Mechanical amplifier

Abstract

A mechanical amplifier comprises a frame 1 extending parallel to a plane XY and perpendicular to a thickness direction Z. The frame comprises a plurality of rigid portions 2a-h connected by a pluralit

Description

Field of the invention The present invention relates to a composite mechanical amplifier for a transducer. In particular, the mechanical amplifier may be formed thin perpendicular to a plane, whilst being constrained to reduce or avoid buckling out of the plane. The mechanical amplifier may be used for applications including mounting input panels to provide haptic feedback. Background Transducers operating on principles such as the piezoelectric effect, the shape memory effect (triggered for example by resistive heating) and so forth may be electrically actuated. This provides for ready integration into electronic devices for a variety of purposes such as providing haptic feedback, precise control of positioning, motion compensation, and so forth. Such transducers have a problem that, although the forces generated may be large, the total range of movement, sometimes termed "stroke length", is typically small. For example, some piezoelectric transducers may have a range of motion in the range of tens of microns. In order to amplify the range of motion, mechanical amplifiers may be used in combination with electrically actuatable transducers such as piezoelectric / shape memory alloy transducers. Issues encountered during the design of a mechanical amplifier include the need to constrain the amplified motion to a desired direction and / or plane, and avoiding buckling of structural elements providing the mechanical amplification. Consequently, mechanical amplifiers used for such purposes have typically been relatively bulky, as well as being often complex and / or expensive to manufacture. CN 112928946 A describes a bridge type flexible displacement amplification mechanism including an upper half bridge flexible amplification module, a lower half bridge flexible amplification module, a rigid connection block and a driver. The upper half-bridge flexible amplification module and the lower half-bridge flexible amplification module are both arranged between the two rigid connecting blocks to form an annular frame, and the driver is arranged between the two rigid connecting blocks in the annular frame. According to this document, accurate constraint design is carried out on the bridge type flexible displacement amplification mechanism; the mechanism is accurate in kinematics constraint, simple and compact in structure and stable and reliable in movement; and the body structure can be integrally machined and does not need to be assembled. US 10,276,776 B2 describes a smart material actuator including a layered web 5 assembly, compensator, smart material device and at least one actuating arm. The web assembly includes a first surface in operable contact with the smart material device and having at least one resilient member in operable connection with the compensator and the actuating arm. Upon activation of the smart material device, the resilient member flexes and the actuating arm moves. The web assembly is formed of 10 joined layers of inner and outer plates. Summary Unless clearly specified to the contrary, or evidently technically incompatible, the following statements are applicable to, and combinable with, the examples in the detailed description hereinafter. According to a first aspect of the invention, there is provided a mechanical amplifier including a frame extending parallel to a plane and perpendicular to a thickness direction. The frame is formed from a number of rigid portions connected together by a number of flexible portions. The frame includes a metal layer having a first thickness and a polymer layer overlying the metal layer and having a second thickness. Each of the metal layer and the polymer layer is a single piece which extends through the number of rigid portions and the number of flexible portions. The frame is configured to receive a transducer connecting between a primary end portion of the number of rigid portions and a secondary end portion of the number of rigid portions. The number of rigid portions include a first rigid portion and a second rigid portion. The frame is configured such that in response to a first displacement between the primary and secondary end portions, a resulting second displacement between the first and second rigid portions is greater than the first displacement. The frame is also configured such that the first thickness is less than 1 mm and the second thickness is sufficient to prevent buckling of the frame in the thickness direction across a range of first displacements for which the frame remains in the elastic regime. In other words, the frame is generally planar and formed from a laminate of the metal layer and the polymer layer, shaped into rigid and flexible portions when viewed along the thickness direction (i.e. in a plan-view). The difference between rigid and flexible portions arises from differences in shape when projected onto the plane. The frame may be formed directly in the desired shape, or may be produced by removing material from a uniform laminate of the metal layer and the polymer layer. When the transducer is received between the primary and secondary end portions, the first displacement will correspond to a length change (extension or contraction) of the transducer. The transducer may be a linear transducer. In other words, the transducer may change length along an axial direction (extension and / or contraction) in response to being actuated. The transducer may be electrically actuated. No rigid portion may be directly connected to another rigid portion. In other words, each rigid portion may be connected to one or more other rigid portions via respective flexible portions. When projected onto the plane perpendicular to the thickness direction, a first projected area corresponding to the metal layer may be substantially coincident with a second projected area corresponding to the polymer layer. Substantially coincident may mean that the Boolean intersection of the first and second projected areas is less than or equal to 10% of the first projected area. The component of the second displacement parallel to the thickness direction may be less than or equal to one tenth of the component of the second displacement parallel to the plane. This condition may only apply whilst deformations of the frame remain in the elastic regime. Whilst deformations of the frame remain in the elastic regime, the component of the first displacement parallel to the thickness direction may also be less than or equal to one tenth of the component of the first displacement parallel to the plane. The frame may consist of the metal layer and the polymer layer. In other words, there may not be any third or further layer (for example having matching or equivalent properties to the metal layer) which is opposed to the metal layer across the polymer layer. Put another way, the laminate forming the frame may have an asymmetric layer structure along the thickness direction, as opposed to a symmetric (or "sandwich") layer structure. The first rigid portion may provide the primary end portion. Alternatively, the first rigid portion may provide the secondary end portion. Alternatively, the second rigid portion may provide the primary or secondary end portion. However, only one of the first and second rigid portions may overlap with the primary and second end portions, otherwise mechanical amplification of the second displacement would not be possible. The number of rigid portions may include a third rigid portion providing the primary end portion and a fourth rigid portion providing the secondary end portion. In other words, the transducer may not be connected to either of the first and second rigid portions, and may instead connect between the third and fourth rigid portions when installed. A first bridge structure may connect the third rigid portion to the fourth rigid portion. The first bridge structure may include the first rigid portion and at least two flexible portions. The first bridge structure may include one or more additional rigid portions and one or more additional flexible portions. The third and fourth rigid portions may be separated along a first direction parallel to the plane. The first direction may correspond to an axial direction of the transducer when received by the frame. The first displacement may be in the first direction. The first bridge structure may be mirror symmetric about a midpoint of a line along the first direction connecting the third and fourth rigid portions (i.e. about a line intersecting the midpoint and directed perpendicular to the first direction). The first bridge structure may include a first arch structure connecting the third rigid portion to the fourth rigid portion via the first rigid portion, one or more first spanning portions of the plurality of rigid portions, and three or more flexible portions. The first arch structure may include an even number of first spanning portions, arranged symmetrically either side of the first rigid portion. The first arch structure may include an a pair (i.e. two) first spanning portions. Each first spanning portion may be elongated along an axial direction. The first bridge structure may also include a second arch structure connecting the third rigid portion to the fourth rigid portion via the first rigid portion, one or more second spanning portions of the plurality of rigid portions, and three or more flexible portions. In this way, the first rigid portion may be common to the first and second arch structures. The second arch structure may include an even number of second spanning portions, arranged symmetrically either side of the first rigid portion. The second arch structure may include an a pair (i.e. two) second spanning portions. Each second spanning portion may be elongated along an axial direction. Each second spanning portion may be oriented parallel to a corresponding first spanning portion of the first arch. A width of each second