Compliant Gripper Research Articles

Soft Robotic Gripper Driven by Flexible Shafts for Simultaneous Grasping and In-Hand Cap Manipulation

Performing a successful robotic grasping to uncertain objects in unstructured environments is challenging. This study presents a new compliant soft robotic gripper for objects handling and cap manipulation through the coordination of three soft fingers and in-hand manipulation. The experiments are conducted to validate that the soft robotic gripper can successfully realize simultaneous grasping and capping manipulations with only one flexible shaft actuation for every single soft finger. Note to Practitioners —Uncertain object manipulation tasks pose significant challenges to a robotic gripper while grasping and capping unknown objects without damaging them. The existing rigid grippers have experienced flexible manipulation through multiple degrees of freedom (DoFs) by complex mechanical structures, and the soft gripper can realize stiffness-compliant manipulation differently. The proposed novel robotic in-hand manipulation can execute grasping and cap manipulation by a single flexible shaft to simultaneously achieve bending and rotational movements. The relationship between stretching force and finger’s curvature can enable a custom design for user-specific applications.

A Novel Variable Stiffness Compliant Robotic Gripper Based on Layer Jamming

Abstract In this paper, we present a novel compliant robotic gripper with three variable stiffness fingers. While the shape morphing of the fingers is cable-driven, the stiffness variation is enabled by layer jamming. The inherent flexibility makes compliant gripper suitable for tasks such as grasping soft and irregular objects. However, their relatively low load capacity due to intrinsic compliance limits their applications. Variable stiffness robotic grippers have the potential to address this challenge as their stiffness can be tuned on demand of tasks. In our design, the compliant backbone of finger is made of 3D-printed PLA materials sandwiched between thin film materials. The workflow of the robotic gripper follows two basic steps. First, the compliant skeleton is driven by a servo motor via a tension cable and bend to a desired shape. Second, upon application of a negative pressure, the finger is stiffened up because friction between contact surfaces of layers that prevents their relative movement increases. As a result, their load capacity will be increased proportionally. Tests for stiffness of individual finger and load capacity of the robotic gripper are conducted to validate capability of the design. The results showed a 180-fold increase in stiffness of individual finger and a 30-fold increase in gripper’s load capacity.

All-Soft Skin-Like Structures for Robotic Locomotion and Transportation.

Human skins are active, smart, and stretchable. Artificial skins that can replicate these properties are promising materials and technologies that will enable lightweight, cost-effective, portable, and deployable soft devices and robots. We show an active, stretchable, and portable artificial skin (ElectroSkin) that combines dielectric elastomer actuators (DEAs) and soft electroadhesives (EAs) in a fully compliant multilayer composite skin-like structure. By taking advantage of the common characteristics of DEA and EA, we define regions of the composite artificial skin as either active or passive. Active areas can be exploited as electromechanical actuators or as electrostatic gripper elements, or both simultaneously. This embedded multimodality delivers a new technology of deformable active skins that can grip and move objects and self-locomote. ElectroSkins can be fabricated using all-soft elastomers and readily available conductive materials. We demonstrate their capabilities in the first soft self-actuating conveyor belt, with a conveyoring speed of 0.28 mm/s, and a pocketable fully soft crawler robot. This new, self-actuating, self-gripping, and self-locomoting soft artificial skin has the potential to significantly impact on functional soft-smart composites, deployable robots, soft-smart conveyoring, and compliant gripping and manipulation applications.

Design and modeling of a compliant gripper with parallel movement of jaws

A compliant gripper with parallel movement of jaws is developed. The gripper is composed of a bar linkage mechanism and a bistable mechanism. After a target object is grasped, the embedded bistable mechanism can maintain the gripping of the object with no energy consumption. Unstable fixation due to rotary motion of opposing gripping arms of traditional tweezer-like grippers is alleviated by the proposed gripper. Kinetostatic models of the device based on chained beam constraint model are derived to assist the design of the device. Grasp function of an object is accomplished while the gripper proceeds from its first stable equilibrium position to its second stable equilibrium position, and release of the object is achieved when the gripper returns back to its first stable equilibrium position.

