The present invention relates to piezoelectric relays, and more particularly to bimorph elements used to construct such relays and a method for driving such bimorph elements.

Background of the Invention

The switching of voltages is typically accomplished through the use of electromechanical relays or solid state devices. Electromechanical relays based on actuators employing magnetic fields present many disadvantages, including large size and weight, high power consumption, and lack of reliability. These disadvantages make such relays inapplicable for constructing systems requiring large numbers of relays such as cross-connect switches for telecommunications. Solid state switches, while much smaller in size and requiring less power than electromechanical switches, present the disadvantage of fragility to many types of real-world operating conditions such as large voltage or current transients. As a result, solid states switches are not suitable building blocks for constructing systems which are subject to such transients.

Relays based on piezoelectric bimorph elements avoid the limitations of the electromechanical relays described above. Bimorph elements typically consist of two plates of piezoelectric material sandwiched between three planar electrodes. The piezoelectric material is

typically a ceramic such as lead zirconate titanate.- The first electrode is located on an outer surface of ^• the first plate, the second electrode is sandwiched between the two plates and the third electrode is located on the outer surface of the second plate.

When a high voltage is applied across one of the plates, the plate changes length. If only one of the two plates is subjected to such a voltage, the bimorph will bend in a direction perpendicular to the plate. This bending effect is used to construct relays.

In a typical piezoelectric relay, one end of the bimorph is mounted in a cantilever manner over a surface. A first contact is mounted on the free end of the bimorph, and a second contact is mounted on the surface. The bending motion of the bimorph is used to make or break a connection between the two contacts.

Relays of this type are particularly well suited to telecommunications applications requiring large numbers of individual relays. In general, the physical size of the relays is much smaller than the size of the equivalent electromagnetic relay. In addition, the power needed to actuate the relay is considerably less than the conventional electromagnetic relay. Finally, a number of individual relays may be constructed from a common bimorph structure.

In a multiple relay switch based on piezoelectric bimorph elements, each relay requires a separate driving circuit which must be capable of supplying voltages of the order of a few hundred volts. Each such driving circuit must include high voltage transistors for switching these voltages. Since high

voltage transistors require substantially more silicon area to fabricate than low voltage transistors, it is important to minimize the number of such transistors needed to drive the bimorph elements. This is particularly important when the driving circuitry for a number of relays is to be incorporated into a single integrated circuit chip. Previous piezoelectric relays typically re-quire a minimum of four high voltage transistors per driving circuit. The cost of these transistors represents a significant fraction of the cost of the piezoelectric relay for many applications in which multiple relays are constructed from a common bimorph structure. Hence, it would be advantageous to provide a piezoelectric relay which required fewer high voltage transistors.

A second problem with previous piezoelectric relays is the large number of electrical connections which must be made to each relay. In these piezoelectric relays, the relays are driven by switching voltages on the top and bottom electrodes thereof. When a plurality of relays are constructed from a single bimorph structure, the driving scheme requires that the top and bottom electrodes of each relay be isolated from the corresponding electrodes of the other relays on the bimorph structure. In addition, separate connections must be made to the top and bottom electrodes of each relay. In systems having a very large number of relays, the sheer volume of connections reduces the reliability of the system.

Hence, it would be advantageous to reduce the number of connections.

In addition, it is more difficult to make the connections to the electrodes on the bottom surface of the bimorph structure. The driving circuitry is

typically mounted on the top surface of the bimorph structure. Connections to the electrodes on the top surface of the bimorph structure can be made by plating conductors on the top surface which connect an integrated circuit containing the driving circuitry to each of the electrodes. However, connections to the electrodes on the bottom surface of the structure typically require that a connection be made through the bimorph structure or around the edges thereof. Such connections are difficult to construct in an easily automated manner. Hence it is important to minimize the number of connections which must be made to electrodes on the bottom surface of the bimorph element.

Broadly, it is an object of the present invention to provide an improved piezoelectric relay element.

