Design and control of a prototype structure that adapts to loading through large shape changes

Adaptive structures have the ability to modify their shape and internal forces through sensing and actuation in order to maintain optimal performance under changing actions. Previous studies have shown that substantial whole-life energy savings with respect to traditional passive designs can be achieved through well-conceived adaptive design strategies. The whole-life energy comprises an embodied part in the material and an operational part for structural adaptation. Structural adaptation through controlled large shape changes allows a significant stress redistribution so that the design is not governed by extreme loads with long return periods. This way, material utilization is maximized and embodied energy is reduced. A design process based on shape optimization has been formulated to obtain shapes that are optimal for each load case. A geometrically non-linear force method is employed to control the structure into required shapes. This paper presents the experimental testing of a small-scale prototype adaptive structure produced by this design process. The structure is a simply supported planar truss. Shape adaptation is achieved through controlled length changes of turnbuckles that strategically replace some of the structural elements. The stress is monitored by strain sensors fitted on some of the truss elements. The nodal coordinates are monitored by an optical tracking system. Numerical predictions and measurements have a minimum Pearson correlation of 0.86 which indicates good accordance. Although scaling effects have to be further investigated, experimental testing on a small-scale prototype has been useful to assess the feasibility of the design and control methods outlined in this work. Results show that stress homogenization through controlled large shape changes is feasible.

Editorial: Left ventricular dysfunction after open repair of simple confenital heart defects in infants and children

This paper presents structural analysis methods to model morphing, achieved via buckling of struts in multi-element structural components. An exact element theory and an energy-based variational approach are used to deduce a closed-form relationship between actuator force and displacement, by firstly assuming an ideal actuator. Subsequently, the flexibility of the actuator is incorporated via a lumped parameter model to account for actuator-structure interaction. The single element actuation concept is extended to multielement frame architecture to produce displacements of an example aircraft tail section, by actuating one or two elements of the assembly. Hybrid actuation, combining large force with large displacement actuators is also introduced, whereby large shape changes can be achieved with optimum demand on the actuators. It is concluded that exact element theory has significant potential in deducing closed-form relationship between the actuator forces and structural displacements. The energy approach, based on the trial function to represent the displacements, is shown to be approximate, but sufficient. These closed-form relationships can be readily used in optimisation routines to achieve an optimal structure to produce structural shape changes.

ABSTRACTThe quasi-binary eutectic NiAl-9% Mo with faceted molybdenum fibers was subjected to both thermal annealing conditions and to annealing under thermal cycling conditions to determine the microstructural stability of this alloy. The static temperature tests were run at 0.85TM- 0.97TMin an argon gas atmosphere. The thermal cycling tests were performed between temperatures of 700°C and 1200°C by induction heating disk-shaped specimens in an argon gas atmosphere using time-temperature heating and cooling profiles to approximate potential engine applications. To quantify microstructural changes, the fiber size and size distribution and number of fibers per unit area were measured as a function of time at temperature. The overall results demonstrate that the directionally solidified eutectic NiAl-9Mo subjected to thermal fatigue conditions exhibits cell boundary coarsening and large shape changes, whereas the microstructure under static stress-free annealing is stable.

System-level aircraft conceptual design can be viewed as a high-dimensional, multidisciplinary design optimization (MDO) problem involving potentially thousands of design variables across multiple subsystems and design conditions. Recent advances in modeling and adjoint-based sensitivity analysis have enabled the use of low-fidelity physics-based models to solve this complex problem in a computationally efficient manner using established MDO algorithms. However, integrating higher-fidelity aerodynamic simulations into comprehensive, large-scale MDO of aircraft concepts remains challenging. This is in large part due to the increased computational cost and the large geometric changes that may occur at the conceptual design stage. Since many mid- to high-fidelity aerodynamic solvers require conforming surface meshes, there is a need for a general deformation method that efficiently updates surface mesh nodes based on changes to the design geometry in a differentiable manner. In this paper, we aim to fill this need by developing such a method using a two-step process: 1) recompute intersections between components given large geometric changes and 2) deform the surface mesh using thin plate splines to conform to the updated intersections. The method is reasonably efficient and all steps are differentiable. We test our method on a simple tube-and-wing geometry for which we translate and rotate the wing with respect to the fuselage. The method performs well for all considered rotation angles (±10 degrees) with over 95% of triangles being of acceptable quality. For translations, the performance is similar up to 1.5 times the chord length. We couple the method with a panel-method aerodynamic solver, confirming its applicability in aircraft conceptual design.

