Abstract:
Electro-active actuators ( 310 ) and methods of manufacturing such actuators are described with such actuators ( 310 ) having a plurality of electro-active segments ( 311 ) arranged along a curved line with each segment ( 311 ) being essentially planar and coupled to adjacent segments ( 311 ) through joint sections ( 312 ) placed at opposite corners of the segment ( 311 ). The actuators ( 310 ) exhibit a displacement in a different direction than the direction of displacement of a segment when seen in isolation. They can be made from a continuous tape of material, preferably by die punching.

Description:
FIELD OF THE INVENION  
       [0001]     This invention relates to curved elements of electro-active material. More particularly, the invention related to electro-active actuators displaying a translational movement when energized.  
       BACKGROUND OF THE IVETION  
       [0002]     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 and piezoresistive material.  
         [0003]     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.  
         [0004]     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 stiffiess of electro-active devices is determined by the electro-active material, which in general is crystalline, ceramic or polymer-based. However, as the electro-active effects are extremely small, e.g. in the order of  1  nrnn, the change in dimensions is relatively small and requires high voltages. Therefore, more complicated electro-active structures have been developed to achieve larger displacements.  
         [0005]     To increase the displacements, several designs have been introduced such as stacks, unimorph or bimnorph benders, recurved benders, corrugated benders, spiral or helical designs.  
         [0006]     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.  
         [0007]     A standard unimorph bender is made up of a flat piezoelectric strip bonded to a metallic shim from one side. Elongation of the strip when voltage is applied, forces the unimorph bender into a bent or curved shape. To increase the displacement range, the bimorph structures, mostly cantilevers-type structures, utilize two laminated piezoelectric layers, thus having two internal 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 been. 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.  
         [0008]     To further increase the maximum displacement of piezoelectric benders, it is known to form stacks and leveraged stacks, chains or extended sheets of benders. Such devices are described for example in the U.S. Pat. Nos. 3,816,774; 4,028,666; 5410,207 and 6,107,726. Stacked recurved actuator designs are described by J. D. Ervin and D. Brei in: “Recurve Piezoelectric-Strain-Ampliying Actuator Architecture”, IEEE/ASME Transactions on Mechatronics, Vol. 3, No.4, December 1998, 293-301.  
         [0009]     Benders, stacks, tubes and other electro-active actuators are employed in a wide array of engineering systems, ranging from mnicro-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 typically are designed to provide a desired actuation displacement or stroke over which a desired force is delivered to a given load.  
         [0010]     Depending upon design, electro-active actuators can generate a rotational or translational displacements or combinations of both movements.  
         [0011]     Comparably large translation displacements have been recently achieved by using a helical structure of coiled piezoelectric tape. Such twice-coiled devices are found to easily exhibit displacement in the order of millimetres on an active length of the order of centimetres.  
         [0012]     Whilst the piling of simple discs or blocks into stacks of electro-active material to achieve a cumulative change of dimension is a comparatively mature technique, it remains a very difficult task to join benders to stacks or to produce helices.  
         [0013]     Therefore, it is an object of the present invention to provide novel configurations of electro-active material that—whilst maintaining similar performance than stacks of benders or twice-coiled benders—are easier to manufacture.  
       SUMMARY OF THE INVENTION  
       [0014]     In view of the above objects, the present invention provides apparatus and methods as claimed in the independent claims.  
         [0015]     According to a first aspect of the invention, there is provided a chain of electro-active segments linked by linkage members or elements. The chain of segments is bent along a non-straight, curved line or contour of uniform or non-uniform curvature, so as to form for example an arc, or a part of a circle, an ellipse, a spiral, a helix or the like.  
