Parallel Gripper Research Articles

Development of the multifunctional rescue robot FUHGA2 and evaluation at the world robot summit 2018

Robots are needed in rescue activities and the inspection of infrastructure. We developed the rescue robot FUHGA2 so that it has various functions. FUHGA2 has high mobility, with main crawlers covering the body and four sub-crawlers, high dexterity using a long six-axis arm, and high searching ability using various sensors. Furthermore, a novel parallel gripper unit with wire-jamming mechanism is mounted at the tip of the arm, which can change its hardness and fit its shape to an object. Additionally, high Maneuvering is achieved by displaying sensor information to the operator clearly and intuitive operation command design. FUHGA2 won the World Robot Summit 2018 Standard Disaster Robotics Challenge, which is a competition for rescue robots. We achieved high scores in all categories of tasks. In this paper, we describe the development of FUHGA2 and the performance evaluation results from the World Robot Summit 2018.

Portable compact suction pad unit for parallel grippers

This paper proposes a portable compact suction pad unit for the parallel grippers of a mobile manipulation robot to place items for product display. In addition, this paper includes verification of the unit’s mechanism. Given that robots have to hold various items, we propose an operation method by which the parallel gripper holding the compact suction pad unit switches between a pinching mode and a vacuum suction mode depending on the item being placed. The compact suction pad unit is self-contained, with all the devices necessary for the suction operation inside the unit. The suction operation is switched on and off wirelessly by an external host system controller. The compact suction pad unit produced a suction force of about 5.6 N at an allowable inclination angle of 15°. We demonstrated in a competition that our proposed operation method of the compact suction pad unit is useful by having it place a beverage in a cup with lid and a boxed lunch.

From Single 2D Depth Image to Gripper 6D Pose Estimation: A Fast and Robust Algorithm for Grabbing Objects in Cluttered Scenes

In this paper, we investigate the problem of grasping previously unseen objects in unstructured environments which are cluttered with multiple objects. Object geometry, reachability, and force-closure analysis are considered to address this problem. A framework is proposed for grasping unknown objects by localizing contact regions on the contours formed by a set of depth edges generated from a single-view 2D depth image. Specifically, contact regions are determined based on edge geometric features derived from analysis of the depth map data. Finally, the performance of the approach is successfully validated by applying it to scenes with both single and multiple objects, in both simulation and experiments. Using sequential processing in MATLAB running on a 4th-generation Intel Core Desktop, simulation results with the benchmark Object Segmentation Database show that the algorithm takes 281 ms on average to generate the 6D robot pose needed to attach with a pair of viable grasping edges that satisfy reachability and force-closure conditions. Experimental results in the Assistive Robotics Laboratory at UCF using a Kinect One sensor and a Baxter manipulator outfitted with a standard parallel gripper showcase the feasibility of the approach in grasping previously unseen objects from uncontrived multi-object settings.

Designing a Mechanical Tool for Robots With Two-Finger Parallel Grippers

This work designs a mechanical tool for robots with 2-finger parallel grippers, which extends the function of the robotic gripper without additional requirements on tool exchangers or other actuators. The fundamental kinematic structure of the mechanical tool is two symmetric parallelograms which transmit the motion of the robotic gripper to the mechanical tool. Four torsion springs are attached to the four inner joints of the two parallelograms to open the tool as the robotic gripper releases. The forces and transmission are analyzed in detail to make sure the tool reacts well with respect to the gripping forces and the spring stiffness. Also, based on the kinematic structure, variety tooltips were designed for the mechanical tool to perform various tasks. The kinematic structure can be a platform to apply various skillful gripper designs. The designed tool could be treated as a normal object and be picked up and used by automatically planned grasps. A robot may locate the tool through the AR markers attached to the tool body, grasp the tool by selecting an automatically planned grasp, and move the tool from any arbitrary pose to a specific pose to grip objects. The robot may also determine the optimal grasps and usage according to the requirements of given tasks.

A novel design of a parallel gripper actuated by a large-stroke shape memory alloy actuator

Current traditional robotic grippers for on-orbit capture missions have several disadvantages: large volume and weight, huge energy consumption and high cost. To overcome these problems, a novel design of parallel gripper actuated by a large-stroke shape memory alloy (SMA) actuator is proposed in this paper. The mechanical design of the gripper is presented and each component is expounded in details. Multiple SMA wires are connected in series to achieve a larger stroke of the actuator. A light-weight, small-volume cross-shear hinge coupler is designed to ensure the symmetry and synchronism of the movement of the two gripping arms. In order to evaluate the working performance of the proposed SMA gripper, three theoretical models are developed for bias element selection, gripping force calculation and capture range analysis. A prototype of the gripper is fabricated and a series of experiments are conducted. The actuation characteristics under different bias forces and different currents are tested. The relationship between the gripping force and the sizes of targets are compared. The experimental observations are in good agreement with the theoretical results which demonstrates the correctness of the proposed models. Research results indicate that the proposed SMA gripper can grasp targets reliably and adaptively.

