Showing papers on "Compliant mechanism published in 2017"

A computational design tool for compliant mechanisms

Abstract: We present a computational tool for designing compliant mechanisms. Our method takes as input a conventional, rigidly-articulated mechanism defining the topology of the compliant design. This input can be both planar or spatial, and we support a number of common joint types which, whenever possible, are automatically replaced with parameterized flexures. As the technical core of our approach, we describe a number of objectives that shape the design space in a meaningful way, including trajectory matching, collision avoidance, lateral stability, resilience to failure, and minimizing motor torque. Optimal designs in this space are obtained as solutions to an equilibrium-constrained minimization problem that we solve using a variant of sensitivity analysis. We demonstrate our method on a set of examples that range from simple four-bar linkages to full-fledged animatronics, and verify the feasibility of our designs by manufacturing physical prototypes.

Design and Development of a Novel Compliant Gripper With Integrated Position and Grasping/Interaction Force Sensing

Abstract: This paper proposes a new compliant gripper with integrated position and force sensors dedicated to automated microassembly tasks. The uniqueness of the gripper is that it possesses a large gripping range with a bidirectional drive, and it is capable of detecting grasping force and environmental interaction forces in horizonal and vertical axes, respectively. This is enabled by a mechanism design based on a rotary flexure bearing. Moreover, a compliant mechanism with two-stage stiffness is designed to provide the force sensing with dual sensitivities and measuring ranges to accommodate the grasp of objects with different sizes and weights. Analytical models are derived to predict the grasping range, force sensing sensitivities, and ranges. These models are verified by conducting finite-element analysis simulations. A proof-of-concept prototype gripper is developed for experimental calibration and performance testing. Results reveal that the single set of strain-gauge force sensor is able to detect both grasping and interaction forces in an alternate manner. The dual-sensitivity, dual-range force sensor provides a solution to large-range gripper with finer and coarser force sensing in a small and large ranges, respectively .

Design and analysis of a multi-notched flexure hinge for compliant mechanisms

Abstract: This article presents a new multi-notched flexure hinge, which consists of two right circular and two parabolic notches, for positioning stages based on compliant mechanisms. First, the configuration of the presented multi-notched hinge is obtained using topology optimization, and the final shape is proposed based on post-processing. Second, the dimensionless empirical equations for the stiffness, rotational precision and stress levels of the flexure hinges are developed using finite element analysis (FEA). Third, based on the established equations, the influences of the geometric parameters on the performance of the flexure hinge are investigated. Finally, to further understand the characteristics of this type of flexure hinge, comparisons with flexure hinges of various shapes are performed in terms of stiffness, rotational precision and stress levels.

Synthesis of auxetic structures using optimization of compliant mechanisms and a micropolar material model

Abstract: Aim of this work is the synthesis of auxetic structures using a topology optimization approach for micropolar (or Cosserat) materials. A distributed compliant mechanism design problem is formulated, adopting a SIMP---like model to approximate the constitutive parameters of 2D micropolar bodies. The robustness of the proposed approach is assessed through numerical examples concerning the optimal design of structures that can expand perpendicularly to an applied tensile stress. The influence of the material characteristic length on the optimal layouts is investigated. Depending on the inherent flexural stiffness of micropolar solids, truss---like solutions typical of Cauchy solids are replaced by curved beam---like material distributions. No homogenization technique is implemented, since the proposed design approach applies to elements made of microstructured material with prescribed properties and not to the material itself.

A 3-D Printed Ti-6Al-4V 3-DOF Compliant Parallel Mechanism for High Precision Manipulation

