GB2380315A

GB2380315A - Bimrph electro-active element with floating central electrode

Info

Publication number: GB2380315A
Application number: GB0123159A
Authority: GB
Inventors: Richard John Topliss
Original assignee: 1 Ltd
Current assignee: 1 Ltd
Priority date: 2001-09-26
Filing date: 2001-09-26
Publication date: 2003-04-02
Also published as: GB0123159D0

Abstract

An electro-active element 20 including: two piezoelectric layers 21; and electrodes 22, 23 extending parallel to the layers, including: two pairs of outer electrodes 22 outside the two layers 21 in two respective regions 24, 25 between which regions the pairs of outer electrodes 22 are electrically isolated; and a centre electrode 23 between the two layers spanning the two regions. The element 20 is poled or activated by a method of simultaneously applying voltages to all the outer electrodes 22 while leaving the centre electrode 23 to float, the voltages applied to the outer electrodes 22 of a first one of the regions 24, 25 being greater than the voltages applied to the outer electrodes 22 of the second one of the regions 24, 25 so that the central electrode 23 floats at an intermediate voltage. The structural arrangement of the element 20 allows the formation of the electric fields necessary for poling or activation in opposite directions for the two layers 21 within each one of the regions 24, 25 without the need for making an electrical connection to the centre electrode 23.

Description

Application of Voltages to an Electro-Active Element The present invention relates to the application of poling or activation voltages to an electro-active element having a bimorph or multi-morph bender construction comprising two or more electro-active layers and electrodes extending parallel to the layers.

Electro-active elements having a bender construction are in themselves known or use as actuators or sensors. Such elements have layers of electro-active material, that is material which changes shape in response to applied electro conditions, or vice versa. The best known and developed type of electro-active material is piezoelectric material which contracts or expands in response to an applied electric field. However, there are other types of electro-active material and the present invention relates to elements which exhibit piezoelectric activity and other forms of electro-activity.

To achieve greater displacements than are achievable from a simple block of electro-active material, electro-active elements having a bender construction are known. In a bender construction, the electro-active element comprises a plurality of electro-active layers. A bimorph bender construction has two electro-active layers, whereas a multi-morph bender construction has more than two electro-active layers.

The element is provided with electrodes ending parallel to the layers to apply an electro field across the layers for activating them. The relative directions of poling of the layers and of the applied electric field are selected to cause a differential expansion of the layers along the length of the element, usually with one layer expanding and another layer contracting. As a result of the layers being constrained by being joined to each other, this differential expansion causes the element to bend sideways in a direction perpendicular to the layers.

For example, Fig. 1 illustrates an element having a known bi-morph bender construction, Fig. 1 being a cross-sectional view taken along the length of the element. The element 1 comprises two parallel layers 2 of piezoelectric material.

Extending parallel to the layers 2 there are a centre electrode 3 between the two layers 2 and outer electrodes 4 outside the layers 2. The layers 2 are poled in

opposite directions across the layers 2 as shown by arrows P The element 1 is electrically activated by applying activation voltages of opposite polarity to the outer electrodes 4, whilst holding the centre electrode 3 at ground, to create an electrical field in the same direction across both layers 2 as shown by the arrow E As a result, one of the layers 2 expands and the other layer 2 contracts Since the layers 2 are constrained by being joined to each other, this differential expansion causes the element 1 to bend perpendicular to the layers 2 causing relative movement of the ends of the device accompanied with relative rotation of the ends. For example, as illustrated in Fig. 2, if one end 5 of the element 1 is fixed, the free end 6 moves along a curve path shown by the arrow A and relatively rotates. Elements with such a known bi-morph bender construction are capable of producing relatively large displacements, for example the order of several millimetres.

Fig. 3 illustrates another element having a known bi-morph bender construction, Fig. 3 being a cross sectional view along the length of the element 10.

The physical arrangement of the element 10 is the same as the element 1 shown in Fig. 1, comprising a two parallel layers 12 of piezoelectric material with a centre electrode 13 between the layers 12 and a pair of outer electrodes 14 outside the layers 12. The element 10 of Fig. 3 is poled differently from the element 1 of Fig. 1. That is to say, the layers 2 are poled in opposite directions in two regions 17 and 18 along the length of the element 10. Each layer 12 is poled in opposite directions within each region 17 and 18, as shown by the arrows P.

