Parallel Robot Manipulator Research Articles

Actuation Redundancy as a Way to Improve the Acceleration Capabilities of 3T and 3T1R Pick-and-Place Parallel Manipulators

This paper analyzes the possible contribution of actuation redundancy in obtaining very high acceleration with parallel robot manipulators. This study is based on redundant and nonredundant Delta/Par4-like manipulators, which are frequently used for pick-and-place applications, and addresses the cases of translational manipulators (also called 3T manipulators) and manipulators with Schoenflies motions (also called 3T1R manipulators). A dynamic model, valid for both redundant and nonredundant manipulators, is used to analyze the moving platform’s acceleration capabilities: (i) at zero speed and in any direction and (ii) at zero speed in the “best” direction. The results show that actuation redundancy makes it possible to homogenize dynamic capabilities throughout the workspace and to increase the moving platform’s accelerations. Designs of redundant Delta/Par4-like manipulators capable of high acceleration pick-and-place trajectories are presented for both 3T and 3T1R manipulators.

Kinematics Analysis of a Parallel Robotic Manipulator

The parallel robotic manipulator has attracted many researchers’ attention and it also has growing applications to different areas. In this paper an algebraic method for solving the direct kinematics analysis problem for a parallel robotic manipulator. Based on the presented algebraic method, the problem is derived into a 40th degree univariate polynomial. All complete sets of 40 solutions to the problem are obtained. The proposed method is exemplified by a numerical example.

Modelling, simulation and control of a redundant parallel robotic manipulator based on invariant manifolds

In this article a systematic approach of modelling and control for a parallel robotic manipulator is presented. Regarding the framework of structured analysis of dynamical systems the derivation of a differential-algebraic model of the mechanical system is straightforward. Using some differential-geometric considerations based on invariant manifolds and the definition of fictitious additional input and output variables a suitable state feedback can be constructed which transforms the differential-algebraic representation into a state-space model for the robotic manipulator. On this basis a classical two-degree-of-freedom (2-DOF) control structure has been designed using the well-known input–output linearization and a linear time-variant Kalman filter-based output feedback. Finally, the control structure including a friction compensation is applied to the robotic system in the laboratory which shows the practical applicability of the proposed procedure.

A study of workspace and singularity characteristics for design of 3-DOF planar parallel robots

A study of workspace and singularity characteristics is presented for two common types of 3-DOF planar parallel robot manipulators. The robots considered feature a kinematic structure with three in-parallel actuated, R-R-R, and R-P-R serial chain geometries. In this study, computer simulations aided with graphic visualization were used to characterize the complete pose workspace (for ranges of both position and orientation) and the singularity inherent to the systems. Parametric studies have also been performed to ascertain the way in which both characteristics vary with respect to various geometric parameters such as pivot locations, link lengths, and size of end-effector. Results are shown by way of a unique composite ratio of the available workspace to the density of singularity within that workspace, which can be used to optimize the geometry of the robots in design.

Modified PSO Method for Robust Control of 3RPS Parallel Manipulators

We propose an effective method to design a modified particle swarm optimization (MPSO) singularity control method for a fully parallel robot manipulator. By adopting MPSO to obtain simple and effective estimated damping values, the result automatically adjusts the damping value around a singular point and greatly improves the accuracy of system responses. This method works by damping accelerations of the end effector, so that accelerations along the degenerated directions are zero at a singular point. These velocities, however, may not be zero in some situations, in which case, fluctuations will be encountered around a singular point. To overcome this drawback, we propose a control scheme that uses both damped acceleration and damped velocity, called the hybrid damped resolved-acceleration control (HDRAC) scheme. The MPSO optimization method can immediately provide optimal damping factors when used in an online application. Our proposed approach offers such superior features as easy implementation, stable convergence characteristics, and good computational efficiency. The main advantage of the HDRAC with MPSO in the 3RPS parallel manipulator control system is that it is not necessary for the system to plan its path for avoiding the singular point; thus, the workspace can be improved. Illustrative examples are provided to show the effectiveness of this HDRAC in practical applications, and experimental results verifying the utility of the proposed control scheme are presented.

