Performance Of Parallel Manipulator Research Articles

Design and Analysis of a Reconfigurable Hybrid Robot for Machining of Large Workpieces

Abstract Large workpieces are important components of core equipment in aerospace and other fields, where the machining mainly focuses on the surfaces and inner cavities. However, it may be unsuitable for existing machining robots to directly achieve integrated machining, that is, not only the high-precision surface machining but also the machining of different inner cavities in a limited space. To satisfy these machining requirements, a new reconfigurable hybrid robot (RHR) is proposed, called the 3PRR-3PSS-UPU RHR, for machining the surface and inner cavity of large workpieces (where P, P, R, S, and U stand for the actuated prismatic joint, passive prismatic joint, revolute joint, spherical joint, and universal joint, respectively). The proposed RHR consists of two parallel manipulators (PMs), in which one is a spatial 3PRR PM with one translational degree-of-freedom (DOF) and the other is a 3PSS-UPU reconfigurable PM (RPM) with different configurations of two rotational and one translational (2R1T) DOFs using locking equipment, which is the main advantage of the designed robot. The inverse kinematics and singularities of two PMs are analyzed. The stiffness performance of the spatial 3PRR PM is compared with that of a moving slider with one translational DOF. By evaluating the workspace and motion/force transmissibility, the kinematic performance of two PMs is presented using several local and global indices, followed by the dimensional optimization of link parameters. Based on the structural characteristics and excellent performance, it can be inferred that the 3PRR-3PSS-UPU RHR has great potential for machining large workpieces.

2PPaRR-2PPaRU: Design, analysis, and optimization of a new 3T1R parallel manipulator with rotational pitch motion

A new 3T1R parallel manipulator (PM) 2PPaRR-2PPaRU with rotational pitch motion is proposed in this paper. In such a mechanism, there are four limbs connected to the moving platform (MP) through several revolute joints and universal joints, and all actuated joints are prismatic joints with an axis along a vertical direction. Mobility and motion characteristics are verified by constraint screw theory. The inverse kinematic solutions and the singularity configurations are derived through kinematic analysis. Next, the workspace analysis and the connection between [Formula: see text] and the volume of the reachable workspace are performed. The local motion/force transmission index is selected to evaluate the local transmission performance of 2PPaRR-2PPaRU PM. Finally, the global motion/force transmission index and the volume of reachable workspace are considered simultaneously to optimize the structural parameters. The 2PPaRR-2PPaRU PM with improved local transmission performance and reachable workspace has been achieved after optimization.

Optimization of the 2PRU-1PRS Parallel Manipulator Based on Workspace and Power Consumption Criteria

In the last few years, parallel manipulators are being increasingly studied and used for different applications. The performance of parallel manipulators is very sensitive to the geometric parameters, so it is essential to optimize them in order to obtain the desired function. We propose two optimization algorithms that consider the size and regularity of the workspace. The first one obtains the geometric parameters combination that results in the biggest and most regular workspace. The second method analyzes the geometric parameters combinations that result in an acceptable size of the workspace—even if it is not the biggest one—and finds out which ones result in the lowest power consumption. Even if the results vary depending on the application and trajectories studied, the proposed methodology can be followed to any type of parallel manipulator, application or trajectory. In this work we focus on the dimension optimization of the geometric parameters of the 2PRU-1PRS Multi-Axial Shaking Table (MAST) for automobile pieces testing purposes.

Effect of Spherical Joint Symmetry Axis Orientation on the Kinematic Performance of Lower Mobility Parallel Manipulators

Parallel manipulators (PMs) are often used as parallel kinematic machines (PKMs), which are more accurate and have more payload capacity compared to their serial counterparts. Spherical joints are frequently used in the construction of PKMs; however, spherical joints have a unique physical constraint, known as cone angle limit, around their axis of symmetry. The present work investigates the effect of changing the orientation of the spherical joint symmetry axis (SJSA) on the kinematic performance of PMs. Workspace index, global conditioning index, kinematic conditioning index and global stiffness index are chosen as performance indices. It is found that changing the orientation of SJSA has a remarkable impact on the kinematic performance of PMs. Also, it is shown that considering the cone angle limits considerably affect the kinematic performance of PMs. To demonstrate the results, two PMs, namely 3-PRS (P, R, and S denote prismatic, revolute, and spherical joints, respectively) and 3-RPS manipulators with spherical joints with adjustable axis of symmetry orientation, are used. The effect of changing SJSA orientation on various performance indices is carried out, and optimal orientation values are determined. Genetic algorithm (GA) optimization technique and traditional method are used. The results obtained using the two methods matched very well. It is found that orientation of the SJSA is one of the important PMs optimization parameters that should be considered during the design process.