The element 10 is electrically activated by applying the same activation voltages as are applied to the element 1 of Fig. 1 to create an electrical field in the same direction across both layers 2 as shown by the arrow E. As a result, within each region 17 and 18, the element 10 bends perpendicular to the layers 12, but with an opposite curvature in the two regions 17 and 18 as a result of the direction of poling being opposite in the two region 17 and 18. This bending causes a relative displacement between the ends 15 and 16. However, provided the two regions 17 and 18 are of equal length and the structure of the element 10 is uniform along its

length, then there is no relative rotation between the ends 15 and 16. For example, as illustrated in Fig. 4 if one end 15 of the element 10 is fixed, the free end 16 is relatively displaced by an amount d without relative rotation of the ends 15 and 16.

Electro-active elements having a bender construction such as those described above, suffer from the problem that it is necessary to make a connection to the centre electrodes arranged between electro-active layers, in order to apply voltages for at least one of poling or activation.

Typically electro-active elements having a bender construction are manufactured by a process in which a layered structure including both the layers of electro-active material and the centre electrode are produced before poling of the electro-active layers. In this case, if the electro-active layers are poled in opposite directions, as in both electro-active elements 1 and 10 described above, then it is necessary to form an electrical connection to the centre electrode during poling. As an alternative to the elements 1 and 10 described above, it is possible to pole the layers in the same direction. However in that case the electric field applied during activation must be in opposite directions for the two layers, so it is necessary to make an electrical connection to the centre electrode during activation.

Providing an electrical connection to the centre electrode presents a difficult manufacturing problem. When the electro-active layers and the centre electrode are produced in common by the same process, for example by extrusion, it is physically difficult to provide an electrical connection to the centre electrode which is isolated from the outer electrodes due to the element being very thin. This difficulty decreases the yield and increases the manufacturing costs of the electro-active element.

One feasible solution to this problem is to manufacture and pole the layers separately and subsequently to affix the poled layers together. However, such a technique is even more inconvenient and costly than forming the layers and centre electrode in common, even when the problem of creating a connection to the centre electrode is taken into account.

Therefore, it would be desirable to avoid the need to form a connection to the

centre electrode, or to at least some of the centre electrodes in a multi-morph element.

According to a first aspect of the present invention, there is provided a method of applying either poling or activation voltages to an electro-active element including: two electro-active layers ; and electrodes extending parallel to the layers, including two pairs of outer electrodes outside the two layers in each of two respective regions between which regions the pairs of outer electrodes are electrically isolated ; and a centre electrode between the two layers spanning the two regions, wherein the method comprises: simultaneously applying voltages to all the outer electrodes while leaving the central electrode to float, the voltages applied to the outer electrodes of a first one of the regions being greater than the voltages applied to the outer electrodes of the second one of the regions so that the central electrode floats at an intermediate voltage.

According to a second aspect of the present invention, there is provided an electro-active element including: two electro-active layers ; and electrodes extending parallel to the layers, including two pairs of outer electrodes outside the two layers in two respective regions between which regions the pairs of outer electrodes are electrically isolated ; and a centre electrode between the two layers spanning the two regions.

The electro-active element in accordance with the present invention has two pairs of electrically isolated, outer electrodes in two regions and a centre electrode which spans the two regions. This structure arrangement allows for both poling and activation voltages to be applied without the need for a connection to the centre electrode.

Considering the regions in isolation, during one of poling or activation it is

necessary to apply voltages which create an electric field in opposite directions across the two electro-active layers. In the prior art, this is achieved by connecting the outer electrodes to voltages of the same polarity and connecting the centre electrode to ground.

However, the structural arrangement of the element in accordance with the present invention avoids the need to connect to the centre electrode use of the method in accordance with the present invention. By simultaneous application of voltages to all the outer electrodes, provided that the voltages applied to the outer electrodes of a first region are greater than the voltages applied to the outer electrodes of the second region, the centre electrode floats at an intermediate voltage, that is between the voltages applied to the outer electrodes in the two regions. Thus the voltage at which the central electrode floats is below the voltage applied to the outer electrodes in the first region, but above the voltage applied to the outer electrodes in the second region.

