WO2002103818A2

WO2002103818A2 - Electro-active elements & devices

Info

Publication number: WO2002103818A2
Application number: PCT/GB2002/002854
Authority: WO
Inventors: Anthony Hooley; Ursula Ruth Lenel; David Henry Pearce; Gareth Mckevitt; Mark Richard Shepherd; Gary Lock
Original assignee: 1... Limited
Priority date: 2001-06-20
Filing date: 2002-06-19
Publication date: 2002-12-27
Also published as: GB2393036B; WO2002103818A3; GB0115064D0; GB0400664D0; GB2393036A

Abstract

An electro-active element comprises at least one elongate member of electro-active material (11) extending along a curve and having two pairs of electrodes (12, 13) spaced across the width W of the elongate member radially of the curve. The electrodes of each pair extend along the length L of the elongate member on opposite sides of the member for developing an electric field across a region of the member therebetween on activation. The material of the elongate member in each region is poled parallel to the electric field to be capable of activation in an expansion-contraction mode with a differential change in length L of the two regions (15, 16) concomitantly with bending of the member parallel to the width of the member so that said bending is concomitant with relative rotation of the ends of the element about an axis on the inner side of the curve. The electro-active element may extend along a helix. The helix may itself extend along a curved axis resulting in a device which twists around the curved axis and concomitantly generates linear displacement between the ends of the device on activation.

Description

ELECTRO- ACTIVE ELEMENTS AND DEVICES

The present invention relates to electro-active elements and electro-active devices formed from such elements. In some aspects, the present invention relates to elements and devices in which, on activation, a relative rotation between the ends of the element occurs. In other aspects, the present invention relates to electro-active devices in which, on activation, relative linear displacement of the ends of the device occurs. In further aspects, the present invention relates to a method of manufacture of an electro-active element, and the element thus manufactured. Electro-active materials are materials which deform in response to applied electrical conditions or, vice versa, have electrical properties which change in response to applied deformation. The best known and most developed type of electro-active material is piezoelectric material, but other types of electro-active material include electrostrictive material and piezoresistive material. Many devices which make use of electro-active properties are known.

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 10^"10 m/V, the change in dimensions is relatively small, typically less than a micron. Therefore, more complicated electro-active structures have been developed to achieve larger displacements.

A known piezoelectric structure is the bender construction, for example a bimorph bender construction consisting of two layers of piezoelectric material or a multimorph bender construction consisting of plural 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, for example one layer expanding and another layer contracting. Due to the layers being constrained by being coupled to one another, this differential change in length is concomitant with bending in the thickness direction perpendicular to the layers, that is about an axis parallel to the width. Accordingly, there is relative displacement of the ends of the device.

As an example, a piezoelectric device 1 having a bimorph bender construction is illustrated in Figs. 1 and 2.

The device 1 is in the form of an elongate tape comprising two layers 2 of piezoelectric material extending parallel to one another. The device has electrodes 3 and 4 extending parallel to the layers 2. In particular, a central electrode 3 is arranged between the layers 2 and outer electrodes 4 are arranged on the outer surfaces of the layers 2. On activation, an electric field is developed across the layers 2 between the electrodes 3 and 4. The material of the piezoelectric layers 2 is poled parallel to that electric field, so that the layers 2 are activated in an expansion- contraction mode and thus change length. In particular, there is a differential change in length between the layers 2 which is concomitant with bending perpendicular to the layers. Fig. 2 illustrates this bending by showing the device 1 as viewed from the side on activation fixed at one end 5, the dotted line 6 showing the original position of the device 1 in the inactive state. The bending causes the other end 8 of the device to be relatively displaced.

In general when a device with a known bender construction is activated, there is a relative displacement between the ends of the device. As the structure bends and the degree of curvature increases, the relative displacement of the ends follows a curve in space, not a straight path.

Typically a device with a known bender construction has dimensions of thickness T, width W and length L which are related as L » W > T. The thickness T is necessarily small, typically less than 1 mm, to allow establishment of a high electric field between the electrodes. Bending is also in the thickness direction, so the small thickness results in a device which is not very stiff. This allows relatively high tip displacement, typically of the order of tens of μm, but generates little force. The force generated may be increased by increasing the width, but increase of the width can cause problems such as dishing of the device in the width direction, so placing a restraint on the width of the device relative to the length. It would be desirable to produce a device capable of generating higher forces for a given size of device.

It would also be desirable to produce a device which is capable of a relative rotation between the ends of the device about a given point on activation.

According to the first aspect of the present invention, there is provided an electro-active element comprising at least one member of electro-active material extending along a curve and having two pairs of electrodes spaced across the width of the member radially of the curve, the electrodes of each pair extending along the length of the member on opposite sides of the member for developing an electric field across a region of the member therebetween on activation, and the material of the member in each region being poled parallel to the electric field to be capable of activation in an expansion-contraction mode with a differential change in length of the two regions concomitantly with bending of the member parallel to the width of the member so that said bending is concomitant with relative rotation of the ends of the element about an axis on the inner side of the curve. Such an element comprises an member having two pairs of electrodes spaced across the width of the member with the electrodes of each pair extending along the length of the member on opposite sides. Thus each pair of electrodes develops an electric field across a respective region of the member on activation, the regions being spaced across the width of the member. Further more, the material of the member is poled parallel to the electric field developed on actuation. As a result, the material in each region is capable of activation in an expansion-contraction mode in which the region undergoes a change in length. In use, the device is activated with a differential change in length of the two regions. As a result of the two regions being constrained because they are coupled together, the differential change in length is concomitant with bending of the member parallel to its width, that is about an axis parallel to its thickness. This contrasts with the bending which occurs in a device having a known bending construction in which the bending occurs parallel to the thickness of the device, that is parallel to the developed electric field. The electro-active element in accordance with the present invention is stiffer, and hence generates a higher force, than a device having a known bender construction of comparable dimensions. This results from the fact that the bending occurs across the width of the member rather than across the thickness. Again, the thickness of the device is desirably small to allow development of high electric fields. Preferably, the thickness of the member is at least an order of magnitude less than the length of the member. However the width, which controls the stiffness, may be selected to be greater than the thickness, preferably considerably greater, for example several mm in a device of thickness less than 1mm. Generation of higher forces is advantageous in many applications where the device is required to move significant masses.

Preferably, the member is elongate, that is with its length along which it extends being greater than its width, preferably by at least an order of magnitude. This is to provide a magnitude of displacement between the ends of the member on activation which is of a significant amount. In addition, in accordance with the first aspect of the present invention, the member extends along a curve. Therefore, the bending of the member which occurs on activation is concomitant with a relative rotation of the ends of the element about an axis on the inner side of the curve. There is also a small change in the radius of the curve along which the member extends, but it is the relative rotation of the ends of the member about the axis of curvature which is particularly useful.

In use, the electro-active element may be electrically activated by application of appropriate activation voltages across each pair of electrodes. The resultant electric field causes bending of the member which generates the relative rotations of the ends of the element because it is curved. In this mode of operation, the electro- active element may be used as an actuator device to drive relative rotation of two objects which may be coupled to the ends of the element. For example, the electro- active element may be used as a device to rotate relatively massive objects, for instance in print-heads, scanners, optical devices or any such equipment. The element may be used as an actuator to drive movements of objects having greater masses than devices of a known bender construction of similar dimensions. Alternatively, the electro-active element may be mechanically activated by mechanically displacing the ends of the element to relatively rotate them. This bends the member causing the development of electric fields between the electrodes, thus generating voltages across each pair of electrodes. These voltages may be interpreted as a measure of the relative rotational displacement of the ends of the element. Thus in this mode of operation, the electro-active element may be used as a sensor device to sense rotational force or displacement.