Adaptive Fuzzy Gaussian Mixture Models for Shape Approximation in Robot Grasping

Robotic grasping has always been a challenging task for both service and industrial robots. The ability of grasp planning for novel objects is necessary for a robot to autonomously perform grasps under unknown environments. In this work, we consider the task of grasp planning for a parallel gripper to grasp a novel object, given an RGB image and its corresponding depth image taken from a single view. In this paper, we show that this problem can be simplified by modeling a novel object as a set of simple shape primitives, such as ellipses. We adopt fuzzy Gaussian mixture models (GMMs) for novel objects’ shape approximation. With the obtained GMM, we decompose the object into several ellipses, while each ellipse is corresponding to a grasping rectangle. After comparing the grasp quality among these rectangles, we will obtain the most proper part for a gripper to grasp. Extensive experiments on a real robotic platform demonstrate that our algorithm assists the robot to grasp a variety of novel objects with good grasp quality and computational efficiency.

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.

Generating regrasp motion toward grasping for tool manipulation automatically by a parallel gripper of single/dual-arm robot

Generating regrasp motion toward grasping for tool manipulation automatically by a parallel gripper of single/dual-arm robot

Mechanical and Finite Element Analysis of Gripper Finger

The finger structure was designed for a two-finger parallel gripper. By design calculations it was clear that the gripper was safe but the finger designed was a critical one. Hence 3D solid model for gripper finger was built with 3D modeling software CATIA V5 and finite element analysis was done with the ANSYS from that the state of gripper fingers with different structure under certain loading conditions was calculated to further improve the gripper finger structure. By the reference to the results, related mechanical properties were compared to obtain stress distribution of gripper with different gripper finger structure. From results it is clear that through structure improvement the stress can be reduced significantly under identical loading and boundary conditions and also deformation get reduced.

Design and testing of a novel 2-DOF compound constant-force parallel gripper

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

Equilateral Three-Finger Caging of Polygonal Objects Using Contact Space Search

Multifinger caging offers a robust object grasping approach. However, while efficient computation of two-finger caging grasps is well developed, the computation of three-finger caging grasps has remained a challenge. This paper considers the caging of polygonal objects with three-finger hands which maintain an equilateral triangle formation during the grasping process. While the c-space of such hands is 4-D, their contact space that represents all two and three finger contacts along the grasped object’s boundary forms a 2-D stratified manifold. The paper describes a caging graph that can be constructed in the hand’s relatively simple contact space. Starting from a desired immobilizing grasp of the polygonal object, the caging graph can be readily searched for the largest finger opening that maintains a three-finger cage about the object to be grasped. Any equilateral finger placement within the corresponding caging regions guarantees robust object grasping. Note to Practitioners —A commonly used industrial gripper is the parallel gripper. While it is well suited for grasping objects that have parallel edges, it is less suited for objects with more complicated geometries, and it relies on high coefficients of friction. This paper considers the robust grasping of polygonal objects using equilateral three-finger robot hands. Such hands are mechanically simple, and enable grasping objects with complicated shapes without relying on contact friction. To safely grasp an object, the hand is first brought to a caging configuration around the object. Once the object is caged, it cannot be accidentally ejected from the hand, and the fingers can be closed safely until an immobilizing grasp is reached. To reach a specific immobilizing grasp, the fingers are placed in the three caging regions associated with that grasp. This paper proposes a method that is both physically intuitive and computationally simple, to compute these caging regions. Although this paper considers grasping polygonal objects using frictionless point fingers, the proposed method can be used for grasping nonpolygonal objects using identical disc fingers, by expanding the object by the finger radius, and then approximating the expanded object by a polygon. Finger’s center can then be treated as a point finger. Frictional jamming of the fingers can overcome using rotating fingers or slightly vibrating the hand.

