WO2016051146A1

WO2016051146A1 - A positioning device

Info

Publication number: WO2016051146A1
Application number: PCT/GB2015/052812
Authority: WO
Inventors: Clifford William HICKS
Original assignee: University Court Of The University Of St Andrews
Priority date: 2014-09-30
Filing date: 2015-09-28
Publication date: 2016-04-07
Also published as: GB2546226A; GB201706911D0; GB201417299D0

Abstract

A positioning device for causing movement relative to a guide (10), the positioning device comprising at least two motion members (20), each member being actuatable between a retracted state and an expanded state, wherein upon actuation between the retracted state and the expanded state a motion member is movable into or able to increase gripping engagement with the guide and simultaneously cause relative movement between the device and the guide.

Description

A POSITIONING DEVICE

Introduction

The present invention relates to a positioning device and in particular to a positioning device operable over a wide temperature range, including cryogenic temperatures, and tolerant of substantial long-term wear.

Background

Over the past decade piezoelectric-driven positioners have come into widespread use in low-temperature experiments. The main application is in scanning probe microscopy, where after a scan is performed in one area, positioners can be used to move a sample or a sensor over distances up to a few millimetres to a new scan area. Another application is in sample alignment. Positioners execute longer-range motion, for example 1 cm, but do not generally need to move with the same speed or precision as scanners.

US5432395 discloses a V-drive linear motor. The motor uses a pair of angled piezoelectric actuators to move a drive shoe in a circular motion. A slide mechanism is used to bring the V-drive linear motor in contact with a metal rod. Upon actuation the motor moves the rod.

Summary of the invention

According to the present invention, there is provided a positioning device for causing movement relative to a guide, the positioning device comprising at least two motion members, each member being actuatable between a retracted state and an expanded state, wherein upon actuation between the retracted state and the expanded state a motion member is movable into or able to increase gripping engagement with the guide and simultaneously cause relative movement between the device and the guide. At least two pairs of motion members may be provided, both members of a given pair being actuatable together to move into or increase gripping engagement with the guide and simultaneously cause relative movement between the device and the guide.

In each pair, the motion members may be operable to act on opposite sides of the guide and at opposing angles. The positioning device may be adapted to: move one motion member into gripping engagement or increased gripping engagement with the guide whilst simultaneously causing relative movement between the device and the guide; move the other motion member into gripping engagement with the guide, so that both motion members grip the guide; actuate the first motion member to reduce gripping engagement with the guide so that the guide is predominantly gripped by the second motion member; and actuate the second motion member to reduce gripping engagement with the guide to cause further relative movement between the guide and the device.

The positioning device may be adapted so that in an initial state the motion members are in contact with the guide.

The positioning device may be adapted so that at an end of each gripping cycle the motion members are in contact with the guide.

The positioning device may comprise at least three pairs of motion members. The positioning device may comprise at least four pairs of motion members. Multiple pairs of motion members may be simultaneously actuated to move into or increase gripping engagement with the guide at multiple, different positions along its length and cause relative movement between the device and the guide.

Each motion member may comprise a piezoelectric stack that is extendable along a longitudinal axis, wherein each stack is arranged so that its longitudinal axis is at a non-perpendicular angle relative to a longitudinal axis of the guide.

The piezoelectric stack may consist of a single extension stack extendible and retractable along its longitudinal axis.

Each motion member may have an end adapted to make static contact with the guide. The end of the motion member may be shaped to mate with the guide. The end of the motion member may comprise a gripper. Biasing means may be provided for biasing the motion members into initial engagement with the guide. The biasing means may comprise at least one spring.

At least two joint substrates may be provided. Each joint substrate may carry two adjacent motion members on the same side of the guide.

Each joint substrate may be paired with another joint substrate, and the paired joint substrates directly face opposite sides of the guide. Means may be provided for allowing each joint substrate to move relative to the guide in response to deformation of at least one of its motion members, and restrict rotation of each joint substrate such that in response to actuation of one motion member, the other moves out of or decreases gripping engagement with the guide.

The means for allowing movement and restricting rotation may comprise at least two parallel flexures connected to each joint substrate, wherein the at least two parallel flexures are located at at least two separate locations along an axis perpendicular to the planes of the flexures. The flexures may join two joint substrates to each other, and/or join two substrates to an intermediate rigid body. Each joint substrate may be joined by at least three flexures to an intermediate rigid body.

The joint substrates of each pair may be joined to each other by four flexures. The biasing means may be provided by deflection of the flexures.

The at least two motion members may be operable to cause linear movement relative to the guide. The guide may comprise a rod or parallel rails. The at least two motion members may be operable to cause rotational movement relative to the guide. The guide may comprise a rod or a hollow cylinder, for example a drum.

According to another aspect of the invention, there is provided a positioning device for causing movement relative to a guide, the positioning device comprising: at least one pair of motion members, each member being actuatable between a retracted state and an expanded state to cause relative movement between the device and the guide;

at least one substrate, each substrate carrying two adjacent motion members on the same side of the guide; and

at least two flexures connected to the at least one substrate for allowing the substrate to move relative to the guide in response to deformation of at least one of its motion members, but restrict rotation, so that in response to actuation of one motion member, the other is able to move out of or decrease gripping engagement with the guide.