         [0016]     Each of segments is thin or two-dimensional, i.e., its width and/or length exceeds its thickness by at least one order of magnitude. Whilst a first of the segments is typically fixed to an immobile support structure, for example a housing, linkage elements or joints bridging the gap between the abutting edges of adjacent segments provide a force-transmitting coupling of subsequent segments. The joints are alternately positioned such that a segment linked at a proximate edge or corner to a preceding segment is linked to a succeeding segment at its distal edge or corner. Effectively, the joints transfer the displacement of the distal edge of one segment to the proximate edge of the subsequent segment. Thus, the displacement of the end of one segment translates into off-setting the base of the following segment and, in fact, of the remaining chain of segments.  
         [0017]     While in principle any bender of electro-active material could be used in the inventive actuator, the segments are preferably bimorphs to increase the available amount of deflection per volt applied.  
         [0018]     If the curvature of the bimorphs alternates between negative and positive curvature, i.e., between convex and concave, the chain of segments can be designed such that the net effect is a cumulative large translational displacement between the first and the last segment of the chain in a direction out of the plane of the curved line along which the successive segments are arranged.  
         [0019]     As the chain of segments follows an arc or a similar curved path, the actuator has a centre opening. This opening can be advantageously used for translational movements of components that require edge-mounting, such as lenses.  
         [0020]     The devices can be assembled segment by segment using for example metallic or plastic linkage elements and suitable adhesives to join adjacent segments. However, the invention also provides a method of manufacturing the actuators described above using green tapes or sheets or electro-active material. Devices in accordance with the present invention can be thus manufactured by applying a pattern of cuts to such a tape or sheet resulting in segments and linkage being effectively one continuous electro-active device with no hetero-material placed between the segments.  
         [0021]     The tape used is preferably very thin. By using thin tape with a thickness below 50 micron or even below 25 micron, the amount of voltage required to operate the device can be significantly reduced. For applications in which the amount of force generated rather than maximum voltage is the most important design consideration, tapes with a thickness of up to lrnm or even higher may be used.  
         [0022]     In a preferred method, two concentric arcs or circles are cut or punched into a sheet of electro-active material together with a set of radial cuts alternately starting from the outer and inner arc or circle and, whilst extending beyond the middle line between the two arcs, not cutting through the opposite arc, thus leaving linkage elements of electro-active material between the segments so generated. In another preferred method, a tape is cut from opposite edges in a comb-like manner and then bent into the desired shape of the actuator. The first method results in a disk-like actuator whilst the second method can be advantageously used to manufacture a ring-like or cylindrical device.  
         [0023]     Electro-active materials for use in the present invention are preferably piezoelectric materials such as PZT.  
         [0024]     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  
       [0025]     In the drawings:  
         [0026]      FIG. 1A  is a schematic side elevation of a known bimorph bender;  
         [0027]      FIG. 1B  a schematic side elevation similar to that of FIG. IA, but illustrating the deflection of the bimorph bender when a drive or deflection voltage is applied thereto;  
         [0028]      FIG. 2  is a schematic perspective view of a stack of recurved bimorph benders according to the prior art;  
         [0029]      FIG. 3A  is a schematic perspective view of an actuator in accordance with a first example of the invention;  
         [0030]      FIG. 3Y   3  is a detail view of the actuator of  FIG. 3A ;  
         [0031]      FIG. 3C  is a schematic perspective view of an actuator similar to that of  FIG. 3A , but illustrating the deflection of the actuator when a drive or deflection voltage is applied thereto;  
         [0032]      FIG. 4A  is a schematic perspective view of an actuator in accordance with a second example of the invention;  
         [0033]      FIG. 4B  is a schematic perspective view of an actuator similar to that of  FIG. 4A , but illustrating the deflection of the actuator when a drive or deflection voltage is applied thereto; and  
         [0034]      FIG. 5  is a flow chart illustrating steps of a method in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0035]     As the invention relates to novel configurations of benders, particularly bimorph benders, the general properties of bimorph benders are described before referring to examples of configurations in accordance with the present invention.  
         [0036]     Referring initially to  FIG. 1A , a typical bimorph  10  includes a 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 . Further, 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.  