Consequently, the present invention allows the generation of an electric field in opposite directions for the two layers within each region (and also in opposite directions within each layer between the two regions). This would be impossible without a connection to the centre electrode if the voltages were applied separately to the two regions or if the same voltages were applied to the outer electrodes in both regions, because in that case the centre electrode would float at the same voltage as the outer electrodes, so no electric field would be generated at all. Therefore, the present invention uses the structure of the element with the centre electrodes spanning both regions and so being forced to float at an intermediate voltage. This is possible because the centre electrode is electrically conductive. On application of the voltages there is an instantaneous current flow which leaves the entire centre electrode at a single potential.

In principle, the method of applying voltages in accordance with the present invention may be applied to the application of either poling or activation voltages.

During the application of the other one of activation or poling voltages, it is simply a matter of applying voltages in an appropriate direction, which again will not require a

connection to the centre electrode. For example, in the case of a bimorph bender construction, during the application of the other one of poling or activation voltages, the direction of the appropriate electric field is the same in both layers. Therefore, for each pair of outer electrodes, voltages of opposite polarity are applied to each electrode of the pair The centre electrode may be left to float at or around ground without the need to apply any voltages thereto However, it is particularly advantageous to apply the method in accordance to the present invention to the application of activation voltages. In this case it is possible to electrically connect the outer electrodes of each pair together after poling This reduces the number of connections necessary, because instead of applying voltages separately to each electrode of a pair, the voltage may be applied to both electrodes of the pair by a single connection.

In principle the voltages applied to the outer electrodes may be of any level provided that the voltages applied to the outer electrodes of the first region are greater than the voltages applied to the outer electrodes of the second region.

However for convenience in the generation of the voltages to be applied, it is preferred that the voltages applied to the outer electrodes of the first region are of positive polarity and the voltages applied to the outer electrodes of the second region are of negative polarity with the voltages applied to the outer electrodes in the two regions being preferably of the same magnitude. In this case, the centre electrode floats at a level at or near ground. Preferably, for symmetry between the two layers the voltages applied to the outer electrodes within each region are of the same magnitude.

The present invention may be applied to an electro-active element which on activation bends with curvature in the same sense in the two regions in an equivalent manner to the known electro-active element I described above or which bends with opposite curvature in the two regions in an equivalent manner to the known electroactive element 10 described above. The mode in which the electro-active element is used can be controlled merely by appropriate selection of the direction of poling and activation between the two regions.

As an example consider the case that the electro-active element has a bimorph bender construction consisting of two electro-active layers and the method in accordance with the present invention is applied during application of activation voltages Thus within each one of the two regions, the two electro-active layers are poled in the same direction, because the electric field generated by the activation voltages will be in opposite directions for the two layers. For the first mode of operation in which the curvature is of the same sense in both regions, the electroactive layers in one region are poled in the opposite direction from the electro-active layer in the other region. Similarly, for the second mode of operation in which the element is activated with opposite curvature in the two regions, then the electroactive layers in both regions are poled in the same direction. The embodiments described in detail below represent other examples.

The present invention may be applied to an electro-active element having a bimorph bender construction in which case the two electro-active layers are the only electro-active layers of the element. The present invention may also be applied to an electro-active element having a multi-morph bender construction in which the electro-active element has further electro-active layers.

The present invention may be applied to an electro-active element which is elongate, in which case the two regions are separated on along the length of the element. In this case, the electro-active element may be straight, as in the known electro-active elements 1 and 10 described above. Alternatively, the present invention may be applied to an electro-active element which is curved. For example the element may be curved in a helix curving around a straight helical axis to form a rotary element which generates relative rotation between its ends on activation, or a helix curving around a helical axis which is itself curved to form a linear element which generates displacement out of the plane of the curve.

To allow better understanding, embodiments of the present invention will now be described by way of non-limitative example, with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of an element having a known bimorph

bender construction ; Fig 2 is a cross-sectional view of the element of Fig 1 on activation ; Fig. 3 is a cross-sectional view of a second element having a known bimorph

of bender construction having a different mode of operation from the element of Fig.