Each substrate may be paired with another substrate, and the paired substrates directly face opposite sides of the guide. Each substrate may have a motion member from two different pairs of motion members.

The at least two flexures may be parallel. The at least two parallel flexures may be located at at least two separate locations along an axis perpendicular to the planes of the flexures.

The flexures may join two substrates to each other, and/or join two substrates to an intermediate rigid body. Each substrate may be joined by at least three flexures to an intermediate rigid body.

Where multiple substrates are provided, each may be joined to at least one other by four or more flexures. Biasing means may be provided for biasing the motion members into initial engagement with the guide. The biasing means may comprise at least one spring. The biasing means may comprise the at least two flexures. In this case, biasing may be provided by deflection of the flexures. The at least two motion members may be operable to cause linear movement relative to the guide. The guide may comprise a rod or parallel rails.

The at least two motion members may be operable to cause rotational movement relative to the guide. The guide may comprise a rod or a hollow cylinder, for example a drum.

Each motion member may comprise a piezoelectric stack. Each piezoelectric stack may be configured for extension only. Each piezoelectric stack may have two sections, one configured for extension and the other for shear.

The positioning device of both aspects of the invention may be included in a positioning system together with a guide for guiding the positioning device. Equally, the positioning device of both aspects of the invention may be included in a scanning- probe microscope.

According to yet another aspect of the invention, there is provided a method for causing relative movement between a positioning device and a guide, the positioning device having at least two motion members, each member being actuatable between a retracted state and an expanded state, the method comprising: actuating at least one motion member to cause it to move into gripping engagement with the guide, and simultaneously cause relative movement between the device and the guide.

At least two pairs of motion members may be provided and the method may comprise actuating both members of a given pair together to move into or increase gripping engagement with the guide. In each pair, the motion members may be operable to act on opposite sides of the guide and at opposing angles.

The method may further involve moving one pair of motion members into gripping engagement or increased gripping engagement with the guide whilst simultaneously causing relative movement between the device and the guide; moving the other pair into gripping engagement with the guide, so that both pairs grip the guide; actuating the first pair to reduce gripping engagement with the guide so that the guide is predominantly gripped by the second pair; and actuating the second pair to reduce gripping engagement with the guide to cause further relative movement between the guide and the device.

Brief Description of the Drawings

Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which:

Figure 1 shows various views of a C-spring linear positioner;

Figure 2(a) is a schematic diagram showing operation of the linear positioner of Figure 1 ;

Figure 2(b) is a schematic diagram showing operation of a modified version of the linear positioner of Figure 1 ;

Figure 3 is section through (a) a pair of flexures having a uniform thickness; (b) two pairs of hinged flexures; and (c) a pair of split flexures;

Figure 4 shows various views of another C-spring linear positioner;

Figure 5 shows various views of a L-spring linear positioner;

Figure 6 shows various views of a U-spring linear positioner;

Figure 7 shows various views an inverted C-spring linear positioner;

Figure 8 shows various views of an inverted L-spring linear positioner;

Figure 9 shows various views of an inverted U-spring linear positioner;

Figure 10 shows various views of a rotary positioner;

Figure 1 1 is a schematic showing the mechanism of operation of a rotary positioner;

Figure 12 shows various types of contact between a rod and a gripper for a rotary positioner;

Figure 13 shows an inverted rotary positioner;

Figure 14(a) shows a linear positioner where the feed direction is z, and the stack alignment is z;

Figure 14(b) shows a linear positioner where the feed direction is z, and the stack alignment is x;

Figure 15(a) shows a rotary positioner where the feed direction is x, and the stack alignment x;

Figure 15(b) shows operation of the rotary positioner of Figure 15(a); Figure 16 shows schematic representations of a range of configurations of positioning devices containing two pairs of stacks depending on whether the positioner is linear or rotary, has a feed direction along x or z, and has x or z stack alignment;

Figure 17 shows various piezo-electric stacks that have a shear portion and an extension portion;

Figure 18 shows various views of a linear positioner with two drive units for causing movement relative to a rod;

Figure 19 shows various views of another linear positioner with two drive units for causing movement relative to a rod, and

Figure 20 shows a linear positioner with multiple piezo-electric stacks All on the same side of a rod.

Detailed description of the invention

Figures 1 (a) and (b) show a linear positioner that has a rod 10 that may be taken to be in a fixed position, and a movable portion 12 that is movable along the rod. The movable portion 12 has first and second identical spring mounts 14 and a top plate 16 and a bottom plate 18 that hold the first and second spring mounts in a fixed working position. This can be seen in Figures 1 (a) and 1 (b). The rod 10 has a cross section arranged to prevent rotation of the movable portion about the rod 10. In the example shown in Figure 1 , the rod 10 has a hexagonal cross section. However, the rod 10 could equally be shaped to have any suitable cross section, for example a square or octagonal cross section. Equally the rod 10 could have features, for example concave features, designed to mate with parts on the movable portion, for example convex features, that allow linear movement but prevent rotation. The angled faces of the rod 10 block motion along the z axis and rotation about the y axis while enabling motion of the movable portion 12 along the x axis.

