Showing papers on "Compliant mechanism published in 2022"

Stiffness-Tunable Soft Gripper with Soft-Rigid Hybrid Actuation for Versatile Manipulations.

Abstract: Highly flexible and environmentally adaptive soft robots have received considerable attention. There remains a demand for soft robots to realize the stiffness modulation and variable workspace for robust and versatile manipulations. This article presents a compact soft gripper with a polylactic acid-based variable stiffness module (VSM) and a rigid retractable mechanism to achieve soft-rigid hybrid actuation. The soft gripper can enhance its stiffness by 18-fold without sacrificing flexibility due to the VSM. A heating circuit is designed to divide the VSM into three regions. Each region can be activated separately for varying flexible segments to amplify the dexterity. Meanwhile, the water-cooling system accelerates the heat exchange, thus reducing the cooling time from ∼400 to 39 s. The rigid retractable mechanism can adjust the initial layout of the gripper to expand the workspace and perform manipulation by opening and closing fingers. The soft finger combined with stiffness tunability can maintain its deformation after being stiffened to realize morphing. Therefore, it can efficiently perform a grasp with a high load and avoid repeated heating and cooling, especially for items with a similar shape. The performance of the gripper is further validated by measuring the grasping force and grasping demonstration with various objects, showing its robustness and dexterity in versatile tasks.

A novel compliant XY micro-positioning stage using bridge-type displacement amplifier embedded with Scott-Russell mechanism

Abstract: This paper presents a new bridge-type compliant displacement amplifier embedded with Scott-Russell mechanism. Based on this design, a novel compliant XY micro-positioning stage is also designed, analyzed and tested to illustrate the advantages of the proposed amplifier. The main feature of the new amplifier is to employ Scott-Russell mechanism to replace one arm of the compound bridge-type mechanism, aiming to improve its lateral stiffness and the natural frequency in the working direction. The compliant XY micro-positioning stage is driven by two novel bridge-type amplifiers with orthogonal distribution. Double parallelogram guide mechanisms are added at the output ends of the bridge-type mechanisms to further lower the parasitic movements of the XY stage. Then, an analytical model based on the stiffness matrix transfer method is established for the static and dynamic characteristics of stage, which are also verified by performing finite element analysis. Finally, an experimental prototype is manufactured and its static and dynamic performances are tested. The test results demonstrate that the stage can realize a workspace of 181.0 μm × 179.5 μm with motion resolution of 20 nm and a maximum coupling error of 1.07%. The natural frequencies of XY stage along the working direction are 178 Hz and 248 Hz respectively when the PZTs are not installed or installed. Additionally, the trajectory tracking performance of the stage is also preliminarily evaluated. All of the results obtained from the analytical model, finite element analysis and experimental tests verify the effectiveness of the novel bridge-type displacement amplifier and the compliant XY micro-positioning stage.

A novel compliant XY micro-positioning stage using bridge-type displacement amplifier embedded with Scott-Russell mechanism

Abstract: This paper presents a new bridge-type compliant displacement amplifier embedded with Scott-Russell mechanism. Based on this design, a novel compliant XY micro-positioning stage is also designed, analyzed and tested to illustrate the advantages of the proposed amplifier. The main feature of the new amplifier is to employ Scott-Russell mechanism to replace one arm of the compound bridge-type mechanism, aiming to improve its lateral stiffness and the natural frequency in the working direction. The compliant XY micro-positioning stage is driven by two novel bridge-type amplifiers with orthogonal distribution. Double parallelogram guide mechanisms are added at the output ends of the bridge-type mechanisms to further lower the parasitic movements of the XY stage. Then, an analytical model based on the stiffness matrix transfer method is established for the static and dynamic characteristics of stage, which are also verified by performing finite element analysis. Finally, an experimental prototype is manufactured and its static and dynamic performances are tested. The test results demonstrate that the stage can realize a workspace of 181.0 μm × 179.5 μm with motion resolution of 20 nm and a maximum coupling error of 1.07%. The natural frequencies of XY stage along the working direction are 178 Hz and 248 Hz respectively when the PZTs are not installed or installed. Additionally, the trajectory tracking performance of the stage is also preliminarily evaluated. All of the results obtained from the analytical model, finite element analysis and experimental tests verify the effectiveness of the novel bridge-type displacement amplifier and the compliant XY micro-positioning stage.