Singularity robustness of the 3RPS parallel manipulator by using the damped-rate resolved-acceleration control

A novel singularity control method for a fully parallel robot manipulator was proposed in this paper. The damped least-square method has frequently been used to solve the singularity problem of resolved-acceleration control schemes. It works by damping accelerations of the end-effector, so that accelerations in the degenerated directions are zero at a singular point. However, its velocities may not be zero in some situations, in those cases they will encounter fluctuations around the singular point. In this paper, the control using damped velocity, being called the damped-rate resolved-acceleration control scheme (DRRAC) is proposed to overcome this drawback. We will show that the DRRAC is asymptotically stable, and discuss its convergent properties. The main advantage of the DRRAC in the 3RPS parallel manipulator control system is not to plan its path to avoid the singular point, and could improve the workspace. Illustrative examples are given to show its effectiveness in practical applications, and experimental results are taken to verify the proposed control scheme.

A New Type of Parallel Robot Manipulator Kinematics Simulation and Analysis of Movement Characteristics

Kinematics simulation and characteristics of a new type of parallel robot manipulator (PRM) with two-dimension movement and one-dimension rotation are analyzed. The new PRM is proved to can realize expected movement, and have smooth movement curves of displacement and velocity. The moving platform center of the PRM can achieve a wide range of swing, and has a larger working space. According to the kinematics theory, the working space and singularity characteristic are analyzed. Simulation and analysis indicated that the working space of the new PRM in the form and position is not singular, and has good maneuverability, thus providing the foundation for practice application and product exploitation.

Computer-aided-symbolic dynamic modeling for Stewart-platform manipulator

SUMMARYThe Stewart platform manipulator is a fully kinematic linkage system that has major mechanical differences from typical serial link robots. It is a six-axis parallel robot manipulator with a high force-to-weight ratio and good positioning accuracy that exceeds that of a conventional serial link robot arm. This study examines the dynamic equations and control methodology for a Stewart platform. Because manual symbolic expansion of Stewart platform robot dynamic equations is tedious, time-consuming, and prone to errors, an automated derivation process is highly desired. The main goal of this work is to present an efficient procedure for computer generation of dynamic equations for a Stewart platform manipulator. As MATLAB has a powerful signal processing toolbox along with symbolic processing capabilities and is widely used as a common technical computing environment in many universities and research laboratories, the objective of this study was to develop a MATLAB-based approach for symbolic computation for a parallel linked robot. Additionally, a computed-torque control methodology is utilized for such a structure. Simulation results demonstrate the effectiveness of the proposed control methodology.

Structural synthesis of maximally regular T3R2-type parallel robots via theory of linear transformations and evolutionary morphology

SUMMARYThe paper presents structural synthesis of maximally regular T3R2-type parallel robotic manipulators (PMs) with five degrees of freedom. The moving platform has three independent translations (T3) and two rotations (R2). A method is proposed for structural synthesis of maximally regular T3R2-type PMs based on the theory of linear transformations and evolutionary morphology. A one-to-one correspondence exists between the actuated joint velocity space and the external velocity space of the moving platform. The Jacobian matrix mapping the two vector spaces of maximally regular T3R2-type PMs presented in this paper is the 5×5 identity matrix throughout the entire workspace. The condition number and the determinant of the Jacobian matrix being equal to one, the manipulator performs very well with regard to force and motion transmission capabilities. Kinematic analysis of maximally regular parallel robots is trivial and no computation is required for real-time control. This paper presents in a unified approach the structural synthesis of PMs with five degrees of freedom with decoupled and uncoupled motions, along with the maximally regular solutions.

Stiffness Analysis Of Multi-Chain Parallel Robotic Systems

The paper presents a new stiffness modelling method for multi-chain parallel robotic manipulators with flexible links and compliant actuating joints. In contrast to other works, the method involves a FEA-based link stiffness evaluation and employs a new solution strategy of the kinetostatic equations, which allows computing the stiffness matrix for singular postures and to take into account influence of the internal forces. The advantages of the developed technique are confirmed by application examples, which deal with stiffness analysis of the Orthoglide manipulator.