It is a further object of the present invention to provide a piezoelectric relay element that can be driven with fewer high voltage transistors than prior art piezoelectric relays.

It is yet a further object of the present invention to provide a piezoelectric relay which requires fewer electrical connections than prior art piezoelectric relays.

It is still another object of the present invention to provide a piezoelectric bimorph element which requires fewer connections to electrodes on the bottom surface thereof.

These and other objects of the present invention will be apparent to those skilled in the art

from the following detailed description of the invention and the accompanying drawings.

Summary of the Invention

The present invention comprises an improved piezoelectric relay which requires fewer transistors and requires fewer connections when used to construct multiple relay modules. The relay includes a bimorph member mounted in a cantilever manner. The bimorph member has a first contact affixed to the free end thereof. The bimorph member comprises first and second substantially planar strips of piezoelectric material. The planar strips are bonded to three planar electrodes. The first electrode is located on the outer surface of the first planar strip. The second planar electrode is sandwiched between the first and second planar strips. The third planar electrode is located on the outer surface of the second planar strip. The free end of said bimorph member is deflected in a direction perpendicular to the planar strips when a potential difference is applied between the first and second electrodes or between the second and third electrodes. A second contact is positioned so as to make electrical connection with the first contact when the free end of said bimorph member is moved to a predetermined position by so applying a potential difference. The relay includes circuitry for generating potential differences to be applied to the electrodes. The potential differences are created by connecting the first electrode to a first constant potential, connecting the third electrode to a second constant potential, and connecting the second electrode either to the first constant potential or the second constant potential in response to a control signal.

Brief Description of the ^* _rawinσs

The advantages of the present invention will become apparent from the following detailed description of the drawings in which:

Figure 1 is a cross-sectional view of a typical prior art piezoelectric relay with its associated driving circuit.

Figure 2 is a top view of a prior art relay module.

Figure 3 is a bottom view of the bimorph structure shown in Figure 2.

Figure 4 is an end view of the relay shown in Figure 2.

Figure 5 is a cross-sectional view of a relay according to the present invention and circuit for driving said relay.

Figure 6 is a top view of a relay module according to the present invention.

Figure 7 is a bottom view of the bimorph structure shown in Figure 6.

Figure 8 is a cross-sectional view of the relay shown in Figure 6.

Detailed Description

The advantages of the present invention will become more apparent after a discussion of a typical

prior art piezoelectric bimorph relay element 112 shown in Figure 1.

Figure 1 is a simplified embodiment of a piezoelectric relay 100 constructed from a bimorph element 112 which is actuated by switching the voltages applied to the top and bottom electrodes thereof. The bimorph element 112 is mounted in a cantilever manner over a mounting surface 114 by attaching one end to a raised portion 113 on the mounting surface 114. The free end of bimorph element 112 includes a first electrical contact 116 which is electrically isolated from bimorph element 112. The contact 116 is brought into contact with a second electrical contact 118 when the free bimorph end on which the first electrical contact 116 is mounted moves toward the surface 114.

The bimorph element 112 typically consists of two planar strips of piezoelectric material 120 and 122 which are bonded to three planar electrodes 124, 126 and 128. The top electrode 124 and the bottom electrode 128 are typically constructed by electroless plating of a conducting material such as nickel on the corresponding piezoelectric strips. Although other methods of bonding the electrodes to the surface of the piezoelectric strips will be apparent to those skilled in the art.

The center electrode 126 is typically a brass shim connected to plated electrodes on the surfaces of the piezoelectric strips which are adjacent to said shim. For clarity, these plated electrodes are omitted from Figure 1. Center electrode 126 may also be constructed by depositing a metallic layer between two sheets of piezoelectric ceramic material such as lead zirconate titanate prior to firing the ceramic

material .

