Compliant Gripper Research Articles

Position/Force Visual-Sensing-Based Robotic Sheet-Like Peg-in-Hole Assembly

This article deals with a sheet-like peg-in-hole robotic assembly task which is usually done in manual operation. Thin-walled deformable objects will introduce some special uncertainties in robotic peg-in-hole assembly, which may make this kind of robotic assembly difficult. To deal with this issue, three four-phase selective compliance articulated robot arm (SCARA)-based peg-in-hole assembly strategies which imitate manual assembly with a hybrid switching force/position control scheme are proposed. Both position/force visual sensing methods are proposed based on a single-lens imaging system in an eye-to-hand configuration, respectively. Based on the developed visual sensing techniques, an iterative learning control (ILC) scheme is designed to approximate the synchronous position control between a SCARA robot and a two-degree of freedom (DoF) hybrid compliant gripper. The experiments are implemented on our built-up SCARA-based robotic peg-in-hole (peg: electrode ear, hole: plastic shell) assembly platform. The results indicate that the visual force sensing nonlinearity is about 3% at the full range, and the proposed robotic assembly strategies are suited to this kind of peg-in-hole assembly from industrial scenarios.

Topology Optimization and Prototype of a Multimaterial-Like Compliant Finger by Varying the Infill Density in 3D Printing.

This study presents a multimaterial topology optimization method for design of multimaterial compliant mechanisms. Traditionally, the objective function in topology optimization for design of structures is to minimize the strain energy (SE). For synthesis of compliant mechanisms, the objective function is usually to maximize the mutual potential energy (MPE). To design an adaptive compliant gripper for grasping size-varied objects, a multicriteria objective function considering both the SE and MPE at two different output ports is proposed in this study. In addition, based on the fact that different infill densities in three-dimensional (3D) printing leads to prototypes with different equivalent mechanical properties, this article proposes that a multimaterial design can be approximated by varying the values of infill densities in different portions of a 3D-printed component, which enables the multimaterial designs to be prototyped using the general low-cost, single-material fused deposition modeling 3D printing machines. The proposed method is used to design and prototype a bi-material compliant finger which is 3D printed using a flexible thermoplastic elastomer with infill densities of 30% and 100%. The experimental results demonstrate that the bi-material finger is a better design in terms of reducing the driving force while increasing the output displacement at the fingertip comparing to the single-material finger design with the same volume and weight. Furthermore, a two-finger gripper with two identical multimaterial-like compliant fingers is prototyped and installed on a six-axis industrial robot. The experimental tests are performed to demonstrate the effectiveness of the presented design.

A Compliant Adaptive Gripper and Its Intrinsic Force Sensing Method

Grasping unstructured objects and sensing the contact force are two vital issues for grippers. However, it is still difficult for most existing grippers to realize these two functions simultaneously. In this article, we revise the traditional fin-ray finger by inserting a series of rigid nodes into the compliant structure and develop an adaptive two-finger gripper. This design linearizes the gripper's deformation-force relationship and enables an intrinsic force sensing ability without any tactile sensor. Experimental results show that the finger has high accuracy in sensing the external force applied at its middle part (average error less than 3%) but much larger errors appear near its two ends. Further experiments indicate that the gripper functions well in sensing the total grasping force (average error less than 8%). Although larger errors are observed in estimating the force distribution at each node, the variation tendency of the sensed force coincides well with the ground truth. Experiments are also carried out on grasping free-form objects and performing pick-and-place operations to further prove the gripper's adaptive grasping and intrinsic force sensing abilities.