The change in radius which occurs is independent of the length of the member and is a function of its curvature. However, the relative rotation of the ends is proportional to the length of the device. Therefore, it is desirable to design a member extending along a long curve. In this case, the force generated decreases with the length of the member, so the construction providing for an increased force, as discussed above, is particularly advantageous compared to a known bender construction. In general, the member may extend along a curve of any shape, although of course regular curves are advantageous from the point of view of designing the member to provide a desired rotation.

In one preferred form, the curve along which the member extends is planar. The use of a planar curve is advantageous because it minimises the thickness of the electro-active element. A simple example of such a planar curve which may be used is the arc of a circle. Another example is a spiral which has the particular advantage that the degree of relative rotation between the ends of the member may be increased, because each turn of the spiral contributes to the overall displacement. In general, a spiral curve allows any desired degree of relative rotation to be achieved by increasing the number of turns of the spiral.

In another preferred form the curve along which the member extends is a helix.

The use of a helix is advantageous because it allows the degree of relative rotation between the ends of the member to be increased, because each turn of the helix contributes to the overall displacement of the ends of the device. In general, any desired degree of relative rotation may be achieved by providing the helix with an appropriate length.

Preferably, for ease of manufacture the electrodes of each pair are on opposite surfaces of the member extending along the length of the member. However, this is not essential as there may be intermediate layers between the electrodes and the member.

Preferably, the material of the member is a piezoelectric material. However, any type of electro-active material other than piezoelectric material may alternatively be used. Electro-active elements in accordance with the present invention may be coupled together to form an electro-active device.

In a first type of device the electro-active elements are coupled to create said relative rotation in parallel on activation.

This first type of device has the advantage of generating a high force equal to the sum of the forces generated by each of the elements of the device. A preferred arrangement is that the electro-active elements are planar and arranged in a stack and coupled at each end to the adjacent elements on both sides. Tins arrangement is particularly easy to manufacture, because the planar elements are themselves easy to manufacture and are easy to stack together. In a second type of device the electro-active elements are coupled to create said relative rotation in series on activation.

This second type of device is advantageous, because the overall relative rotation produced by the device is equal to the sum of the relative rotation generated by each element of the device. In a preferred arrangement, the electro-active elements are planar and arranged in a stack and coupled at each end to an adjacent element. This arrangement is particularly advantageous for ease of manufacture.

The planar elements are themselves easy to manufacture, for example by cutting the members of electro-active material f om a sheet. Furthermore, the planar elements are easily stacked. In accordance with the present invention, it is possible to form a device comprising an electro-active structure which extends along a minor axis and which, on actuation, twists around the minor axis. The structure may be constituted by an electro-active element comprising a member extending along a curve which is helix, as described above. Alternatively the structure may be constituted by the second type of device described above in which the electro-active elements are coupled to create relative rotation in series.

A particularly advantageous type of device in accordance with the first aspect of the present invention comprises such a structure which twists around an axis on activation in which that axis is curved rather than straight. Similarly, in accordance to the second aspect of the present invention there is provided an electro-active device comprising an electro-active structure comprising electro-active portions disposed successively along a curved minor axis, the electro-active portions comprising a member of electro-active material having two pairs of electrodes across the width of the member radially of the minor axis, the electrodes of each pair extending along the length of the member on opposite sides of the member for developing an electric field across a region of the member therebetween on activation, and the material of the member in each region being poled parallel to the electric field to be capable of activation in an expansion-contraction mode with a differential change in length of the two regions concomitantly with bending of the member around the minor axis parallel to the width of the member, the portions being arranged, on activation, for the structure to twist around the minor axis and concomitantly for relative linear displacement of the ends of the structure to occur.

In the second aspect of the invention, the electro-active portions may be separate elements coupled together, corresponding to the second type of device in accordance to the first aspect of the present invention described above. Alternatively, the electro-active portions in devices in accordance with the second aspect of the present invention may be finite portions of a continuous electro-active member curving around the minor axis, for example in a helix which corresponds to the helical form of the electro-active element in accordance to the first aspect of the present invention. Such an electro-active device is advantageous because on activation it provides relative linear displacement between the ends of the structure. The relative displacement between the ends of the device occurs concomitantly with the twist of the structure around the minor axis on activation, because of the fact that the device extends along a curved minor axis. The electro-active device uses the physical principal that twisting of a curved object causes displacement perpendicular to the local curve, and vice versa displacement of the ends of a curved object causes twisting along its length. The displacement is equivalent to a change in the orientation of the minor axis of the structure relative to its original orientation. Considering any given small section of the structure along the curved minor axis it is easy to visualise how twist of that given section rotates adjacent sections and hence relatively displaces them in opposite directions perpendicular to the local curve of the given section, because the adjacent sections extend at an angle to the given section as a result of the curve of the minor axis. Therefore twisting of the given section is concomitant with a relative displacement of the adjacent sections perpendicular to the plane of the curve. The degree of relative displacement is proportional to the degree of curvature in the given section and the magnitude of the twisting. The overall displacement of the device is the combination of the displacement of each section. Thus the overall displacement on activation is a relative displacement of the ends of the structure.

For minor axes which extend along a regular curve around a major axis, such as along an arc of a circle or a helix, on activation each section produces displacement in the same direction parallel to the major axis. Therefore, the overall relative displacement of the end of the structure is a linear displacement parallel to the major axis. Therefore an electro-active device of this type can produce displacement which is linear in space.

The degree of displacement is proportional to the length of the structure along the minor axis, because each section of the structure contributes to the overall displacement. Therefore any desired degree of displacement may be achieved by suitable design of the device, in particular by selection of the length of the structure along the minor axis and of the type of structure which controls the magnitude of the twisting-field response. As a result of the structure extending along a minor axis which is curved, a relatively compact device may be produced. In general, the curve along which the minor axis extends may be of any shape whatsoever. One possibility is for the curve along which the minor axis extends to be planar, for example as the arc of a circle or a spiral. In this case, the displacement on activation occurs perpendicular to the plane of the curve. The thickness of the device in the direction in which relative displacement occurs is merely the thickness of the electro-active structure so a relatively thin device may be produced. Another possibility is for the curve along which the minor axis extends to be a helix. In this case, each helical turn of the structure contributes towards displacement in the direction along the geometrical major axis around which the helix is formed. Therefore a large degree of displacement may be achieved proportional to the number of helical turns, therefore producing relatively high displacement for a relatively compact device.

In accordance with the third aspect of the present invention, there is provided a method of manufacture of an electro-active element, the method comprising: forming a member of sintered electro-active material; applying two pairs of electrodes spaced across the width of the member with the electrodes of each pair extending along the length of the member on opposite sides of the member for developing an electric field across a region of the member therebetween; and applying poling voltages across each of the pairs of electrodes to pole the material of the member in each region parallel to the electric field. The method allows manufacture of an electro-active element in accordance with the first aspect of the present invention and comprising a single member. The method also allows manufacture of a similar electro-active element comprising a single member extending along a straight line. Thus in accordance with the fourth aspect of the present invention, there is provided an electro active element which may be manufactured in accordance with the method of the third aspect of the present invention, that is an electro-active element comprising a single member of electro- active material having two pairs of electrodes applied after sintering of the material of the member, the pairs being spaced across the width of the member, the electrodes of each pair extending along the length of the member on opposite sides of the member for developing an electric field across a region of the member therebetween on activation, and the material of the member in each region being poled parallel to the electric field to be capable of activation in an expansion-contraction mode with a differential change in length of the two regions concomitantly with bending of the member parallel to the width of the member. The structure of the electro-active element in the third and fourth aspect of the present invention provides the advantage that the manufacture of the element is eased and the cost of manufacture is reduced compared to a known bender construction. In particular, the structure of the element allows the electrodes to be applied after firing of the material of the member because there are no buried electrodes. Formation of the member itself is simplified, because the electrodes need not be provided at this stage. For example, the member may be formed by any suitable method, for example by cutting it from a sheet. Subsequently, after firing of the member, the pairs of electrodes are applied. This step is also easy to perform, for example by electroless deposition or by depositing a metallic ink. Thus it is not necessary to co-fire the electrodes whilst sintering the member. This advantage is achieved because the electro-active elements in accordance with the present invention do not include any central electrodes buried within the element. In contrast, there are buried electrodes in many known electro-active devices. For example the device 1 having a known bender construction illustrated in Figs. 1 and 2 has a central electrode 3 buried between two piezoelectric layers 2. This advantage of the present invention results from the geometry of the device in which the pairs of electrodes are spaced across the width of the member rather than being spaced across the thickness.