Each of the strips of piezoelectric material 120 and 122 is polarized such that the application of an electrical field across the strip will result in a change in the length of the strip. If a potential is applied across only one of the strips, bimorph element 112 will bend causing the free end thereof to move in a direction which is perpendicular to the surface of the piezoelectric strips. The direction of motion will be toward the strip whose length decreased.

This polarization is typically accomplished by applying voltages between the two electrodes on each side of the piezoelectric sheet while cooling the piezoelectric sheet in question from a temperature above the Curie point of the piezoelectric material to a temperature below said Curie point. Alternatively, the polarization of some piezoelectric materials may be carried out at room temperature by applying significantly larger potentials across the piezoelectric strips then those needed if the materials are heated above their Curie point.

After polarization, the direction of the applied electrical field relative to the direction of polarization determines whether the length of the strip will increase or decrease. If the electric field produced by the potentials on the electrodes is in the same direction as the electric field used to polarize the piezoelectric strip, the piezoelectric strip will decrease in length.

Referring to bimorph element 112, the polarization of the piezoelectric material in strip 120 is in the same direction as that of the material in

strip 122. The electric fields used to actuate the relay are typically generated by the application of an electrical potential between the electrodes 124 and 126 simultaneously with the application of the opposite potential between the electrodes 126 and 128. This potential causes one of the strips to shorten and the other to elongate. As a result, the bimorph will either bend toward the surface 114 or away from said surface depending on the direction the electrical fields generated. Typically, one direction is used to close the relay contacts, the other is used to move the contacts away from each other. In principle, this second motion can be used to cause a second set of contacts 130 and 132 to close thus implementing a single pole double throw relay.

A typical driving circuit 200 for operating relay 100 in this manner is also shown in Figure 1. The circuit has two states which are specified by a signal on control line 129. In the first state, a first potential, V, is applied to electrode 126, and a second potential, ground, is applied to electrodes 124 and 128. In the second state, the first potential is applied to electrodes 124 and 128, and the second potential is applied to electrode 126.

The circuitry operates in the following manner. For the purposes of explanation, assume that in Figure 1 all transistors are MOS enhancement mode field effect transistors and also assume that when the transistors are "on" they are conducting current and when they are "off" they are not conducting current.

Referring to Figure 1, the input of an inverter 214 is connected to an input terminal 129, to the gate of transistor 204 and the input of an inverter 212.

The output of inverter 214 is coupled to the gate of a transistor 202. The drains of transistors 202 and 208 are connected to V. The sources of transistors 204 and 206 are connected to ground. The drain of transistor 208 and the source of transistor 206 are connected to each other as veil as being coupled to the center electrode. The source of transistor 202 is connected to drain of transistor 204 as well as being connected to the top and bottom electrodes 124 and 126.

When an input signal is provided to inverter 214 that is higher than the threshold voltage of the transistors and is V _t greater tahn V, transistors 202 and 206 will be on, and transistor 204 and 208 will be turned off. Here, V _t is the threshold voltage of transistor 202. Through this action a potential of V is applied to the top and bottom electrodes 124, 128 via line 150 and a potential of ground is applied to center electrode 126 thereby causing the relay 112 to bend downward.

When a signal is provided that is below the threshold voltage, the transistors 202 and 206 will be turned off and transistors 204 and 208 will be turned on. Through this action V is applied to the center electrode 126 and the top and bottom electrodes 124, 128 are connected to ground potential causing the relay 112 to bend upward.

As is seen, the circuit 200 requires at least four high voltage transistors 202, 204, 206, and 208 and three inverters 210, 212 and 214 to drive both the top and bottom electrodes 124 and 128 between V and ground potential. Furthermore, inverters 210 and 214 must be high voltage devices. In many applications, the cost of these transistors represents a significant

fraction of the cost of the relay 100.

In addition to requiring expensive driving circuitry, the prior art bimorph element 112 requires that connections be made to each of the electrodes.

