WO2003063262A2 - Curved electro-active actuators - Google Patents

Abstract

Electro-active actuators and methods of manufacturing such actuators are described having a longitudinally extended layer of electro-active material bent to form a non-straight, curved line or contour of uniform or non-uniform curvature, which line or contour having section with a mean radius of curvature to thickness ratio r/t close to 0.5. The layer is preferably monolithic with no buried electrode layer or layers, thus facilitating manufacture.

Description

CURNED ELECTRO-ACTIVE ACTUATORS FIELD OF THE INVENTION This invention relates to curved elements of electro-active material. More particularly, the invention related to electro-active actuators displaying a movement when energized.

BACKGROUND OF THE INVENTION Electro-active materials are materials that deform or change their dimensions in response to applied electrical conditions or, vice versa, have electrical properties that change in response to applied mechanical forces. The best-known and most used type of electro-active material is piezoelectric material, but other types of electro- active material include electrostrictive material.

Many devices that make use of electro-active materials are known. The simplest piezoelectric device is a block of pre-poled, i.e., pre-oriented, piezoelectric material activated in an expansion-contraction mode by applying an activation voltage in direction of the poling.

Because piezoelectric devices are capacitive in nature, they exhibit a number of desirable mechanical and electrical characteristics. They have a very efficient coupling of energy from applied charge to mechanical strain, leading to a high bandwidth, a large force output and negligible resistive heating. Due to their capacitive nature, these devices draw their least current at zero rate of displacement. The electro-active material, which in general is crystalline, ceramic or polymer- based, determines the stiffness of electro-active devices. However, as the electro- active effects are extremely small, e.g. in the order of 1 nm/V, the change in dimensions is relatively small and requires high voltages. Therefore, more complicated electro-active structures have been developed to achieve larger displacements.

To increase the displacements, several designs have been introduced such as stacks, unimorph or bimorph benders, recurved benders, corrugated benders, spiral or helical designs. For example, piezoelectric multilayer stacks can be fabricated by joining multiple piezoelectric rings or plates, such that the total displacement of the stack is the sum of the displacements of each individual plate. Inner electrodes separate adjacent plates. The stacks provide vertical displacement in accordance with their piezoelectric charge coefficients and the potential applied. Several hundred plates are necessary to provide total displacements of 10 or more micron.

A standard unimorph bender is made up of a flat piezoelectric strip bonded to a non-active, purely elastic layer, e.g. a metallic shim, from one side. When a voltage is applied across the thickness of the piezoelectric layer, longitudinal and transverse strain develop. The elastic layer opposes the transverse strain, thus causing a bending deformation. To achieve an acceptable level of performance in a unimorph actuator, the non-active layer has a thickness in the same order of magnitude as the electro- active layer.

To increase the displacement range of the unimorph benders, the art has turned mostly to utilizing bimorph structures. Bimorphs have at least two laminated piezoelectric layers, thus having at least one internal and two external electrodes to which voltages of opposite polarisation is applied. The application of an electric field across the two outer layers causes one layer to expand while the other contracts. This results in a bending motion with relatively wide displacements at the tip of the cantilever beam. In a cantilever configuration, the displacement of the tip is related to the length of the cantilever, the applied voltage and the thickness of the cantilever. Cantilever-based piezoelectric actuators require lengths on the order of 25 mm or more to achieve a free deflection of 0.3 mm. It should be noted that a reinforced bimorph, i.e., a bimorph having a centre shim, actually consists of nine layers: two piezo-ceramic layers, four electrode layers, two adhesive layers and the centre shim. It is accepted knowledge in the art that a bimorph bender exhibits more displacement than a unimorph bender of equal dimensions. Thus, modern applications of bender-type electro-active structures have focussed more or less exclusively on bimorph structures while often referring in passing to unimorphs. However, bimorphs, particularly curved bimorphs, require at least one central electrode and are therefore difficult to manufacture. It is known that the bonding layer causes both an increase in hysteresis and a degradation of the displacement characteristics, in addition to delamination problems at the electrode interface layer. Also, the fabrication process, cutting, polishing, electroding, and bonding is rather laborious and costly. Any monolithic bender is expected to greatly reduce such - problems. The known monolithic devices consist of a piezoelectrically active layer and a reduced passive layer formed in a high-temperature reduction treatment. An example of such a monolithic device is known as "rainbow" actuator.

