Showing papers on "Compliant mechanism published in 2018"

Topology Optimized Design, Fabrication, and Characterization of a Soft Cable-Driven Gripper

Abstract: Soft-bodied robots, due to their intrinsic compliance, have shown great potential for operating within unstructured environment and interacting with unknown objects. This letter deals with automatic design and fabrication of soft robots. From a structure point of view, we synthesize a soft cable-driven gripper by recasting its mechanical design as a topology optimization problem. Building on previous work on compliant mechanism optimization, we model the interactions between the gripper and objects more practically, in form of pressure loadings and friction tractions, and furthermore, we investigate how the interaction uncertainties affect the optimization solution by varying the contact location and area. The optimized soft fingers were three-dimensionally printed and then assembled to build a gripper. The experiments show that the gripper can handle a large range of unknown objects of different shapes and weights (up to 1 kg), with different grasping modes. This letter represents an important step toward leveraging the full potential of the freeform design space to generate novel soft-bodied robots.

Modeling Large Deflections of Initially Curved Beams in Compliant Mechanisms Using Chained Beam-Constraint-Model

Abstract: Understanding and analyzing large and nonlinear deflections are the major challenges of designing compliant mechanisms. Initially, curved beams can offer potential advantages to designers of compliant mechanisms and provide useful alternatives to initially straight beams. However, the literature on analysis and design using such beams is rather limited. This paper presents a general and accurate method for modeling large planar deflections of initially curved beams of uniform cross section, which can be easily adapted to curved beams of various shapes. This method discretizes a curved beam into a few elements and models each element as a circular-arc beam using the beam constraint model (BCM), which is termed as the chained BCM (CBCM). Two different discretization schemes are provided for the method, among which the equal discretization is suitable for circular-arc beams and the unequal discretization is for curved beams of other shapes. Compliant mechanisms utilizing initially curved beams of circular-arc, cosine and parabola shapes are modeled to demonstrate the effectiveness of CBCM for initially curved beams of various shapes. The method is also accurate enough to capture the relevant nonlinear load-deflection characteristics.

Optimal Design of a Soft Robotic Gripper for Grasping Unknown Objects.

Abstract: This study presents the design of an underactuated, two-finger, motor-driven compliant gripper for grasping size-varied unknown objects. The gripper module consists of one main frame struc...

Optimum Design of a Piezo-Actuated Triaxial Compliant Mechanism for Nanocutting

Abstract: A novel piezo-actuated compliant mechanism is developed to obtain triaxial translational motions with decoupled features for nanocutting. Analytical modeling of the working performance followed by Pareto-based multiobjective optimization is conducted for determining dimensions of the mechanism. Finite element analysis on the designed mechanism verifies the accuracy of the developed model, accordingly demonstrating the effectiveness of the optimal design process. Open-loop test on the prototype shows that proper strokes with low coupling and high natural frequencies are obtained as estimated. Low tracking error in closed-loop test suggests that the developed mechanism can precisely follow the desired trajectory to form complicated nanostructures. Finally, closed-loop based nanosculpturing is conducted, demonstrating the effectiveness of the developed triaxial motion system for nanocutting well.

Stress‐based multi‐material topology optimization of compliant mechanisms

Abstract: In this paper, a level-set-based method is presented to deal with the multi-material topology optimization of compliant mechanisms with stress constraints. A novel stress-based multi-material topology optimization model of compliant mechanisms is proposed. In this model, the multi-material level set topology description model and the separable stress interpolation scheme are adopted. The weighted sum method is used to deal with the multi-objective optimization of the output displacement and compliance of compliant mechanisms. The penalty of stresses is also considered in the objective function to control the local stress level in different materials. To solve the optimization problem, the parametric level set method is employed and the sensitivity analysis is conducted. Application of the method is demonstrated by two numerical examples. Results show that the multi-material structures without undesirable de facto hinges can be obtained. The output displacement and compliance of the compliant mechanisms are optimized, and stress constraints in different materials are simultaneously satisfied.

