Actuator device based on an electroactive or photoactive polymer

Abstract

An actuator device comprises an electroactive or photoactive polymer arrangement having an effective length over which expansion or contraction is induced by actuation. The effective length is greater than the maximum linear physical dimension of the space occupied by the polymer arrangement. In this way, a compact design is provided which can support a large actuation displacement.

Claims

1. An actuator device comprising: a stack of at least a first and a second polymer units connected by an intermediate layer, wherein the first polymer unit has a first end and a second end, wherein the second end is opposite the first end, wherein the second polymer unit has a first end and a second end, wherein the second end is opposite the first end, wherein the intermediate layer has a first end and a second end, wherein the first end is opposite the second end, wherein the first end of the first polymer unit is connected to the first end of the intermediate layer, and the second end of the second polymer unit is connected to the second end of the intermediate layer, and wherein the intermediate layer comprises at least a third polymer unit; and a drive unit, wherein the drive unit is arranged to operate in at least a first, a second, and a third modes, wherein in the first mode, the first polymer unit and the second polymer unit are driven off and the third polymer unit is driven on, wherein in the second mode, the first and the second polymer units are driven on and the third polymer unit is driven off, and wherein in the third mode, the first, second and third polymer units are driven off, and the first, second, and third polymer units have the same length.

2. The actuator device as claimed in claim 1, further comprising: a fixed end at the first end of the second polymer unit; and a free end at the second end of the first polymer unit.

3. The actuator device as claimed in claim 2, wherein the free end has a rest position in the third mode, wherein the free end is displaced in a first direction in the first mode, and wherein the free end is displaced in a second direction in the second mode.

4. The actuator device as claimed in claim 1, wherein the layers of the stack comprise a single folded layer.

5. The actuator device as claimed in claim 1, wherein the polymer units are arranged within a plane.

6. The actuator device as claimed in claim 1, further comprising a clamp arrangement for clamping the stack in the stacking direction.

7. The actuator device as claimed in claim 1, wherein each polymer unit comprises an electroactive polymer and actuation comprises electrical driving.

8. The actuator device as claimed in claim 1, wherein at least a portion of the first polymer unit overlaps at least a portion of the second polymer unit.

9. The actuator device as claimed in claim 1, wherein each of the first second, and third polymer units is electroactive.

10. The actuator device as claimed in claim 1, wherein each of the first second, and third polymer units is photoactive.

11. The actuator device as claimed in claim 3, wherein the first direction is opposite the second direction.

12. The actuator device as claimed in claim 4, wherein a plurality of segmented electrodes are arranged to define the first, second, and third polymer units.

13. The actuator device as claimed in claim 5, wherein an effective length of the actuator device is along a direction within the plane.

14. The actuator device as claimed in claim 6, wherein the clamp arrangement is arranged to prevent bucking.

15. The actuator device as claimed in claim 1, wherein each polymer unit comprises a photoactive polymer and actuation comprises optical driving.

16. An actuator device comprising: a first polymer unit, the first polymer unit comprising a first end and a second end; a second polymer unit, the second polymer unit comprising a third end and a fourth end; an intermediate layer, the intermediate layer comprising a fifth end a sixth end, wherein the first end is connected to the fifth end, wherein the fourth end is connected to sixth end, and wherein the intermediate layer comprises at least a third polymer unit; and a drive unit, wherein the drive unit is arranged to operate in at least a first mode, a second mode, and a third mode, wherein, in the first mode, the first polymer unit and the second polymer unit are driven off and the third polymer unit is driven on, wherein, in the second mode, the first polymer unit and the second polymer unit are driven on and the third polymer unit is driven off, and wherein in the third mode, the first polymer unit, the second polymer unit, and the third polymer unit are driven off, and the first polymer unit, the second polymer unit and the third polymer unit have the same length.

17. The actuator device as claimed in claim 16, wherein the first end is aligned with the third end.

18. The actuator device as claimed in claim 16, wherein each polymer unit comprises an electroactive polymer and actuation comprises electrical driving.

