GB2365205A

GB2365205A - Helical piezoelectric deice

Info

Publication number: GB2365205A
Application number: GB0124970A
Authority: GB
Inventors: Anthony Hooley
Original assignee: 1 Ltd
Current assignee: 1 Ltd
Priority date: 1997-09-05
Filing date: 1998-09-04
Publication date: 2002-02-13
Anticipated expiration: 2018-09-04
Also published as: GB2364965A; GB0124964D0; GB2365206A; GB0124972D0; GB2365251A; GB0124978D0; GB2365205B; GB2364965B; GB2365206B; GB0124970D0; GB2365251B

Abstract

A unimorph or bimorph piezoelectric 'bender' formed into a flat- (or edge-) wound helix 50,54,55. The device may be formed by the extrusion of "green" piezoelectric material e.g. PZT.

Description

2365205 Piezoelectric Device This invention relates to piezoelectric

devices. More particularly, the invention relates to novel constructions of piezoelectric devices for use as electromechanical drivers.

Transducers which convert electrical energy to mechanical energy are well known and come in a wide variety of forms perhaps the most common of which is the loudspeaker (which converts electrical signals into the motion of a piston or like device the movement of which is caused to displace air so as ultimately to "change" the electrical signal into audible sound).

Linear actuators of many varieties are also well known, examples including hydraulic, pneumatic and internal-combustion cylinders, electromagnetic solenoids, linear motors of many kinds, piezoelectric and magnetostrictive actuators, and inch worm devices.

Where a relatively small, self-contained, compact and electricallyoperated linear actuator is required capable of movement of merely a few millimetres and/or of the application of small forces in the Newton range, then solenoids are generally preferred. It would be attractive to utilise a piezoelectric device instead, but unfortunately the present-day piezoelectric devices generally have a problem producing millimetre-range displacements if direct acting, and even 'stacks' of piezoelectric plates produce only small deflections with practical applied voltages.

Moreover, piezo devices become quite bulky if used in the 'bender' mode; in this mode it is usual to provide an elongate two-layer cantilever beam made of a piezoelectric unimorph (a single shape-changing layer on a shape-fixed layer) or bimorph (two shape-changing layers back to back), which beam bends significantly as the activating voltage is applied, but such a beam necessarily extends some distance away from the axis of output movement.

Nevertheless, some use of piezoelectric benders Mi place of solenoids has been made, particularly in the application of pneumatic valves, where a multilayer bimorph has been used to provide reasonable deflection from relatively low operating voltages. Moreover, in GB-A-2,322,232 there is described a helical tape-wound bender geometry suited for applying radial "squeezing" pressures to a translator mounted therewithin via a linear bearing. This helical bender has electrodes on its radially inner and outer surfaces which are split half way along the axis of the helix and are cross-coupled to produce, when driven, a reduction in helix radius at one end and an fficrease in helix radius at the opposite end. The present invention concerns a more generally useful manner of substituting a piezoelectric device for a solenoid - of retaining the compact cylindrical shape of the solenoid with the main length of the actuator aligned along (rather than at right angles as in a classical bender) to the direction of movement.

The helically-wound piezo device described in the aforementioned Specification is one wherein the piezoelectric material is tape-wound, as though a length of tape or ribbon had been wound in the conventional fashion around the outside of a cylinder. By contrast, in this aspect of the present invention a piezoelectric bender is made in the form of aflat(or edge-) wound helical coil, with the direction of the applied electric field being between the two surfaces of the flatwinding (i.e. nominally aligned along the direction of the axis of the helical coil).

