Parallel Link Manipulator Research Articles

３自由度パラレルリンクマニピュレータの順運動学に関する高速・高精度演算法

This study deals with a calculation method for the forward kinematics of a 3 DOF parallel link manipulator. As a method for such a calculation, up to the present, the Newton method, or Newton-Raphson method alternatively, has been generally used. Nevertheless, this method is difficult to apply to on-line control systems, since it requires significant computation time and a large capacity of computer memory for calculations. In this study, therefore, a new algorithm for calculating forward kinematics is proposed. Fundamentally, the algorithm consists of two processes. The first is to calculate in a very short time an approximate solution of the kinematics. The second is to compensate the approximate solution in order to obtain the accurate one. It was confirmed that the computation time for the new algorithm was decreased to as little as 1/69-1/81 that for the Newton method. It was also confirmed experimentally that the algorithm was successfully applicable to an on-line control system.

Position control of a parallel link manipulator using electro-rheological valve actuators

This paper presents a position control of a three degree-of-freedom (d.o.f.) parallel link manipulator using ER (electro-rheological) valve actuators. Following the composition of an ER fluid consisting of chemically treated starch particles and silicon oil, Bingham model of the ER fluid is empirically obtained as a function of electric fields. Three sets of cylindrical ER valves are then manufactured, and field-dependent pressure drops and response time are evaluated. A three d.o.f. closed-loop parallel link manipulator activated by ER valves bridge circuits is constructed, and its governing equations of motion for heave, roll and pitch are derived using Lagrange equation. Subsequently, sliding mode controllers are formulated by considering nonlinear characteristics of the system and also parameter uncertainties due to the load change on the platform. Computer simulation and experimental implementation of the controllers are undertaken in order to demonstrate effective position tracking responses of the proposed control system. In addition, a comparative work for tracking responses between the sliding mode controller and PID controller is made.

Development of micromanipulator and haptic interface for networked micromanipulation

Telemicromanipulation systems with haptic feedback, which are connected through a network, are proposed. It is based on scaled bilateral teleoperation systems between different structures. These systems are composed of an original 6 degree of freedom (DOF) parallel link manipulator to carry out micromanipulation and a 6-DOF haptic interface with force feedback. A parallel mechanism is adopted as a slave micromanipulator because of its good features of accuracy and stiffness. The system modeling and control of the parallel manipulator system are conducted. Parallel manipulator feasibility as a micromanipulator, positioning accuracy and device control characteristics are investigated. A haptic master interface is developed for micromanipulation systems. System modeling and a model reference adaptive controller are applied to compensate friction force, which spoils free motion performance and force response isotropy of the haptic interface. These systems aim to make the micromanipulation more productive constructing a better human interface through the microenvironment force and scale expansion.

Kinematic and Singularity Analyses of a Six DOF 6-3-3 Parallel Link Machine Tool

A parallel link manipulator is considered to be an important construction feature of machine tools in the future. To construct a parallel link machine tool which can be practically applied for machining, the analysis of singularity is essential. A six degrees-of-freedom 6-3-3 parallel link machine tool was considered to investigate the effects of the design parameters on the singularity of the system. The U matrix method, which is based on the Plucker coordinate system, is adopted to investigate the singularity of the machine tool. The results show that a little variation of the orientation of the tool frame can effectively avoid the singular condition during the machining processes. This result is important for the applications of parallel link machine tools in real machining processes.

Computation of stiffness and stiffness bounds for parallel link manipulators

Parallel link spatial mechanisms in general, and Stewart Platforms in particular, are increasingly being studied for possible use in multi-axis machine-tools. An important consideration in the design of such machines is their stiffness. For a given design, stiffness varies with the direction in which it is computed, the posture (or configuration) of the mechanism and the direction of the actuation or disturbing force. This paper addresses the problem of finding the minimum and maximum stiffnesses and the directions in which they occur for a manipulator in a given posture. In addition, the computation of stiffness in an arbitrary direction is also discussed. Since engineers are often interested in the response of the mechanism in the direction of perturbation (called single-dimensional or engineering stiffness in this paper), this paper proves the computed bounds (maximum and minimum) are tight for such a definition. Stiffness computed using the algebraic formulae derived are compared to those obtained from a Finite Element Analysis model to demonstrate correctness of formulation. Finally, minimum, maximum, arbitrary direction and single-dimensional stiffness maps, are produced for a Stewart Platform and their use in machine tool design is discussed.

