A New Cable-Based Parallel Robot with Three Degrees of Freedom

In this paper, a new cable-based parallel robot is introduced. In this robot, the cables are used to not only actuate the end-effector but apply the necessary kinematic constrains to provide three pure translational degrees of freedom. In order to maintain tension in the cables, a collapsible element called “spine” is used between the end-effector and the robot’s base. The kinematic analysis of this robot is similar to that of a rigid link parallel manipulator as long as the cables are in tension. The rigidity of this robot which corresponds to having all cables in tension is studied thoroughly and it is proved that a single spine with a finite force is sufficient to guarantee rigidity for any external load at any position of the workspace.     

Optimal regulation of a cable suspended robot equipped with cable interfering avoidance controller

Closed-loop regulation of a spatial cable suspended robot is performed in this paper subject to maximizing the Dynamic Load Carrying Capacity (DLCC) of the end-effector while the cable interference is avoided actively. Optimization is performed between two predefined boundaries and considering the cable interference constraint. This constraint is satisfied by designing a controller which prevents the cables’ collision. The overall formulation of the closed-loop optimal control based on Feedback linearization is derived in this paper for planning the optimal path with the highest load capacity. Then a complementary adaptive controller is designed and implemented to the main controller which is responsible for providing cable interfering avoidance. The efficiency of the designed controller for preventing the cables’ collision is shown by performing and analyzing some comparative simulations conducted on an under constrained cable robot with six cables and six DOFs. All results related to regulation, tracking and DLCC are compared between the simple optimal closed-loop system and the system which is equipped with the proposed cable interfering avoidance controller. It is proved that the planned path satisfies cable interference constraint while its DLCCs are optimized.     

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.     

Trajectory planning of a suspended cable driven parallel robot with reconfigurable end effector

In this paper, a new suspended cable driven parallel robot (CDPR) with reconfigurable end effector is presented. This robot has been conceived for pick and place operations in industrial environments. For such applications, the possibility to change the configuration of the cables at the end-effector level is a promising way to avoid collisions with obstacles in the approaching phases, while reducing at the same time the duration of motion in the remaining part of the trajectory. An optimized trajectory planning algorithm is proposed, which implements a pick and place operation in the operational space with dynamic on-line reconfiguration of the end effector. The results on a simplified scenario demonstrate the ability of the system to obtain reduced movement times together with obstacle avoidance.     

Design,modelling and validation of a novel extra slender continuum robot for in-situ inspection and repair in aeroengine

In-situ aeroengine maintenance works are highly beneficial as it can significantly reduce the current maintenance cycle which is extensive and costly due to the disassembly requirement of engines from aircraft. However, navigating in/out via inspection ports and performing multi-axis movements with end-effectors in constrained environments (e.g. combustion chamber) is fairly challenging. A novel extra-slender (diameter-to-length ratio  < 0.02) dual-stage continuum robot (16 degree-of-freedom) is proposed to navigate in and out confined environments and perform required configuration shapes for repair operations. Firstly, the robot design presents several innovative mechatronic solutions: (i) dual-stage tendon-driven structure with bevelled disks to perform required shapes and to provide selective stiffness for carrying high payloads; (ii) various rigid-compliant combined joints to enable different flexibility and stiffness in each stage; (iii) three commanding cables for each 2-DoF section to minimise the number of actuators with precise actuation. Secondly, a segment-scaled piecewise-constant-curvature-theory based kinematic model and a Kirchhoff-elastic-rod-theory based static model are established by considering the applied forces/moments (friction, actuation, gravity and external load), where the friction coefficient is modelled as a function of bending angle. Finally, experiments were carried out to validate the proposed static modelling and to evaluate the robot capabilities of performing the predefined shape and stiffness.     

Dynamic Load Carrying Capacity of Flexible Cable Suspended Robot: Robust Feedback Linearization Control Approach

In this paper dynamic load carrying capacity (DLCC) of a cable robot equipped with a closed loop control system based on feedback linearization, is calculated for both rigid and flexible joint systems. This parameter is the most important character of a cable robot since the main application of this kind of robots is their high load carrying capacity. First of all the dynamic equations required for control approach are represented and then the formulation of control approach is driven based on feedback linearization method which is the most suitable control algorithm for nonlinear dynamic systems like robots. This method provides a perfect accuracy and also satisfies the Lyapunov stability since any desired pole placement can be achieved by using suitable gain for controller. Flexible joint cable robot is also analyzed in this paper and its stability is ensured by implementing robust control for the designed control system. DLCC of the robot is calculated considering motor torque constrain and accuracy constrain. Finally a simulation study is done for two samples of rigid cable robot, a planar complete constrained sample with three cables and 2 degrees of freedom and a spatial unconstrained case with six cables and 6 degrees of freedom. Simulation studies continue with the same spatial robot but flexible joint characteristics. Not only the DLCC of the mentioned robots are calculated but also required motors torque and desired angular velocity of the motors are calculated in the closed loop condition for a predefined trajectory. The effectiveness of the designed controller is shown by the aid of simulation results as well as comparison between rigid and flexible systems.     