Each spring mount 14 has a generally C-shaped block that defines an inner surface and an outer surface. Piezoelectric stacks 20 are positioned on this C-shaped block, so it may be termed a piezo substrate. The piezo substrate is coupled to two corner blocks 22 via flexures 24 located on its outer surface at four connection points, two at each end. In their natural state, i.e. before the movable portion is mounted on the rod, the flexures 24 are all parallel. The flexures 24 are thin, wide foils that allow the piezo substrates to move slightly towards and away from the rod. The parallel flexures 24 provide motion restriction to the piezo substrate and are located at two separate locations along an axis perpendicular to the planes of the flexures. By the plane of the flexure, it is meant the plane of the foil that forms the flexure 24.

Each piezo substrate is sandwiched between a pair of stiffening plates 26, which enhance their stiffness. The stiffening plates 26 are not an intrinsic requirement, however incorporating stiffening plates in this manner reduces the rigidity required from the piezo substrate and allows for more compact construction. Each combination of piezo substrate and two stiffening plates constitutes a rigid unit that may be termed an oscillating unit, because it will move in and out along the y axis as the positioning device operates. The combination of the four corners 22, top plate 16, and bottom plate 18 constitute another rigid unit that may be termed an intermediate unit, because it connects the two oscillating units, through the flexures 24. The spring constants of the flexures are such that a reasonable internal load is maintained over the full range of thermally-induced flexing.

First and second piezoelectric extension stacks 20 are positioned symmetrically on an inner surface of each piezo substrate, so that the positioner has two pairs of stacks, a right hand pair and a left hand pair. Each stack has a longitudinal axis 30 along which it can extend or retract. These longitudinal axes 30 are indicated in Figure 1 (b). The first and second stacks are positioned so that their longitudinal axes form an angle and neither is perpendicular to the rod. The first and second angled stacks on the same piezo substrate are mirror images of each other. One stack is at an acute angle relative to the rod and the other is at an obtuse angle (the obtuse angle being 180° - the acute angle). In the example shown in Figure 1 , the first stack is at an acute angle to the rod and the second stack is at an obtuse angle to the rod. Each stack is terminated by a gripper 32 designed to make contact with the rod. Each gripper 32 has a jaw that is shaped to mate with and fit onto the rod. The four grippers 32 form a cavity for the rod. Figure 2(a) is a schematic representation of the mechanism of operation of the positioner of Figure 1 . The stiffening and top and bottom plates are not shown. In this device, all the flexures 24 are deflected outwards due to engagement of the stacks 20 with the rod 10. Initially, both pairs of stacks are inactivated and both pairs of grippers 32 are in contact with the rod 10, as shown in Figure 2(a)(i). The flexures 24 are always deflected outward, so that they are always applying a force that biases the oscillating units towards the rod 10. The bias force is transmitted through the stacks and grippers to the rod 10.

The flexures 24 allow the piezo substrates to move inward and outward along the y axis. Relative motion between the piezo substrates and corner blocks 22 along the x or z axis, or relative rotation about any axis, would require stretching or shearing the flexures. However, the spring constants for these motions are much higher than for bending, and so stretching or shearing is prevented. In this regard, it is noted that the further apart the flexures are (along the y-axis), the greater rotational stiffness they provide. So in practice the flexures have to be positioned to allow them to provide enough rotational stiffness to allow the retracted member to come out of contact with the rod, under the bias force.

The right-hand pair of stacks 34 is then activated, and extended, as shown in Figure 2(a)(ii). In this state, the piezo substrates are pushed slightly away from the rod 10, as can be seen from the increased angle of deflection of the flexures. Because the flexures 24 do not allow rotation of the piezo substrates about the z axis, the left-hand pair of stacks is pulled out of contact with the rod 10. Also, because the stacks are mounted at an angle, the positioner takes a step to the right, that is, the movable portion moves rightward with respect to the rod 10.

In subsequent steps the pair of stacks on the left is first extended, Figure 2 (a)(iii), so that both pairs of stacks are in a gripping engagement with the rod. Then the right pair 34 is retracted, Figure 2(a)(iv) so that it is moved to a position in which it does not prevent movement, for example to a position in which it is out of engagement with the rod (as shown), or in only light contact with the rod. In Figure 2(a)(ii) the bias force is carried by the right-hand pair of stacks 34; in Figure 2(a)(iv), it has been transferred predominantly to the left-hand pair of stacks 36. The movable portion remains at substantially the same position relative to the guide rod between steps (ii) and (iv). Upon retraction of the left pair 36 , as shown in Figure 2(a)(v), the positioner takes another step to the right, and the flexures allow movement of the piezo substrates back to their original position, so that the positioner returns to its initial state in which both pairs of stacks 34 and 36 are inactivated, i.e. retracted, and in contact with the rod. Because the actuation axes of the stacks are at oblique angles to the rod, clamping and feeding motions are combined into a single motion. The forward step size depends on the choice of angle for the stacks. The forward step is repeated twice over the cycle.