These connections are particularly cumbersome in relay modules in which a plurality of relays are constructed from a single bimorph structure. Such a relay is shown in Figures 2-4 at 250. Relay 250 is substantially the same as the relay described in U.S. patent application Serial No. 896,792, filed August 15, 1986, in the names of John D. Harnden, Jr. and William P. Kornrumpf entitled "Piezoelectric Switch" and assigned to the General Electric Corporation.

Referring to Figures 2-4, relay 250 is constructed from a bimorph structure 252 mounted on a pedestal 254 over a surface 256. Figure 2 is a top view of bimorph structure 252. Figure 3 is a bottom view of the bimorph structure. Figure 4 is an end view of relay 250. The bimorph structure consists of top and bottom piezoelectric plates 251 and 253, respectively, which are bonded to a continuous center electrode 274.

Relay 250 includes a plurality of bimorph actuators 260. Each bimorph actuator 260 provides a means for causing a first contact 262 mounted thereon to move relative to a stationary second contact 264 when potentials are applied to electrodes included in the bimorph actuator in question. The individual bimorph actuators are constructed by dividing the bimorph structure into a plurality of "fingers" after the two piezoelectric plates have been bonded to center electrode 274.

In this prior art design, each bimorph actuator 260 includes three electrodes, a top electrode 270, a bottom electrode 272, and a center electrode. The center electrodes of all of the bimorph actuators are connected together. In this design, the center electrodes 274 are in the form of a continuous metal plate bonded to the top and bottom piezoelectric plates and substantially coextensive therewith.

Each bimorph actuator is caused to move by applying a potential to either the top electrode 270 or the bottom electrode 272 corresponding to the bimorph actuator in question. In this design, the center electrode 274 is held at ground potential. The circuitry for applying the potentials to the top and bottom electrodes is preferably contained in an integrated circuit chip 280 mounted on the top surface of the bimorph structure. Connections to electrodes 270 is provided by depositing conductors on the top surface of bimorph structure 252 as shown in Figure 2.

One problem with this prior art design is that connections must also be made between the bottom electrodes 272 and chip 280. This requires that connecting wires 282 be run from the bottom surface of the bimorph structure 252 to the top surface thereof. In the above cited U.S. Patent application, wires 282 connect chip 280 to bottom electrodes 272 around the edge of bimorph structure 252. These connections are difficult to automate, and, hence, significantly increase the cost of the relay module.

The present invention provides a means for reducing both the number of driving transistors needed to control a piezoelectric relay based on a bimorph actuator and, in addition, substantially reduces the

number of connections which must be made to control a multiple relay nodule.

Figure 5 shows a relay element 322 according to the present invention with a driving circuit 300 coupled thereto. Relay element 322 consists of a bimorph actuator 323 which is cantilever mounted over a surface 321 by connecting one end thereof to a pedestal

327. Bimorph actuator 323 includes top and bottom electrodes 332 and 334, respectively, a center electrode 339, and two piezoelectric strips 328 and 330. As is seen, piezoelectric strip 328 is sandwiched between top electrode 332 and center electrode 339. Piezoelectric strip 330 is sandwiched between center electrode 339 and bottom electrode 334.

Relay element 322 differs from prior art relay elements in that it is driven by applying potentials to the center electrode 339, top electrode 328 being held at a constant potential designated as V and bottom electrode 330 being held at a second constant potential designated as ground. This results in a potential difference appearing across only one of the two piezoelectric strips at any given time. If center electrode 339 is switched to a potential of ground, a potential difference appears across piezoelectric strip

328. The direction of polarization of piezoelectric strip 328 is chosen such that this potential difference causes the length piezoelectric strip 328 to decrease thus causing the bimorph actuator 323 to move upward which, in turn, separates contacts 335 and 337. Similarly, if center electrode 339 is switched to a potential of V, a potential difference appears across piezoelectric strip 330. The polarization of piezoelectric strip 330 is chosen such that this potential difference cause the length of piezoelectric

strip 330 to decrease thus causing the bimorph actuator 323 to move downward, which, in turn, causes contacts 335 and 337 to move into contact with one another.