Maximum DLCC of Spatial Cable Robot for a Predefined Trajectory Within the Workspace Using Closed Loop Optimal Control Approach

One of the most important applications of cable robots is load carrying along a specific path. Control procedure of cable robots is more challenging compared to linkage robots since cables can’t afford pressure. Meanwhile carrying the heaviest possible payload for this kind of robots is desired. In this paper a nonlinear optimal control is proposed in order to control the end-effector within a predefined trajectory while the highest Dynamic Load Carrying Capacity (DLCC) can be carried. This purpose is met by applying optimum torque distribution among the motors with acceptable tracking accuracy. Besides, other algorithms are applied to make sure that the allowable workspace constraint is also satisfied. Since the dynamics of the robot is nonlinear, feedback linearization approach is employed in order to control the end-effector on its desirable path in a closed loop way while Linear Quadratic Regulator (LOR) method is used in order to optimize its controlling gains since the state space is linearized by the feedback linearization. The proposed algorithm is supported by doing some simulation studies on a two Degrees of Freedom (DOF) constrained planar cable robot as well as a six DOFs under constrained cable suspended robot and their DLCCs are calculated by satisfying the motor torque, tracking error and allowable workspace constraints. The results including the angular velocity, motors’ torque, actual tracking of the end-effector and the DLCC of the robot are calculated and verified using experimental tests done on the cable robot. Comparison of the results of open loop simulation results, closed loop simulation results and experimental tests, shows that the results are improved by applying the proposed algorithm. This is the result of tuning the motors’ torque and accuracy in a way that the highest DLCC can be achieved.     

Inverse Kinematics and Workspace Analysis of a 3 DOF Flexible Parallel Humanoid Neck Robot

To mimic the human neck’s three degree-of-freedom (DOF) rotation motion, we present a novel bio-inspired cable driven parallel robot with a flexible spine. Although there exists many parallel robotic platform that can mimic the human neck motion, most of them have only two DOF, with the yaw motion being actuated separately. The presented flexible parallel humanoid neck robot employs a column compression spring as the main body of cervical vertebra and four cables as neck muscles to connect the base and moving platform. The pitch and roll movements of moving platform are realized by the two dimensional lateral bending motion of the flexible spring, and a bearing located at the top of the compression spring and embedded in the moving platform is used to achieve the yaw motion of the moving platform. By combing the force and torque balance equations with the lateral bending statics of the spring, inverse kinematics and optimizing the cable placements to minimize the actuating cable force are investigated. Moreover, the translational workspace corresponding to pitch and roll movements and rotational workspace corresponding to yaw movement are analyzed with positive cable tension constraint. Extensive simulations were performed and demonstrated the feasibility and effectiveness of the proposed inverse kinematics and workspace analysis of the novel 3 DOF flexible parallel humanoid neck robot.     

Design of a large-scale cable-driven robot with translational motion

The design of a new cable-driven robot for large-scale manipulation is presented with focus on the tension condition in the cables. In this robot, the arrangement of the cables is such that the moving platform has three translational motions. The robot has potentials for large-scale robotic manipulations, machining of large parts and material handling. The design analysis presented here is towards the synthesis of the robot as well as the sizing of the actuators and cables. The synthesis of this robot is dependent on the results of the tensionable workspace analysis previously published by the Alikhani et al. [6]. The analysis of the cable forces is presented in detail, which is then used to size the actuators. For this purpose, a geometrical approach is used to represent the capability of the end-effector for applying forces and moments as convex polyhedra. The design problem is then reduced to the sizing of these polyhedra according to the design requirements and manufacturing limitations. A prototype is also designed and fabricated, which is presented at the end to further elaborate on the proposed approach.     

一种基于任务的机器人全局并行算法研究及实现

本文提出了一种基于任务的机器人全局并行算法，结合主从结构的MIMD并行处理平台将机器人控制中的运动学、动力学、控制律等基本计算任务分别进行任务划分，将划分好的子任务统一用工作池方式实现全局的动态调度．采用流水线及集中式动态调度策略，在一个由5个DSP处理器组成的同构型松耦合MIMD并行处理平台上对平面机器人进行了并行实时仿真实验，取得了满意的并行性能指标．     

Cable suspended planar robots with redundant cables: controllers with positive tensions

Cable-suspended robots are structurally similar to parallel actuated robots but with the fundamental difference that cables can only pull the end-effector but not push it. From a scientific point of view, this feature makes feedback control of cable-suspended robots more challenging than their counterpart parallel-actuated robots. In the case with redundant cables, feedback control laws can be designed to make all tensions positive while attaining desired control performance. This paper presents approaches to design positive tension controllers for cable suspended robots with redundant cables. Their effectiveness is demonstrated through simulations and experiments on a three degree-of-freedom cable suspended robots.     

