Force-closure Condition Research Articles

A Fast Grasp Planning Algorithm for Humanoid Robot Hands.

Grasp planning is crucial for robots to perform precision grasping tasks, where determining the grasp points significantly impacts the performance of the robotic hand. Currently, the majority of grasp planning methods based on analytic approaches solve the problem by transforming it into a nonlinear constrained planning problem. This method often requires performing convex hull computations, which tend to have high computational complexity. This paper proposes a new algorithm for calculating multi-finger force-closure grasps of three-dimensional objects based on humanoid multi-fingered hands. Firstly, sufficient conditions for the multi-finger force-closure grasps of three-dimensional objects are derived from a point contact model with friction. These three-dimensional force-closure conditions are then transformed into two-dimensional plane conditions, leading to a simple algorithm for multi-finger force-closure determination. This method is purely based on geometric analysis, resulting in low computational demands and enabling the rapid assessment of force-closure grasps, which are beneficial for real-time applications. Finally, the algorithm is validated through two case studies, demonstrating its feasibility and effectiveness.

Enhancing Robotic Grasping Detection Accuracy With the R2CNN Algorithm and Force-Closure

AbstractThis study aims to use an improved rotational region convolutional neural network (R2CNN) algorithm to detect the grasping bounding box for the robotic arm that reaches supermarket goods. This algorithm can calculate the final predicted grasping bounding box without any additional architecture, which significantly improves the speed of grasp inferences. In this study, we added the force-closure condition so that the final grasping bounding box could achieve grasping stability in a physical sense. We experimentally demonstrated that deep model-treated object detection and grasping detection are the same tasks. We used transfer learning to improve the prediction accuracy of the grasping bounding box. In particular, the ResNet-101 network weights, which were originally used in object detection, were used to continue training with the Cornell dataset. In terms of grasping detection, we used the trained model weights that were originally used in object detection as the features of the to-be-grasped objects and fed them to the network for continuous training. For 2828 test images, this method achieved nearly 98% accuracy and a speed of 14–17 frames per second.

Minimum Friction Coefficient-Based Precision Manipulation Workspace Analysis of the Three-Fingered Metamorphic Hand

Abstract Reconfigurable robotic hands can constitute one of the future trends of dexterous manipulator design, as they can strike a balance between precision, force exertion, flexibility, and adaptability. However, the feasible manipulation workspace of a reconfigurable robotic hand, the metamorphic hand, is complex as the finger operation planes alter with the reconfigurable palm’s motions. Different useful workspace approaches and grasp quality metrics have been introduced, but a precision manipulation workspace (PMW) approach for reconfigurable robotic hands has yet to be presented. This paper presents a hand workspace taxonomy based on previous studies, and a new approach to obtaining a PMW of a robotic hand which satisfies three properties: force closure, singularity avoidance, and interference avoidance. A grasp quality metric, termed the minimum friction coefficient (MFC), is introduced to indicate the force-closure conditions of a robotic hand’s configurations. Unlike the previous grasp quality metrics targeting online grasp-planning tasks, this MFC-based measure focuses on the offline design of robotic hands. This method is essential for conducting grasp planning, design optimization, and actuation reduction for reconfigurable robotic hands. Further, the approach is applied to a three-fingered metamorphic hand, and the results are studied thoroughly.

Optimal Contact Points for an Octopus Arm

We investigate the optimality of the configurations of a tentacle-like soft manipulator ensuring a planar force-closure condition. In particular, the optimization is performed with respect to both to the control strategies and to the set of contact points ensuring a planar, frictionless, first-order force-closure of the target. The case of an elliptic target object is discussed in detail, and numerical simulation are presented.

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.

Collaborative manipulation with multiple dual-arm robots under human guidance

This paper presents the control strategy and experimental results of multiple dual-arm robots cooperatively manipulating an object whose load and size is beyond the capability of a single robot. The multi-robot system is required to stably grasp and transport the load through frictional contacts. The human operator provides motion guidance by directly manipulating the load. Through contact force sensing, the multi-arm/multi-robot system infers the human intent for motion while maintaining the contact force closure condition. The contact forces at the end effectors need to be carefully managed to avoid possible loss of contact. This is particularly challenging during motion, where the motion-induced inertial force adds a disturbance to the static stable grasp condition. We propose a feedforward force compensation scheme using support vector regression to estimate the inertial force. This enables the robots to coordinate their motion to maintain contacts, avoid slippage, and follow human hand-guided motion. Experimental results with three Baxter robots are included to show the efficacy and limitation of the proposed method.

Screw-Based Modeling of Soft Manipulators With Tendon and Fluidic Actuation

A screw-based formulation of the kinematics, differential kinematics, and statics of soft manipulators is presented, which introduces the soft robotics counterpart to the fundamental geometric theory of robotics developed since Brockett's original work on the subject. As far as the actuation is concerned, the embedded tendon and fluidic actuation are modeled within the same screw-based framework, and the screw-system to which they belong is shown. Furthermore, the active and passive motion subspaces are clearly differentiated, and guidelines for the manipulable and force-closure conditions are developed. Finally, the model is validated through experiments using the soft manipulator for minimally invasive surgery STIFF-FLOP.

