Soft-rigid coupling grippers:Collaboration strategies and integrated fabrication methods

Continuously increasing applications of robot technologies in unstructured environments put higher requirements on the robotic grippers'performance,such as interaction capability,output force range,and controllability.However,currently,it is hard for either rigid or soft grippers to meet these requirements,as single soft or rigid structures alone are difficult to effectively overcome/alleviate their inherent defects,e.g.,low compliance of rigid structures and low output force of soft structures.To deal with these difficulties,soft-rigid coupling grippers,or hybrid grippers are proposed.Technically,the soft-rigid coupling is a promising design that combines soft and rigid structures,in order to exploit their respective advantages,such as the strength of rigid structures and compliance of soft structures,in the same set of the gripper system.For the first time,herein,this paper systematically discusses the collaboration strategies of the existing hybrid robotic grippers,by classifying them as Rigid-activesoft-passive,Rigid-passive-soft-active,and Rigid-active-soft-active.At the same time,we introduce the integrated fabrication methods of hybrid grippers,through which the soft and rigid structures with great stiffness and property differences can be coupled together to construct a stable system.Also,possible performance improvements on soft-rigid coupling design for gripper systems are discussed.

Enhancing interaction performance of soft pneumatic-networks grippers by skeleton topology optimization

The inherent compliance of soft materials imbues robots,generally referred to as soft robots,with particular advantages in producing adaptive and safe interactions.However,the mainstream design paradigms of soft robots have been focused on pursuing large free motions only,usually at the expense of greatly decreased stiffness,leading to limited capability of withstanding external loads in interactive scenarios.There is a pressing need to incorporate the interaction specifications at the design stage to embody soft robots with not only proper deformability but equally importantly,considerable stiffness to perform complex tasks in practical applications.Here,inspired by the dexterity of human hands,we propose a computational design framework for soft grippers with a focus on improving their interaction performance in power grasping or precision grasping mode.The design paradigm rests on attaching a relatively stiffer skeleton layer to the parametric pneumatic networks based actuator which is widely used due to the geometric advantage,and the skeleton layout is designed for customized interaction conditions by a level set based topology optimization approach.As expected,the optimized skeleton layouts exhibit specified structural features highly relevant to the predefined concentrated loads for precision grip or distributed loads for power grip,which physically implies the compromise between deformability and stiffness.Since the interaction forces are difficult to measure in situ,we devise power and precision grasping scenarios and evaluate the critical actuation pressure of the object’s falling instead.The experiments qualitatively demonstrate the superiority of each specified design.This work represents an initial step toward the rational design for interaction in soft robots.

A Mathematical Modeling Method Elucidating the Integrated Gripping Performance of Ant Mandibles and Bio-inspired Grippers

The ability to grip unhatched eggs is a skill exploited by the ants Harpegnathos venator,as they care their brood in tunneled nests,which is of extreme difficulty to keep the eggs intact while gripping.In this paper we propose a mathematical modeling method to elucidate the mechanism of such a gripping behavior in the ant mandibles.The new method can be subdivided into following steps.As a preliminary,the concavity geometry and mandible kinematics are examined experimentally.Second,coordinate transformation is used to predict the real-time spatial topology of the concavity.Third,we come up with a new method to quantify the workspace required to grip and the contact area between the concavity and ant egg.Our model indicates that the biaxial rotation fashion with specialized concavities can reduce workspace by 40%and increase contact area by 53%on average compared with the uniaxial rotation pattern,which augments success rate of gentle gripping.This methodology may have applications in evaluating mechanical performance in both natural and artificial grippers.

A Track-type Inverted Climbing Robot with Bio-inspired Spiny Grippers

To enable the capacity of climbing robots to work on steep surfaces,especially on inverted surfaces,is a fundamental but challenging task.This capacity can extend the reachable workspace and applications of climbing robots.A track-type inverted climbing robot called SpinyCrawler was developed in this paper.Using a spiny track with an opposed gripping mechanism,the robot was experimentally demonstrated to have the ability of generating considerable adhesion to achieve stable inverted climbing.First,to guarantee reliable attachment of the robot on rough ceilings,a spiny gripper inspired by the opposed gripping prolegs of caterpillars is designed,and a gripping model of the interaction between spines and the ceiling asperities is established and analyzed.Second,a spiny track is developed by assembling dozens of spiny grippers to enable continuous attachment.A cam mechanism is introduced in the robot design without extra actuators to achieve stable attachment and easy detachment during continuous climbing.Finally,climbing experiments are conducted on different surfaces,using a SpinyCrawler prototype.Experimental results demonstrated stable climbing ability on various rough inverted and vertical surfaces,including concrete walls,crushed stone walls,sandpaper walls,brick walls,and brick ceilings.