Optimal Design of a Motor-Driven Three-Finger Soft Robotic Gripper

This article presents an innovative, motor-driven, three-finger compliant gripper for adaptive grasping of size-varied delicate objects. An optimized compliant finger design is identified numerically through a topology optimization method. A stepper motor is used to actuate three identical compliant fingers, which can operate through elastic bending deformation. Finite-element models are developed to investigate the maximum equivalent stress, input force, and output displacement relations corresponding to the amount of input displacement of the compliant finger. Simulation results show that the proposed finger design is with a lower driving force and a lower maximum equivalent stress during operation comparing to one previous design. The proposed compliant finger is prototyped by three-dimensional printing using thermoplastic elastomer. Experimental results for the input displacement versus input force and input displacement versus geometric advantage relationships of the prototyped finger agree well with simulation results. The developed three-finger soft robotic gripper is mounted on an industrial robot arm to demonstrate its capability in handing size-varied delicate objects, such as egg, fruits, and glass products. Experimental results show that the proposed three-finger gripper can be used to grip object with a maximum weight of 4.2 kg and a maximum object size of 140 mm. The overall weight of the developed three-finger soft robotic gripper is 1.2 kg. The load capacity of the developed gripper can vary according to the friction between gripper and object. The maximum payload of the gripper can be increased to 9.5 kg when an additional antislip foam tape is applied on the grip surfaces of the compliant fingers.

The design of a compliant shape-preserving ring

Compliant shape-preserving mechanisms fill a small niche within the established research field of compliant mechanisms. To date no report exists on a compliant shape-preserving ring, even though these mechanisms can give rise to a multitude of practical applications. They can essentially act as novel sealing mechanisms, compliant grippers or find use in medical applications. This paper presents three compliant shape-preserving rings that maintain their circular shape up to 99% for their full range-of-motion. The best design can expand in diameter by ≈ 45%. A prototype of a compliant shape-preserving ring was constructed from PETG (Polyethylene Terephtalate Glycol-modified) using fused-filament-fabrication. The experimental evaluation of this prototype showed a good agreement with the numerical model describing the design. A novel compliant scissor mechanism was obtained in the design process and this mechanism can prove useful in the design of other compliant shape-preserving mechanisms.

Compliant bipolar electrostatic gripper using 3D-printed-layered elastic probes

A 3D-printed-layered structure for forming the compliant bipolar electrostatic gripper is proposed. A prototype gripper module, which consists of an array of elastically deformable bipolar micro-probes, is fabricated using a conventional 3D printer. Experiments are conducted to examine the attractive force and the pick-and-place performance of the prototype. Experimental results show that the prototype module can generate the maximum attractive pressure of 87.1 Pa, which can pick and hold a piece of xerographic paper of 31.2 mg successfully. The effects of residual charges and the probe-tips flattening process on the attractive force are also discussed.

Design of compliant gripper for surgical applications

ABSTRACT This article presents a novel design of monolithic compliant grippers for surgical applications. Compliant mechanisms are jointless mechanism and overcome the major traditional issues such as wear, friction and assembly error. Performance of compliant mechanism depends on the elastic body deformation to transmit motion between the linkages. In this article, the proposed gripper is designed with three different parts with different intended functions: tilting actuator, linear actuator and gripping jaws. Gripping jaws are developed to cut through shearing and collect tissues. A linear actuator transmits the input force to the gripping jaws. A tilting actuator tilts the whole gripper up and down to increase the shear force to cut the tissues. A conceptual mechanism is developed using topology optimisation for these concepts independently. Later, these parts are assembled and developed as a final solid model using SolidWorks. The structural performance of the gripper is investigated thoroughly using finite-element method.

On Alternative Uses of Structural Compliance for the Development of Adaptive Robot Grippers and Hands.