Abstract: This paper presents the synthesis and evaluation of a 3-D printed three degrees-of-freedom (DOF) spatial-motion compliant parallel mechanism (CPM). The CPM was synthesized by the beam-based structural optimization method and a prototype was fabricated by the electron beam melting (EBM) technology with Ti6Al4V material. The mechanical characteristics of 3-D printed compliant mechanism for precision applications, i.e., stiffness property, dynamic response, and large elastic deformation, were experimentally evaluated. Most importantly, a coefficient factor of 1.27 was proposed to determine the effective thickness for 3-D printed compliant mechanisms with 0.5 mm thick flexures. Using the effective thickness, the characteristics of the 3-D printed CPM have shown to agree with the prediction, with a maximum deviation of 10.5%. The Ti6Al4V CPM is able to achieve the large work range up to 4 mm of linear displacement, 6 degrees of angular displacements, fast dynamic response of 119 Hz, good decoupled motions, and high non-actuating stiffness. A 3-DOF manipulator was built based on the 3-D printed CPM and actuated by three voice-coil motors. Experimental results have shown that the 3-DOF manipulator could achieve repeatable motions with resolution of 20 nm for the translation along the Z -axis, 0.14 arcsecond for the rotation about the X -axis, and 0.12 arcsecond for the rotation about the Y -axis. In conclusion, EBM technology is suitable for fabricating compliant mechanisms in precision manipulator systems; the mechanical characteristics of 3-D printed compliant mechanisms are predictable when an effective thickness is used.

Full closed-loop controls of micro/nano positioning system with nonlinear hysteresis using micro-vision system

Abstract: The design and full closed-loop controls of micro/nano positioning system are presented in this paper. The micro/nano positioning system mainly consists of a 3-DOF compliant mechanism, a micro-vision system and three piezoelectric actuators. In order to compensate for nonlinear hysteresis in the piezoelectric actuators, an adaptive controller using direct inverse modeling approach based on a modified Prandtl–Ishlinskii model is presented. Instead of using traditional displacement sensors, a micro-vision system (MVS) is used to obtain feedback signals online. Based on the micro-vision system, a full closed-loop tracking control is developed to enhance the positioning accuracy of micro/nano positioning system. Taking into account the coupling motions, another full closed-loop tracking control with a decoupling feedforward controller is also proposed. Experimental results demonstrate that the full closed-loop controls can improve the positioning accuracy when compared with an open-loop control. Furthermore, the decoupling feedforward controller can improve the tracking performance of full closed-loop tracking control as well.

Design and multi-objective optimization for a broad self-amplified 2-DOF monolithic mechanism

Abstract: This paper proposes a structural design and multi-objective optimization of a two-degree-of-freedom (DOF) monolithic mechanism. The mechanism is designed based on compliant mechanism with flexure hinge and is compact in size (126 mm by 107 mm). Unlike traditional one-lever mechanisms, a new double-lever mechanism is developed to increase the working travel amplification ratio of the monolithic mechanism. The ideal amplification ratio, the working travel, the statics and the dynamics of the mechanism are taken into consideration. The effects of design variables on the output responses such as the displacement and first natural frequency are investigated via finite-element analysis based on response surface methodology. The fuzzy-logic-based Taguchi method is then used to simultaneously optimize the displacement and the first natural frequency. Experimental validations are conducted to verify the optimal results, which are compared to those of the original design. On using a finite-element method, the validation results indicated that the displacement and frequency are enhanced by up to 12.47% and 33.27%, respectively, over those of the original design. The experiment results are in a good agreement with the simulations. It also revealed that the developed fuzzy-logic-based Taguchi method is an effectively systematic reasoning approach for optimizing the multiple quality characteristics of compliant mechanisms. It was noted that the working travel/displacement of the double-lever mechanism is much larger than that of the traditional one-lever mechanism. It leads to the conclusion that the proposed mechanism has good performances for manipulations and positioning systems.

A semi-analytical modeling method for the static and dynamic analysis of complex compliant mechanism

Abstract: A semi-analytical modeling method towards the static and dynamic analyses for a class of flexure hinge-based compliant mechanisms or their composed systems is presented to provide accurate and efficient solutions. It is realized by firstly transforming the theoretical compliance matrix of a flexure hinge into a unified elemental stiffness matrix of a variable cross-section beam. Then, the semi-analytical finite element model of complex compliant mechanisms is established based on Lagrange’s equation taking the flexure hinge, the flexible beam and the lumped mass as the minimum elements. Shearing effects of the flexure hinge and rotary inertia of the flexible beam are included to enhance the modeling accuracy. A comparison of the method with another existing theoretical method and the finite element software ANSYS for two exemplary compliant mechanisms reveals a maximum deviation of less than 8% regarding the static displacement and the fundamental frequency but with a much substantial reduction of degrees of freedom. The results suggest the presented method is applicable to time-critical scenarios such as dynamic topology optimization and real-time feedback control simulation for complex compliant mechanisms.

