Showing papers on "Compliant mechanism published in 2005"

Design of multimaterial compliant mechanisms using level-set methods

Abstract: A monolithic compliant mechanism transmits applied forces from specified input ports to output ports by elastic deformation of its comprising materials, fulfilling required functions analogous to a rigid-body mechanism. In this paper, we propose a level-set method for designing monolithic compliant mechanisms made of multiple materials as an optimization of continuum heterogeneous structures. Central to the method is a multiphase level-set model that precisely specifies the distinct material regions and their sharp interfaces as well as the geometric boundary of the structure. Combined with the classical shape derivatives, the level-set method yields an Eulerian computational system of geometric partial differential equations, capable of performing topological changes and capturing geometric evolutions at the interface and the boundary. The proposed method is demonstrated for single-input and single-output mechanisms and illustrated with several two-dimensional examples of synthetics of multimaterial mechanisms of force inverters and gripping and clamping devices. An analysis on the formation of de facto hinges is presented based on the shape gradient information. A scheme to ensure a well-connected topology of the mechanism during the process of optimization is also presented.

Dynamic Modeling of Compliant Mechanisms Based on the Pseudo-Rigid-Body Model

Abstract: Based on the principle of dynamic equivalence, a new dynamic model of compliant mechanisms is developed using the pseudo-rigid-body model. The dynamic equation of general planar compliant mechanisms is derived. The natural frequency of a compliant mechanism is obtained in the example of a planar compliant parallel-guiding mechanism. The numerical results show the effectiveness and advantage of the proposed method compared with the methods of FEA and flexible mechanisms.

Compliant mechanism design using multi-objective topology optimization scheme of continuum structures

Abstract: Topology optimization problems for compliant mechanisms using a density interpolation scheme, the rational approximation of material properties (RAMP) method, and a globally convergent version of the method of moving asymptotes (GCMMA) are primarily discussed. First, a new multi-objective formulation is proposed for topology optimization of compliant mechanisms, in which the maximization of mutual energy (flexibility) and the minimization of mean compliance (stiffness) are considered simultaneously. The formulation of one-node connected hinges, as well as checkerboards and mesh-dependency, is typically encountered in the design of compliant mechanisms. A new hybrid-filtering scheme is proposed to solve numerical instabilities, which can not only eliminate checkerboards and mesh-dependency efficiently, but also prevent one-node connected hinges from occurring in the resulting mechanisms to some extent. Several numerical applications are performed to demonstrate the validity of the methods presented in this paper.

Design and Application of Compliant Mechanisms for Surgical Tools

Abstract: This paper introduces the benefits of exploiting elasticity in the engineering design of surgical tools, in general, and of minimally invasive procedures, in particular. Compliant mechanisms are jointless mechanisms that rely on elastic deformation to transmit forces and motion. The lack of traditional joints in these single-piece flexible structures offers many benefits, including the absence of wear debris, pinch points, crevices, and lubrication. Such systems are particularly amenable to embedded sensing for haptic feedback and embedded actuation with active-material actuators. The paper provides an overview of design synthesis methods developed at the Compliant Systems Design Laboratory and focuses specifically on surgical applications. Compliant systems have potential to integrate. well within the constraints of laparoscopic procedures and telerobotic surgery. A load-path representation is used within a genetic algorithm to solve two gripper example problems. In addition, the paper illustrates the design and construction of an organ (kidney) manipulator for use in minimally invasive procedures.