In manufacture of known devices having a buried central electrode, the central electrode is typically applied to the piezoelectric material in the green state i.e. before sintering. Thus the material of the electrodes needs to be capable of withstanding the temperature sufficient to sintering the piezoelectric material. Typically the material might be noble metal or a noble metal alloy, such as platinum or silver-palladium. Such materials are very expensive and typically form the major part of the cost of the device. In contrast, in the present invention, as a result of applying the electrodes after sintering, it is possible to use a material which is not capable of withstanding the temperatures during sintering allowing a much cheaper material to be selected. Thus the overall cost of manufacture of the electro-active element is reduced considerably as compared to known devices which require co- firing of a central electrode with the piezoelectric material.

The method in accordance with the third aspect of the present invention may be used to manufacture an electro-active element in which the member is straight.

Alternatively, the method may be used to manufacture an electro-active element in which the member extends along a curve, thereby forming an electro- active element in accordance with the first aspect of the present invention. The advantage of not needing to co-fire the central electrode is of even greater benefit in comparison with a curved device having a known bender construction in which it would be necessary to perform the curving steps with the central electrode present. A further advantage is achievable by forming the member by cutting the member from a piece of sintered electro-active material. For example, a planar member may be cut from a disc or a helical member may be cut from a cylinder or tube.

In contrast, to manufacture a curved device having a conventional bender construction it would be necessary to deform and curve the multi-layer structure including the co-planar layers of piezoelectric material and at least the central electrode. This creates a number of problems. Firstly, the deformation process induces unwanted stresses which can lead to cracks and weakness in the final device. Also, as the device is shaped in the pre-sintering state, the shrinkage which occurs during sintering makes it difficult to provide the resultant element with accurate final dimensions due to the curved geometry. This is further hampered because the stresses created in deforming the material also affect the shrinkage in an unpredictable possibly non-linear manner. In accordance with the present invention the problems are avoided by cutting the member from a piece of electro-active material.

To allow better understanding, embodiments of the present invention will now be described by way of non-limitative example with reference to the accompanying drawing in which:

Fig.1 is a perspective view of a device having a known bender construction;

Fig. 2 is a side view of the device of Fig. 1 on activation;

Fig. 3 is a perspective view of a portion of an electro-active element in accordance with the present invention;

Fig. 4 is a perspective view of a straight electro-active element having the construction of the portion of Fig. 3;

Fig. 5 is a top plan view of the element of Fig. 4 on activation;

Fig. 6 is a perspective view of an electro-active element in accordance with the present invention extending along an arc of a circle;

Fig. 7 is a top plan view of the element of Fig. 6;

Fig. 8 is a perspective view of a device comprising a plurality of the elements of Fig. 6 connected in parallel;

Fig. 9 is a perspective view of a device comprising a plurality of the elements of Fig. 6 connected in series;

Fig. 10 is a perspective view of a further device comprising a plurality of the elements of Fig. 6 connected in series;

Fig. 11 is a top plan view of an electro-active element extending along a spiral; Fig. 12 is a perspective view of an electro-active element extending along a helix;

Fig. 13 is a perspective view of a device comprising two inter- threaded electro-active elements extending along a helix;

Fig. 14 is a perspective view of an electro-active element extending along a helix curving around an axis which is itself curved; Fig. 15 is a plan view of a device comprising a plurality of the elements illustrated in Fig. 6 coupled in series along an axis which is curved; and

Figs. 16 and 17 are perspective views of the device of Fig. 15;

Fig. 18 is a plan view of a further device comprising a plurality of the elements illustrated in Fig. 6 coupled in series along an axis which is curved; and

Figs. 19 and 20 are perspective views of the device of Fig. 18.

In the following description, some of the embodiments are described with reference to axes which are imaginary, but are nonetheless useful for visualising and defining the elements and devices. The electro-active elements which are embodiments of the invention all have a common construction along the length of the elements. Therefore, this construction will first be described with reference to Fig. 3 which shows a portion 10 of an electro-active element along the length L of the element. Any of the electro-active elements described below may be considered as a plurality of adjacent portions 10 as illustrated in Fig. 3 disposed successively along the length of the element, which may be straight or curved as described further below. The construction illustrated in Fig. 3 is preferably uniform along the entire length of the electro-active elements described below in order to provide uniform properties on activation. Alternatively, the electro-active elements may be designed with some variation along the length of the element, for example in the cross-section dimensions or type of material used.

The electro-active portion 10 comprises a member 11 of electro-active material extending along the length L of the portion 10. The member has a rectangular cross-section, although it will be appreciated that other cross-sections could be used. Electrodes 12, 13 are provided on the opposed surfaces of the member 11 on opposite sides across the thickness T of the member 11. The electrodes are arranged as two pairs 12, 13 spaced across the width W of the member with a gap 14 therebetween so that the pairs 12, 13 of electrodes are electrically isolated from each other. The electrodes 12,13 extend along the length L of the member 11 and thus along the length L of the overall electro-active element. In many devices the electrodes will extend along the entire length of the electro-active element, hi other devices there may be a plurality of electrodes separated along the length of the element, the electrodes each extending along portions of the element to allow independent activation of those portions.

On activation, an electric field E is developed between the electrodes of each pair 12, 13 in the respective regions 15 and 16 of the member 11 therebetween. Thus the electric field E is developed across the thickness T of the member 11. The material of the member 11 in the regions 15, 16 is poled parallel to the electric field, i.e. in the direction P across the thickness direction t. Such poling is achieved during manufacture by applying poling voltages across each pair of electrodes 12, 13 of sufficient magnitude to pole the material of the member 11 in the regions 15, 16. As a result of the poling direction P and the direction of the electric field E being parallel, the material of the member 11 in the regions 15 and 16 is activated in an expansion-contraction mode. Thus on activation the regions 15, 16 undergo a change in length along the length L of the portion 10. In fact there is a differential change in length between the portions 15 and 16, preferably with one of the portions 15 or 16 expanding and the other of the portions 15 or 16 contracting. As a result of the regions 15, 16 being constrained by being coupled together along the length L of the portion 10 because they are regions of the same member 11, such a differential change in length is concomitant with bending of the member parallel to the width W.

This is similar to the bending which occurs in devices of a known bender construction where the bending is also driven by a differential change in length. However the bending of the portion 10 occurs across the width W, that is about an axis parallel to the thickness direction or the direction in which the electric field E is developed. This is because the pairs 12, 13 of electrodes and the regions 15, 16 therebetween are spaced across the width W of the device. In contrast, in the known bender construction, the layers which undergo a differential change in length are spaced across the thickness T of the device, so bending occurs in the direction of the thickness T perpendicular to the layers, that is about an axis parallel to the width, as illustrated above with reference to Fig. 2.

