GB2376796A

GB2376796A - Motors and generators using a coiled-coil piezoelectric device

Info

Publication number: GB2376796A
Application number: GB0115072A
Authority: GB
Inventors: Mark Richard Shepherd; Anthony Hooley; Gareth Mckevitt; David Pearce; Ursula Ruth Lenel; Irving Alexander Bienek; David Livingstone
Original assignee: 1 Ltd
Current assignee: 1 Ltd
Priority date: 2001-06-20
Filing date: 2001-06-20
Publication date: 2002-12-24
Anticipated expiration: 2021-06-20
Also published as: GB2376796B; GB0115072D0

Abstract

Various types of motor and generator use an electro-active device 11 comprising an electro-active structure in the form of a continuous electro-active member 12 curving in a helix around a minor axis 13 which is in itself curved for example in a helix around a major axis 14. The continuous member 12 has a bender construction of a plurality of layers 21 and 22 including at least one layer of electro-active material so that it bends, on activation, around the minor axis 13. Concomitantly with the bending, the electro-active structure twist around the minor axis. Concomitantly with that twisting, relative displacement of the ends 16 of the device 11 occurs due to the combination of the twisting around the minor axis 13 and the fact that the minor axis 13 is curved.

Description

Motors and Generators Using An Electro-Active Device The present invention relates to motors and generators using an electro-active device, that is devices employing an electro-active structure arranged on activation to cause, or generate a voltage in response to, relative displacement of the ends of the structure.

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 mit, 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 electro-active structure is the bender construction, for example a bimorph bender construction. With a bender construction, the electro-active structure comprises a plurality of layers at least one of which is of electro-active material. On activation, the layers deform 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, the differential change in length causes the bender to bend perpendicular to the layers. Accordingly there is a relative displacement of the ends of the structure. However, the relative displacement does not follow a linear path in space. As the structure bends and the degree of curvature increases, the relative displacement of the ends follows a curve in space. Furthermore, to achieve relatively large displacement, it is necessary to increase the length of the structure which therefore becomes inconvenient. For

example, to achieve a displacement of the order of 0. 1mm with a bimorph bender construction, a structure of length around 5cm is typically needed.

Many other known constructions for electro-active devices suffer from problems such as those described above, for example that the displacement provided by the device is non-linear in space or that the device can provides a displacement which is of low magnitude in absolute terms or which can only be of high magnitude if the device has an inconvenient structure.

According to a first aspect of the present invention, there is provided a motor including an electro-active device arranged to generate mechanical displacement on electrical actuation of the electro-active device, the electro-active device comprising an electro-active structure extending along a curved minor axis and arranged, on electrical activation, for the structure to twist around the minor axis and concomitantly for relative displacement of the ends of the structure to occur.

According to a second aspect of the present invention, there is provided a generator including an electro-active device arranged to generate electrical energy on mechanical actuation of the electro-active device, the electro-active device comprising an electro-active structure extending along a curved minor axis and arranged to be mechanically activated by relative displacement of the ends of the structure and, concomitantly, for the structure to twist around the minor axis causing the electrical energy to be generated.

First, the operation of the electro-active device will be considered. 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.

The device uses an electro-active structure which twists on activation.

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 all 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 in accordance to the present invention 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 acme ! vend 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.

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 summary, the electro-active device can provide a relative displacement which is linear in space. Furthermore, the displacement can be of any desired magnitude by appropriate selection of the form of the electro-active structure and the curve along which the minor axis extends. This is achieved with an electro-active device which has a convenient physical arrangement as compared to known electroactive devices such as a straight bender construction.

In accordance with the present invention such an electro-active device is used in a motor or a generator, thereby making use of the advantages noted above. The linear displacement simplifies the design of the motor or generator. This, and the possibility of achieving large displacements, allows the application of the electroactive device to types of motor and generator for which the known electro-active devices discussed above would not suffice.

In the case of a motor, the electro-active device is, in use, electrically activated to produce relative displacement of the ends of the structure of the electroactive device, the motor being arranged to output mechanical displacement driven by the relative displacement of the electro-active device.

In the case of a generator, the electro-active device is, in use, mechanically activated by relative displacement of the ends of the structure of the electro-active device to generate electrical energy. The generator may be electrically connected to a battery to store the generated electrical energy.

The electro-active device may be used in many types of motor and generator, for example of the types of the hereinafter described embodiments.

Preferably, the electro-active structure of the electro-active device comprises electro-active portions disposed successively along the minor axis and arranged to

bend, on activation, around the minor axis.

The electro-active structure is arranged with portions which bend on activation around the minor axis concomitantly with twisting of the structure around the minor axis. As a result, the electro-active portion may have any construction which bends on activation. The preferred construction is the known bender construction comprising a plurality of layers including at least one layer of electroactive material, preferably a bimorph bender construction having two layers. Such a construction is well known and understood as applied to a straight bender and particularly easy to manufacture. The same benefits are obtained when the bender construction is applied to the portions of the present invention. However, any other construction which provides bending on activation may be used.

Preferably, the electro-active structure comprises a continuous electro-active member curving around the minor axis, said electro-active portions being adjacent finite portions of the continuous member.

This structure is particularly easy to manufacture, for example by winding a deformable continuous electro-active member into shape.

Preferably wherein the continuous electro-active member curves in a helix around the minor axis.

By using a continuous electro-active member which curves in a helix around the minor axis a number of advantages are achieved. Firstly, it is easy to provide a structure which is regular along the length of the minor axis and hence provide the same degree of twisting along the entire length of the minor axis. Secondly, the helix is easy to manufacture, for example by winding a deformable continuous member into shape or by making a helical cut in a tubular electro-active member. Thirdly, the device is compact as the helical turns of the member around the minor axis may be packed closely together.

However the electro-active structure may alternatively comprise a continuous electro-active member having a different shape which provides for bending around the minor axis concomitantly with twisting around the minor axis. For example it may comprise a continuous member having the shape of a flat member twisted

around the minor axis. Furthermore, instead of comprising a continuous electroactive member, the electro-active structure may comprise a plurality of electro-active portions coupled together.