Determining Force-Closure Grasps Reachable by a Given Hand

The paper presents an approach to find contact points on an object surface that are reachable by a given hand and such that the resulting grasp satisfies the force-closure condition. This is a very common problem that still requires a practical solution. The proposed method is based on the computation of a set of independent contact regions on the object boundary such that a finger contact on each region produces a force-closure grasp, and then this set of regions is iteratively recomputed while looking for a set of contact points that are reachable by a given hand. The search is done guided by a cost function that indicates the proximity of the hand fingertips to a candidate set of grasping contact points. The approach has been implemented for the Schunk Anthropomorphic Hand and planar objects, and application examples are included to illustrate its performance.

Cooperative Enclosing and Grasping of an Object by Decentralized Mobile Robots Using Local Observation

This paper discusses the design of a decentralized capturing behavior by multiple mobile robots. The design is based on a gradient descent method using local information. The task of capturing a target is divided into two subtasks; the enclosing subtask and the grasping subtask. An analysis of the convergence of the local control policy in the enclosing subtask is provided, while ensuring that the neighborhood relation of the robot system is preserved. In the grasping subtask, the force-closure condition in decentralized form is used to design a local objective function. A local estimation of the shape of the object is introduced so that each robot can decide how to move on the basis of only the available local information. The proposed local control policies were evaluated using simulations and the flexibility of the system was verified owing to the decentralized nature of the system. The enclosing subtask was implemented using multiple mobile robots with local observation from omni-directional CCD cameras.

Force Closure Analysis for the Forging Gripping Mechanisms

The gripping contact forces distributed between the interface of the tongs and the work-piece of the gripper of forging robots are considered as an equivalent resultant force whose contact model is friction point contact. So the gripping operation is similar to multi fingered robot hands, and the operation theory of N robot fingers to grasp an object can be used for the analysis of the force-closure and the optimization of contact forces of heavy gripping mechanisms. After the configuration of heavy duty gripping mechanisms is analyzed, the position distribution of the resultant contact forces is optimized, the iterative algorithm of linear constrained gradient flows is used to calculate and analyze force-closure conditions. The force closure features for different forging robots and in different operation conditions and mechanisms are analyzed. Simulation and experimental results demonstrate the effectiveness of the force-closure method.

Calculation of Contact Forces of Large-Scale Heavy Forging Manipulator Grippers

Different from dexterous robotic hands, the gripper of heavy forging manipulator is an underconstrained mechanism whose tongs are free in a small wiggling range. However, for both a dexterous robotic hand and a heavy gripper, the force closure condition: the force and the torque equilibrium, must be satisfied without exception to maintain the grasping/gripping stability. This paper presents a gripping model for the heavy forging gripper with equivalent friction points, which is similar to a grasp model of multifingered robot hands including four contact points. A gripping force optimization method is proposed for the calculation of contact forces between gripper tongs and forged object. The comparison between the calculation results and the experimental results demonstrates the effectiveness of the proposed calculation method.

Workspace Determination of General 6-d.o.f. Cable Manipulators

This paper addresses workspace determination of general 6-d.o.f. cable-driven parallel manipulators with more than seven cables. The workspace under study is called force-closure workspace, which is defined as the set of end-effector poses satisfying the force-closure condition. Having force-closure in a specific end-effector pose means that any external wrench applied to the end-effector can be balanced through a set of non-negative cable forces under any motion condition of the end-effector. In other words, the inverse dynamics problem of the manipulator always has a feasible solution at any pose in the force-closure workspace. The workspace can be determined by the Jacobian matrix and, thus, it is consistent with the usual definition of workspace in the robotics literature. A systematic method of determining whether or not a given end-effector pose is in the workspace is proposed. Based on this method, the shape, boundary, dimensions and volume of the workspace of a 6-d.o.f., eight-cable manipulator are discussed.

A method of verifying force-closure condition for general cable manipulators with seven cables

This paper introduces a systematic method of verifying the force-closure condition for general 6-DOF cable-driven manipulators with seven cables. Force-closure is defined as that the inverse dynamics problem has a feasible solution for any external wrench exerted on the end-effector. For any given configuration of a 7-cable manipulator, the method can easily determine, by examining the Jacobian matrix of the manipulator, whether a solution of all-positive cable forces exists. The necessary and sufficient conditions of the proposed method are mathematically proven. The paper also addresses the application of the proposed method for determination of the force-closure workspace. As a numerical example, the shape, boundary, dimensions, and volume of the force-closure workspace of a 6-DOF, 7-cable manipulator are discussed.