The electro-active portion 10 may be electrically activated by applying an activation voltage across each of the pairs 12, 13 or electrodes. In this case, the activation voltages generate the electric fields E in the regions 15 and 16 thereby causing bending of the element 11. In this mode of operation, an electro-active element of which the portion 10 is part may be used as an actuator to drive relative displacement of objects which may be coupled to the ends of the element. Alternatively, the electro-active portion 10 may be mechanically activated by displacing the ends of the electro-active element of which the portion 10 forms part relatively in the direction of the width W, thus causing the electro-active portion 10 to bend. Thus the regions 15, 16 are mechanically deformed with a differential change in length which generates the electric field E and produces activation voltages across each pair of electrodes 12, 13. Thus generated activation voltages may be detected to sense the bending of the device and hence the applied force or displacement. In this mode of operation the electro-active element of which the portion 10 forms part may be used as a sensor.

The pairs of electrodes 12, 13 are electrically connected to a circuit 17. In use the circuit 17 may apply the activation voltages to electrically activate the portion 10 or may detect the activation voltages generated by mechanical activation of the portion 10.

The material of the member 11 is preferably piezoelectric material. The piezoelectric material may be any suitable material, for example a piezoelectric ceramic such as lead zirconate titanate (PZT) or a piezoelectric polymer such as polyvinylidenefluori.de (PVDF). However, the material of the member 11 may be any other type of electro-active material for example piezoresistive material, in which the electrical resistance changes as the material is deformed or strained, or electro strictive material, which constricts on application of an electric field.

The relative directions of the electric field E and the poling direction P are preferably selected to be in the same direction in one of the regions 15 and 16 and in opposite directions in the other of the regions 15 and 16, so that on activation one of the regions 15 and 16 expands and the other of the regions 15 and 16 contracts. This may be achieved by selection of the relative directions of the poling voltage applied during manufacture to pole the material of the member 11 and the activation voltages applied or developed on activation. One possibility is for the poling voltages to be applied across each pair 12, 13 of electrodes with the same polarity to pole the regions 15 and 16 in parallel directions, in which case the activation voltages are developed across each pair of electrodes 12, 13 in opposite directions.

It will be appreciated that instead of comprising a single member 11 as illustrated in Fig. 3, it would be possible to form the portion 10 of a plurality of members arranged in parallel along the length L of the portion and each having the construction of the single portion 10 illustrated in Fig. 10 so that they are activated in parallel.

As to the relative dimensions of the construction illustrated in Fig. 3, it is preferable that the length L (of the entire electro-active element of which the portion 10 forms a part), the width W and thickness T are arranged such that L » W > T. The thickness of the portion 10 is necessarily small to allow development of a high electric field across the member 11 for a given activation voltage. The width W is selected to be greater than the thickness t, preferably considerably greater. For instance a typical thickness T might be less than 1mm in which case the width W might be several mm.

Since bending occurs across the width W, the portion 10 is stiff in the direction of bending. This means that the force generated is increased, albeit associated with a reduction in the displacement. Thus a particular advantage of electro-active elements having the construction illustrated in Fig. 3 is that they provide higher forces, albeit with a lower displacement, as compared to an equivalent device having a known bender construction of comparable dimensions. To illustrate this, the device 1 having the known bender construction illustrated in Fig. 1 will be compared with a straight device 21 illustrated in Fig. 4. The electro-active element 21 has the construction of the portion 10 illustrated in Fig. 3 which may be considered as a part of the electro-active element 21 along its length L. Thus the electro-active element 21 comprises an elongate member of electro-active material 22 having two pairs 23, 24 of electrodes spaced across the width of the member 22 and extending along the length of the member 22. The electro-active element 21 on activation bends in the width direction as illustrated in Fig. 5 which is a view from above showing the displacement d of the free end 26 with respect to the opposite end 25 which is held fixed, the inactive position of the electro-active element 21 being illustrated by the dotted line 27.

The electro-active element 21 is straight along its length L in order to provide a comparison with the known device 1 illustrated in Fig. 1. However, the following comparison of devices in accordance with the present invention and the devices having a known bender construction applies equally to the further electro-active elements described below which extend along a curve.

The devices 1 and 21 will be compared for typical dimensions of length L of 20mm, width W of 5mm and thickness T of 1mm. With these dimensions, the electro-active element 21 achieves a force of over 4N with a displacement of 16μm, whereas the device 1 having a known bender construction generates a force of less than IN with a tip displacement of 82μm. Similarly, for a width W of 2.5mm instead of 5mm (with the other dimensions remaining the same), the electro-active element 21 generates a force of about 2Vι times the force generated by the known device 1 and generates a displacement d of about 0.4 times the displacement d of the known device 1. In general, the product of the force and displacement factors is unity. However, the construction of electro-active elements in accordance with the present invention as illustrated in Fig. 3 gives the considerable advantage that it is easier to design a device which generates a high force. Higher forces are advantageous in many applications where it is required to drive movement of significant masses, for example in a loudspeaker where a diaphragm displaces air, or to provide significant force, as for instance in the closure of a switch or relay.

There will now be described a number of different electro-active elements which extend along their length L along a curve. They each have the same construction as the portion 10 illustrated in Fig. 3 which may be considered as a part of the electro-active elements along their length. In the electro-active elements, the pairs of electrodes are spaced radially of the curve so that the bending on activation increases or decreases the radius of curvature. Thus, on activation, as well as a small change in radius, a significant rotation of one end of the element with respect to the other occurs around an axis on the inner side of the curve. This axis will be the geometrical axis of the element for a regular curve, but could in general be any axis.

The change in radius is a function of the curvature of the element and is independent of its length. However, the relative rotation of the ends is a function of length, increasing with length. As a result, very significant rotations can be achieved by appropriate selection of the length of the curve.

The advantages referred to above of generating high force as compared to a device having a bender construction with equivalent dimensions also apply to such curved elements. However, in the case of a curved element which is very long, for example extending along a spiral or helix, this advantage is particularly significant, because the long length of the device produces a corresponding reduction in the force generated.

Figs. 6 and 7 illustrate an electro-active element 28 which extends along its length along an arc of a circle about an axis 32. The electro-active element 28 has the same construction as the portion 10 illustrated in Fig. 3. Thus, the electro-active element 28 comprises an elongate member 31 of electro-active material extending along an arc of a circle about an axis 32, with the width of the device extending radially between an inner circumference 33 and an outer circumference 34. Two pairs of electrodes 35 spaced across the width W of the member 31 radially of the curve are formed extending along the length L of the elongate member 31 on both the upper and lower surfaces which both appear as illustrated in Fig. 7. Half-way across the width W of the element 28, the pairs of electrodes 35 are separated by a gap 36 extending along the length L of the element 28. The element 28 extends almost all the way around the axis 32 with only a small gap 37 between its ends 38 and 39. A device comprising a smaller arc of a circle, for instance a semi-circle, would be possible but would generate less rotation. On activation, the electro-active element 28 bends parallel to its width, so its ends 38 and 39 rotate relative to each other around the axis 32. For instance, if the end 38 is fixed then the other end 39 rotates in the direction of the arrow 29. Alternatively, if the element 28 is fixed half-way along its length at the position shown by the dotted line 30, then each of the ends 38 and 39 rotate creating a pincer movement. As a refinement, the electrodes 35 may be split along the position of the dotted line 30 to allow each arm of the pincer to be independently activated. In a rotary actuator device, this would allow improved control of the object being rotated. Electro-active elements in accordance with the present invention may be coupled together to form an electro-active device. The electro-active element 28 of Fig. 6 is particularly suitable for this as a result of extending along a planar curve, because a plurality of such elements 28 may be easily arranged in a stack. As an example, Fig. 8 illustrates an electro-active device 50 in which a plurality of the elements 28 of Fig. 6 are arranged in a stack. Fig. 8 shows the device 50 as having five elements 28 but this is merely for illustration and any number may be provided. The individual elements 28 are coupled in parallel by couplings 51. The couplings 51 couple the elements 28 at each end to the adjacent elements 28 on both sides so that the relative rotation generated by each element 28 between its ends on activation occurs in parallel. The couplings 51 may be of any suitable form, for example an adhesive, some form of adherent interlayer or a mechanical coupling. As an alternative, the coupling 51 may be replaced by coupling extending along the entire length of the elements 28. In use of the device 50, the individual elements 28 are activated together, for example by providing common activation voltages to each element 28. The resultant relative displacement of the ends 52 and 53 of the device 50 have the same magnitude as the displacement of an individual element 28, but the force generated is equal to the sum of the forces generated by each element 28. Thus the device 50 provides a high force.