spanning portion may be less than a width of any first spanning portion. Width may be the dimension within the plane and perpendicular to the axial direction of a first / second spanning portion. A second bridge structure may connects the third rigid portion to the fourth rigid portion. The second bridge structure may include the second rigid portion and at least two flexible portions. The second bridge structure may be configured analogously to any of the hereinbefore described configurations of the first bridge structure. For example, the second bridge structure may include a single arch structure or a dualarch structure as described in relation to the first bridge structure. The second bridge structure may be mirror symmetric about the midpoint of the line along the first direction connecting the third and fourth rigid portions. The first bridge structure and the second bridge structure may both be mirror symmetric about the midpoint of the line along the first direction connecting the third and fourth rigid portions. The first bridge structure and the second bridge structure may be mirror symmetric about the line along the first direction connecting the third and fourth rigid portions. The mechanical amplifier may be configured such that the second displacement in response to the first displacement is greater than or equal to two times the first displacement. The second displacement in response to the first displacement may be greater than or equal to three times the first displacement. The second displacement in response to the first displacement may be greater than or equal to three times the first displacement. The second displacement in response to the first displacement may be greater than or equal to four times the first displacement. The second displacement in response to the first displacement may be greater than or equal to five times the first displacement. The second displacement in response to the first displacement may be greater than or equal to ten times the first displacement. Any of the conditions described hereinbefore for the second displacement may be applied instead to the ratios of the component of the second displacement parallel to the plane and the component of the first displacement parallel to the plane. The metal layer may be formed of a steel, an aluminium alloy, or a titanium alloy. The metal layer may be formed from any metal, or alloy thereof, having a stiffness (i.e. Youngs Modulus) which is 20 or more times a stiffness of the polymer layer. Equally or alternatively, the polymer layer may be formed from any material having a stiffness which is 20 or fewer times a stiffness of the metal layer. The polymer layer may include, or be formed from, one or more of polyvinyl chloride (PVC), polypropylene (PP), polyester, polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), nylon, , high-density polyethylene (HDPE), and co-polymers thereof. Preferably, the polymer layer is formed from a polymer which is suitable for injection moulding such as, for example, PP, nylon, or any other polymer known to be compatible with an injection moulding process. The frame may have been formed by insert-injection moulding the polymer layer over the metal layer. The use of insert-injection moulding to form the frame will be readily apparent from inspection of the material and microstructure of the polymer layer. In particular, polymer chains will exhibit local alignments resulting from flow patterns during the filling of the mould. The frame may have been formed by insert-casting the polymer layer over the metal layer. For example, the metal layer may be placed into a mould, then a two (or more) component mixture may be poured into the mould to cover the metal layer, before curing to form the polymer layer. The curing may occur naturally at standard temperature and pressure, for example by reaction and / or drying. Curing may additionally or alternatively be assisted and / or accelerated by application or heat and / or light (for example UV light). Examples of a two (or more) component mixture include commercially available two-component resin systems. The use of insertcasting will be readily apparent from inspection of the material and microstructure of the polymer layer. The metal layer may include a number of flaps and / or protrusions extending into the polymer layer. In this way the polymer layer may flow around the flaps / protrusions when molten / uncured, before subsequently solidifying / curing to securely grip the metal layer. This may reduce the possibility of delamination between the metal layer and the polymer layer. Flaps and / or protrusions of the metal layer may have been formed by a stamping process conducted before, during or after delineation of the shape of the metal layer. The frame may have been formed by printing the polymer layer over the metal layer. The use of printing will be readily apparent from inspection of the material and microstructure of the polymer layer. Printing may include 3D printing (also termed additive manufacturing or any type suitable for printing the material of the polymer layer. The metal layer may have been formed by selective material removal from sheet metal. The metal layer may have been shaped by laser cutting. The use of laser cutting to delineate the shape of the metal layer will remain apparent from inspection of the metal layer. For example, a heat affected zone will be evident unless the metal layer has been subjected to re-crystallisation after cutting to shape (which is neither technically nor commercially practical). Alternatives to laser cutting include, without being limited to, water-jet cutting, stamping, electrical discharge machining (EDM), chemical etching (using a mask), and so forth. Use of any specific technique can be readily determined from inspection of the cut edges of the metal layer and / or the microstructure of the metal layer proximate to the edges. Laser cutting is preferred as a balance between throughput and maintaining the mechanical properties of the metal in the flexible portions. Alternatively, the metal layer and the polymer layer may each have been formed separately, and the frame may have been formed by chemically or thermally bonding the polymer layer to the metal layer. Bonding a pre-shaped polymer layer to a preshaped metal layer will be readily apparent from inspection of the interface between the layers and the edges of the frame. Alternatively, the frame may have been formed by selective material removal from a uniform laminate of the metal layer and polymer layer. The uniform laminate may have been shaped using one or more of laser cutting, water-jet cutting, stamping and so forth. The uniform laminate may have been formed by chemically or thermally bonding a uniform polymer layer of the second thickness to a uniform metal layer of the first thickness. Selective material removal from a uniform laminate to delineate the shape of the frame may be readily distinguished from the preceding examples by inspection of the interface between the layers and the edges of the frame. A total thickness of the mechanical amplifier may be less than or equal to 3 mm. The total thickness of the mechanical amplifier may be less than or equal to 2 mm. The total thickness of the mechanical amplifier may be less than or equal to 1.8 mm. The mechanical amplifier may also include a flexure connecting the primary end portion to the secondary end portion. The flexure may be integrally formed with the metal layer. The flexure may be a separate piece connected to the frame. The polymer layer may not overlie the flexure. The flexure may serve to control pre-stressing (in tension or compression) of a transducer received by the frame. The flexure may additionally or alternatively serve to provide a restoring force for a unidirectionally actuatable transducer such as, for example, a shape memory alloy wire. An actuator assembly may include the mechanical amplifier and a transducer. The transducer may be received by the frame and may connect between the primary end portion and the secondary end portion. The transducer may be a piezoelectric transducer. Alternatively, the transducer may be a shape memory alloy (SMA) transducer. A SMA transducer may take the form of a single SMA wire opposed by a spring. For example a flexure connecting the primary end portion to the secondary end portion. Alternatively, an SMA transducer may take the form of two or more SMA wires arranged to oppose one another when actuated. Any form of linear (i.e. length changing) transducer may be included in the actuator assembly. Electrically actuatable transducers are preferred. The transducer may be pre-stressed in the actuator assembly. The transducer may be pre-stressed in tension. The transducer may be pre-stressed in compression. Pre-stress may be applied if a natural length of the transducer is different from a natural (unstressed) distance between the primary end portion and the secondary end portion. A first displacement may be applied so that the distance between the primary end portion and the secondary end portion matches the natural length of the transducer for the duration of bonding or otherwise attaching the transducer to the frame. When released, actuator assembly will adopt an equilibrium deformation in which the transducer and the frame are both pre-stressed. Within the actuator assembly, the first and second displacements may be determined with reference to the equilibrium frame shape, which is not necessarily identical to an unstrained state of the frame. The actuator assembly may be coupled between a device structure / frame / casing and a user input surface. The user input surface may include, or take the form of, a touch pad. The user input surface may include, or take the form of, a touch panel. The user input surface may include, or take the form of, a touchscreen. The user input surface may include, or take the form of, one or more discrete buttons, sliders, and / or other user input controls. The user input surface may include, or take the form of, a smart surface. The actuator assembly may be used in devices including, but not