Analysis of the wrench-closure workspace of planar parallel cable-driven mechanisms

The mobile platform of a parallel cable-driven mechanism is connected in parallel to a base by lightweight links, such as cables. Since the cables can only work in tension, the set of poses of the mobile platform for which the cables can balance any external wrench, i.e., for which the platform of the mechanism is fully constrained, is often limited or even nonexistent. Thus, the study and determination of this set of poses, called the wrench-closure workspace (WCW), is an important issue for parallel cable-driven mechanisms. In this paper, the case of planar parallel cable-driven mechanisms is addressed. Theorems that characterize the poses of the WCW are proposed. Then, these theorems are used to disclose the parts of the reachable workspace which belong to the WCW. Finally, an efficient algorithm that determines the constant-orientation cross-sections of these parts is introduced.     

A new reconfigurable parallel mechanism using novel lockable joints for large scale manufacturing

A new lockable spherical joint is proposed in this paper, and it can be used as a revolute joint, a universal joint or a spherical joint. Three locking methods are introduced to construct the lockable spherical joint. Based on the proposed lockable spherical joint, a new reconfigurable parallel mechanism (RPM) with large positioning workspace is presented. The RPM has a tripod architecture with a lockable joint in each limb, which enables it three types of parallel mechanisms in six motion cases. Mobility analysis of the six motion cases is conducted. The new RPM can realize both translation and rotation by changing operative modes, which can be employed as machine tools, fixtures or manipulators. Based on the new RPM, two modular reconfigurable manufacturing systems are designed for aircraft assembly production, and a reconfiguration strategy is presented.     

A novel dynamic modelling approach for parallel mechanisms analysis

A novel approach (recursive matrix method), which is used for kinematic and dynamic analysis of a 3-DOF parallel mechanism with revolute actuators, is established in this paper. The active links of the mechanism are actuated by three electric motors and have three independent motions. Knowing the evolution of movable platform, first we develop the positions, velocities and accelerations of all elements of the mechanism. An inverse dynamic problem is solved using the principle of virtual work. Finally, recursive relations and graphs for the torques of three actuators are determined. It showed the efficiency of the proposed method by the example.     

Dynamic Modeling and Experimental Validation of a 3-PRR Parallel Manipulator with Flexible Intermediate Links

This paper presents the development of structural dynamic equations of motion for a 3-PRR parallel manipulator with three flexible intermediate links, based on the assumed mode method. Lagrange’s equation is used to derive the dynamic model of the manipulator system. Flexible intermediate links are modeled as Euler–Bernoulli beams with pinned–pinned boundary conditions. Dynamic equations of motion of a 3-PRR parallel manipulator with three flexible links are developed by adopting the assumed mode method. The effect of concentrated rotational inertia at both ends of intermediate links is included in this model. Numerical simulations of vibration responses, coupling forces and inertial forces are presented. The corresponding frequency spectra analysis is performed using the Fast Fourier Transform (FFT). Experimental modal tests are performed to validate the theoretical model through comparison and analysis of modal characteristics of the flexible manipulator system.     

Design and kinetostatic analysis of a new parallel manipulator

This paper proposes an innovative design for a parallel manipulator that can be applied to a machine tool. The proposed parallel manipulator has three degrees of freedom (DOFs), including the rotations of a moving platform about the x and y axes and a translation of this platform along the z-axis. A passive link is introduced into this new parallel manipulator in order to increase the stiffness of the system and eliminate any unexpected motion. Both direct and inverse kinematic problems are investigated, and a dynamic model using a Newton–Euler approach is implemented. The global system stiffness of the proposed parallel manipulator, which considers the compliance of links and joints, is formulated and the kinetostatic analysis is conducted. Finally, a case study is presented to demonstrate the applications of the kinematic and dynamic models and to verify the concept of the new design.     