PARA-Arm: A five-bar parallel manipulator with singularity-perturbed design

We propose a new singularity-perturbed design approach to a planar five-bar parallel-link manipulator. The design allows us to operate the manipulator either in parallel or in serial branch mode, and also to exchange those modes. Thus, it is possible to merge some of the well-known advantages of serial and parallel manipulators. We study two basic singularity-perturbed designs and show that one of them is preferable. A feasibility study through computer simulation, including maximum torque requirement, is also presented.

ゴム人工筋６自由度パラレルリンクマニピュレータの積分型確率ファジィ制御

A robust control approach is described for controlling the position and orientation of a six-DOF parallel link manipulator, in which one prismatic link consists of a rubber muscle actuator, so that the controlled object contains several nonlinear factors. The position of the link is also assumed to be measured by using a linear potentiometer. As a result, the associated measurement system contains some measurement noises. The control system is construted using a stochastic fuzzy controller with an integlator so as to remove a tracking error. To design the feedback gains of the stochastic fuzzy controller, a mmn-max control strategy is adopted to the systems, with unknown physical parameters. The effectiveness of the proposed method is illustrated by using some actual experiments.

Singularity-consistent path planning and motion control through instantaneous self-motion singularities of parallel-link manipulators

The newly proposed singularity-consistent path tracking approach is applied to nonredundant parallel-link manipulators. We analyze the singularities of such manipulators under the assumption that the output-link moves on a pre-defined and parameterized path. Special attention is paid to the so-called instantaneous self-motion type singularity. We propose a velocity-command generator type closed-loop controller that guarantees asymptotic stability when tracking paths through such singularities. As a comprehensive analytical example we use a planar five bar mechanism. Results from computer simulations with the five bar mechanism and the HEXA parallel robot are also presented. © 1997 John Wiley & Sons, Inc.

Force Control of Parallel Link Manipulator.

We propose a force control algorithm for a Stewart Platform parallel link manipulators. First, we design a control scheme for a one degree-of-freedom hydraulic actuator, then we extend the control scheme to the parallel link manipulator. The hybrid force/position control scheme for the parallel link manipulator is also proposed. The control scheme design is based on a velocity based force control algorithm so that we can use velocity-controlled hydraulic actuators as parallel link manipulators. The proposed algorithm is applied to an experimental system and the results illustrate the validity of the system.

Computation of Parallel Link Manipulator Dynamics.

This paper proposes an algorithm, for calculating the inverse dynamics of link manipulators. First, we derive a method to calculate the inverse dynamics of a general type of parallel link mechanism using the method proposed by Luh and Zeng., which is developed for a simple closed link mechanism and yields the inverse dynamics of the simple closed link mechanism using the Newton-Euler method of serial link manipulators. We then derive a calculation algorithm for the inverse dynamics of a Stewart Platform. The algorithm is applied to a dynamic simulation of a Stewart Platform by a unit-vector method, and the simulation results demonstrate the validity of the method.

Input/Output Force Analysis of Parallel Link Manipulator.