limited to, a tablet computer, a mobile phone, a handheld electronic device, a laptop computer, a display screen, a peripheral for a computer, a game controller and so forth. The actuator assembly is not limited to input / output applications, and may be used instead to provide precise positional control. For example, in devices / applications including but not limited to optical systems, cameras, vibration and / or motion compensation, speakers, micro-stages, and so forth. According to a second aspect of the invention, there is provided a method of using the mechanical amplifier, or the actuator assembly including the mechanical amplifier, to generate haptic excitations. The method of the second aspect may include features corresponding to any features of the mechanical amplifier of the first aspect and / or an actuator assembly including the mechanical amplifier of the first aspect. Definitions applicable to the mechanical amplifier of the first aspect and / or an actuator assembly including the mechanical amplifier of the first aspect (and / or features of either) may be equally applicable to the method of the second aspect (and / or features thereof). According to a third aspect of the invention, there is provided a method of manufacturing the mechanical amplifier. The method of the third aspect may include features corresponding to any features of the mechanical amplifier of the first aspect and / or an actuator assembly including the mechanical amplifier of the first aspect. Definitions applicable to the mechanical 5 amplifier of the first aspect and / or an actuator assembly including the mechanical amplifier of the first aspect (and / or features of either) may be equally applicable to the method of the third aspect (and / or features thereof). Brief Description of the drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figures 1A and IB schematically illustrate a first example of a mechanical amplifier; Figures 2A to 2C schematically illustrate an insert injection moulding process; Figures 3A to 3D schematically illustrate a metal layer for a second example of a mechanical amplifier; Figure 4A to 4C schematically illustrate the second example of a mechanical amplifier; Figures 5A and 5B schematically illustrate an actuator assembly; Figure 6 schematically illustrates integration of an actuator assembly to provide haptic excitations for a touch panel; Figure 7 schematically illustrates a third example of a mechanical amplifier; and Figures 8A and 8B schematically illustrate a modification of the first example of a mechanical amplifier. Detailed description In the following description, like parts are denoted by like reference numerals. The design of mechanical amplifiers must balance multiple constraints and considerations. For example, a mechanical amplifier needs to be stiff enough to minimise losses from the desired movement due to compliance of the materials used to form the mechanical amplifier, yet compliant enough that the output force of the transducer can deform the mechanical amplifier with sufficient resultant force to move a load (in many cases at a desired response speed / frequency). In addition, it is often required that the displacement of the transducer be amplified in a well-controlled plane or direction. These issues are often compounded when a mechanical amplifier is required to be thin, for example 3 mm of less, in a thickness direction, due to issues arising from out-of-plane buckling of structural elements of the mechanical amplifier. This can make it difficult or impossible to find the necessary balance of stiffness, amplification and constraint. The mechanical amplifier of the present specification arises, at least in part, from the inventors' realisation that a specific type of composite structure may be employed to produce thinner mechanical amplifiers suitable for a range of applications. The present specification also relates to simple, cost effective and scalable manufacturing methods for producing mechanical amplifiers. First mechanical amplifier Referring to Figures 1A and IB, the present invention will be described with reference to a first example of a mechanical amplifier (hereinafter the "first mechanical amplifier"). Figure 1A shows a side view of the first mechanical amplifier and Figure IB shows a plan view of the first mechanical amplifier. The first mechanical amplifier takes the form of a frame 1 extending parallel to a plane (x-y as illustrated) and perpendicular to a thickness direction (z-direction as illustrated). The frame 1 is formed from a number of rigid portions 2 connected together by a number of flexible portions 3. The frame 1 is formed from a metal layer 4 having a first thickness tm and a polymer layer 5 which overlies the metal layer 4 and has a second thickness tP. An overall thickness t of the frame 1 is the sum of the first and second thickness, t = tm + tP. Each of the metal layer 4 and the polymer layer 5 is a single piece which extends through all of the rigid portions 2 and flexible portions 3. In other words, the frame 1 is generally planar and is formed from a laminate of the metal layer 4 and the polymer layer 5, shaped into rigid portions 2 and flexible portions 3 when viewed along the thickness direction (i.e. along the z-axis as illustrated). The difference between rigid portions 2 and flexible portions 3 arises from differences in shape when projected onto the plane (x-y as illustrated). In particular, the flexible portions 3 are narrower, allowing the overall frame to deform in an articulated fashion (in the x-y plane) with the narrower flexible portions 3 serving as hinges for substantially rigid rotations / translations of the rigid portions 2. Preferably, the deformation of the frame 1 is substantially within the plane (x-y as illustrated). As discussed further hereinafter, the frame 1 may be formed directly in the desired shape, or may be produced by removing material from a uniform laminate of the metal layer 4 and the polymer layer 5. The first mechanical amplifier shown in Figures 1A and IB has a frame 1 including eight rigid portions 2a, ..., 2h, connected by eight flexible portions 3a, ..., 3h to form a shape which is approximately an octagon, longer along a first in-plane direction (x-axis as illustrated) than along a second in-plane direction (y-axis as illustrated). This general shape for a mechanical actuator is known, for example from CN 112928946 A. The novelty of the first mechanical actuator does not arise from the specific shape, which is used here solely for the purpose of providing a specific example. Instead, one key contribution (in addition to others explained hereinafter) lies in the formation of the frame 1 from the laminate of the metal layer 4 and polymer layer 5. The resulting composite structure provides improved capacity to balance the stiffness of the frame 1 against the output force and range of motion of a transducer 6 (Figure 5A), whilst also preventing undesirable out-of-plane buckling. The principle of using a laminate of the metal layer 4 and the polymer layer 5 to form the frame 1 is generally applicable to any previously known or new shape of mechanical actuator. The frame 1 is configured to receive a transducer 6 (Figure 5A) connecting between a primary end portion 7 and a secondary end portion 8 of the rigid portions 2. In the first mechanical amplifier shown in Figures 1A and IB, the primary end portion 7 corresponds to the rigid portion 2c, which includes a first recess 8 to receive a first end 9 (Figure 5A) of the transducer 6. The secondary end portion 10 corresponds to the rigid portion 2d in this example, and includes a second recess 11 to receive a second, opposite end 12 (Figure 5A) of the transducer 6. In the illustrated example, the first and second recesses 8, 11 extend through the entire thickness tP of the polymer layer 5, exposing the underlying metal layer 4 so that the ends 9, 12 of the transducer 6 may be attached directly between the metal layer 4 of the primary end portion 7 and the metal layer 4 of the secondary end portion 10. In other examples, the first and second recesses 8, 11 need not extend through the entire thickness tP of the polymer layer 5. The rigid portions 2 also include a first rigid portion 13 and a second rigid portion 14, which correspond to the rigid portions 2 of the frame 1 between which the output amplified movement is produced. Typically, one of the first rigid portion 13 and the second rigid portion 14 may be fixed to a supporting structure 15 (Figure 6), in order to produce movement of a load (not shown) connected to the other of the first and second rigid portions 13, 14 relative to the supporting structure 15. Whilst in some examples both the first rigid portion 13 and a second rigid portion 14 may be free to move, in the following discussion the first rigid portion 13 may also be referred to as the "mobile" rigid portion 13 and the second rigid portion 14 may also be referred to as the "fixed" rigid portion 14. In general, either of the first rigid portion 13 or the second rigid portion 14 may be fixed, whilst the other is free to move. In the first mechanical amplifier shown in Figures 1A and IB, the first rigid portion 13 corresponds to the rigid portion 2a, and the second rigid portion 14 corresponds to the rigid portion 2b. In other words, in the first mechanical amplifier shown in Figures 1A and IB the transducer 6 is not connected to the first rigid portion 2a, 13 or the second rigid portion 2b, 14, and instead will be connected between a third rigid portion 2c providing the primary end portion 7 and a fourth rigid portion 2d providing the secondary end portion 10. In the first mechanical amplifier shown in Figures 1A and IB, a first "bridge" structure 16 and a second "bridge" structure 17 connect the third rigid portion 2c to the fourth rigid portion 2d. The first bridge structure 16 in the illustrated example includes the first rigid portion 2a, 13 as a central element, connected to the third rigid portion 2c providing the primary end portion 7 by rigid portion 2e and flexible portions 3c and 3e, and connected to the fourth rigid