Benders, stacks, tubes and other electro-active actuators are employed in a wide array of engineering systems, ranging from micro-positioning applications and acoustic wave processing to printing applications. Generally, actuators are used in such applications to generate force and effect displacement, for example, to move levers or other force transmitting devices, pistons or diaphragms, to accurately position components, or to enable similar system functions. Actuators employed for such functions are typically designed to provide a desired displacement or stroke over which a desired force is delivered to a given load.

Depending upon design, electro-active actuators can generate a rotational or translational displacements or combinations of both movements.

Comparably large translation displacements have been recently achieved by using a helical structure of coiled piezoelectric tape. Such twice-coiled or "super- helical" devices are found to easily exhibit displacement in the order of millimetres on an active length of the order of centimetres. These structures and variations thereof are described, for example, in the published international patent application WO-0147041 or by D. H. Pearce et al in : Sensors and Actuators A 100 (2002), 281-286. It is therefore seen as an object of the present invention to provide novel configurations of electro-active material that - whilst maintaining similar performance to that of stacks, benders or twice-coiled benders - are easier to manufacture.

More specifically, it is an object of the invention to provide such curved electro-active actuators of relatively small dimensions. SUMMARY OF THE INVENTION In view of the above objects, the present invention provides apparatus and methods as claimed in the independent claims.

According to a first aspect of the invention, there is provided a longitudinally extended layer of electro-active material bent to form a non-straight, curved line or contour of uniform or non-uniform curvature, having section with a small mean radius r to layer thickness t ratio.

The layer is preferably monolithic in the sense that it has no buried conductive layer or layers. Hence, electrodes to activate the electro-active layer are found exclusively on the surface of the layer. It will be appreciated by those skilled in the art that having a monolithic electro-active bender with displacement characteristics close to those of a bimorph of similar dimensions is advantageous in view of manufacturing such devices.

The radius of curvature is measured at a half-thickness point of the extended layer. Hence, the theoretical radius r of curvature has to be greater than half the value oft (r = 0.5 t). A practical limit to the ratio r/t appears to be closer to 0.6. Though the value of r/t approaches infinity in the limit of a non-curved flat layer, the advantage offered by this invention reduces at a much higher values of r/t. Preferable upper limits of the ratio r/t are hence 4, 3 or even only 2. Particularly advantageous are values of r/t in the region of 1.2.

In strict mathematical terms, the radius of curvature may change from point to point along the length of the layer. However, as the efficiency of the monolithic layer of is found to decrease with increasing value r/t, it is the design object of this invention to have low r/t values at as many points as possible, i.e. over as long stretches or sections of the layer as possible. To operate effectively, the parts where the ratio r/t is in the above range form a substantial amount of the curved actuator. It is particularly desirable to form a curved actuator with at least 50 % of its length being parts or sections wound such that the radius of curvature has a value close to the thickness of the layer. Practical devices are however likely to include straight sections or section with a large r/t ratio. Normally the layer will be curved in a direction perpendicular to the thickness of the layer that is with the axis of curvature perpendicular to the thickness direction. However in general any curve could be used. In the general case, the thickness of the layer of electro-active material may be measured in the direction along the radius of curvature, mat is perpendicular to me axis of curvature.

A very suitable form of generating long sections with small radii of curvature are wave-shaped benders and helical shaped benders. As it is known from the international patent application WO-0147041 that a coiled helix exhibits large displacements compared to conventional benders. In another preferred variant, the bender has an essentially linear but corrugated or wavy shape. In other embodiments, a corrugated linear bender may be folded or bent into an arc, a zigzag, serpentine or into circular or helical shapes.