Aircraft Structural Morphing Using Tendon-Actuated Compliant Cellular Trusses

Abstract: Recently, smoothly-deforming aircraft structures have been investigated for their ability to adapt to varying flight conditions. Researchers aim to achieve large changes in the shape of the wings: area changes of up to 50% and aspect-ratio changes of up to 200% are being pursued. The research described in this paper aims to develop a structural concept capable of achieving continuous stable deformations over a large range of aircraft shapes. The basic concept underlying the approach is a compliant cellular truss, with tendons used as active elements. The truss members of the unit cell are connected through compliant joints such that only modest bending moments may be transmitted from one member to another. Actuation is achieved by pulling on one set of cables while releasing another set. The tendonactuated compliant truss can be made to behave locally, and temporarily, as a nearmechanism, by releasing appropriate cables. As a result, in the absence of aerodynamic forces, the structure can be morphed using relatively low forces. The cables are reeled in or released in a controlled manner while the structure is loaded, hence, the stability of the structure can be maintained in any intermediate position. Highly-distributed actuation also enables the simultaneous achievement of global shape changes as the accumulation of local ones, while the use of compliant joints rather than true rotating joints eliminates binding as a significant concern. A six-noded octahedral cell with diagonal tendon actuation is developed for a bending type deformation in the wing. Initial cell geometry is determined by “strain matching” to the local morphing deformation required by the application. A finite element analysis is performed on a wing made of these unit cells and sized for a representative UAV weighing 3000 lbs. The areas of the individual truss members are sized so that they don’t fail or buckle under the air loads, while deflection at the wing tip is reduced. The octahedral unit cell is capable of achieving smooth deformations of the truss structure. The cell size is dictated by the available space and the morphing strain. The cell sizes are reasonable for strains on the order of 10% to 15% and get smaller for larger strains. Additional cell shapes are being investigated for larger area changes through a process of topology optimization using genetic algorithms. Numerous other technical challenges remain, including the details of actuation and a robust skin.

Analysis and design of parallel mechanisms with flexure joints

Abstract: Flexure joints are frequently used in precision-motion stages and microrobotic mechanisms due to their monolithic construction. The joint compliance, however, can affect the static and dynamic performance of the overall mechanism. In this paper, we consider the analysis and design of general platform-type parallel mechanisms containing flexure joints. Based on static performance measures such as task-space stiffness and manipulability, and constraints such as joint stress, mechanism size, and workspace volume, we pose the design problem as a multiobjective optimization. We first calculate the Pareto frontier, which can then be used to select the desired design parameters based on secondary criteria, such as performance sensitivity and dynamic characteristics. To facilitate design iteration, we apply the pseudo rigid-body approach with a lumped approximation of the flexure joints. A planar mechanism is used to illustrate the analysis and design techniques.

Synthesis of Compliant Mechanisms for Path Generation using Genetic Algorithm

Abstract: In this paper is described a procedure to synthesize the optimal topology, shape, and size of compliant continua for a given nonlinear output path. The path is prescribed using a finite number of distinct precision points much in accordance with the synthesis for path generation in traditional kinematics. Geometrically nonlinear analysis is employed to model large displacements of the constituent members. It is also essential to employ nonlinear analysis to allow the output port to negotiate the prescribed path accurately. The topology synthesis problem is addressed in its original binary form in that the corresponding design variables are only allowed to assume values of 0 for no material and 1 for the material present at a site in the design region. Shape and size design variables are modeled using continuous functions. Owing to the discrete nature of topology design variables, since gradient based optimization methods cannot be employed, a genetic algorithm is used that utilizes only the objective values to approach an optimum solution. A notable advantage of a genetic algorithm over its gradient based counterparts is the implicit circumvention of nonconvergence in the large displacement analysis, which is another reason why a genetic algorithm is chosen for optimization. The least squared objective is used to compare the design and desired output responses. To allow a user to specify preference for a precision point, individual multiple least squared objectives, same in number as the precision points are used. The multiple objectives are solved using Nondominated Sorting in Genetic Algorithm (NSGA-II) to yield a set of pareto optimal solutions. Thus, multiple solutions for compliant mechanisms can be obtained such that a mechanism can traverse one or some precision points among those specified more precisely. To traverse the entire path, a solution that minimizes the sum of individual least square objectives may be chosen. Synthesis examples are presented to demonstrate the usefulness of the proposed method that is capable of generating a solution that can be manufactured as is without requiring any interpretation.