An evaluation approach for motion-force interaction performance of parallel manipulators with closed-loop passive limbs

Motion/force transmissibility and constrainability are common and widely used measures for performance evaluation and dimension optimization of parallel manipulators (PMs). However, the transmission and constraint indices in current use do not seem simple or intuitive for PMs with closed-loop passive limbs due to the difficulties in the wrench identification. To deal with this problem, this paper presents a blocking-and-actuating strategy to evaluate the motion-force interaction performance of such PMs. On this basis, the distal wrenches and proximal wrenches are identified inside the closed-loop passive limbs. Note that both of these wrenches are physically available, and they are distinct from the pure transmission and pure constraint wrenches. Examples of identifying these wrenches inside the passive limbs of some typical PMs are presented. Several performance indices are then defined based on the power coefficient via screw theory. Based on this foundation, this paper proposes a generalized approach for the coupled motion/force transmissibility and constrainability evaluation of PMs with closed-loop passive limbs. The motion-force interaction performance evaluation of a spatial 3-P(SS)S (P, actuated prismatic joint; S, spherical joint) PM with closed-loop passive limbs is finally derived to illustrate the generality and validity of this evaluation approach. Furthermore, results show that the proposed approach can reveal the effect of the structural parameters inside closed-loop passive limbs on the motion-force interaction performance.

Comparison of metaheuristic optimization algorithms for dimensional synthesis of a spherical parallel manipulator

It is well known that the performance of parallel manipulators is highly dependent on their geometry, therefore the dimensional synthesis plays an important role in the design of a manipulator. In this work, the dimensional synthesis of a spherical parallel manipulator is carried out by means of a multi-objective optimization. This manipulator is planned to be a prosthetic wrist. The considered objective functions were the dexterity, the variability of the dexterity, the maximum required torque and the stiffness. The optimization was performed using three different metaheuristic algorithms: NSGA-II, OMOPSO, MOEA/D. After performing a set of simulations, the results show a clearly better performance of the MOEA/D algorithm. The optimized geometry of the manipulator allowed to design a virtual prototype of a prosthetic wrist.

Reliable and Failure-Free Workspaces for Motion Planning Algorithms for Parallel Manipulators Under Geometrical Uncertainties

Manufacturing tolerances and other uncertainties may play an important role in the performance of parallel manipulators since they can affect the distance to a singular configuration. Motion planning strategies for parallel manipulators under uncertainty require decision making approaches for classifying reliable regions within the workspace. In this paper, we address fail free and reliable motion planning for parallel manipulators. Failure is related to parallel kinematic singularities in the motion equations or to ill-conditioning of the Jacobian matrices. Monte Carlo algorithm is employed to compute failure probabilities for a dense grid of manipulator workspace configurations. The inverse condition number of the Jacobian matrix is used to compute the distance between each configuration and a singularity. For supporting motion planning strategies, not only failure maps are constructed but also reliable and failure-free workspaces are obtained. On the one hand, the reliable workspace is obtained by minimizing the failure probabilities subject to a minimal workspace area. Differently, a failure-free workspace is found by maximizing the workspace area subject to a probability of failure equal to zero. A 3RRR manipulator is used as a case study. For this case study, the usage of the reliable strategy can be useful for robustifying motion planning algorithm without a significant reduction of the reliable regions within the workspace.

Kinematic Optimization of Parallel Manipulators with a Desired Workspace

A kinematic optimization method of parallel manipulators is presented in this paper. A desired workspace for a parallel manipulator is usually an essential requirement in a practical application. Additionally, a good kinematic performance and/or a relatively small physical size would be of great significance. A dexterity index is utilized to measure the kinematic performance of parallel manipulators. A method to define the physical size of a parallel manipulator is introduced. The cost function is then formulated as a linear combination of a dexterity index and a physical size measurement. The Optimization Tool Box of MATLAB is applied to solve the optimization problem. Finally, a general Stewart-Gough platform is taken as an example to specify the design methodology.

