This invention relates to a piezoelectric drive assembly. Piezoelectric drive assemblies are known for use for example in the precise positioning of objects in high-accuracy scientific and engineering apparatuses.

A piezoelectric material possesses unique properties, as is well known. The principal one that is exploited in a piezoelectric drive assembly is that on the application of a control voltage across the ends of an element made from a piezoelectric material controlled expansion and contraction of the element arises.

The expansion/contraction are linearly related to the applied voltage and this makes the piezoelectric materials inherently suitable in the manufacture of "analogue" drive assemblies, in which the motion of a part is related to the applied waveform.

Piezoelectric materials however may also be employed in order to manufacture drive assemblies in which the movement may be effected as a series of discrete steps, thereby giving rise to a digital output motion.

One way of achieving such a digital output is to produce so-called actuator "stacks" in which a plurality of layers of piezoelectric materials are secured together. The wiring of the stacks may be arranged so as to cause extension of the stack (caused in turn by extension in the same direction of a plurality of the layers in the stack); or a shearing motion in which the stack moves transversely relative to its longitudinal axis (which is also the direction of the elongation described above).

Through the choice of applied voltages and the switching of the layers of the stack the stack may be caused to extend in a straight line (in which case it may clamp a driven member forming part of the drive assembly); or it may move according to a motion involving translational movement (relative to the longitudinal axis) while the stack is spaced from the driven member.

In a drive assembly construction comprising two or more stacks of this kind it is possible to arrange for a clamping stack to be clamping a driven member while an incrementing stack moves in a translational fashion as indicated. Subsequent advancement of the incrementing stack to engage the driven member and releasing of the clamping stack followed by further translational movement of the incrementing stack causes stepper- type translational motion of the driven member. Repetitions of this sequence of actions cause the driven member to move a chosen distance in a predetermined direction (assuming of course that the translational movements of the incrementing stack all occur so as to move the driven member in the same direction).

When the driven member is close to a desired position (e.g. so as to locate a target object supported by it suitably close to the operative part of a scientific measuring device) the mode of operation of the drive assembly may be changed to that of an analogue type, in order to achieve fine positioning of the driven member.

The analogue mode may be attained by engaging the driven member only by stacks that are driven in the translational movement direction. This permits highly accurate control of the final positioning of the driven member and any object supported by it; but the analogue mode is only available over very short distances of movement of the driven member. For this reason the positioning of an object typically is achieved through a combination of the stepper mode (for coarse positioning) and the analogue mode (for final positioning, as aforesaid). US2003/0085633 discloses an example of the foregoing type of drive assembly. In this disclosure sets of actuator stacks are arranged in lines respectively on opposite sides of the driven member, the movement of which is constrained to a straight-line form by guide rollers that engage its sides. In the assembly of US2003/0085633 each set of actuator stacks consists of four stacks of which at any given time two are driven in the clamping mode and two in the translational movement mode described above, when the motion of the driven member is of the stepper type. When on the other hand analogue mode is required selected stacks are driven to engage the driven member and confer translational movement in forward and reverse directions as required.

The arrangement of US2003/0085633 suffers numerous disadvantages, because of its design. Firstly the requirement to locate sets of the stacks on opposite sides of the driven member means that the design is not compact. This is a distinct problem since in the kinds of apparatus in which the drive assemblies are intended to be used there is usually extremely limited space.

Furthermore in the device of US2003/0085633 the rectangular cross-section driven member is surrounded on all sides by other parts of the apparatus. On two sides the actuator stacks are present; and on two further sides the guide rollers limit access to the driven member. This can result in numerous drawbacks, as described below.

An aim of the invention is to overcome or at least ameliorate one or more of the problems in prior art piezoelectric drive assembly designs.

According to the invention in a broad aspect there is provided a piezoelectric drive assembly comprising a frame supporting at least two piezoelectric actuator stacks, the frame including a plurality of frame elements and the actuator stacks protruding from a first said frame element in adjacent relation to one another so as to provide for a stepper- type piezomotor action caused by operation of sets of the piezoelectric actuator stacks in clamping and shearing modes respectively; and an elongate, first driven member that is acted on by the actuator stacks for advancement in the direction of its major dimension, characterised in that the drive assembly includes a further frame element that is juxtaposed relative to the driven member and that reacts forces applied to the first driven member by the actuator stacks.

The piezoelectric actuator stacks preferably are formed as "pairs", ie. elements constructed from two distinct materials stacked one on another so as to confer desired characteristics. However other constructions of the stacks are possible within the scope of the invention.

Such an arrangement requires only half the number of actuator stacks as are present in the US2003/0085633 assembly, since there is a requirement for only a single actuator stack extending along one side of the driven member parallel to its elongate axis. Aside from reducing the cost and complexity of the drive assembly such an arrangement reduces the dimension of the assembly measured in the direction parallel to the extent of the actuator stacks. This makes the assembly of the invention highly suitable for incorporation into apparatuses in which the space for manipulating and positioning devices is limited. The further element provides the reactive force components that in US2003/0085633 are provided by a second set of actuator stacks; and the clamping and shearing forces are generated in toto by the single set of actuator stacks extending along one side of the driven member.