An effective method of synthesizing compliant adaptive structures using load path representation

Abstract: The synthesis of shape morphing compliant mechanism is inherently different from the typical single output design problems, due to the multiple output points along the morphing boundary. We have previously developed a genetic algorithm (GA)-based synthesis approach, incorporating a binary ground structure parameterization, to systematically design shape morphing compliant mechanisms. However, the approach is ineffective due to issues such as the generation of disconnected structures and the need to choose an initial mesh. In this paper, we present the 'load path representation,' which is developed to overcome the issues encountered using the binary ground structure parameterization. The performance of the load path approach over the binary ground structure approach is demonstrated through several design examples. The results have shown that the load path approach offers several advantages, such as (a) eliminating the need of an initial ground structure, (b) ensuring structural connectivity, and (c) yielding solutions that generate desired shape change efficiently.

Fully compliant tensural bistable micromechanisms (FTBM)

Abstract: A new class of bistable mechanisms, the fully compliant tensural bistable micromechanism (FTBM) class, is introduced. The class consists of linear bistable micromechanisms that undergo tension loads, in addition to the bending loads present, through their range of motion. Proof-of-concept designs fabricated in two different microelectromechanical systems (MEMS) surface micromachining processes were demonstrated. Three sets of refined designs within the FTBM class were designed using optimization methods linked with nonlinear finite element analysis (FEA), then fabricated and tested. Measured force and displacement performance are compared to values obtained by FEA. On-chip actuation of the bistable mechanisms was achieved using thermomechanical in-plane microactuators (TIMs). The FTBM class of bistable mechanisms explores a relatively new design space for fully compliant micromechanisms, and mechanisms from this class have promise in such applications as micro shutter positioning, microvalves, and electrical microrelays. [1448].

Sparse monolithic compliant mechanisms using continuum structural topology optimization

Abstract: A formulation for design of continuous, hinge-free compliant mechanisms is developed and examined within a continuum structural topology optimization framework. The formulation makes use of two distinctly different sets of springs, the first of which are artificial springs of relatively large stiffness attached to the input and output ports of the mechanism model, and the second of which are springs attached only to the output port with smaller stiffnesses that represent the resistance of the workpiece as it is manipulated by the mechanism. The proposed formulation involves solving two nested optimization problems. In the inner problem the arrangement of a constrained amount of structural material is optimized to maximize the mechanism's mutual potential energy in response to a force loading at the input port while working against the stiff artificial springs on the input and output ports. As the relative stiffness of the artificial springs increases, the material continuity of the mechanism also increases to the point where de facto ‘hinge’ regions are eliminated. In the outer problem, the artificial springs are removed and one solves for an appropriate amount of structural material that yields the desired finite deformation compliance characteristics of the mechanism when working against the real workpiece resistance. Different aspects of the proposed formulation are demonstrated on a number of examples and discussed. Copyright © 2004 John Wiley & Sons, Ltd.

Topology design of large displacement compliant mechanisms with multiple materials and multiple output ports

Abstract: Topology optimization of compliant mechanisms is presented in this paper wherein the layout design problem is addressed in its original binary or discrete (0-1) form. Design variables are modeled as discrete variables and allowed to assume values pertaining only to their void (0) or solid (1) states. Due to this discrete nature, a genetic algorithm is employed as an optimization routine. Using the barrier assignment approach, the search algorithm is extended to use with multiple materials. The layout design of compliant mechanisms is performed wherein displacements at multiple points (ports) in the design region are maximized along the respective prescribed directions. With multiple output ports and multiple materials, additional freedom in motion and force transduction can be achieved with compliant mechanisms. Geometrically large deformation analysis is employed to compute the displacement-based multiple objectives that are extremized using Nondominated Sorting in Genetic Algorithms (or NSGA). With genetic algorithms, buckling or snap through like issues with nonconvergent solutions in the population when computing nonlinear deformations can be implicitly circumvented.