Figure 2(b) shows a schematic representation of the mechanism of operation of a modified version of the positioner of Figure 1 . In this case, the flexures 24 are shown as being deflected inwards and the biasing force is provided dominantly by a tensioning spring connected between the two oscillating units, and the requirement for the flexures to provide this function is removed. The flexures still, as in Figure 2(a), substantially prevent linear motion of the piezo substrates along the x and z axes, or rotation about any axis, relative to the corners. However, relative to the configuration shown in Figure 2(a), the maximum deflection of the flexures is reduced. Figure 3 shows three possible flexure shapes (a) uniform-thickness flexures; (b) hinged flexures, in which at least one portion of the flexure is narrower than other portions, and (c) split flexures, in which each flexure is split into two or more closely-spaced flexures. The spring constant for deflecting a flexure is o t³ , where t is the thickness of the flexure, while that for stretching the flexure is o t. The deflection spring constant sets the bias force (in cases where the bias force is provided by the flexures and not by a separate tensioning spring). The stretching spring constant sets k_T, the torsional spring constant for rotation of the piezo substrates about the z axis. With a non-uniform flexure thickness or split flexures, k_T can be increased while maintaining k. Figure 4 shows a schematic representation of a linear positioner, shown in a way to simplify explanation of various alternative arrangements, which are illustrated in Figures 5 to 9. The positioner of Figure 4 may be termed a C-spring linear positioner, because the piezo substrates are generally C-shaped. As described previously, it has two oscillating units, each coupled by four flexures to an intermediate body. The oscillating units are biased against the rod, either by deflection of the flexures or by a separate tensioning spring between the oscillating units. Piezoelectric stacks are positioned on the inner surface of each oscillating unit, in the configuration described above. Figures 4 to 9 all take the same format and interpretation. Panels (a) are schematic representations of the rod or rails, stacks, grippers, and spring mount(s). Stiffening, top, and bottom plates are removed. Panels (b) show stiffening plate(s), which are repeated on the bottom, and are bonded to the spring mount(s) over the hatched area(s). Panels (a) are reproduced in panels (b) in outline form to show the positioning of the stiffening plates. Panels (c) show the top plate (repeated on the bottom as the bottom plate, which has substantially the same form as the top plate), and the hatched area(s) again indicate bonding to the spring mount(s). As necessary, the thickness of the spring mount(s) is varied to allow bonding to the plates represented in panels (b) and (c), and with clearance provided between the plates represented in panels (b) and those represented in panels (c).

Figure 5 shows a positioner configuration which may be termed an L-spring linear positioner. It is comprised of two oscillating units, each coupled by three flexures to an intermediate unit. The physical arrangement and operation of the piezoelectric stacks is as described previously. The oscillating units are biased against the rod either by deflection of the flexures or by a tensioning spring between the oscillating units.

The piezo substrates 40 are generally L-shaped parts that face each other to define the inner cavity. Each oscillating unit has a piezo substrate sandwiched between two stiffening plates. The intermediate unit 42 has two edge blocks, the top plate, and the bottom plate. There is no functional difference between the edge blocks in this positioner configuration and the corner blocks in the C-spring configuration: the edge blocks may be considered as two corner blocks that have been merged. Holes are provided in the edge blocks and piezo substrates for receiving the rod. To centre the load generated by flexure deflection along the centre line in the figure, the flexures may have unequal lengths and/or thicknesses. One possible scheme is indicated in Figure 5. In this case, the two flexures on one side of each piezo substrate are thinner than the single flexure on the other side.

Figure 6 shows a positioner configuration that may be termed a U-spring linear positioner. This has two oscillating units, coupled to each other by four flexures. There is no intermediate unit. The physical arrangement of the piezoelectric stacks is as described previously. The oscillating units are biased against the rod either by deflection of the flexures or by a tensioning spring between the oscillating units. In Figure 6, the piezo substrates are generally U-shaped parts 44 and 46. The first piezo substrate 44 is generally smaller than the second 46, and fits substantially inside the second piezo substrate. The flexures link the outer surface of the smaller substrate to the inner surface of the larger substrate. The first piezo substrate 44 is sandwiched between two stiffening plates, shown in Figure 6(b), to form an inner oscillating unit. The top plate shown in Figure 6(c) (which, again, is repeated on the bottom) is bonded to the second piezo-substrate 46, to form an outer oscillating unit. Figures 7 to 9 show schematic representations of inverted positioners. In these configurations, the positioner is located between, and in operation moves along, two longitudinally extending rails 48, rather than a centrally located rod. The two oscillating units of each configuration are biased outwards and against the rails, either by deflection of the flexures, or by a compression spring between the oscillating units.

The configuration represented in Figure 7 may be termed an inverted C-spring linear positioner; the configuration in Figure 8 an inverted L-spring linear positioner, and the configuration in Figure 9 an inverted U-spring linear positioner. Although the piezo substrates in these configurations may no longer possess the same C, L, and U-like shapes of the non-inverted configurations of Figures 5 to 7, the configurations are functionally related: the inverted C-spring positioner contains two oscillating units each coupled by four flexures to an intermediate unit, the inverted L-spring positioner contains two oscillating units each coupled by three flexures to an intermediate unit, and the inverted U-spring positioner contains two oscillating units linked to each other by four flexures, and does not contain an intermediate unit.