To allow better understanding, embodiments of the present invention will now be described by way of non-limitative examples with reference to the accompanying drawings in which : Fig. 1 is a plan view of a first electro-active device ; Fig. 2 is a side view of a second electro-active device; Fig. 3 is a perspective view of a portion of either the first device of Fig. 1 or the second device of Fig. 2; Fig. 4 is a side-view of a first linear motor in accordance with the present invention; Fig. 5 is a side-view of a second linear motor in accordance with the present invention; Fig. 6 is a side-view of an alternative ratchet track usable in the linear motors of Figs. 4 and 5; Fig. 7 is a side-view of a third linear motor in accordance with the present invention; Fig. 8 is a schematic view of a fourth linear motor in accordance-With the present invention; Fig. 9 is a side view of a fifth linear motor in accordance with the present invention; Fig. 10 is a schematic view of a first rotary motor in accordance with the present invention; Fig. 11 is a schematic view of a second rotary motor in accordance with the present invention; Fig. 12 is a side-view of an orbital motor in accordance with the present invention; Fig. 13 is a side-view of a first generator in accordance with the present invention;

Fig. 14 is a side-view of a second generator; Fig. 15 is a side-view of a third generator; Fig. 16 is a cross-sectional view of a fourth generator which includes a battery; Fig. 17 is a cross-sectional view of a sixth generator embodied in an electronic device; and Fig. 18 is a side-view of a seventh generator.

The present invention relates to motors and generators using an electro-active device. For clarity, the electro-active device will first be described, followed by motors and generators using the electro-active device.

In the following description, the electro-active devices are described with reference to minor and major axes which are imaginary, but are nonetheless useful for visualising and defining the devices.

A first electro-active device 1 in accordance with the present invention is illustrated in Fig. 1. The device 1 comprises a structure consisting of a continuous electro-active member 2 curving in a helix around a minor axis 3 so that the structure extends along the minor axis 3. The minor axis 3 is curved, extending in a curve which is an arc of a circle around a geometrical major axis 4 perpendicular to the plane of the minor axis 3, i. e out of the plane of the paper in Fig. 1. As me minor curve 3 is planar, the thickness of the device parallel to the major axis 4 is merely the thickness of the helical structure of the electro-active member 2.

A second electro-active device 11 in accordance with the present invention is illustrated in Fig. 2. The device 2 comprises a structure consisting of a continuous electro-active member 12 to curving in a helix around a minor axis 13 so that the structure extends along the minor axis 13. The minor axis 13 is curved, extending in a curve which is a helix around a geometrical major axis 14. The electro-active device 11 is illustrated in Fig. 2 with a minor axis which extends along of a helix of three turns merely for illustration, any number of turns being possible.

Fig. 3 illustrates a portion 20 of either the continuous member 2 of the first device 1 of Fig. 1 or the continuous member 12 of the second device 11 of Fig. 2.

The construction of the portion 20 being the same for both the first device 1 and the second device 2 the electro-active portion 20 is a finite portion of the continuous member 2 or 12 and hence the electro-active member 2 or 12 may be considered as a plurality of adjacent portions 20 as illustrated in Fig. 3 disposed successively along the minor axis 3 or 13. Hence, the portion 20 extends along part of a helical curve around the minor axis 3 or 13 as shown in Fig. 3.

Fig. 3 illustrates the construction of the electro-active portion 20. This construction is preferably uniform along the entire length of the minor axis 3 or 13 in order to provide uniform properties on activation. Alternatively, the device 1 or 11 may be designed with some variation along the length of the minor axis 3 or 13, either in the construction of the continuous member 2 or 20 or in the shape of the curve of the continuous member 2 or 20 around the minor axis 3 or 13.

The electro-active portion 20 has a bimorph bender construction comprising two layers 21,22 of electro-active material extending along the length of the portion 20. The layers 21, 22 of electro-active material both face the minor axis 3 or l3. The electro-active layers 21 or 22 preferably extend, across the width of the portion 20, parallel to the minor axis 3 or 13, although there may be some distortion of the electro-active portion 20 of the continuous member 2 or 12 due to the nature of the curve around the minor axis 3 or 13. Alternatively, the layers 21 or 22 may extend, across the width of the portion 20, at an angle to the minor axis 3 or 13 so that one edge along the electro-active portion 20 is closer to the minor axis 3 or 13 than the opposite edge.

The material of the electro-active layers 21 or 22 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 polyvinylidenefluoride (PVDF). However, the material of the electro-active layers 21,22 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 electrostrictive material, which constricts on application of an electric field.

The electro-active portion 20 further comprises electrodes 23 to 25 extending parallel to the layers 21,22 of piezoelectric material. Outer electrodes 23,24 are provided outside the electro-active layers 21,22 on opposite sides of the electricactive portion 20. A centre electrode 25 is provided between the electro-active layers 21 and 22. The electrodes 23 to 25 are used to apply poling voltages and to operate electro-active portion 20 in a bending mode. On electrical activation, activation voltages are applied to the electrodes 23 to 25 and conversely on mechanical activation voltages are developed on the electrodes 23 to 25. On activation, the electro-active layers 21 and 22 undergo a differential change in length concomitant with bending of the portion 20 due to the constraint of the layers being coupled together at their interface formed by the centre electrode 25. For maximum displacement, on activation one of the electro-active layers 21 or 22 expands and the other one of the electro-active layers 21 and 22 contracts The relative direction an magnitude of the activation and poling voltages may be selected in the same manner as for known linear electro-active devices having a bender construction. For example, poling voltages of sufficient magnitude to pole the electro-active layers 21 and 22 may be applied in opposite directions across the electro-active layers 21 and 22 by grounding the centre electrode 25 and applying poling voltages of the same polarity to both the outer electrodes 23,24. In this case, the electro-activer portion 20 is electrically activated by applying activation voltages in the same direction across the electro-active layers 21 and 22 by applying voltages of opposite polarity to the two outer electrodes 23 and 24.

On activation, the electro-active portion 20 bends around the minor axis 3 or 13, either towards or away from the minor axis 3,13 depending on the polarity of the activation voltages. On electrical activation, the activation voltages are applied from a circuit 26 through external terminals 27 electrically connected to the electrodes 23 to 25 in the manner known for known straight piezoelectric devices having a bender construction. On mechanical activation, the activation voltages developed at the electrodes 23 to 25 are fed to the circuit 26.

Electrical connection to the electrodes 23 to 25 may be made in the same way

as is known for known straight devices having a bender construction, in principle at any point along the length of the device of which the portion 20 forms part but preferably at the end. The preferred technique is to provide the electrodes with fingers (not shown) extending at the end of the device at different lateral positions across the width of the device as known for straight devices having a bender construction.