Optimization of a two degrees of freedom compliant mechanism using Taguchi method-based grey relational analysis

Abstract: This paper presents a multi-response optimal design for new two degrees of freedom compliant mechanism (TDCM) by the use of the hybrid statistical optimization techniques such as Taguchi method, response surface methodology, grey relational analysis and entropy weighting measurement technique. The design parameters like various thicknesses of flexure hinges play a vital role in determining its quality characteristics of TDCM. The quality characteristics of TDCM are assessed by measuring the displacement and first natural frequency. The experimental trials are designed by the Taguchi’s L 25 orthogonal array. A hybrid approach of grey-Taguchi based response surface methodology and entropy measurement is then combined to maximize both the displacement and first natural frequency simultaneously. Response surface methodology is utilized for modeling the relationship between design parameters and two responses with grey relational grade. Entropy measurement technique is employed for calculating the weight corresponding to each of quality characteristics. Analysis of variance (ANOVA) is conducted to determine the significant parameters affecting the responses. ANOVA and confirmation tests are conducted to validate the prediction accuracy and the statistical adequacy of the developed mathematical models. The experimental results were in a good agreement with the simulation results from ANSYS. The proposed methodology is expected to use for related micropositioning compliant mechanisms.

Simplified modelling and development of a bi-directionally adjustable constant-force compliant gripper:

Abstract: This paper proposes the design of a wholly mechanical constant-force gripper that can accommodate the imprecise manipulation of brittle/delicate objects by the actuation. This was achieved by desig...

Aerial manipulator with a compliant arm for bridge inspection

Abstract: This paper presents the design, development and testing of a 4-DoF aerial manipulator for bridge inspection, where the arm is placed at the upper part of the multirotor body. The manipulator joints are equipped with a compliant mechanism that allows the contact with the environment reducing the influence over the platform stability. The transmission mechanism consists of two pairs of springs and a potentiometer for measuring the angular deflection between the servo and the joint angular position, which allows the estimation of the contact forces. Experimental tests have been done with the aerial manipulator placing the end effector at different points in the lower part of a bridge girder, which is needed by bridge inspectors to measure girder's deflections over time.

Evaluation of a Compliant Droop-Nose Morphing Wing Tip via Experimental Tests

Abstract: A morphing droop-nose wing tip with span of 1.3 m and target 2 deg droop was designed, manufactured, and tested as part of the European project Novel Air Vehicle Configurations: From Fluttering Wings to Morphing Flight. The morphing droop-nose device featured a fiberglass composite skin with optimized three-dimensional thickness distribution, which was supported by topology-optimized superelastic nickel titanium and aluminum internal compliant mechanisms. The tests included ground and low-speed wind-tunnel tests (55 m/s) with the aim of assessing the structural performance and the design chain through measurements of shape, strains, surface pressures, and total forces/moments on the model. Comparisons with the experimental results were used for validation of the computational models and optimization tools. The morphing device was able to change shape while resisting the external loads, and it was identified that the distribution of strain and shape accuracy may be improved through the use of a concurrent...

Design of flexure hinges based on stress-constrained topology optimization:

Abstract: Stress concentration is one of the disadvantages of flexure hinges. It limits the range of motion and reduces the fatigue life of mechanisms. This article designs flexure hinges by using stress-con...

Optimal design of a soft robotic gripper with high mechanical advantage for grasping irregular objects

Abstract: This study presents a soft robotic gripper for grasping irregular objects. The optimal design is based on the proposed topology optimization and size optimization methods with the objective to maximize the mechanical advantage (MA, which is defined as the ratio of output force to the input force) of the analyzed compliant mechanism. The optimal design is prototyped using silicon rubber material. Experimental tests including MA test, geometric advantage (GA, which is defined as the ratio of output displacement to the input displacement) test, adaptability test, and grasping test are carried out to investigate the design. A performance index has also been proposed to evaluate the grasping performance of the grippers. The results show the developed gripper is with the highest performance index, which represents the developed gripper is with better adaptability, faster response, higher payload and stability in overall.