1, Fig. 4 is a cross-sectional view of the element of Fig. 3 on activation ; Fig. 5 is a perspective view of an electro-active element in accordance with the present invention ; Fig 6. is a schematic side view of the element of Fig. 5 during poling for a first mode of operation; Fig. 7 is a schematic side view of the element of Fig. 5 during activation in the first mode of operation; Fig. 8 is a schematic side view of the electro-active element of Fig. 5 during poling for second mode of operation, Fig. 9 is a schematic side view of an electro-active element in according to the present invention having a multi-morph bender construction during poling; Fig 10 is a schematic side view of the electro-active element of Fig. 9 during activation ; Fig. 11 is a schematic view of an electro-active element which extends along a helix around a straight axis; and Fig 12 is a schematic view of an electro-active element which extends along a helix around a curved axis.

An electro active element 20 in accordance with the present invention is shown in perspective view in Fig. 5. The element 20 has a bimorph bender construction comprising two parallel layers 21 of piezoelectric material. The element 20 is an elongate rectangle as viewed perpendicularly to the layers 20 which are therefore uniform along the length of the element 20. The element 20 has outer electrodes 22 and a centre electrode 23 extending parallel to the layers 21. The thickness of the layers 21 and of the electrodes 22 and 23 in the direction perpendicular to the layer 21 is shown grossly exaggerated in the figures for clarity.

The outer electrodes 22 are arranged as two pairs with the outer electrodes 22 of each pair outside the two layers 21 on opposites sides of the elements 20. Each pair of outer electrodes 22 is in a respective region 24 and 25 along the length of the element 20, separated by a gap 26 so that the pairs of outer electrodes 22 are electrically isolated between the two regions 24 and 25. The two regions 24 and 25 are of equal length. The gap 26 is shown in Fig. 5 with exaggerated length for clarity and may be of any size sufficient to isolate the two pairs of outer electrodes 22. To prevent the gap 26 having a significant effect on the operation of element 21, the gap 26 is preferably shorter than the length of the elements 20 by at least an order of magnitude.

The centre electrode 23 is continuous along the entire length of the element 20 so that it spans both regions 24 and 25.

The electro-active element 20 is shown with the outer electrodes electrically connected to a circuit 27 for supply of poling and activation voltages to the outer electrodes. As described in more detail below, the structural arrangement of the electroactive element 20 avoids the need to form an electrical connection to the centre electrode 23.

During manufacture, poling voltages are applied to the outer electrode 22, the poling voltages being sufficient to pole the layers 21, that is to permanently re-orient the crystals of the layers 21 parallel to the electric field created by the poling voltages. During use of the element 20, activation voltages are applied to the outer electrodes 22 to create an electric field across the layers 21 effective to activate the layers.

The direction of the electric field during activation is parallel to the poling direction so that the layers 21 expand or contract depending on whether the applied electric field during activation is in the same or the opposite direction to the poling direction. In each of the regions 24 and 25, the relative directions of the poling voltages and the activation voltages are selected to cause one of the layers 21 to expand and the other of the layers to contract. This causes the element 20 to bend within the respective regions 24 and 25 due to the layers 21 being constrained by

being joined together through the centre electrode 23. Different combinations of poling and activation voltages may be selected depending upon the desired mode of operation The relative directions of the poling voltages and the activation voltages is between the two regions 24 and 25 are selected to cause the element 20 to bend either with curvature in the same sense in the two regions 24 and 25 or with opposite curvature in the two regions 24 and 25.

The method of applying poling and activation voltages to the element 20 will now be described.

In the preferred method, for a first mode of operation the electro-active element 20 is poled as illustrated schematically in Fig 6 In both regions 24 and 25, poling voltages of opposite polarity are applied to the outer electrodes 22 of each pair Also, the poling voltages applied to the outer electrodes 22 on the same side of the element 20 are of the same polarity for each region 24 and 25. Therefore, the poling directions P are the same for the two layers 21 within each region 24 and 25 and are also in the same direction for both regions 24 and 25.

A voltage is induced on the centre electrode 23 which is intermediate between the voltages applied to the outer electrodes 22 on either side of the element 20.

Therefore assuming that the magnitude of the applied poling voltages is the same as between the outer electrode of each pair, the centre electrode floats at OV, as though it were grounded.

Fig. 7 schematically illustrates the activation voltages applied using the method in accordance with the present invention. In Fig. 7, the outer electrodes 22 of each pair are shown electrically connected together by a conductive portion 28 extending across the opposite ends of element 20 along its length. By electrically connecting the outer electrodes 22 of each pair together it is possible to apply the activation voltages to each pair through a single electrical connection. The conductive portions 28 may be applied after poling. Alternatively the outer electrodes 22 and the conductive portion 28 may be applied together after poling to form an integral electrode, for example by electro-plating each region 24 and 25 of the element 20 including the end faces which would be cheap and easy to perform.