19. The actuator device as claimed in claim 16, wherein each polymer unit comprises a photoactive polymer and actuation comprises optical driving.

20. The actuator device as claimed in claim 16, wherein at least a portion of the first polymer unit overlaps at least a portion of the second polymer unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a known electroactive polymer device which is not clamped;

(3) FIG. 2 shows a known electroactive polymer device which is constrained by a backing layer;

(4) FIG. 3 shows a first example of EAP actuator device in two different modes;

(5) FIG. 4 shows a second example of EAP actuator device in three different modes;

(6) FIG. 5 shows a third example of EAP actuator device;

(7) FIG. 6 shows a fourth example of EAP actuator device;

(8) FIG. 7 shows a fifth example of EAP actuator device in two different modes; and

(9) FIG. 8 shows a sixth example of EAP actuator device in two different modes.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(10) The invention provides an actuator device comprising an electroactive or photoactive polymer arrangement having an effective length over which expansion or contraction is induced by actuation. The effective length is greater than the maximum linear physical dimension of the space occupied by the electroactive or photoactive polymer arrangement. In this way, a compact design is provided which can support a large actuation displacement.

(11) The detailed description below is based on various EAP actuator designs, but the same approaches apply equally to photoactive polymer designs.

(12) FIG. 3 shows a first possible example of EAP actuator device, which does not fall within the scope of the invention, in two different modes.

(13) The device comprises an electroactive polymer arrangement 20 which comprises a stack of two overlapping electroactive polymer units 22, 24 connected by an intermediate layer 26. The end of the top electroactive polymer unit is connected to a first end (the left end in FIG. 3) of the intermediate layer 26, and the bottom electroactive polymer unit 34 is connected to an opposite end (the right end in FIG. 3) of the intermediate layer 26.

(14) This defines a zig-zag structure, with a staggered overlapped pair of electroactive polymer units. The actuator is fixed at one end, such as the left end of the bottom actuator 24 as shown in FIG. 3.

(15) The actuator has an effective length which contributes to the overall expansion or contraction which is at least the combination of the lengths of the two electroactive polymer units 22, 24. This is thus greater than the maximum linear physical dimension of the space occupied by the electroactive polymer arrangement.

(16) This physical dimension is typically simply the length between end points. The maximum physical dimension may be taken to be that which is present when all EAP units are turned off (which will equate to the length L shown in FIG. 3) or it may be taken to be the greatest length which arises as between all actuation states. In this latter case, the maximum physical dimension for the example of FIG. 3 is the overall length in FIG. 3(b).

(17) As explained above, the effective length Le may be considered to be the original length (of a single linear actuator) which would give rise to an observed absolute change in length for a given actuation signal.

(18) For a series connection of actuator devices, the effective length will be equal to the sum of the lengths which contribute in a series manner to the overall displacement, which lengths are measured when the device is in a non-actuated mode. Thus, for the example of FIG. 3, the effective length is the sum of the lengths of the two EAP units in their at-rest positions in FIG. 3(a), or the sum of all three units in FIG. 3(a) if they are all electroactive polymer units (these different options are discussed below). The effective length is the lengths in the non-actuated mode which contribute in a combinatory way to the expansion or contraction when actuated.

(19) Indeed, in the example of FIG. 3, the intermediate layer 26 may also be an electroactive polymer unit, such that the stack comprises a top, a middle, and a bottom electroactive polymer unit. By having a three-layer stack, the middle unit can be in its short mode when the top and bottom units are in their long mode. In this example, the middle unit contracts when the device switches from the short mode to the long mode, but this reduction in length contributes in a combinatory way to an increase in overall length of the device.

(20) In this way, in a first mode shown in FIG. 3(a), the top and bottom electroactive polymer units 22, 24 are driven off (in their short mode) and the middle electroactive polymer unit 26 is driven on (in its long mode). Note that alternative designs may be driven on to their short mode and off to their long mode.