A Variable Stiffness Robotic Gripper Based on Structure-Controlled Principle

This paper presents a novel structure-controlled variable stiffness robotic gripper that enables adaptive gripping of soft and rigid objects with a wide range of compliance. With the structure-controllable principle, the stiffness is controlled by the mechanical structure configurations rather than by material properties or electronic means. The principle is realized by changing the effective second moment of area of the gripper finger through rotating a built-in flexure hinge shaft. Based on this principle, the states of the stiffness can be continuously, instead of discretely, studied and assessed over the intermediate states from compliant to almost completely rigid. A variable stiffness mechanism has been developed to demonstrate the validity of the proposed principle. It enables that the finger stiffness and gripping position are independently controlled. With the introduction of flexure hinges, the undesired lateral buckling resulted from the rotation of a normal leaf spring is eliminated. In addition, a two-finger parallel gripper with this variable stiffness mechanism is developed which can provide the grasping stiffness according to the grasping task requirements. The effectiveness of the gripper has been demonstrated to handle the objects range from light, fragile to heavy, rigid without using any feedback loop or soft pads. Note to Practitioners —This work was inspired by the fact that the capability of variable stiffness in the robot actuator can reduce the undesired impacts to the robot arms. It can also allow a safer interaction between robot and human. Gripping of objects with uncertainties in shapes and position, as well as large variations in fragility and weight can also benefit from the concept of the variable stiffness. However, existing designs of variable stiffness grippers either have limited stiffness range or bulky configurations. They compromised the practical applications. This paper introduces a design in that a rotating flexure hinge shaft is embedded inside the robotic gripper finger. The mechanical stiffness of such fingers can be varied by changing the rotation angle of the flexure hinges. We present the working principle supported by mathematical models in the design and development. We also show an example design of a two-finger parallel gripper equipped with the VSFs. Extensive experiments demonstrated that the gripper is effective in gripping objects with wide range of uncertainties. Such gripper design avoids the use of soft pads as well as closed-loop control and high-precision sensors. In the future work, we shall implement such grippers for actual industrial applications.

Force sensing drill jig for robotic assisted drilling

PurposeThe purpose of this paper is to present a novel force sensing jig for robot-assisted drilling used to drill holes for the fastening of floating nut plates in aircraft assembly.Design/methodology/approachThe paper describes the drill jig, which consists of a parallel gripper, peg-in-hole pins and a back-plate with a recess where a Polydimethylsiloxane cone is placed on top of a force sensor. As the jig approaches the part, the force sensor registers the applied force until it reaches steady state, which indicates full contact between the jig and the part. The peg-in-hole pins then lock into a pre-existing hole, which provides a mechanical reference, and the support plate provides back support during drilling.FindingsPositional accuracy and the repeatability of the system were successfully placed within the specification for accuracy and repeatability (0.1 mm tolerance and 0.2 mm tolerance, respectively).Practical implicationsThe drill jig can be integrated into existing robot drilling solutions and modified for specific applications. The integration of the force sensor provides data for engineers to monitor and analyze forces during drilling. The design of the force sensing drill jig is particularly suited to industrial prototype robot drilling end-effectors for small and medium manufacturers.Originality/valueThe key novelties of this drilling jig are in the compact assembly, modular design and inclusion of force sensing and back support features.

Study on bolts tightening using a tool grasped by a 4-finger 12 d.o.f. hand with 6-axis force sensors on fingertip

Introduction of robots is progressing for the purpose of reducing work load and enhancing productivity in industrial fields where there are many mechanical work. In addition, it is expected that introducing robots that work in place of humans in extreme conditions such as space and disaster sites. To improve the dexterity of industrial robots, we investigate robotic assembly of fixtures used in the production lines. We not use a parallel gripper or dedicated hand, but use a 4-finger 12 d.o.f. hand with a camera attached on the palm and a 6-axis force sensor on the fingertip. In this paper, we investigate assembling fixture with bolts. The fixture used this time can't be assembled by the hand alone because the counter bore is done. Accordingly we grasp the tool made with 3D printer by the hand and tighten bolts. Also, in order to improve the accuracy of bolt insertion and reduce the time, we improve the fixture installation position correction method. We use a camera for object recognition and use a fingertip 6-axis force sensors to check the tightening torque and reliably grasp.

Adaptive Learning of Hand Movement in Human Demonstration for Robot Action

This paper describes a process for adaptive learning of hand movements in human demonstration for manipulation actions by robots using Dynamic Movement Primitives (DMPs) framework. The process includes 1) tracking hand movement from human demonstration, 2) segmenting hand movement, 3) adaptive learning with DMPs framework. We implement a extended DMPs model with a modified formulation for hand movement data observed from human demonstration including hand 3D position, orientation and fingers distance. We evaluate the generated movements by DMPs model which is reproduced without changes or adapted to change of goal of the movement. The adapted movement data is used to control a robot arm by spatial position and orientation of its end-effector with a parallel gripper.

