WO2004095596A1

WO2004095596A1 - Electro-active device

Info

Publication number: WO2004095596A1
Application number: PCT/GB2004/001600
Authority: WO
Inventors: Anthony Hooley; Ursula Ruth Lenel; David Henry Pearce
Original assignee: 1... Limited
Priority date: 2003-04-16
Filing date: 2004-04-13
Publication date: 2004-11-04
Also published as: GB2416426B; GB2416426A; GB0522722D0; GB0308810D0; TW200524189A

Abstract

A device (20) comprises a plurality of electro-active benders (21), each having two sections arranged to bend, on activation, with opposite curvature in the two sections, in an arrangement in which the displacements of individual benders (21), on activation, contribute to an overall, relative displacement between a proximal end and a distal end of the device (20). The mass of benders (21) in the arrangement decreases from the proximal end to the distal end of said device (20).

Description

ELECTRO-ACTΓVE DEVICE

The present invention relates to an electro-active device, for example a piezoelectric device, having an arrangement of benders in which the displacement of each bender, when activated, contributes to an overall displacement of the device. The present invention is of particular application to benders that, when activated, bend with opposite curvature in two sections, also referred to as a "recurved" bender.

Piezoelectric material deforms on electrical activation by an applied electric field, or, vice versa, generates an electrical field in response to applied deformation. There are known many types of piezoelectric device which use piezoelectric material arranged with electrodes to apply or detect the electric field. Piezoelectric material belongs to the wider class of materials known as electro-active materials also including electrostrictive and piezoresistive material. For the purpose of this invention, all of these materials are collectively referred to as piezoelectric material.

The most simple type of piezoelectric device is a block of piezoelectric material activated in an expansion-contraction mode by applying an activation voltage in the direction of poling. However, as the piezoelectric effect is small, of the order of 10^" ¹⁰m/V, the change in dimensions is relatively small, less than 0.1 micron for a typical device. Therefore piezoelectric devices having a more complicated structure have been developed to achieve larger displacements For example a known construction for a piezoelectric device is the bender construction, for example a bimorph bender construction consisting of two layers of piezoelectric material or a multimorph bender construction consisting of more than two layers of piezoelectric material. With a bender construction, the layers are activated in an expansion-contraction mode with a differential change in length between the layers, usually with one layer expanding and another layer contracting. Due to the layers being constrained by being mechanically coupled to one another, this differential change in length is concomitant with bending in the thickness direction around an axis across the layers. Accordingly, there is relative displacement of the ends of the device.

The article "Recurve Piezoelectric-Strain- Amplifying Actuator Architecture", Ervin & Brei, IEEE/ASME Transactions on Mechatronics, Vol.3 No.4, December 1998 discloses details of recurved benders that, when activated, bend with opposite curvature in two sections. The recurved benders are limited to approximately half of the displacement of ordinary benders. To achieve greater displacements, the article further discloses arrays mechanically coupled in series and parallel such that the total displacement is equal to the sum of the displacements of the devices connected in series, h the light of the above known devices, it is seen as desirable to further improve their efficiency and performance particular for high frequency applications such as drive unit for acoustic loudspeakers with a bandwidth of 20 Hz to 20 KHz.

Accordingly, in a first aspect of the present invention there is provided a device comprising a plurality of recurved benders or bender segments arranged such that the displacement of the recurved benders contribute to a relative displacement between a proximal end and a distal end of the device, wherein the mass of benders in the arrangement gradually decreases from the proximal (fixed) end towards the distal (moving) end of the arrangement.