         [0037]     When an electrical potential or voltage from a source  15  is applied across each of the piezo-ceramic elements  11  and  12 , for example, between outer and inner electrodes  14 ,  13 , as shown on  FIG. 1B , the element or plate  11  contracts and element or plate  12  expands, so that birnorph 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 on  FIG. 1B .  
         [0038]     Referring now to  FIG. 2 , a known technique to stack benders is illustrated. The individual benders  21  are of the recurved type cantilevers as described for example by J. D. Ervin and D. Brei in: “Recurve Piezoelectric-Strain-Amplifying Actuator Architecture”, IEE/ASME Transactions on Mechatronics, Vol. 3, No.4, December 1998, 293-301. The benders are joined via joint sections  22  with the major face of each bender facing one another. The joint are described to include layers of epoxy adhesive with metal end caps (not shown).  
         [0039]     The displacement of each bender  21  adds to the overall displacement of the stack and, in principle, any amount of displacement can be achieved stacking benders in sufficient number. However, to stack ceramic material is a difficult task and, while accomplished with stacks of simple disks piezoelectric material, stacks of benders  21  manufactured on as commercial products are rare. The difficulties arise mainly out of the need to laminate or bond a plurality of heterogeneous layers at the joint sections  22 .  
         [0040]     The invention, therefore, provides multiple-bender configurations where the electroding and laminating of the benders can be performed on a flat layer of electro-active material. It will be appreciated by those skilled in the art that the problems usually associated with the manufacture of complex ceramic structures are thus greatly reduced.  
         [0041]     A first embodiment of the invention is described making reference to  FIG. 3A , which shows an open ring or disk  310  of sixteen piezoelectric segments  311 . For clarity, dotted lines  320  are drawn to close the ring indicative of the flat shape the actuator  310  resumes in its rest state. The first three of the sixteen segments  311  are labelled to clarify relationships between adjacent segments  311 . Each segment  311  is an essentially flat rectangular bimorph bender of lead zirconate titanate (PZT). As such, the segments  311  are arranged to bend in a direction perpendicular to the faces of the segments  311 . It will be appreciated that other shapes and electro-active materials could be used. For example, other bender structures comprising a plurality of layers arranged to create bending perpendicular to the layers could be used.  
         [0042]     As illustrated, the segments  311  are preferably of uniform size and shape. However, as this preference is mainly motivated by the resulting ease of manufacturing, other design considerations may lead to the use of two or more different types of segments.  
         [0043]     Successive segments  311  are placed along an arc of a circle, essentially forming a part-disk with a centre opening. Segments  311  may be placed along other two- or three-dimensional curves such as ellipsoids, spirals or helices.  
         [0044]     Each of the segments  311  (with the obvious exception of the first and the last segment) are linked to a preceding and succeeding segment  311  at lateral edges  311   a  and  311   b . The lateral edges  311   a  and  311   b  of two adjacent segments  311  face each other, thus forming a narrow slit or gap. The chain segments are linked via joints  312 . Each segment  31  is joined at a proximate corner and a distal corner to its adjacent neighbours. Thus the joints  312  along the distal edges  320  of the segments  311  and the joints along the proximate edges  320  of the segments  311  are arranged along respective curved lines which are parallel one another.  
         [0045]     To take full advantage of the displacement of each of the segments  311 , the joints are placed at the extreme ends of the segments  311  to increase the length of the slits or gaps between adjacent segments  311 . On the other hand, the slits are designed with minimum width so that a maximum number of segments can be placed along a given length of chain  310 .  
         [0046]     In this embodiment, the joints  312  are formed of the same electro-active material as the segments  311 , in fact, cut from the same sheet of base material. However, it is alternatively possible to have segments  311  joined via portions of heteromaterial such as metal or adhesives.  