Compliant thin-walled joint based on zygoptera nonlinear geometry

Abstract: This paper introduces a Compliant thin-walled joint (CTWJ) that expands the group of existing compliant joints. The CTWJ design is based on the nonlinear geometry of the zygoptera animal. With a thin-walled structure, the CTWJ allows a considerably large range of motion in the x-and-y axes. In addition, the thin-walled structure is then filled by polydimethylsiloxane material to reinforce the stiffness of the CTWJ. First, design of experiment methodology is used for the sensitive analysis of the width and the thickness to the strain of joint. The range of motion, the strain, the buckling behavior, and the first natural frequency of CTWJ are investigated via finite element analysis and experiments. The behavior of the CTWJ is subsequently compared with the conventional compliant joints to realize the efficient performance of the CTWJ. The results revealed that the CTWJ has a range of motion and strain energy larger than those of traditional compliant joints. Finally, an example of vibration isolator is modeled by using the CTWJ as planar spring. It is believed that the CTWJ has a great potential for the development of compliant mechanisms in terms of large range of motions in mutliple axes.

Design of Large-Displacement Compliant Mechanisms by Topology Optimization Incorporating Modified Additive Hyperelasticity Technique

Abstract: This paper is focused on the topology design of compliant mechanisms undergoing large displacement (over 20% of the structural dimension). Based on the artificial spring model and the geometrically nonlinear finite element analysis, the optimization problem is formulated so as to maximize the output displacement under a given material volume constraint. A modified additive hyperelasticity technique is proposed to circumvent numerical instabilities that occurred in the low-density or intermediate-density elements during the optimization process. Compared to the previous method, the modified technique is very effective and can provide more accurate response analysis for the large-displacement compliant mechanism. The whole optimization process is carried out by the gradient-based mathematical programming method. Numerical examples of a force-inverting mechanism and a microgripping mechanism are presented. The obtained optimal solutions verify the applicability of the proposed numerical techniques and show the necessity of considering large displacement in the design problem.

Constraint-force-based approach of modelling compliant mechanisms: Principle and application

Abstract: Numerous works have been conducted on modelling basic compliant elements such as wire beams, and closed-form analytical models of most basic compliant elements have been well developed. However, the modelling of complex compliant mechanisms is still a challenging work. This paper proposes a constraint-force-based (CFB) modelling approach to model compliant mechanisms with a particular emphasis on modelling complex compliant mechanisms. The proposed CFB modelling approach can be regarded as an improved free-body- diagram (FBD) based modelling approach, and can be extended to a development of the screw-theory-based design approach. A compliant mechanism can be decomposed into rigid stages and compliant modules. A compliant module can offer elastic forces due to its deformation. Such elastic forces are regarded as variable constraint forces in the CFB modelling approach. Additionally, the CFB modelling approach defines external forces applied on a compliant mechanism as constant constraint forces. If a compliant mechanism is at static equilibrium, all the rigid stages are also at static equilibrium under the influence of the variable and constant constraint forces. Therefore, the constraint force equilibrium equations for all the rigid stages can be obtained, and the analytical model of the compliant mechanism can be derived based on the constraint force equilibrium equations. The CFB modelling approach can model a compliant mechanism linearly and nonlinearly, can obtain displacements of any points of the rigid stages, and allows external forces to be exerted on any positions of the rigid stages. Compared with the FBD based modelling approach, the CFB modelling approach does not need to identify the possible deformed configuration of a complex compliant mechanism to obtain the geometric compatibility conditions and the force equilibrium equations. Additionally, the mathematical expressions in the CFB approach have an easily understood physical meaning. Using the CFB modelling approach, the variable constraint forces of three compliant modules, a wire beam, a four-beam compliant module and an eight-beam compliant module, have been derived in this paper. Based on these variable constraint forces, the linear and non-linear models of a decoupled XYZ compliant parallel mechanism are derived, and verified by FEA simulations and experimental tests.