Automated Extraction of 3D Printed Parts From Unfused PA12 Powder Using a One-Shot 3D Printed Compliant Gripper

Automated individual part extraction from powder based 3D printers has the potential to save time and cost compared with fully manual part extraction, or part sorting following automated bulk separation of parts and unfused powder. This work details the development of a novel one-shot 3D printed compliant gripping mechanism, able to extract individual solid parts from unfused PA12 powder. It was found that the unfused powder causes grip slip and instability as well as an increase in the perceived object width. A new toothed digit geometry was created, able to reduce the duration of the instability, and localised vibration was shown to eliminate the initial period of increased strain. A combination of toothed geometry and localised vibration showed a relative strain output on the gripper almost identical to that of the same grasp with no powder present. This allows individual solid parts to be extracted from unfused powder in a known pose, ready for automation of subsequent post-processing steps.

Design of a Parallel Gripper Based on Topology Synthesis and Evolutionary Optimization

Abstract A compliant gripper with nearly parallel gripping motion is developed by a topology synthesis and a dimensional synthesis approach. The topology synthesis process can generate linkage-type compliant mechanisms. Suitable boundary conditions of the topology synthesis process are selected to achieve the desired functions of the device. The dimensional synthesis is based on an evolutionary optimal design process. To meet various design goals, a nondominated multi-objective genetic algorithm is selected for the optimal design process. A kinetostaic model based on the chained beam constraint model is developed for force–displacement analysis of the designs. Efficiency and accuracy of the design approach are proved by experiments. Appropriate linkage types of compliant mechanisms may be discovered by the topology optimization process before moving on to the dimensional synthesis to obtain final designs.

On material selection for topology optimized compliant mechanisms

Although compliant mechanism design is a thoroughly studied field, surprisingly little information can be found in literature regarding selection of optimal materials. This paper is intended to fill this gap. Density-based, geometrically robust, stress constrained topology optimization based on a total Lagrangian FEM formulation is used for investigation of a compliant inverter and a compliant gripper example. Changes in handling of the projection parameters and the stress constraints are proposed for improved algorithmic stability and accurate representation of the stresses and material stiffness in topology optimization. Large-scale optimization studies are carried out, varying elastic modulus and allowable stress in a wide range. A comparison with an Ashby chart shows the characteristics of the best suited material. It is shown, that an optimal Young’s modulus and a minimum required material strength (depending on the modulus) can be identified from the results. Altering critical optimization parameters, e.g. allowable volume fraction and the minimum length scale, their influence on the optimal material choice is investigated. Guidelines for compliant mechanism designers for efficient selection of suited materials are developed.

Spider-Inspired Electrohydraulic Actuators for Fast, Soft-Actuated Joints.

The impressive locomotion and manipulation capabilities of spiders have led to a host of bioinspired robotic designs aiming to reproduce their functionalities; however, current actuation mechanisms are deficient in either speed, force output, displacement, or efficiency. Here—using inspiration from the hydraulic mechanism used in spider legs—soft‐actuated joints are developed that use electrostatic forces to locally pressurize a hydraulic fluid, and cause flexion of a segmented structure. The result is a lightweight, low‐profile articulating mechanism capable of fast operation, high forces, and large displacement; these devices are termed spider‐inspired electrohydraulic soft‐actuated (SES) joints. SES joints with rotation angles up to 70°, blocked torques up to 70 mN m, and specific torques up to 21 N m kg−1 are demonstrated. SES joints demonstrate high speed operation, with measured roll‐off frequencies up to 24 Hz and specific power as high as 230 W kg−1—similar to human muscle. The versatility of these devices is illustrated by combining SES joints to create a bidirectional joint, an artificial limb with independently addressable joints, and a compliant gripper. The lightweight, low‐profile design, and high performance of these devices, makes them well‐suited toward the development of articulating robotic systems that can rapidly maneuver.