A via hole 333 provides a path for connection to center electrode 339. This path connects center electrode 339 to driving circuit 300 which is used to switch center electrode 339 between the potential at which the top electrode is held and the potential at which the bottom electrode is held.

The present invention reduces both the number of driving transistors needed to operate bimorph actuator 323 and the number of electrical connections needed to fabricate a multiple relay module from bimorph actuators according to the present invention. Firstly, by driving the center electrode 339, the circuit associated therewith is simpler and utilizes half as many transistors as the circuit 200 shown in Figure 1, since only the center electrode need be switched between two potentials. A simple driving circuit for accomplishing this is shown at 300 in Figure 5. Circuit 300 includes transistors 302 and 304 and inverter 306.

In the preferred embodiment, transistors 302 and 304 are MOS enhancement mode transistors. Transistors 302 and 304 are said to be "on" when they are conducting current between the source and drain thereof and "off" when they are not so conducting current.

The gate of transistor 302 is coupled to the output of inverter 306. The drain of transistor 302 is coupled to V and top electrode 332. The source of transistor 302 is coupled to the drain of transistor

304 and center electrode 339. The gate of transistor 304 is attached to the input line 129 and the input of inverter 306. Bottom electrode 334 is coupled to the source of transistor 304 and ground.

Accordingly, when an input signal is provided that exceeds the threshold voltage of transistor 304, transistor 304 is on, while transistor 302 would be off due to the action of inverter 306. In this state, ground would be provided to center electrode 339, top electrode 332 and bottom electrode 334 being held at V and ground, respectively. As explained above, this results in bimorph actuator 323 bending upward.

If on the other hand, the input signal is below the threshold voltage, transistor 302 is turned on and center electrode 334 will be at a potential of V, top electrode 332 and bottom electrode 334 being held at V and ground, respectively. As explained above, this results in bimorph actuator 323 bending downward.

Driving circuit 300 utilizes two transistors and one inverter as compared to the four transistors and three inverters that are necessary to implement circuit 200 shown in Figure 1. Therefore it has been shown that by driving the center electrode 339 there is a significant reduction in the number of transistors compared to driving the top and bottom electrodes as shown in Figure 1.

In addition to reducing the number of driving transistors, the present invention substantially reduces the number of connections which must be made to construct a multiple relay module such as that discussed with reference to Figures 2-4, above. A multiple relay module according to the present

invention is shown in Figures 6-8 at 400. Relay module 400 is constructed from a bimorph structure 402 which is mounted over a surface 404 by attaching the bimorph structure to a pedestal 406. Figure 6 is a top view of module relay 400. Figure 7 is a bottom view of bimorph structure 402. And, Figure 8 is a cross-sectional view of relay 400 module taken along line 408-408' shown in Figure 6.

The bimorph structure 402 includes first and second piezoelectric plates 401 and 403 which are polarized in the same direction. Bimorph structure 402 is divided into a plurality of "fingers" in a manner analogous to bimorph structure 252 shown in Figures 2- 4.

Each finger is used to construct a bimorph actuator 410 which includes three electrodes, a top electrode 411, a center electrode 412, and a bottom electrode 413. All of the top electrodes 411 are connected together and held at a first potential. These electrodes are preferably constructed by depositing a metal layer on the top surface of bimorph structure 402. The layer in question may be photo- patterned so as to leave a non-metalized region 415 in which an integrated circuit chip 416 can be mounted and other conducting lines such as conductor 417 can be deposited without connecting said conductors to the top electrodes 411.

Similarly, all of the bottom electrodes 413 are connected to together and held at a second potential. The bottom electrodes 413 are preferably constructed from a single conducting layer deposited on the bottom surface of bimorph structure 402. Since, only one connection need be made to the bottom layer, no photo-

patterning of the bottom layer is needed. This single connection for connecting the bottom layer to the second potential has been omitted from the figures for clarity. However, it will be apparent to those skilled in the art that the connection in question can be made by providing a via hole to the bottom surface so that the bottom metal layer 413 can be connected to chip 416 or by merely making a single soldered connection to layer 413.