Survey on Recent Designs of Compliant Micro-/Nano-Positioning Stages

Abstract: Micromanipulation is a hot topic due to its enabling role in various research fields. In order to perform a high precision operation at a small scale, compliant mechanisms have been proposed and applied for decades. In microscale manipulation, micro-/nano-positioning is the most fundamental operation because a precision positioning is the premise of subsequent operations. This paper is concentrated on reviewing the state-of-the-art research on complaint micro-/nano-positioning stage design in recent years. It involves the major processes and components for designing a compliant positioning stage, e.g., actuator selection, stroke amplifier design, connecting scheme of the multi-DOF stage and structure optimization. The review provides a reference to design a compliant micro-/nano-positioning stage for pertinent applications.

Topology Synthesis and Optimal Design of an Adaptive Compliant Gripper to Maximize Output Displacement

Abstract: This paper presents the optimal design process of an innovative adaptive compliant gripper (ACG) for fast handling of objects with size and shape variations. An efficient soft-add topology optimization algorithm is developed to synthesize the optimal topology of the ACG. Unlike traditional hard-kill and soft-kill methods, the elements are equivalent to be numerically added into the analysis domain through the proposed soft-add scheme. A size optimization method incorporating Augmented Lagrange Multiplier (ALM) method, Simplex method, and nonlinear finite element analysis with the objective to maximize geometric advantage (which is defined as the ratio of output displacement to input displacement) of the analyzed compliant mechanism is carried out to optimize the design. The dynamic performance and contact behavior of the ACG is analyzed by using explicit dynamic finite element analysis. Three designs are prototyped using silicon rubber material. Experimental tests are performed, and the results agree well with the simulation models. The outcomes of this study provide numerical methods for design and analysis of compliant mechanisms with better computational efficiency, as well as to develop an innovative adaptive compliant gripper for fast grasping of unknown objects.

Design, Pseudostatic Model, and PVDF-Based Motion Sensing of a Piezo-Actuated XYZ Flexure Manipulator

Abstract: This paper develops a compact piezo-actuated XYZ flexure mechanism with an integrated polyvinylidene fluoride (PVDF) based displacement sensor. The sensing scheme is, by sticking a shaped PVDF film on the guiding flexible beams and using the kinematic relationship of the clamped–sliding flexible beam, widely available in compliant mechanisms. The optimal shape of PVDF is found by maximizing the sensitivity with an analytical sensing model. A pseudostatic model is also established to predict and optimize the kinetostatics and dynamics of the manipulator without calculating the elastic/kinetic energies or using Lagrange's equation. Thus, the dynamic modeling is simplified as a statics-similar problem. A comparison of the theoretical model with finite-element analysis reveals its high accuracy and substantially concise step. Finally, the manipulator is tested having the stroke range of 112 μ m × 112 μ m × 123 μ m and the resonance frequency of 682 Hz × 687 Hz × 3.88 kHz with relatively large stroke range and high frequency in the Z -motion. Moreover, the experimental output of the proposed PVDF sensor matches well with the laser sensor, providing a new way with ignorable volume, high sensitivity, large bandwidth, and low cost to measure the displacement of compliant mechanisms, in which the space is usually confined and assembling a bulky transducer is difficult.

Design, fabrication and testing of a 2 DOF compliant flexural microgripper

Abstract: This paper presents the development of a monolithic two degrees of freedom (2 DOF), piezoelectric actuated microgripper for the manipulation of micro-objects. Micromanipulation and microassembly are the major subjects of interest in recent times and are becoming increasingly important in many domains. An effort is being made to develop a novel 2 DOF microgripper, each jaw being able to move independently to grasp and rotate objects of micro sizes. Microgripper is developed based on the compliant mechanism. The designed 2 DOF compliant microgripper is modeled using FEM and PRBM approach further validated experimentally. The microgripper is actuated using APA 120-S piezoelectric stack actuators. The displacement of the microgripper and the gripping force is measured by image processing technique using LabVIEW tools. The microgripper is subjected to various tests to measure the displacement amplification ratio and micromanipulation experiments. Wire of various sizes are used to test the grasping and rotating sequence of the microgripper. The theoretical, simulation and experimental results reveal the good performance of the microgripper.