Cascade control of a hydraulically driven 6-DOF parallel robot manipulator based on a sliding mode

A cascade-control algorithm based on a sliding mode in the legspace is proposed to realize the trajectory tracking control of a hydraulically driven six degrees of freedom (6-DOF) parallel robot manipulator. The cascade controller, which consists of an inner loop and an outer loop, is applied to separate the hydraulic dynamics from the mechanical part so that the designed controller takes into account not only the mechanical dynamics but also the hydraulic dynamics of the manipulator. The experimental results investigate the effectiveness of the proposed approaches and demonstrate the satisfactory position tracking behaviour of the parallel manipulator for the proposed cascade controller based on a sliding mode.

Optimizing fault tolerance to joint jam in the design of parallel robot manipulators

A methodology is developed to modify the kinematic layout of parallel manipulators to provide fault tolerance to active-joint jam at a large number of end-effector poses. The modification is based on equipping each branch of the manipulator with a redundant backup active joint to which the actuation is switched in case of a joint jam. An efficient optimization procedure based on linear algebra is developed to determine the optimum location and direction of each backup joint. The procedure is applied successfully on a 6-degree of freedom example manipulator.

Structural synthesis of fully-isotropic parallel robots with Schönflies motions via theory of linear transformations and evolutionary morphology

Abstract The paper presents structural synthesis of fully-isotropic parallel robotic manipulators (PMs) with Schonflies motions. The moving platform of a parallel manipulator with Schonflies motions (PMSM) has four degrees of freedom, which are three independent translations and one rotation about an axis of fixed direction. A method is proposed for structural synthesis of fully-isotropic PMSMs based on the theory of linear transformations and the evolutionary morphology. A one-to-one correspondence exists between the actuated joint velocity space and the external velocity space of the moving platform. The Jacobian matrix mapping the two vector spaces of fully-isotropic PMSMs presented in this paper is the identity 4 × 4 matrix throughout the entire workspace. The condition number and the determinant of the Jacobian matrix being equal to one, the manipulator performs very well with regard to force and motion transmission capabilities. The synthesis method proposed in this paper allows us to obtain structural solutions of PMSMs with decoupled and uncoupled motions, along with the fully-isotropic solutions in a systematic manner. Overconstrained/isostatic solutions with elementary/complex and identical/different legs are obtained. Uncoupled and fully-isotropic PMSMs have the advantage of simple command and important energy-saving due to the fact that, for a unidirectional motion, only one motor works as in a serial translational manipulator.

Structural synthesis of serial platform manipulators

In this paper, structural synthesis of serial platform manipulators is considered. Serial platform manipulators are created according to the development of the platforms and closed loops. Also a new structural formula of mobility of parallel Cartesian platform robot manipulators is presented. Structural synthesis of serial platform manipulators with lower and higher kinematic pairs with respect to their structures are also examined. Structural synthesis of parallel Cartesian platform robot manipulators is also introduced. History of structural formulas DOF are presented as a table with equations, authors, years and some commentaries. New and revised methods for structural synthesis of serial platform manipulators and parallel Cartesian platform robot manipulators are illustrated along with examples.

Modelling and Control of a Parallel Redundant Manipulator with Hydraulic Actuators

The parallel robotic manipulator, MULTIPOD, is specially designed for carrying out drilling tasks in mines. It is based on two parallel three-degree-of-freedom mechanisms in serial connection. The structure that provides excellent stiffness is hydraulically driven and has six degrees of mobility. The pose of the end-effector is described by five degrees of freedom. In this paper, complete kinematic and dynamic analyses of the manipulator are carried out. In addition, a proportional controller equipped with hydraulic force feedback is designed for the robot. The simulated results that demonstrate the behaviour of the manipulator are presented in this paper.