Another advantage of the arrangement of the invention is that it readily permits a construction in which at least one elongate side of the driven member is uncovered and hence readily accessible. This in turn facilitates the joining to the driven member of further articles such as but not limited to further drive assemblies that optionally but not necessarily fall within the scope of the invention.

This advantage is not available, or is available only to a limited extent, in prior art designs in which the complexity of the structures supporting the piezoelectric elements prevents or hinders access to the driven member.

Preferably the further frame element includes a groove and the first driven member includes a protrusion of complementary shape to the groove, the protrusion extending along at least part of the length in the major dimension of the first driven member and being received in the groove.

The use of a further frame element to provide forces that react those generated in the actuator stacks means that the further frame element can additionally perform a secondary function, of providing a groove or other shaped formation that stabilises and guides the driven member as it moves.

This in turn means that there is no need for the separate guide rollers provided in US2003/0085633. This further reduces the cost and complexity of the assembly, while maintaining ample stability and accuracy in its operation. The use of the groove furthermore constrains the driven member against "lifting" that in some prior art arrangements is a consequence of the stepper action, as the actuator stacks engage and press against the driven member. The provision of a further frame element that lies opposite the actuator stacks, and hence reacts the forces resulting from the stepper action, means that the driven member cannot move relative to the actuator stacks.

Conveniently the groove and the protrusion are substantially of "V" cross-section. V-section grooves and protrusions are simple to manufacture and provide good stability. In particular the preferred groove shape eliminates unwanted lateral motion of the driven member. This can be a further problem in some prior art designs.

In an alternative embodiment of the invention the further frame element includes formed therein or secured thereto a first groove; the first driven member includes formed therein or secured thereto a further groove; and the drive assembly includes one or more rotative members received in the grooves so as to interconnect the grooves and permit relative movement between the first driven member and the further frame member.

This arrangement advantageously ensures that the coefficient of friction on the one hand between the first driven member and the further frame member has a low value; and on the other hand between the piezoelectric actuator stacks and the first driven member is relatively high. Ensuring that the "stack-driven member" coefficient of friction is higher than the "driven member- further frame member ¹¹ coefficient ensures that the motions of the piezoelectric actuator stacks translate into movements of the first driven member.

Preferably the assembly includes a plurality of the rotative members retained rotatably captive in a cage member lying between the first and further grooves.

This arrangement facilitates manufacturing of the assembly and/or replacement of the rotative members. In a particularly preferred embodiment of the invention the or each rotative member is a roller bearing member.

In a preferred embodiment of the invention the first and further frame elements are interconnected by an intermediate frame member. Preferably the intermediate frame member includes first and second sections that are spaced from one another and are interconnected by one or more resiliently deformable connections so as to define a spring in the kinematic path between the first and further frame elements.

An advantage of this arrangement is that a degree of "float" may exist in the operation of the assembly. This permits the assembly to accommodate e.g. forces acting on the driven member tending to cause the aforesaid problem of "lifting"; or to damp shocks that may from time to time be experienced. In more detail the arrangement provides a spring pre-force that acts on the driven member in order to retain it in the assembly. This pre-force is arranged, through the design and configuration of the components, to prevent the problem of lifting that arises in the prior art designs, while simultaneously accommodating unexpected impulses and providing a degree of tolerance of eg. mis-alignment stacks or other parts of the assembly.

Conveniently the or each resiliently deformable connection includes a resiliently deformable leaf member that at opposite ends is connected respectively to the first frame element and the intermediate frame element, the leaf member being capable of bending movement at against at least one said end against the resilience of the leaf.

Such an arrangement is a beneficially efficient way of creating a suspension system interconnecting the first and further frame elements.

In an alternative, and more preferred, embodiment of the invention the first frame member includes first and second sections that are spaced from one another and are interconnected by one or more resiliently deformable connections so as to define an in- line spring in the kinematic path between the frame and the actuator stacks.

Such an arrangement shifts the location at which the spring in the kinematic path acts compared to the arrangement in which the spring lies between the first and further frame elements. As a result the spring force coincides better with the lines of action of the forces imparted by the piezoelectric elements. In turn this means that the frame is less likely to be subject to torques that distort it and hence reduce the accuracy of the device.

Conveniently the or each resiliently deformable connection includes a resiliently deformable leaf member that at opposite ends is connected respectively to the first and second sections of the first frame member, the leaf member being capable of bending movement at at feast one said end against the resilience of the leaf.

This aspect of the invention may be essentially the same, regardless of whether one is considering the spring interconnecting the first and further frame elements or the version of the invention in which the spring interconnects first and second sections of the first frame member. However numerous other designs of spring may, in the alternative, be employed. The drive assembly of the invention moreover optionally may include a further, pre-tensioning spring acting between the first and second sections of the first frame member so as to urge the said first and second sections apart from one another against the force of the in-line spring.

This feature of the invention may be employed to ensure that the driven member is always subject to a relatively light force, transmitted via the piezoelectric elements, that prevents unintended movement of the first driven member.