Underwater Dynamic Modeling for a Cable-Driven Soft Robot Arm

Abstract: Soft robotics has attracted great attention because of its potential to overcome safety issues and produce a more harmonious cooperative environment for human beings and robots. One of the main challenges hindering its wider application to the daily routine is the dynamic modeling of its compliant mechanisms. In this paper, to further investigate the soft robot arm's performance and extend the utilization in water or other dense and viscous mediums, the dynamic model based on Kane's theory is proposed. This model takes complicated hydrodynamics into account with reasonable simplification, considering the physical conditions. Compared with the previous work, we adopt the Column friction model to compensate for the actuation force's loss in the transmission process. The proposed dynamic model is validated by comparing the theoretical results of both the dynamic responses and steady-state poses with the experimental results under different conditions. Given the modification method for the computed dynamic equation, the presented dynamics can adapt to variable environments and serve as the platform for the controller design for a soft robot working in a complex environment.

On a simplified nonlinear analytical model for the characterisation and design optimisation of a compliant XY micro-motion stage

Abstract: Compliant micro-positioning stages offer low-cost high precision and repeatability but limited workspace and nonlinear behaviour The conventional modelling techniques used to characterise micro-motion stages are often either complex or inaccurate for large displacements New methods have recently been developed with satisfying results However, the presented models often focus on one part of the stage characterisation This paper presents an analytical model used to characterise a compliant XY micro-motion stage in terms of stiffness and working range, taking into account the stress and buckling limitations, motion loss and parasitic displacements The presented model combines a 6-degree-of-freedom (DOF) linear model and a simplified 2-DOF nonlinear static model As a case study, this model is used for the design of a micro-motion stage which is intended to be the fine positioning system for a hybrid miniaturised product assembly system The results generated by the analytical model, the finite element analysis (FEA) and the experimental testing are all in agreement The analytical model is therefore proven to be suitable for a full characterisation and design optimisation; reducing the computation time from a few hours to a few minutes when using MATLAB rather than FEA software Its ability to predict the output displacement as a function of the input displacement with a maximum error of less than 2% also makes it suitable for open-loop control The travel range of the fabricated stage is greater than ±23 mm 2 and the maximum cross-coupling error is less than 25%

A pseudo-static model for dynamic analysis of distributed compliant mechanisms

Abstract: This paper presents a pseudo-static modeling methodology for dynamic analysis of distributed compliant mechanisms to provide accurate and efficient solutions. First, a dynamic stiffness matrix of the flexible beam is deduced, which has the same definition and a similar form as the traditional static compliance/stiffness matrix but is frequency dependent. Second, the pseudo-static modeling procedure for the dynamic analysis is implemented in a statics-similar way based on D'alembert's principle. Then, all the kinematic, static and dynamic performances of compliant mechanisms can be analyzed based on the pseudo-static model. The superiority of the proposed method is that when it is used for the dynamic modeling of compliant mechanisms, the traditional dynamic modeling procedures, such as calculation of the elastic and kinetic energies as well as using Lagrange's equation, are avoided and the dynamic modeling is converted to a statics-similar problem. Comparison of the proposed method with an elastic-beam-based model in previous literature and finite element analysis for an exemplary XY precision positioning stage reveals its high accuracy and easy operation.

Optimal design of compliant mechanisms using functionally graded materials

Abstract: This research applies topology optimization to create feasible functionally graded compliant mechanism designs with the aim of improving structural performance compared to traditional homogeneous compliant mechanism designs. Converged functionally graded designs will also be compared with two-material compliant mechanism designs. Structural performance is assessed with respect to mechanical/geometric advantage and stress distributions. Two design problems are presented – a gripper and a mechanical inverter. A novel modified solid isotropic material with penalization (SIMP) method is introduced for representing local element material properties in functionally graded structures. The method of moving asymptotes (MMA) is used in conjunction with adjoint sensitivity analysis to find the optimal distribution of material properties. Geometric non-linear analysis is used to solve the mechanics problem based on the Neo-Hookean model for hyperelastic materials. Functionally graded materials (FGMs) have material properties that vary based on spatial position. Here, FGMs are implemented using two different resource constraints – one on the mechanism’s volume and the other on the integral of the Young’s modulus distribution throughout the design domain. Tensile tests are performed to obtain the material properties used in the analysis. Results suggest that FGMs can achieve the desired improvements in mechanical/geometric advantage when compared to both homogeneous and two-material mechanisms.