Preferably, adjacent elements 28 are poled in opposite directions so that the activation voltages necessary to activate each successive element 28 are of alternate polarity. As a result, the polarity of the activation voltages applied to adjacent electrodes on two adjacent elements 28 are at the same polarity. This means that it is not necessary to electrically isolate the electrodes of adjacent elements 28 to prevent shorting or arcing. Figs. 9 and 10 illustrate electro-active devices 55 and 55a, respectively, comprising a plurality of the electro-active elements 28 of the type illustrated in Fig. 6 coupled in series by couplings 56 and 56a, respectively. For each device 55 and 55a, the elements 28 are disposed successively along the axis 32 about which relative rotation of the ends of the elements 28 occurs. The devices 55 and 55 a are illustrated as comprising five electro-active elements 28 but this is merely for illustration and any number of elements 28 may be provided.

The individual electro-active elements 28 are coupled in series through the stack by couplings 56 and 56a. In the device 55 of Fig. 9, successive couplings 56 are arranged at alternate ends of the elements 28 as one progresses through the stack of element 28 and couple adjacent, overlying ends of the elements 28. In the device 55a of Fig. 10, the couplings 56a couple opposite ends of adjacent elements 28 to each other, with the couplings 56a extending across the gap 37 between the ends 38 and 39 of the elements 28. Thus, in both devices 55 and 55a, each end of the elements 28 in the stack is coupled to an opposite one of the adjacent elements 28. The couplings 56 and 56a may take any suitable form, such as adhesive, an adherent interlayer, a piece of material or a mechanical coupling.

In use, the individual elements 28 are activated so that the relative rotation of each individual element 28 adds in series through the stack of elements 28. In the device 55 of Fig. 9, as the stack of elements 28 overlie one another with the couplings 56 at alternate ends, this means that the relative rotations of successive elements 28 are in the alternate senses around the axis 32. In the device 55a of Fig 10, as the opposite ends of the adjacent elements 28 are coupled, the relative rotation of each element 28 is in the same sense around the axis 32. The ends of the device 55 are the free ends 57 and 58 of the elements 28 at either end of the stack. On activation, the ends 57 and 58 of the device 55 undergo a relative rotation which is the sum of the individual relative rotations generated between the ends of each element 28 in the device 55. Thus the device 55 is capable of generating a considerable relative rotation between its ends 57 and 58. In general, any relative rotation may be generated by selecting an appropriate number of elements 28. The force generated is similar to that of a single element.

Fig. 11 illustrates an electro-active element 41 comprising an elongate member 40 of electro-active material which extends in a spiral around axis 48. Thus element 41 is another example of an electro-active element in which the curve along which the member extends is planar. The electro-active element 41 has the same construction along its length as the portion 10 illustrated in Fig. 3. For clarity, in the main view of Fig. 11 neither the gap between electrodes nor the gap between adjacent spiral turns is shown, this being illustrated in the insert for the section shown in dotted lines. The insert shows portions 42 of three turns of the spiral of the element 41 which are separated by gaps 43. Along each length 42, the elongate member 40 is provided with two pairs of electrodes 45 spaced radially across the width of the elongate member 40 with a gap 44 in between.

On activation of the electro-active element 41 a considerable degree of relative rotation occurs between the outer end 46 and the inner end 47 of the elongate member 40 about the axis 48 because each of the turns of the spiral along which the element 41 extends contributes to the relative rotation of the ends 46 and 47. Fig. 12 illustrates an electro-active element 60 comprising an elongate member 61 of electro-active material which extends along a curve which is a helix about an axis 62.

Along its length, the electro-active element 60 has the same construction as the portion 10 of Fig. 3. The elongate member 61 is provided with two pairs of electrodes 63 and 64 spaced across the width of the elongate member radially of the helical curve about the axis 62, the electrodes 63 and 64 extending along the length of the member 61 on opposite sides. On activation, bending of the electro-active elements 60 occurs around the axis 62 causing a relative rotation of the ends 65 and 66 of the electro-active element 60 about the axis 62. A considerable degree of relative rotation between the end 65 and 66 can be generated, because each of the turns of the helix contributes towards the overall relative rotation. In general, any degree of relative rotation may be achieved by selection of an appropriate length for the element 60. As described above, the construction of the electro-active element 60 means that it generates a higher force, albeit with lower displacement, as compared to the use of a known bender construction of the same dimensions. As compared to a helical device having a known bender construction, the electro-active element 60 of Fig. 12 generates a higher force, but less rotation per helical term. The generation of a higher force is in itself advantageous. Furthermore, there is an additional advantage as compared to the use of a known bender construction that a higher number of helical terms can be packed into a given length along the axis 62. This results from the fact that the bender 61 is oriented with its thickness extending parallel to the axis 62, whereas with a conventional bender construction, the width of the bender would extend parallel to the axis 62.

For comparison one can consider an elongate member with a width W of 4mm and thickness T of 1mm curving in a helix with a length along the axis 62 of 40mm. If a known bender construction were used with the width of the tape parallel to the axis 62 then eight helical turns would be present. In contrast, an element in accordance with the present invention would have twenty turns. Thus although a helical element in accordance with the present invention produces less rotation per turn, this can be compensated for with a higher number of turns such that the total rotation between the end 65 and 66 of the element 60 may be of the order of that of a helical device with the same length formed from a known bender construction having similar dimensions.

In addition, the greater force currently available with the construction used in the present invention is compounded by the closer packing of material, which also improves the force generated. Thus as compared to a helical device having a known bender construction, the force generated by an equivalent device in accordance with the present invention may be greater by as much as an order of magnitude. For example, with the dimensions given above, a helical device with a known bender construction provides a relative tip rotation of 14 degrees with a blocking moment of 9Nmm. In contrast, a helical element in accordance with the present invention provides 62% of this rotation and four times the blocking moment. Similarly, if the thickness T of the elongate member 61 is halved to 0.5mm (so that it is the same thickness as a single layer of the equivalent known member construction which allows the device to be operated at the same voltage without any loss of electric field), the rotation is 83% and the blocking moment is twice that of the known bender construction. It will be noted that, unlike the case of a straight element as described above, for the curved geometry the product of the factors by which the force and displacement of the element in accordance with the present invention are related to those of the known bender construction is greater than unity. The actual value of the product depends on the choice of geometry, but for the sample given above the product is 2.5 and 1.7 for thicknesses of 1mm and 0.5 mm respectively. Thus not only do the helical devices in accordance with the present invention provide the advantage of making it possible to design the device to produce a higher output force, the reduction in displacement of the device need not be reduced by a corresponding amount. It is possible to design helical electro-active elements in accordance with the present invention which produce a higher force and displacement than is obtainable with the equivalent known bender construction.