It will be appreciated that other bender constructions could equally be applied to the portion 20, for example a unimorph bender construction comprising a layer of electro-active material and an inactive layer or a multimorph bender construction comprising a plurality of layers of electro-active material.

Whilst the bender construction illustrated in Fig. 3 is preferred for simplicity and ease of manufacture, it will be appreciated that the continuous numbers 2 or 12 could in fact have any construction which bends around the minor axis 3 or 13 on activation. For example, the continuous members could be electro-active elements of the type described in the application being filed simultaneously with this application entitled"Electro-Active Elements and Devices"in which the elements have two pairs of electrodes extending along the length of the member for bending across the width on activation.

On activation, the electro-active portions 20 of the continuous member 2 or 12 bend around the minor axis 3 or 13. As a result of the continuous electro-active member 2 or 12 curving around the minor axis 3 or 13, in particular in a helix, such bending is concomitant with twisting of the continuous member 2 or 12 around the minor axis 3 or 13. This may be visualised as the turns of the continuous member 2 or 12 as the bending tightening or loosening causing a twist of the structure of the member 2 or 12 along the minor axis 3 or 13. The twist of the continuous member 2 or 12 occurs along the entire length of the minor axis 3 or 13 causing a relative rotation of the ends of the structure labelled 5 and 6 in the first device 1 of Fig. 1 and 15 and 16 in the second device 11 of Fig. 2.

It will be appreciated that the continuous member 2 or 12 could curve around the minor axis 3 or 13 in curves other than a helix to produce such twisting, for

example by having the shape as though formed by twisting a flat member round the minor axis. It will also be appreciated that other structures other than a continuous member could be applied to produce twisting around th minor axis. For example the electro-active structure could consist of a plurality of electro-active portion disposed successively along the minor axis and coupled together so that the bending of each individual portion twists the adjacent portion around the minor axis causing twisting of the structure as a whole. Alternatively the electro-active structure could be a device of the type described in the application being filed simultaneously with this application entitled"Piezoelectric Devices"which comprises a plurality of electro-active torsional actuators which may comprise electro-active elements activated in shear mode.

Considering the first device 1 of Fig. 1, the twisting of the continuous member 2 around the minor axis 3 is concomitant with relative displacement of the ends of the device 5 and 6 perpendicular to the curve of the minor axis 3, that is parallel to the major axis 4. The relative displacement of the ends 5 and 6 derives from the twisting of the continuous member 2 around the minor axis 3 in combination with the curve of the minor axis 3. 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 a similar manner, on activation of the second device of Fig. 2, the twisting of the continuous member 12 around the minor axis 13 is concomitant with displacement of the ends of the device 15 and 16 parallel to the major axis 14.

Again, this relative displacement derives from the rotation of the continuous member 12 around the minor axis 13 in combination with the curve of the minor axis 13. In this case, the relative displacement caused by any given small section of the structure along the minor axis 13 causes relative displacement of the ends of that section perpendicular to the local curve of the minor axis 13. The overall displacement of the ends 15,16 of the device 11 is the sum of the displacements of all the sections which results in an overall relative displacement parallel to the major axis 14.

The exact construction and dimensions of the member 2 or 12 and the form of

the electro-active structure may be freely varied to produce the desired response. A suitable member 2 or 12 has a 0.5 mm thickness tape wound as a 4 mm diameter minor helix around the minor axis 3 or 13. When this forms the first device 1 in which the minor curve extends around about three quarters of a circle of 30 mm diameter the observed displacement is about 6mm. Similarly if this structure was used to form the second device 11 in which the minor curve extends along a 20 turn helix of diameter 30mm, this would produce displacement of around 120mm.

In general, the minor axis, along which the structure of devices in accordance with the present invention extend, may follow any curve and the resultant displacement of the ends of the structure will be the sum of the displacement caused by each section of the structure along the curve. However, curves which are regular such as the curve of the minor axis of the first and second devices 1 and 11 are preferred so that all sections of the device caused relative displacement in a common direction and also because design and manufacture are thereby simplified.

The first and second devices 1 and 11 may be electrically activated to create mechanical displacement between the ends 5 and 6 or 15 and 16. This mode of operation is used when the devices 1 and 11 are used in a motor. Also the devices 1 and 11 may be mechanically activated in which case relative displacement of the ends 5 and 6 or 15 and 16 causes an electrical voltage to be developed across the electrodes 23 to 25. This mode of operation is used when the devices 1 and 11 are used in a generator.

Manufacture of the electro-active devices 1 and 11 will now be described.

The preferred method of manufacture is to initially form the electro-active structure extending along a straight minor axis and subsequently to bend the straight electro-active structure so that the minor axis along which it extends becomes curved.

To form the continuous member 2 or 12 as an electro-active structure along a straight minor axis there are two preferred techniques.

The first preferred technique is to initially form the continuous member 2 or 12 as a straight member and subsequently to deform it to curve around the straight minor axis. The bender construction of the continuous member 2 or 12 is in itself

known and the continuous member 2 or 12 may be formed by applying any of the known techniques for manufacturing a device having a bender construction. For example, the continuous member 12 may be initially manufactured by co-extrusion of the layers 21 and 22 of plasticised material or by co-calendering of the layers 21 and 22. Alternatively, the continuous member 2 or 12 may be made through lamination of thin layers 21 and 22. These thinner layers may be made by any suitable route, such as high shear mixing of a ceramic powder, polymer and solvent mixer followed by co-extrusion and calendering. Alternatively, techniques such as tape casting or the process called the Solutech process known in the field of ceramics may be used.

The electrodes may be formed as an integral part of the manufacture of the continuous member 2 or 12, for example by being in co-extruded or co-calendered.

Further electrodes, which may be activation layers 23 to 25 or may be terminal electrodes to allow access to the electrodes 23 to 25, may be applied by printing, by electro-less plating, through fired-on silver past or by any other appropriate technique.

The second preferred technique is to initially manufacture the continuous member as a cylinder or other tube with a multi-layered bender construction of electro-active layers 21 and 22 and electrodes 23 to 25 and subsequently6 cut the member along the helical line to leave the continuous member 2 or 12 extending in a helix around the axis of the cylinder or tube which then constitutes the minor axis.

Subsequently the straight structure is bent to curve the minor axis along which the structure extends.