Adaptive robot hands are typically created by introducing structural compliance either in their joints (e.g., implementation of flexures joints) or in their finger-pads. In this paper, we present a series of alternative uses of structural compliance for the development of simple, adaptive, compliant and/or under-actuated robot grippers and hands that can efficiently and robustly execute a variety of grasping and dexterous, in-hand manipulation tasks. The proposed designs utilize only one actuator per finger to control multiple degrees of freedom and they retain the superior grasping capabilities of the adaptive grasping mechanisms even under significant object pose or other environmental uncertainties. More specifically, in this work, we introduce, discuss, and evaluate: (a) a design of pre-shaped, compliant robot fingers that adapts/conforms to the object geometry, (b) a hyper-adaptive finger-pad design that maximizes the area of the contact patches between the hand and the object, maximizing also grasp stability, and (c) a design that executes compliance adjustable manipulation tasks that can be predetermined by tuning the in-series compliance of the tendon routing system and by appropriately selecting the imposed tendon loads. The grippers are experimentally tested and their efficiency is validated using three different types of tests: (i) grasping tests that involve different everyday objects, (ii) grasp quality tests that estimate the contact area between the grippers and the objects grasped, and (iii) dexterous, in-hand manipulation experiments to evaluate the manipulation capabilities of the Compliance Adjustable Manipulation (CAM) hand. The devices employ mechanical adaptability to facilitate and simplify the efficient execution of robust grasping and dexterous, in-hand manipulation tasks.

Pneumatic Hyperelastic Actuators for Grasping Curved Organic Objects

Soft robotic grippers often incorporate pneumatically-driven actuators that can elastically deform to grasp delicate, curved organic objects with minimal surface damage. The complexity of the actuator geometry and the nonlinear stress–strain behavior of the stretchable material during inflation make it difficult to predict actuator performance prior to prototype fabrication. In this work, a scalable modular elastic air-driven actuator made from polydimethylsiloxane (PDMS) is developed for a mechanically compliant robotic gripper that grasps individual horticultural plants and fungi during automated harvesting. The key geometric design parameters include the expandable surface area and wall thickness of the deformable structure used to make contact with the target object. The impact of these parameters on actuator displacement is initially explored through simulation using the Mooney–Rivlin model of hyperelastic materials. In addition, several actuator prototypes with varying expandable wall thicknesses are fabricated using a multistep soft-lithography molding process and are inserted in a closed ring assembly for experimental testing. The gripper performance is evaluated in terms of contact force, contact area with the target, and maximum payload before slippage. The viability of the gripper with PDMS actuators for horticultural harvesting applications is illustrated by gently grasping a variety of mushroom caps.

Design of a Monolithic Double-Slider Based Compliant Gripper with Large Displacement and Anti-Buckling Ability.

In a micro-manipulation system, the compliant gripper is used for gripping, handling and assembling of objects. Large displacement and anti-buckling characteristics are desired in the design of the gripper. In this paper, a compliant gripper with these two characteristics is proposed, modelled and verified. The large displacement is enabled by using distributed compliance in a double-slider kinematic mechanism. An inverted flexure arrangement enables the anti-buckling of the gripper when closing the two jaws. A pseudo-rigid-body model (PRBM) method with the help of virtual work principle is employed to obtain several desired analytical relations including the amplification coefficient and kinetostatics. The results of the finite element analysis (FEA) are shown to be consistent with the results of the derived analytical model. An experimental test was carried out through a milling machined aluminium alloy prototype, the results of which verify the good performance of the compliant gripper.

Retraction Note to: Input Displacement Neuro-fuzzy Control and Object Recognition by Compliant Multi-fingered Passively Adaptive Robotic Gripper

The requirement for new flexible adaptive grippers is the ability to detect and recognize objects in their environments. It is known that robotic manipulators are highly nonlinear systems, and an accurate mathematical model is difficult to obtain, thus making it difficult make decision strategies using conventional techniques. Here, an adaptive neuro fuzzy inference system (ANFIS) for controlling input displacement and object recognition of a new adaptive compliant gripper is presented. The grasping function of the proposed adaptive multi-fingered gripper relies on the physical contact of the finger with an object. This design of the each finger has embedded sensors as part of its structure. The use of embedded sensors in a robot gripper gives the control system the ability to control input displacement of the gripper and to recognize particular shapes of the grasping objects. Fuzzy based controllers develop a control signal according to grasping object shape which yields on the firing of the rule base. The selection of the proper rule base depending on the situation can be achieved by using an ANFIS strategy, which becomes an integrated method of approach for the control purposes. In the designed ANFIS scheme, neural network techniques are used to select a proper rule base, which is achieved using the back propagation algorithm. The simulation results presented in this paper show the effectiveness of the developed method.