In an embodiment of the invention the pre-tensioning spring includes a coil spring received in a recess defined in at least one of the first and second sections of the first frame member and located so as to interconnect the said first and second sections. In other arrangements within the scope of the invention other types of spring may be employed, and the invention is not limited to the designs illustrated in the drawings.

Preferably the drive assembly includes an adjuster for adjusting the force exerted by the pre-tensioning spring. It is also preferable that the adjuster is or includes a screw that is threadedly received in a bore in one of the first and second frame portions and against which the pre-tensioning spring bears such that rotation of the screw in the bore causes selective compression and relaxation of the spring.

The foregoing features of the invention give rise to advantages, as set out elsewhere herein, that are not available in prior art designs.

In preferred embodiments of the invention the first driven member includes one or more attachments for attaching thereto a further drive assembly that is capable of causing movement of a further driven member in a direction perpendicular to the direction of movement of the first driven member.

Such a feature permits the building up, in a modular fashion, of connected, plural examples of the assembly of the invention (or combinations of the assembly of the invention with other assemblies causing movement of a supported object). This in turn opens the possibility of multiple degree-of-movement control (since one of the assemblies may be used to control movement in an x-direction, and the other in a y- direction). This advantage of the invention cannot arise in the device of US2008/0085633 since it is not possible to gain access to a free side of the driven member in order to attach a further assembly to it and thereby provide multiple degree of freedom movement in an economical manner.

To the foregoing end the invention also resides in a drive assembly including secured thereto using the said one or more attachments at least one said further drive assembly so as to define a multi-drive assembly. In a further preferred embodiment of the invention the further drive assembly includes one or more attachments for attaching thereto an additional drive assembly that is capable of causing movement of an additional driven member in a direction perpendicular to the directions of movement of the first and further driven members. This opens the possibility of providing a three degree of movement device, using which it is possible finely to control the positioning of an object in x-, y- and z-directions.

The preferred form of the or each attachment includes one or more bores that is threaded for receipt therein of a screw and/or one or more plain bores for receipt therein of a locating member and/or one or more grooves formed in the surface of the first driven member for receipt therein of a protuberance. However other forms of attachment are possible within the scope of the invention.

Conveniently the drive assembly includes one or more linear travel limiters for the first driven member, the or each linear travel limiter including a protuberance protruding from one of the frame and the first driven member and a recess formed in the other of the frame and the first driven member and in which the protuberance is slideably receivable so as to limit the advancement of the first driven member. This feature can be employed for example to ensure that the first driven member cannot in one movement be driven more than a predetermined distance. This means that the first driven member can be prevented from inadvertently being driven out of the groove relative to which it sits. If desired extra travel limitations (eg. so as to prevent collisions with objects in the vicinity of the drive assembly) can be set by the user or the system designer when programming driving signals or driving software, or when designing or building the driver electronics Preferably the extent of protrusion of the or at least one said protuberance is adjustable so as to permit its selective receipt in or disengagement from the recess. This means that the first driven member can be released from the drive assembly if required.

In more detail, preferably the or at least one said protuberance protrudes from the frame and the frame includes a cavity, at least a part of the protuberance being moveable into and out of the cavity in order to cause selective receipt of the protuberance in the recess or its disengagement therefrom.

Additionally or alternatively the drive assembly may include an optional torque limiter including a protuberance protruding from one of the frame and the first driven member and a recess formed in the other of the frame and the first driven member, wherein the fit of the protuberance in the recess is such as to limit rotation of the first driven member when acted on by the piezoelectric actuator stacks to a predetermined amount. In a particularly efficient embodiment of the invention the drive assembly includes a common protuberance defining or forming part of a said linear travel limiter and the torque limiter. Such an arrangement is desirable because the protuberance is dual-purpose; but it is equally possible, within the scope of the invention, to include separate elements respectively performing the linear travel limiting and torque limiting functions.

In the preferred embodiment of the invention the or each driven member includes an elongate bar formed substantially from Si ₃ N ₄ . Preferably also the groove is formed from or faced with Si ₃ N ₄ such that contact between the driven member and the groove occurs as contact between respective Si ₃ N ₄ parts of the drive assembly.

Contact between two members made of or from Si ₃ N ₄ has been found to exhibit a very low coefficient of friction, thereby providing for highly efficient sliding contact between the driven member and the groove.

It is also preferable that the first driven member includes at least a region facing the piezoelectric actuator stacks that is faced or coated with AI ₂ O ₃ ; and that at least a region of the end of each actuator stack facing the first drive member is faced or coated with AI ₂ O ₃ .

The use of an AI ₂ O ₃ coating on each of the side of the driven member facing the actuator stacks, and the ends of the actuator stacks themselves, provides for high-friction contact between these components. This means that both the clamping effect (when a stack is operated in this way) and the shearing motion are efficiently conferred.

Preferably the drive assembly of the invention includes a shoe member that is removably securable to the first frame element and to which the piezoelectric actuator stacks are secured.