Designs of Compliant Mechanism-Based Force Sensors: A Review

Abstract: Sensing the interaction force in robot-assisted manipulation draws lots of attentions in related research fields. In order to protect the manipulation objects and to build the real-time force feedback for system control, force sensors have been proposed and applied. The sensing structures of most proposed force sensors possess the characteristics of compliant mechanisms. This paper reviews the designs of compliant mechanism-based force sensors in the past few years. Main sensing principles, scales and compliance, compliant mechanism designs of the force sensors have been studied and concluded. According to the researching status, the future developments of compliant mechanism-based force sensors have been also introduced. This paper aims at providing references of designing a high-performance compliant mechanism-based force sensor.

LARG: A Lightweight Robotic Gripper With 3-D Topology Optimized Adaptive Fingers

Abstract: Adaptive grasping is an important approach for robotic grippers to handle objects with irregular shapes. Compared to rigid-link-based adaptive grippers, the continuum-structure grippers benefit from their structural compliance and have thus a higher degree of adaptive grasping freedom. Based on this advantage, we have developed a continuum-structure-based double-finger gripper in this article to achieve the adaptive grasping. To improve the design efficiency, a 3-D-topology-optimization-based design method was adopted in this article, which realized the adaptive-grasping function of the robotic finger by introducing an additional spring into the design problem. The proposed robotic gripper was selective-laser-sintered with the material polyamide (PA2200) and was actuated by a linear motor. Experiments were also conducted to evaluate the grasping performance and load capacity of the developed gripper. Results have shown that the gripper could successfully grasp objects of different shapes and materials. In addition, with a total weight of only 180 g, the developed gripper can achieve a maximum grasping payload of 8.8 kg, which is about 49 times of its self-weight. From the methodological point of view, this work has successfully demonstrated the feasibility of optimization-based automatic design of robotic grippers.

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 <50 N, strain >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.

A Soft and Bistable Gripper with Adjustable Energy Barrier for Fast Capture in Space.

Abstract: Actuators for fast capture are essential in the tasks of space structure assembly and space debris disposal. To avoid damage and rebound caused by collision, the mechanical devices for capture or docking impose very strict restrictions on the collision speed. The gripper made of soft material can realize compliant grasping, but its actuating speed and driving mode should adapt to the scenarios of grasping moving objects in space. By harnessing the rapid occurrence of structural instability and tuning its triggering conditions, we present a soft and bistable gripper for dynamic capture. The gripper deforms on the collision with other objects, and it absorbs the kinetic energy of the objects to trigger an instability, and then achieve fast grasping as well as cushioning. This process does not need any other input energy, and it greatly simplifies the conventional driving devices so as to realize the miniaturized and light-weight gripping actuation. The proper pre-deformation to the bistable structure of the gripper enables one to dynamically adjust the energy barrier for triggering the onset of instability to achieve the optimal grasping and buffering effect according to the kinetic characteristics of targets. After finishing one grasping task, the bistable gripper can automatically return to its initial state and release the target via a self-designed cable-driven mechanism. The ground-testing experiment demonstrates that the proposed soft gripper is capable to grasp, transfer, and release moving targets, and it thus possesses great potential to fulfill challenging operations in space missions.

Novel design of a piezoelectrically actuated compliant microgripper with high area-usage efficiency

Abstract: Microgrippers act as an end effector in micromanipulation systems, completing the pick-transport-release operations during the working process. This paper presents the design, modeling, optimization, simulation, and experiment of a novel piezoelectrically actuated compliant microgripper for micromanipulation and microassembly. Considering the space and cost constraints of the micromanipulation system, the area-usage efficiency and piezoelectric actuator utilization efficiency are introduced to evaluate its performance. A three-stage amplification mechanism based on bridge-type and leverage mechanisms arranged in series is introduced to achieve a large jaw displacement. The displacement amplification ratio of the microgripper is analyzed via the pseudo-rigid-body model approach. Optimization based on response surface analysis was conducted to determine the structural parameters of the compliant mechanism. Finite-element analysis is performed to evaluate the gripper performance. Moreover, a gripper prototype was fabricated for the experimental test. The investigation results indicate that the gripper allows a maximum gripping displacement of 548.42 μm, a first natural frequency of 334 Hz, and a motion resolution of ±0.75 μm. In comparison with previous designs, the reported microgripper has the advantages of a larger displacement, higher area-usage efficiency, and better PEA utilization efficiency. The micromanipulation capability of the developed gripper was demonstrated by gripping three tiny objects of different sizes and shapes.