portion 2d providing the secondary end portion 10 by rigid portion 2f and flexible portions 3a and 3f. The rigid portions 2e and 2f may also be referred to as first spanning portions 18. The rigid portions 2e, 2a, 2f and flexible portions 3c, 3e, 3a, 3f form a single "arch" first bridge structure 16. The second bridge structure 17 is similarly configured, with the second rigid portion 2b, 14 providing the central element, connecting between the third rigid portion 2c providing the primary end portion 7 and the fourth rigid portion 2d providing the secondary end portion 10 in a single arch which also includes rigid portions 2g and 2h as first spanning portions 18, and flexible portions 3d, 3g, 3b and 3h. Of course, the illustrated first mechanical amplifier is only one example of the shape of frame 1, and in other examples there may be overlap between one (but not both) of the first and second rigid portions 13, 14 and the primary and secondary end portions 7, 10 (see for example Figure 7). In general, the constraints on the frame 1 are that: • The frame 1 is formed from the laminate of the metal layer 1 and the polymer layer 5, shaped to form a number of rigid portions 2 connected together by flexible portions 3; • In response to a first displacement dtrans between the primary end portion 7 and the secondary end portion 10, a resulting second displacement damp between the first rigid portion 13 and the second rigid portion 14 is greater than the first displacement dtrans, i.e. damp >dtrans; • The first thickness tm, of the metal layer 4, is less than 1 mm; and • The second thickness tP is sufficient to prevent buckling of the frame 1 in the thickness direction (z in Figure 1A) across a range of first displacements dtrans for which the frame 1 remains in the elastic regime. In other words, to prevent buckling provided the first displacement dtrans remains small enough that no part of the frame 1 undergoes permanent deformation (whether plastically, or in any other irreversible manner such as cracking, tearing and so forth). Within these constraints, the shape of the frame 1 may be varied according to requirements of the specific application. The first mechanical amplifier shown in Figures 1A and IB is simply one example of the frame 1. In the illustrated example, the first displacement dtrans is oriented along the illustrated x-axis, shown by the arrows with reference numeral 19, and the second displacement damp is oriented along the illustrated y-axis, shown by the arrows with reference numeral 20. Preferably, any component of the second displacement damp parallel to the thickness direction (z-axis as illustrated in Figure 1A) is constrained to be less than or equal to one tenth of a component of the second displacement damp parallel to the plane (x-y as illustrated in Figure IB). This condition may only apply whilst deformations of the frame 1 remain in the elastic regime. Optionally, whilst deformations of the frame 1 remain in the elastic regime, the component of the first displacement dtrans parallel to the thickness direction may also be constrained to be less than or equal to one tenth of the component of the first displacement dtrans parallel to the plane. In this way, the stiffness of the frame 1 is dominated by the metal layer 4. For example, the metal layer is formed of a steel, an aluminium alloy, a titanium alloy, or any other metal alloy system known for use in structural parts. Alloys which possesses good fatigue resistance and high strength are preferable, because these properties will be beneficial for the flexible portions 3. The polymer layer 5 may include one or more (for example mixed as a blend) of polyvinyl chloride (PVC), polypropylene (PP), polyester, polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), nylon, high-density polyethylene (HDPE), and co-polymers thereof. Steels typically have Youngs modulus in the region of 200 GPa, aluminium alloys in the region of 65 GPa, and titantium alloys in the region of 100 GPa. By comparison, polymers typically have a Youngs modulus in the region of a few GPa and often considerably less. Thus, the stiffness of the frame 1 may be controlled using the thickness of the metal layer 4, essentially independently of the thickness tP and / or material selection of the polymer layer 5. By varying the thickness of the metal layer 4 and the width of the flexible portions 3, the force required for (elastically) deforming the frame 1 may be balanced against the amount of deformation in the frame 1 to obtain the desired amplification ratio damp / dtrans and output force for moving a load. Of course this could be done for the metal layer 4 on its own. However, especially for smaller transducers 6 providing smaller force outputs and intended for use in thin and / or miniaturised applications, the thickness tm of the metal layer 4 will become so small that buckling becomes a significant issue. Buckling of a beam-like structural element may be approximated by the Euler formula. For a thin metal layer 4 which can be elastically deformed in plane without large forces, the smallest second moment of area, and hence the controlling factor for buckling, will be in the thickness direction (z-axis in Figure 1A). The thickness tm cannot be increased to resist buckling without also increasing the force needed to bend the flexible portions 3, without reducing the thickness of the flexible portions 3 in the x-y plane to the point where reliable and repeatable manufacturing becomes impossible (or at least prohibitively expensive). The inventors have realised that the resistance to buckling may be provided by adding the thicker (tP >tm) polymer layer 5. Whilst the stiffness (Young modulus) E of the polymer layer 5 is low, the critical force for buckling varies in proportion to the product of stiffness E and second moment of area I. Because the second moment of area I of a rectangular cross section increases in proportion to the cube of the thickness t, the thickness of the polymer layer tP may be increased to prevent buckling of the frame 1, without excessively increasing the force output of the transducer 6 needed to deform the frame 1 in the desired plane (x-y as illustrated), leaving increased resultant force to drive movement of a load connected to the fixed (second) rigid portion 14. Part of the inventors present contribution lies in the development of practical fabrication methods for frames 1 formed from a laminate of a metal layer 4 and polymer layer 5, as described hereinafter. In this way, low profile and / or miniaturised mechanical amplifiers may be produced to operate in combination with low profile and / or miniaturised force transducers. For example, one application is in integration of transducers to provide haptic excitations to touchscreens, laptop touch pads, and other applications where there is a space constraint. For such applications, for such applications the total thickness t = tm + tP of the mechanical amplifier is preferably less than or equal to 3 mm, and may be less than or equal to 2 mm, for example 1.8 mm. In the first mechanical amplifier shown in Figures 1A and IB, the frame 1 consists of the metal layer 4 and the polymer layer 5, shaped to form the rigid portions 2 and flexible portions 3. This may also be the case for many other example of frames 1 for mechanical amplifiers according to the present specification. In other words, there may not be any third or further layer (not shown - for example having matching or equivalent properties to the metal layer 4) which is opposed to the metal layer 4 across the polymer layer. Put another way, the laminate forming the frame 1 may have an asymmetric layer structure along the thickness direction, as opposed to a symmetric (or "sandwich") layer structure. Of course, in other examples, additional layers may be included. In general, no rigid portion 2 is directly connected to another rigid portion 2. In other words, each rigid portion 2 may be connected to one or more other rigid portions 2 via respective flexible portions 3. If two rigid elements are attached directly together, then in effect this would constitute a single rigid portion 2. In the general case, when projected onto the plane (x-y as illustrated) perpendicular to the thickness direction (z-axis as illustration), a first projected area corresponding to the metal layer 4 is preferably substantially coincident with a second projected area corresponding to the polymer layer 5. Substantially coincident may mean that the Boolean intersection of the first and second projected areas is less than or equal to 10% of the first projected area. Such differences may correspond to structures such as the first and second recesses 8, 11 of the first mechanical amplifier shown in Figure 1A and IB, and other similar structures concerned with attachment of the frame 1 to other components (or to facilitate manufacturing). The key point is that the metal layer 4 and polymer layer 5 should be strongly connected along their interface, and continuous throughout the frame 1. Additionally, when forming the polymer layer 5 it may be beneficial to incorporate cavities, scallops, bevels, drafting and so forth (not shown). For example, to ensure good polymer melt flow during manufacturing (see for example the description of insert injection moulding hereinafter). Such features may be included without otherwise affecting the functional performance of the frame 1. Although any ratio damp / dtrans greater than unity represents amplification, for practical amplification, a ratio of greater than equal to two is preferable. Ratios greater than or equal to ten are possible, the principle constraint being that larger ratios will typically require more force, leaving a smaller resultant force to move a load. Actuator assembly An actuator assembly is formed by attaching a transducer 6 to connect between the primary end portion 7 and the secondary end portion 10 of the frame 1. Attachment of the transducer may use adhesives, or any other suitable means for forming a strong attachment. The primary end portion 7 and the secondary end portion 10 of the frame 1 may optionally