Passive compliant wafer stage for single-step nano-imprint lithography

Abstract: Nano-imprint lithography, which has the advantages of simplicity, low cost, high replication fidelity and relatively high throughput, requires surface contact between a template with nano patterns and a wafer that transfers the patterns. This article presents the wafer stage for single-step nano-imprint lithography. The wafer stage has a six degree-of-freedom compliant mechanism for complete contact between the surface of the template and the surface of the wafer. The compliant mechanism consists of an inner mechanism for in-plane motion and an outer mechanism for out-of-plane motion. The inner and outer mechanisms have symmetric flexures, which were machined monolithically, onto each plane to cope with thermal deformation. The wafer stage, which was designed to satisfy stiffness requirements, is analyzed with the aid of a dynamic model for a flexure mechanism and a finite element method. Experiments were conducted on the wafer stage in a nano-imprint machine, and nano patterns with linewidths of 100 and ...

Compliant Joint Design Principles for High Compressive Load Situations

Abstract: Buckling failure has been a major obstacle in designing compliant joints in high compression applications. This paper describes two principles, isolation and inversion, that can be successfully applied to many compliant joints to increase their ability to withstand high compressive loads by avoiding buckling-prone loading conditions. Isolation and inversion give rise to a new breed of compliant joints called high compression compliant mechanisms (HCCM). HCCMs have many of the inherent advantages of compliant mechanisms with the additional qualities of high load-bearing joints. This added robustness in compression can be achieved without adversely affecting the kinematic behavior of the joint.

MARIONET: An exotendon-driven rotary series elastic actuator for exerting joint torque

Abstract: A cable-driven, rotary series elastic actuator named MARIONET (moment arm adjustment for remote induction of net effective torque) is introduced as a novel means to deliver torque to a joint. Its advantages include remote actuation, independent control of compliance and equilibrium, and in future versions, the ability to span multiple joints. This cable-driven, compliant mechanism should prove very useful in a variety of human-robot interactions. Here we present a single joint device evaluated in terms of its position and torque step responses, its ability to follow a minimum jerk trajectory, and its ability to create torque fields. Results show that this device behaves as planned with several important limitations. We conclude with potential applications of this type of mechanism.

Topological Synthesis of Compliant Mechanisms Using Spanning Tree Theory

Abstract: This paper introduces the spanning tree theory to the topological synthesis of compliant mechanisms, in which spanning trees connect all the vertices together using a minimum number of edges. A valid topology is regarded as a network connecting input, output, support, and intermediate nodes, which contains at least one spanning tree among the introduced nodes. Invalid disconnected topologies can be weeded out if no spanning tree is included. No further deformation analysis and performance evaluation is needed to invalidate disconnected topologies. Problem-dependent objectives are optimized for topological synthesis of compliant mechanisms. Constraints about maximum input displacement and force, maximum stress and overlapping connections are directly imposed during optimization process. The discrete optimization problem is solved by genetic algorithm with penalty function handling constraints. Two examples are given to verify the effectiveness of the proposed synthesis procedure.

Shape design of pin-jointed multistable compliant mechanisms using snapthrough behavior

Abstract: A general approach is presented for generating pin-jointed multistable compliant mechanisms using snapthrough behavior. An optimization problem is formulated for minimizing the total structural volume under constraints on the displacements at the specified nodes, stiffnesses at initial and final states, and load factors to lead to snapthrough behavior. The design variables are cross-sectional areas and the nodal coordinates. It is shown in the numerical examples that several mechanisms can be naturally found as a result of optimization starting from randomly selected initial solutions. It is also shown that no local bifurcation point exists along the equilibrium path, and the obtained mechanism is not sensitive to initial imperfections.