A Novel Flexure Piezomotor With Minimized Backward and Nonlinear Motion Effect

Abstract: Continuous, smooth, and highly linear displacement output with centimeter-scale stroke and nanometer-scale resolution is greatly attractive for an ultraprecision positioning system. On the foundation of piezoelectric actuation and compliant mechanism, a novel linear piezomotor with minimized backward and nonlinear motion property is designed. Specifically, a driving unit capable of $xy$ -direction decoupling displacement output is proposed to make the generated motion linear, and a flexure mechanism is introduced into the contact part to make the motion output continuous and nonbackward. Combined with double driving units and the flexure contact mechanism, a motion generation strategy is presented. For performing the motion generation strategy well, the kinematics model of the piezomotor is established. Then, the dimension parameters are optimized. After theoretical derivations, the piezomotor is analyzed and evaluated by finite-element analysis simulation. Finally, a proper control waveform is designed, and the motion generation performance tests are successfully carried out. The results uniformly confirm that the proposed piezomotor can output ultraprecision motion with 8-nm resolution and 10-mm stroke in a continuous and smooth manner, and the backward and nonlinear effects are successfully minimized within 100 Hz.

Mechanical Performance of Lightweight-Designed Honeycomb Structures Fabricated Using Multijet Fusion Additive Manufacturing Technology.

Abstract: Cellular structures including three-dimensional lattices and two-dimensional honeycombs have significant benefits in achieving optimal mechanical performance with light weighting. Recently developed design techniques integrated with additive manufacturing (AM) technologies have enhanced the possibility of fabricating intricate geometries such as honeycomb structures. Generally, failure initiates from the sharp edges in honeycomb structures, which leads to a reduction in stiffness and energy absorption performance. By material quantity, these hinges account for a large amount of material in cells. Therefore, redesigning of honeycomb structures is needed, which can improve aforementioned characteristics. However, this increases the design complexity of honeycombs, such that novel manufacturing techniques such as AM has to be employed. This research attempts to investigate the optimal material distribution of three different topologies of honeycomb structures (hexagonal, triangular, and square) with nine different design configurations. To achieve this, higher amount of material was distributed at nodes in the form of fillets while keeping overall weight of the structure constant. Furthermore, these design configurations were analyzed in terms of stiffness, energy absorption, and the failure behavior by performing finite element analysis and experimental tests on the samples manufactured using Multijet fusion AM technology. It was found that adding material to the edges can improve the mechanical properties of honeycombs such as stiffness and energy absorption efficiency. Furthermore, the failure mechanism is changed due to redistribution of material in the structure. The design configurations without fillets suffer from brittle failure at the start of the plastic deformation, whereas the configurations with increased material proportion at the nodes have larger plastic deformation zones, which improves the energy absorption efficiency.

Optimized design of a compact multi-stage displacement amplification mechanism with enhanced efficiency

Abstract: Mechanically amplifying micro-stroke of actuators such as piezoelectric stacks through compliant mechanisms is an effective solution for use in micro/nano manipulation, precision positioning, gripping and manufacturing with large enough workspaces. This paper proposes a novel type of hybrid three-stage compliant displacement amplifier with a compact structure by synthesizing bridge-type and lever-type compliant mechanisms. A new index is introduced to measure its displacement amplification efficiency by considering the whole size and input stiffness. To facilitate design, the dynamic stiffness model is also derived to capture its kinetostatics and dynamics on both time and frequency domain. Then, the key structural parameters are efficiently optimized with the Pareto multi-objective optimization strategy. The capacity curve in terms of the resonance frequency, displacement amplification ratio and load capacity (output stiffness) is provided as well in a form of the Pareto optimal solution set. Experimental tests of two prototypes indicate a high displacement amplification efficiency of the current design, of which one prototype exhibits the output displacement of 0.7 mm and the resonance frequency of 874 Hz with a compact size of only 57 mm × 50 mm × 10 mm.

Enhancing Dynamic Bandwidth of Amplified Piezoelectric Actuators by a Hybrid Lever and Bridge-Type Compliant Mechanism

Abstract: Ongoing interests in high-speed precision actuation continuously sparks great attention on developing fast amplified piezoelectric actuators (APAs) with compliant mechanisms. A new type of APA with enhanced resonance frequency is herein reported based on a hybrid compliant amplifying mechanism. A two-stage displacement flexure amplifier is proposed by synthesizing the lever-type and semi bridge-type compliant mechanisms in a compact configuration, promising to a well tradeoff between the displacement amplification ratio and dynamic bandwidth. The static and dynamic performances are experimentally evaluated. The resonance frequency of 2.1 kHz, displacement amplification ratio of 6, and step response time of around 0.4 ms are realized with a compact size of 50 mm × 44 mm × 7 mm. Another contribution of this paper is to develop a comprehensive two-port dynamic stiffness model to predict the static and dynamic behaviors of the compliant amplifier. The modeling approach presented here differs from previous studies in that it enables the traditional transfer matrix method to formulate both the kinetostatics and dynamics of compliant mechanisms including serial-parallel branches and rigid bodies.