Study of manipulator operations maneuvered by a ROV in virtual environments

It is extremely difficult to operate an underwater manipulator carried on a remotely operated vehicle (ROV) in the deep sea. In order to investigate underwater manipulator operations maneuvered by a remotely operated vehicle (ROV), this paper develops a virtual underwater manipulator and its carrier, the ROV, via the three-dimensional simulator “Webots” to animate operational tasks. The ROV model refers to the structure of a typical ROV which operates below the ocean's surface down to 1000m, while the graphical and kinematic models of the manipulator refer to a master-slave servo hydraulic manipulator with 7 functions, consisting of six degrees of freedom (DOFs) and a parallel gripper for manipulations. This study uses the virtual platform to challenge an operator to conduct different tasks by using the manipulator, including two switch-based and one master arm-based operation, and combined by remote and autonomous operations. When conducting the operations, we have considered uncertain external disturbances that stem from subsea environments. In order to demonstrate the feasibility and effectiveness of the virtual platform, we have designed two typical underwater operational tasks for training the operator: grasping a machine part in a basket, and making a circle at a specified position on the seabed. The tasks are completed by coordinating the underwater manipulator, the submersible vehicle, and a pan-tilt camera. Three operators were invited for the training operations, and their operating results are presented.

Modeling and Calibration of a Tactile Sensor for Robust Grasping

Robust grasping of everyday objects is still an open problem in robotics due to uncertainties affecting object physical properties like weight and friction. The present paper proposes to exploit the perception data provided by a tactile sensor to obtain useful information on the contact state, like normal and tangential components of the contact force. A novel mechanical model of the contact between the soft fingertip and the grasped object is here presented and supported by both FEM analysis and experimental verification. The proposed algorithm to extract such information from tactile raw data, based on this model, is simple enough to allow implementation of the grasp control strategy on the control hardware embedded into a standard robotic parallel gripper, so as to mimic human reactive grip responses, which occur when the control of an held object appears uncertain.

Variable-Grasping-Mode Underactuated Soft Gripper With Environmental Contact-Based Operation

A novel robotic gripper with soft surfaces and underactuated joints was proposed. The soft surface was fabricated from a deformable rubber bag filled with incompressible fluid and a microgripper inside the fluid. A ratchet was installed at the underactuated joint so that the joint's rotation caused by contact with an environment, such as a supporting surface, can be preserved, and the actions of scooping and enveloping an object are realized. With one actuator, the gripper realized three modes, i.e., parallel gripper, pinching, and enveloping. The range of graspable objects was wide and included soft, rigid, deformable, fragile, small (boundary length less than 30 mm), large (more than 80 mm long), thin (less than 0.5 mm), and heavy (more than 3 kg) objects.

Real-Time Grasping Planning for Robotic Bin-Picking and Kitting Applications

Multi-fingered robotic hands can potentially offer a higher degree of dexterity for grasping a variety of parts than a traditional parallel gripper or a custom end of arm tool. However, real-time grasp planning for complex robotic hands is a challenging task. In this paper, we present real-time grasping planning algorithms that utilize two simple yet effective dimension reduction strategies. The first strategy is the “intersected volume” computation which places the robotic palm relative to the object as well as its environment. The second strategy is the “finger curling planes” which decomposes a grasp contact computation problem in high-dimensional configuration space into several independent grasp contact computation problems in low-dimensional configuration space. Our algorithms have been demonstrated with simulation, physical experiments, and robotic bin-picking and kitting applications on two robotic platforms with three different robotic hands.

Development of a Virtual Platform for Telepresence Control of an Underwater Manipulator Mounted on a Submersible Vehicle

This paper develops a virtual platform of an underwater manipulator mounted on a submersible vehicle via the three-dimensional simulator “Webots” for teleoperation through a replica master arm. The graphical, kinematic, and dynamic models of the manipulator refer to a master–slave servo hydraulic manipulator with seven functions, consisting of six degrees of freedom and a parallel gripper, while the “Jiaolong” deep-manned submersible vehicle, operating below the sea surface down to 7000 m, is chosen as the underwater manipulator carrier. This study uses the virtual platform for training an operator to telepresence control the virtual manipulator to complete basic tasks in subsea environments. When training the operator, one has to consider uncertain external disturbances and the visual impacts that stem from subsea environments. In order to demonstrate the feasibility and effectiveness of the virtual platform, one designs two typical underwater operational tasks: grasping a marine organism sample and reaching at a given position. This paper presents the comparative studies: 1) the performances demonstrated by remotely controlling the virtual manipulator and the real manipulator; 2) the operating performances delivered by three operators before and after training when using the platform.