To deform the member and structure, there must exist in the initially formed member a sufficient degree of flexibility. Suitably deformable electro-active materials are known, typically including constituent polymers which enhance the deformability. With such materials after shaping, the constituent polymers are burnt

out, typically at up to 600 C and the material is then densified through further sintering at higher temperature, typically 1000 C to 1200 C. In this case, the electroactive structure is initially formed with enlarged dimensions to allow for linear

shrinkage which occurs during sintering, typically of around 12 to 25%.

The curving of the straight member and the bending of the structure may be performed around formers. The formers are subsequently removed either physically or by destruction of the former for example by melting, burning or dissolving.

Now will be described various embodiments of motor and generator using the electro-active device of the type described above. In all the following embodiments the electro-active device is illustrated as being of the type of the second device 11 described above with an electro-active structure curving in a helix, but this merely for illustration, and any of the types of electro-active device described above may alternatively be used.

Next, there will be described embodiments of motors in which the electroactive device is arranged to generate mechanical displacement on electrical actuation of the electro-active device. Thus the motor may be used to drive the displacement of some further object.

The embodiments are linear motors, rotary motors and an orbital motor.

The electro-active device of the type described above is by itself a very simple form of linear motor, because on electrical activation it provides relative displacement between the ends of the electro-active structure. Although relatively large relative displacements may be achieved as compared to many knównelectro- active devices, the range of relative displacement will be restricted by the structure of any given device, typically to the order of tens or hundreds of milimeters. The hereinafter described embodiments of linear motor are capable of achieving a displacement greater than the maximum relative displacement of the ends of the structure of a single electro-active device through the use of a directional couplings.

In each case the directional coupling is coupled to the electro-active device to convert relative displacement of the ends of the structure into linear displacement in a predetermined direction.

The first linear motor 40 is illustrated in Fig. 4. The linear motor 40 comprises an electro-active device 41 of the type described above in combination with a ratchet track 42 and a pair of pawl elements 43, each coupled to a respective

end of the structure of the electro-active device 41. The ratchet track 42 extends parallel to the direction of relative displacement of the ends of the structure of the electro-active device 41.

The ends of the pawl elements 43 and 44 engage the ratchet track 42 so that they may be driven along the ratchet track in one direction A but are restrained from moving in the opposite direction B along the ratchet track 42. The end plates 43 and 44 are provided with sufficient flexibility to function as pawl elements with the ratchet track 42. As an alternative, the end plates could be provided with a specifically formed structure to engage the ratchet track 42.

During relative displacement of the ends of the structure of the electro-active device towards one another, the pawl element 43 at one end of the electro-active device 41 is restrained by the ratchet track 42, whereas the second pawl element 44 at the opposite end of the electro-active device 41 is driven along the ratchet track 42 in the direction A. Conversely, during the reciprocal motion of the electro-active device 41 in which the ends of the electro-active device 41 move away from each other, the second pawl element 44 is restricted by the ratchet track 42 whilst the first pawl element 43 is driven along the ratchet track 42. Thus reciprocal relative displacement of the ends of the electro-active device causes the pawl elements 43 and 44 to be alternately driven along the ratchet track 42, thereby creating overall linear displacement of the electro-active device 41 in the direction A. Thus the ratchet track 42 and pawl elements 43 and 44 together constitute a directional coupling.

The linear mechanical displacement of the electro-active device 41 of the linear motor 40 may be used to drive linear displacement of a further object, for example by coupling the further object to the electro-active device 41 or to either of the end plates 43 or 44.

A control circuit 45 is electrically connected to the electro-active device 41.

In use, the control circuit 45 applies activation voltages to the electrodes of the structure of the electro-active device 41 to produce the desired output displacement of the electro-active device 41. For example, to achieve a motion of the centre of the electro-active device 41 at a constant rate, the applied activation voltages will have a

saw-tooth waveform rising and falling at a constant rate. In general, the average speed of displacement of the electro-active device 41 is equal to the frequency of the applied activation voltage multiplied by the overall range of relative displacement of the ends of the structure of the electro-active device 41.

Due to the nature of the ratchet arrangement, the end plates 43 and 44 undergo intermittent linear motion because they only move during a respect of one of the expansion and contraction strokes of the electro-active device 41. To achieve a continuous linear displacement, the output of the motor 40 may be taken from the center of the structure of the electro-active device 41. Alternatively, a further electroactive device may be employed as in the second linear motor 50 illustrated in Fig. 5.

The second linear motor 50 comprises an electro-active device 41 and a ratchet arrangement consisting of a ratchet track 42 and pawl elements 43 and 44 which are arranged in an identical manner to the corresponding elements of the first linear motor 40, so the description thereof will not be repeated. In addition, a second electro-active device 51 is coupled to the pawl element 43 at one end of the structure of the first electro-active device 41. The opposite end of the second structure of the electro-active device 51 is coupled to an output plate 52. The output plate 52 has smaller dimensions than the end plates 43 and 44 coupled to the first electro-active

device 41 so that the output plate 52 does not engage the ratchet track 42' : The output of the second electro-active device 50 is taken from the output plate 52, for example by coupling the output plate to an object to be driven. The second electro-active device 51 is used to generate a continuous motion of the output plate 52, compensating for the intermittent motion of the end plate 43 to which the second electro-active device 51 is coupled. In particular, the second electro-active device 51 is driven to have a displacement of magnitude half that of the displacement of the first electro-active device 41, and the second electro-active device 51 is in use driven in anti-phase with the first electro-active device 41 so that the second electroactive device 51 expands as the first electro-active device 41 contracts and vice versa.

In the second linear motor 50, the electro-active devices 41 and 51 are electrically connected to a control circuit 53 which provides appropriate activation

voltages to the electrodes of the structure of the electro-active devices 41 and 51 to generate the desired motion. For example, to obtain motion at a constant rate, the electro-active devices 41 and 51 are driven with activation voltages having a sawtooth waveform so that the electro-active devices 41 and 51 expand and contract at a constant rate. In general, the average speed of displacement of the output plate 42 is equal to the frequency of the applied activation voltage multiplied by the range of relative displacement between the ends of the structure of the first electro-active device 41.

Of course, the ratchet track 42 of the first and second linear motors 40 and 50 provide for uni-directional displacement, that is in direction A. There will be now be described some linear motors in which the directionality of the ratchet arrangement may be selected in order to produce a bi-directional linear motor. This could be achieved by switching a mechanical system, but in the following embodiments the ratchet track is formed from electro-active elements which are actuatable to select the directionality.