A Hybrid Soft Robotic Surgical Gripper System for Delicate Nerve Manipulation in Digital Nerve Repair Surgery

In digital nerve repair surgery, handling the digital nerves using traditional forceps requires surgeons to be extremely cautious in order to minimize unintended iatrogenic nerve trauma. These injuries are mainly caused by excessively forceful manipulation with metallic rigid forceps. Soft pneumatic actuators have been increasingly adopted to broaden the biomedical applications of conventional rigid structures due to their inherent excellent compliance and compressibility. Existing soft grippers, however, face barriers to their use in digital nerve repair, due to their large prototype size or limited gripping force. In this paper, a new two-arm hybrid soft surgical gripper system is proposed to reduce the risk of excessive stress to nerves and facilitate surgeons’ delicate nerve manipulation in digital nerve repair surgery. It consists of two hybrid soft surgical grippers, two stiffness-tunable positioning arms, and a pedal-based control system. The inflated soft pneumatic gripping actuator is capable of providing compliant gripping and soft interaction with nerve tissues. This can prevent sudden overgripping force stimulation. The ability to position two surgical grippers is provided by two stiffness-tunable arms combining six pneumatic locking actuators. The inflation of the soft pneumatic actuator is investigated using a theoretical model and finite element analysis. Cadaver experiments, rodent experiments, and histopathological studies are conducted to validate that the proposed surgical gripper system is capable of completing required digital nerve manipulations in digital nerve repair surgery and exhibits very low disruption to nerve tissues.

Development of a Novel Modular Compliant Gripper for Manipulation of Micro Objects.

This paper proposes a modular gripping mechanism for the manipulation of multiple objects. The proposed micro gripper combines traditional machining techniques with MEMS technologies to produce a modular mechanism consisting of a sturdy, compliant aluminium base and replaceable end-effectors. This creates an easily-customisable solution for micro manipulation with an array of different micro tips for different applications. We have provided the kinematic analysis for the gripper to predict the output and have also optimised design parameters based on FEA (finite element analysis) simulation and the effects of altering flexure beam lengths. The gripper is operated by a piezo actuator capable of 18 m displacement at 150 V of applied DC voltage. This is then amplified by a factor of 8.1 to a maximum tip displacement of 154 m. This is achieved by incorporating bridge and lever amplifying techniques into the design. An initial experimental analysis of the micro gripper is provided to investigate the performance of the micro gripper and to gauge the accuracy of the theory and simulation through comparison with experimental results.

Design, analysis, and testing of a novel compliant underactuated gripper.

This paper proposes a novel compliant underactuated gripper with multiple working modes. Based on the pseudo-rigid-body method, a static analysis of different working modes is carried out, establishing an analytical relationship between the output grasping forces and the input load. For the enveloped grasping mode, an algorithm to determine the static equilibrium position is given. Furthermore, a parametric optimization algorithm based on gradient descent is designed to obtain the maximum grasping forces. The effectiveness of the multiple grasping modes, the grasping force models, and the optimization algorithm are verified by a dynamic simulation package and finite element analysis as well as by experimental tests. Finally, various grasping experiments are conducted to further validate each working mode, the stability of grasping, and the ability to protect fragile objects.

Efficient Insertion of Partially Flexible Objects in Precision Assembly

This paper proposes an efficient strategy for the insertion of the partially flexible object in precision assembly. The partially flexible object refers to the component with unevenly distributed flexibilities: coupling rigidity and flexibility. This paper focuses on the insertion of one class of partially flexible objects: rigid shapes connected by a compliant mechanism. We first analyze the horizontal compliance of the compliant mechanism and build a model to relate its state and force. The insertion is separated into two stages according to the insertion type. The first stage is a compliant insertion and we estimate the insertion direction based on the built model, horizontally compensate resorting to the horizontal compliance and the updated direction, and efficiently plan the vertical insertion in an aggressive strategy regarding the uncertainties caused by the compliant mechanism and predicting the future insertion. The second stage is a hybrid insertion with both rigid and compliant gripping and its features include that the object states are not precisely measurable and the motion of a part of the object is not directly controllable. To solve it, we qualitatively and quantitatively analyze all possible configurations and, taking advantage of the insertion property, conclude one insertion posture based on which a control strategy is proposed. Additionally, a conservative insertion strategy is planned resorting to the past execution performance and the current state evaluation. Experiments are carried out to demonstrate the validation of the proposed method. Note to Practitioners —This paper was motivated by the problem of inserting partially flexible objects in precision assembly, but it also applies to other components with unevenly distributed flexibilities. This approach has benefits in terms of assembling rigid-compliant-rigid sets of the objects and modeling the behavior of many similar objects. The following key steps are mentioned to apply the proposed method. The practitioners need to first compute the insertion compliance based on the model that is built in this paper to relate the deformation and force. The determination of the object configuration in hybrid insertion is then demanded considering the system compliance. The final steps are to obtain the object states according to the force output and efficiently plan the insertion depths considering the past performance, the current evaluation, and the future prediction. In future research, we will address the simultaneous assembly of multiple partially flexible objects.