Other configurations are possible. The minimum requirement is that the oscillating units each be coupled either to each other, or jointly to an intermediate unit. To provide the required stiffness against relative rotations about the z axis, each such coupling should have at least two flexures, located at at least two well-separated locations along the y axis. Incorporating an intermediate unit is useful because the intermediate unit will move along a generally straighter line than the oscillating units, and so is a more natural attachment point for objects affixed to the positioning device. Figure 10 shows a positioner configuration that may be termed a C-spring rotary positioner. Like the C-spring linear positioner, the C-spring rotary positioner contains two oscillating units, each coupled by four flexures to an intermediate unit. As before, in their normal, relaxed state, in the absence of the rod, the flexures are all parallel. The oscillating units each contain a piezo substrate sandwiched between two stiffening plates. The intermediate unit contains four corner blocks, each bonded to a top and a bottom plate to fix their relative positions. The oscillating units are biased against a centrally located rod, either by deflection of the flexures or by a tensioning spring between the oscillating units.

The rod of the C-spring rotary positioner extends along the z axis. Over the length where the rod may contact the grippers, the rod is rotationally symmetric about the z axis. Holes are provided in the top and/or bottom plates to allow the rod to pass. In this configuration, a pair of piezoelectric stacks is affixed to an inner surface of each piezo substrate. Each stack is terminated by a gripper that contacts the rod. Within each oscillating unit, the first stack is offset in the z direction relative to the second stack, such that one is in an upper position and the other in a lower position. The grippers terminating the upper stacks, and likewise the grippers terminating the lower stacks, are positioned so as to contact the rod at points or over areas that are generally at the same height (that is, z position), and generally opposed to each other with respect to both the centre axis of the rod and the x-z plane that runs through the centre axis of the rod. The stacks are positioned so as to effectively transmit the bias force to grippers so positioned.

The axes of extension of the upper stacks are parallel to each other, and at an angle in the x-y plane with respect to the y axis. The axes of extension of the lower stacks are also parallel, and at the opposite angle with respect to the y axis as the axes of extension of the upper stacks.

Figure 1 1 shows steps for operating a rotary positioner. In this Figure the intermediate unit (which includes the four corners) is taken to be at a fixed location, such that operation drives rotation of the rod; however the rod may also be considered fixed, such that operation drives rotation of the intermediate unit. Initially all of the flexures are deflected slightly outwards and both the upper and lower pairs of stacks are inactivated (Figure 1 1 (i)). In this state, both the upper and lower pairs of stacks are retracted, and both pairs of grippers are in contact with the rod. Upon activation, the upper pair of stacks extends and pushes against opposite sides of the rod, as shown in Figure 1 1 (ii). Because the extension axes of the upper stacks are angled with respect to the y axis, extension of each upper stack generates displacements in both the x and y directions. The displacements in the y direction push the oscillating units outward. This is accommodated by further deflection of the flexures outwards. Because the flexures prevent the oscillating units from rotating about the x axis, the lower stacks are pulled out of contact with the rod. The flexures also prevent the oscillating units moving linearly in the x direction, so the x displacements generated by the stacks drive a counter-clockwise rotation step of the rod.

In Figure 1 1 (iii), the lower stacks are extended, and their grippers come into contact with the rod. In Figure 11 (iv), the upper stacks are retracted, such that the bias force is transmitted by the lower pair of stacks. Between steps (ii) and (iv), there is no substantial rotation of the rod. In Figure 1 1 (v), the lower stacks are retracted, returning the stacks to the state of Figure 1 1 (i), in which the flexures are only slightly deflected outwards, and causing the rod to take another counter clockwise rotational step. Figure 12 shows different types of contact between the rod and gripper that can be implemented. Figure 12(a) shows a rolling contact. During operation of the positioning device the rod rolls against the gripper face. Figure 12(b) shows a stack provided with a shoe gripper. The shoe is curved to conform to the rod. When all stacks are terminated by such shoe grippers, the grippers form a cavity which contains the rod. As such, relative rotation between the rod and positioner about the y axis, and relative translation between the rod and positioner in the x direction, are prevented. A pivot is provided for the shoe flexure, such that as the rod is rotated at each rotation step, the shoe pivots to maintain a large-area, non-rolling contact with the rod. Figure 12(c) shows a shoe gripper that is provided with a V-shaped protrusion that mates with a V- shaped groove in the rod. As such, relative translation between the rod and positioner in the z direction is prevented.

A rotary positioner may also be implemented in the L-spring and U-spring configurations, previously described. Holes may need to be provided in the stiffening, top, and/or bottom plates to allow the rod to pass. A rotary positioner may also be implemented in an inverted configuration. Figure 13 shows the rotary equivalent of the inverted C-spring linear positioner, shown in Figure 7. The arrangement and operation of the stacks is essentially the same as the positioner of Figure 1 1 , except that the oscillating units are biased outward (either by deflection of the flexures or a compression spring between the oscillating units), so that the grippers are biased against the internal walls of a drum rather than a centrally located rod. The gripper-drum contact will generally be at a larger radius than the gripper-rod contact in the configuration of Figure 11 , so that the inverted rotator will generate generally larger torques.