On the influence of local and global stress constraint and filtering radius on the design of hinge-free compliant mechanisms

Abstract: Distributed compliant mechanisms are components that use elastic strain to obtain a desired kinematic behavior. Compliant mechanisms obtained via topology optimization using the standard approach of minimizing/maximizing the output displacement with a spring at the output port, representing the stiffness of the external medium, usually contain one-node connected hinges. Those hinges are undesired since an ideal compliant mechanism should be a continuous part. This work compares the use of two strategies for stress constrained problems: local and global stress constraints, and analyses their influence in eliminating the one-node connected hinges. Also, the influence of spatial filtering in eliminating the hinges is studied. An Augmented Lagrangian formulation is used to couple the objective function and constraints, and the resulting optimization problem is solved by using an algorithm based on the classical optimality criteria approach. Two compliant mechanisms problems are studied by varying the stress limit and filtering radius. It is observed that a proper combination of filtering radius and stress limit can eliminate one-node connected hinges.

Development of a multistage compliant mechanism with new boundary constraint.

Abstract: This paper presents a piezo-actuated compliant mechanism with a new boundary constraint to provide concurrent large workspace and high dynamic frequency for precision positioning or other flexible manipulation applications. A two-stage rhombus-type displacement amplifier with the "sliding-sliding" boundary constraint is presented to maximize the dynamic frequency while retaining a large output displacement. The vibration mode is also improved by the designed boundary constraint. A theoretical kinematic model of the compliant mechanism is established to optimize the geometric parameters, and a prototype is fabricated with a compact dimension of 60 mm × 60 mm × 12 mm. The experimental testing shows that the maximum stroke is approximately 0.6 mm and the output stiffness is 1.1 N/μm with the fundamental frequency of larger than 2.2 kHz. Lastly, the excellent performance of the presented compliant mechanism is compared with several mechanisms in the previous literature. As a conclusion, the presented boundary constraint strategy provides a new way to balance the trade-off between the frequency response and the stroke range widely existed in compliant mechanisms.

Kinematic Synthesis of a D-Drive MEMS Device through the Rigid-Body Replacement Method

Abstract: In this paper, a microsystem with prescribed functional capabilities is designed and simulated. In particular, the development of a straight line path generator MEMS device is presented. A new procedure is suggested for avoiding branch or circuit problems in the kinematic synthesis problem. Then, Ball’s point detection is used to validate the obtained pseudo-rigid body model (PRBM). A compliant Micro Electro Mechanical System (MEMS) device is obtained from the PRBM through the rigid-body replacement method, by making use of Conjugate Surfaces Flexure Hinges (CSFHs). Finally, the functional capability of the device is investigated by means of FEA simulations and experimental testing at the macroscale.

Design, Analysis and Testing of a New Compliant Compound Constant-Force Mechanism

Abstract: This paper presents the design and testing of a novel flexure-based compliant compound constant-force mechanism (CCFM). One uniqueness of the proposed mechanism lies in that it achieves both constant-force input and constant-force output, which is enabled by integrating two types of sub-mechanisms termed active and passive constant-force structures, respectively. Unlike conventional structures, the active constant-force structure allows the reduction on input force requirement and thus the enlargement of motion stroke provided that the maximum stress of the material is within allowable value. While the passive one offers a safe environmental interaction during the contact process. Analytical model of the proposed CCFM is derived which is verified by simulation study with finite element analysis (FEA). A prototype mechanism is fabricated by a 3D printer to demonstrate the performance of the proposed CCFM design. Experimental results reveal the effectiveness of the reported CCFM.