As a result of the mass of the benders decreasing from the proximal end to the distal end, the moving mass of the device as a whole is decreased as compared to an equivalent device in which the benders are all identical. The moving mass of each bender is the mass of a device that has the same maximum kinetic energy Emax as the bender when moving at its maximum velocity Vmax. The maximum kinetic energy can be calculated as half of the integral of the local mass elements times the square of the local velocity when integrated over the length of the bender. Using the moving mass, the integral can be replaced with the product of moving mass times the square of the maximum local velocity, which is the velocity of the moving or distal tip of the device. In general, the moving mass is equal or smaller than the actual mass of the device. Although the amount of the reduction would need to be calculated for any given device, in qualitative terms, the reduction in moving mass can be understood as resulting from the benders at the distal end which move most having a lower mass as compared to an equivalent device in which the benders are all identical. For example, in the type of embodiments described in detail below, for a device in which all the benders have the same mass, the moving mass is approximately a third of its total mass, whereas the embodiments can achieve a moving mass of around 0.2. The maximum speed of the actuator arrangement can then be increased by a factor that is approximately the square root of the reduction in the moving mass, for example the square root of 1.5 when the moving mass is reduced from 0.3 to 0.2 of the total mass.. The reduction in moving mass of a device in accordance with the present invention as compared to an equivalent device in which the benders all the same mass provides the advantage of increasing the maximum possible velocity of the distal end of the device. This increases the maximum amplitude response at a desired frequency or the maximum frequency response for a desired amplitude. Similarly, a device in accordance with the present invention may have a relatively high efficiency in the sense that more energy may be transferred to the load rather than being wasted moving the mass of the device itself, as compared to an equivalent device in which the benders have the same mass. Accordingly, a device in accordance with the present invention is well suited for high-speed dynamic applications.

The moving mass of a bender may be reduced by reducing its length, width, or thickness or through a combination of two or more of these alterations of its physical dimensions as a function of the distance from the proximate end

The reduction in width and/or length may create a stack of prismatic or pyramidal shape.

In embodiments of the invention in which the thickness of the benders is varied to reduce the mass, it is preferred to reduce the thickness in integral numbers of piezoelectric layers, more preferable in even numbers. For example, the first bender, closest to the mounting point, may have sixteen layers of electro-active material whilst the final distal bender, closest to the moving object, could be reduced to two layers. Hence the same drive voltage can be applied across each layer, having constant thickness, while the total thickness of each bender element varies.

In another preferred embodiment of the invention, the benders or bender segments are made of monocrystalline material or structured thin film material.

The device takes preferably the form of a stack with each bender mounted with its proximal end onto the distal end of the previous bender and extending in the opposite direction. Thus the benders are stacked anti-parallel to act in series.

The benders are preferably separate elements comiected together. However as an alternative the benders could be portions of a continuous piezoelectric element. As an example of this alternative, the device could be of the type disclosed in WO-03/038919 comprising a continuous piezoelectric element extending alternately back and forth to form a plurality of parallel, straight portions arranged in a stack, the straight portions each comprising a bender in accordance with the present invention.

These and other features of the inventions will be apparent from the following detailed description of non-limitative examples making reference to the following drawings, throughout which like parts are designated by like reference numerals and characters.

In the drawings:

Figs. 1 A,B show a vertical cross-section of a known recurved bender in a non- activated and activated state, respectively;

Fig. 2 shows a stack of recurved benders with varied thickness in accordance with an example of the present invention;

Figs. 3 A,B show a schematic top view and vertical cross-section, respectively, of a stack of recurved benders with varied length in accordance with another example of the present invention;

Figs. 4A,B show a schematic top view and vertical cross-section, respectively, of a stack of recurved benders with varied width in accordance with another example of the present invention; Figs. 5A,B show a schematic top view and vertical cross-section, respectively, of a stack of recurved benders with varied length and width in accordance with another example of the present invention;

Figs. 6A,B show a schematic top view and vertical cross-section, respectively, of a stack of tapered recurved benders with varied length and width in accordance with another example of the present invention;

Fig. 7 illustrates a possible electrode layout of benders used in the present invention.