For all the positioners described so far, the axes have been chosen such that (i) the flexures are parallel to the xz plane, and (ii) the junctions between the flexures and the bodies the flexures join are extended in the z direction. This convention will be retained for the remainder of this description of the invention. The piezoelectric stacks have been angled so as to generate displacements in the xy plane. The y component of the displacement moves the oscillating units in and out, while the x component drives motion of the rod. This component may be termed the feed direction. For the positioners presented so far, the feed direction has been x. Feed motion in the x direction can generate either linear translation in the x direction, or rotation about the z axis.

For the linear positioners described so far, the stacks within each oscillating unit have been at distinct x positions and substantially the same z position; the stacks may be described as aligned along the x axis. For the rotary positioners described so far, the stacks have been at distinct z positions and substantially the same x position, which may be described as z stack alignment.

For both rotary and linear positioners, the feed direction can either be x or z, and the stack alignment can either be x or z. These two choices may be made independently. For example, Figure 14(a) shows a linear positioner where the feed direction is z, and the stack alignment is z. The operation of the stacks is as shown in Figure 2, but the rod is now extended along the z axis, and the positioner drives linear translation in the z direction. Figure 14(b) shows a linear positioner where the feed direction is z, and the stack alignment is x. The rod is extended along z, but has a generally double-lobe cross section. The left-hand pair of grippers contacts the left-hand lobe, while the right- hand pair of grippers contacts the right-hand lobe.

Figure 15(a) shows a rotary positioner where the feed direction is x, and the stack alignment x. In this case, the gripper-rod contacts are at small angles with respect to the y axis, to avoid collision between the left-hand and right-hand grippers. The line from the centre axis of the rod to the centre of the contact areas (where the centre of the contact area is taken to be the centre of the force distribution within the contact area) will generally be at an angle from the y axis. The angle between the stack longitudinal axes and the y axis can be labelled θ₂. This positioner will operate as shown in the Figure as long as θ₂ is substantially larger than The operation of this positioner is shown in Figure 15(b).

Figure 15(b)(i) shows the initial state of the positioner, in which all stacks are retracted and all are in contact (through their grippers) with the rod. In Figure 15(b)(ii), stacks A and D are extended, causing the rod to take a clockwise rotational step, and causing stacks B and C to move out of contact with the rod. Between Figures 15(b)(ii) and 15(b)(iii), stacks B and C are extended, then A and D retracted, and there is no substantial net rotation of the rod. In Figure 15(b)(iv), stacks B and C have been retracted, causing the rod to take another clockwise rotation step.

Figure 16 shows schematic representations of the range of configurations of positioning devices containing two pairs of stacks that is possible by choosing independently whether the positioner is linear or rotary, has the feed direction along x or z, and has x or z stack alignment. In the figure, the angling of the stacks is not shown. Instead, the arrows indicate the feed direction of each stack that is associated with extension in the y direction. Two dark-coloured and two light-coloured stacks are shown representing the stacks being activated at the same time, following the sequence shown in Figure 2 for linear positioners, and Figure 1 1 or Figure 15, as appropriate, for rotary positioners. Linear positioners with x and z feed directions will drive linear translation along x and z, respectively. Rotary positioners with x and z feed directions will drive rotation about the z and x axes, respectively. In addition to the choices of linear versus rotary, x versus z feed, and x versus z stack alignment, the configuration of the oscillating units, intermediate unit, and flexures can be chosen to be C, L, U, inverted C, inverted L, or inverted U. The positioners described so far have contained extension stacks mounted at an angle, to drive simultaneous clamping and feed motions. Alternatively, the stacks can be a combination of extension-type and shear-type stacks, as shown in Figure 17(a). In this case, the extension-type portion drives the clamping motion, and the shear-type portion the feed motion. This is shown in Figures 17(b) and 17(c), which depict a C-spring linear positioner incorporating shear-type stacks.

Figure 18 is an example of a linear positioner with two drive units for causing movement relative to a rod. The positioner has first and second identical piezo substrates, each provided with first and second recesses. The first and second recesses directly face each other so that they are symmetrically positioned about the rod. First and second piezoelectric stacks are affixed to the surface of each C-shaped recess. In each recess, the first and second stacks are positioned symmetrically such that the longitudinal axes of the stacks form an angle. One stack is at an acute angle relative to the rod and the other is at an obtuse angle (the obtuse angle being 180° - the acute angle). In the example shown in Figure 18, the first stack is at an acute angle to the rod and the second stack is at an obtuse angle to the rod. Each stack is terminated by a contact portion or gripper to make contact with the rod. A corner block is connected to each end of the each piezo substrate via at least one flexure. The flexures allow oscillating movement in the y direction of the piezo substrates. The piezoelectric stacks are biased into engagement with the rod in their natural inactivated state either by deflection of the flexures or by a tensioning spring between the two piezo substrates.

