Description:F O R M 2

THE PATENTS ACT, 1970
(39 of 1970)

COMPLETE SPECIFICATION
(See section 10 and rule 13)

TITLE
MODULAR CONFIGURABLE LINEAR ACTUATOR UNDER THERMAL LOAD

INVENTORS
PRASANNA, Hariprasad Mohan - Indian Citizen
SHANKAR, Balakrishnan - Indian Citizen
MENON, Hrishikesh Gopakumar- Indian Citizen
Department of Mechanical Engineering
AMRITA VISHWA VIDYAPEETHAM
Amritapuri Campus, Kollam, Kerala 690525, India

APPLICANT
AMRITA VISHWA VIDYAPEETHAM
Amritapuri Campus, Kollam, Kerala 690525, India

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED:

MODULAR CONFIGURABLE LINEAR ACTUATOR UNDER THERMAL LOAD
CROSS-REFERENCES TO RELATED APPLICATION
None.
FIELD OF THE INVENTION
The disclosure generally relates to thermally responsive structures and more specifically relates to a thermally active linear actuator.
DESCRIPTION OF THE RELATED ART
The Poisson's ratio of the material dictates its shrinkage or expansion in any general scenario involving the deformation of structures. Poisson ratio is defined as the lateral strain to longitudinal strain ratio. The Poisson's ratio is discovered to be positive for the traditional materials. Auxetic structures, a class of structures that has been the subject of extensive study and research for the past three decades, are observed to display a negative Poisson's ratio behavior because of their inherent geometry. Parallel to this, manufacturing procedures underwent a revolution with the introduction of additive manufacturing techniques like 3D and 4D printing, photon lithography, etc. This has had a direct impact on auxetic meta-materials research because it is now much easier to make them.
Since the last decade, mechanical meta-materials have been studied for their potential usage as smart structures.Meta-materials have gained popularity in the fields of soft robotics, acoustics, programmable structures, and self-reconfigurable devices due to their distinct deformation modes and when combined with soft materials.
US Pat No. US10549505B2 constructs an active auxetic lattice that undergoes physical transformation when subjected to external stimuli such as heat or moisture. By arranging and interconnecting flexible-composite films in form of two dimensional and three dimensional structure, heat-active lattices portrayed autonomous shape changing behaviour. The difference in the rates of thermal expansion in the primary and secondary layer of the film was maintained significantly large in order to attain curved/buckled shape change.
By altering the external pressure or temperature, a structure proposed in UK Pat No. GB2455167A shows tuning of auxetic or non-auxetic behaviour. These systems are configured in ways that give riseunconventional behaviours such as negative Poissons’ ratio, negative thermal expansion and/or negative compressibility.
US Pat No. US8302696B2puts forth a passive auxetic actuator wherein the negative Poissons ratio behaviour of the constituent auxetic unit cells manifests as an overall actuation in the structure.
The majority of the research in this area focuses on mechanical characterization, energy absorption, and failure investigations, with more recent studies emphasizing active intelligent meta-structures, soft robotics, and actuators.
SUMMARY OF THE INVENTION
Embodiments of the present invention relate to a thermally activated linear actuator that uses a modified re-entrant honeycomb cell with curved cell walls. The actuator comprises multiple unit cells arranged in a pre-defined manner. Each unit cell having a specific set of parameters, including span, curvature depth, in-plane thickness of the cell wall, length and breadth of the horizontal connector, and curvature and span of the curved edges.
In various embodiments the linear actuator is designed to produce a positive or negative displacementunder thermal load about an axis based on the placement of materials in the bi-material strips which depends on the material’s elastic modulus and thermal expansion coefficient.
The actuator comprises a periodic arrangement of a single unit cell in a linear fashion, resulting in a simple yet effective linear actuator. The number of unit cells maybe changed based on the requirement. The unit cell geometry may also be varied to obtain desirable behaviour, and the actuator may be formed out of multiple cells of varying geometries to achieve a functionally graded linear actuation.
The unit cell is constructed by arranging bi-material strips curved outwards and uni-material strips curved inwards. Under thermal loading, the bi-material strips lend the whole unit cell a change in shape which then cascades with every additional unit cell in the actuator to give rise to a globalized linear actuation. The extent of actuation depends not only on the unit cell parameters but also on the number of unit cells in the design. The actuation may be increased by increasing the number of unit cells.
For attaching the curved strips of the bi-material together, a thermally conductive glue may be used. The thermally conductive glue or adhesives may also be used to attach the bi-material and uni-material strips together. The actuator may be fabricated through multi-material 3-D printing or CNC machining, depending on the suitability of the materials.
Further embodiments may contain various combinations of geometries and aspects along-with/substituting the existential ones under the norms and conditions of the invention. The actuator may find applications in various fields, such as robotics, aerospace, and medical devices, where linear actuation is required.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention has other advantages and features, which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:
Figure 1Aillustrates the structure of linear actuator showing the arrangement of unit cellsalong anX axis.
Figure 1B illustrates the structure of a type 1 unit cell which shows a positive expansion.
Figure 1C illustrates the structure of a type 2 unit cell which shows a negative expansion.
Figure 2 illustratesthe combination of cascading effect with individual cell deformation of type 1 linear actuator under thermal load.
Figure 3A illustrates an example of deformation mechanism involved under thermal load for type 1 actuation.
Figure 3B illustrates an example of deformation mechanism involved under thermal load for type 2 actuation.
DETAILED DESCRIPTION OF THE EMBODIMENTS
While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.
Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.” Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.
The invention in its various embodiments discloses a linear actuator configured to attain a positive or negative displacement about an axis under thermal load.In various embodiments the linear actuator (100) as shown in FIG. 1Ahas aplurality of modified re-entrant honey comb unit cells (101) each connected to one another by horizontal connectors (102) along an axis of the actuator.The linear edges of re-entrant honey comb unit cells are modified with curvilinear edges. As shown in FIG. 1A, the left hand side of the actuator is anchored, while the double arrow shows direction of actuation.Under thermal load, deformation in each of the modified unit cells (101) is cascaded with every additional unit cell in the actuator to give rise to a globalized linear actuation in a direction of the axis.
In various embodiments, the unit cell (101) under thermal load, exhibits an expansion that leads to a positive displacement of the linear actuator along the axis, which may be categorized as a type 1 actuation. In various other embodiments, the unit cell under thermal load, exhibits a contraction that leads to a negative displacement of the linear actuator along the axis, which may be categorized as type 2 actuation.
In various embodiments the honey combunit cell is modified, wherein the opposite re-entrant edges are replaced by curvilinear edges (103) that are curved inwards and the parallel edges (104,105,106,107) are replaced by curvilinear edges that are curved outwards.
In various embodiments, here after, the curved re-entrant edges of the modified unit cell are termed as the horizontal edgesand the other two curved sides are termed as vertical edges.
The modified unit cell comprises two curved bi-material strips (104,105,106,107) and two curved uni-material strips(103), wherein the uni-material strips forms the re-entrant horizontal edges and the bi-material strips forms the vertical edges.The horizontal re-entrant edges are made of a first material and the vertical bi-material strips are made up of a second and a third material. For a type 1 positive actuation, the second material withlower elastic modulus and higher thermal expansion coefficient is placed in the exterior and the third material withhigher elastic modulus and lower thermal expansion coefficient value is placed in the interior of the curved bi-material strip. For a type 2 negative actuation, the second material is the one with higher elastic modulus and lower thermal expansion coefficient value, which is placed in the exterior and the third material is one with lower elastic modulus and higher thermal expansion coefficient, is placed in the interior of the curved bi-material strip. The elastic modulus of the first material used as the uni-material strip is lower than or equal to the elastic modulus of the second or third material.