In that case, during poling separate electrodes not connected to the element 20 would be used In accordance with the method of the present invention, the activation voltages are applied to all the outer electrodes 22 simultaneously, while leaving the centre electrode 23 to float. By applying activation voltages to the outer electrodes 22 in the first region 24 of positive polarity and applying activation voltages to the outer electrodes 22 in the second region 25 of negative polarity, an intermediate voltage is induced on the centre electrode 23. Assuming the voltages applied to the outer electrodes 22 are of the same magnitude in each region 24 and 25, then the centre electrode 24 floats that OV as though it were grounded. As a result, the electric field E applied across the layers 21 by the activation voltages is, within each one of the regions 24 and 25, in opposite directions for the two layers 21 and 22. As the poling direction P is in the same direction for each layer 21, this causes the electro-active element 20 to bend in each region. Furthermore, as a result of the poling direction P being in the same direction in each region 24 and 25, the element 20 bends with curvature in an opposite sense in the two regions 24 and 25.

Therefore, in this mode of operation, the electro-active element 20 bends in an identical manner to the known electro-active element 10 described above with reference Fig. 3.

The generation of an electric field E in appropriate directions for activation without making a connection to the centre electrode 23, is only achievable because of the presence of a centre electrode 23 spanning both regions 24 and 24. As the centre electrode 23 is electrically conductive the simultaneous application of voltages to the outer electrodes 22 in the two regions 24 and 25 causes a current to flow instantaneously along the centre electrode 23 to place it at a constant potential because it is electrically conductive. If the centre electrode 23 was not present or was split between the two regions 24 and 25, then no electric field would be generated across the layers 21 between the pair of outer electrodes in either one of the regions 24 and 25. Therefore the element 20 would not operate.

In an alternative mode of operation, the electro active element 20 is poled as

illustrated schematically in Fig. 8 In this case a poling voltage of positive polarity is applied to one outer electrode 22 of each pair in the respective regions 24 and 25 and a poling voltage of opposite polarity is applied to the other outer electrode 22 of each pair. As a result, within each region 24 and 25, the poling direction P is in the same direction for each layer 21. However, in contrast to the poling for the first mode of operation as illustrated in Fig. 6, the poling voltages are applied with opposite polarity for the outer electrodes on each side of the element 20, so that within each layer 21 the poling direction P is in the opposite direction for the two regions 24 and

25.

In the second mode of operation, activation voltages are applied in an identical manner as in the first mode of operation, as illustrated in Fig. 7 and described in detail above. As a result of the poling direction P being in opposite directions for the two regions 24 and 25, in combination with the electric field E applied by the activation voltages being in opposite directions in the two regions 24 and 25, the electro-active element 20 bends with a curvature in the same sense in each region 24 and 25. Therefore, in the second mode of operation, the electro-active element 20 bends in an identical way to the known electro-active element 1 described above with reference to Figs. 1 and 2.

The above-described method of applying poling and activation voltages may be modified by applying poling voltages in the directions described above for activation voltages and by applying activation voltages in the directions as described above for poling voltages. Such a reversal means that the method in accordance with the present invention in which the voltages apply to the outer electrodes of the first region is greater than the voltages applied to the outer electrodes of the second region relates to the application of poling voltages. However, despite the reversal, the element 20 bends in exactly the same mode of operations as described above.

In the embodiment describe above, the voltages applied to the electrodes 22 are voltages of the same magnitude of positive and negative polarity for ease of generation of the voltages. However, the voltages applied to the outer electrodes 22 may be of any level provided that the potential difference between the outer

electrodes 22 is sufficient to create the desired electric field for poling or activation.

Similarly, whilst it is preferred that the applied voltages are all of the same magnitude for symmetry across the element 10, it is possible to vary the magnitude of the applied voltages either between the two regions 24 and 25 or between the outer electrodes 22 within a single region 24 or 25 if desired. The electro-active element 10 as described above has a construction which is symmetrical between the two regions 24 and 25 which is referred for predictability of design and operation, but this is not essential.