To best visualise the geometry of this structure, consider first a conventional elongate rectangular unimorph or bimorph piezoelectric 'bender', with a thickness t (smallest dimension), width w (intermediate dimension) and length 1 (greatest dimension). The morph is constructed of two layers (which together have the total thickness t), and the applied electric field is in the direction of the thickness, so that if V volts are applied then the electric field in the morph has magnitude VIt. Such a field will cause the morph to bend preferentially in a direction at right angles to the length and width dimensions, such that the thickness direction lies within that plane of bending. Next, consider the undeflected rectangular morph lying flat on a horizontal surface, with the thickness t vertically aligned, and placed with its side at one end adjacent to and touching a cylinder of diameter d standing on the surface with its axis also vertical. Now imagine that the morph is flexible, and with the end touching the cylinder held stationary, the morph is edgewound around the cylinder (i.e. in the plane of the surface) so that it forms after one turn an annulus - a circle of inner diameter d and outer diameter approximately d + 2w. If, as the morph is wound around the cylinder, it is raised by at least a height t per turn then it is possible to continue winding it into a continuous helix with each turn being on top of its preceding turn, thus giving a helix of pitch p (wherein this case p= t). Ifthepitchp is made greater than t then there will be a space between each turn of width.P-t. That describes the geometry of the helical bender of the invention; the thickness (or smallest dimension) is aligned substantially axially along the helix, the width (or intermediate dimension) is aligned radially along the helix, and the length (or greatest dimension) is aligned helically along the helix, and in operation the morph is polarised in the thickness direction.

Note, incidentally, that it is not being suggested that the above method is necessarily a practical means of constructing such a helical morph: this is merely a description to portray the desired geometry (however, it is in practice possible to make a helical morph in much this manner if the winding into a helix is done while the ceramic layers of the morph are still in the green or unsintered state, and in practice if the green ceramic is suitably plasticised).

Therefore, the invention provides a unimorph or bimorph piezoelectric bender' formed into a flat- (or edge-) wound helix.

More particularly the invention provides a unimorph or bimorph (morph) piezoelectric 'bender' (bender) formed into a 'flat-wound' helix - i.e. where the thickness or smallest dimension of the bender cross section is aligned axially along the helix, the width or intermediate dimension of the bender is aligned radially along the helix, and the length or greatest dimension of the bender is aligned helically along the helix - where the pitch of the helix is greater than the thickness of the bender, and where the morph is polarised in the thickness direction (i.e. nominally along the axis of the helix), and wherein electrically-conductive electrodes are deposited along the length of the bender on both of the largest surface area sides of the bender (i.e. on either side of the thickness direction) and are drivable by an electrical signal so as to cause the helical bender to exhibit a dimensional change in the axial direction when so driven.

In use the morph is polarised in the thickness direction (i.e. nominally along the axis of the helix - a direction approximately parallel to the axis of the helix).

Consider the effect of applying an electric field in this direction along the thickness dimension. Each part of the helical bender will try to bend nominally in the thickness direction, and this attempt will, because of the helical geometry, cause the entire helix to lengthen or shorten (if the pitch p is not greater than the thickness t then the helix will not be able easily to shorten). There will also be a small - deflection orthogonal to the thickness direction which will mostly contribute to a slight increase or decrease in the diameter of the helical coil, but this effect will be small compared to the length change in the helix, due in part to the structure and alignment of the morph relative to the helix. Depending on the chosen materials, the achievable deflection along the direction of helix axis, per size of helical coil, is considerable compared to that achievable using a comparably compact bender beam or stack. For example, a practical morph of width 8nim and thickness l min formed into a helix of inner diameter 16min and outer diameter 32mm, with 12 turns at 2nun pitch, will have overall cylindrical dimensions of 32min diameter and 24min length, but will have the equivalent bender-length of a cantilever beam bender (of the same width and thickness) of approximately 91 Omm (nearly a metre), and will deflect well over 10= with practical drive voltages. This is quite comparable to the performance possible with a similarly-sized solenoid (though perhaps with somewhat less output operating force), and such a helical bender has the great advantage over a solenoid that once actuated it requires no static holding current, and therefore dissipates essentially zero energy as heat, whilst still producing a static output force.

The bender also has very small inductance, and is free from the inductive switching transients that are a problem with solenoids.

in operation the morph is, as just described, polarised in the thickness direction, and it is convenient to effect this utilising electricallyconductive electrodes deposited along the length of the bender on both of the largest surface area sides of the bender (i.e. on either side of the thickness direction), and then driving these by a suitable electrical signal so as to cause the helical bender to exhibit the required associated dimensional change in the axial direction.