There now follows a description of preferred embodiments of the invention, with reference being made to the accompanying drawings in which:

Figure 1 is a perspective view from one side of an assembly according to the invention;

Figure 2 is a perspective view from the opposite side of the Figure 1 device;

Figure 3 is a view similar to the Figure 1 view, omitting a driven member visible in Figure 1 in order to illustrate part of an attachment arrangement;

Figure 4 is a perspective view showing two assemblies, each in accordance with the invention, that are secured together to define a multi-drive assembly;

Figure 5 is a perspective view of a second embodiment of the invention;

Figure 6 is a view similar to Figure 5 that for ease of viewing omits parts of the frame;

Figure 7 shows in perspective view a plurality of the Figure 5 drive assemblies that are secured together to provide controlled movement of plural-driven members in mutually orthogonal directions;

Figure 8 shows in perspective view the reverse of the Figure 5 assembly and in particular a key-way that is of benefit when considering certain ways of mounting the assembly;

Figure 9 is a perspective view of a third, and presently most preferred, embodiment of the invention (omitting, for ease of illustration, the driven member thereof);

Figure 10 is an end elevational view of the Figure 9 device, including the driven member; and

Figure 1 1 shows in perspective view the Figure 9 / Figure 10 embodiment in a partly dismantled state, in order to illustrate optional features of the invention.

Referring to the drawings a first, presently less preferred embodiment of a piezoelectric drive assembly 10 comprises a frame 1 1 supporting a plurality (four in the embodiment shown, although other numbers are within the scope of the invention) of piezoelectric actuator stacks 12. The frame 11 is essentially rigid and preferably is manufactured from a metal such as aluminium, titanium, alloys of such metals or steels (especially but not limited to stainless steels). The worker of skill however will be aware of a wide range of materials (particularly metals) within the scope of the invention that meet the requirements of being an easy to machine, non magnetic, vacuum compatible, high elasticity modulus material.

Aluminium in particular provides these desirable characteristics. Frame 11 is made from a single piece of the chosen material in order to achieve a high stiffness. Frame 11 could be manufactured from multiple components, but the use of a one-piece construction is preferred.

Frame 11 although manufactured from a single piece of material may be considered in functional terms as a series of interconnected frame elements or portions. A first frame element 13 has protruding therefrom the actuator stacks 12. First frame element 13 in the preferred embodiment as illustrated adopts approximately the shape of a horseshoe or staple, with the piezoelectric actuator stacks 12 protruding parallel to lateral arms 13a, 13b of the frame element from an intermediate portion 13c that interconnects the arms. The actuator stacks 12 are rigidly secured to the intermediate portion 13c and may be e.g. of the kind disclosed in US2003/0085633 or may be of other designs that would be known to the worker of skill in the art.

The frame element 13 may adopt a different shape than that shown, but the staple-shape has been found to provide a good mixture of stability and ease of manufacture.

As is visible in the figures, in the preferred embodiment four of the actuator stacks 12 are provided one adjacent another so as to define a linear set. The actuator stacks are electrically connected (typically using hard-wiring methods) to driver circuits that generate the voltages needed to provide for either a stepper mode of operation or an analogue mode, as described hereinabove. The precise electrical connection arrangements will be known to the worker of skill. They may be for example similar to those described in US2003/0085633, or they may adopt different designs.

More or fewer of the actuator stacks 12 than the four shown may if desired be provided within the scope of the invention. The actuator stacks 12 are juxtaposed relative to a driven member 14 that in the preferred embodiment shown takes the form of an elongate, approximately rectangular- section bar. The driven member 14 is described in more detail below and in a preferred embodiment of the invention is made from a ceramic material (although this need not necessarily be so). It may serve in use of the assembly 10 to support any of a range of objects the precise positioning of which, relative to e.g. an optical or other sensing device, is required. Alternatively the driven member 14 may be connected e.g. to drive the mechanism of a further assembly the precise detail of which would be known to the worker of skill.

The assembly includes a further frame element 16 that is secured in the frame 11 in a manner described hereinbelow. Further frame element 16 is juxtaposed relative to the driven member 14 such that the driven member lies in a space defined between the first and further frame elements.

The operation of the drive assembly 10 is based on controlling the extension and retraction of the stacks 12 in such a way that they sequentially engage and disengage the driven member 14. Not all the stacks 12 contact the driven member simultaneously during operation of the drive assembly 10.

In more detail, the control of the stacks 12 is such as to cause pairs of them to be "clamping" or "shearing" stacks respectively.

Typically the first and third (counted from one end) of the linear series of stacks 12 shown in the drawings would act as clamping stacks and the second and fourth stacks 12 as shearing stacks (or vice versa).

The driven member would be held by the clamping stacks until operation of the shearing stacks initiates. At this point the clamping stacks would withdraw from contact with the driven member 14 in order to permit its longitudinal advancement caused by the shearing stacks.

Following completion of the shearing action the clamping stacks again would advance and clamp the driven member 14. This would prevent the latter from moving while the shearing stacks retract / reverse in preparation for a subsequent shearing - induced movement of the driven member. The above-outlined process repeats until the driven member occupies a chosen position.