         [0047]     Adjacent segments  311  display a different sense of curvatures when bending. When a voltage is applied, adjacent segments  311  bent as indicated in  FIG. 3B , which shows two segments  311  of the actuator  310  of  FIG. 3A . The dotted line  320  indicates where the first segment  311  is located when zero voltage is applied.  
         [0048]     The segments  311  bend in the direction of the thickness and, hence, perpendicular to the face of the benders. The benders are arranged and actuated to alternately exhibit positive or negative curvatures of bending, as indicated in  FIG. 3B  by encircled “+” and “−”, respectively.  
         [0049]     The cumulative effect on the actuator is shown in  FIG. 3C . Though each segment  3   11  basically exhibits the typical rotational bender motion, the sum of the all displacements along the curve the segments  311  are placed on results in a translational movement perpendicular to the main face of the actuator  310 . One way to visualise the displacement is that the bending of successive segments  311  with alternate curvatures causes twisting of the actuator  310  around the curved line along which successive segments  311  are arranged, which twisting causes displacement of the actuator  310  out of the plane of the curved line. The arrow  323  indicates such cumulative displacement out of the plane defined by lines  320 .  
         [0050]     Referring now to another embodiment of the invention, as shown in  FIGS. 4A and 4B , an actuator  410  comprising a plurality of segments  411  of piezoelectric material arranged in a part circle and joined via bridge elements from the same material. This embodiment and the previously described embodiment of  FIG. 3  differ in that the segments  411  are arranged parallel to the main axis of displacement giving the actuator  410  an essentially cylindrical shape. As above, dotted lines  420  indicate the base plane out of which the actuator bends when energized. The segments  411  when seen in isolation from their respective neighbours bend alternately towards or away from the centre of the cylinder defined by the arrangement of segments. An arrow  423  in  FIG. 4B  indicates the direction of the cumulative motion of the actuator  410  being essentially parallel to the main axis of this cylinder.  
         [0051]     Making now reference to  FIG. 5 , a method of manufacturing the electro-active actuators according to the present invention is described.  
         [0052]     As a first step  51  a sheet or tape of electro-active material is prepared using any of the known methods of making such sheets or tapes. Examples of such methods include tape casting, compaction, injection molding or co-extrusion. The latter two methods are, for example, described by C. Near et al. in: SPIE Vol. 3326, 323-331 and by A. T. Crummn et al. in: SPIE Vol. 3224 (March 1998), 20-27, respectively.  
         [0053]     For example, in a typical tape casting process a slurry of PZT powder is prepared including the powder and solvent (on an alcoholic base), together with binders, plasticizers and dispersing agents. After ball-miling to ensure a complete de-agglomeration of its solid particles, sieving and degassing, the slurry is cast onto a polymer carrier. A system of Doctor blades is typically used to produce a tape of uniform thickness. After drying, this tape is in a state often referred to as “green” in the art.  
         [0054]     The green tape is within limits flexible, compliant and can be easily cut using blades or any other cutting process  52 , such as laser cutting. Because of its speed, a punch die process using a pre-formed die with sharp edges in accordance to the pattern to be cut from the tape or sheet is preferably employed to cut segments and links in a single step from the remaining material.  
         [0055]     Electrodes are applied by a process known as screen-printing using an “ink” (paste) of platinum or silver palladium powder mixed with a binder and a solvent. After applying the paste, the tape is dried again at about 100 degrees Celsius. For a bimorph structure, two single layers are stacked and pressed at 70 degrees Celsius. At this stage  53 , the tape is still compliant and can be formed into more three-dimensional shapes such as helices or spirals.  
         [0056]     After applying the electrodes and the lamination, the tapes buned-out at 600 to 800 degrees Celsius to evaporate organic residues. And as a final step  54 , the material is hardened or sintered at temperatures above 1000 degrees Celsius.  
         [0057]     After sintering the devices are polarized applying a several hundred volts across the electrodes.  
         [0058]     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.