A modified post-processing technique to design a compliant based microgripper with a plunger using topological optimization

Abstract: The precision of microobject manipulation is predominantly based on the appropriate design of micromanipulation devices such as microgrippers. A compliant mechanism-based microgripper is an appropriate choice to achieve a highly precise and controlled motion. This research article proposes a refined technique to design a compliant-based microgripper with a plunger. The topological optimization technique has been adopted in this research work to develop the conceptual design of the mechanism. Flexure hinges are introduced in the topologically optimized design to overcome the senseless regions developed during the optimization process which is highly complicated to manufacture. Various flexure hinge contours such as rectangular, circular, and elliptical are introduced in the conceptual design domain, and their effects are investigated. Various parameters of flexure hinges are considered; the stress, the displacements, and the strain energy stored in the mechanism are studied through finite element analysis (FEA). In addition to FEA, experimental verification of the design was also performed. Both results are convincing about the structural performance of the microgripper design. In general, microdevices possess higher surface forces than volumetric forces; hence, this design is introduced with a plunger segment which is used to push the microobject for an active release during micromanipulation.

Optimized cross-spring pivot configurations with minimized parasitic shifts and stiffness variations investigated via nonlinear FEA

Abstract: Compliant mechanisms are nowadays a well-established means of achieving ultra-high precision, albeit at the expense of complex kinematics with the presence of parasitic motions. Diverse design configurations of compliant rotational joints called cross-spring pivots are hence studied in this work by applying various analytical and numerical approaches. Depending on the required precision and loading conditions, the limits of applicability of the available analysis tools, validated with nonlinear finite element calculations tuned with experimental data reported in literature, are established. The variation of design parameters allows, in turn, establishing design configurations of the studied mechanism that allow attaining minimized parasitic shifts and slight variations of its rotational stiffness, even when a broad range of rotations and varying transversal loads are considered, creating thus the preconditions for their application in high-precision micropositioning applications.

Development of a highly efficient bridge-type mechanism based on negative stiffness

Abstract: The bridge-type mechanism is one of the most widely used displacement amplifiers in micro-scale applications. However, a bridge-type mechanism with an external load always works in an energy-inefficient situation due to the storage of strain energy in the flexural hinges, which can be validated by the analytical model established in this paper. In fact, in the majority of cases, the energy efficiency of this type of mechanism is only about 50%. To solve this problem, a highly efficient bridge-type mechanism based on negative stiffness is proposed in this paper, which features compact size, simple design, symmetric structure, and also high efficiency. The potential energy in the negative stiffness mechanism acts as an additional energy stream to maintain the total potential energy constant in the system, i.e. the input energy from the actuator can be totally transformed into the output energy, therefore the energy efficiency is close to 100% in an ideal situation. To validate the feasibility of the proposed solution, a prototype of the highly efficient bridge-type mechanism is fabricated. The experimental results show that the efficiency has been improved to 90% approximately when the negative stiffness mechanism is employed. The proposed design can be employed and extended to other compliant mechanisms where high efficiency is required.

A novel flexure beam module with low stiffness loss in compliant mechanisms

Abstract: Flexure mechanisms provide guided motion via elastic deformation of thin beams. Due to the employment of compliant elements, these mechanisms cannot sufficiently maintain acceptable constraint stiffness level in the entire range of motion. The stiffness deterioration afflicts the performance of flexure mechanisms in terms of motion range, accuracy and constraint characteristics. This paper presents a novel flexure beam module with improved constraint behavior in beam-based flexure mechanisms. The proposed module alleviates the problem of stiffness loss in large displacements and provides a better motion performance. The mathematical model governing the static behavior of the module is developed using the principle of virtual work. The geometric nonlinearity associated with large midplane stretching is taken into account. Closed-form solutions are derived for load-displacement relationships, providing a powerful design tool for the novel flexure. Also a nonlinear expression is obtained for the strain energy of the flexure in terms of end displacements. The functionality of the presented module is exploited in a multi-beam parallelogram mechanism. The constraint behavior of the parallelogram is analytically quantified and considerable improvements in stiffnesses and error motions are observed. The analytical results provided in this paper are verified via finite element simulations. The proposed novel module can be used as the building block of more complex flexures to improve their stiffness characteristics, diminish their error motions and widen their stability region.