include features such as protrusions and / or recesses (not shown) arranged to cooperate with corresponding features of a housing of the transducer in order to provide additional strength to the attachment. For the example of the first mechanical amplifier shown in Figures 1A and IB, the transducer 6 would be received into the recesses 8, 11. The transducer 6 is preferably an electrically actuatable transducer such as, for example, a piezoelectric transducer, a shape memory alloy (SMA) transducer and so forth. These are examples of linear transducers, which change their length (expanding or contracting) when actuated. However, other types of transducer may be used, for example, arranged to curve in the plane (x-y as illustrated), which may still be converted to a (predominantly) linear second displacement damp using an appropriately shaped frame 1. A SMA transducer may take the form of a single SMA wire (not shown) opposed by a spring to provide a restoring force, for example a flexure 40 (Figure 8A) connecting the primary end portion 7 to the secondary end portion 10. In another example, an SMA transducer may take the form of two or more SMA wires (not shown) arranged to oppose one another when actuated (contraction of one causes extension of the other). Fabrication by insert injection moulding The frames 1 of mechanical amplifiers according to the present specification (including the first mechanical amplifier shown in Figures 1A and IB) are preferably formed using an insert injection moulding process to directly cast the plastic layer 5 onto the metal layer 4. A brief summary of insert injection moulding is provided to aid the understanding of the advantageous effects in relation to fabrication of frames 1 for mechanical amplifiers in accordance with the present specification. Referring also to Figures 2A, 2B and 2C, steps of an insert injection moulding process for forming a frame 1 are illustrated. Referring in particular to Figure 2A, similarly to conventional injection moulding, the mould is formed of at least a first part 21 and a second part 22 which can be brought together to define a cavity 23 having a well-defined shape and volume. Between the first and second parts 21, 22, there are also included at least one inlet 24 and at least one outlet 25 in communication with the cavity 23. In the example shown in Figure 2A, one inlet 24 and one outlet 25 are formed in the second part 22. The numbers and positions of inlets 24 and outlets 25 are selected depending on the specific shape of the cavity 23, to ensure even filling without trapping gas. In insert injection moulding, one or more inserts, typically formed of metal, are placed into the mould before closing the parts 21, 22 to seal the cavity 23. In this case, the metal layer 4 for a frame 1 is placed into the first part 21. The first part 21 is preferably a tight fit around the edges of the metal layer 4 to minimise any gaps around the edge of the metal layer 4 (in plan view, for example as illustrated in Figure IB). There may be some differences between the shape of the parts 21, 22 and the metal layer 4, for example, to leave some areas of the metal layer 4 uncovered (or covered by a reduced thickness of polymer layer 4) to form recesses such as the first and second recesses 8, 11 illustrated in Figures 1A and IB. The metal layer 4 is pre-formed by selective material removal from sheet metal. Preferably, the metal layer 4 is cut to the required shape by laser cutting. The use of laser cutting to delineate the shape of the metal layer 4 will remain apparent from inspection of the metal layer 4. For example, a heat affected zone will be evident unless the metal layer 4 has been subjected to re-crystallisation after cutting to shape (which is neither technically nor commercially practical). Laser cutting is preferred due to the combination of speed (throughout), precision, and avoidance of significant mechanical straining in the region of the cut edge compared to shearing processes such as stamping. These considerations are particularly relevant for accurate and repeatable forming of the flexible portions 3 which are narrower in-plane. Alternatives to laser cutting include, without being limited to, water-jet cutting, stamping, electrical discharge machining (EDM), chemical etching (using a mask), and so forth. Use of any specific technique can be readily determined from inspection of the cut edges of the metal layer and / or the microstructure of the metal layer proximate to the edges. Shearing processes such as stamping or rolling can still be used where precision and / or fatigue lifetime of the frame 1 are subject to lower requirements. Formation of the metal layer 4 is not restricted to subtractive processes. In further examples, the metal layer 4 could be produced by 3D printing / additive manufacturing processes such as, for example, laser sintering of metallic powders. A surface of the metal layer 4 facing inwards (toward the cavity 23) may also be mechanically and / or chemically treated to improve adhesion to the polymer layer 4. For example, the metal layer 4 may be roughened by sandblasting (of the sheet metal or the shaped metal layer 4) or similar processes. Alternatively, a rough, porous coating may be applied (to the sheet metal or the shaped metal layer 4), for example a metal layer 4 formed of aluminium or titanium could be anodised. Referring in particular to Figure 2B, once the parts 21, 22 are sealed to form the cavity 23 with the metal layer 4 inside, molten polymer 26 is injected under pressure into the cavity 23 via the one or more inlets 24. As the molten polymer 26 fills the cavity 23, gas 27 from the cavity (typically air) is exhausted via the one or more outlets 25. Suitable polymers include poly-propylene (PP), nylon, high density polyethylene (HDPE), or any other polymers, blends or co-polymers known to be compatible with an injection moulding process. Referring in particular to Figure 2C, once the molten polymer 26 has filled the cavity 23, for example after injection of a calibrated volume, the polymer 26 is allowed to cool and solidify to form the polymer layer 4. The parts 21, 22 are then separated to allow extraction of the completed frame 1. Some finishing processes may be applied to remove artefacts of the injection moulding such as "flash" around the join of the parts 21, 22, and / or to remove any excess material 28 left at sites of inlets 24 or outlets 25. Whether finishing is necessary will depend on the application. The use of insert-injection moulding to form the frame 1 will be readily apparent, even after finishing to remove gross artefacts such as flash / excess material 28. For example, from inspection of the material and microstructure of the polymer layer 4. In particular, polymer chains will exhibit local alignments resulting from flow patterns during the filling of the mould cavity 23. Formation by insert injection moulding ensures a complete (barring any residual bubbles), smooth and strong interface between the polymer layer 5 and the metal layer 4, because the polymer layer 5 has solidified directly onto the metal layer 4. Insert injection moulding also provides excellent alignment of the polymer layer 5 over the metal layer 4. Fabrication by insert casting Although insert inject moulding is preferred, more conventional insert casting may also be used. For example, the pre-shaped metal layer 4 (cut as described hereinbefore) may be placed into a mould (not shown). A two (or more) component mixture is subsequently poured into the mould to cover the metal layer 4, before curing to form the polymer layer 5. The curing may occur naturally at standard temperature and pressure, for example by chemical reaction and / or drying. Curing may additionally or alternatively be assisted and / or accelerated by application or heat and / or light (for example UV light). Examples of a two (or more) component mixture include commercially available two-component resin systems. Alternatively, molten polymer 26 could be poured into the mould. The use of insert-casting will be readily apparent from inspection of the material and microstructure of the polymer layer. Insert casting can experience issues with trapped gas bubbles forming voids, which can be mitigated to an extent by conducting curing / solidification under reduced pressure. Insert casting provides many of the advantages of insert injection moulding. The throughput of insert casting will typically be lower compared to injection moulding, though this may be mitigated by reduced tooling costs for smaller volumes and / or bespoke applications. Fabrication by additive manufacturing Another approach for fabrication of the frames 1 is to print the polymer layer 5 directly onto a metal layer 4 pre-shaped as described hereinbefore. This will still provide a direct interface between the polymer layer 5 and the metal layer 4 (or any coating applied to the metal layer 4). The use of printing will be readily apparent from inspection of the material and microstructure of the polymer layer 5. Printing may include 3D printing (also termed additive manufacturing) of any type suitable for printing the material of the polymer layer 5. Printing the polymer layer 5 may be most suited to production of prototype parts, larger frames, small volumes of customised mechanical actuators, repairs and so forth. Fabrication as separate layers Alternatively, the metal layer 4 and the polymer layer 5 can be formed separately, and the frame 1 completed by bonding the polymer layer 5 to the metal layer 4. The bonding may be chemical, thermal, mechanical or a combination thereof. The metal layer 4 may be cut to the required shape as described hereinbefore. The polymer layer 5 may be formed to the required shape by injection moulding, casting, printing and so forth, or may be cut out from a polymer sheet using any of the techniques already mentioned (or any other suitable subtractive process). Bonding a pre-shaped polymer layer 5 to a pre-shaped metal layer 4 will be readily apparent from inspection of the interface between the layers and the edges of the frame. This approach is not