For attaching the curved strips of the bi-material together, thermally conductive glue may be used. Similarly, for attaching the bi-material embodiment with the uni-material embodiment thermally conductive adhesives may be used. The actuator may be fabricated through multi-material 3-D printing or CNC machining, depending on the suitability of the materials.
The unit cell parameters such as its span (?) and curvature depth (ß) of thehorizontal edges, and the curvature (f) and span(?)of the vertical edges, length (d) and breadth (c) of the horizontal connector,and in-plane thickness of the cell wall (t) contribute to the deformation of the unit cell.
In various embodiments the modified unit cell having the bi-material strips and the uni-material strips, the parameters like, elastic modulus(E) and the thermal expansion coefficient(a) of the materials, and the placement of the materials used in the formation of bi-material strips determine the type of actuation accomplished by the actuator. The modified unit cell comprising bi-material strip having material with lower elastic modulus and higher thermal expansion coefficient (104a,105a) placed in the exterior of the curved bi-material strip, and material with higher elastic modulus and lower thermal expansion coefficient (104b,105b) placed in the interior side of the curved bi-material strip as shown in FIG 1B, exhibits a type 1 actuation. The modified unit cell comprising bi-material strip having material with higher elastic modulus and lower thermal expansion coefficient (106a,107a) placed in the exterior of the curved bi-material strip, and material with lower elastic modulus and higher thermal expansion coefficient (106b,107b) placed in the interior side of the curved bi-material strip as shown in FIG 1C, exhibits a type 2 actuation.
For type 1 actuation the material placement in the bi-material strips in the exterior part and interior part, and the uni-material is selected based on the equations,
E_5,E_6=E_2,E_3=E_1,E_4 (1)
a_1,a_4>a_2,a_3 (2)
For type 2 actuation the material placement in the bi-material strips in the exterior part and interior part, and the uni-material is selected based on the equations
E_5,E_6=E_1,E_4=E_2,E_3 (3) ? a?_2,a_3>a_1,a_4 (4)where E is the Youngs modulus (elastic modulus) and a is the thermal expansion coefficient of the material. Subscripts1 and 4 represents materials placed in the exterior of bi-material strip, 2 and 3 represents materials placed in the interior of bi-material strip, and 5 and 6 represents materials of upper and lower uni-material strips respectively. The interior materials of the left and right bi-material stripsmay be of the same material as well as the exterior materials of the left and right bi-materials may be the same material for both type 1 and type 2 linear actuation. The materials used for the two uni-material strips may also be the same.
In various embodiments, the asymmetry in the unit cell structure about the X direction and Y direction contributes to the deformation space for the actuator.Each unit cell in the actuator can be divided into four sub-zones/quadrants for design modification. Upper section of the unit cell constituting the quadrants 1 and 2 can have a different design compared to quadrants 3 and 4. This will make the structure asymmetric about the X-axis which would open up novel deformation space for the actuator. Similarly, quadrants 2 and 3 are designed differently in comparison to quadrants 1 and 4 makes the unit cell have a Y-axis asymmetry. In some other embodiments, all the four quadrants could be designed independently and differently. Additionally, multiple unit cells with varying parameters and material choices could be arranged in the same actuator to obtain peculiar and unique deformations.
In various embodiments, a thermal load leads to anet tip deflection of the horizontal connector of each unit cellalong the horizontal axis. Each unit cell has two horizontal connectors. Both the left as well as the right connector of each unit cell displacesalong the horizontal axis leading to the actuation behavior. This effect cascades with every additional unit cell and exhibits a higher globalized linear actuation.The extent of actuation depends on the unit cell parameters and the number of unit cells in the design.
In various embodiments, the displacement of each horizontal connector along its axis is determined by two factors.The first one is the deformation of the unit cell which lends the uni-material a buckling behaviour, as a result of which the horizontal connector is displaced along its axis. This deformation is the result of the thermal response of bi-material strips in the unit cells and is referred as the cell tip deflection (d). The second factor is the rigid body displacement of the particular unit cell in consideration, due to the cascading effect of the individual deformations of each unit cell prior to it in the actuator.The cumulative deformation of the all the unit cells prior acts on the subject unit cell to cause a rigid body displacement on it. The linear actuation can be tracked by observing this cascading effect. This cascade deflection (?) is taken as the second parameter.The rigid body displacement of each unit cell due to the cascading effect is added to the cell tip deflection to produce the resultant cascading deflection that will act on the next adjacent cell as shown in FIG 2. Thus the effective cascaded deflection at the nth unit cell can be represented as the sum of the cascaded deflection till the (n-1)th unit cell and the cell tip deflection of the nth cell
?_n=?_(n-1)+ d_n (5)
For an actuator having 'n' unit cells, the final tip deflection is the result of the cascading effect produced till the n-1th unit cell (including the n-1th unit cell) and the deformation of the nth cell. If all the unit cells are same in the actuator, the final tip deflection can be calculated through a summation of the individual cell tip deflections.
?_n= ?_(i=1)^n¦d_i (6)
In certain embodiments the unit cell may have the first material as iron or steel, and the second material as copper or aluminium. The third material is same as the first material. The coefficient of thermal expansion (CTE) for copper is 1.7 × 10–5/°C and CTE for aluminium is 2.3×10-5/°C. Also the CTE for iron is 1.2×10-5/oC and for steel the CTE is 1×10-5/oC. The material placed in the exterior of the bi-material strip (copper or aluminium) has a higherthermal expansion coefficient than the material placed in the interior of the bi-material strip (iron or steel). Upon thermal excitation second material expands more than the third material producing a further expansion of the curvature outwards. The expansion is enhanced by the buckling effect of the uni-material strip which is same as the third material. This causes a positive displacement of the cell along the axis. The unit cell tip displacement is cascaded with every additional cell tip displacement and results in a global linear displacement in the positive direction which is a type 1 actuation.
In certain embodiments the second material at the exterior of the bi-material stripmay be iron or steel and the third materialat the interior of the bi-material stripmay be copper or aluminium. The first material forming the uni-material strip may be the same as that of the third material. In this case, upon thermal excitation, the interior material of the bi-material strip expands more than the exterior material which causes lessening the curvature of the bi-material strip. This results in a negative displacement of the cell along the axis. The unit cell shape change is cascaded with every unit cell but in the negative direction along the axis which is a type 2 actuation.
In another embodiment the cellular structure may be made of polymers. The first and third material may be Polytetrafluoroethylene (PTFE), and the second material may be Poly-(methyl methacrylate) (PMMA). On thermal excitation PMMA, having a higher thermal expansion coefficient than PTFE expands more which results in a positive displacement of the cell along the axis. This causes a type 1 linear thermal actuation. In another aspect the first and third material may bePoly-(methyl methacrylate) (PMMA), and the second material may be Polytetrafluoroethylene (PTFE).Upon thermal activation the PMMA expands more than PTFE. PMMA being the interior material here shrinks the overall cell structure which results in a negative displacement along the axis.
The invention has several advantages as set forth herein. The inventive device offers a wide range of controllable actuation options, including expansion under thermal load or contraction to any useful engineering requirements, in various fields, such as robotics, aerospace, and medical devices, where linear actuation is required.