Fig. 6 illustrates a portion of an alternative ratchet track 60 which may replace the ratchet track 42 in the first and second linear motors 40 and 50 of Fig. 4 and 5. The portion of the ratchet track 60 comprises a wall 64 and a pair of electroactive benders 61 and 62 facing in opposition directions along the linearextent of the ratchet track 60. The benders 61 and 62 may have any construction which causes them to bend when they are actuated, but preferably have a conventional bender construction. In such a bender construction, the benders 61 and 62 comprise a plurality of layers at least one of which is of electro-active material, the layers being arranged to undergo a differential change in length when the benders 61 and 62 are actuated. This causes bending of the benders 61 and 62 perpendicular to the direction in which the layers extend. The bender construction may be a uni-morph bender construction consisting of a single electro-active layer and an inactive layer, a bi-morph bender construction consisting of two electro-active layers or a multimorph bender construction consisting of many electro-active layers. The electroactive material is preferably piezoelectric material, although other types of electro-

active material, such as electro-strictive may be used.

The benders 61 and 62 are connected to a control circuit 63 for providing activation voltages to actuate the benders 61 and 62. In use, the control circuit 63 selectively actuates the first bender 61 or the second bender 62. On actuation, the benders 61 and 62 bend to a curved shape as illustrated by the bold and dotted outlines in Fig. 6. Accordingly, when actuated, the benders 61 and 62 may act as a ratchet tooth, because they present a gently rising slope when approached along the length of the bender 61 or 62 from the ends 65 attached to the track walls 64 to the free ends 66. In contrast, when approached from the opposite direction, the free end 66 presents a barrier above the level of the track wall 64 which is sufficient to catch a pawl element. Therefore, when the first bender 61 is actuated (bold outline in Fig.

6), it forms a ratchet tooth allowing movement of a pawl element in a first direction A along the ratchet track 60. On the other hand, when the second bender 62 is actuated (dotted outline in Fig. 6), it forms a ratchet tooth allowing movement of a pawl element in the opposite direction along the ratchet track 60.

The portion of the ratchet track 60 illustrated in Fig. 6 is repeated along the length of the ratchet track 60. By selective actuation of the first or second bender 61 or 62 of its portion, the directionality of the ratchet track is selected.

Alternatively, the pawl elements of the ratchet arrangement may Be electroactively actuatable to select the functionality of the ratchet arrangement. An example of such an arrangement is the third linear motor 70 illustrated in Fig. 7. The third linear motor 70 comprises an electro-active device 41 which is coupled between end plates 43 and 44 which have the same arrangement as the first linear motor 40 of Fig.

4 and are operated in the same manner.

The third linear motor 70 differs from the first linear motor 40 in the following way. In place of the ratchet tracks 42 of the first linear motor 40, the third linear motor 70 has two parallel ratchet tracks 71 and 72 which are both unidirectional, but which are oriented to allow motion of a pawl element in opposite directions. The end plates 43 and 44 do not themselves engage the ratchet tracks 71 and 72. Instead, benders 73 and 74 are coupled to the end plates 43 and 44, at a

position to engage a respective one of the ratchet tracks 71 and 72 when actuated and to disengage the ratchet track 71 and 72 when not actuated. The benders 73 and 74 are selectively actuated so that either the first bender 73 engages the first ratchet track 71 (bold outline in Fig. 7) or the second bender 41 is actuated to engage the second ratchet track 72 (dotted outline in Fig. 7). When the benders 73 or 74 engage the respective ratchet track 71 or 72 they act as pawl elements allowing motion in a respective direction A or B due to the opposite directionality of ratchet track 71 or 72.

Other directional couplings from the ratchet arrangements of the first, second and third linear motors 40, 50 and 70 may be employed. For example, the fourth linear motor 80 illustrated in Fig. 8 uses a sprag drive 81 as a directional coupling.

In particular, the fourth linear motor 80 comprises an electro-active device 82 of the type described above which is coupled at a first end 83 to a fixed support 84 and is coupled at the opposite end 85 to the sprag drive 81. The sprag drive 81 is in itself of conventional construction to provide a coupling to a rod 86 which allows the expansion of the electro-active device 82 to drive displacement of the rod 86 in a first direction C, but resists displacement of the rod 86 in the opposite direction D during contraction of the electro-active device 82.

Fig. 9 illustrates a fifth linear motor 90 using a further type of directional coupling, as follows. In particular, the fifth electro-active device 90 comprises an electro-active device 91 coupled at each end of its structure to a respective end plate 92 and 93 in a similar manner to the electro-active device 41 and pawl elements 43 and 44 of the first linear motor 40.

The fifth linear motor 90 further includes a track 94 forming a guide means for movement of the electro-active device 91 therealong. The track 94 extends parallel to direction of relative displacement of the ends of the structure of the electro-active device 91. The end plates 92 and 93 are piezoelectric disc actuators which are actuatable in a dishing mode to change their diameter to selectively engage or disengage the track 94.

The electro-active device 91 and end plates 92 and 93 are electrically

connected to a control circuit 95 for applying activation voltages to operate the fifth linear motor 90 as follows. Activation voltages are applied to the electro-active device 91 to cause it to alternately expand and contract so that the ends of the structure of the electro-active device 91 are relatively displaced in opposite directions. The end-plates 92 and 93 are alternately actuated so that one of the end plates 92 and 93 expands to engage the track 94 during expansion of the electroactive device 91 and the other of the end plates 92 and 93 expands to engage the track 94 during contraction of the electro-active device 91. This causes the electroactive device 91 to move along the track 94 in an equivalent manner to the electroactive device 41 of the first linear motor 40. However, the fifth linear motor 90 is bidirectional, the direction of motion being selected by the choice of which of the end plates 92 and 93 are actuated during expansion and contraction, respectively, of the electro-active device 91.

The track 94 is provided with teeth to assist engagement of the end plates 92 and 93, thereby preventing slippage of the end plates 92 and 93 along the track 94.

As the diameter change of the end plates 92 and 93 is relatively small, the size of the teeth in the track is correspondingly small, but with the benefit of providing very fine movement resolution.

In the fifth linear motor 90, the end plates 92 and 93 constitute engagement means for selectively engaging the track 94 which constitutes a guide means for movement of the electro-active device 91. Other types of engagement means may be coupled to the ends of the electro-active device 91 to provide a bi-directional linear motor for movement along other types of guide means. For example, the engagement means may be electro-magnets for engaging a magnetizable guide means, for example a metal pipe or track. Alternatively, the engagement means could be a mechanical clamping system to engage a guide means such as a rail, track or pipe wall. By providing the engagement means with some flexibility in the coupling to the electro-active device 91, it is possible to provide a linear motor which is movable along a curved line, rather than a straight line. This would allow the linear motor to be used in cable laying and pipe inspection.