Figs. 1 A, B illustrate a known piezoelectric bender 10 having a bimorph bender construction showing a cross-sectional view taken along the length of the bender 10. The piezoelectric bender 10 is similar to the known type of bender disclosed in "Recurve Piezoelectric-Strain- Amplifying Actuator Architecture", Ervin & Brei, IEEE/ASME Transactions on Mechatronics, Vol.3 No.4, December 1998. The bender 10 comprises two parallel layers 12 of piezoelectric material. Extending parallel to the layers 12 along the entire length of the bender 10, there are a centre electrode 13 between the two layers 12 and outer electrodes 14 outside the layers 12. The layers 12 are each poled in opposite directions in two sections 15 and 16 along the length of each element 10, with the layers 12 being poled in opposite directions to each other within each section 15 and 16, as shown by the arrows P. The sections 15 and 16 are the same length, that is substantially half the length of the bender 10. The bender 10 is electrically activated by applying activation voltages of opposite polarity to the outer electrodes 14, whilst holding the centre electrode 13 at ground, to create an electrical field in the same direction across both layers 12 as shown by the arrows E. As a result of the relative directions of poling P and of the electric field E, within each section 15 and 16 the bender 10 bends as shown in Fig. IB, with an opposite curvature in the two sections 15 and 16. Thus, as illustrated in Fig. IB, this bending causes a relative displacement by an amount d between the ends 17 and 18 of the bender 10. Furthermore, as a result of the two sections 15 and 16 being of equal length and the structure of the bender 10 being uniform along its length, there is no relative rotation between the ends 17 and 18, i.e, the ends of the bender 10 remain in parallel. Hereinafter benders which, on activation, bend with opposite curvature in two sections, in a similar manner to the bender 10 will be referred to as "recurved" benders. Whilst the above known benders and devices constructed therefrom have certain advantageous properties, they are rarely used in present-day industrial applications. However, there will now be described devices in accordance with the present invention employing benders of the same type as the bender 10 but with an improved arrangement. In Fig. 2, there is shown a cross-section of a device 20 comprising a stack of recurved benders 21 which bend on activation in the same manner as the bender 10 of Fig, 1. The device 20 includes four benders 21, but any number could in fact be present. Each bender 21 comprises two or more laminated layers 22 of ceramic PZT separated by layers of electrodes (not shown). The proximal bender 21 (lowermost in Fig. 2) is mounted onto a support 23, and so in use is fixed, whereas in use the distal bender 21 (uppermost in Fig. 2) moves. The number of layers within each bender 21 decreases from the proximal bender 21 to the distal bender 21 from sixteen to two layers. The layers have uniform size and thickness, and hence the number of layers in each bender is proportional its thickness and mass. Thus the mass of the benders 21 gradually decreases from the proximal bender 21 to the distal bender 21 and as a result the moving mass is decreased to around 0.2 of the actual mass, as compared to around 0.3 for an equivalent device formed from identical benders.

The benders 21 are linked by copper spacer elements 24 which each connect together a pair of adjacent benders 21, the spacer elements 24 being positioned at alternate ends of the benders 21 for each successive adjacent pairs of benders 21. Accordingly, the displacement of each individual bender 21 contributes to an overall relative displacement between the proximal end of the device 20, ie the fixed end of the proximal bender 21, and the distal end of the device 20, ie the free end of the distal bender 21. The spacer elements 24 also separate each adjacent pair of benders 21, the thickness of the spacer elements 24 varying with the maximum displacement of the bender mounted on it. Thus a collision of benders 21 is avoided when the device 20 is operated in a contraction or "pull" mode. The spacers are joined with the benders by an adhesive such as epoxy glue.

It is possible to combine devices 20 of the type shown in Fig. 2 in a horizontal or parallel arrangement for applications requiring an increased amount of force.

In Figs. 3 A, B there is shown a schematic top view and a vertical cross-section through another device 30 comprising a stack of benders 31. The device 30 has a similar arrangement to the device 20 of Fig. 2 except as follows. The benders 31 have equal thickness, but the length of the benders 31 is successively reduced from the proximal bender 31 to the distal bender 31. Thus the mass of the benders 31 gradually decreases from the proximal bender 31 to the distal bender 31.

The benders 31 are separated by copper spacer elements 34. The thickness of the spacer elements 34 varies with the maximum displacement of the bender 31 mounted on it, the spacer elements 34 decreasing in thickness from the proximal bender 31 to the distal bender 31.

The reduction of the mass can also be achieved by reducing the width of subsequent benders as in the device 40 illustrated in Figs. 4A,B, which show a schematic top view and a vertical cross-section, respectively, through the device 40. The device 40 comprises a stack of benders 41 having a width which gradually decreases from the proximal bender 41 to the distal bender 41, but otherwise having essentially the same arrangement as the devices 20 and 30 of Figs. 2 and 3. The benders 41 are linked by spacer elements 44 each having equal thickness and length.