Electro-active materials for use in the present invention are preferably piezoelectric materials such as PZT. These and other aspects of inventions will be apparent from the following detailed description of non-limitative examples making reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 A is a schematic cross-section of a known bimorph bender;

FIG. IB is the bimorph bender of FIG. 1 A shown in an activated state; FIG. 2 is a schematic cross-section of a known unimorph bender; FIG. 3 A is a schematic cross-section of a bender illustrating features of the present invention; FIG. 3B is a graph illustrating features of the present invention;

FIG. 4 is schematic cross-section of a corrugated bender; FIG. 5 is schematic cross-section of a coiled helical bender; FIG. 6A is a schematic perspective view of a first stage of an extrusion process suitable for manufacturing coiled helical benders; FIG. 6B is a schematic cross-section of the extrusion tool of FIG. 6A; FIG. 7 is a schematic perspective view of a second stage of an extrusion process suitable for manufachiring coiled helical benders;

FIG. 8 is a schematic perspective view of a third stage of an extrusion process suitable for manufacturing coiled helical benders; and FIG. 9 is a flow chart summarizing the steps of FIG.6 to FIG.8.

DETAILED DESCRIPTION Before referring to the novel configurations of benders, the salient features of known bender designs are illustrated in FIGs. 1 and 2.

A typical bimorph bender is shown in FIGs. 1 A and IB. A bimorph bender 10 is known to include at least one pair of piezoelectric or piezo-ceramic plates 11 and 12, which may be elongated and are interposed between an inner electrode 13 or electrically conductive shim and outer electrodes 14. The piezo-ceramic elements or plates 11 and 12 are shown electrically poled or polarized in a common direction, as represented by the arrows 111 and 121, respectively. When an electrical potential or voltage from a source 15 is applied across both of the piezo-ceramic elements 11 and 12 through the outer and inner electrodes 14, 13, as shown on FIG. IB, the element or plate 11 contracts and element or plate 12 expands, so that bimorph cantilever 10 bends in the direction perpendicular to its longitudinal axis. The absolute displacement depends on the strength of the electric voltage applied to the bender and its material properties. If the polarity of the voltage applied from source 15 to bimorph leaf 10 is reversed, the direction of bending of the leaf will be opposite to that shown in FIG. IB.

Referring now to FIG. 2, there is shown a basic unimorph configuration 20. The ummorph bender includes only one piezoelectric layer 21 having a top face electrode 22 and a bottom face electrode 23. The lower electrode 23 is used to counter the strain in the piezoelectric layer when an electric field is applied across it. Therefore, the electrode 23 is shown to have a much greater thickness than the upper electrode 22 which is used solely as an electrically conductive layer. An electrical power source 25 is used to supply the appropriate voltage to the electrodes. When energized the unimorph bends as shown.

The general features of a bender in accordance with the invention are shown in FIG. 3 A. The bender of FIG. 3 A includes an electro-active layer 31 sandwiched between two electrode layers 32, 33. In contrast to the unimorph configuration shown in FIG. 2, both electrodes are used exclusively to shape the electrical field across the active layer 31. Also, in contrast to the known unimorph benders, the electro-active layer includes no passive layer, but forms a single homogeneous active layer. Therefore it appears appropriate to refer to a device in accordance with the invention as "monomorph". The monomorphic structures of this invention are distinguishable from other monolithic devices by either its geometric features, in particular the r/t ratio as described below, or by the absence of a passive layer, i.e., a layer of material that is not electro-active.

In FIG. 3 A, the thickness of the electro-active layer is denoted as t. At t/2 a dashed curve 34 is shown. This dashed circle 34 indicates the curvature at one point on the centre line of the layer 31. Its radius is denoted as the mean radius r. As the section of the bender has a semi-circular shape, all points located on the midpoint line or plane 34 of this example have an equal radius r of curvature. In general this is not the case and the curvature may change from point to point along the midplane.

It is an important feature of the invention to decrease the ratio of r/t. From the definitions of r and t it follows that this ratio is larger than 0.5 - a value that is approached as the bend layer 31 degenerates into a solid semi-cylindrical shape. A second limiting case is a perfectly flat layer where r and the ratio of r/t are infinite. It is the aim of the invention to use configurations that approach the lower limiting value, hence are close to 0.5. The graph of FIG. 3B shows the amount of energy that can be extracted per unit volume of the device as the ratio r/t is increased from 0.5. The calculation shows a distinct peak at a ratio r/t of 1.2. In general, it should therefore be a design objective to remain close to that value for a device used dynamically.