One method of constructmig such a bender involves co-extrusion of two (or more) layers of plasticized piezoelectric material, typically a lead zirconium titanate (PbZTi, or M) ceramic, to form a unimorph, bimorph or multimorph. The extrusion is effected through a rectangular aperture nozzle of exit dimensions w x t and so arranged internally that extrudate issues from the exit aperture at a rate which is a flinction of position across the 'W' dimension of the aperture; the effect of this is that upon so exiting the extrudate "curls", and in fact forms into a circle or helix if so coerced by means of an external cylindrical former of diameter 'V' and winding arrangement, with inner diameter d and outer diameter d+2w. In order successfully to achieve such co-extrusion it is necessary to grind very finely and uniformly the PZT powder (obtained, for exwnple, from Morgan- Matroc), and to mix it thoroughly with a suitable plasticizer (eg polyvinyl acetate, PVA) and water.

The separate layers are loaded with more or less silver oxide to make a unimorph - active layers with a small proportion of silver oxide, say 2%, and conductive inactive layers with somewhat more silver oxide, say 20%. For a twolayer device, one active and one inactive layer are co-extruded together such that their total thickness is = t and each has a thickness of t/2 after extrusion. For a multilayer device, active layers are alternated with inactive layers, each of nominal thickness t/n, to form a total bender thickness of t after extrusion.

Once the two- or multilayer extrudate has been extruded and "wound" onto the former of diameter d, it is sintered on the former in a furnace at a temperature in the region of 900-1,000T. Surface electrodes can then be added to any external active layers by, for example, the sputtering of a conductive material such as silver or aluminium.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying diagrammatic Drawing in which Fig. 1 shows a perspective view of a form of piezoelectric helical bender of the 5 invention; Fig. 1 shows a helical flat-wound bender of some suitable diameter, thickness, pitch and width (shown respectively at 58, 57, 59 and 56).

The bender is comprised of a top (as viewed) layer (54) and a bottom (as viewed) layer (55) bonded together at their interface (50). If both layers 54,55 are piezoelectric then the bender is a bimorph- if only one layer is piezoelectric then it is a unimorph.

The helix extends or contracts along the direction of the axis (shown as dashed line 51) depending on the polarity of the electrical drive voltage applied between conductive electrodes (not shown) deposited one on the top (as viewed) face of the top layer 54 and one on the bottom (as viewed) face of the bottom layer 55. To allow easy application of load forces, the top and bottom turns of the helix may be flattened out somewhat as indicated in the Figure, or they may be ground flat.

Claims

1. A unimorph or bimorph piezoelectric 'bender' formed into a flat- (or edge-) wound helix.

2. A bender as claimed in Claim 1, wherein: the thickness or smallest dimension of the bender cross section is aligned axially along the helix, the width or intermediate dimension of the bender is aligned radially along the helix, and the length or greatest dimension of the bender is aligned helically along the helix; wherein the pitch of the helix is greater than the thickness of the bender, and the morph is polarised in the thickness direction, nominally along the axis of the helix; and wherein electrically-conductive electrodes are deposited along the length of the bender on both of the largest surface area sides of the bender, on either side of the thickness direction, and are drivable by an electrical signal so as to cause the helical bender to exhibit a dimensional change in the axial direction when so driven.

3. A bender as claimed in either of Claims 1 and 2, wherein the piezoelectric material forming the bender is a lead zirconium titanate (PZT) ceramic.

4. A bender as claimed in any of Claims 1 to 3 and substantially as described hereinbefore.

5. A method of constructing a bender as claimed in any of Claims 1 to 4, in which two (or more) layers of plasticized piezoelectric material are co-extruded to form a unimorph, bimorph or multimorph.

6. A method as claimed in Claim 5, in which the co-extrusion is effected through a rectangular aperture nozzle of exit dimensions w x t and so arranged internally that extrudate issues from the exit aperture at a rate which is a function of position across the 'W' dimension of the aperture, the effect of this being that upon so exiting the extrudate "curls", and forms itself into a circle or helix if so coerced by means of an external cylindrical former of diameter "d" and winding arrangement, 5 with inner diameter d and outer diameter d+2w.

7. A method as claimed in either of Claims 5 and 6, in which the separate layers are loaded with more or less potentially conductive material active layers with a small proportion of material and conductive inactive layers with somewhat more 10 material.

8. A method as claimed mi any of Claims 5 to 7, in which the wound two- or multilayer extrudate is sintered on the former, and then surface electrodes are provided by sputtering on any external active layers.

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9. A method as claimed in any of Claims 5 to 8 and substantially as described hereinbefore.

10. A bender made by a process as claimed in any of Claims 5 to 9.