Connected to each piezo substrate is a pair of stiffening plates that prevent bending of the substrates but do not impede oscillation. The hatched areas in Figure 18(b) show where the stiffening plates are bonded to the blocks. Top and bottom joining plates are bonded to the four corner blocks on the top and bottom side respectively, as shown in Figure 18(c). The joining plates are bonded to the corner blocks without contacting the stiffening plates. The joining plates and the four corner blocks form an intermediate unit. The oscillating units, each comprised of a piezo substrate and two stiffening plates, do not need to be provided with high torsional stiffness, and so may be connected to the intermediate unit by only two, or even a single, flexure. High torsional stiffness is not necessary because there are always at least two piezos, on opposite sides of the bias force axis, in contact with the rod.

In Figure 18, the stacks arearranged in ABAB order (as shown in Figure 19(a)), where the A stacks are operated together, and likewise the B stacks, so that multiple pairs of stacks can be simultaneously actuated to move into or increase gripping engagement with the guide at multiple, different positions along its length and cause relative movement between the device and the guide (as shown in Figure 19(b)). The stacks could also be arranged in ABBA order, as shown in Figure 19(c). Because the two inner stacks are angled in the same direction, they could also be replaced with a single stack, as shown in Figure 19(c).

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, whilst in all of the embodiments described with reference to the drawings pairs of piezoelectric stacks are on opposing sides of a guide, in some cases they could be provided on the same side of the guide, as shown in Figure 20. In this case, tensioning springs between the guide and piezo substrate provide the biasing force. The stacks labelled A are operated together, as are the stacks labelled B. The bias force is centred so as to always be within a contact area or between two contact points between the stacks and the rods, so that rotation of the piezo substrate relative to the guide is prevented without the requirement for parallel flexures. This can be done if the positioner contains at least three stacks, at least two of which are operated together. Equally, although in all the examples, each piezoelectric stack is an extension only stack, in some cases, the piezoelectric stack could have a section for extension and a section for shear. Accordingly, the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

1 . A positioning device for causing movement relative to a guide, the positioning device comprising at least two motion members, each member being actuatable between a retracted state and an expanded state, wherein upon actuation between the retracted state and the expanded state a motion member is movable into or able to increase gripping engagement with the guide and simultaneously cause relative movement between the device and the guide.

A positioning device as claimed in claim 1 wherein at least two pairs of motion members are provided, both members of a given pair being actuatable together to move into or increase gripping engagement with the guide and simultaneously cause relative movement between the device and the guide.

A positioning device as claimed in claim 2 wherein in each pair the motion members are operable to act on opposite sides of the guide and at opposing angles.

A positioning device as claimed in any of the preceding claims adapted to move one motion member into gripping engagement or increased gripping engagement with the guide whilst simultaneously causing relative movement between the device and the guide; move the other motion member into gripping engagement with the guide, so that both motion members grip the guide; actuate the first motion member to reduce gripping engagement with the guide so that the guide is predominantly gripped by the second motion member; and actuate the second motion member to reduce gripping engagement with the guide to cause further relative movement between the guide and the device.

A positioning device as claimed in claim 2 or claim 3 adapted to move one pair of motion members into gripping engagement or increased gripping engagement with the guide whilst simultaneously causing relative movement between the device and the guide; move the other pair of motion members into gripping engagement with the guide, so that both pairs of motion members grip the guide; actuate the first pair of motion members to reduce gripping engagement with the guide so that the guide is predominantly gripped by the second pair of motion members; and actuate the second pair of motion members to reduce gripping engagement with the guide to cause further relative movement between the guide and the device.

6. A positioning device as claimed in any of the preceding claims comprising at least three pairs of motion members.

7. A positioning device as claimed in any of the preceding claims comprising at least four pairs of motion members.

8. A positioning device as claimed in any of claims 2 to 7 wherein in multiple pairs of motion members are simultaneously actuated to move into or increase gripping engagement with the guide at multiple, different positions along its length and cause relative movement between the device and the guide.

9. A positioning device as claimed in any of the preceding claims, wherein each motion member comprises a piezoelectric stack that is extendable along a longitudinal axis, wherein each stack is arranged so that its longitudinal axis is at a non-perpendicular angle relative to a longitudinal axis of the guide.

10. A positioning device as claimed in claim 9 wherein the piezoelectric stack consists of a single extension stack extendible and retractable along its longitudinal axis.

1 1 . A positioning device as claimed in any of the preceding claims, wherein each motion member has an end adapted to make static contact with the guide.

12. A positioning device as claimed in claim 1 1 wherein the end of the motion member is shaped to mate with the guide.

13. A positioning device as claimed in claim 1 1 or claim 12 wherein the end of the motion member comprises a gripper.

14. A positioning device as claimed in any of the preceding claims comprising biasing means for biasing the motion members into initial engagement with the guide.

15. A positioning device as claimed in claim 14 wherein the biasing means comprise at least one spring.

16. A positioning device as claimed in any of the preceding claims comprising at least one joint substrate, each joint substrate carrying at least two adjacent motion members on the same side of the guide.

17. A positioning device as claimed in claim 16 wherein each joint substrate is paired with another joint substrate, and the paired joint substrates directly face opposite sides of the guide.