The electro-active element 60 of Fig. 12 suffers from the disadvantage that the electrodes 63 or 64 which oppose each other on adjacent helical turns of the element 60 have an activation voltage which is necessarily of opposite polarity. Accordingly, there is a risk of shorting or arcing. To prevent this, the helical turns must be spaced by a sufficient amount, which reduces the compactness of the element 60, or an insulator must be provided therebetween, which makes manufacture more difficult.

Another way to avoid this problem is to form a device from two interthreaded helical elements. As an example such a device 70 is illustrated in Fig. 13. The device 70 comprises two electro-active elements 67 and 68 each having an identical construction to the electro-active element 60 of Fig. 12 except that they extend in a helix around a minor axis 69 which is curved, rather than being straight like the minor axis 62 of Fig. 12.

The elecfro-active elements 67 and 68 are interthreaded so that they curve around the same minor axis 69. The electro-active elements 67 and 68 are poled so that the activation voltages for corresponding electrodes of the two elements 67 and 68 are of opposite polarity. As a result, the adjacent electrodes of the two elements 67 and 68 are at the same voltage. Thus there is no risk of shorting, so the device 70 may be packed tightly with the elements 67 and 68 close together or even touching. Also, as both electro-active elements 67 and 68 are activated in parallel, both elements 67 and 68 contribute to the force generated, thereby allowing a high total force to be generated by the device 70.

It will be appreciated that the elecfro-active device 55 of Fig. 9 and the electro-active element 60 of Fig. 12 twist around the axes 32 and 62, respectively, this twist along the length of the axes 32 and 62 being concomitant with generation of relative rotation between the ends 57 and 58 or 65 and 66. There will now be described devices corresponding to the device 55 of Fig. 9 and the element 60 of Fig. 11, but which differ in that the axis around which the twist occurs is curved. This means that on activation of the devices there is a relative linear displacement at the ends of the device concomitant with twisting.

Fig. 14 illustrates an electro-active device 80 comprising an elongate elecfro- active element 81 which curves in a helix around a minor axis 82, which minor axis 82 itself extends in a curve consisting of an arc of a circle about a major axis 83. The electro-active device 80 has the same construction as the portions 10 illustrated in Fig. 3. Thus, as illustrated in the insert in Fig. 14 showing a portion of the device 80, the member 81 is provided with two pairs of electrodes 84 and 85 spaced across the width of the member 81 radially of the minor axis 82 separated by a gap 87. Consequently, on activation, the elongate member 81 bends around the minor axis 82 around which the elongate member 81 curves in a helix. Such bending is concomitant with twisting of the elongate member 81 around the minor axis 82. This may be visualised as the turns of the elongate member 81 tightening or loosening causing a twist of the structure of the helical member 81 around the minor axis 82. The twist occurs along the entire length of the minor axis 82 causing a relative rotation of the ends 86 and 88 of the device. Thus the device 80 may be considered as comprising a plurality of elecfro-active portions disposed successively along the minor axis which portions are the successive finite portions of the elongate member 81 which extends continuously between its ends 86 and 88.

The twisting of the elongate member 81 around the minor axis 82 is concomitant with relative displacement of the ends 86 and 88 of the device perpendicular to the curve of the minor axis 82, that is parallel to the major axis 83. This relative displacement of the ends 86 and 88 derives from the twisting of the continuous member 81 around the minor axis 82 in combination with the curve of the minor axis 82. It is an inevitable result that twisting of a curved object causes relative displacement of the ends of that object perpendicular to the local curve of the object.

In general, similar devices to the device 80 of Fig. 14 could be arranged along a minor axis which follows any curve. Then, the resultant displacement of the end of the device would be the sum of the displacement caused by each section of the device along the curve. However, curves which are regular, such as an arc of a circle as in the device 80 of Fig. 14 or a helix, are preferred so that all sections of the device cause relative displacement in a common direction and so that design and manufacture is simplified.

The device 80 may be electrically activated to create a mechanical linear displacement between the ends 86 and 87. Alternatively the device 80 may be mechanically activated in which case the relative displacement of the ends 86 and 87 causes an electrical voltage to be developed across the pairs of electrodes 84 and 85. In the case of electrical activation, the ends 86 and 87 of the device 80 are used to drive relative movement of further objects which may be coupled to the ends 86 and 87. Similarly, in the case of mechanical activation, the ends 86 and 87 are moved by obj ects to which they may be coupled. The use of the construction illustrated in Fig. 3 provides a number of advantages to the device 80, as compared to an equivalent device having a known bender construction, in which the device would be arranged with the layers in the thickness direction spaced radially of a minor axis itself curved around a major axis. Firstly, the device in accordance with the present invention provides the advantage of allowing a higher force to be produced. In terms of the rotational moment about the axis 82, the same comments apply to the device 80 as were made above in connection with the elecfro-active element 60 of Fig. 12. The higher rotational moments create a higher force in the relative displacement of the ends 86 and 87. Thus the device 80 can be designed to provide an equivalent displacement, with a greater force output as compared to the equivalent device using the known bender construction. Therefore, the device in accordance with the present invention is particularly suitable for use where it is desired to generate high force, such as for loudspeakers, solenoid replacements in relays, locking devices and industrial automation.

As in the case of the electro-active element 60 of Fig. 12, the product of the factors relating the displacement and force of the device 80 to those of the equivalent device employing a known bender construction is greater than unity, as a result of the greater mass of effective electro-active material which can be packed in a given space. As an example, one can consider a device which fits in a space of diameter 85mm and height 25mm extending along a curve which is an arc of a circle as illustrated in Fig. 14. An appropriately designed device using the known bender construction would produce a linear relative displacement between its ends of about 20mm with a blocking force of about 0.3N. Depending on the thickness T and configuration selected, the equivalent device 80 in accordance with the present invention can produce a similar relative displacement between its ends 86 and 87 with over three times the force using an elongate member 81 with the same outer dimensions as the known bender construction.

The amount of displacement between the ends of the device available depends on the degree of rotation achieved about the minor axis and the length of the device. The degree of rotation may be controlled by selecting the dimensions of the elongate member and the configuration of the helix along which it extends around the minor axis. The length of the device may be controlled by selection of the curve along which the minor axis extends. In general any form of curve may be used. Planar curves, such as an arc of a circle as in the device 80 of Fig. 14, are advantageous in that they limit the thickness of the device as a whole to the thickness of the helical member. One example of a planar curve which provides a greater displacement than the device 80 is a spiral, because then each turn of the spiral contributes to the overall displacement. Another possible curve along which the minor axis may extend is a helix. This produces considerable relative displacements between the ends of the device, because each of the turns of the helix contributes towards the overall relative displacement.

Fig. 15 illustrates another electro-active device 90 which produces linear displacement on activation by twisting of a structure extending along a curved minor axis. In particular, the device 90 comprises a plurahty of elecfro-active elements 28 as illustrated in Fig. 6 arranged in a stack with the elements 28 disposed successively along a minor axis 91 which extends in a curve around a major axis 92. Fig. 16 illustrates the case that the minor axis 91 extends along a curve which is an arc of a circle around the major axis 92. Fig. 17 illustrates the case that the minor axis 91 extends in a curve which is a helix of less than one turn around the major axis 92. The individual elements 28 are each oriented at a slight angle to each other so that the axis 32 of each element 28 is coaxial with the minor axis 92.

The elements 28 are coupled in series through the stack by couplings 93. Progressing along the stack, the successive couplings 93 are arranged at alternate ends of the elements 28 and couple adjacent, overlying ends of the elements 28.