In the above described embodiment, the present invention is applied10 a bimorph electro-active element 20 comprising only two layers. However, the present invention may equally be applied to two layers within an electro-active element having a multi-morph bender construction of more than two layers. In such a case, the present invention may be applied to respective sets of two layers from the total number of layers. Of course, the directions of the applied fields for activation and poling differ from the simple case of a bimorph bender construction depending upon how the two layers to which the present invention is applied are intended to bend in operation, but the same principles apply. In particular, the two layers within the electro-active element have the same construction as shown in Fig. 5 with outer electrodes separated between two regions and a centre electrode spanning the two regions. Poling and/or activation voltages are applied simultaneously to both the outer electrodes accordance to with the method of the present invention. Of course, in the multi-morph case the"outer"electrodes of the present invention may be buried within the overall construction of layers, but it remains possible to avoid the need for connections to the centre electrodes in between the outer electrodes.

As an example of the multi-morph case, Figs. 9 and 10 illustrate schematically an electro-active element 30 comprising six parallel electro-active layers 31. Electrodes 37 and 38 extends parallel to the layers 31 on the surface of the layer 31 to 36. For each set of two layers 31a and 31b the electrodes 37 and 38 have the same arrangement as the electro-active element 30 shown in Fig. 5. In particular, each set of two layers 31 a and 31 b has outer electrodes 33 in two regions 35 and 36

along the length of element 30 separated by a gap 37, as well as a centre electrode 34 extending along the entire length of the element 30 spanning both region 35 and 36.

The outer electrodes 33 within the layered structure between sets of two layers 31 la and 3 lb are used in common as outer electrodes 33 for both sets of two layers on either side thereof.

Fig. 9 schematically illustrates the voltages applied to the outer electrodes 33 during poling and Fig. 10 schematically illustrates the voltages applied during activation in order to operate the electro-active element 30 in a mode in which it bends with a constant curvature along the length of the element 30, so that it bends in the same manner as the electro-active element 10 of Fig. 1. In particular in applying all the voltages to the outer electrodes 33, the centre electrodes 34 between each set of two layers 31 a and 31 b are left to float. For various different sets of two layers 3 la and 3 lb the method in accordance with the present invention is used by simultaneously applying voltages of positive polarity to the outer electrodes of a first region 35 (or 36) and voltages of negative polarity to the outer electrodes 33 of the other region 36 (or 35) to cause the centre electrode to float at ground, that is at an intermediate voltage. In particular, during poling, this method is applied to the two outer sets of electrodes 31 la and 31 b, but not to the centre set of electrodes 31 a and 31 b, whereas during activation this method is applied to all the sets of electrodes 31 la and31b.

Thus with a multi-morph bender construction, although the directions of the applied voltages may vary depending on the desired activation of the layers during operation, the present invention may in general be applied to two layers within a multi-layer bender construction to achieve the same advantage of avoiding the need to make a connection to the centre electrode between the two layers concerned.

Although it remains necessary to connect to the outer electrodes buried within the layers of the multi-morph bender construction the overall number of connections necessary is reduced, because it is not necessary to connect to the centre electrodes.

The embodiments described above are all elements which are straight elongate and flat. However the present invention may equally be applied to any other

form of element having a bender construction consisting of two or more electroactive layers.

As examples of this Figs. 11 and 12 illustrate elongate electro active elements 40 and 50, respectively, which are curved in a helix. The electro-active element 40 of Fig. 11 is curved in a helix around a straight axis 41, whereas the electro-active element 50 of Fig. 12 is curved around an axis 51 which is itself curved. The electroactive elements 40 and 50 have the same construction along their length as the straight electro-active elements 20 and 30 described above, in particular with the outer electrodes of the electro-active elements 40 and 50 separated at a point 44 and 54, respectively, along the length of elements 40 and 50 The layers of the electroactive elements 40 and 50 face the axes 41 and 51, respectively, so that on activation the electro-active elements 40 and 50 bend around the respective axes 41 and 51 so that the elements 40 and 50 as a whole twist around those axes 41 and 51. Thus the

electro-active element 40 of Fig. 11 may be used as a rotary device, because it bends 42 and 43 relatively rotate around the axis 41. The electro-active element 50 of Fig 12 generates relative motion between its ends 52 and 53 as a result of the axis 51 being curved, in the manner which is described in detail in WO-A-01/47318.