It is believed that in the devices based on the invention, the d33 effect has a predominant role in causing the displacement. In known devices, particularly the bimorph cantilever and other devices based thereupon, it is the d31 effect that contributes most to the displacement of the bender. Both d33 and d31 are well known coefficients to describe the volume change of a piezoelectric material in relation to the electrical field applied to it. From the above general structure, there can be derived a number of geometrical configurations that firstly increase the curvature 1/r (given that increasing the thickness is straight-forward) and secondly increase the section of the layer exhibiting significant curvature.

In FIG. 4, a wave shaped structure is shown. Such structures per se are known for example as bimorphs with an apparently larger ratio r/t from the U.S. patent no. 3,816,774. The piezoelectric layer made of PZT is interposed between two electrodes 42, 43. The electrodes are at least one order, preferably two orders of magnitude thinner than the active layer 41. The whole structure has a wave form assembled from semi-circular sections as shown in FIG.4 in a longitudinal cross-section. Both electrodes are connected to a voltage source (not shown). By applying an appropriate voltage across the layer 41, the free end of the wave shaped structure is displaced. Relative to its total active length, the bender of FIG. 4 has many sections that are strongly curved over more than 75 per cent of the length.

Another known geometry providing large sections of high curvature, i.e., small radii of curvature are described in the published international patent application WO-01/47041. The devices of WO-01/47041 include what is referred to as "twice- coiled" structures having a helically wound piezoelectric tape with one (minor) helix being bend itself into a curve, spiral or (major) helix. In FIG. 5, a twice-coiled helix is shown including a monolithic layer 51 firstly coiled into a minor helix with a radius to thickness ratio of under 2 and then bent into the three-quarter winding of the major helix 50. The electro-active layer is activated using a first (outer) electrode 52 and a second (inner) electrode 53 to which a voltage is supplied in a manner well known and illustrated above. When keeping one end rigidly mounted, the other end of the actuator moves in direction of the axis of the major helix 50. The three-quarter-turn major helix 50 is shown to better illustrate the device. However, adding more turns by selecting an appropriate pitch angle and radius of the major helix can produce devices exhibiting larger displacement. As above, the piezoelectric tape has a very small ratio of r/t for most of its length.

It will be appreciated that the present invention offers the possibility to efficiently build a variety of curved piezoelectric device that previously depended on bi-morph structures to generate sufficient displacement. In most cases, it is anticipated that redesigning the known device to one having smaller radii of curvature suffices for a transition to a monomorph device. The devices according to this invention can therefore be built with a characteristic length (total diameter or length) of 1cm or less and layer thickness in the range of 0.3cm, even 0.1 cm and less.

A process of manufacturing such a device usually includes the step of a manufacturing a flat straight "green" tape having the desired thickness, width and length. The tape in its green state is then wound round some plastically deformable former or mandrel. The former with the tape is then wound round a second former having an outer diameter that matches the desired inner diameter of the major helix or bent of the device.

It is apparent to a skilled person that manufacturing such structures is a delicate task given that the most efficient piezoelectric materials are brittle ceramics requiring electrodes, polarization and sintering. The difficulties are compounded by the fact that a high displacement device required a bimorph configuration with central electrodes. By implementing these structures using a monomorphic layer with surface electrodes in accordance with the present invention the manufacturing can be simplified and, hence, made more cost efficient. In fact, it may be possible to employ an extrusion process to mass- manufacture such structures. Extrusion can be designed as a continuous process, in which a plastically deformable rod is pulled off a reel at one end, PZT 'dough' that has been mixed with binders and/or plasticizer is fed into an Archimedes screw (or other ram) and is extruded onto the rod whilst at various points electrode ink and resinate are printed onto the rod and PZT and at the other end of the process the formed PZT/electrode combination tube is helically cut forming for example the minor helix of the double-wound actuator described above. After sfretching the minor coil it can be wound into a secondary coil and finally cut into single turn lengths and placed on a tray ready for sintering. The individual stages are now described in more detail.