Optimal custom design of both symmetric and unsymmetrical hexapod robots for aeronautics applications

The Stewart parallel mechanism is used in various applications due to its high load-carrying capacity, accuracy and stiffness, such as flight simulation, spaceship aligning, radar and satellite antenna orientation, rehabilitation applications, parallel machine tools. However, the use of such parallel robots is not widespread due to three factors: the limited workspace, the singularity configurations existing inside the workspace, and the high cost. In this work, an approach to support the design of a cost-effective Stewart platform-based mechanism for specific applications and to facilitate the choice of suitable components (e.g., linear actuators and base and mobile plates) is presented. The optimal design proposed in this work has multiple objectives. In detail, it intends to maximize the payload and minimize the forces at each leg needed to counteract external forces applied to the mobile platform during positioning or manufacturing, or, in general, during specific applications. The approach also aims at avoiding reduction of the robot workspace through a kinematic optimization. Both symmetric and unsymmetrical geometries have been analysed to show how the optimal design approach can lead to effective results with different robot configurations. Moreover, these objectives are achieved through a dynamic optimization and several optimization algorithms were compared in terms of defined performance indexes.     

Delayed reference control for multi-degree-of-freedom elastic systems: Theory and experimentation

The objective of this paper is to provide an experimental proof of the effectiveness and ease of implementation of a non-time based control strategy for the simultaneous active vibration control and path tracking of multi-degree-of-freedom linear systems. What is peculiar in the proposed scheme, named DRC, is that it allows reducing elastic vibrations while guaranteeing coordinated motion among the system rigid-body degrees of freedom, and hence the accurate tracking of desired paths through space. The DRC is here applied to damp the oscillation of a load suspended to the moving platform of an Adept Quattro parallel robot by means of a cable.     

Climbing parallel robot: a computational and experimental study of its performance around structural nodes

This paper presents a study of the kinematics and dynamics performance of a climbing parallel robot (CPR) to avoid nodes on structural frames. To avoid a structural node, a CPR can acquire some determined postures. A series of postures can be combined to generate the convenient movements to climb along the structural node. The postures of a CPR must be studied to detect its feasibility, because the postures can drive the robot near its singular configurations. Also, the forces originated in the actuators to evade the structural nodes are evaluated. Therefore, the goal of this paper is to show that a Stewart-Gough (S-G) parallel platform can be used as a climbing robot, because a CPR can avoid structural nodes easily and elegantly, in contrast with other types of robots. To support the simulation results presented in the first part of this paper, an experimental testbed has been developed to study the dynamic performance of the CPR prototype around a structural node. The results obtained are very interesting, and show that an industrial potential to use the parallel S-G robot as a climbing robot exists.     

Optimal design of a parallel assembling robot with large payload-to-mass ratio

Assembly in a confined space, such as the cabin of an aircraft or train, demands the assembling device to be with compact structure, satisfactory kinematics and excellent load carrying capability. A six degree-of-freedom (DoF) parallel robot is proposed and designed for such assembling tasks in this paper. Specially, internal structure changes introduced by topology optimization are considered and the multi-objective optimization reaching large load-to-mass ratio is implemented. First, external dimensions of the base and the ratio of the remaining volume to the complete volume are the inputs. Once a set of input variables are given, the topology optimization will be performed by FEA software to form the structure of the base. The stiffness and mass of the base, being the outputs, are obtained numerically by software. Then, the meta models are established by the response surface model (RSM) method. On this basis, stiffness and mass models of the robot are built by the semi-analytical method. The optimal design is implemented by Pareto-based multi-objective optimization. Different arrangements of the objectives are compared. The results show that kinematic indices on the Pareto fronts are all at a satisfactory level. The optimization having payload-to-mass ratio as objective leads to the optimum with higher stiffness along z-axis and smaller mass. The design 6-DoF parallel assembly robot can carry up to 42.06 kg loads while the mass is only 12.002 kg.     