(21) In a second mode shown in FIG. 3(b), the top and bottom electroactive polymer units 22, 24 are driven on (in their long mode) and the middle electroactive polymer unit is driven off (in its short mode).

(22) In this design, in the first mode, the three electroactive polymer units have the same length. Thus, they have dissimilar inherent lengths (i.e. when all are turned off).

(23) It can be assumed that the displacement D of an individual unit follows the relationship:
D=L

(24) where L is the at rest length and c is the (engineering) strain.

(25) This design thus makes use of a multi-body stack of EAP units with alternating mechanical connections in combination with an alternating on-off driving signal. If the driving signal is not alternated the multi-body stack has the same displacement as a single actuator. By alternating the driving signals of the single actuators the total tip displacement is multiplied by the number of actuators.

(26) In a preferred embodiment, the length of the shorter actuator when extended (V=on) is equal to the length L of the longer actuator when in its non-extended mode (V=off). Therefore the length of the shorter EAP unit 26 when non-extended is L(1/(1+)).

(27) In this manner, the stack displays the smallest external dimension (L) and the maximum possible forward displacement.

(28) A controller is used to drive the actuator segments in an alternating way to have a maximum (static or reciprocating) tip movement. By varying the number of activated segments the tip displacement can be varied incrementally between zero and the maximum displacement.

(29) The structure of FIG. 3 which has n=3 electroactive polymer units gives a displacement difference between FIG. 3(a) and FIG. 3(b) of:
D=L((1/1+)+2)

(30) For small , this approximates D=L3.

(31) The effective length as defined above of the design of FIG. 3 is thus 3L, where L is the at rest length of the units 22,24 but the at rest length of the unit 26 is L/(1+).

(32) More generally, for a stack of n layers (where n is an odd number, so that there are (n1)/2 middle units and 1+(n1)/2 top/bottom units):
D=L(((n1)/2(1/1+))+((n1)/2)+1)

(33) The effective length as defined above is simply nL.

(34) For small , this approximates D=Ln.

(35) The number of EAP units in the stack can be chosen based on the displacement needed. Thus, the design may have 3, 5, 7 or 9 units, or indeed any odd number of units. A slightly different design may have an even number of units greater than 3, in which case the actuation tip is at the same end of the device as the fixed connection.

(36) Note that each individual EAP unit may comprise a multi-layer structure.

(37) If the tip needs to provide an actuation force, a means for clamping the stack in the z-direction (i.e. the stacking direction) may be used, to prevent the stack buckling. This can be implemented by adding a downwards clamping force at the top and coating the surfaces of the actuators with very low friction material or adding an adhesive material between the stack layers with very low shear modulus but high tensile modulus in z-direction. This may for example comprise an array of thin vertical wires.

(38) FIG. 4 shows a second example of EAP actuator device in three different modes.

(39) In the example of FIG. 3, the electroactive polymer units have different lengths. In FIG. 4(a), there is shown a third mode in which the top, middle and bottom electroactive polymer units are all driven off. This may be considered to be a third mode, and it is a rest mode for the electroactive polymer units. In this mode, the three electroactive polymer units have the same effective length. Thus, they have the same inherent lengths and can all be of the same design. The actuator tip 30 is at the non-coupled end of the top unit 22, and the non-coupled end of the bottom unit 24 is fixed.

(40) FIG. 4(b) shows a first mode (similar to FIG. 3(a)) in which the middle unit is actuated and thus expanded. The actuator tip 30 moves to one side relative to its previous position.

(41) FIG. 4(c) shows a second mode (similar to FIG. 3(b)) in which the top and bottom units are actuated and thus expanded. The actuator tip 30 moves to the other side relative to its previous position.

(42) In this case, the actuation is to both sides of the device. Thus, the first and second modes provide actuation to opposite sides with respect to the rest position of the third mode of operation.

(43) This example thus provides a multi-body stack having EAP actuators with alternating mechanical connections in combination with separate driving signals per EAP unit. In this case, the amplitude of the displacement that can be reached is more than for FIG. 3 but not in one direction only. The use of units of similar lengths is advantageous for manufacturing and resonance driving.