An Enhanced Ray-Shooting Approach to Force-Closure Problems

Force-closure is a fundamental topic in grasping research. Relevant problems include force-closure test, quality evaluation, and grasp planning. Implementing the well-known force-closure condition that the origin of the wrench space lies in the interior of the convex hull of primitive wrenches, Liu presented a ray-shooting approach to force-closure test. Because of its high efficiency in 3D work space and no limitation on the contact number of a grasp, this approach is advanced. Achieving some new results of convex analysis, this paper enhances the above approach in three aspects. (a) The exactness is completed. In order to avoid trouble or mistakes, the dimension of the convex hull of primitive wrenches is taken into account, which is always ignored until now. (b) The efficiency is increased. A shortcut which skips some steps of the original force-closure test is found. (c) The scope is extended. Our simplified ray-shooting approach yields a grasp stability index suitable for grasp planning. Numerical examples in fixturing and grasping show the enhancement superiority.

Force-closure workspace analysis of cable-driven parallel mechanisms

A cable-driven parallel mechanism (CDPM) possesses a number of promising advantages over the conventional rigid-link mechanisms, such as simple and light-weight mechanical structure, high-loading capacity, and large reachable workspace. However, the formulations and results obtained for the rigid-link mechanisms cannot be directly applied to CDPMs due to the unilateral property of cables. This paper focuses on the workspace analysis of fully restrained positioning mechanisms. Because the cable tension is the most essential issue to constrain the moving platform, the force-closure workspace is mainly studied. A general approach is proposed to check the force-closure condition. This condition is expressed in terms of the convex hull which encloses the origin. However, such a condition is formulated and expressed in high dimensions. To simplify the analysis, a recursive dimension reduction algorithm is proposed to check convex hulls in one dimension spaces. This algorithm is verified through simulation results of various CDPMs.

Contact Force Prediction and Force Closure Analysis of a Fixtured Rigid Workpiece With Friction

Analysis and characterization of workpiece-fixture contact forces are important in fixture design since they define the fixture stability during clamping and strongly influence workpiece accuracy during manufacturing. This paper presents a method for predicting and analyzing the normal and frictional contact forces between workpiece-fixture contacts. The fixture and workpiece are considered to be rigid bodies, and the model solution is solved as a constrained quadratic optimization by applying the minimum norm principle. The model reveals some intricate properties of the passive contact forces, including the potential of a locator release and the history dependency during a sequence of clamping and/or external force loading. Further, a notion of passive force closure is considered to characterize the passive nature of the fixture forces. Geometric conditions for two types of passive force closure (concordant and discordant closures) are provided, showing a complication of released locator under clamping with a limited role in force closure. Model predictions are shown to be in good agreement with known results of an elastic-contact model prediction and experimental measurements. The passive force closure conditions are illustrated with examples. This presented method is conceptually simple and computationally efficient. It is particularly useful in the early stages of fixture design and process planning.

A human-like real-time grasp synthesis method for humanoid robot hands

This paper addresses a real-time grasp synthesis of multi-fingered robot hands to find grasp configurations which satisfy the force closure condition of arbitrary shaped objects. We propose a fast and efficient grasp synthesis algorithm for planar polygonal objects, which yields the contact locations on a given polygonal object to obtain a force closure grasp by a multi-fingered robot hand. For an optimum grasp and real-time computation, we develop the preference and the hibernation process and assign the physical constraints of a humanoid hand to the motion of each finger. The preferences consist of each sublayer reflecting the primitive preference similar to the conditional behaviors of humans for given objectives and their arrangements are adjusted by the heuristics of human grasping. The proposed method reduces the computational time significantly at the sacrifice of global optimality, and enables grasp posture to be changeable within 2-finger and 3-finger grasp. The performance of the presented algorithm is evaluated via simulation studies to obtain the force-closure grasps of polygonal objects with fingertip grasps. The architecture suggested is verified through experimental implementation to our developed robot hand system by solving 2- or 3-finger grasp synthesis.

Static Analysis of Deformable Object Grasping Based on Bounded Force Closure.

A static analysis of deformable object grasping based on bounded force closure is presented. First, we introduce a new concept of bounded force closure, which is an extension of force closure condition. Secondly, we investigate the necessary conditions for the bounded force closure in order to derive the properties of bounded force closure grasping. Thirdly, the deformation of rod-like objects is formulated. Then, we propose a procedure to evaluate the stability of deformable object grasping. Finally, some numerical examples are shown in order to demonstrate the effectiveness of our proposed method.

A Linear Complementarity Approach to the Frictionless Gripper Problem

This article deals with static unilateral phenomena arising in grasping mechanisms and caused by the possible loss of contact between elastic grippers and rigid objects. The problem is formulated as a linear complementarity problem (LCP) that is derived from the equilibrium equations of forces and the compatibility equations for the displacements, as well as from the complementarity inequality restriction describing the contact. A variant of the infeasibility-reducing, complementary pivot al gorithm of Lemke is used for the numerical solution of the arising LCP. Form and force closure conditions are fulfilled in an auto matic way from the solution of the aforementioned LCP. The used algorithm guarantees rapid convergence to the solution of the problem. The theory is illustrated by numerical examples.