Potentialities of optimal design methods and associated numerical tools for the development of new micro- and nanointelligent systems based on structural compliance - An example -

11 pagesInternational audienceThis paper deals with the interest and potential use of intelligent structures mainly based on compliant mechanisms (and optionally including smart materials), for the development of new micro- and nano-robotics devices. The state of the art in optimal design methods for the synthesis of intelligent compliant structures is briefly done. Then, we present the optimal method developed at CEA LIST, called FlexIn, and its new and still in development functionalities, which will be illustrated by a few simple design examples. An opening will be given about the possibility to address the field of Nanorobotics, while adding functionalities to the optimal design method

Study of compliant mechanisms and flexible hinges in topology optimization

This thesis presents a comprehensive study on the application of compliant mechanisms and flexible hinges in topology optimization. Compliant mechanisms are a promising approach for achieving desired functionalities and structural flexibility in engineering designs. By exploiting the inherent elasticity of materials, compliant mechanisms offer advantages such as reduced complexity, improved reliability, and enhanced performance. Topology optimization, conversely, allows obtaining compliant mechanisms with reduced weight through the creation of holes, thus achieving an optimized design. In this work, we explore the integration of compliant mechanisms and flexible hinges within the framework of topology optimization, aiming to propose a method of improvement for the design efficiency and performance of structures in the aerospace field. The thesis begins with a thorough literature review of compliant mechanisms and their role in current aerospace applications. Various design principles and analysis techniques are examined to establish a solid foundation for the subsequent chapters. The study then focuses on the implementation of mathematical models and computational algorithms to incorporate compliant mechanisms and flexible hinges into the topology optimization process. To validate the proposed approach, a series of numerical experiments are conducted. Various case studies are considered, including a gripping and inverter mechanisms. The results demonstrate the effectiveness of compliant mechanisms and flexible hinges in enhancing the performance of optimized structures. The compliant mechanisms exhibit improved flexibility, adaptability, and energy absorption capabilities enabling smooth and controlled motion. Overall, this thesis significantly contributes to the understanding and implementation of compliant mechanisms and their integration with topology optimization techniques. The study not only showcases their potential for creating innovative and efficient designs across various engineering disciplines but also emphasizes their particular relevance in the aerospace field. By exploring the application of compliant mechanisms and topology optimization in aerospace engineering, it has been seen that this cutting-edge technology is opened up for new avenues for further research and development

Surgical Applications of Compliant Mechanisms:A Review

Current surgical devices are mostly rigid and are made of stiff materials, even though their predominant use is on soft and wet tissues. With the emergence of compliant mechanisms (CMs), surgical tools can be designed to be flexible and made using soft materials. CMs offer many advantages such as monolithic fabrication, high precision, no wear, no friction, and no need for lubrication. It is therefore beneficial to consolidate the developments in this field and point to challenges ahead. With this objective, in this article, we review the application of CMs to surgical interventions. The scope of the review covers five aspects that are important in the development of surgical devices: (i) conceptual design and synthesis, (ii) analysis, (iii) materials, (iv) maim facturing, and (v) actuation. Furthermore, the surgical applications of CMs are assessed by classification into five major groups, namely, (i) grasping and cutting, (ii) reachability and steerability, (iii) transmission, (iv) sensing, and (v) implants and deployable devices. The scope and prospects of surgical devices using CMs are also discussed