(44) In particular, units of similar inherent length have similar resonance frequencies.

(45) The tip displacement varies between:
D=(n1)/2L; and
D=((n1)/2+1)L

(46) Thus, the total range is equal to nL.

(47) FIG. 5 shows a third example of EAP actuator device. In this design, the intermediate unit 26 is a rigid part. The stack then comprises two overlapping electroactive polymer units 22, 24 and a rigid connection piece between them. This design enables all of the electroactive polymer units to be driven in the same way, so simplifies the drive scheme and the electrode arrangement.

(48) The device is again operable in at least first and second modes. In a first mode, the two electroactive polymer units are driven off. In a second mode shown in FIG. 5, the two electroactive polymer units are driven on.

(49) The stack thus has alternating layers of EAP units 22, 24 and a stiff passive material 26. The end of the first (upper) EAP unit 22 is connected to one end of the passive layer. This layer is connected to the second (lower) EAP unit at the other end, creating a net displacement proportional to nL where n is the number of EAP layers only.

(50) Alternatively, the displacement may be expressed as:
D=((n1)/2+1)L, where n is the total number of layers including EAP units and passive layers.

(51) In a preferred embodiment, the length of the passive material is equal to the length of the longest actuator when in its non-extended mode (no voltage applied). In this manner, the stack displays the smallest external dimension (L) and the maximum possible displacement.

(52) The EAP units are again driven in an alternating way to have a maximum (static or reciprocating) tip movement. By varying the number of activated segments the tip displacement can be varied incrementally between zero and the maximum displacement.

(53) This example makes the driving scheme more simple, for example with only two wires, and allows for thin film electronics in embedded each passive layer.

(54) All of the designs above may be extended to more layers, with either an odd or even number of units, as explained above.

(55) In all of these stack designs, the layers of the stack may comprise a single folded layer, with segmented electrodes to define the different electroactive polymer units.

(56) FIG. 6 shows a single folded EAP sheet with segmented electrodes, and non-electrode areas located at the bends.

(57) The single folded sheet 60 defines all three EAP units. If no electrodes are provided in the middle unit, it may be a static layer as in the example of FIG. 5. Between adjacent layers in the stack, there is a low friction interface 62. This may function as a clamp arrangement for clamping the stack in the stacking direction to prevent bucking. The low friction enables relative sliding. The interface 62 may also provide electric shielding between the different electrode arrangements.

(58) Each driven unit in the stack has a top electrode 64 and a bottom electrode 66. In FIG. 6, all three units are driven, so there are six electrode lines 67, and drive signals are applied between each pair. FIG. 6 schematically shows a drive unit 68 for delivering the required drive signals to the electrode lines 67. As shown in FIG. 6, when the top and bottom electrode pairs have a zero signal, the middle pair has a high voltage, and when the top and bottom electrode pairs have a high voltage signal, the middle pair is at zero.

(59) The bends in the sheet 60 have no metallization.

(60) The top and bottom outer surface have further sheets 70 to prevent buckling.

(61) One end of the device functions as the movable tip, and there is a tip interface 72 for interfacing with a device to be actuated. The tip interface is connected to the end of the sheet 60 and also to the top anti-buckling layer 70. The other end of the sheet is attached to the bottom anti-buckling layer by glue 74 and this defines a static point for the design of FIG. 6.

(62) The electroactive polymer units are planar in the design above, and the effective length is along a direction within the plane.

(63) The examples above make use of a three-layer stack. More layers may be added to the stack to increase the displacement. However, a more simple structure (not within the scope of the invention as claimed) may also make use of a two layer stack without intermediate layer. In a first mode, the bottom actuatable polymer layer is driven off and the top actuatable polymer layer is driven on; whereas in a second mode the bottom polymer layer is driven on and the top polymer is driven off By having one end of the top polymer layer externally fixed, and the other end of the top polymer layer attached to the bottom polymer layer, the free end of the bottom polymer becomes the actuator tip. It moves each side of the external fixation by the expansion amount, depending which polymer layer is actuated.