preferred due to the need to obtain precise alignments of the layers 4, 5. Slight offsets, particularly around the flexible portions 3, may reduce the effectiveness of the polymer layer 5 to prevent out-of-place buckling of the frame 1. Additionally, there may be difficulties in applying even pressure to obtain a consistent bonding across the interface. Fabrication from a uniform laminate An alternative approach, which does not suffer from issues of aligning the metal layer 4 and polymer layer 5, is to form the frame 1 by selective material removal from a uniform laminate of the metal layer 4 and polymer layer 5. The uniform laminate may be cut to the desired shape for the frame 1 using one or more of laser cutting, water-jet cutting, stamping, conventional machining (such as milling) and so forth. The uniform laminate may have been formed by chemically or thermally bonding a uniform polymer layer of the second thickness tP to a uniform metal layer of the first thickness tm. Selective material removal from a uniform laminate to delineate the shape of the frame 1 may be readily distinguished from the preceding examples by inspection of the interface between the layers 4, 5, and the edges of the frame 1. Second mechanical amplifier Referring also to Figures 3A to 4C, a second example of a mechanical amplifier is shown (hereinafter the "second mechanical amplifier".) The second mechanical amplifier is the same as the first mechanical amplifier shown in Figures 1A and IB, except for the shape of the frame lb formed from the metal layer 4b and polymer layer 5b. Figures 3A to 3D show the metal layer 4b prior to deposition of the polymer layer 5b. Figure 3A is a perspective view, Figure 3B is a plan view along the illustrated z-axis, Figure 3C is a side view along the illustrated y-axis, and Figure 3D is a side view along the illustrated x-axis. For visual purposes, in Figures 3A to 3D, portions of the metal layer 4b have been given the same labels as the corresponding rigid portions which they belong to, even though each rigid portion 2 is in fact formed by the combination of the respective regions of both the metal layer 4b and the polymer layer 5b. Figures 4A to 4C show the frame lb of the second mechanical amplifier after deposition of the polymer layer 5b. Figure 4A is a perspective view, Figure 4B is a plan view along the illustrated z-axis, and Figure 4C is a cross-section along the line labelled A-A* in Figure 4B. The frame lb of the second mechanical amplifier has the same shape as the frame 1 of the first mechanical amplifier, except that the first bridge structure 16 and second bridge structure 17 are each modified to have a two arch structure, with the first rigid portion 2a, 13 belonging to both arches of the first bridge structure 16 and the second rigid portion 2b, 14 belonging to both arches of the second bridge structure 17. In addition to the first rigid portion 2a, 13, a second arch structure of the first bridge structure 16 is formed by: • a rigid portion 2i which connects the first rigid portion 2a, 13 to the third rigid portion 2c providing the primary end portion 7 via flexible portions 3i and 3j; and • a rigid portion 2j which connects the first rigid portion 2a, 13 to the fourth rigid portion 2d providing the secondary end portion 10 via flexible portions 3k and 31. In addition to the second rigid portion 2b, 14, a second arch structure of the second bridge structure 17 is formed by: • a rigid portion 2k which connects the second rigid portion 2b, 14 to the third rigid portion 2c providing the primary end portion 7 via flexible portions 3p and 3q; and • a rigid portion 21 which connects the second rigid portion 2b, 14to the fourth rigid portion 2d providing the secondary end portion 10 via flexible portions 3m and 3n. The rigid portions 2i, 2j, 2k and 21 may also be referred to collectively as second spanning portions 28. Each of the second spanning portions 2i, 2j, 2k, 21, 28 extends parallel to the corresponding first spanning portion 2e, 2f, 2g, 2h, 18. This helps to constrain the second displacement damp to a linear path across the range of movement of the mechanical amplifier (whilst within the elastic regime). Given the purpose of the second spanning portions 2i, 2j, 2k, 21, 28 as guiding elements, a width of each second spanning portion 2i, 2j, 2k, 21, 28 (within the plane, x-y as illustrated), may be less than a width of any first spanning portion 2e, 2f, 2g, 2h, 18. This configuration is illustrated in Figures 3A to 4C, but is not essential. Referring in particular to Figures 3A to 3D, the metal layer 4b of the second mechanical amplifier includes a number of (optional) flaps and / or protrusions 29, 30 extending into the polymer layer 5b. In particular, a pair of flaps 29 is within each of the primary end portion 2c, 7, the secondary end portion 2d, 10 and each first spanning portion 2e, 2f, 2g, 2h, 18. Within each pair, the flaps 29 are angled in opposite directions, such that separating the metal layer 4b and polymer layer 5b is impossible without permanently deforming / breaking one or both. In addition, a number of curved protrusions 30 are formed around the outer perimeter of the frame lb, within each of the primary end portion 2c, 7, the secondary end portion 2d, 10, the first rigid portion 2a, 13, and the second rigid portion 2b, 14. Each curved protrusion 30 is formed by bending a protrusion from the edge of the metal layer 4b back on itself so that the distal end lies over the metal layer 4b. Referring in particular to Figures 1A and 4B, in this example the curved protrusions 30 extend in the thickness direction (z-axis as illustrated) for a distance equal to the thickness of the polymer layer 5b. The curved protrusions 30 grip the polymer layer 5b to oppose any lateral or perpendicular separation of the parts. Additionally, because they extend through the thickness of the polymer payer 5b, the curved protrusions 30 also have a secondary function to help ensure the metal layer 4b is held in the correct position within the injection mould tool. The illustrated examples of flaps 29 and protrusions 30 of the metal layer 4b may preferably be formed by a stamping process conducted before, during or after processing to delineate the shape of the metal layer 5. Although the numbers, shapes and / or position of flaps / protrusions 29, 30 may be varied in other examples, the effect is to provide an improved distribution of loading, spreading stress throughout the thickness t of the frame lb, and reducing the stress acting at the interface between the polymer layer 5b and the metal layer 4b (at least the parts other than the flaps 29 and protrusions 30). As with any composite structure, the frames 1, lb of the present specification are only as good as the interface between the two different material types. The flaps 29 and / or protrusions 30 can substantially improve the stress distribution, with positive effects including but not limited to reduced possibility of delamination between the metal layer 4b and the polymer layer 5b. The flaps 29 and protrusions 30 illustrated in the metal layer 4b are only usable with the insert injection moulding and insert casting methods described hereinbefore. In these processes, the polymer layer may flow around the flaps 29 and protrusions 30 when molten / uncured, before subsequently solidifying / curing to securely grip the metal layer 4b. Similar flaps 29 and / or protrusions 30 may be added to the metal layer 4 of the first mechanical amplifier 4, or indeed the metal layer 4 of any other frame 1 in accordance with the present specification. Flaps 29 and / or protrusions 30 can be used in other methods of fabrication, though the shapes would be more restricted to ensure that the polymer layer 5b could be formed over / applied to the metal layer 4b. The frame lb of the second mechanical amplifier also includes a first through-hole 31 passing through the first rigid portion 2a, 13 in the thickness direction, and a second through-hole 32 similarly formed in the second rigid portion 2b, 14. To facilitate connection to a load and / or supporting structure 15 (Figure 6), either or both of the first rigid portion 2a, 13 and the second rigid portion 2b, 14 may include a reduced thickness portion 33. For example, as illustrated in Figure 4A for the second rigid portion 2b, 14. Actuator assembly Referring also to Figures 5A and 5B, an actuator assembly 34 is shown in perspective view. Figure 5A illustrates assembly, whilst Figure 5B shows the completed actuator assembly 34. To form the actuator assembly 34, a transducer 6, in this example a piezoelectric transducer, is received into the recesses 8, 11 of the frame lb of the second mechanical amplifier and bonded securely in position. In the illustrated example, the piezoelectric transducer 6 is actuatable by electrical stimuli received via an electrical connection cable 34. This is only one example, and an actuator assembly 34 may be created by connecting any type of transducer 6 described herein between the primary end portion 7 and secondary end portion 10 of a frame 1, lb in accordance with the present specification. Example application - haptic touch pad As one example of use of mechanical amplifiers according to the present specification, and referring also to Figure 6, the integration of the actuator assembly 34 to provide haptic excitations to a touch pad is shown. A supporting structure 15 is provided in the form of a backplate. A number of holes 34 are formed through protrusions provided around the perimeter of the backplate 15 to allow the assembly to be secured to the structure of a laptop computer or similar device which uses a touch pad for input. A raised lip 35 is formed substantially around the perimeter of the backplate 15, and the perimeter of the touchpad (not shown) will be supported by the raised lip when the touchpad is installed. In the illustrated example, the lip 35 is formed in two parts, separated by a gap along each short edge. Cables (not shown) connecting to the touchpad may be routed through the gap(s) between the parts of the lip 35. Alternatively, the backplate 15 may