Topology Optimization for Design of a 3D-Printed Constant-Force Compliant Finger

A compliant constant-force mechanism is a passive force regulation device that can generate a nearly constant output force over a range of input or output displacements while without the use of sensors and feedback control. In topology synthesis of a compliant constant-force mechanism for a given input displacement range, the constant output force can be achieved by maintaining the output displacement to be nearly a same value while the input displacement increases. In order to further control the desired output displacement for the compliant constant-force mechanism before contacting the object, this article introduces a new composite objective function that can consider both the output force (with contact) and the output displacement (without contact) of the synthesized compliant mechanism. The sensitivity for the proposed objective function with respect to the element density is derived while considering the effect of nonlinearity in the large deformation condition. The proposed topology optimization method is used to design an innovative constant-force compliant finger, and its prototype is manufactured by 3-D printing using a flexible thermoplastic elastomer. The experimental results show the developed constant-force compliant finger can provide a nearly constant output force of 41.9 N over the input displacement ranging from 15 to 30 mm while the maximum and average force variations within the constant-force range are 2.2% and 0.9%, respectively. In addition, the developed constant-force compliant finger is used to design a three-fingered constant-force compliant gripper that can be used in robotic grasping of fragile objects.

Closed-Structure Compliant Gripper With Morphologically Optimized Multi-Material Fingertips for Aerial Grasping

Aerial grasping empowers unmanned aerial vehicles to find applications beyond structured logistics. However, it brings a number of challenges including inaccurate positioning of the end effector and limited energy sources. Moreover, solutions so far have difficulty in handling a variety of objects. A novel closed structure compliant gripper was developed to address the challenges above. The gripper has a large self-centering work envelope and is normally-closed for passive grasping. Introduction of compliance as a form of morphological computation was also considered to enhance grasping capabilities, where multi-material 3D printing would facilitate rapid design changes based on target application. To grasp different objects, the gripper has hot-swappable 3D printed fingertips which are optimized with a multi-objective Bayesian Optimization process using physical bench experiments mimicking drone grasping and ascent on a common object set. The morphology of the fingertip including tip width, curvature and distribution of soft and hard material on contact surface, are optimized with a bench test that mimics quadcopter takeoff and landing. The best design from optimization shows an improvement of more than 10% from the initial design in successful grasp operations, demonstrated by field tests with a quadcopter.

Hybrid hinge structure with elastic hinge on self-folding of 4D printing using a fused deposition modeling 3D printer

Recently, a new technology has come to the fore, namely four-dimensional (4D) printing. Conventional research has, however, mostly been in a glass state after a shape change such as self-folding of 4D printing, because the heat shrinkage or shape memory effect of polymers is used. Therefore, the 4D printed hinges are rigid and cannot be folded or unfolded after 4D printing below the glass transition temperature of the shape memory polymer (SMP). Hence, in this study, we proposed a hybrid hinge structure that is elastic and can be largely deformed as an origami structure even after 4D printing. On the hybrid hinge, a soft elastomer hinge was arranged beside the rigid 4D printing hinges to achieving the elastic folding deformation after self-folding. We confirmed that the samples recovered to almost their original shape without failure after 500 cycles of folding. They exhibited high durability and elasticity. Finally, the usefulness of the hybrid hinge was demonstrated by fabricating a miura-ori and an origami compliant mechanism gripper, which could be deformed as origami structure below the glass transition temperature of the SMP. We believe that our 4D printing method can be applied for developing self-folding actuators and robots, which require origami deformation.

Optimizing compliant gripper mechanism design by employing an effective bi-algorithm: fuzzy logic and ANFIS

Optimizing compliant gripper mechanism design by employing an effective bi-algorithm: fuzzy logic and ANFIS

Analytical Model and Computing Optimization of a Compliant Gripper for the Assembly System of Mini Direct-Current Motor

This study proposes a combination of kinematics-based design method, statistic method, and TLBO algorithm to solve computing optimization for a compliant gripper. It is employed in an assembly system of mini direct current motor. First, the kinematic models of the gripper are developed. The static model is paid attention. Next, the dynamic model is established based on Lagrange's principle. Then, a multi-criteria optimization for the gripper is conducted by Taguchi method integrated with TLBO algorithm. Finally, the FEA and experiments are implemented to verify the optimal results and evaluate the performances of the compliant gripper. The results indicated that theoretical models are in good accord with the results from simulations and experiments. Additionally, the performance of the present method is superior to PSO algorithm. The results revealed that the compliant gripper allows a displacement up to 3000 µm and the amplification ratio of 12 times. The compliant gripper is a potential application for the mini direct-current motor assembly system.