(64) Thus, the intermediate layer is not needed for all possible examples of device. The advantage it provides is that the actuator tip can be positioned remotely from the external fixation.

(65) In a further set of examples, the electroactive polymer arrangement is coiled, and the outer tip of the coil comprises the actuation tip. This provides an alternative way to reduce the linear size of the electroactive polymer arrangement while maintaining an effective length larger than that linear size.

(66) FIG. 7 shows a first coiled design. The maximum external dimension of the unit is the outer diameter, shown as L. The coil is fixed in the center. When the coil is actuated, the free end moves. In FIG. 7, the free end moves in a straight line.

(67) FIG. 8 shows a second coiled design. The maximum external dimension of the unit is again the outer diameter, shown as L. The coil is again fixed in the center. In FIG. 8, the free end moves in a curve.

(68) To constrain the movement in the manner desired, an external guide rail arrangement is used.

(69) In all examples, incremental tip displacement may be enabled by using a simple on-off driving scheme.

(70) The individual units in the stack may for example have dimensions of 10 mm10 mm0.5 mm. A typical and non-limiting size range may be from 50 mm50 mm2 mm to 2 mm2 mm0.1 mm. The shape in plan view may be square, but it may instead be an elongate rectangle.

(71) Materials suitable for the EAP layer are known. Electro-active polymers include, but are not limited to, the sub-classes: piezoelectric polymers, electromechanical polymers, relaxor ferroelectric polymers, electrostrictive polymers, dielectric elastomers, liquid crystal elastomers, conjugated polymers, Ionic Polymer Metal Composites, ionic gels and polymer gels.

(72) The sub-class electrostrictive polymers includes, but is not limited to:

(73) Polyvinylidene fluoride (PVDF), Polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), Polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (PVDF-TrFE-CFE), Polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) (PVDF-TrFE-CTFE), Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyurethanes or blends thereof.

(74) The sub-class dielectric elastomers includes, but is not limited to: acrylates, polyurethanes, silicones.

(75) The sub-class conjugated polymers includes, but is not limited to:

(76) polypyrrole, poly-3,4-ethylenedioxythiophene, poly(p-phenylene sulfide), polyanilines.

(77) Additional passive layers may be provided for influencing the behavior of the EAP layer in response to an applied electric field.

(78) The EAP layer of each unit may be sandwiched between electrodes. The electrodes may be stretchable so that they follow the deformation of the EAP material layer. Materials suitable for the electrodes are also known, and may for example be selected from the group consisting of thin metal films, such as gold, copper, or aluminum or organic conductors such as carbon black, carbon nanotubes, graphene, poly-aniline (PANI), poly(3,4-ethylenedioxythiophene) (PEDOT), e.g. poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). Metalized polyester films may also be used, such as metalized polyethylene terephthalate (PET), for example using an aluminum coating.

(79) The materials for the different layers will be selected for example taking account of the elastic moduli (Young's moduli) of the different layers.

(80) Additional layers to those discussed above may be used to adapt the electrical or mechanical behavior of the device, such as additional polymer layers.

(81) The EAP devices may be electric field driven devices or ionic devices. Ionic devices may be based on ionic polymer-metal composites (IPMCs) or conjugated polymers. An ionic polymer-metal composite (IPMC) is a synthetic composite nanomaterial that displays artificial muscle behavior under an applied voltage or electric field.

(82) IPMCs are composed of an ionic polymer like Nafion or Flemion whose surfaces are chemically plated or physically coated with conductors such as platinum or gold, or carbon-based electrodes. Under an applied voltage, ion migration and redistribution due to the imposed voltage across a strip of IPMCs result in a bending deformation. The polymer is a solvent swollen ion-exchange polymer membrane. The field causes cations travel to cathode side together with water. This leads to reorganization of hydrophilic clusters and to polymer expansion. Strain in the cathode area leads to stress in rest of the polymer matrix resulting in bending towards the anode. Reversing the applied voltage inverts the bending.