A parallel robotic attachment and its remote manipulation

This paper discusses a 3-dof (degree of freedom) parallel robotic attachment and its remote manipulation. This attachment is designed as a tripod that provides two rotary motions and one linear motion. The attachment can be mounted onto a variety of machines for different applications, including CNC milling machines, industrial robots, and CMM. Java technologies are used to develop a remote manipulation system for the parallel robotic attachment, including remote monitoring and control. The main difference of this system from the existing web-based or internet-based remote systems is the way to control the motion of the machine from a remote site. Instead of using a camera for monitoring, the tripod is modeled using 3D computer graphics with behavioral control nodes embedded. Compared with camera-based solutions, network traffic is largely reduced, thereby making real-time remote device manipulation practical on the web. Our parallel robotic attachment is one type of parallel kinematic mechanisms (PKM). With PKM emerging as a new way of building flexible systems or agile machines, its advantage over serial mechanism is also presented.     

Synthesis on forward kinematics problem algebraic modeling for the planar parallel manipulator: displacement-based equation systems

Based on a proven exact method which solves the forward kinematics problem (FKP) this article investigates the FKP formulation specifically applied to planar parallel manipulators. It focuses on the displacement-based equation systems. The majority of planar tripods can modeled by the 3-RPR parallel manipulator, which is a tripod constituted by a fixed base and a triangular mobile platform attached to three kinematics chains with linear (prismatic) actuators located between two revolute joints. In order to implement the algebraic method, the parallel manipulator kinematics are formulated as polynomial equation systems where the number of equations is equal to or exceeds the number of unknowns. Three geometrical formulations are derived to model the difficult FKP. The selected proven algebraic method uses Gröbner bases from which it constructs an equivalent univariate system. Then, the real roots are isolated using this last system. Each real solution exactly corresponds to one manipulator assembly mode, which is also called a manipulator posture. The FKP resolution of the planar 3-RPR parallel manipulator outputs six complex solutions which become a proven real solution number upper bound. In several typical examples, the resolution performances (computation times and memory usage) are given. It is then possible to compare the models and to reject one. Moreover, a number of real solutions are obtained and the corresponding postures drawn. The algebraic method is exact and produces certified results.     

Conceptual development of an enhanced tripod mechanism for machine tool

In this paper, a spatial three degrees of freedom parallel mechanism enhanced by a passive leg is proposed. The proposed parallel mechanism can be used in several applications, e.g. motion simulator, micromanipulator and machine tools. First, the geometric model of the three degrees of freedom parallel mechanism is addressed, in which a fourth kinematic link—a passive link connecting the base center to the platform center—is introduced. This last link is used to constrain the motion of the platform to only three degrees of freedom, i.e. the degree of freedom of the mechanism depends on the passive leg. The passive leg also enhances the global stiffness of the structure and distributes the torque from machining. Second, the kinematic analysis with the consideration of link flexibility is conducted. A kinetostatic model of the three degrees of freedom parallel mechanism with a passive link is then established and analyzed using lumped-parameter model. With the proposed method, a significant effect of the link flexibility on the mechanism's precision has been demonstrated. The influence of the change of structure parameters, including material properties, on the system behavior is discussed. Compliance mapping is also illustrated. The kinetostatic model proposed in the paper can be extended for optimal design and control of parallel kinematic machines. Finally, design optimization is conducted using genetic algorithms and some design guideline is given.     

Kinematic design of a six-DOF parallel-kinematics Machine with decoupled-motion architecture

The design of a new six-degree-of-freedom (6-DOF) parallel-kinematics machine (PKM) has been proposed. Different from the conventional Stewart-Gough platform which has six extensible legs, the new PKM employs three identical RPRS legs to support the moving platform. Since all joint axes, excluding the three spherical joints at the leg ends, are parallel to each other and perpendicular to the base plane, this 6-DOF PKM presents a promising platform structure with decoupled-motion architecture (DMA) such that translation in a horizontal plane and rotation about a vertical axis are driven by the three active revolute joints, while translation in the vertical direction and rotation about horizontal axes are driven by the three active prismatic joints. As a result, this 6-DOF 3RPRS PKM with DMA has simple kinematics, large cylindrical reachable workspace, and high stiffness in the vertical direction. These features make it appropriate for light machining and heavy parts assembly tasks. Because of the DMA, a projection technique is employed for its kinematics analysis. By projecting the manipulator onto horizontal directions and vertical planes, the kinematics issues such as the displacement, singularity, and workspace analysis are significantly simplified.     