The center electrodes 412 are preferably constructed by depositing a metal layer between piezoelectric plates 401 and 403 while said plates are in the form of a green ceramic to form a sandwich-like structure. The metal layer in question is patterned such that it does not extend into the central region 419 of the plates. This sandwich structure is then fired to form bimorph structure 402 having a region which lacks the center electrode. After firing, cuts 425 are made from the edge of bimorph structure 402 into region 419. These cuts form gaps which isolate the center electrodes 412 of each bimorph actuator 410 from the corresponding center electrodes of the neighboring bimorph actuators. Access to the various center electrodes 412 is preferably made by way of a via hole 420 which may be punched in the top piezoelectric plate 401 while said piezoelectric plate is still in the form of a green ceramic. Conducting lines 417 from chip 416 to each center electrode 412 are deposited on the top surface of piezoelectric plate 401 to allow the individual bimorph actuators to be driven by the circuitry contained in chip 416.

Each bimorph actuator 410 includes a first relay contact 422 on the bottom surface of piezoelectric plate 403 which is caused to move

relative to a corresponding contact 424 which is fixed to surface 404. Relay contact 422 is electrically isolated from electrode 413.

Each of the relays in relay module 400 operate in a manner analogous to that described with reference to the single relay shown in Figure 5. When the center electrode 412 of a bimorph actuator 410 is connected to the second potential (i.e., the potential of the bottom electrode) by the circuitry in chip 416, a potential difference appears across the portion of the top piezoelectric plate 401 included in said actuator. As a result, the bimorph actuator in question bends upward separating contacts 422 and 424. Similarly, if the center electrode in question is connected to the first potential (i.e., the potential of the top electrode 411) , the potential difference will appear across the portion of the second piezoelectric plate 403 which is included in the bimorph actuator in question. This will cause said actuator to bend downward which, in turn, will bring contacts 422 and 424 into contact with each other thus making a connection in an electrical circuit in which said contacts are included. For clarity, the electrical circuit in question has been omitted from the drawings.

It will be apparent from a comparison of relay module 400 and the relay module 250 shown in Figures 2- 4 that the present invention requires significantly fewer electrical connections to implement. The module 250 requires two connections per bimorph actuator, one to the bottom electrode on each bimorph actuator and one to the top electrode. The present invention requires only a single connection to the center electrode on each bimorph actuator.

In addition, all of the connections between the driving circuitry and the individual bimorph actuators can be accomplished by depositing conductors on the top surface of the bimorph structure. As noted above, in the nodule 250 shown in Figures 2-4, the connections to the bottom electrodes on the bimorph actuators had to be made by wires connecting the chip on the top surface of the bimorph structure to the electrodes on the bottom surface thereof. These connections are difficult to automate and, hence, substantially increase the cost of fabricating the module 250.

Accordingly, the present invention represents a significant improvement in reliability and costs over previous piezoelectric relays.

Modifications to the present invention can be made and it would be understood by one ordinarily skilled in the art that those modifications would still be within the scope and spirit of the present invention. For example the V and ground potentials shown can be a variety of values and still be utilized with the present invention. Similarly, one ordinarily skilled in the art recognizes that the components utilized for the driving circuit can be active switching devices other than FET's. It is also clear that via hole 33 is only one of many ways to attach the circuit 300 to the center electrode 339 of the present invention. In addition multi-fingered relay modules having different geometrical shapes can be constructed according to the present invention. For example, a single sided comb-like structure in which the fingers extend from a common spine in only one direction will be apparent to those skilled in the art.

Accordingly, while this invention has been

described by means of a specific illustrative embodiment, the principles thereof are capable of a vide range of modification by those skilled in the art Hence, the present invention is to be limited only by the scope of the following claims.