It will be appreciated that the electro-active device in accordance with the present invention may be applied to many other types of linear motor, in addition to the linear motors described above.

The electro-active device may also be used to drive a rotary motor arranged to generate rotary mechanical displacement. To achieve this, a mechanical coupling may be used to convert the linear relative displacement of the ends of the electroactive device into rotary displacement. Such a mechanical coupling is preferably directional in order to convert reciprocal relative displacement of the ends of the structure of the electro-active device into rotary displacement in a predetermined sense. A simple example of such a directional coupling is a crank or a"Scotch-yoke" (also known as a"Geneva mechanism").

Figs. 10 and 11 illustrate first and second rotary motors on 100 and 110, respectively in which the directional mechanical coupling is a rotary sprag drive.

In the first rotary motor 100 of Fig. 10, an electro-active device 101 has one end 102 coupled to a fixed support 103 and the other end 104 coupled to sprag wheel 105 of a sprag drive 106. The sprag drive 106 is of conventional construction with the sprag wheel 105 converting relative linear displacement of the ends 102 and 104 of the electro-active device 101 in one direction into rotary motion of the shaft 107 of the sprag drive 106, whilst resisting rotary motion of the shaft 107 during relative displacement of the ends 102 and 104 of the electro-active device 101 in the opposite direction. A control circuit 108 is electrically connected to the electrodes of the electro-active device 101 for providing an activation voltage to activate the electroactive device 101 in any desired manner.

The second rotary motor 110 illustrated in Fig. 11 has a similar construction in that a electro-active device 111 is coupled at one end 112 to fixed support 113 and at the opposite end 114 to a sprag drive 115. However, in this case the electro-active device 111 is coupled to two separate sprag wheels 117 and 118 which each convert relative linear displacement of the ends 112 and 114 of the electro-active device 111 in opposite directions into rotation of the shaft 116 of the sprag drive 115 in a common direction. A control circuit 119 is electrically connected to the electrodes of

the structure of the electro-active device 111 to provide activation voltages to activate the electro-active device 111. For example, to drive rotary displacement of the shaft 116 at a constant rate, the activation voltages may have saw-tooth waveform rising and falling at a constant rate.

Another directional coupling which may be used to convert reciprocal relative displacement of an electro-active device into relative rotation is a screw and ratchet mechanism of the type commonly found in spinning-top toys and in"Yankee" screwdrivers. Such a screw mechanism comprises a helical threaded rod, preferably with a large helix angle, and nut with a matching helical thread mated with the helical thread on the rod. One of the rod and the nut are coupled to the electro-active device to be driven reciprocally and linearly, while the other of the rod and thread are coupled to a shaft which is thus caused to rotate. The coupling further includes a ratchet mechanism which allows operation when the electro-active device is driven in a forward direction and slips when the electro-active device is driven in the reverse direction. Thus the shaft is caused to rotate during linear displacement of the ends of the electro-active device in one of the two reciprocal directions. By use of a second screw and ratchet mechanism in the coupling, the rotation of the shaft can be driven by relative displacement of the ends of the electro-active device in both directions.

Further rotational couplings which could be used are a conventional worm gear system which has the advantage of providing high mechanical advantage or a rack and pinion mechanism or indeed any other type of coupling.

A rotary motor in accordance with the present invention can be connected to a transmission system that provides a mechanical advantage through the gear ratio, reverse the rotation direction and/or change the orientation of the output shaft.

The electro-active device of the type described above can also be used in an orbital motor capable of driving an orbital member around an orbital path. Fig. 12 illustrates an orbital motor 120 of this type.

The orbital motor 120 comprises three electro-active devices 121 coupled at one end to a fixed support 122 and at the other end to an orbital member 124 which is an eccentric coupled to drive a rotary shaft 125. The electro-active devices 121 are

arranged around the orbital member 124 to provide for relative displacement between the ends of the devices 121 in different directions in a common plane (parallel to the plane of the diagram in Fig. 12).

A control circuit 126 is electrically connected to the electrodes of the structures of the electro-active devices 121 and arranged to provide activation voltages to activate the electro-active devices 121. In use, the activation voltages are arranged to drive electro-active devices 121 with different phases, preferably with a fixed phase relationship between the electro-active devices 121. The result of this is to drive the orbital member around an orbital path 123. In the orbital motor 120 of Fig. 12, the magnitude of the displacement of each of the electro-active devices 121 is the same so that the orbital path is a circular path in order to rotate the orbital member 124 around the shaft 125. However, by varying the relative magnitude of the displacements of the electro-active devices 121, it would be possible to drive the orbital member 124 around orbital paths of other shapes. Although the electro-active device 120 is shown as having three electro-active devices 121, for illustration, any number of electro-active devices 121 may in fact be used.

There will now be described embodiments of the present invention which are generators in which an electro-active device of the types described above is used. In such generators, the electro-active device is mechanically actuated to cause relative displacement between the ends of the structure of the electro-active device. Such mechanical actuation causes the electro-active device to output electrical energy, for example as a direct or alternating current or voltage across the electrodes of the structure of the electro-active device. In general, the ends of the structure of the electro-active device will be coupled to respective elements which are relatively movable and in use move to mechanically activate the electro-active device. The electro-active device is typically connected to an electrical circuit which receives the generated power for supply to another circuit or for storage, for example in a battery cell. Usually, the electro-active device will be used to convert a reciprocal relative displacement between the ends of the device into an alternating current or voltage, in which case, the electrical circuit may include a rectifier to convert the alternating

input into a direct current or voltage.

First there will be described generators which are used to generate electrical energy from an element that is driven to reciprocate. The first generator 130 illustrated in Fig. 13 comprises an electro-active device 131. One end 132 of the electro-active device 131 is coupled to a fixed support 133, whilst the other end 134 of the electro-active device 131 is connected to a further element 135 which is movable relative to the support 133. The element 135 may be coupled to literally any reciprocating drive source to mechanically activate the electro-active device 131.

The electrodes of the structure of the electro-active device 131 are electrically connected to a circuit 136 for receiving the generated electrical energy.