Design and Control of 6-DOF Mechanism for Twin-Frame Mobile Robot

A new lightweight six-legged robot that uses a simple mechanism and can move and work with high efficiency has been developed. This robot consists of two leg-bases with three legs each, and walks by moving each leg-base alternately. These leg-bases are connected to each other with a 6 degrees of freedom (DOF) mechanism. While designing this robot, the output force, velocity, and workspace of various connection mechanisms were compared, and the results showed that good performance could be achieved with a serial/parallel hybrid mechanism. The serial/parallel hybrid mechanism consists of three 6-DOF serially linked arms positioned with radial symmetry about the center of each leg-base; each leg-base is composed of two active and four passive joints. Walking experiments with this robot confirmed that this mechanism has satisfactory performance not only as a walking robot, but also as an active walking platform. Furthermore, in this robot, the entire leg-drive mechanism acts as a 6-axis force sensor, and individual sensors at the feet are not necessary. The forces and moments can be calculated from the changes in the joint angles. Experiments conducted verified that smooth contact with the ground by the swing-leg and successful switching from swing to support leg can be achieved using this force control and force measurement method.     

The NIST robocrane

The Robot Systems Division of the National Institute of Standards and Technology (NIST) has been experimenting for several years with new concepts for robot cranes. These concepts utilize the basic idea of the Stewart platform parallel link manipulator. The unique feature of the NIST approach is to use cables as the parallel links and to use winches as the actuators. As long as the cables are all in tension, the load is kinematically constrained and the cables resist perturbing forces and moments with equal stiffness to both positive and negative loads. The result is that the suspended load is constrained with a mechanical stiffness determined by the elasticity of the cables, the suspended weight, and the geometry of the mechanism. Based on these concepts, a revolutionary new type of robot crane, the NIST ROBOCRANE, has been developed that can control the position, velocity, and force of tools and heavy machinery in all six degrees of freedom (x, y, z, roll, pitch, and yaw). Depending on what is suspended from its work platform, the ROBOCRANE can perform a variety of tasks. Examples are: cutting, excavating and grading, shaping and finishing, lifting, and positioning. A 6-m version of the ROBOCRANE has been built and critical performance characteristics analyzed.     

Dynamics of a six degrees-of-freedom parallel robot actuated by three two-wheel carts

This paper deals with a new, six degrees-of-freedom parallel manipulator comprising a platform, three links and three two-wheel carts supporting the links. The wheels are independently driven, allowing the manipulator a workspace which is limited only by the lengths of the links and the size of the plane on which the carts move. The direct and inverse dynamic problems of the manipulator are solved. It is shown that the Jacobian associated with the direct problem becomes identically singular when used to solve the inverse problem, and hence must be redefined; and that once redefined, it losses its standard structure and cannot be used to solve the direct problem. Three solution methods to the inverse problem are presented and are shown to lead to indistinguishable results.     

任务并行编程模型研究与进展

任务并行编程模型是近年来多核平台上广泛研究和使用的并行编程模型,旨在简化并行编程和提高多核利用率.首先,介绍了任务并行编程模型的基本编程接口和支持机制;然后,从3个角度,即并行性表达、数据管理和任务调度介绍任务并行编程模型的研究问题、困难和最新研究成果;最后展望了任务并行未来的研究方向.     

基于代理的并行空间查询语言

目前针对并行空间数据处理的研究主要集中在空间数据划分及其在其基础上的并行空间算法,对空间并行数据库平台本身的可用性,如应用程序的开发模式、高并发请求支持等研究较少。为此,对开源并行关系数据库查询语言进行空间查询扩展,提出一种基于代理的并行空间查询语言,并实现相应的并行数据库平台原型。基于该平台开发标准的网络地图绘图服务,在高并发环境下使用该服务对海量矢量数据进行实时渲染。实验结果表明,该平台具有与传统关系数据库一致的开发应用模式,可提供无缝的衔接方式,在海量数据高并发的情况下具有较高的可用性及查询性能。     

Dynamic Modeling and Decentralized Control of a 3 PRS Parallel Mechanism Based on Constrained Robotic Analysis