Present and future robot control development—An industrial perspective

Robot control is a key competence for robot manufacturers and a lot of development is made to increase robot performance, reduce robot cost and introduce new functionalities. Examples of development areas that get big attention today are multi robot control, safe control, force control, 3D vision, remote robot supervision and wireless communication. The application benefits from these developments are discussed as well as the technical challenges that the robot manufacturers meet. Model-based control is now a key technology for the control of industrial robots and models and control schemes are continuously refined to meet the requirements on higher performance even when the cost pressure leads to the design of robot mechanics that is more difficult to control. Driving forces for the future development of robots can be found in, for example, new robot applications in the automotive industry, especially for the final assembly, in small and medium size enterprises, in foundries, in food industry and in the processing and assembly of large structures. Some scenarios on future robot control development are proposed. One scenario is that light-weight robot concepts could have an impact on future car manufacturing and on future automation of small and medium size enterprises (SMEs). Such a development could result in modular robots and in control schemes using sensors in the robot arm structure, sensors that could also be used for the implementation of redundant safe control. Introducing highly modular robots will increase the need of robot installation support, making Plug and Play functionality even more important. One possibility to obtain a highly modular robot program could be to use a recently developed new type of parallel kinematic robot structure with large work space in relation to the robot foot print. For further efficient use of robots, the scenario of adaptive robot performance is introduced. This means that the robot control is optimised with respect to the thermal and fatigue load on the robot for the specific program that the robot performs. The main conclusion of the presentation is that industrial robot development is far away from its limits and that a lot of research and development is needed to obtain a more widely use of robot automation in industry.     

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.     

Design and analysis of the statically balanced direct-drive robot manipulator

A practical architecture, using a four-bar linkage, is considered for the University of Minnesota direct-drive robot (Kazerooni, H., Kim, S.: A new architecture for direct drive robots. In Proc. IEEE International Conference on Robotics and Automation, Philadelphia, Pennsylvania, April 1988). This statically balanced direct-drive robot has been constructed for stability analysis of the robot in constrained manipulation (Kazerooni, H. et al.: Fundamentals of robust compliant motion for robot manipulators. IEEE J. Robotics Automation 2: 1986; Kazerooni, H.: On the robot compliant motion control. ASME J. Dynamic Systems Msmt Control; 111 (3): September 1989. Kazerooni, H. et al.: Theory and experiments on robot compliant motion control. ASME J. Dynamic Systems Msmt Control, June 1990). As a result of the elimination of the gravity forces (without any counterweights), smaller actuators and, consequently, smaller amplifiers were chosen. The motors yield acceleration of 5 g at the robot end point without overheating. High torque, low speed, brushless AC synchronous motors are used to power the robot. Graphite-epoxy composite material is used for construction of the robot links. A 4-node parallel processor has been used to control the robot. A compliant motion control method has been derived and experimentally verified to guarantee stable constrained maneuvers for the robot. As part of the research work, a general criterion has been derived to guarantee the stability of robot manipulators in constrained maneuvers.     

Kinematic analysis and error modeling of TAU parallel robot

The TAU robot presents a new configuration of parallel robots with three degrees of freedom. This robotic configuration is well adapted to perform with a high precision and high stiffness within a large working range compared with a serial robot. It has the advantages of both parallel robots and serial robots. In this paper, the kinematic modeling and error modeling are established with all errors considered using Jacobian matrix method for the robot. Meanwhile, a very effective Jacobian approximation method is introduced to calculate the forward kinematic problem instead of Newton–Raphson method. It denotes that a closed form solution can be obtained instead of a numerical solution. A full size Jacobian matrix is used in carrying out error analysis, error budget, and model parameter estimation and identification. Simulation results indicate that both Jacobian matrix and Jacobian approximation method are correct and with a level of accuracy of micron meters. ADAMS's simulation results are used in verifying the established models.     

Cable inspection robot for cable‐stayed bridges: Design,analysis, and application

As the most important component of cable‐stayed bridges, cable safety has been of crucial public concern. In this paper, a new robot system for the inspection of stay cables is proposed. The robot not only replaces human workers in carrying out risky tasks in a hazardous environment but also increases operational efficiency by eliminating costly erection of scaffolding or dragging of winches. The designed robot is composed of two equally spaced modules, joined by connecting bars to form a closed hexagonal body that clasps the cable. For safe landing in case of an electrical interruption or malfunction, a gas damper with a slider‐crank mechanism is proposed to use up the extra energy generated by gravity when the robot slips down. To conserve energy, a landing method based on back electromotive force is introduced. Laboratory and field experiments verified that the robot can stably climb random inclined cables and land smoothly upon electrical malfunction. Finally, along with an application example, the vision inspection system based on charge‐coupled device cameras, operating modes of the robot, control methods, and feasibility are discussed in detail. The field applications on two cable‐stayed bridges indicate that such a low‐cost robot system can improve the efficiency of inspection operations and satisfy the requirements of actual cable inspection. © 2011 Wiley Periodicals, Inc.