Fig. 14 illustrates a second generator 140 which may be used to generate power from waves. The second generator 140 comprises an electro-active device 141 which is coupled at one end 142 to a fixed support 143. The other end 144 of the electro-active device 141 is coupled to a mass 145 and a float 146. The electrodes of the structure of the electro-active device 141 are connected to an electrical circuit 147 to receive electrical energy generated by the electro-active device 141.

In use, the second generator 140 is positioned with the float floating at or near the surface of a body of water 148 such as a lake or the sea. As waves on the surface of the water 148 pass the generator 140, they drive the float 146 upwardsReciprocal motion after passage of the wave is driven by the weight 145. As alternative to the use of the weight 145, any means to drive reciprocal motion of the electro-active device 141 may be used. In this way, reciprocating motion of the end 144 of the electro-active device 141 relative to the opposite end 142 is driven by the float 146, allowing the conversion of energy from the waves into electrical energy.

There will now be described embodiments of generators which harness vibrational energy. In these generators, one end of an electro-active device is coupled to a vibrational element which is likely to vibrate in normal use, for example any type of portable electronic device. The vibration may be produced internally, e. g. from operation of moving parts of the device, or externally, e. g. from the device being transported. The other end of the electro-active device is coupled to a mass

which is free to move relative to the vibrational element. Accordingly, when the vibrational element is vibrated, this causes one end of the electro-active device to be displaced, whereas the inertia of the mass limits the displacement of the other end of the electro-active device. As a result, the ends of the electro-active device are relatively displaced by the vibration of the vibrational element.

The third generator 150 illustrated in Fig. 15 is an example of this type of generator in its most simple form. The third generator 150 comprises an electroactive device 151 coupled at one end 152 to a vibrational element 153 which may be literally any type of element which might vibrate. The other end 154 of the electroactive device 151 is coupled to a mass 155. The electrodes of the structure of the electro-active device 151 are electrically connected to a circuit 156 which receives the generated electrical energy.

The fourth generator 160 illustrated in Fig. 16 is another generator of this type which harnesses vibrational energy. The fourth generator 160 comprises an electro-active device 161 coupled at one end 162 to a casing having the physical arrangement of a standard battery, such as a DC or AA cell. The other end 164 of the electro-active device 161 is mechanically coupled to a battery cell 165. The electroactive device 161 and the battery cell 165 are both disposed inside the casing 163 with the battery cell 165 free to move inside the casing 163. Thus the baftery cell 165 acts as mass causing the generator 160 to operate as described above and when casing 163 is vibrated.

The electro-active device 161 is electrically coupled through a rectifier circuit 166 to the battery cell 165 to store in the battery cell 165 the electrical energy generated by the electro-active device 161. The battery cell 165 is also electrically connected to contacts 167 and 168 provided as part of the casing 163 with the same physical arrangement as a standard battery. As a consequence, the generator 160 may be used as a standard battery in a portable electronic device, but will get recharged by motion of the casing 163 such as may be expected during normal use of the portable electronic device.

It will be appreciated that modifications to the fourth generator 160 are

possible. For example the battery cell 165 might be arranged inside the electroactive device 164 with the structure of the electro-active device 164 curving around the battery cell. This would produce a very compact arrangement. Whilst it is advantageous if the battery cell 165 acts as the mass coupled to the end 164 of the electro-active device 161 this is not essential, and there could instead be a separate mass coupled to that end 164 of the electro-active device 161.

The fifth generator 170 illustrated in Fig. 17 is another generator of the type which harnesses vibrational energy.

The fifth generator 170 comprises an electro-active device 171 coupled at one end 172 to a housing 173 and mechanically coupled at the other end 174 to a printed circuit board 175. The housing 173 may be the housing for literally any type of electronic device, such as a mobile telephone. The printed circuit board 175 provides the circuitry for the electronic device in the normal way.

The printed circuit board 175 is relatively movable within the casing 173, for example by being slidably mounted between guides 176. The electro-active device 171 is electrically connected to the printed circuit board 175 which is provided with circuitry to receive the generated electrical energy and store it in a battery 177 provided in the housing 173. The generator 170 allows the printed circuit board 175 of any electronic device to provide the mass allowing an electro-active device to generate electrical energy on vibration of the device. This energy is subsequently stored in the electronic device which is therefore made to be re-chargeable by the vibration which is expected during normal use.

A further charging application would be in mobile computers where an electro-active device could be placed under each key. The electro-active device could replace the keyboard spring. Each key press would mechanically actuate the electro-active device allowing the generation of electrical energy which could be used to recharge the battery of the mobile computer.

When the electro-active device is used in a generator, the electro-active device is provided with mechanical properties appropriate for the expected form of the mechanical activation. For example, the electro-active device may be pre-

arranged to have a resonant frequency equal to the expected frequency of mechanical activation. Such properties may be controlled to some extend by selection of the type of electro-active material and the design of the structure, for example the width, thickness of the electro-active layers. Alternatively, further inactive layers may be used in the structure, or the structure of the electro-active device may be embedded in a resilient material for example a polymer.

In addition, a resilient biassing ring means may be coupled in parallel with the electro-active device, for example between the relatively movable elements to which the electro-active device is coupled. Any type of resilient biasing means, for example a spring, may be used. One purpose of the resilient biasing means is to control the resonant frequency of the combined system consisting of the resilient biasing means and the electro-active device in parallel with one another, for example to cause vibration to persist after an initial impulse. Another purpose of the resilient biasing means is to return the electro-active device to its initial state when the driving force is expected to be applied in one direction only, for example from the impact of waves on a shore, or a rotating cam. Otherwise, it is necessary to rely on the stiffness of the electro-active device to cause it to return to its initial state when the driving force is removed.

The resilient biasing means may force the electro-active device inti'a stressed configuration, for example to increase or decrease the distance between the ends as compared to the relaxed, inactive state of the device. In this way the full stroke of the electro-active device may be harnessed by a force acting in one direction only by arranging the driving force to be applied against the resilient biasing means. Thus the force moves the free end of the electro-active device from its stressed initial position with maximum (or minimum) displacement through its relaxed position to another stressed position with minimum (or maximum) relative displacement.

Subsequently the resilient bus means returns the electro-active device to its initial stressed state.

An additional resilient biasing means allows the electro-active device to be operated by larger forces that electro-active device can carry on its own. The

additional energy stored in the resilient biasing means is retrieved as the electroactive device is returned to its initial position.