The Reproducing Kernel Particle Method (RKPM) is one of several so-called meshfree methods of structural analysis and has been applied in this paper to the gradient-based shape optimization of two-dimensional automotive components. As indicated by the term meshfree, no mesh is required, but rather a field of points, or particles, are distributed within the domain of the problem. Three standard linear examples are chosen to allow a comprehensive comparison between the optimization with the RKPM and the state-of-art optimization tool in Unigraphics Version 18 (UG V18). The results show that the optimization analysis with the RKPM is able to accommodate very large shape changes required in the optimization process without doing particle re-adaptation, and provides accurate solutions of the objective and constraint functions and their gradients, therefore facilitates a fast convergence of the gradient-based optimization algorithm.

A finite element based shape optimization program has been developed for three‐dimensional shell structures which allows for multiple edge movement and large shape changes. This program automatically performs shape optimization by linking together adaptive mesh generation, substructuring and optimization algorithms with a commercial finite element analysis program, MSC/NASTRAN. This paper examines the robustness of the shape optimization program by varying the initial shape and design parameters for three examples. This is performed to determine the conditions under which similar optimum shapes will be generated for different initial conditions.

Morphing technology enables aerospace vehicles to achieve a broader range of operational modes. Computational aeroelastic and design analysis tools of these vehicles must be able to handle arbitrarily large deformations and shape changes. The objective of this study is to improve aircraft performance, expand its flight envelope, replace conventional control surfaces, reduce drag, and reduce vibration/flutter by morphing technology. This paper presents a revolutionary approach to solve aeroelastic instabilities of air vehicles undergoing large deformation and arbitrarily large shape change. The technology was implemented in a CFD code and demonstrated for several solid-fluid interaction and morphing telescope wing problems. This innovative technology casts structure/body-dynamics equations in an Eulerian reference frame, so that the dependent variables for the fluid and solids are continuous and coupled. The volume of solid tracks the evolution of solid-fluid interface, while implicit formulation embeds the interaction forces into the solution procedure. The method is devoid of troublesome moving mesh or tedious grid regeneration issues. The developed technology will provide the lacking design and analysis capability in conceptualizing how vehicles should safely change shape during flight.

Highly redundant modular robots may undergo large shape changes which significantly affect the geometry and dynamics of the robot. In these motions, the shape change may induce a tipping or rolling behavior of the robot. The paper describes the dynamic modeling, locomotion planning, control and simulation of such rolling motions for the Tetrobot modular robots. The motion is described by the path profiles of controlled nodes, the tipping criteria and dynamic tipping motion and an impact-reaction model of contact with the ground. These phases of motion are described using Newton-Euler dynamic equations and the principle of conservation of angular momentum. In the paper, a two-phase planning and switching control sequence is introduced to achieve stable and reliable motion of a Tetrobot modular robot. Simulation results illustrate the tipping behavior of a tetrahedron, the dynamic contact and rolling of an icosahedral Tetrobot and dynamic control of the rolling Tetrobot. The resulting models are useful to analyze and control both intentional rolling as a new mode of mobility as well as the avoidance of unintentional tipping and rolling during task execution.

Materials and structures with shape morphing capabilities are attractive in aerospace, maritime vehicles and robotics: such systems offer lighter weight, better distributed stresses, simpler kinematics, smaller numbers of actuators and higher reliability. Current morphing materials are either very compliant to achieve large shape changes (metamaterials, compliant mechanisms, hydrogels), or very stiff but with infinitesimal shape changes that require large actuation forces (metallic or composite sandwich panels with piezoelectric actuation). Morphing efficiency and structural stiffness are therefore mutually exclusive properties in current engineering morphing materials, which limit the range of their application. Here we propose a design for a stiff morphing beam made of “meta-elements” with unusual combinations of elastic properties, and which can be actuated from a simple push/pull at the base. To capture the performance of the beam we propose and use three metrics: The “morphing amplitude”, the “morphing compliance” and “structural stiffness”. We develop nonlinear finite elements to explore a wide range of designs, and to show some inevitable tradeoffs of stiffness and morphing. To aid the design of the structure we propose ternary diagrams to select optimum combinations of properties for the meta-elements. Finally, we fabricated and tested three prototypes of stiff morphing beams to validate the approach.