(83) If the plated electrodes are arranged in a non-symmetric configuration, the imposed voltage can induce all kinds of deformations such as twisting, rolling, torsioning, turning, and non-symmetric bending deformation.

(84) As mentioned above, the mechanical designs explained above may also be applied to photoactive materials. Such photo-mechanically responsive materials for example consist of photo-mechanically responsive molecules embedded in a polymer matrix. The photo-mechanically responsive molecules change shape as a function of an applied illumination with light of a certain wavelength.

(85) The most common photo-mechanical materials are driven by a shape change between two isomeric states in the photoresponsive molecules present in the material. The switch between the trans configuration is driven by illuminating with light corresponding to the absorption wavelength of the molecule in the trans state while the switch back to the metastable cis form can be thermally induced or once again triggered by illuminating with light of a wavelength which corresponds to the absorption wavelength of the molecule in its cis state.

(86) Such materials include but not limited to anthracenes, diarylethenes, spiropyrans and azobenzenes, including substituted azobenzenes such as aminoazobenzenes and pseudostilbenes. These photo-mechanically responsive molecules are embedded in a polymer material, either via doping of host polymers with such functional photo-mechanically responsive molecules or via covalently bonded molecules with photo-mechanical functionality in the polymer via polymerization. This includes polymerization of azofunctionalized monomers or post-functionalization of polymers with a different backbone, creating side-chain azofunctionalized polymers.

(87) Polymers can be amorphous or liquid crystalline (LC) in nature. Where amorphous polymers contract equally in all directions, LC polymers can be made to deform in a preferential direction, greatly increasing the effect in that direction. Liquid crystal elastomers (LCE's) are especially suitable materials for photo mechanical materials, since they can have LC induced directionality in an all solid state polymer material.

(88) This invention is of particular interest for actuators which are designed to have an in plane linear expansion response. Some types of material (for example ionic EAPs have a bending response or an in-plane expansion response depending how they are driven or constrained. Thus ionic EAPs and photo actuated materials may be configured to provide the desired in plane expansion, for example by actuating from both opposite sides of a layer of material.

(89) The invention can be applied in many EAP and photoactive polymer applications, including examples where a passive matrix array of actuators is of interest.

(90) In many applications the main function of the product relies on the (local) manipulation of human tissue, or the actuation of tissue contacting interfaces. In such applications EAP actuators for example provide unique benefits mainly because of the small form factor, the flexibility and the high energy density. Hence EAP's and photoactive polymers can be easily integrated in soft, 3D-shaped and/or miniature products and interfaces. Examples of such applications are:

(91) Skin cosmetic treatments such as skin actuation devices in the form of a responsive polymer based skin patches which apply a constant or cyclic stretch to the skin in order to tension the skin or to reduce wrinkles;

(92) Respiratory devices with a patient interface mask which has a responsive polymer-based active cushion or seal, to provide an alternating normal pressure to the skin which reduces or prevents facial red marks;

(93) Electric shavers with an adaptive shaving head. The height of the skin contacting surfaces can be adjusted using responsive polymer actuators in order to influence the balance between closeness and irritation;

(94) Oral cleaning devices such as an air floss with a dynamic nozzle actuator to improve the reach of the spray, especially in the spaces between the teeth. Alternatively, toothbrushes may be provided with activated tufts;

(95) Consumer electronics devices or touch panels which provide local haptic feedback via an array of responsive polymer transducers which is integrated in or near the user interface;

(96) Catheters with a steerable tip to enable easy navigation in tortuous blood vessels.

(97) Another category of relevant application which benefits from such actuators relates to the modification of light. Optical elements such as lenses, reflective surfaces, gratings etc. can be made adaptive by shape or position adaptation using these actuators. Here one benefit of EAPs for example is a lower power consumption.

(98) Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.