A Novel Pneumatic Soft Gripper with a Jointed Endoskeleton Structure

In current research on soft grippers,pneumatically actuated soft grippers are generally fabricated using fully soft materials,which have the advantage of flexibility as well as the disadvantages of a small gripping force and slow response speed.To improve these characteristics,a novel pneumatic soft gripper with a jointed endoskeleton structure(E-Gripper)is developed,in which the muscle actuating function has been separated from the force bearing function.The soft action of an E-Gripper finger is performed by some air chambers surrounded by multilayer rubber embedded in the restraining fiber.The gripping force is borne and transferred by the rigid endoskeleton within the E-Gripper finger Thus,the gripping force and action response speed can be increased while the flexibility is maintained.Through experiments,the bending angle of each finger segment,response time,and gripping force of the E-Gripper have been measured,which provides a basis for designing and controlling the soft gripper The test results have shown that the maximum gripping force of the E-Gripper can be 35 N,which is three times greater than that of a fully soft gripper(FS-Gripper)of the same size.At the maximum charging pressure of 150 kPa,the response time is1.123 s faster than that of the FS-Gripper.The research results indicate that the flexibility of a pneumatic soft gripper is not only maintained in the case of the E-Gripper,but its gripping force is also obviously increased,and the response time is reduced.The E-Gripper thus shows great potential for future development and applications.

Soft Robotics:Morphology and Morphology-inspired Motion Strategy

Robotics has aroused huge attention since the 1950s.Irrespective of the uniqueness that industrial applications exhibit,conventional rigid robots have displayed noticeable limitations,particularly in safe cooperation as well as with environmental adaption.Accordingly,scientists have shifted their focus on soft robotics to apply this type of robots more effectively in unstructured environments.For decades,they have been committed to exploring sub-fields of soft robotics(e.g.,cutting-edge techniques in design and fabrication,accurate modeling,as well as advanced control algorithms).Although scientists have made many different efforts,they share the common goal of enhancing applicability.The presented paper aims to brief the progress of soft robotic research for readers interested in this field,and clarify how an appropriate control algorithm can be produced for soft robots with specific morphologies.This paper,instead of enumerating existing modeling or control methods of a certain soft robot prototype,interprets for the relationship between morphology and morphology-dependent motion strategy,attempts to delve into the common issues in a particular class of soft robots,and elucidates a generic solution to enhance their performance.

Review and Prospect of the Shuttleless Loom Development

In this paper, a review of the shuttleless looms’ development shows that three types of shuttleless looms: rapier, Jet and gripper projectile, are now equally matchable with each other. A prospect of the shuttleless loom’s development is also dealt with. It can be predicted that both rapier and Jet looms will be developed further and the future development of a new type of modern weaving looms will be prosperous in the coming 21st century.

Research on vacuumgripper based on fuzzy control for micromanipulators

This paper presents a vacuum gripper （as an actuator of an intelligent micromanipulator） for micro objects （with a diameter of 100 - 300μm） assembly tasks. The gripper is composed of a vacuum unit and a control unit. The vacuum unit with a proportional valve and a pressure sensor, and the control unit with a PC ＋ MCU two-layered control architecture are designed. The mechanical structure, workflow and major programs of the micro-gripper are presented. This paper discusses the major components of the adhesion force acting on micro objects. Some equations of the operation conditions m three phases of pick, hold and place are derived by mechanics analysis. The pneumatic system＇s pressure loss is inevitable. There are some formulas for calculating the amount of the pressure loss, but parameters in formulas are diffficult to be quantified and evaluated. To control the working pressure accurately, a pressure controller based on fuzzy logic is designed. With MATLAB＇s fuzzy logic toolbox, simulation experiments are performed to validate the performance of the fuzzy PD controller. The gripper is characterized by a steady and reliable performance and a simple structure, and it is suitable for handling micro objects with a sub-millimeter size.