Consequently, the two ends of each element 28 are coupled to an opposite one of the adjacent elements 28. The coupling 93 may take any suitable form, such as an adhesive, an adherent interlayer or a mechanical coupling, but is preferably a simple piece of inactive material adhered to each element 28. The major curve around the major axis 92 may be introduced by the shape of the couplings 93, for example by providing them with a wedged shape. The couplings 93 have sufficient stiffness to ensure that the device maintains its curved shape. Electrical connections are also provided between the electrodes of the individual elements, preferably incorporated into the couplings 93. Thus the device 90 of Fig. 15 is equivalent to the device 55 of Fig. 9, except that the axis along which successive elements 28 are disposed is curved.

Fig. 18 illustrates another electro-active device 100 which produces linear displacement on activation by twisting of a structure extending along a curved minor axis. In particular, the device 100 comprises a plurality of electro-active elements 28 as illustrated in Fig. 9 arranged in a stack with the elements 28 disposed successively along a minor axis 101 which extends in a curve around a major axis 92. Fig. 19 illustrates the case that the minor axis 101 extends along a curve which is an arc of a circle around the major axis 102. Fig. 20 illustrates the case that the minor axis 101 extends in a curve which is a helix of less than one turn around the major axis 102. The individual elements 28 are each oriented at a slight angle to each other so that the axis 32 of each element 28 is coaxial with the minor axis 102.

The elements 28 are coupled in series through the stack by couplings 103. The couplings 103 couple opposite ends of the adjacent elements 28. Consequently, the two ends of each element 28 are coupled to an opposite one of the adjacent elements 28 on either side thereof. The coupling 103 may take any suitable form, such as an adhesive, an adherent interlayer or a mechanical coupling, but is preferably a simple piece of inactive material adhered to each element 28. The major curve around the major axis 102 may be introduced by the shape of the couplings 103, for example by providing them with a wedged shape. The couplings 103 have sufficient stiffness to ensure that the device maintains its curved shape. Electrical connections are also provided between the electrodes of the individual elements, preferably incorporated into the couplings 103. Thus the device 100 of Fig. 18 is equivalent to the device 55 of Fig. 10, except that the axis along which successive elements 28 are disposed is curved. In use of the devices 90 and 100, the individual elements 28 are activated to generate a relative rotation which adds in series for successive elements 28, so that the devices 90 and 100 as a whole twist around the minor axis 91 and 101. The ends 94 and 104 are relatively linearly displaced parallel to the major axes 92 and 102 as a result of the twisting of the structure of the devices 90 and 100 around the minor axes 91 and 101 in combination with the curve of the minor axis 91 and 101. Thus linear displacement occurs for exactly the same reasons as described above with reference to the device 80 of Fig. 14. The devices 90 and 100 can produce considerable displacement, of the order of millimetres. However, it is easy to manufacture, because the individual elements, being flat, are themselves easy to manufacture and couple together.

In general all the elecfro-active elements and devices of the present invention may be combined together for example by being stacked or nested to form a larger device. Similarly the elecfro-active elements and devices may be integrated into larger devices with other components. The method of manufacture of the electro-active elements described above will now be described.

In general, the electro-active elements are simply new configurations of electro-active material and electrodes, so may be manufactured using the same techniques as are known for manufacturing known constructions of elecfro-active elements.

For example, a conventional technique for manufacture of a known bender construction of co-firing the piezoelectric material and the electrodes may be applied. In this method, the elements are formed from a piezoelectric ceramic to which the electrodes are applied before firing (i.e. in the green state), the material and electrodes then being co-fired to sinter the piezoelectric material.

When this known technique is applied to manufacture of curved elements in accordance with the present invention, the preferred method of manufacture is to initially form the elecfro-active element extending along a straight line and subsequently to bend the straight elecfro-active element into the desired curve. The elongate member may be first formed as a straight member by any suitable technique, for example by co-extrusion or co-calendering of the member of piezoelectric material with the electrodes, or by initially forming the member of electro-active material by any known technique and subsequently applying the electrodes, for example by printing, electroless plating or as a metallic paste. Subsequently the straight element is bent into the desired form. To thus deform the member, there must exist a sufficient degree of flexibility in the initially-formed member. Suitably deformable elecfro-active materials are known, typically including constituent polymers which enhance the deformability. With such materials, after shaping, the constituent polymers are burnt out by firing the member typically at up to 600° C and the material is then densified through further sintering at higher temperature, typically 1,000° C to 1,200° C. In this case, the electro-active element is initially formed with enlarged dimensions to allow for shrinkage which occurs during sintering, typically of around 12 to 25%.

The curving of the straight member may be performed around a former. The former is subsequently removed either physically or by destruction of the former for example by melting, burning or dissolving.

However, the techniques described above, whilst entirely technically feasible, do suffer from problems, as follows. As the electrodes are co-fired with the member of electro-active material, they must be formed from a material which is capable of withstanding the temperatures which are sufficient to sinter the material of the elongate member. Suitable materials usually need to be noble metal or noble metals alloys, such as platinum or silver palladium. Such materials are very expensive. A similar problem is encountered in manufacture of the known bender technique, in which the cost of the materials for the electrodes typically forms a major part of the cost of the device.

Also, bending of the straight member can induce unwanted stresses which can lead to deformation or cracks. Since the bending occurs before sintering it is not always easy to account for the shrinkage occurring during sintering, especially because this is affected by the stresses caused by curving the element. This problem is also encountered in deforming a member having a known bender construction, but in that case is exacerbated by the presence of the central electrodes.

However, the construction of the electro-active element described above comprising a single elongate member allow manufacture using a method which avoids the above problems, as a result of the fact that there are no buried electrodes (in contrast to the known bender construction). Therefore it is possible to manufacture the electro-active element by first forming the elongate member of sintered electro-active material and subsequently applying the electrodes to opposite surfaces of the sintered elongate member using the techniques described above. It is possible to tell whether the electrodes were applied before or after sintering by using a conventional metallographic examination, because electrodes subjected to a sintering process have a characteristic pore structure, as is known in the art.

As the electrodes are applied after sintering, it is not necessary to make the electrodes from a material which is capable of withstanding temperatures sufficient to sinter the electro-active material. The electrodes may be made of any suitable metal. This means that the cost of the electrodes is greatly reduced as compared to the expensive materials capable of withstanding sintering.

To form the elongate member of sintered material, it is possible to initially form the elongate member into the desired shape and subsequently to sinter it. However, in this case, the problem remains of accurately obtaining a device of desired dimensions because of the limited predictability of shrinkage during sintering. Therefore, the preferred technique for forming the elongate member comprises cutting the member from a larger piece of sintered material. For example elements which are planar (for example the element 21 of Fig. 4, the element.28 of Fig. 7 and the element 41 of Fig. 11) may be formed by cutting them from a sheet of sintered material. This is particularly advantageous, because such sheets are commercially available.

Electro-active elements having more complicated structures may also be formed by cutting from an appropriately shaped piece of elecfro-active material. For example, the helical element 60 of Fig. 12 may be cut from a cylindrical tube. In this case the thickness of the tube becomes the width of the resultant member 61. The thickness of the resultant element 61 is determined by the pitch angle of the helical cut. The initial piezoelectric tube may be made by any known method, a suitable known method being extrusion. The cut may be made on fully sintered material, but is preferably carried out before sintering so that the material is softer and easier to cut. Li the latter case, the helical element is preferably sintered on a suitable former to define the helical geometry, in particular the gap between helical turns, for example in the grooves of a threaded former.