Performance analysis and optimization of a five-degrees-of-freedom compliant hybrid parallel micromanipulator

There are generally two main directions for the investigation and development of parallel manipulators, namely macro/meso stream and micro/nano stream, in which the former one has been thoroughly investigated in recent decades, while the latter one still remains many performance related open issues that significantly affect their application potentials in critical situations such as high-precision automated cell manipulation. Improving the overall performance of parallel manipulators is the bridge to connect the academia and industry for the great development and real-world application. This research is to develop a novel methodology called performance decomposition and integration for governing the design optimization process of complicated micromanipulator. A new five degrees-of-freedom (DOF) compliant hybrid parallel micromanipulator which is configured with five identical PSS limbs and one constraining UPU limb is proposed as a case study. The performance visualization, finite element analysis, and dimensional optimization are implemented. The proposed methodology is applicable for the design improvement of different kinds of compliant/parallel mechanisms. Performance is a critical topic for the further improvement of compliant parallel mechanisms.The research attempts to propose a paradigm called performance decomposition and integration to manage the overall performance of these mechanisms in a higher level.A CHPMM is proposed as an example to showcase part of the principle of PDI in the process of design and optimization.The proposed method can be well integrated with the methodology of system hybridization through the specific methods of mechanism hybridization, actuation hybridization and optimization hybridization.For the future work, a physical prototype will be manufactured to further improve the overall performance in aspects of manipulation and control under the guidance of the proposed methods.

2-DOF Synchronous Control of 6PUS-UPU Parallel Manipulator

To meet the increasing demands on the control performance of parallel manipulator, servo control strategy for parallel manipulator was researched systematically. Combined 2-DOF control with Internal Model Control, the servo controller for parallel manipulator was optimized as smoothly and efficiently as possible. On the basis of the original current loop and speed loop, the position loop was redesigned to have both good position tracking characteristics and anti-disturbance capacity. The synchronous control strategy combined position signal and force signal was proposed. The actual experiments show that 2-DOF synchronous control has better stability and position tracking performance compared with the traditional PID control. The control performance of parallel manipulator is improved effectively.

Mechanism and actuation hybridization for a four degrees-of-freedom parallel manipulator

Performance is paramount to parallel manipulators. Improving the overall performance of parallel manipulator is the bridge to connect the academia and industry for the great development and real-world application of parallel manipulators. For the past research, scholars put more emphasis on optimizing the performance of parallel manipulators in different aspects, e.g. dimensional optimization, error calibration, trajectory planning. This research, a methodology which is called as System Hybridization is proposed to manage the performance of parallel manipulator in a higher level. Through a systematic method, the global capabilities of parallel manipulator can be significantly improved. A case study of a four degrees-of-freedom (DOF) parallel manipulator is proposed to well explain the proposed paradigm in the way of Mechanism Hybridization and Actuation Hybridization. It demonstrates that the hybrid actuation based on hybrid mechanism performs better than the mechanism with the identical actuators in term of stiffness in the main directions, which proves the proposed method is meaningful to improve the performance of parallel manipulator.

Force/Position Hybrid Control of 6PUS-UPU Redundant Driven Parallel Manipulator Based on 2-DOF Internal Model Control

To improve control performance of parallel manipulator, servo control is optimized according to 2-DOF control with Internal Model Control. Position loop of the controller is redesigned based on the original current loop and speed loop. The hybrid force/position control strategy on the basis of cross-coupling is proposed. With mechatronic cosimulation system, it is proved that the force/position hybrid control on the basis of 2-DOF Internal Model Control has better stability and position precision compared with the traditional PID control. The control precision and stability of the parallel manipulator are improved effectively in actual experiment. Two types of compliance operation including peg in hole and surface tracking are realized in 6PUS-UPU parallel manipulator and both have good applicable effect.

Design and analysis of kinematically redundant parallel manipulators with configurable platforms

Redundancy can, in general, improve the ability and performance of parallel manipulators by implementing the redundant degrees of freedom to optimize a secondary objective function. Almost all published researches in the area of parallel manipulators redundancy were focused on the design and analysis of redundant parallel manipulators with rigid (nonconfigurable) platforms and on grasping hands to be attached to the platforms. Conventional grippers usually are not appropriate to grasp irregular or large objects. Very few studies focused on the idea of using a configurable platform as a grasping device. This paper highlights the idea of using configurable platforms in both planar and spatial redundant parallel manipulators, and generalizes their analysis. The configurable platform is actually a closed kinematic chain of mobility equal to the degree of redundancy of the manipulator. The additional redundant degrees of freedom are used in reconfiguring the shape of the platform itself. Several designs of kinematically redundant planar and spatial parallel manipulators with configurable platform are presented. Such designs can be used as a grasping device especially for irregular or large objects or even as a micro-positioning device after grasping the object. Screw algebra is used to develop a general framework that can be adapted to analyze the kinematics of any general-geometry planar or spatial kinematically redundant parallel manipulator with configurable platform.