A device 50 that combines a stepwise reduction in both, length and width of the constituent benders 51 is illustrated in Figs. 5A,B. The device 50 is shown in a schematic top view in Fig. 5 A and in a vertical cross-section in Fig. 5B.

The dimensions of the benders 51 are reduced successively in width and length from the proximal bender 51 to the distal bender 51, while the thickness of each bender 51 is maintained. Thus the mass of the benders 51 gradually decreases from the proximal bender 51 to the distal bender 51. Otherwise the device 50 has the same arrangement as the devices 20 and 30 of Figs. 2 and 3. At the end of each bender 51 there is mounted a spacer element 54 that carries the following bender.

It is possible to combine the example of Fig. 5 with the one shown in Fig. 2 to create a stack with benders with varied length, width and thickness.

Fig. 6 shows a device 60 comprising a stack of benders 61 in a similar arrangement to the preceding device 50 of Fig. 5, in particular with the width and the length of each of the benders 61 being reduced successively from the proximal bender 51 to the distal bender 51. In addition, the benders 61 are tapered along their longitudinal axis. Hence the width of the actuator 60 is continuously reduced along its length from its proximate end 63 to its tip 65. The benders 21, 31, 41, 51 and 61 of each devices 20, 30, 40, 50 and 60 described above can be manufactured from "green" laminated ceramic tape. The tape can be produced in accordance with well known manufacturing techniques. In its green state, it is then cut or folded into suitable shapes for the individual benders 21, 31, 41, 51 and 61. Then the tape is burned out and sintered at high temperatures (600 to 1200 degrees Celsius). Outer electrodes may be applied after the sintering and then poling is performed. These manufacturing steps are known per se and are regarded to be well within the scope of person skilled in the art. The resultant benders 21, 31, 41, 51 and 61 are then simply connected together by spacer elements 24, 34,44, 54 and 64 to form the devices 20, 30, 40, 50 and 60 which may be mounted onto a chassis.

Alternatively, sheets of monocrystalline piezoelectric material such as lead magnesium niobate-lead titanate or lead zinc niobate-lead titanate single crystals (PMN- PT, PZN-PT) that have piezoelectric and dielectric properties superior to those of other known ceramics may be used to fabricate the individual layers of such structures as described. These materials are commercially available from a number of sources.

A particularly advantageous electrode pattern for the individual benders 21, 31, 41, 51 and 61 is shown in Fig. 7. The top face of a sheet of tape is indicated by a dashed circumferential line 710. By screen-printing or other known methods two electrodes are applied to the face. Each electrode includes a main area segment 711 and leads 712 along an edge of the bender 71 terminating in contacts 713 at two opposite corners. The main area segments 711 of both electrodes effectively divide the bender 71 into the two sections required for activation as a recurved bender. Contacts can be made to each electrode from two sides of the bender facilitating the voltage supply, particularly to inner electrodes.

Claims

1. A device comprising a plurality of electro-active benders in an arrangement in which the displacements of individual benders, on activation, contribute to an overall, relative displacement between a proximal end and a distal end of the device, wherein the mass of benders in the arrangement decreases from the proximal end to the distal end of said device.

2. The device of claim 1, in which the benders are separate elements connected together.

3. The device of claim 1 or 2, wherein the benders include at least one bender having two sections arranged to bend, on activation, with opposite curvature in the two sections.

4. The device of any one of the preceding claims, wherein the benders in the arrangement vary in width, length, thickness or any combination thereof.

5. The device of any one of the preceding claims, wherein the benders are tapered along a longitudinal axis.

6. The device of any one of the preceding claims, wherein the device comprises a stack of benders in which each adjacent pair of benders is linked at ends of the benders, which ends alternate for each successive adjacent pairs of benders.

7. The device of claim 5, wherein the benders are stacked with varying distances between adjacent benders.

8. The device of claim 5, wherein the benders are stacked with varying distances between adjacent benders such distances being proportional to the maximum displacement of said benders.

9. The device of any one of the preceding claims, wherein the benders are pieces of ceramic tape.

10. The device of any one of the preceding claims, wherein the benders comprise sheets of monocrystalline piezoelectric material.