Experimental and theoretical results are presented on the evolution of large elastic deformation, non-uniform curvature, shape changes and geometric instability in substrates of Si wafers with metal films. The critical diameter and thickness of the Si wafer, for which large deformation and shape instability occur, are identified, as functions of the line tension in the film (which is the product of the biaxial stress in the film and the film thickness). Observations of the curvature and shape variations along the wafer diameter and geometry-dependence of the shape instability compare favorably with those predicted by detailed finite element analyses.

We have designed and synthesized a novel [2]pseudorotaxane-based molecular machine in which the interconversion between end-to-interior and end-to-end loop structures is reversibly controlled by electrochemical stimuli. Cucurbit[8]uril (CB[8]) and the thread molecule 3(4+) with an electron-rich hydroxynaphthalene unit and two electron-deficient viologen units form the 1:1 complex 4(4+) with an end-to-interior loop structure, which is reversibly converted into an end-to-end structure upon reduction. Large changes in shape and size of the molecule accompany the reversible redox process. The key feature of the machine-like behavior is the reversible interconversion between an intramolecular charge-transfer complex and viologen cation radical dimer inside CB[8] triggered by electrochemical stimuli.

As walking robots are requested to perform tasks in rough terrain, the development of actuators capable to flexibly adapt to the unstructured environment becomes more and more necessary. The conventional mechanisms with stiff joints make the robots more complex, heavy, large and expensive. Shape Memory Alloys are a category of artificial muscles which can be used as actuators for a walking robot. Even if they can exhibit large changes in shape when heated and cooled, only one part of their de-formation can be used, if we want to maximize the actuator life. This is why smart mechanisms that can convert the small strain of the wire into large motion are necessary. In this paper, an example of using Shape Memory Alloys as actuators for a hexapod walking micro-robot is presented. A leg mechanism that can convert the small strain of these actuators in large motion is also discussed. DOI: http://dx.doi.org/10.5755/j01.mech.20.1.3531

This research is concerned with the design of adaptive structures for achieving global multishape morphing aerodynamic configurations by using thin-walled structures. The proposed methodologies pursue two threads toward global optimization of morphing structures, by providing means of aerodynamic enhancement using efficient structural shape optimization. A heuristic approach is proposed in this work that enables morphing through a range of stable cambered airfoils to achieve aerodynamic properties for different maneuvers, with the benefit of lowpowered actuation control. This allows large changes in shape by exploiting a range of incremental nonlinear structural solutions while keeping prescribed flow improvements. Such a heuristic argument provides the basis for global shape control of three-dimensional wings and is applied to aerodynamic design to provide enhanced roll control. A hierarchical strategy is employed herein, interleaving parameterization enhancement followed by structural optimization into the aerodynamic design process, such that the design paradigm, in conjunction with global approximation techniques, is emphasized by enhanced roll while drag is minimized. This figure of merit is complemented by structural metrics and constraints to maintain product integrity.

Monitoring the shape of morphing is essential for their effective and safe operation. However, current sensing systems such as fiber optic sensors are expensive, rigid, and unsuitable for monitoring large shape changes without being susceptible to failure or performance degradation. Therefore, a new class of sensors that does not suffer from these serious limitations is presented. The proposed sensor system relies in its operation on a specially configured distributed network of wires that are embedded in the composite fabric of these structures. The output of the sensor network is wirelessly transmitted to a control processor to compute the linear and angular deflections, the shape, and maps of the strain distribution and power flow over the entire surface of the morphing. The deflection and shape information are vital to ascertain that the structure is properly deployed and that its surfaces are operating wrinkle-free. The strain map ensures that the structure is not loaded excessively to adversely affect its service life. While the power flow map provides a metric that uniquely identifies the structural health in a manner that mimics biological systems which tend to redistribute the load and redirect its path away from the injured sites. The equations governing the operation of the sensor network are developed for a beam-like morphing structure using the non-linear theory of finite elements. The resulting equations will provide the sensor with its unique interpolation capabilities that make it possible to map the linear and angular deflection and strain fields as well as the power flow distribution over the entire surface of the morphing structure. The theoretical and experimental characteristics of the sensor network are determined under static and dynamic loading conditions. The results obtained are used to demonstrate the merits and potential of this new class of sensors as a viable means for monitoring the static and dynamic deflections of 1-D morphing structures. Integration of the proposed sensor network with the supporting electronics and with arrays of flexible actuators will enable the development of a self-contained, actively controlled, and autonomously operating new generation of morphing.