EXAMPLES
Example 1:Type 1 and Type 2 actuation and its operation under thermal load
9 unit cells of dimensions wereconnected to each other and subjected to a changing temperature field. Both the bi-material strips have PTFE (Polytetrafluoroethylene) with elastic modulus E=500MPa, and coefficient of thermal expansion a=246x10-6 at its exterior and PMMA (poly-(methyl methacrylate)) (E=3100MPa, a=68x10-6) at its interior. Figure 3A shows the material placement for type 1 actuation.The overall dimension of the actuator was200 mm. The left extreme of the actuator waspinned and the rest of it wasleft free. When the temperature raised to 100°C from the ambient temperature (considered to be 30°C here), the actuator exhibited a tip displacement of 15.4mm. The structure was analyzed using ABAQUS 6.14 using a non-linear modelling approach taking into consideration the first order shear deformation effects also. Meshing wasdone with S4R: 4 node general-purpose shell element.
For type 2 actuation, 9 unit cells of dimensions were connected to each other. Here, on both sides, PMMA(poly-(methyl methacrylate)) was at the exterior of the bi-material strips and PTFE (Polytetrafluoroethylene) at its interior side.Figure. 3B represents this material placement for type 2 actuation. The experiment was conducted under the same thermal conditions as above, with overall dimension being 200mm. The actuator tip was displaced by 10.86 mm in the negative direction of the horizontal axis.
, Claims:WE CLAIM:
1. A temperature responsive linear actuator configured to produce a positive or a negative displacement about an axis thereof (100) comprising:
a plurality of unit cells(101) lined-up together horizontally each connected to one another by a horizontal connector (102);
wherein, each unit cell is a modified re-entrant honeycomb auxetic structure;
wherein the unit cell is an arrangement of two curved bi-material strips (104,105,106,107), wherein the bi-material strips are placed roughly across the axis, and two curved uni-material strips (103) placed roughly along the axis linked together by the uni-material, and forming a single actuating element;
wherein the uni-material strips are of a first material forming the horizontal re-entrant edges curved inwards and the bi-material strips form the vertical edges curved outwards;
wherein the bi-material strips are of a second material (104a,105a) with lower elastic modulus and higher thermal expansion coefficient in the exterior and, and of a third material (104b,105b) with higher elastic modulus and lower thermal expansion coefficient value in the interior; or
wherein the bi- material strips are of a second material (106a,107a) with higher elastic modulus and lower thermal expansion coefficient in the exterior of the strip and of a third material (106b,107b) with lower elastic modulus and higher thermal expansion coefficient value in the interior; wherein the horizontal connectors are uni-material strips.