A Multi-Scale Grasp Detector Based on Fully Matching Model

Robotic grasping is an essential problem at both the household and industrial levels,and unstructured objects have always been difficult for grippers.Parallel-plate grippers and algorithms,focusing on partial information of objects,are one of the widely used approaches.However,most works predict single-size grasp rectangles for fixed cameras and gripper sizes.In this paper,a multi-scale grasp detector is proposed to predict grasp rectangles with different sizes on RGB-D or RGB images in real-time for hand-eye cameras and various parallel-plate grippers.The detector extracts feature maps of multiple scales and conducts predictions on each scale independently.To guarantee independence between scales and efficiency,fully matching model and background classifier are applied in the network.Based on analysis of the Cornell Grasp Dataset,the fully matching model canmatch all labeled grasp rectangles.Furthermore,background classification,along with angle classification and box regression,functions as hard negative mining and background predictor.The detector is trained and tested on the augmented dataset,which includes images of 320×320 pixels and grasp rectangles ranging from 20 tomore than 320 pixels.It performs up to 98.87% accuracy on image-wise dataset and 97.83% on object-wise split dataset at a speed of more than 22 frames per second.In addition,the detector,which is trained on a single-object dataset,can predict grasps on multiple objects.

Vision-Based Action Control System of a Multi-Finger Mechanical Gripper

A study about the action control of a dexterous mechanical gripper based on stereo-vision system was proposed. The vision-based system was used to replace the data-glove for gesture measurement. The stereo vision theory was applied to calculate the 3D information of the hand gesture. The information was used to generate the grasping action parameters of a 3-finger dexterous mechanical gripper. Combined with a force feedback device, a closed control loop could be constructed. The test for the precision of the algorithms and action control simulation result were shown in the paper.

Using Microgripper in Development of Automatic Adhesive Glue Transferring and Binding Microassembly System

A system using microgripper for gluing and adhesive bonding in automatic microassembly was designed, implemented, and tested. The development of system is guided by axiomatic design principle. With a compliant PU microgripper, regional-edge-statistics (RES) algorithm, and PD controller, a visual-servoing system was implemented for gripping micro object, gluing adhesive, and operating adhesive bonding. The RES algorithm estimated and tracked a gripper’s centroid to implement a visual-servoing control in the microassembly operation. The main specifications of the system are: gripping range of 60~80μm, working space of 7mm×5.74mm×15mm, system bandwidth of 15Hz. In the performance test, a copper rod with diameter 60μm was automatically gripped and transported for transferring glue and bonding. The 60μm copper rod was dipped into a glue container and moved, pressed and bonding to a copper rod of 380μm. The amount of binding glue was estimated about 5.7nl.

Bionic soft robotic gripper with feedback control for adaptive grasping and capturing applications

Robots are playing an increasingly important role in engineering applications.Soft robots have promising applications in several fields due to their inherent advantages of compliance,low density,and soft interactions.A soft gripper based on bio-inspiration is proposed in this study.We analyze the cushioning and energy absorption mechanism of human fingertips in detail and provide insights for designing a soft gripper with a variable stiffness structure.We investigate the grasping modes through a large deformation modeling approach,which is verified through experiments.The characteristics of the three grasping modes are quantified through testing and can provide guidance for robotics manipulation.First,the adaptability of the soft gripper is verified by grasping multi-scale and extremely soft objects.Second,a cushioning model of the soft gripper is proposed,and the effectiveness of cushioning is verified by grasping extremely sharp objects and living organisms.Notably,we validate the advantages of the variable stiffness of the soft gripper,and the results show that the soft robot can robustly complete assemblies with a gap of only 0.1 mm.Owing to the unstructured nature of the engineering environment,the soft gripper can be applied in complex environments based on the abovementioned experimental analysis.Finally,we design the soft robotics system with feedback capture based on the inspiration of human catching behavior.The feasibility of engineering applications is initially verified through fast capture experiments on moving objects.The design concept of this robot can provide new insights for bionic machinery.