As is apparent from the arrangement illustrated, any forces generated on extension or shearing movement of the actuator stacks 12 may be transmitted to the driven member 14 via any such stacks for the time being in contact with the driven member 14. By reason of juxtaposition of the further frame element 16 in contact with the driven member 14 such forces are reacted by reason of the connection between the further and first frame elements. This arrangement gives rise to the principal advantages of the invention set out above.

Further frame element 16 extends parallel to the elongate dimension of the driven member 14 and is of the same length as the first frame element. As shown the further frame element 16 includes extending along its side that lies adjacent the driven member 14 an elongate groove 17.

Groove 17 essentially is of "V" cross-section in the preferred embodiment shown in the figures. In the base of the V-shape the groove 17 is formed to include a part-cylindrical recess. This is the result of machining operations needed to form the groove. The part- cylindrical recess assists to relieve stresses in the material of further frame element 16.

The driven member 14 includes extending along its length towards the groove 17 a protrusion 18. Protrusion 18 is of complementary cross section to the groove 17 and is received in and in sliding contact with it.

As will be apparent from the drawings it is not necessary for the profiles of the groove 17 and protrusion 18 to be of the V-shape shown, and any shape that serves to constrain and guide the driven member 14 relative to the further element 16 while permitting sliding movement and providing for reacting of forces as explained may be considered as lying within the scope of the invention. The V-shaped cross section adopted in the preferred embodiment however has been found to be particularly suitable for simultaneous guiding/stabilising and force reacting.

The frame 11 includes an intermediate frame member 19 that interconnects the first and further frame elements. The intermediate frame member 19 includes first 21 and second

22 sections that are spaced from one another and are interconnected by one or more resiliently deformable connections so as to define a spring in the kinematic path between the first and further frame elements.

The resiliently deformable connections take the form, in the preferred embodiment of the invention, of respective, resiliently deformable leaf members 23 each of which is at an opposite end 24, 26 connected respectively to the second section 22 and the first section

21 of the intermediate frame element 19. The leaf members 23 are capable of bending movement at at least one end 24, 26 and preferably at both ends. Since the intermediate frame element is manufactured from the (resiliently deformable) metal of the first frame element 13 such bending occurs against the resilience of the material employed. In consequence the leaf members define a pair of suspension springs that provide the various effects noted hereinabove.

The leaf members 23 as explained provide a spring pre-force acting on the driven member 14. This prevents driven member 14 from falling out of the groove 17.

In an exemplary embodiment of the invention the pre-force may be eg. 1ON, being equivalent to the force required to move the driven member 14. Thus the assembly 10 provides a relatively large (compared to the prior art) holding force in a compact construction.

As is apparent from study of the drawing figures the existence of the further frame element 16 causes the pre-force to act in a direction that opposes forces applied by the stacks 12. Since in the preferred embodiment shown at any time two of the stacks occupy an extended position engaging the driven member the design of the assembly 10 prevents unwanted "lifting" (lateral movements) of the driven member 14. The driven member 14 therefore moves only in a direction parallel to its longitudinal axis. Such motion is predictable and avoids a problem, extant in the prior art, of the lifting phenomenon causing eg. inaccurate positioning of target structures.

Other forms of resiliently deformable connection between the first and second sections 21 , 22 of the intermediate frame element 19, or at other locations in the kinematic chain defined by the frame 11 , may be provided; or the resiliently deformable connection may be omitted entirely if desired. An example is shown in Figure 9, in which instead of interconnecting parts 21 , 22 of the intermediate frame element 22 the leaf members 23" interconnect first 13a and second 13b sections of a modified form 13" of the first frame element. The designs and arrangements of the leaf members 23" are similar to the versions visible in Figure 2 such that they are resiliently deformably retained at their respective ends 24", 26" by reason of being formed integrally with hinge portions, at the ends 24", 26", that are integral with the material of the sections 13a and 13b of first frame element 13". As a result of positioning of the leaf members 23" as shown in Figure 9 the spring force provided by them acts in line with the line of action of forces exerted by the piezoelectric elements 12. This in turn means that when the piezoelectric stacks 12 are activated there is in the Figure 9 embodiment a considerably reduced tendency than in the Figure 2 arrangement for the frame 13" to distort by "opening out. This effect in Figure 2 is a result of the spring force being mis-aligned with the actuator stacks and hence the latter causing a moment to act on the springs defined by the leaf members 23. Such a moment cannot be generated by the stacks 12 in the Figure 9 design.

Figures 9 and 11 show another optional feature in the form of an additional pre- tensioning spring 55.

Spring 55 preferably is a cylindrical coil spring as shown, although other designs of resiliently deformable element may be employed instead. At one end spring 55 bears against first section 13a of first frame element 13" and at the other end against second frame element 13b. The latter is hollowed to define a cuboidal cavity 52 that accommodates the length of the spring 55 without requiring an increase in the overall dimensions of the drive assembly 10". Other shapes of the spring and cavity are possible within the scope of the invention; and the cavity may additionally or alternatively be formed in the first section 13a. Moreover if desired the surfaces against which the ends of the spring 55 bear may be formed including locating recesses the purpose of which would be to assure correct alignment of the spring 55.