Design and analysis of a 2-DOF compliant gripper with constant-force flexure mechanism

This paper presents the design and testing of a novel flexure-based compliant parallel gripper with constant driving force. One uniqueness of the gripper lies in that it achieves two-degree-of-freedom (2-DOF) independent constant driving force in grasping and rolling operations. In each direction, the grasping and rolling operation is executed with the constant driving force. The parallel-kinematic flexure mechanism design enables nearly decoupled operations in 2-DOF manipulation. The constant driving force property enlarges the grasping range by reducing the required driving force. Analytical modeling of the gripper mechanism is carried out based on pseudo-rigid-body method, which is verified by conducting simulation study with nonlinear finite-element analysis (FEA). Parametric study is conducted to investigate the influence of each design variable on the gripper performance. To demonstrate the performance of the gripper, a prototype is fabricated by 3D printer. Experimental results reveal that the devised gripper owns a good constant driving force property during grasping.

Topology design and analysis of compliant mechanisms with composite laminated plates

To improve the small deformation and high stress level in hinge zones of compliant mechanisms with isotropic material, a topology optimization method of compliant mechanisms with composite laminated plates was proposed. Based on the anisotropy and designability of composite laminated plates, a topology optimization model of compliant mechanisms with composite laminated plates was built to maximize the deformable capability. Numerical examples of designing compliant inverters and grippers were investigated to demonstrate the effectiveness of the proposed method. The influence mechanism of layer sequences on topologic shapes, deformation and loading capability were also discussed. The results showed that the deformable capability and stress levels of compliant mechanisms with composite laminated plates were further improved by a reasonable configuration of layer sequences.

Topology and Size–Shape Optimization of an Adaptive Compliant Gripper with High Mechanical Advantage for Grasping Irregular Objects

SummaryThis study presents an optimal design procedure including topology optimization and size–shape optimization methods to maximize mechanical advantage (which is defined as the ratio of output force to input force) of the synthesized compliant mechanism. The formulation of the topology optimization method to design compliant mechanisms with multiple output ports is presented. The topology-optimized result is used as the initial design domain for subsequent size–shape optimization process. The proposed optimal design procedure is used to synthesize an adaptive compliant gripper with high mechanical advantage. The proposed gripper is a monolithic two-finger design and is prototyped using silicon rubber. Experimental studies including mechanical advantage test, object grasping test, and payload test are carried out to evaluate the design. The results show that the proposed adaptive complaint gripper assembly can effectively grasp irregular objects up to 2.7 kg.

Neural network–based sliding-mode control of a tendon sheath–actuated compliant rescue manipulator

The novel contribution of this article is to propose a neural network–based sliding-mode control strategy for improving the position-control performance of a tendon sheath–actuated compliant rescue manipulator. Structural design of a rescue robot with slender and compliant mechanical structure is introduced. The developed robot is capable of drilling into the narrow space under debris and accommodating complicated configuration in ruins. Dynamics modeling and parameters identification of a compliant gripper with flexible tendon sheath transmission are researched and discussed. Moreover, the neural network–based sliding-mode control scheme developed based on radial basis function network is proposed to improve the position-control accuracy of the gripper with modeling uncertainties and external disturbances. The stability of the proposed control system is demonstrated using Lyapunov stability theory. Further experimental investigation including trajectory-tracking experiments and step-response experiments are conducted to confirm the effectiveness of the proposed neural network–based sliding-mode control scheme. Experimental results show that the proposed neural network–based sliding-mode control scheme is superior to cascaded proportional–integral–derivative controller and conventional sliding-mode controller in position-control application.