Robotic Measurement and Control for Chiropractic Research

The precision and programmability of robotic manipulators makes them suitable for biomechanics research, particularly when an experimental procedure must be accurately repeated multiple times. This paper describes a robotic system used to investigate biomechanical mechanisms of stroke in humans. A parallel robot manipulator is used to reproduce chiropractic manipulations on animal subjects using a 3-D vision system. An algorithm for calibrating the system is proposed and tested on the robot. An iterative learning control scheme is then introduced to improve positional accuracy. Experimental results demonstrate that the calibration procedure and learning scheme are both effective.

動揺安定台の実用化に関する研究 : 動揺シミュレーションによる減揺効果の確認

In this research, we designed and produced on-board Horizontal Stabilizing Platform for making it easy to usher in technology established ashore. The platform consists of four-axis parallel robot manipulators with powerful torque and quick response. The control system adopted two types, one is PID feed back control system and the other one is the auto regressive model feed foreword system. In fact, we experimented in order to verify the performance of Horizontal Stabilizing Platform, and verified validity of this stabilizing platform.

Mobility and spatiality of parallel robots revisited via theory of linear transformations

Abstract Parallel robots are extensively used for various applications including manipulation, machining, guiding, testing and control. The mechanical architecture of parallel robots is based on parallel mechanisms in which a mobile platform is connected to a reference element by at least two legs. Mobility and spatiality are the main structural and kinematic parameters of a parallel robot. These two parameters are defined via the theory of linear transformation and can be easily determined by inspection using the definitions, properties and theorems introduced in this paper. An analytical method to compute these parameters is also presented just for verification and for a better understanding of their meanings. The new formalism presented in this paper is based on spatiality of an elementary open kinematic chain and relative spatiality between two elements of a closed kinematic chain. As far as we are aware, this paper demonstrates for the first time a new formula for calculation of general (full-cycle) mobility of parallel robots that overcomes the drawbacks of Chebychev–Grubler–Kutzbach's mobility criterion largely used for mobility calculation of multi-loop mechanisms. This new formula is easily applicable to parallel robotic manipulators with elementary or complex legs and mobility calculation does not involve the setting up of instantaneous constraint systems associated to the parallel mechanism.

APPLYING EFFICIENT COMPUTATION OF THE MASS MATRIX FOR DECOUPLING CONTROL OF COMPLEX PARALLEL MANIPULATORS

Most challenging aspects in the control of complex parallel mechanisms or robotic manipulators is how to deal with the high system nonlinearity and with the coupled dynamics. These aspects can be satisfied by integrating the pose dependent inertia or mass matrix into the control scheme. To satisfy the requirements of real-time application, this paper presents a high efficient method to compute the mass matrix of arbitrarily complex parallel manipulators. Model simplification become dispensable, which leads necessarily to the improvement of control accuracy.

Analysis of Active Joint Failure in Parallel Robot Manipulators

In this study, the effect of active joint failure on the mobility, velocity, and static force of parallel robot manipulators is investigated. Two catastrophic active joint failure types are considered: joint jam and actuator force loss. To investigate the effect of failure on mobility, the Gru¨bler’s mobility equation is modified to take into account the kinematic constraints imposed by various branches in the manipulator. In the case of joint jam, the manipulator loses the ability to move and apply force in a specific portion of its task space; while in the case of actuator force loss, the manipulator gains an unconstrained motion in a specific portion of the task space in which an externally applied force cannot be resisted by the actuator forces. The effect of joint jam and actuator force loss on the velocity and on the force capabilities of parallel manipulators is investigated by examining the change in the Jacobian matrix, its inverse, and transposes. It is shown that the reduced velocity and force capabilities after joint jam and loss of actuator force could be determined using the null space vectors of the transpose of the Jacobian matrix and its inverse. Computer simulation is conducted to demonstrate the application of the developed methodology in determining the post-failure trajectory of a 3-3 six-degree-of-freedom Stewart-Gough manipulator, when encountering active joint jam and actuator force loss.