Tailoring the in-plane and out-of-plane stiffness of soft fingers by endoskeleton topology optimization for stable grasping

The intrinsic compliance of soft materials endows soft robots with great advantages to achieve large deformation and adaptive interactions in grasping tasks.However,current soft grippers usually focus on the in-plane large deformation and load capacity but ignore the effect of out-of-plane external loads,which may lead to instability in practical scenarios.This problem calls for stiffness design along multiple directions to withstand not only in-plane interacting forces with objects,but also unexpected outof-plane loads.In this paper,we design a new type of soft finger by embedding an endoskeleton inside the widely-used PneuNets actuator,and the endoskeleton layout is optimized to achieve a remarkable bending deflection and limited lateral deflection under combined external in-plane and out-of-plane loads.Based on the multi-objective topology optimization approach,the key structural features of the optimized endoskeleton are extracted and parameterized.The multi-material soft fingers are fabricated by the silicone compound mold method.Static and dynamic experiment results validate that the soft gripper with endoskeleton embedded exhibits remarkably improved out-of-plane stiffness,without sacrificing the in-plane bending flexibility,and leads to more stable grasping.

Additive manufacturing of flexible 3D surface electrodes for electrostatic adhesion control and smart robotic gripping

Mechanically flexible surface structures with embedded conductive electrodes are attractive in contact-based devices,such as those used in reversible dry/adhesion and tactile sensing.Geometrical shapes of the surface structures strongly determine the contact behavior and therefore the resulting adhesion and sensing functionalities;however,available features are often restricted by fabrication techniques.Here,we additively manufacture elastomeric structure arrays with diverse angles,shapes,and sizes;this is followed by integration of conductive nanowire electrodes.The fabricated flexible three-dimensional(3D)surface electrodes are mechanically compliant and electrically conductive,providing multifunctional ability to sense touch and to switch adhesion via a combined effect of shear-and electro adhesives.We designed soft,anisotropic flexible structures to mimic the gecko’s reversible adhesion,which is governed by van der Waals forces;we integrated nanowires to further manipulate the localized electric field among the adjacent flexible 3D surface electrodes to provide additional means to digitally tune the electrostatic attraction at the contact interface.In addition,the composite surface can sense the contact force via capacitive sensing.Using our flexible 3D surface electrodes,we demonstrate a complete soft gripper that can grasp diverse convex objects,including metal,ceramic,and plastic products,as well as fresh fruits,and that exhibits 72%greater electroadhesive gripping force when voltage is applied.

A 0.5-meter-scale,high-load,soft-enclosed gripper capable of grasping the human body

Developing large,soft grippers with high omnidirectional load(above 40 kg)has always been challenging.We address this challenge by developing a powerful soft gripper that can grasp the human body based on a soft-enclosed grasping structure and a soft-rigid coupling structure.The envelope size of the proposed soft gripper is 611.6 mm×559 mm×490.7 mm,the maximum grasping size is 417 mm,and the payload on the human body is more than 90 kg,which has exceeded most existing soft grippers.Furthermore,the grasping force prediction of the gripper is achieved through theoretical modeling.The primary contribution of this work is to overcome the size and payload limits of current soft grippers and implement a human-grasping experiment based on the soft-grasping method.

Rigid-Soft Coupled Robotic Gripper for Adaptable Grasping

Inspired by the morphology of human fingers,this paper proposes an underactuated rigid-soft coupled robotic gripper whose finger is designed as the combination of a rigid skeleton and a soft tissue.Different from the current grippers who have multi-point contact or line contact with the target objects,the proposed robotic gripper enables surface contact and leads to flexible grasping and robust holding.The actuated mechanism,which is the palm of proposed gripper,is optimized for excellent operability based on a mathematical model.Soft material selection and rigid skeleton structure of fingers are then analyzed through a series of dynamic simulations by RecurDyn and Adams.After above design process including topology analysis,actuated mechanism optimization,soft material selection and rigid skeleton analysis,the rigid-soft coupled robotic gripper is fabricated via 3D printing.Finally,the grasping and holding capabilities are validated by experiments testing the stiffness of a single finger and the impact resistance of the gripper.Experimental results show that the proposed rigid-soft coupled robotic gripper can adapt to objects with different properties(shape,size,weight and softness)and hold them steadily.It confirms the feasibility of the design procedure,as well as the compliant and dexterous grasping capabilities of proposed rigid-soft coupled gripper.