2. The linear actuator as claimed in claim 1, comprising unit cells with bi-material strips having material with lower elastic modulus and higher thermal expansion coefficient in the exterior and, having material with higher elastic modulus and lower thermal expansion coefficient value in the interior, configured to exhibit positive expansion(101a) when thermally excited.

3. The linear actuator as claimed in claim 1, comprising unit cells with bi- material strips having material with higher elastic modulus and lower thermal expansion coefficient in the exterior of the strip and having material with lower elastic modulus and higher thermal expansion coefficient value in the interior configured to exhibit negative expansion(101b) when thermally excited.

4. The linear actuator as claimed in claim 1, wherein the first material, the second material or the third materialis a metalor a polymer.

5. The linear actuator as claimed in claim 4, wherein the first material is a metal selected from iron or steel, and the second material is copper or aluminium or an alloy thereof, and the third material is same as the first material.

6. The linear actuator as claimed in claim 4, wherein the first and the third material are PTFE, and the second material is PMMA.

7. The linear actuator as claimed in claim 4, wherein the first and the third material are PMMA, and the second material is PTFE.

8. The unit cell structure as claimed in claim 1, wherein the bi- material strips and uni-material strips are attached together with thermal adhesives or fabricated via a 3D printing technique.

9. The thermally excited linear actuator structure as claimed in claim 1,wherein the extent of actuation is increased by increasing the number of unit cells in the actuator.

Dr V. SHANKAR
IN/PA-1733
For and on behalf of the Applicants