include additional through-holes for routing of cables and / or other connectors. The actuator assembly 34 is received into a recess 36 shaped and sized to receive the actuator assembly 34 and allow deformation within its operational range of movement. In other examples, a through-hole may be used instead of recess 36. The second rigid portion 14 is fixed (directly or indirectly) to the backplate 15. The fixing to the backplate 15 may be performed using adhesive to form a bond and / or by mechanical fixing using the second through-hole 32. The touchpad (not shown) is then placed abutting the lip 35, with a surface for user interaction facing away from the backplate 15. The touch pad is then connected to the first rigid portion 13. The fixing to the touch pad may be performed using adhesive to form a bond and / or by mechanical fixing using the first through-hole 31. In this way, actuation of the transducer 6 will result in the amplified second displacement damp being imparted to the touchpad. The touchpad is also coupled to the backplate 15 via a number of couplings 37. The touchpad is constrained from movement in the negative z-direction as illustrated by abutting the lip 35. The touchpad is constrained from movement in the positive z-direction by at least the couplings 37, and potentially also by an overlying lip (not shown). The couplings 37 also support a limited range of lateral movement, so that movement of the mobile, first rigid portion 13 can be transferred to the touchpad. The trackpad may operate in any known way, for example, it may be a capacitive touchpad, a piezoelectric touchpad, a resistive touchpad, or may use combination such as, for example, combined capacitive and piezoelectric measurements, to detect user touches and / or presses. A detailed example of suitable couplings 37 is found in WO 2023 / 223012 Al, referring in particular to the example illustrated in of Figures 6A to 6C and the corresponding description. Other options of couplings 37 suitable for lateral haptic excitations may be found in Figures 8A to 12G and the corresponding description of WO 2023 / 223012 Al. Using the second mechanical amplifier with a piezoelectric transducer 6, an actuator assembly having a total thickness t = 1.8 mm has been produced which is capable of amplifying to a ratio of damP / dtrans >6 and still providing perceptible haptic excitations to a trackpad having a mass of approximately 40 g. This experimentally produced second mechanical amplifier had a metal layer 4b of laser cut steel with thickness tm = 300 pm = 0.3 mm (sheet thickness, not including the flaps 29 and protrusions 30) and a polymer layer 5b formed of 3D printed resin material, MA-R.103 (RTM), having equivalent properties to polypropylene and thickness tP = 1,500 pm = 1.5 mm. The polymer layer 5b was 3D printed separately, and then bonded to the laser cut metal layer 4b. The lateral dimensions were 60 mm by 35.6 mm. Although application has been described to provide haptic excitations to a touchpad for a laptop or similar device, the same principles may be applied to any other combination of a device and a user input surface. In the general case, the actuator assembly 34 may be coupled between a device structure / frame / casing 15 and a user input surface (not shown). The user input surface may include, or take the form of, a touch pad, a touchscreen, one or more discrete buttons, sliders, and / or other user input controls, a smart surface, and so forth. The device may include, without being limited to, a laptop computer, a tablet computer, a mobile phone, a camera, a handheld device, a kitchen appliance, industrial / plant machinery, a medical device and so forth. Third mechanical amplifier In the first and second mechanical amplifiers, there is no overlap between the first and second rigid portions 13, 14 and the primary and secondary end portions 7, 10. However, as described hereinbefore this is not essential, and in other examples, one of the first rigid portion 13 or the second rigid portion 14 may provide one of the primary end portion 7 or the secondary end portion 11. For example, referring also to Figure 7, a third example of a mechanical amplifier is shown in a plan view (along the illustrated z-axis). The frame lc of the third mechanical amplifier includes a first rigid portion 2a, 13 which also provides the primary end portion 7. The first rigid portion 2a, 7, 13 includes first recess 8. A second rigid portion 2b, 14 extends along the illustrated y-axis and is coupled to the first rigid portion 2a, 7, 13 by a flexible portion 3a. A third rigid portion 3c provides the secondary end portion 10. The third rigid portion 2c, 10 is "J" shaped, and is connected to the second rigid portion along the illustrated x-direction by a flexible connector 3b. The third rigid portion 2c, 10 includes a second recess 11. A transducer 6 may be received into the first and second recesses 8, 11 to connect the first rigid portion 2a, 7, 13 to the third rigid portion 2c, 10 along the illustrated x-axis. With the first rigid portion 2a, 7, 13 fixed, a first displacement dtrans (corresponding to arrows labelled 37) will cause an amplified second displacement damp of a distal end 38 through an arc roughly centred on the flexible portion 3a (corresponding to arrow labelled 39). Pre-stressing the transducer Some types of transducer 6 may have improved performance, for example faster responsivity or better return to zero-position, when the transducer is pre-stressed. Applied pre-stress may be tensile or compressive, depending on the type of transducer 6. One way to implement pre-stressing is simply to design the shape of the frame 1, lb, lc such that a natural length Lt of the transducer 6 is different to an unstressed separation Lo between the primary end portion 7 and the secondary end portion 10. The frame 1, lb, lc is then elastically deformed to the natural length At of the transducer 6 and the transducer 6 is attached. Once the attachment is secure (for example once adhesive has dried / cured), a force deforming the frame 1, lb, lc is released and the actuator assembly will move to an equilibrium state (the zero for first displacement dtrans) in which the transducer 6 is pre-stressed. This approach has the advantage of simplicity. Another approach, which may also help to provide additional restoring force to an equilibrium position, is to include a flexure connecting the primary end portion 7 to the secondary end portion 10. For example, referring also to Figures 8A and 8B a modified first mechanical amplifier is shown. Figure 8A shows a plan view (along the illustrated z-axis) and Figure 8B shows a cross-section along the line labelled B-B* in Figure 8A. The frame Id of the modified first mechanical amplifier additionally includes a flexure 40 connecting the primary end portion 7 to the secondary end portion 10. The flexure 40 includes a serpentine segment so as to not provide excessive resistance to the first displacement dtrans. The flexure 40 may be integrally formed with the metal layer 4. Alternatively, the flexure 40 may be a separate piece connected to the metal layer 4. In this way, the flexure 40 may serve to provide additional control over pre-stressing (in tension or compression) of a transducer 6 received by the frame Id, see in particular Figure 8B. The flexure 40 may additionally serve to provide both pretension and a restoring force for a unidirectionally actuatable transducer such as, for example, a shape memory alloy (SMA) wire. Modifications It will be appreciated that many modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture, mounting and use of mechanical amplifiers for use with transducers, and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. The actuator assembly 34 may be used in devices including, but not limited to, a tablet computer, a mobile phone, a handheld electronic device, a laptop computer, a display screen, a peripheral for a computer, a game controller and so forth. The actuator assembly 34 is not limited to input / output applications, and may be used instead to provide precise positional control. For example, in devices / applications including but not limited to optical systems, cameras, vibration and / or motion compensation, speakers, micro-stages, and so forth. The described and illustrated examples have used an articulated structure comprising rigid portions 2 connected by short, hinge-like flexible portions 3. However, other types of flexible portions may be used, for example, longer, elongated flexures. These are also within the scope of the present disclosure provided that they are formed from the laminate of the metal layer 4 and the polymer layer 5, and satisfy the other conditions described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicant hereby gives notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. A mechanical amplifier comprising:a frame extending parallel to a plane and perpendicular to a thickness direction, the frame formed from a plurality of rigid portions connected together by a plurality of flexible portions, the frame comprising a metal layer having a first thickness and a polymer layer overlying the metal layer and having a second thickness;wherein each of the metal layer and the polymer layer is a single piece which extends through the plurality of rigid portions and the plurality of flexible portions;wherein the frame is configured to receive a transducer connecting between a primary end portion of the plurality of rigid portions and a secondary end portion of the plurality of rigid portions;wherein the plurality of rigid portions comprises a first rigid portion and a second rigid portion;wherein the frame is configured such that:in response to a first displacement between the primary and secondary end portions, a resulting second displacement between the first and second rigid portions is greater than the first displacement; andthe first thickness is less than 1 mm and the second thickness is sufficient to prevent buckling of the frame in the thickness direction across a range of first displacements for which the frame remains in the elastic regime.