Similarly, the present invention may be applied to electro-active elements having other shape, including elements which are not elongate.

To manufacture electro-active elements in accordance to the present invention, the same techniques as are known for conventional electro-active elements having a bender construction may be applied. Preferably the layers 21 and the centre electrode 23 are produced together, for example by extrusion or calendaring. In this case, the outer electrodes 22 may also be produced together with the layers 21. As an alternative, the outer electrodes 22 may be applied as separate electrodes for example by electroless plating or printing. Additional external terminals are then applied in a conventional manner to allow electrical connection to the outer electrodes 22.

Alternatively, the layers 21 and the electrodes 22 and 23 may be individually laminated on one another.

The outer electrodes 22 may advantageously be applied initially as a

continuous electrode extending along the entire length of the element 20.

Subsequently a portion of the continuous electrodes 22 is removed to create the gaps 26 between the two regions 24 and 25, for example, by a mechanical process or by etching. Where this process is applied and the layers 21 are each poled in the same direction in both regions 24 and 25, for example in the poling illustrated in Fig. 6, then the poling may be performed prior to removal of the central portion of the outer electrodes 22 to create the gap 26.

Preferably the material is piezoelectric material, for example a piezoelectric

ceramics such as lead zirconate titanate (PZT) or a polymer such as polyvinylideneflouride (PVDF).

To manufacture an electro-active element having a multi-morph bender construction, individual sets of two layers may be manufactured as described above for the element 20 having a bimorph bender construction an subsequently laminated or fixed together.

Claims

1. A method of applying either poling or activation voltages to an electro-active element including: two electro-active layers ; and electrodes extending parallel to the layers, including: two pairs of outer electrodes outside the two layers in each of two respective regions between which regions the pairs of outer electrodes are electrically isolated ; and a centre electrode between the two layers spanning the two regions, wherein the method comprises: simultaneously applying voltages to all the outer electrodes while leaving the central electrode to float, the voltages applied to the outer electrodes of a first one of the regions being greater than the voltages applied to the outer electrodes of the second one of the regions so that the central electrode floats at an intermediate voltage.

2. A method according to claim 1, wherein the voltages applied to the outer electrodes within each region are of the same magnitude.

3. A method according to claim 1 or 2, wherein the voltages applied to the outer electrodes of the first region are of positive polarity and the voltages applied to the outer electrodes of the second region are of negative polarity.

4. A method according to any one of the preceding claims, wherein the voltages applied to the outer electrodes of the first region are of the same magnitude as the voltages applied to the outer electrodes of the second region.

5. A method according to any one of the preceding claims, being a method of applying activation voltages to the electro-active element.

6. A method according to any one of claims 1 to 5, being a method of applying poling voltages to the electro-active element.

7. An electro-active element including: two electro-active layers; and electrodes extending parallel to the layers, including: two pairs of outer electrodes outside the two layers in two respective regions between which regions the pairs of outer electrodes are electrically isolated ; and a centre electrode between the two layers spanning the two regions.

8 An electro-active element according to claim 7, wherein the outer electrodes of each pair are electrically connected together.

9 An electro-active element according to claim 8, wherein the electro-active layers in one region are poled in the opposite direction from the electro-active layers in the other region.

10. An electro-active element according to claim 8, wherein the electro-active layers in one region are poled in the same direction as the elector-active layers in the other region.

11. An electro-active element according to any one of claims 7 to 10, wherein the electro-active layers comprise piezoelectric material.

12. An electro-active element according to any one of claims 7 to 11, wherein the two electro-active layers are the only electro-active layers of the element.

13. An electro-active element according to any one of claims 7 to 11, wherein the electro-active element has further electro-active layers.

14. An electro-active element according to any one of claim 7 to 13, wherein the electro-active element is elongate and the two regions are separated along the length of the element.

15. An electro-active element according to claim 14, wherein the electro-active layers are uniform along the length of the element and the regions are of equal length.

16. An electro-active element according to claim 14 or 15, wherein the electroactive element is straight.

17. An electro-active element according to claim 14 or 15, wherein the electroactive element is curved.

18. An electro-active element according to claim 17, wherein the electro-active element is curved in a helix.

19. An electro-active element according to claim 18, wherein the electro-active element is curved in a helix having a curved helical axis.