The first stage of the process as shown in FIG. 6 A requires a thermoplastic rod 61, used as the carrier on which the device is formed, to be pulled off a reel (not shown) at a constant speed. A platinum resinate is then roller-printed using a drum 62 onto the surface of the rod. The drum 62 rotates round the rod 61, covering the circumference in a thin layer of the resinate. The rod 61 continues by passing through the centre of a conventional extrusion tool (generally 63). As the rod 61 passes through this extrusion tool, a layer of PZT dough 64 is co-extruded onto the surface of the rod 61. The resinate, being still wet (and inherently not adhering very well to the polyethylene rod material), is picked up onto the inner surface of the PZT layer 64, so that the rod now has a well-controlled layer of PZT with an inner electrode around it.

Once the rod with its layer 64 of PZT has emerged from the extrusion tool 63 a further print roller 72 applies resinate to the outer surface of the PZT layer 64 to form thereon the outer electrode, as shown in Fig. 7. After the outer electrode has been printed, the resulting cylinder of green elecfroded tape is then cut into a helix using a cutter 73 that rotates as the layered rod 61 advances. The angle of the cutter 73 must be accurately set to correlate with the speed of the rod 61 and the rotational speed of the cutter to achieve the required helix pitch. This stage produces a helically-cut tube 81; the next stage adjusts it and curves it, to make the required strongly curved and coiled monomorph. Fig. 8 shows this: having been cut into a helix, the structure 81 is gently heated, to soften the thermoplastic PE rod 61, and is then stretched by means not shown to set the appropriate gap and pitch angle of the bimorph (the tensioning at this stage is also used to pull the structure through the machine). Then, whilst still warm, the stretched structure 81 is bent by winding it around a post 83 to form the secondary turn of the helix.

And finally the thus strongly curved and coiled structure is then sliced to form the completed geometry - here depicted as a single turn of coiled PZT/electrode composite, and the individual coiled-coils thus formed are placed in a tray 84 for a subsequent sintering step (not illustrated). After sintering the devices are polarized applying a several hundred volts across the electrodes.

In FIG. 9 the above steps are summarized as steps 91 to 98 in a flowchart.

The above-described process can be varied with respect to compositions, binder and electrode material, temperatures etc. Also the electroding step may be set at a different stage within the process. However, these and other variants of the process are considered to be well within the scope of the present invention.

It will be appreciated by a skilled person that the newly found advantageous range of r/t can equally applied to bimorphs or more complex electro-active benders adding more intermediate steps to the manufacturing process described above.

Claims

1. An electro-active actuator comprising an extended layer of electro-active material, wherein the layer is curved, characterized in that at one or more portions along the length of the layer, the ratio r/t of the radius of curvature r of the layer to the thickness t of the layer is in the range of 0.5 to 4.

2. The actuator of claim 1 , wherein the ratio r/t is in the range of 0.5 to 4 for at least 30 per cent of the length of the layer.

3. The actuator of claim 1, wherein the ratio r/t is in the range of 0.5 to 4 for at least 50 per cent of the length of the layer.

4. The actuator of claim 1, wherein the ratio r/t is in the range of 0.5 to 4 for at least 70 per cent of the length of the layer.

5. The actuator of claim 1, wherein the ratio r/t is in the range of 0.6 to 3.

6. The actuator of claim 1 , wherein the ratio r/t is in the range of 0.6 to 2.

7. The actuator of claim 1, essentially being a monomorph actuator.

8. The actuator of any one of the preceding claims, wherein the layer is a monolithic layer of electro-active material having electrodes only on its outer surfaces.

9. The actuator of claim 8, wherein the thickness of the electrodes is 10 per cent or less of the thickness t of the electro-active layer.

10. The actuator of claim 8, wherein the thickness of the electrodes is one per cent or less of the thickness t of the electro-active layer.