2. The mechanical amplifier of claim 1, wherein the component of the second displacement parallel to the thickness direction is less than or equal to one tenth of the component of the second displacement parallel to the plane.

3. The mechanical amplifier of claims 1 or 2, wherein the frame consists of the metal layer and the polymer layer.

4. The mechanical amplifier of any one of claims 1 to 3, wherein the first rigid portion provides the primary end portion or wherein the first rigid portion provides the secondary end portion.

5. The mechanical amplifier of any one of claims 1 to 3, wherein the plurality of rigid portions comprises a third rigid portion providing the primary end portion and a fourth rigid portion providing the secondary end portion.

6. The mechanical amplifier of claim 5, wherein a first bridge structure connects the third rigid portion to the fourth rigid portion, the first bridge structure comprising the first rigid portion and at least two flexible portions.

7. The mechanical amplifier of claim 6, wherein the first bridge structure comprises a first arch structure connecting the third rigid portion to the fourth rigid portion via the first rigid portion, one or more first spanning portions of the plurality of rigid portions, and three or more flexible portions.

8. The mechanical amplifier of claim 7, wherein the first bridge structure further comprises a second arch structure connecting the third rigid portion to the fourth rigid portion via the first rigid portion, one or more second spanning portions of the plurality of rigid portions, and three or more flexible portions.

9. The mechanical amplifier of claim 8, wherein a width of each second spanning portion is less than a width of any first spanning portion.

10. The mechanical amplifier of any one of claims 5 to 9, wherein a second bridge structure connects the third rigid portion to the fourth rigid portion, the second bridge structure comprising the second rigid portion and at least two flexible portions.

11. The mechanical amplifier of any one of claims 1 to 10, configured such that the second displacement in response to the first displacement is greater than or equal to two times the first displacement.

12. The mechanical amplifier of any one of claims 1 to 11, wherein the metal layer is formed of a steel, an aluminium alloy, or a titanium alloy.

13. The mechanical amplifier of any one of claims 1 to 12, wherein the polymer layer comprises one or more of polyvinyl chloride (PVC), polypropylene (PP), polyester, polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), nylon, , high-density polyethylene (HDPE), and co-polymers thereof.

14. The mechanical amplifier of any one of claims 1 to 13, wherein the frame is formed by insert-injection moulding the polymer layer over the metal layer.

15. The mechanical amplifier of any one of claims 1 to 13, wherein the frame is formed by insert-casting the polymer layer over the metal layer.

16. The mechanical amplifier of claim 14 or claim 15, wherein the metal layer comprises a plurality of flaps and / or protrusions extending into the polymer layer.

17. The mechanical amplifier of any one of claims 1 to 13, wherein the frame is formed by printing the polymer layer over the metal layer.

18. The mechanical amplifier of any one of claims 1 to 17, wherein the metal layer is formed by selective material removal from sheet metal.

19. The mechanical amplifier of any one of claims 1 to 18, wherein a total thickness of the mechanical amplifier is less than or equal to 3 mm.

20. The mechanical amplifier of any one of claims 1 to 19, further comprising a flexure connecting the primary end portion to the secondary end portion.

21. An actuator assembly comprising:the mechanical amplifier of any one of claims 1 to 20; anda transducer received by the frame and connecting between the primary end portion and the secondary end portion.

22. The actuator assembly of claim 21, wherein the transducer is a piezoelectric transducer.

23. The actuator assembly of claim 21 or claim 22, wherein the transducer is prestressed.

24. Use of the mechanical amplifier of any one of claims 1 to 21 or the actuator assembly of any one of claims 22 to 24 to generate haptic excitations.

25. A method of manufacturing the mechanical amplifier of any one of claims 1 to 21 or the actuator assembly of any one of claims 22 to 24.

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