Design of Soft Grippers with Modular Actuated Embedded Constraints

Underactuated, modular and compliant hands and grippers are interesting solutions in grasping and manipulation tasks due to their robustness, versatility, and adaptability to uncertainties. However, this type of robotic hand does not usually have enough dexterity in grasping. The implementation of some specific features that can be represented as “embedded constraints” allows to reduce uncertainty and to exploit the role of the environment during the grasp. An example that has these characteristics is the Soft ScoopGripper a gripper that has a rigid flat surface in addition to a pair of modular fingers. In this paper, we propose an upgraded version of the Soft ScoopGripper, developed starting from the limits shown by the starting device. The new design exploits a modular structure to increase the adaptability to the shape of the objects that have to be grasped. In the proposed device the embedded constraint is no rigid neither unactuated and is composed of an alternation of rigid and soft modules, which increase versatility. Moreover, the use of soft material such as thermoplastic polyurethane (TPU) reduces the risk of damage to the object being grasped. In the paper, the main design choices have been exploited and a finite element method (FEM) analysis through static simulation supports a characterization of the proposed solution. A complete prototype and some preliminary tests have been presented.

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

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 Novel Simple, Adaptive, and Versatile Soft-Robotic Compliant Two-Finger Gripper With an Inherently Gentle Touch

AbstractIn soft robotics, there is still a great need for a universal but simple gripper that realizes a high level of adaptability as well as a gentle touch to a wide variety of unknown objects of different size, shape, stiffness, and weight without the use of sensors or vision. Various, mostly complex grippers already exist based on certain actuation concepts. However, each solution has specific limitations, especially regarding gripping different soft and delicate objects. Therefore, this paper introduces a new approach to design a simple, adaptive, and versatile soft robotic two-finger gripper that is based on compliant mechanisms. More specifically, an inherently gentle touch is realized by utilizing an optimally synthesized mechanism with distributed compliance in combination with a conventional linear actuator. It is shown by finite elements method (FEM) simulations that the gripper realizes a high force and motion transmission at the same time. Furthermore, it is demonstrated by tests with a gripper prototype that reliable, safe, and fast grasping as well as manipulation are possible for a wide variety of objects. It is shown that beside regular and stiff objects also very challenging objects can be easily gripped, e.g., small, irregular, soft, and squeezable objects like fruits, berries, and vegetables. Moreover, it is confirmed that the developed compliant two-finger gripper can be used beneficially without sensors and control for differently sized and shaped objects with a comparable weight.

Soft Robotic Gripper With Compliant Cell Stacks for Industrial Part Handling

Robot object grasping and handling requires accurate grasp pose estimation and gripper/end-effector design, tailored to individual objects. When object shape is unknown, cannot be estimated, or is highly complex, parallel grippers can provide insufficient grip. Compliant grippers can circumvent these issues through the use of soft or flexible materials that adapt to the shape of the object. This letter proposes a 3D printable soft gripper design for handling complex shapes. The compliant properties of the gripper enable contour conformation, yet offer tunable mechanical properties (i.e., directional stiffness). Objects that have complex shape, such as non-constant curvature, convex and/or concave shape can be grasped blind (i.e., without grasp pose estimation). The motivation behind the gripper design is handling of industrial parts, such as jet and Diesel engine components. (Dis)assembly, cleaning and inspection of such engines is a complex, manual task that can benefit from (semi-)automated robotic handling. The complex shape of each component, however, limits where and how it can be grasped. The proposed soft gripper design is tunable by compliant cell stacks that deform to the shape of the handled object. Individual compliant cells and cell stacks are characterized and a detailed experimental analysis of more than 600 grasps with seven different industrial parts evaluates the approach.