It will be appreciated that a resilient biasing means may be applied to any of the generators described above. Furthermore, the sixth generator 180 illustrated in Fig. 18 uses such a resilient biasing means in the form of a spring 186. In particular, the sixth generator 180 comprises an electro-active device 181 one end 182 of which is coupled to a fixed support 183 and the other end 184 of which is coupled to an element 185 movable with respect to the support 183. The movable element 185 in use receives the applied force. The spring 186 is also coupled between the support 183 and the element 185 in parallel with the electro-active device 181.

Claims

1. A motor including an electro-active device arranged to generate mechanical displacement on electrical actuation of the electro-active device, the electro-active device comprising an electro-active structure extending along a curved minor axis and arranged, on electrical activation, for the structure to twist around the minor axis and concomitantly for relative displacement of the ends of the structure to occur.

2. A motor as claimed in claim 1 and being a linear motor arranged to generate linear mechanical displacement.

3. A motor as claimed in claim 2, further comprising a directional coupling coupled to the electro-active device to convert reciprocal relative displacement of the ends of the structure of the electro-active device into linear displacement in a predetermined direction.

4. A motor as claimed in claim 3, wherein the directional coupling is a ratchet arrangement.

5. A motor as claimed in claim 4, wherein the ratchet arrangement comprises a ratchet track and a respective pawl element coupled to each end of the structure of the electro-active device and engageable with the track.

6. A motor as claimed in claim 4 or 5, wherein the ratchet arrangement is selectively bi-directional so that the motor is bi-directional.

7. A motor as claimed in claim 6, wherein the ratchet track is formed from a plurality of electro-active elements which are actuatable to select the directionality of the ratchet arrangement.

8. A motor as claimed in claim 6, wherein the pawl elements are electroactively actuatable to select the directionality of the ratchet arrangement.

9. A motor as claimed in claim 3, wherein the directional coupling is a sprag drive coupled to one end of the stucture of the electro-active device.

10. A motor as claimed in claim 3, wherein the directional coupling comprises a respective engagement means coupled to each end of the structure of the electro-active device, each respective engagement means being for selectively engaging a linearly extending guide means during relative displacement of the ends of the structure in opposite directions.

11. A motor as claimed in claim 9, wherein the engagement means comprises an electro-active element actuatable to engage the guide means.

12. A motor as claimed in claim 1 and being a rotary motor arranged to generate rotary mechanical displacement.

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13. A motor as claimed in claim 12, further comprising a directional coupling coupled to the electro-active device to convert reciprocal relative displacement of the ends of the structure of the electro-active device into rotary displacement in a predetermined sense.

14. A motor as claimed in claim 13, wherein the directional coupling is a rotary sprag drive

15. A motor as claimed in claim 1, comprising a plurality of electro-active devices each comprising an electro-active structure extending along a curved minor axis and arranged, on electrical activation, for the structure to twist around the minor

axis and concomitantly for relative displacement of the ends of the structure to occur, the electro-active devices being connected to an orbital member with alignments providing for the relative displacement of the devices to'occur in different directions in a common plane, whereby electrical activation of the electro-active devices with differing phases is capable of driving the orbital member around an orbital path.

16. A motor as claimed in claim 15, wherein the orbital member is an eccentric coupled to drive a rotary shaft so that the motor constitutes a rotary motor.

17. A generator including an electro-active device arranged to generate electrical energy on mechanical actuation of the electro-active device, the electroactive device comprising an electro-active structure extending along a curved minor axis and arranged to be mechanically activated by relative displacement of the ends of the structure and, concomitantly, for the structure to twist around the minor axis causing the electrical energy to be generated.

18. A generator as claimed in claim 17, wherein one end of the electroactive device is coupled to a float.

19. A generator as claimed in claim 18, wherein said one end of the electro-active device is also coupled to a mass.

20. A generator as claimed in claim 17, wherein one end of the electroactive device is coupled to a mass.

21. A generator as claimed in claim 17, wherein the ends of the structure of the electro-active device are mechanically coupled to respective elements which are relatively movable.

22. A generator as claimed in claim 21, wherein a first one of the

elements includes a float.

23. A generator as claimed in claim 22, wherein a first one of the elements further includes a mass.

24. A generator as claimed in claim 21, wherein a first one of the elements includes a mass.

25. A generator as claimed in claim 21, wherein the first one of the elements is a battery cell to which the electro-active device is also electrically connected to store the generated electrical energy.

26. A generator according to claim 21 or 25, wherein the second element is a casing having the physical arrangement of a standard battery, and inside which casing are disposed the electro-active device and the battery cell electrically connected to the electro-active device to store the generated electrical energy.

27. A generator as claimed in claim 21, wherein the first element is a printed circuit board and the second element is a housing for the printed-circuit board.

28. A generator as claimed in any one of claims 21 to 27, wherein a second one of the elements is a support.

29. A generator according to any one of claims 21 to 28, further comprising a resilient biasing means coupled between the relatively movable elements.

30. A motor or a generator as claimed in any one of the preceding claims, wherein the electro-active structure comprises electro-active portions disposed

successively along the minor axis and arranged to bend, on activation, around the minor axis.

31. A motor or a generator as claimed in claim 30, wherein the electroactive structure comprises a continuous electro-active member curving around the minor axis, said electro-active portions being adjacent finite portions of the continuous member.

32. A motor or a generator as claimed in claim 31, wherein the continuous electro-active member curves in a helix around the minor axis.

33. A motor or a generator as claimed in any one of claims 30 to 32, wherein the successive electro-active portions have a bender construction of a plurality of layers including at least one layer of electro-active material.

34. A motor or a generator as claimed in claim 33, wherein the electroactive portions have a bimorph bender construction of two layers of electro-active material or a multimorph bender construction of more than two layers of electroactive material.

35. A motor or a generator as claimed in any one of the preceding claims, wherein the electro-active structure includes electrodes for development of an electric voltage thereacross on activation of the electro-active structure.

36. A motor or a generator as claimed in any one of the preceding claims, wherein the minor axis extends in curve which is a helix.

37. A motor or a generator as claimed in any one of the preceding claims, wherein the minor axis extends in curve which is planar.

38. A motor or a generator as claimed in any one of the preceding claims,

wherein the electro-active structure includes piezoelectric material.

39. A motor or a generator as claimed in claim 38, wherein the piezoelectric material is a piezoelectric ceramic or a piezoelectric polymer.

40. A motor or a generator as claimed in claim 39, wherein the piezoelectric material is lead zirconate titanate (PZT) or polyvinylidenefluoride (PVDF).

41. A motor constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawings.

42. A generator constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawings.