Showing papers on "Compliant mechanism published in 2020"

Kinetostatic and Dynamic Modeling of Flexure-Based Compliant Mechanisms: A Survey

Abstract: Flexure-based compliant mechanisms are becoming increasingly promising in precision engineering, robotics, and other applications due to the excellent advantages of no friction, no backlash, no wear, and minimal requirement of assembly. Because compliant mechanisms have inherent coupling of kinematic-mechanical behaviors with large deflections and/or complex serial-parallel configurations, the kinetostatic and dynamic analyses are challenging in comparison to their rigid-body counterparts. To address these challenges, a variety of techniques have been reported in a growing stream of publications. This paper surveys and compares the conceptual ideas, key advances, and applicable scopes, and open problems of the state-of-the-art kinetostatic and dynamic modeling methods for compliant mechanisms in terms of small and large deflections. Future challenges are discussed and new opportunities for extended study are highlighted as well. The presented review provides a guide on how to select suitable modeling approaches for those engaged in the field of compliant mechanisms.

A Spatial Deployable Three-DOF Compliant Nano-Positioner With a Three-Stage Motion Amplification Mechanism

Abstract: This article presents the design, analysis, and prototype test of a novel spatial deployable three-degree of freedom (DOF) compliant nano-positioner with a three-stage motion amplification mechanism (MAM). Inspired by deployable structures, a new design concept, namely monolithically spatial compliant mechanism (MSCM) is proposed to minimize the overall structure. Based on MSCM, a folding operation is employed uniquely by arranging three typical kinds of basic MAM modules with two sets of hooke joints. Due to the spatial structure, the dimensions in horizontal plane is reduced by 60.94%. Furthermore, the proposed nano-positioner demonstrates the simultaneous design of large-ratio amplification mechanism, compact, highly flexible, and assembly-free spatial XY platform with integrated Z platform. Analytical modeling is carried out, and finite element analysis is conducted to optimize the geometric parameters. A prototype is fabricated to verify the performances of the nano-positioner through tests. Experimental results demonstrate that the maximum displacements in $x$ -, $y$ -, and $z$ -axes can reach 177.33, 179.30, and 17.45 $\mu$ m, respectively. The motion amplification ratios in the $x$ - and $y$ -axes can reach 10.19 and 10.30, respectively. Moreover, by adopting proportional-integral-derivative feedback controller, the closed-loop control experiments are conducted. The results show that the motion resolution in three axes can all reach 5 nm. As the MSCM has been verified to be feasible and favorable, it can be anticipated that the design concept will contribute to the multiformity and development of compliant mechanisms.

Deterministic snap-through buckling and energy trapping in axially-loaded notched strips for compliant building blocks

Abstract: Harnessing elastic instabilities has enabled recent advances in new classes of materials and devices due to the characteristics of amplifying force and augmented motion. Achieving these enhanced effects usually relies on using buckled beam or strips as the building block. In response to such a need, we investigate the contact-induced energy trapping of axially-loaded strips. To achieve the feature of energy trapping, we implement an "imperfection by design" approach to trigger a controllable and predictable interactive buckling in axially-loaded strips. By combining finite-element simulations and desktop-scale experiments, we found that the contact of strip elements can be induced by strategically controlled the number, the location and the layout of local predefined geometric defects, leading to a deterministic on-demand snap-through buckling response compared to the ones without such geometric defects. Our study thereby opens avenues for the design of the next generation of compliant mechanisms with high fidelity and low sensitivity over a wide range of length scales.

Optimal Design of a Compliant Constant-Force Mechanism to Deliver a Nearly Constant Output Force Over a Range of Input Displacements.

Abstract: This study presents an optimal design procedure, including topology and geometry optimization methods to design a compliant constant-force mechanism, which can generate a nearly constant output for...

Compliant, Large-Strain, and Self-Sensing Twisted String Actuators.

Abstract: Twisted string actuators (TSAs) convert rotational motion from twisting into linear motion. They are known for high energy efficiency, and large linear strain and stress outputs. Although they have been successfully applied as the moving mechanism for different robot applications, their potential in soft robotics is mainly challenged by two aspects: First, the conventional strings of TSAs are stiff and strong but not compliant. Second, precise control of TSAs predominantly relies on external position or force sensors. Because of these, TSA-driven robots are often rigid and bulky. In this study, we propose the design, modeling, and robotic application of TSAs that are compliant, can produce large strain, and are capable of self-sensing during twisting-induced actuation. The design is realized by replacing conventional stiff strings with compliant, thermally activated, and conductive supercoiled polymer strings. Experiments show that the developed TSAs have normalized stiffness of 30%, and position self-sensing capability during twisting. The quasi-static actuation and self-sensing properties are accurately captured by the Preisach hysteresis operators. In particular, both the twisting-induced actuation and thermally induced actuation are considered. Finally, the proposed TSAs are successfully demonstrated in a low-cost three-dimensionally printed compliant robotic gripper.

BLT Gripper: An Adaptive Gripper With Active Transition Capability Between Precise Pinch and Compliant Grasp

Abstract: Achieving both the precise pinching and compliant grasping capabilities is a challenging goal for most robotic hands and grippers. Moreover, an active transition between the pinching pose and the grasping pose is difficult for limited degrees-of-freedom (DOF) hands or grippers. Even when using high DOF robotic hands, it requires a substantial amount of control effort and information from tactile sensors. In this letter, a 3-finger, 5-DOF adaptive gripper with active transition capability is presented. Each finger is composed of a minimum number of components using one rigid link, one belt, one fingertip frame and one motor for flexion motion. This simple and unique finger structure enables precise parallel pinching and highly compliant stable grasping with evenly distributed pressure. The other two motors are used for fingertip angle adjustment and change of the finger orientation respectively. Thorough kinematic and force analysis with detailed descriptions of the mechanical design clearly shows controllable transition property and stable grasping performance. The experiments including the grasping force and pressure measurement verify the performance of the proposed gripper and prove the practical usefulness for real-world applications.

Large-Deflection Analysis of General Beams in Contact-Aided Compliant Mechanisms Using Chained Pseudo-Rigid-Body Model

Abstract: The nonlinear analysis and design of contact-aided compliant mechanisms (CCMs) are challenging. This paper presents a nonlinear method for analyzing the deformation of general beams that contact rigid surfaces in CCMs. The large deflection of the general beam is modeled by using the chained pseudo-rigid-body model. A geometry constraint from the contact surface is developed to constrain the beam’s deformed configuration. The contact analysis problem is formulated based on the principle of minimum potential energy and solved using an optimization algorithm. Besides, a novel technique based on the principle of work and energy is proposed to calculate the reaction force/moment of displacement-loaded cases. Several analysis examples of the compliant mechanisms with straight or curved beams are used to verify the proposed method. The results show that the proposed method and technique can evaluate the deformation of beam-based CCMs and the reaction force/moment with acceptable accuracy, respectively.

Virtual and physical prototyping of a beam-based variable stiffness actuator for safe human-machine interaction

Abstract: This paper reports about the virtual and physical prototyping of an antagonistic Variable Stiffness Actuator (VSA) to be used on robotic arms specifically realized for physical human-robot interaction. Such antagonistic actuation system, which comprises purposely conceived Compliant Transmission Elements (CTEs) characterized by a nonlinear relation between the deflection and the applied torque, allows to simultaneously control both the joint’s position and stiffness. The CTE’s beams geometry, namely slender spline beams, has been defined by means of an automatic routine leveraging on Matlab and ANSYS and allowing for the shape optimization of complex flexures. The synthesized springs are characterized by a predefined quadratic torque-deflection characteristic, which is shown to guarantee a precise stiffness modulation while avoiding the need for a joint’s position sensor. After shape optimization, the CTE is fabricated via additive manufacturing and subsequently tested. The acquired data show a very good consistency with the numerical results, although highlighting a non-negligible hysteresis due to material damping. Therefore, in order to cope with such unavoidable effect along with other parameter uncertainties and unmodeled effects (e.g. static friction), a robust feedback controller is proposed, allowing for the simultaneous and decoupled regulation of joint position and stiffness. Finally, a VSA prototype is produced and tested. Experimental results confirm that the VSA behaves as expected.

Designing 2D auxetic structures using multi-objective topology optimization

Abstract: The behaviour of the auxetic structure under external load was regarded as the behaviour of the compliant (flexible) mechanism. The multi-objective topological optimization, based on genetic algorithms and the finite element method, was used to find the optimal shape of such two-dimensional compliant mechanism in this study. The optimization was performed on a quarter of a double-symmetric representative unit cell, which is a building block of the symmetrical auxetic structure. Static linear computational simulations were performed to determine the mechanical response of cell topologies. The proposed method leads to a set of best solutions positioned on the Pareto front with different topologies and gives a broad overview of possible designs of new auxetic structures. The method is highly effective and can be easily extended to large deformation formulations, nonlinear elasticity or elasto-plasticity.

Plant Movements as Concept Generators for the Development of Biomimetic Compliant Mechanisms.

Abstract: Plant movements are of increasing interest for biomimetic approaches where hinge-free compliant mechanisms (flexible structures) for applications, for example, in architecture, soft robotics, and medicine are developed. In this article, we first concisely summarize the knowledge on plant movement principles and show how the different modes of actuation, that is, the driving forces of motion, can be used in biomimetic approaches for the development of motile technical systems. We then emphasize on current developments and breakthroughs in the field, that is, the technical implementation of plant movement principles through additive manufacturing, the development of structures capable of tracking movements (tropisms), and the development of structures that can perform multiple movement steps. Regarding the additive manufacturing section, we present original results on the successful transfer of several plant movement principles into 3D printed hygroscopic shape-changing structures ("4D printing"). The resulting systems include edge growth-driven actuation (as known from the petals of the lily flower), bending scale-like structures with functional bilayer setups (inspired from pinecones), modular aperture architectures (as can be similarly seen in moss peristomes), snap-through elastic instability actuation (as known from Venus flytrap snap-traps), and origami-like curved-folding kinematic amplification (inspired by the carnivorous waterwheel plant). Our novel biomimetic compliant mechanisms highlight the feasibility of modern printing techniques for designing and developing versatile tailored motion responses for technical applications. We then focus on persisting challenges in the field, that is, how to speed-boost intrinsically slow hydraulically actuated structures and how to achieve functional resilience and robustness, before we propose the establishment of a motion design catalog in the conclusion.

Topology optimization of fluidic pressure-loaded structures and compliant mechanisms using the Darcy method

Abstract: In various applications, design problems involving structures and compliant mechanisms experience fluidic pressure loads. During topology optimization of such design problems, these loads adapt their direction and location with the evolution of the design, which poses various challenges. A new density-based topology optimization approach using Darcy’s law in conjunction with a drainage term is presented to provide a continuous and consistent treatment of design-dependent fluidic pressure loads. The porosity of each finite element and its drainage term are related to its density variable using a Heaviside function, yielding a smooth transition between the solid and void phases. A design-dependent pressure field is established using Darcy’s law and the associated PDE is solved using the finite element method. Further, the obtained pressure field is used to determine the consistent nodal loads. The approach provides a computationally inexpensive evaluation of load sensitivities using the adjoint-variable method. To show the efficacy and robustness of the proposed method, numerical examples related to fluidic pressure-loaded stiff structures and small-deformation compliant mechanisms are solved. For the structures, compliance is minimized, whereas for the mechanisms, a multi-criteria objective is minimized with given resource constraints.

Modeling and Task-Oriented Optimization of Contact-Aided Continuum Robots

Abstract: Steerable catheters, as one type of continuum robot, has been popularly used to reach targets to treat atrial fibrillation. Its tip orientation relative to the surface normal of anatomy affects the ablation efficiency and lesion volume. Traditional catheters with a constant deflection have to rely on the reaction force from the tissue to reach targets, and their tip orientation capability is always limited. To overcome this challenge, we propose a miniaturized tendon actuated continuum robot with asymmetric contact-aided compliant mechanisms (CCMs) to generate asymmetric bends. It can adapt to the confined space and improve the tip orientation capability. The general configuration with the customized contact-aided segments and spatial arrangements is investigated. A mechanical model considering self-contacts and tendon interactions is built to describe free bends and constrained bends. A task-oriented optimization based on the mechanical model is implemented to find the best asymmetric configuration with unevenly distributed CCMs. Experiments are carried out to validate the proposed design, model, and optimization. Comparisons of continuum robots with and without CCMs by experiments and simulation demonstrate its potential advantages to improve the deflection and tip orientation in the confined space.

A novel design and fabrication of a micro-gripper for manipulation of micro-scale parts actuated by a bending piezoelectric

Abstract: In this study, a novel micro-gripper using a piezoelectric actuator was designed and improved by the design of experiments (DOE) approach. Using a bending PZT actuator connected to the micro-gripper by a rigid wedge can be considered as a novel approach in this field. Almost all of the similar grippers in this category were former actuated by a piezo-stack which has some limitations and difficulties like fabrication in MEMS proportions. The basic design was borrowed from compliant mechanisms that are suitable for MEMS application and easy to manufacture in micro-scale because of the intrinsic integration characteristic. Since stress concentration is common in flexure hinge compliant mechanisms, our focus was to consider strength as an important factor in our design. Finite element analysis tools were used to implement the DOE based on two criteria; minimizing stress concentration and maximizing the output displacement in the micro-gripper structure as much as possible with the consideration of the total size of the gripper. The experiment was performed to validate the simulation results and experiment results agreed well with the simulation one. The slight geometrical discrepancy in significant portions of structure like flexure hinges partially contributes to the accumulated error between the simulation and the experiments.

A Fully Compliant Tristable Mechanism Employing Both Tensural and Compresural Segments

Abstract: A multistable compliant mechanism is a device that can hold several distinct positions through the storage and release of the strain energy associated with deflections of the flexible members. This self-locking capability can benefit many applications such as threshold acceleration sensing, overload protection, and shape reconfiguration. This work presents a novel class of fully compliant tristable mechanisms called tensural–compresural tristable mechanisms (TCTMs), which forms three stable equilibrium positions through unique utilization of both tensural segments and compresural segments. To identify feasible designs, a kinetostatic model is developed using the chained beam-constraint-model (CBCM) for both tensural segments and compresural segments. Two TCTM designs accompanied with a prototype are presented to demonstrate the feasibility of this new tristable configuration and the effectiveness of the kinetostatic model.

Computational parametric analysis and experimental investigations of a compact flexure-based microgripper

Abstract: This paper presents a flexure-based piezoelectric actuated microgripper for high precision grasping/releasing tasks. The design of the microgripper consists of a three-stage amplification and transmission mechanism, and the parallel grasping technique. A bridge-type mechanism and two sequential lever-type mechanisms are symmetrically connected to amplify the output displacement of the embedded piezoelectric actuator. The parallelogram mechanisms assist in linearizing the output displacement of both jaws of the microgripper. The computational analysis is conducted to investigate the effect of the dimensional parameters on the characteristics of the microgripper. A computational parametric optimization methodology is established to achieve the required attributes of the microgripper. The design optimization resulted in a compact design, a high displacement amplification ratio, and a large output displacement of the microgripper. The experimental studies are conducted to investigate the key characteristics of the microgripper such as the displacement amplification ratio, the output displacement, tracking performance. Further, the parasitic motion, input-end and output-end motion resolution of the microgripper are identified. The experimental results indicate that the compact microgripper can achieve a high displacement amplification ratio and large output displacement with a high positioning accuracy.

Topology optimization of distributed flexure hinges with desired performance

Abstract: Traditional notch flexure hinges have been widely used in precision engineering, but the lumped notch configuration limits the range of motion. This article presents a synthesis method for ...

Stress-based topology optimization of compliant mechanisms design using geometrical and material nonlinearities

Abstract: In this work, a density-based method is applied for synthesizing compliant mechanisms using topology optimization. This kind of mechanisms uses the elastic strain as the basis for kinematic actuation and it is widely used in precision mechanical devices, in biomedical engineering, and recently in MicroElectroMechanical Systems (MEMS). Geometrical and material (compressible hyperelasticity) nonlinearities are taken into account to obtain mechanisms near real-world applications. A strength criterion for the optimization problem is applied, to design compliant mechanisms that fulfill the desired kinematic tasks while complying with a stress threshold. The addition of a stress constraint to the formulation also aims to alleviate the appearance of hinges in the optimized design. Employing benchmark examples, we investigate the influence of a nonlinear formulation with a stress constraint in the final designs. It is shown that material nonlinearity plays an important role for stress constraint problems. The use of a projection scheme helps to obtain optimized topologies with a high level of discreteness. The Method of Moving Asymptotes (MMA) is applied for design variables updating and the required derivatives are calculated analytically by the adjoint method.