More than one spring 55 if required may be provided in a corresponding plurality of cavities 52 spaced along the first frame element 13. The primary purpose of any additional pre-tensioning spring such as that shown in Figure 4 is to ensure that even when the piezoelectric stacks 12 are not activated the first driven member is subjected to a constant force, transmitted via the stacks 12, so as to prevent unwanted movement of it.

Thus for example the first driven member is prevented from falling out of the drive assembly by the forces applied by the leaf springs 23". The purpose of the additional pre-tensioning spring 55 is to achieve a higher pre-force level than would be achievable using the leaf springs 23" alone. The latter would yield and undergo plastic deformation if excessively strained in order to increase the pre-force, and the use of an additional resiliency deformable element such as the spring 55 obviates this potential problem.

In this regard it might be possible to increase the spring force applied via leaf springs such as springs 23 of Figure 2 by thickening them but this option might be sub-optimal. This is because internal forces resulting from using thickened leaf springs would give rise to the plastic deformation problems indicated above.

The driven member 14 includes one or more attachments, in the form of a series of three apertures 27, 28, 29, for attaching thereto a further drive assembly that is capable of causing movement of a further driven member in a direction perpendicular to the direction of movement of the first driven member.

At least one of the apertures, labelled 28 in the embodiment shown in the drawings, is formed including received therein a threaded bushing in which is receivable a bolt or similar threaded fastener that may be employed to secure such a further drive assembly.

The additional attachment bores 27, 29 are shown omitting a threaded bushing sleeve and being formed directly in the ceramic material of the driven member therefore are plain (ie. unthreaded) since it is difficult to form threads in ceramic materials owing to their low ductility and high brittleness. Such plain apertures may be used to receive locating or guiding pins protruding from further structures, as described, supported by the driven member 14.

In an alternative arrangement one or more of the apertures 27, 29 could be provided with the threaded bushing members as desired. Any of the apertures shown could optionally include one or more further features that facilitate the positionally accurate securing together of two assemblies. Moreover a greater or lesser number of the attachments, that need not take the form of the apertures shown, may be present.

Other means of securing two or more assemblies together therefore are within the scope of the invention, and may include clips, clamps and interengaging formations of a wide variety of designs as would be within the ability of the worker of skill to devise.

As an example of one of the many ways of securing the assemblies together, and as best shown in Figure 3, the intermediate frame element 19 may include a through-going bore 31. The bore 31 permits a bolt or screw to extend through the intermediate element 19 so as to be screwed into an aperture such as aperture 28.

Figure 4 shows the result of such securing, in the form of a multi-drive assembly. In Figure 4 a further assembly 10' is secured onto the driven member 14 of assembly 10 by way of such a screw or bolt (that is not visible in Figure 4).

In the arrangement illustrated the assembly 10' is identical to assembly 10, although this need not necessarily be so. The components of assembly 10' visible in Figure 4 are identified by the same numerals as their counterparts in assembly 10, except that the numerals are "primed".

The direction in which driven member 14' of further assembly 10' extends (and hence the direction in which it may be driven in use of the multi-drive assembly) is perpendicular to that of driven member 14. In consequence Figure 4 shows a two degree of movement multi-drive assembly, in which further assembly 10' being secured on the driven member 14 of assembly 10 is itself moved on driving of member 14. Driving of member 14' relative to the remainder of further assembly 10' causes movement of an object supported by member 14' in a direction perpendicular to that of movement of member 14; and controlled x- and y-axis movement may result.

Since member 14' also includes attachments in the form of apertures 27', 28', 29' and 34' at least a third assembly may be attached thereto. If the third assembly is orientated so that its driven member extends in a direction that is perpendicular to the two directions provided by the arrangement of Figure 4, x-, y- and z-axis (three degree of movement) controlled movement becomes available. In the preferred embodiment of the invention the driven member 14 is a bar formed substantially from Si ₃ N ₄ ; and the groove 17 is similarly formed from or faced with Si ₃ N ₄ . As explained, this provides for very low sliding resistance between the driven member 14 and the surface of groove 17. In the arrangement visible in the figures the further frame element 16 is formed from Si ₃ N ₄ as a discrete component and during manufacture of the assembly 10 is secured using screws 32 to the intermediate frame element 19. This permits further frame element 16 and the remainder of the frame 1 1 to be made from differing materials so as to reduce the cost of the assembly 10 as a whole. The driven member 14 includes secured extending along its side that faces the actuator stacks 12 an elongate plate 33 made from or at least coated with AI ₂ O ₃ . The ends of each of the actuator stacks 12 that face the driven member 14 are similarly provided with plates 34 of or coated with AI ₂ O ₃ secured thereto. The plates 33, 34 engage one another during operation of the actuator stacks 12. The use of AI ₂ O ₃ provides for a high coefficient of friction between the ends of the stacks 12 and the driven member 14 so as to enhance the transmission of forces from the stacks 12 to the driven member 14.