In this paper, a novel dynamic model is developed for analysis and control of a 3-PRS (Prismatic, Revolute, Spherical) parallel mechanism. The dynamic model is formulated in the joint space and three holonomic constraint equations are derived to specify the coupling relationships between the actuated legs and the moving platform. The associated constraint forces are determined to be internal forces and it leads to a novel model where the dynamics of the moving platform can be separated from that of the actuated legs. By utilizing the developed model, the constraint forces can be computed more efficiently as compared to the conventional approach. After compensation of the coupling dynamics, each actuated legs could be considered as a decoupled system. The coupling behavior of the parallel mechanism can then be easily illustrated by the proposed model. To facilitate real-time control implementation, the derived model can be further simplified and a decentralized control scheme integrated with a disturbance observer is proposed. The computational efficiency and tracking performances for the proposed control method are then validated by computer simulations.     

Mobility analysis and kinematics of the semi-general 2(3-RPS) series-parallel manipulator

This work reports on the kinematics of a series-parallel manipulator built with two zero-torsion tangential parallel manipulators assembled in series connection. Although this mechanism has been widely studied, there are some topics that must be revised, e.g. the mobility analysis here reported shows that the robot under study is not precisely a six degrees of freedom spatial mechanism as it has been commonly considered. Furthermore, the traditional hexagonal coupler platform is replaced with a three-dimensional platform which yields a mechanism with a more general topology. The forward and inverse displacement analyses of the robot are obtained in semi-closed form solutions based on simple closure equations which are generated upon the coordinates of three points embedded to the moving platform while the input–output equations of velocity and acceleration of the semi-general series-parallel manipulator are easily derived by resorting to reciprocal-screw theory. A case study is included in order to show the application of the method of kinematic analysis.     

Optimal design of a 3-PUPU parallel robot with compliant hinges for micromanipulation in a cubic workspace

In recent years, nanotechnology has been developing rapidly due to its potential applications in various fields that new materials and products are produced. In this paper, a novel macro/micro 3-DOF parallel platform is proposed for micro positioning applications. The kinematics model of the dual parallel mechanism system is established by the stiffness model with individual wide-range flexure hinge and the vector-loop equation. The inverse solutions and parasitic rotations of the moving platform are obtained and analyzed, which are based on a parallel mechanism with real parameters. The reachable and usable workspace of the macro motion and micro motion of the mechanism are plotted and analyzed. Finally, based on the analysis of parasitic rotations and usable workspace of micro motion, an optimization for the parallel manipulator is presented. The investigations of this paper will provide suggestions to improve the structure and control algorithm optimization for the dual parallel mechanism in order to achieve the features of both larger workspace and higher motion precision.     

Dynamics of the spherical 3- \mathit{U\underline{P}S/}S parallel mechanism with prismatic actuators

Recursive relations in kinematics and dynamics of the symmetric spherical 3- parallel mechanism having three prismatic actuators are established in this paper. Controlled by three forces, the parallel manipulator is a 3-DOF mechanical system with three parallel legs connecting to the moving platform. Knowing the position and the rotation motion of the platform, we develop first the inverse kinematics problem and determine the position, velocity, and acceleration of each manipulator’s link. Further, the inverse dynamic problem is solved using an approach based on the principle of virtual work, but it has been verified using the results in the framework of the Lagrange equations with their multipliers. Finally, compact matrix relations and graphs of simulation for the input forces and powers are obtained.     

Kinematic analysis of the 3-CUP parallel mechanism

This paper investigates the kinematics of a parallel mechanism that is composed of three identical CUP legs evenly distributed on the fixed base. The platform of the mechanism has three degrees-of-freedom, namely: two rotations and one translation along the axis perpendicular to the base. The paper obtains closed form solutions for the inverse and forward kinematics problems. Furthermore, the Jacobian matrix is determined in order to solve the instantaneous kinematics analysis. It is used for the identification of the singular configurations of the mechanism, which are investigated by applying screw theory. The parasitic motions of the platform are determined by means of a workspace analysis. This paper uses several simulations and numerical examples to prove the accuracy of the analytical results.