Surrogate models for the design and control of soft mechanical systems

Soft mechanical systems constitute stretchable skins, tissue-like appendages, fibers and fluids, and utilize material deformation to transmit forces or motion to perform a mechanical task. These systems may possess infinite degrees of freedom with finite modes of actuation and sensing, and this creates challenges in modeling, design and controls. This thesis explores the use of surrogate models to approximate the complex physics between the inputs and outputs of a soft mechanical system composed of a ubiquitous soft building block known as Fiber Reinforced Elastomeric Enclosures (FREEs). Towards this the thesis is divided into two parts, with the first part investigating reduced order models for design and the other part investigating reinforcement learning (RL) framework for controls. The reduced order models for design is motivated by the need for repeated quick and accurate evaluation of the system performance. Two mechanics-based models are investigated: (a) A Pseudo Rigid Body model (PRB) with lumped spring and link elements, and (b) a Homogenized Strain Induced (HIS) model that can be implemented in a finite element framework. The parameters of the two models are fit either directly with experiments on FREE prototypes or with a high fidelity robust finite element model. These models capture fundamental insights on design by isolating a fundamental dyad building block of contracting FREEs that can be configured to either obtain large stroke (displacement) or large force. Furthermore, the thesis proposes a novel building block-based design framework where soft FREE actuators are systematically integrated in a compliant system to yield a given motion requirement. The design process is deemed useful in shape morphing adaptive structures such as airfoils, soft skins, and wearable devices for the upper extremities. Soft robotic systems such as manipulators are challenging to control because of their flexibility, ability to undergo large spatial deformations that are dependent on the external load. The second part of this work focuses on the control of a unique soft continuum arm known as the BR2 manipulator using reinforcement learning (RL). The BR2 manipulator has a unique parallel architecture with a combined bending mode and torsional modes, and its inherent asymmetric nature precludes well defined analytical models to capture its forward kinematics. Two RL-based frameworks are evaluated on the BR2 manipulator and their efficacy in carrying out position control using simple state feedback is reported in this work. The results highlight external load invariance of the learnt control policies which is a significant factor for deformable continuum arms for applications involving pick and place operations. The manipulator is deemed useful in berry harvesting and other agricultural applications

Design of functionally graded compliant mechanisms using topology optimization

This research applies topology optimization to create feasible functionally graded complaint mechanism designs with the aim of improving structural performance compared to traditional homogeneous compliant mechanism designs. Structural performance is assessed with respect to mechanical/geometric advantage and stress distributions. A novel modified solid isotropic material with penalization (SIMP) method is adopted for representing local element material properties in FGM structures. The method of moving asymptotes (MMA) is used in conjunction with adjoint sensitivity analysis to find the optimal distribution of material properties. Functionally graded materials (FGMs) have material properties that vary based on spatial position. Here, FGMs are implemented using two different resource constraints \textendash \ one on the mechanism's volume and the other on the integral of the Young's modulus distribution throughout the design domain. Two sets of results are presented \textendash \ polymeric and metallic designs. Geometric non-linear analysis based on the Neo-Hookean model for hyperelastic materials is used to solve the mechanics problem for polymeric designs, whereas analysis of metallic materials is solved using conventional linear finite element analysis (FEA). Tensile tests are performed to obtain the material properties used in the analysis. To ensure an accurate representation when using linear FEA, metallic designs are subject to stress constraints. A novel method of stress-based design for FGM structures is presented where local yield strength is a function of local Young's modulus. Results suggest that FGMs can achieve the desired improvements in structural performance for certain designs and can also have a favorable effect on the von Mises stress distribution

Variational approach to relaxed topological optimization: closed form solutions for structural problems in a sequential pseudo-time framework

The work explores a specific scenario for structural computational optimization based on the following elements: (a) a relaxed optimization setting considering the ersatz (bi-material) approximation, (b) a treatment based on a non-smoothed characteristic function field as a topological design variable, (c) the consistent derivation of a relaxed topological derivative whose determination is simple, general and efficient, (d) formulation of the overall increasing cost function topological sensitivity as a suitable optimality criterion, and (e) consideration of a pseudo-time framework for the problem solution, ruled by the problem constraint evolution. In this setting, it is shown that the optimization problem can be analytically solved in a variational framework, leading to, nonlinear, closed-form algebraic solutions for the characteristic function, which are then solved, in every time-step, via fixed point methods based on a pseudo-energy cutting algorithm combined with the exact fulfillment of the constraint, at every iteration of the non-linear algorithm, via a bisection method. The issue of the ill-posedness (mesh dependency) of the topological solution, is then easily solved via a Laplacian smoothing of that pseudo-energy. In the aforementioned context, a number of (3D) topological structural optimization benchmarks are solved, and the solutions obtained with the explored closed-form solution method, are analyzed, and compared, with their solution through an alternative level set method. Although the obtained results, in terms of the cost function and topology designs, are very similar in both methods, the associated computational cost is about five times smaller in the closed-form solution method this possibly being one of its advantages. Some comments, about the possible application of the method to other topological optimization problems, as well as envisaged modifications of the explored method to improve its performance close the workPeer ReviewedPostprint (author's final draft