This paper proposes a method to evaluate the kinematic structure of parallel link manipulators, based on the output force/torque of the mechanisms. As is well known, the relationship between the actuator force/torque and the output force/torque of manipulators is expressed by a manipulator Jacobian. In the conventional method based on the Jacobian matrix, both force and torque are analyzed simultaneously. However, it is not natural to deal with both force and torque simultaneously, since the force and torque have different dimensions. In this paper, we consider the output force and the output torque independently, and introduce the translatability and the rotatability of the manipulators. The kinematic structure is then analyzed based on the translatability and the rotatability of the manipulators. The translatability and the rotatability depend on the maximum output force and output torque of the manipulators, respectively, for given actuator forces. The proposed method is more natural than the conventional method and is useful for the design of the manipulator kinematic structure. An example illustrates the method.

Dynamic control of a parallel link manipulator using a CMAC neural network

A multiple neural network structure called a cascaded CMAC (Cerebellar Model Arithmetic Computer) is proposed which is able to perform dynamic control of a parallel link manipulator. The cascaded CMAC network achieves faster learning than conventional neural networks, and captures both general trends and fine details of an unknown nonlinear mapping. Simulation results are included in this paper.

The NIST robocrane

AbstractThe 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.

Ｓｔｅｗａｒｔ Ｐｌａｔｆｏｒｍ型パラレルリンクマニピュレータのＦｏｒｗａｒｄ Ｋｉｎｅｍａｔｉｃｓ計算アルゴリズム

This paper proposes a method to calculate the forward kinematics of a Stewart Plaftorm type of parallel link manipulators. The Stewart Platform consists of a base platform, a moving platform, and linear actuators connecting the both platforms. First we introduce a virtual parallel link mechanism, having the same kinematic structure as the original parallel mechanism. We assume that the moving platform of the virtual mechanism is directly driven by actuators and the limbs connecting both platforms have no actuators. A trajectory tracking control system is designed for this virtual mechanism based on the resolved motion rate control, so that the displacements of limbs are controlled by the actuators driving the moving platform. We calculate the forward kinematics by numerical simulation of the control system ; the position and orientation of the moving platform of the virtual mechanism is a solution for given displacements of limbs, when the tracking error of limbs converges to zero. The method is effective for a time varying displacements of limbs as long as the displacements are differentiable about time. The position and orientation of the moving platform is obtained without error, once the tracking error converges to zero. The simulation results illustrate the method.

The NIST SPIDER, A Robot Crane.

The Robot Systems Division of the National Institute of Standards and Technology 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. So 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 SPIDER (Stewart Platform Instrumented Drive Environmental Robot) 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 SPIDER can perform a variety of tasks. Examples are: cutting, excavating and grading, shaping and finishing, lifting and positioning. A 6 m version of the SPIDER has been built and critical performance characteristics analyzed.

Stiffness Study of a Parallel Link Robot Crane for Shipbuilding Applications

This describes the first phase of an effort to develop a robot crane for shipbuilding applications. The focus of this phase is on the study of the stiffness characteristics of this robot as a function of its geometry payload and height. A brief description of the design of the main part of the robot crane consisting of a six-wire parallel link manipulator is given. The stiffness of the manipulator to side loads and moments was studied. The nonlinear and linearized mathematical model of the manipulator stiffness matrix is derived. Stiffness measurement tests were conducted using a small size laboratory model. The results of these tests for various external loads, heights, and payloads are given. Computer simulation and theoretical results are also discussed.

Computer simulation of a parallel link manipulator

A parallel link manipulator has been designed, which may be used as a robot wrist. The dynamic equations of the system have been formulated rigorously without assuming that the displacements and rotations are small. In computer simulation, it is shown that this manipulator can be used to perform tasks such as position control, path tracing, and force control. For each task, the control algorithm is formulated and tested.

Characteristics and Mechanism Analysis of Parallel Link Manipulator

In this paper the characteristics of the parallel link manipulator are discussed in comparison with a conventional serial link manipulator in terms of kinematics and statics. The parallel mechanism is analyzed by using a singular value decomposition of the transposed Jacobian for static case. In this analysis a performance index is introduced that indicates how the mechanism produces force and moment at its end. A suboptimal mechanism is proposed that has a better performance index than the conventional Stewart platform that was originally proposed as a parallel 1ink mechanism.