114 is energized to move the contacts 110 and 112 closer together, while the second actuator 116 is energized to move them farther apart. Alternatively, if the actuator length is decreased when the actuator is energized, the second actuator 116 is energized to move the contacts 110 and 112 closer together, while the first actuator 114 is energized to move them farther apart. In a further embodiment, the actuators in FIG. 7 are bi-directional.

FIG. 8 is a further sectional view of a latching relay of the present invention, showing an exemplary circuit routing. Referring to FIG. 8 , circuits 702 and 704 pass through vias in the circuit layer 102 and are electrically coupled the first actuator 114 . The circuits terminate in a pad on the outer surface of the circuit layer. Circuit 706 is electrically connected to the first contact 110 . Control signals may be attached to the pads of circuits 702 and 704 using solder balls 708 and 710 . Similarly, connection can be made to the contact circuit 706 using solder ball 712 . Corresponding circuits 718 and 716 pass through vias in the circuit layer 102 and are electrically coupled the second actuator 116 . Circuit 714 is electrically connected to the second contact 112 . Control signals may be attached to the circuits 716 and 718 using solder balls 724 and 722 . Similarly, connection can be made to the contact circuit 714 using solder ball 720 . Dielectric material 726 and 728 provides electrical insulation between the various circuits.

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.

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.

Claims

1. An electrical relay, comprising:a first electrical contact, having a wettable surface; a first conducting liquid droplet in wetted contact with the first electrical contact; a second electrical contact, spaced from the first electrical contact and having a wettable surface facing the wettable surface of the first electrical contact; a second conducting liquid droplet in wetted contact with the second electrical contact; and a first actuator in a rest position, coupled to the first electrical contact and operable to move the first electrical contact towards the second electrical contact, to cause the first and second conducting liquid droplets to coalesce and complete an electrical circuit between the first and second electrical contacts, and away from the second electrical contact, to cause the first and second conducting liquid droplets to separate and break the electrical circuit.

2. An electrical relay in accordance with claim 1, wherein the first actuator is a piezoelectric actuator.

3. An electrical relay in accordance with claim 1, wherein the first and second conducting liquid droplets are liquid metal droplets.

4. An electrical relay in accordance with claim 1, wherein the volumes of the first and second conducting liquid droplets are such that coalesced droplets remain coalesced when the actuator is returned to its rest position, and separated droplets remain separated when the actuator is returned to its rest position.

5. An electrical relay in accordance with claim 1, wherein the wettable surfaces of the first and second electrical contacts are stepped.

6. An electrical relay in accordance with claim 1, further comprising a second actuator, coupled to the second electrical contact and operable to move the second electrical contact towards the first electrical contact, to cause the first and second conducting liquid droplets to coalesce and complete an electrical circuit, and away from the first electrical contact, to cause the first and second conducting liquid droplets to separate and break the electrical circuit.

7. An electrical relay in accordance with claim 6, wherein the second actuator is a piezoelectric actuator.

8. An electrical relay in accordance with claim 6, further comprising:a circuit substrate supporting electrical connections to the first and second actuators and the first and second electrical contacts; a cap layer; and a switching layer positioned between the circuit substrate and the cap layer and having a cavity formed therein; wherein the first and second actuators and the first and second electrical contacts are positioned within the cavity formed in the switching layer.

9. An electrical relay in accordance with claim 8, wherein at least one of the electrical connections to the first and second electrical contacts passes through the circuit substrate and terminates in a solder ball.

10. An electrical relay in accordance with claim 8, wherein at least one of the electrical connections to the first and second electrical contacts is a trace deposited on the surface of the circuit substrate.

11. An electrical relay in accordance with claim 8, wherein at least one the electrical connections to the first and second electrical contacts terminates at an edge of the switching layer.

12. An electrical relay in accordance with claim 8, manufactured by a method of micro-machining.

13. A method for switching an electrical circuit between a first contact and a second contact in a relay, the first contact supporting a first conducting liquid droplet and the second contact supporting a second conducting liquid droplet, the method comprising:if the electrical circuit is to be completed: energizing a first actuator to move the first contact and second contact closer together so that the first and second conducting liquid droplets coalesce to complete the electrical circuit; and if the electrical circuit is to be broken: energizing the first actuator to move the first contact and the second contact farther apart so that the first and second conducting liquid droplets are separated to break the electrical circuit.

14. A method for switching an electrical circuit between a first contact and a second contact in a relay, the first contact supporting a first conducting liquid droplet and the second contact supporting a second conducting liquid droplet, the method comprising:if the electrical circuit is to be completed: energizing a first actuator to move the first contact and second contact closer together so that the first and second conducting liquid droplets coalesce to complete the electrical circuit; and if the electrical circuit is to be broken: energizing a second actuator to move the first contact and the second contact farther apart so that the first and second conducting liquid droplets are separated to break the electrical circuit.

15. A method in accordance with claim 14, wherein the first actuator is attached to the first contact and the second actuator is attached to the second contact, further comprising:if the electrical circuit is to be completed: energizing the second actuator to move the first contact and second contact closer together so that the first and second conducting liquid droplets coalesce to complete the electrical circuit; and if the electrical circuit is to be broken: energizing the first actuator to move the first contact and the second contact farther apart so that the first and second conducting liquid droplets are separated to break the electrical circuit.

16. A method in accordance with claim 14, further comprising:if the electrical circuit is to be completed: de-energizing the first actuator after the conducting liquid droplets coalesce; and if the electrical circuit is to be broken: de-energizing the second actuator after the conducting liquid droplets separate.

17. A method in accordance with claim 14, wherein the first actuator is a piezoelectric actuator and wherein energizing the first actuator comprises applying an electrical voltage across the piezoelectric actuator.

18. A method in accordance with claim 14, wherein the first actuator is a magnetorestrictive actuator and wherein energizing the first actuator comprises applying an electrical voltage to generate an electromagnetic field across the magnetorestrictive actuator.