Design of Compliant Mechanisms: Applications to MEMS

Compliant mechanisms are single-piece flexible structures that deliver the desired motion by undergoing elastic deformation as opposed to jointed rigid body motions of conventional mechanisms. Compliance in design leads to jointless, no-assembly (Fig. 1), monolithic mechanical devices and is particularly suited for applications with small range of motions. The compliant windshield wiper shown in Fig. 1 illustrates this paradigm of no-assembly. Conventional flexural mechanisms employ flexural joints that connect relatively rigid links as depicted in Fig. 2. Reduced fatigue life, high stress concentration and difficulty in fabrication are some of the drawbacks of flexural joints. Our focus is on designing compliant mechanisms with distributed compliance which employs flexural links (see Fig. 3) and have no joints (neither pin nor flexural joints) for improved reliability, performance, and ease of manufacture. Distributed compliant mechanisms derive their flexibility due to topology and shape of the material continuum rather than concentrated flexion at few regions. This paper focuses on the unique methodology employed to design jointless mechanisms with distributed compliance. The paper also illustrates a compliant stroke amplification mechanism that was recently designed, fabricated and tested for MEMS application.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44061/1/10470_2004_Article_353448.pd

Development Of a Novel Multi-disciplinary Design Optimization Scheme For Micro Compliant Devices

The focus of this research is on the development of a novel multi-disciplinary design optimization scheme for micro-compliant devices. Topology optimization is a powerful tool that can address the need for a systematic method to design MEMS. It is expected that systematic design methods will make the design of micro devices transparent to the user and thus spur their use. Although topology optimization of MEMS devices with embedded actuation has received a great deal of attention among researchers recently, there is not a significant amount of literature available on the subject. The limited literature available addresses multi-physics topology optimization, which employs the homogenization method. However, the products of this method inherit the drawbacks of homogenized material discretization, including checkerboard pattern, gray-scale material and narrow flexural hinges in the optimum solution. In this thesis, a new topology optimization scheme is introduced that addresses the specific needs of MEMS domain. A new discretization approach with frame-ground structure is introduced. This approach offers significant conceptual and practical advantages to the compliant MEMS optimization problem, including compatibility with MEMS fabrication processes. The design spaces of compliant mechanisms are non-convex and it is critical to employ an algorithm capable of converging to the global optimum without the need to evaluate gradients of objective function. In this thesis, an efficient real-coded genetic algorithm is implemented, which shows a better repeatability and converges to very similar solutions in different runs. This new method of optimization facilitates the use of a coarse subdivision of the design domain rather than the homogenized material method, for the same resolution of shape definition. Therefore, the topology optimization scheme developed in this thesis significantly reduces the computational burden without compromising the sharpness of the shape definition. As the problem of compliant mechanism design is posed as a set of conflicting objectives, a well-posed multi-criteria objective function is introduced which avoids one objective dominating the solution. Moreover, the formulation is modified to incorporate electro-thermal boundaries and enables the optimization of the compliant mechanisms to transfer maximum motion or maximum force at the output. A number of design examples are used to demonstrate the ability of the procedure to generate non-intuitive topologies. Their performance is verified using ANSYS and compared with results from the homogenization method and designs reported in the available literature