Compliant gripper design, prototyping, and modeling using screw theory formulation

This article investigates some aspects related to the design, modeling, prototyping, and testing of soft–rigid tendon-driven grippers. As a case study, we present the design and development of a two-finger soft gripper and exploit it as an example to demonstrate the application scenario of our mathematical model based on screw theory. A mathematical formulation based on screw theory is then presented to model gripper dynamics. The proposed formulation is the extension of a model previously introduced including the mechanical system dynamics. In this type of gripper, it is possible to achieve different behaviors, e.g., different fingertip trajectories, equivalent fingertip stiffness ellipsoids, etc., while keeping the same kinematic structure of the gripper and varying the properties of its passive deformable joints. These properties can be varied in the prototype by properly regulating some manufacturing parameters, such as percentage of printing infill density in a 3D printing process. We performed experiments with the prototype of the gripper and an optical tracking system to validate the proposed mathematical formulation, and to compare its results with other simplified formulations. We furthermore identified the main performance of the gripper in terms of payload and maximum horizontal resisted force, and verified the capabilities of the gripper to grasp objects with different shapes and weights.

Topology optimization of compliant mechanisms considering strain variance

In this work, compliant mechanisms are designed by using multi-objective topology optimization, where maximization of the output displacement and minimization of the strain are considered simultaneously. To quantify the strain, we consider typical measures of strain, which are based on the p-norm, and a new class of strain quantifying functions, which are based on the variance of the strain. The topology optimization problem is formulated using the Solid Isotropic Material with Penalization (SIMP) method, and the sensitivities with respect to design changes are derived using the adjoint method. Since nearly void regions may be highly strained, these regions are excluded in the objective function by a projection method. In the numerical examples, compliant grippers and inverters are designed, and the tradeoff between the output displacement and the strain function is investigated. The numerical results show that distributed compliant mechanisms without lumped hinges can be obtained when including the variance of the strain in the objective function.

Design and test of a compact compliant gripper using the Scott–Russell mechanism

This paper presents the design, modeling, fabrication, and test of a monolithic compliant gripper for micro-manipulation applications. A compact compliant mechanism that enables in-principle straight-line parallel jaw motion is obtained, by combining the Scott–Russell mechanism and the parallelogram mechanism. The right-circular corner-filleted (RCCF) flexure hinge is adopted to achieve a large displacement of lumped-compliance joints. A pseudo-rigid-body model (PRBM) method with the help of the virtual work principle is performed to obtain parametric analytical models including the amplification coefficient and kinetostatics. Finite element analysis (FEA) is conducted to validate the analytical model and capture adverse parasitic motions of jaws. A monolithic prototype was fabricated, the test results of which show satisfactory performances.

A Variable-Structure Robot Hand That Uses the Environment to Achieve General Purpose Grasps

Modern robotic grippers are either specialized and simple, complex and general purpose, or soft and compliant hands. The first provide high reliability but are limited in the range of objects they grasp. Compliant or soft grippers can grasp wide ranges of objects, but are not yet reliable enough for real-world applications. This letter presents a novel variable-structure general purpose robotic hand. The planar-acting hand has a minimalistic structure that can adapt itself against the environment to fit a wide range of objects. The single motor, multi-finger hand utilizes a novel principle of re-arranging its structure prior to grasping. This is achieved by pressing the hand against the environment and performing a series of adjustment moves, to best suit the hand for the object to be grasped. The design and method of operation of the hand is explained. Then, a technique for determining the set of adjustments needed to re-arrange the hand according to the desired grasp is sketched. Real-world experiments are performed with the hand, showing its ability to re-arrange itself and grasp previously unseen objects. Source code for the simulations and experiments is supplemented to the paper, as well as a video clip of the variable-structure hand in action.