Thus the plate 33 and plates 34 constitute a high friction engagement between the actuator stacks 12 and the driven member 14. The coefficient of friction between these engagement components is noticeably greater than the coefficient of friction between the groove 17 and the protrusion 18 of driven member 14. This in turn promotes successful operation of the driven assembly through sequential engagements of the ends of the stacks 12 and the driven member 14. Figures 5 to 8 illustrate a second embodiment of the invention.

The drive assemblies 10 shown in Figures 5 to 8 are in many respects similar to the assemblies 10 visible in Figures 1 to 4. In the following description therefore only the differences evident in the second embodiment are described in detail.

The components of the Figures 5 to 8 embodiment that are counterparts to parts of the Figures 1 to 4 embodiment are identified by the same reference numerals.

As best illustrated in Figure 5 (which is a perspective view of an entire drive assembly 10 in accordance with the second embodiment of the invention) and Figure 6 (which omits the frame portion 13c) further frame element 16 includes extending along its length a first groove 36. Groove 36 is formed in a facing strip 42 that extends along the further frame element 13. In the embodiment illustrated facing strip 42 is attached to the further frame element 16 by any convenient means such as by gluing. Facing strip 42 and further frame element 16 may thus be made from different materials so as to reduce the cost of the assembly as a whole.

Groove 36 is juxtaposed relative to driven member 14 and as shown defines a part- circular concave shape in cross-section.

The face of driven member 14 that lies opposite the groove 36 is also formed with a concave, part-circular further groove 37 extending along its length. Groove 37 is of the same profile as groove 36 in the embodiment shown, although this need not necessarily be so.

The grooves 36, 37 are aligned with one another so as to act as raceways for a linear series of rotative bearing members in the form of rigid ball bearings (of which one, 38, is visible in the figures). The ball bearings 38 interconnect the further frame element 16 and the driven member 14 so as to provide for a very low friction connection between these parts. As explained above the motion of the piezoelectric stack pairs in causing motion of the driven member relies on the coefficient of friction between the stack pairs 12 and the driven member 14 on the one hand being lower than the coefficient of friction between the further frame member 16 and the driven member 14 on the other. The use of roller bearings 36 (or other rotative members) is one way, of several, of achieving this.

The roller bearings 36 are as shown retained in a rotatably captive fashion in bearing cage member 39.

In the embodiment shown cage member 39 is formed as an elongate plate of a suitably stiff material such as any of a range of metal alloys, or a composite or polymeric material. Cage member 39 includes extending at regular intervals along its length a linear series of circular perforations 41. The dimensions and edge profiles of the perforations 41 are such as to retain the ball bearings 38 in the cage member such that a part of each ball bearing 38 protrudes on either side of the cage member 39 so as to be engageable with the grooves 36, 37. The ball bearings 38 therefore space the driven member 14 and the further frame member 16 apart from one another in a fashion that through the rolling action of the bearings 38 promotes low-friction movement between these parts of the assembly 10. One advantage of employing the bearing cage member 39 is that it promotes easy replacement of the ball bearings 38 as a group for example in the event of one or more of them becoming worn. Figure 5 (partially) and Figure 6 additionally show a further, optional feature of the invention in the form of a shoe 43 on which the piezoelectric actuator stacks 12 are mounted.

As best seen in Figure 6 shoe 43 is an elongate, stiff (eg. metal) bar that extends parallel to the driven member 14. The piezoelectric stacks 12 are secured in a line protruding from shoe 43 towards the driven member 14.

The intermediate frame portion 13c is perforated by apertures 44 (Figure 8) through which retaining screws 46 extend for the purpose of engaging threaded bores (not shown in the drawings) formed in the shoe 43. The shoe 43 as a result is releasably securable in the assembly 10. This arrangement beneficially permits removal of all the piezoelectric stacks together. This in turn permits them to be repaired or (more probably) replaced as a group. This is likely to be advantageous since the stacks 12 tend to wear all at the same rate.

Figures 5 to 8 illustrate another optional feature of the invention in the form of concave, elongate locating groove 47 formed in the surface of driven member 14 lying remote from the frame 13. Groove 47 is an alternative to the apertures 27, 29 employed in the first embodiment of the invention for the purpose of locating another object (such as but not limited to another of the assemblies 10) to the driven member 14. Such an object may include a protuberance of complementary shape to elongate groove 47. Groove 47 is essentially a milled slot having a concave rear wall. Such a formation may, depending on the precise material chosen for the driven member 14, be easier to machine than the plain bores 27, 29.

Figure 8 shows the existence of a further, elongate key-way 48 formed in the side of frame 13 lying remote from driven member 14. Key-way 48 is similar to groove 47 and serves a similar purpose of acting as a locator for one or more protuberances formed on a further object to be secured to the assembly 10.

In Figure 8 key-way 48 is shown extending in a direction perpendicular to that of groove 47, although this need not necessarily be so. Groove 47 and key-way 48 each may be formed including bores, apertures, protuberances or other formations intended to assist in securing objects together. In the drawings exemplary bores 49 are shown in the centres of the groove 47 and key-way 48 respectively. Other arrangements however are as indicated possible within the scope of the invention.