Application of the elecfrodes after sintering may be performed by any suitable method, for instance elecfroless deposition or printing of a metallic ink. A preferred method is to feed the elongate member through two pairs of inked rollers, thereby producing twin electrode tracks on the upper and lower surfaces of the elongate member. Alternatively, an electrode pre-cursor may be deposited on the elongate member prior to sintering. The pre-cursor may for instance be an activator for elecfroless deposition. After sintering, the electrodes may be produced by immersing the activated member in suitable elecfroless plating solutions.

An alternative method of manufacture of the helical member 61 of the element 60 of Fig. 12 is extrusion of a suitable plastercised ceramic paste through a slit-shaped die in such a way that more material exits on one side of the slit than the other so that the extruded tape curves to form the helix directly.

After formation of the elecfro-active element with the electrodes using any of the above techniques, poling voltages sufficient to pole the material of the elongate member are applied across each of the pairs of electrodes to pole the region inbetween in the appropriate direction. After manufacture of the individual elecfro-active element as described above, the elements may be coupled together to form larger devices such as the device 50 of Fig. 8, the device 55 on Fig. 9 or the device 90 of Fig. 15 by any suitable coupling.

Claims

CLAΓMS

1. A piezoelectric actuator comprising at least one member of piezoelectric ceramic material extending along a curve and having two pairs of electrodes spaced across the width of the member radially of the curve, the elecfrodes of each pair extending along the length of the member on opposite sides of the member for developing an electric field across a region of the member therebetween on activation, and the material of the member in each region being poled parallel to the electric field to be capable of activation in an expansion- contraction mode with a differential change in length of the two regions concomitantly with bending of the member parallel to the width of the member so that said bending is concomitant with relative rotation of the ends of the element about an axis on the inner side of the curve.

2. A piezoelectric actuator according to claim 1, wherein the curve along which the member extends is planar.

3. A piezoelectric actuator according to claim 2, wherein the curve along which the member extends is the arc of a circle.

4. A piezoelectric actuator according to claim 2, wherein the curve along which the member extends is a spiral.

5. A piezoelectric actuator according to claim 1, wherein the curve along which the member extends is a helix.

6. A piezoelectric actuator according to claim 5, wherein the helix curves around a minor axis which is curved.

7. A piezoelectric actuator device comprising a pair of piezoelectric actuators as claimed in claim 5 or 6 inter-threaded with each other.

8. A piezoelectric actuator according to any one of the preceding claims, wherein the electrodes of each pair are on opposite surfaces of the member extending along the length of the member.

9. A piezoelectric actuator according to any one of the preceding claims, wherein the thickness of the member between the opposite sides on which the electrodes are arranged is at least an order of magnitude less than the length of the member.

10. A piezoelectric actuator according to any one of the preceding claims, wherein the length of the member is at least an order of magnitude greater then the width of the member.

11. A piezoelectric actuator according to any one of the preceding claims, wherein the width of the member is greater than the thickness of the member between the opposite sides on which the electrodes are arranged.

12. A piezoelectric actuator according to any one of the preceding claims, wherein the piezoelectric ceramic material is replaced by a piezoelectric polymer.

13. A piezoelectric actuator device comprising a plurality of piezoelectric actuators elecfro-active elements as claimed in any one of the preceding claims coupled together.

14. An electro-active device comprising a structure of electro-active portions coupled together, the electro-active portions comprising a member of elecfro-active material extending along a curve and having two pairs of electrodes spaced across the width of the member radially of the curve, the elecfrodes of each pair extending along the length of the member on opposite sides of the member for developing an electric field across a region of the member therebetween on activation, and the material of the member in each region being poled parallel to the electric field to be capable of activation in an expansion-contraction mode with a differential change in length of the two regions concomitantly with bending of the member parallel to the width of the member so that said bending is concomitant with relative rotation of the ends of the element about an axis on the inner side of the curve.

15. A device according to claim 13 or 14, wherein the piezoelectric actuators are coupled to create said relative rotation in parallel on activation.

16. A device according to claim 15, wherein the piezoelectric actuators are planar and arranged in a stack and coupled at each end to the adjacent elements on both sides.

17. A device according to claim 13 or 14, wherein the piezoelectric actuators are coupled to create said relative rotation in series around a common axis on activation.

18. A device according to claim 17, wherein the piezoelectric actuators are planar and arranged in a stack and coupled at each end to an adjacent element.

19. A device according to claim 17 or 18, wherein said common axis is curved.

20. An electro-active device comprising an elecfro-active structure comprising elecfro-active portions disposed successively along a curved minor axis, the elecfro- active portions comprising a member of elecfro-active material having two pairs of electrodes across the width of the member radially of the minor axis, the electrodes of each pair extending along the length of the member on opposite sides of the member for developing an electric field across a region of the member therebetween on activation, and the material of the member in each region being poled parallel to the electric field to be capable of activation in an expansion-contraction mode with a differential change in length of the two regions concomitantly with bending of the member around the minor axis parallel to the width of the member, the portions being arranged, on activation, for the structure to twist around the minor axis and concomitantly for relative linear displacement of the ends of the structure to occur.

21. An electro-active device as claimed in claim 20, wherein the electro-active portions are separate elements coupled together

22. An electro-active device as claimed in claim 20, wherein the electro-active structure comprises a continuous elecfro-active member curving around the minor axis, said electro-active portions being adjacent finite portions of the continuous member.

23. An elecfro-active device as claimed in claim 22, wherein the continuous elecfro-active member curves in a helix around the minor axis.

24. An electro-active device as claimed in any one of claims 20 to 23, wherein the minor axis extends in curve which is a helix.

25. An elecfro-active device as claimed in any one of claims 20 to 24, wherein the minor axis extends in curve which is planar.

26. A method of manufacture of an electro-active element, the method comprising: forming a member of sintered elecfro-active material; applying two pairs of elecfrodes spaced across the width of the member with the electrodes of each pair extending along the length of the member on opposite sides of the member for developing an electric field across a region of the member therebetween; and applying poling voltages across each of the pairs of electrodes to pole the material of the member in each region parallel to the electric field.

27. A method as claimed in claim 26, wherein said step of forming a member comprises cutting the member from a piece of sintered elecfro-active material.

28. A method as claimed in claim 26, wherein said step of forming a member of sintered electro-active material comprises: forming a member of sintered electro-active material; and sintering the member.

29. An elecfro-active element comprising a single member of ceramic elecfro-active material having two pairs of elecfrodes applied after sintering of the material of the member, the pairs being spaced across the width of the member, the elecfrodes of each pair extending along the length of the member on opposite sides of the member for developing an electric field across a region of the member therebetween on activation, and the material of the member in each region being poled parallel to the electric field to be capable of activation in an expansion-contraction mode with a differential change in length of the two regions concomitantly with bending of the member parallel to the width of the member.

30. An elecfro-active element according to claim 29, wherein the electrodes are made of a material which is incapable of withstanding temperatures sufficient to sinter the material of the member.

31. An elecfro-active element according to claim 29 or 30, wherein the member extends along its length along a curve with the pairs of electrodes spaced radially of the curve so that said bending is concomitant with relative rotation of the ends of the element about a point on the inner side of the curve.

32. An elecfro-active element according to any one of claims 29 to 31, wherein the electrodes of each pair are on opposite surfaces of the member extending along the length of the member.

33. An elecfro-active element according to any one of claims 29 to 32, wherein the length of the member is at least an order of magnitude greater then the thickness of the member between the opposite sides on which the elecfrodes are arranged.

34. An elecfro-active element according to any one of claims 29 to 33, wherein the length of the member is at least an order of magnitude greater then the width of the member.

35. An elecfro-active element according to any one of claims 29 to 34, wherein the width of the member is greater than the thickness of the member between the opposite sides on which the electrodes are arranged.

36. An elecfro-active element according to any one of claims 29 to 35, wherein the material of the member is a piezoelectric material.