Mechanical fish robot exploiting vibration modes for locomotion

Abstract: Compliant mechanisms whose dominant modes of vibration match the desired kinematics for locomotion in a given environment are used to mimic motions of a living animal Mechanisms are simple and mechanically robust They may have as few as one actuator, which excites the compliant portion to vibrate in a natural mode that results in motion that mimics a living animal Additional actuators may drive directional elements such as flippers Models for compliant mechanism bodies were derived and used to identify actuator, material, and geometrical properties of the required mechanisms The design and fabrication techniques of mechanisms implementing these ideas is also presented Experiments found that important features of fish-swimming kinematics can be captured qualitatively by compliant mechanisms The resulting mechanism swimming performance was ⅓ of the real fish performance, comparable to current robotic fish A compliant mechanism approach to biomimetic locomotion has significant advantages since mechanisms are simpler and more robust than traditional mechanisms used for biomimetic robots and performance achieved is comparable Radio control is straightforward The flexible material may be uniform, but need not be Active materials, such as piezoelectric materials may be used to change the stiffness or other mechanical properties of the flexible portion The material may be distributed uniformly or non-uniformly The tail portion may be tapered or uniform or variable cross-section

Initial thoughts on weight penalty effects in shape-adaptable systems

Abstract: If a structural system has to be subjected to high loads and large geometry changes, according to the state of the art, articulated systems with discrete actuators are used, with the articulated systems consisting of stiff members connected by hinges. As an alternative, smart structures technologies can supply solutions based on the deliberate use of structural flexibility and on distributed actuation. In order to assess the advantages, which can be expected from such solutions, a thorough comparison must be made between the properties of compliant mechanisms and the conventional ones. A crucial aspect of this comparison, on which this contribution is focused, is the impact on the system’s structural weight. The first part of the paper deals with the relevance of weight penalty effects in shapeadaptable systems, with a special focus on airfoil shape control. A quantitative analysis of weight penalty effects in pin-jointed articulated structures follows. The criterion on which this analysis is based allows a characterization of general validity, not restricted to a particular example, and can be applied to other mechanisms or hinge architectures, providing a sound way of assessing the lightweight potential of a given concept and allowing a consistent comparison between different design philosophies. An extension to weight penalty effects in compliant systems shows a higher degree of complexity and could not be addressed in the same detail in this study. Anyway, some peculiar aspects are discussed in the final part of the paper, which can serve as a basis for future developments in this sense. In particular, the dependence of weight penalty effects on the system’s range of motion as well as on the loaddependence of the mechanism’s kinematics is addressed. Even if the presented results can be of direct significance to the designer of conventional articulated mechanisms, the primary relevance of this work is to be seen in the long term. Its main target is to provide a basis for the analysis of the potential offered by the compliant mechanisms and smart materials for the realization of light shape-adaptable structures and to give an impulse to research efforts aiming at developing suitable optimization procedures as well as formulating proper design rules for such kind of systems.

Accurate low DOF modeling of a planar compliant mechanism with flexure hinges: the equivalent beam methodology

Abstract: A methodology for accurate and efficient finite elements method (FEM) simulations of planar compliant mechanisms with flexure hinges is presented. First, using symmetry/antisymmetry boundary conditions and 3D elements, one-eighth of a single hinge is simulated to determine its true stress/stiffness characteristics. A set of fictitious beams is derived, which have the identical characteristics. This set is used in conjunction with other beams that model relatively stiff links to generate an equivalent model of an entire mechanism consisting of the beam elements only. The model has a low number of degrees-of-freedom (DOF) and appears to be more accurate than any 2D FEM models, even those with very large number of DOF. The methodology has been developed specifically for the right circular flexure hinge; however, it can be applied to all types of revolute flexure hinges.

Design and analysis of planar compliant microgripper based on kinematic approach

Abstract: The overall design scope of most microgrippers developed in recent years lacks a systematic mechanism design approach. Accordingly, the main objective of this investigation is to establish a new design concept in order to enhance the design scope of microgrippers. Using a systematic design procedure which particularly stresses planar compliant mechanisms, this study presents an atlas of 28 different planar compliant linkages for two-finger microgrippers. The preliminary FEM simulation results are in good agreement with the expected kinematic motion. Moreover, the stress analysis also points out that the relationship between the direction of driving force and orientation of deflected compliant joints is one of the crucial factors for designing the compliant microgripper mechanism. Hence, the mechanism design concept presented in this study can be integrated into the design of micro-scale actuating devices.