Fabrication and experimental characterization of a bistable tensegrity-like unit for lattice metamaterials

Abstract: The peculiar nonlinear mechanical behaviour of tensegrity structures and their engineering applications have attracted considerable interest in the last two decades. However, the difficulties in their traditional fabrication and assembling methods represent current limitations to their widespread use. This paper presents a novel design and fabrication procedure for bistable tensegrity-like units. Starting from the classical triangular tensegrity prism and using stereolithography technology, a double tensegrity-like unit was designed and realised monolithically as a compliant mechanism. High repeatability of compression tests confirmed the activation of the designed bistable twisting mechanism in large displacements, proving that the bistability of a tensegrity-like unit with null selfstress and no cables can dependably be achieved. Numerical simulations showed that a reduced-order stick-and-spring model is able to provide predictions on the nonlinear mechanical behaviour of the unit in close agreement with experimental results. Low relative density and bistable characteristics make this type of tensegrity-like unit suitable to manufacture highly-customisable multistable metamaterials. The proposed procedure could be applied to transform and additively manufacture other types of tensegrity structures with different nonlinear responses into corresponding tensegrity-like versions.

Configuration design and experimental verification of a variable constant-force compliant mechanism

Abstract: Abstract The realizing of variable output constant force has received wide attention. To achieve a force regulation in an economic way, a configuration of the constant force mechanism (CFM) referring to positive and negative stiffness combination method is proposed in this paper. By adjusting preloading displacement applied on positive-stiffness structure of the CFM, the variable constant force output can be realized. The force–displacement expression of the CFM in the non-preloaded condition is deduced by the established analytical models. Furthermore, parametric sensitivity analysis with several architectural parameters are conducted for optimizing physical structures. Finally, the correctness of the proposed principle is verified by experimental studies. The observed experimental results show that the CFM under different preloading displacements can provide required output constant force, which is consistent with proposed hypothesis.

Extended Dynamic Stiffness Model for Analyzing Flexure-Hinge Mechanisms With Lumped Compliance

Abstract: This paper introduces an extended dynamic stiffness modeling approach for concurrent kinetostatic and dynamic analyses of planar flexure-hinge mechanisms with lumped compliance. First, two novel dynamic stiffness matrices are derived for two types of flexure hinge connected to rigid bodies by shifting the end node to the mass center of rigid bodies considering the geometric effect of rigid motion. A straightforward modeling procedure is then proposed for the whole compliant mechanism based on d'Alembert's principle by selecting the displacements at both the mass center of rigid bodies and the rest end nodes of flexure hinges as the hybrid state variables. With the presented method, the statics and dynamics of flexure-hinge mechanisms with irregular-shaped rigid bodies in complex serial-parallel configurations can be analyzed in a concise form. The presented method is compared with other theoretical models, finite element simulation, and experiments for three case studies of a bridge-type compliant mechanism, a leveraged XY precision positioning stage, and a Scott–Russell-mechanism-based XYθ flexure manipulator. The results reveal the easy operation and well prediction accuracy of the presented method.

Force Enhanced Multi-Twisted and Coiled Actuator and Its Application in Temperature Self-Adaptive Tensegrity Mechanisms

Abstract: Twisted and coiled actuators (TCAs) are a type of thermal-driven actuator with high energy density and display the enormous application potential to drive soft robots. However, the existing soft robots driven by TCAs are scarce and can only realize the grabbing and creeping movement almost without load capacity. The main reason is that the TCA fabricated by traditional self-coiling method has low force and long thermal response time. Aiming to improve the output force of TCA, the fabrication and force enhancement principle of multitwisted and coiled actuators (MTCAs) are developed. Compared with the traditional single-twisted and coiled actuators, the output force of the MTCA can be increased to more than three times with the same fiber. The proposed MTCA with a mass of 0.1 g achieves deformations of 14.46% of its length under load of 1200 g. Besides, we expand the application field of the MTCA into the self-adaptive mechanisms. Two temperature self-adaptive tensegrity mechanisms with self-folded and self-upraised ability are designed and tested to further display the excellent performances of the MTCA. This article not only proposed the MTCA with significantly improved output force, but also promoted the application of the TCAs in temperature self-adaptive mechanisms.