18. A positioning device as claimed in claim 16 or claim 17 comprising means for allowing each joint substrate to move relative to the guide in response to actuation of at least one of its motion members, and restrict rotation of each joint substrate such that in response to actuation of one motion member, the other moves out of or decreases gripping engagement with the guide.

19. A positioning device as claimed in claim 18, wherein the means for allowing movement and restricting rotation comprise at least two parallel flexures connected to each joint substrate.

20. A positioning device as claimed in claim 19, wherein the at least two flexures, preferably parallel flexures, are located at at least two separate locations along an axis perpendicular to the planes of the flexures.

21 . A positioning device as claimed in claim 20, wherein the flexures join two joint substrates to each other, and/or join both substrates to an intermediate rigid body.

22. A positioning device as claimed in any of claims 16 to 21 , where each joint substrate is joined by at least three flexures to an intermediate rigid body.

23. A positioning device as claimed in claim 17 or any claim dependent on claim 17, wherein joint substrates of each pair are joined to each other by four flexures.

24. A positioning device as claimed in any of claims 14 or any claim dependent on claim 14, where the biasing means comprises one or more flexures.

25. A positioning device as claimed in any of the preceding claims wherein the at least two motion members are operable to cause linear movement relative to the guide.

26. A positioning device as claimed in claim 25, wherein the guide comprises a rod or parallel rails.

27. A positioning device as claimed in any of claims 1 to 26 wherein the at least two motion members are operable to cause rotational movement relative to the guide.

28. A positioning device as claimed in claim 27, wherein the guide comprises a rod or a hollow cylinder, for example a drum.

29. A positioning device for causing movement relative to a guide, the positioning device comprising:

at least one pair of motion members, each member being actuatable between a retracted state and an expanded state to cause relative movement between the device and the guide;

at least one joint substrate, each joint substrate carrying two adjacent motion members on the same side of the guide; and

at least two flexures connected to the at least one joint substrate for allowing each joint substrate to move relative to the guide in response to actuation of at least one of its motion members, and restrict rotation of each joint substrate such that in response to actuation of one motion member, the other moves out of or decreases gripping engagement with the guide.

30. A positioning device as claimed in claim 29 wherein each joint substrate is paired with another joint substrate, and the paired joint substrates directly face opposite sides of the guide.

31 . A positioning device as claimed in claim 29 or claim 30 wherein each joint substrate has a motion member from two different pairs of motion members.

32. A positioning device as claimed in any of claims 29 to 31 , wherein the at least two flexures are parallel.

33. A positioning device as claimed in any of claims 29 to 32 wherein the at least two flexures are located at at least two separate locations along an axis perpendicular to the planes of the flexures.

34. A positioning device as claimed in any of claims 29 to 33, wherein the flexures join two joint substrates to each other, and/or join both substrates to an intermediate rigid body.

35. A positioning device as claimed in any of claims 29 to 34, where each joint substrate is joined by at least three flexures to an intermediate rigid body

36. A positioning device as claimed in any of claims 29 to 35, where the joint substrates of each pair are joined to each other by four flexures.

37. A positioning device as claimed in any of claims 29 to 36, where the biasing means are provided by deflection of the flexures.

38. A positioning device as claimed in any of claims 29 to 37 wherein the at least two motion members are operable to cause linear movement relative to the guide.

39. A positioning device as claimed in claim 38, wherein the guide comprises a rod or parallel rails.

40. A positioning device as claimed in any of claims 29 to 39 wherein the at least two motion members are operable to cause rotational movement relative to the guide.

41 . A positioning device as claimed in claim 40, wherein the guide comprises a rod or a hollow cylinder, for example a drum.

42. A positioning device as claimed in any of claims 29 to 41 , wherein each motion member comprises a piezoelectric stack.

43. A positioning device as claimed in claim 42, wherein each piezoelectric stack is configured for extension only.

44. A positioning device as claimed in claim 42, wherein each piezoelectric stack has two sections, one configured for extension and the other for shear.

45. A positioning system comprising a positioning device as claimed in any preceding claims and a guide for guiding the positioning device.

46. A scanning-probe microscope comprising a positioning device or positioning system as claimed in any of the preceding claims.

47. A method for causing relative movement between a positioning device and a guide, the positioning device having at least two motion members, each member being actuatable between a retracted state and an expanded state, the method comprising: actuating at least one motion member to cause it to move into gripping engagement with the guide, and simultaneously cause relative movement between the device and the guide.

48. A method as claimed in claim 47 wherein at least two pairs of motion members are provided, the method comprising: actuating both members of a given pair together to move into or increase gripping engagement with the guide.

49. A method as claimed in claim 48 wherein in each pair the motion members are operable to act on opposite sides of the guide and at opposing angles.

50. A method as claimed in claim 48 or claim 49 comprising:

moving one pair of motion members into gripping engagement or increased gripping engagement with the guide whilst simultaneously causing relative movement between the device and the guide;

moving the other pair into gripping engagement with the guide, so that both pairs grip the guide;

actuating the first pair to reduce gripping engagement with the guide so that the guide is predominantly gripped by the second pair; and

actuating the second pair to reduce gripping engagement with the guide to cause further relative movement between the guide and the device.