A compliant end-effector for microscribing

Abstract: A new end-effector was designed and built for microscribing processes using principles of compliant mechanisms. Initial testing of a chemomechanical microscribing process showed that low forces produce lines that are significantly smoother, more uniform, and exhibit less material chipping. From the testing it is apparent that there is a need for a specialized precision end-effector that (1) is passively controlled, (2) has low axial stiffness, and (3) has high lateral stiffness. To meet these needs a three segment folded-beam compliant mechanism was chosen. This design is passively controlled, has a high axial/lateral stiffness ratio, is ideal for clean sensitive applications, and can be designed for a range of low forces. The axial stiffness of the end-effector was modeled using both the compliant mechanism pseudo-rigid body model and linear-elastic beam theory. The lateral stiffness was modeled using FEA techniques. Ratios of lateral stiffness to axial stiffness were found to be nearly 1000:1. A fatigue analysis was also performed and it was determined that the mechanism could reach approximately 1 billion cycles before failure. The compliant end-effector design is a significant improvement over existing scribing alternatives and can produce smooth and uniform scribed lines that exhibit less material chipping.

Design of a piezoelectric actuator and compliant mechanism combination for maximum energy efficiency

Abstract: The combined optimization of a compliant mechanism and a piezoelectric stack actuator for maximum energy conversion efficiency is considered. The analysis assumes all components to be free from dissipation and that the piezoelectric stack actuator is driven by an ideal sinusoidal voltage source. The energy conversion efficiency is defined as the ratio of the output mechanical energy to the input electric energy. Using linear two-port models, an analytical expression for the maximum energy conversion efficiency is derived. It is shown that the optimization of the piezoelectric stack actuator can be decoupled from the topology optimization of the compliant mechanism. Computational verification of the analytical results is presented for two ground structures modeled using frame elements. The trade-off between displacement amplification and maximization of the energy conversion efficiency is examined.

Simulation-Based Design Under Uncertainty for Compliant Microelectromechanical Systems

Abstract: SIMULATION-BASED DESIGN UNDER UNCERTAINTY FOR COMPLIANT MICROELECTROMECHANICAL SYSTEMS Jonathan W. Wittwer Department of Mechanical Engineering Doctor of Philosophy The high cost of experimentation and product development in the field of microelectromechanical systems (MEMS) has led to a greater emphasis on simulation-based design for increasing first-pass design success and reliability. The use of compliant or flexible mechanisms can help eliminate friction, wear, and backlash, but compliant MEMS are sensitive to variations in material properties and geometry. This dissertation proposes approaches for design stage uncertainty analysis, model validation, and robust optimization of nonlinear compliant MEMS to account for critical process uncertainties including residual stress, layer thicknesses, edge bias, and material stiffness. Methods for simulating and mitigating the effects of non-idealities such joint clearances, semi-rigid supports, non-ideal loading, and asymmetry are also presented. Approaches are demonstrated and experimentally validated using bistable micromechanisms and thermal microactuators as examples.

A Three Degree-of-Freedom Model for Self-Retracting Fully Compliant Bistable Micromechanisms

Abstract: A three degree-of-freedom (3DOF) pseudo-rigid-body model (PRBM) has been developed and used in the design of a new class of self-retracting fully compliant bistable micro-mechanism (SRFBM). The SRFBM provides small-displacement linear travel bistability and is suitable for low-power microswitching applications. The design process involved a combination of single and multiple degree-of-freedom PRBM and finite element models to quickly proceed from a concept rigid-body mechanism to fully compliant fabrication-ready geometry. The 3DOF model presented here was developed to more accurately model the behavior of the tensural pivots-a new class of compliant segment used to avoid combined compressive loading of flexible segments. Four SRFBM designs were fabricated and tested for bistability, on-chip actuation, critical force, and fatigue tests. These tests validate the models used in the design process and demonstrate the functionality and reliability of the SRFBM.