Figure 7 shows three of the assemblies 10a, 10b, 10c of the second embodiment of the invention secured together such that their driven members 14a, 14b, 14c are moveable in mutually orthogonal directions. This permits the positioning of eg. a microscope target in X-, y- and z-directions.

In Figure 7 an optional adapter bracket 51 is present for the purpose of interconnecting the drive assemblies 10a and 10b.

As indicated elsewhere herein, the embodiment of Figures 9 to 11 includes various optional features that may be included in drive assemblies in accordance with the invention. Figure 11 for clarity omits the frame 13" forming part of the drive assembly 10".

Figures 9 to 11 illustrate a pair 53, 54 of rigid cylindrical protuberances in the embodiment shown spaced apart from one another and protruding from the intermediate frame element 19 towards the first driven member 14. The cylinders 53, 54 may be eg. machined from a metal.

The free ends of the protuberances 53, 54 are received in respective mutually parallel, elongate slots 56, 57 formed in the face of driven member 14 that lies adjacent intermediate frame element 19. The slots 56, 57 are spaced from one another along the length of the member 14.

The width of each slot 56, 57 is only slightly greater than the diameter of the respective cylindrical protuberance 53, 54 received in it. The slots 56, 57 are each closed at one end 56a, 57a that lies "inboard' of the free end of the driven member 14 at which the slot in question initiates at a respective open end 56b, 57b. The protuberances 53, 54 and the slots 56, 57 are slideable one relative to the other. The overall effect of the arrangement is to ensure that movement of the driven member 14 in either direction parallel to its longitudinal axis is limited to no more than the length of one or other of the slots 56, 57. In practice the slots are of the same length as each other so that movement of the driven member 14 is limited to the same extent in each direction.

Limitation of the movement of the driven member 14 occurs by reason of one or other of the protuberances 53, 54 engaging its associated closed slot end 56a, 57a as appropriate.

The combination of the protuberances 53, 54 and the slots 56, 57 among other things means that the chance of the driven member 14 unintentionally colliding with items in the vicinity of the drive assembly 10" is much reduced. Moreover there is no possibility of the driven member 14 falling or being driven out of the drive assembly 10" in the event of the piezoelectric stacks 12 being withdrawn from contact with the driven member.

As best illustrated by Figure 10, which is an end elevational view of the assembly 10", the protuberances 43, 54 and the slots 56, 57 additionally serve a further purpose in resisting any torque tending to rotate the driven member 14 about its longitudinal axis.

In this regard the piezoelectric stacks 12 may apply a force that is not completely in line with the line of action of the force transferred to the rotative members 38. This tends to rotate the driven member 14 as explained.

In high-precision positioning operations such rotation is undesirable. The relatively close fit, however, between the walls of each slot 56, 57 and the exterior surface of the associated protuberance 53, 54 means that only a very small amount of such rotation is possible before engagement of the protuberances 53, 54 and the slots 56, 57 occurs.

Although in the embodiment shown and illustrated the protuberances 53, 54 perform dual functions of limiting both longitudinal travel and unwanted rotation of the driven member, in other embodiments of the invention such functions could be provided by distinct components if desired.

Although not shown in Figures 9 to 11 there could be provided in the frame 13 a respective cavity for receiving the non-exposed end of each protuberance.

The protuberances could be moveable into and out of the associated cavities to a certain extent, by reason of the chosen cavity sizes and the provision of detent arrangements by which limited movement of the protuberances is made possible.

Such moveability of the protuberances permits their selective withdrawal so as to disable the travel-limiting function described above and thereby permit removal of the driven member 14 from the drive assembly 10". Movement of the protuberances 53, 54 between the advanced positions shown in the drawings and positions in which they are partially retracted inside the cavities so as to permit such removal of the driven member 14 may be effected using an appropriate tool or implement. Figure 11 illustrates yet another optional feature of the invention, that is an adjuster in the form of a grub screw 58 that is threadedly received in a through-going, threaded bore 59 formed in first frame member 13".

The location and diameter of the bore 59 are such that it defines a seat for the spring 55 thereby assuring correct alignment of the latter. One end of the spring 51 bears on an end of the grub screw 58 in the bore 59. It will be apparent therefore that rotation of the grub screw may be employed to adjust the magnitude of the pre-tensioning force applied by the spring 55. This feature of the invention beneficially permits adjustment of the performance of the drive assembly, for example during testing or when the drive assembly is moved from one environment (eg. a vacuum) to another (eg. a cryogenic chamber).

The embodiments of Figures 5 to 8 and 9 to 11 include several optional features as described. Any of these may, if necessary following modification as would occur to the worker of skill, be included in other embodiments illustrated or described, either singly or in combination. Similarly the optional features described in relation to the first embodiment of the invention may be included, as necessary in modified form, in the second or the third embodiment, either singly or in combination as would be understandable by the worker of skill. Overall the assembly 10 of the invention provides good performance and numerous design/installation advantages that are not available in the prior art arrangements.

The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.