Design and Grasping Force Modeling for a Soft Robotic Gripper with Multi-stem Twining

To improve the grasping power of soft robots,inspired by the scene of intertwined and interdependent vine branches safely clinging to habitats in a violent storm and the phenomenon of large grasping force after being entangled by aquatic plants,this paper proposes a soft robotic gripper with multi-stem twining.The proposed robotic gripper can realize a larger contact area of surrounding or containing object and more layers of a twining object than the current twining gripping methods.It not only retains the adaptive advantages of twining grasping but also improves the grasping force.First,based on the mechanical characteristics of the multi-stem twining of the gripper,the twining grasping model is developed.Then,the force on the fiber is deduced by using the twining theory,and the axial force of the gripper is analyzed based on the equivalent model of the rubber ring.Finally,the torsion experiments of fibers and the grasping experiments of the gripper are designed and conducted.The torsion experiment of fibers verifies the influence of a different number of fiber ropes and fiber torque on the grasping force,and the grasping experiment reflects the large load of the gripper and the high adaptability and practicability under different tasks.

Modeling and Experimental Evaluation of a Bionic Soft Pneumatic Gripper with Joint Actuator

The pneumatic gripper in industrial applications has the advantages of structure simplicity and great adaptability,but its gripping power is usually limited due to the low modulus of soft materials.To address this problem,a novel bionic pneumatic gripper inspired by spider legs is proposed.The design has two pairs of symmetrical fingers,each finger consists of two pneumatic actuated joints,two rigid links and one pneumatic soft pad.The rigid link connects the pneumatic chamber which is enclosed in a retractable shell to increase the actuation pressure and the gripping force.The compressibility and elasticity of the soft joint and pad enable the gripper to grasp fragile objects without damage.The modeling of the bionic gripper is developed,and the parameters of the joint actuators are optimized accordingly.The prototype is manufactured and tested with the developed experimental platform,where the gripping force,flexibility and adaptability are evaluated.The results indicate that the designed gripper can grasp irregular and fragile items in sizes from 40 to 140 mm without damage,and the lifting weight is up to 15 N.

Novel gripper module and method for automated assembly of miniature parts

During assembly process,the miniature part needs to be fixed in its assembly position.In some occasions where adhesive is used,the joining force is not established due to the adhesive curing process,in that case the locking of parts is required.Manual locking is difficult to meet the increasing demand for mass production.To solve this problem and realize fully automatic assembly,a novel gripper module was designed and corresponding locking method was proposed.Thanks to the functional integration,the gripper module is capable of manipulating and locking the part.This module is integrated into the assembly system and plays a crucial role in automatic assembly.The locking method for automatic assembly is based on the integration of the part picking up and the locking unit releasing.After being placed accurately at its desired position,the miniature part can be automatically locked by releasing the locking unit.The innovative structure and mechanism of the gripper module convert the spring force into the locking force of the miniature part,ensuring non-rigid locking and suitable small locking force.Locking principle,flexibility and limitations of the proposed method were clarified in detail.Moreover,an effective compensation strategy was used to achieve accurate and stable pickup of the part,which increased the reliability of the assembly process.During automatic locking,the disturbances to the part due to the eccentric load were analyzed.The effectiveness of the gripper module and proposed method was verified by experiment.Experimental results indicated that the modular system integrated with the gripper module could meet the requirements of fully automatic assembly.Manual locking is replaced by automatic locking,and workers are liberated from tedious manual operations.The improvement of automation level enables assembly equipment to be applied to mass production scenarios.

A Variable Stiffness Soft Gripper Using Granular Jamming and Biologically Inspired Pneumatic Muscles

As the domains, in which robots operate change the objects a robot may be required to grasp and manipulate, are likely to vary sig- nificantly and often. Furthermore there is increasing likelihood that in the future robots will work collaboratively alongside people. There has therefore been interest in the development of biologically inspired robot designs which take inspiration from nature. This paper pre- sents the design and testing of a variable stiffness, three fingered soft gripper, which uses pneumatic muscles to actuate the fingers and granular jamming to vary their stiffness. This gripper is able to adjust its stiffness depending upon how fragile/deformable the object being grasped is. It is also lightweight and low inertia, making it better suited to operation near people. Each finger is formed from a cylindrical rubber bladder filled with a granular material. It is shown how decreasing the pressure inside the finger increases the jamming effect and raises finger stiffness. The paper shows experimentally how the finger stiffness can be increased from 21 N·m^-1 to 71 N·m^-1. The paper also describes the kinematics of the fingers and demonstrates how they can be position-controlled at a range of different stiffness values.