Showing papers on "Parallel manipulator published in 1988"

The optimum kinematic design of a spherical three-degree-of-freedom parallel manipulator

Abstract: Optimisation de la cinematique d'un manipulateur utilisant un mecanisme parallele spherique a trois degres de liberte

Kinematic analysis of a three degrees of freedom in-parallel actuated manipulator

Abstract: The dynamic analysis of a three-degrees-of-freedom in-parallel actuated manipulator is presented. The equations of motion have been formulated in joint-space using the Lagrangian approach. The analysis provides the solution to predict the forces required to actuate the links so that the manipulator follows a predetermined trajectory. A dynamic simulation program illustrates the influence of the link dynamics on the actuating force required. An example of tracing a helical path is chosen to illustrate the dynamic simulation and to show that the Cartesian position of the moving platform may be controlled at a sacrifice of orientation freedoms. The dynamic analysis provides a basis for future theoretical research to develop the control scheme, for experimental research to estimate the inertia parameters, and for design optimization of the prototype manipulator. >

Inverse dynamic analysis and simulation of a platform type of robot

Abstract: This article presents an algorithm to solve the inverse dynamics for platform type of manipulators using Newton-Euler equations of motion. We found that the inverse dynamics of the system is governed by thirty-six linear equations. The number of these simultaneous equations can be reduced to six, if a proper sequence is taken. The relationships between the actuating forces and the shape of the structure are analyzed. Based on the algorithm, computer code for simulation was developed. Three cases were studied. As a result, configurations which minimize the actuating forces are suggested. It is also found that the fluctuation of the driving forces is lesser when the path is closer to the center of the base. These results are believed to be useful in the design and control of this type of manipulating devices.

Singular configurations of parallel manipulators and Grassmann geometry

Abstract: Parallel manipulators have a specific mechanical architecture where all the links are connected both at the basis and at the gripper of the robot. By changing the lengths of these links we are able to control the positions and orientations of the gripper. In general, for a given set of links lengths there is only one position for the gripper. But in some cases more than one solution may be found for the position of the gripper : this is a singular configuration. To determine these singular configurations the classical method is to find the roots of the determinant of the jacobian matrix. In our case this matrix is complex and it seems to be impossible to find these roots. We propose here a new method based on Grassmann line-geometry. If we consider the set of lines of P3, it constitutes a linear variety of rank 6. We show that a singular configuration is obtained when the variety spanned by the lines associated to the robot links has a rank less than 6. An important feature of the varieties of this geometry is that they can be described by simple geometric rules. Thus to find the singular configurations of parallel manipulators we have to find the configuration where the robot matches these rules. Such an analysis is performed on a special parallel manipulator and we show that we find all the well-known singular configurations but also new ones.

Kinematic analysis for a six-degree-of-freedom 3-PRPS parallel mechanism

Abstract: A Stewart-platform-based six-degree-of-freedom parallel mechanism is presented that can be used as a general-purpose spatial manipulator arm. The system consists of an output platform which is connected to a fixed base by means of three PRPS (parameters P, R, and S denote the prismatic, revolute, and spherical joints) subchains. All prismatic joints in this mechanism are active inputs which control the platform's motion. The author provides a detailed investigation describing the mechanism and analyzing its forward and reverse position functions. A closed-form solution is presented to obtain the required inputs for a desired position and orientation of the output platform. A forward position analysis of this mechanism is formulated which can be solved numerically to determine the platform's position and orientation for a set of given inputs. The author examines the workspace and uses screw theory to identify the geometric singularities of the manipulator in order to avoid undesirable robot configurations. >

Robot calibration and compensation

Abstract: A method is presented for calibrating and compensating for the kinematic errors in robot manipulators. A method of selecting a set of independent kinematic errors for modeling any geometric errors in a manipulator's structure is developed. A calibration algorithm is presented for finding the values of these kinematic errors by measuring the end-effector position. These kinematic errors are experimentally determined for a PUMA 560 six-joint manipulator. Two general-purpose compensation algorithms are developed and the improvement in the Cartesian position of the end-effector is experimentally measured and these results are presented. The results show that the positioning accuracy of a robot manipulator can be substantially improved using a relatively simple technique for measuring the Cartesian position of a tool attached to the end of the manipulator. >

Augmented Object and Reduced Effective Inertia in Robot Systems

Abstract: The paper investigates the dynamic characteristics and control of robot systems involving combinations of parallel and serial mechanical structures, e.g. multiple manipulators and macro/micro-manipulators. A framework for the analysis and control of multiple manipulator systems with respect to the dynamic behavior of the manipulated object is developed. A multi-effector/object system is treated as an augmented object representing the total masses and inertias perceived at some operational point. This system is actuated by the total effector forces acting at that point. The allocation of forces is based on the minimization of the total joint actuator efforts. For serial structures, the effective inertial characteristics of a combined macro/micro-manipulator are shown to be dominated by the inertial characteristics of the micro-manipulator. A new approach for a dextrous dynamic coordination of such mechanisms based on treating the combined system as a single redundant manipulator while minimizing of the deviation from the neutral (mid-range) joint positions of the micro-manipulator is proposed.

Force-feedback control of parallel manipulators

Abstract: The main features of a parallel manipulator are summarized, and it is shown that their passive compliance can be completely controlled. This feature is particularly important for improving the efficiency of force-feedback control methods and the stability. A prototype parallel manipulator is designed which includes force sensing. A novel force-feedback method is implemented and assembly experiments are performed with parts of various shape and rigidity and with very tight clearance (less than 10 mu ). One purpose of these experiments was to determine the importance of the passive compliance of the manipulator. It is shown clearly that the force-feedback method greatly improves the insertion process. >

Initial Experiments on the Control of a Two-Link Manipulator with a Very Flexible Forearm

Abstract: Understanding how to control the flexibility in mechanical manipulator allows us to increase their performance and to reliably design and control larger, lighter, and more power-efficient manipulators. To date, a number of papers and some experiments have addressed the control of single-link flexible manipulators. In order to understand fully and to control a larger, more useful class of manipulators, we need to extend our knowledge further to the two-link configuration. To this end, this paper addresses the dynamics and control of a two-link manipulator with a very flexible forearm. Further, to verify the theoretical developments, an experimental planar two-link flexible manipulator system has been constructed. A nonlinear model of the experimental manipulator is developed using an assumed-modes description for the deflections in the forearm link. The design of an initial collocated proportional-plus-derivative controller is presented and implemented on the experimental manipulator. For a step command of 10° in each joint, the simulation and experimental responses are found to be in excellent agreement.

Parallel manipulators. Part 2 : Theory. Singular configurations and Grassmann geometry

Abstract: Parallel manipulators have a specific mechanical architecture where all the links are connected both at the basis and at the gripper of the robot. By changing the lengths of these links we are able to control the positions and orientations of the gripper. In general, for a given set of links lengths there is only one position for the gripper. But in some cases more than one solution may be found for the position of the gripper : this is a singular configuration. To determine these singular configurations the classical method is to find the roots of the determinant of the jacobian matrix. In our case this matrix is complex and it seems to be impossible to find these roots. We propose here a new method based on Grassmann line-geometry. If we consider the set of lines of P3, it constitutes a linear variety of rank 6. We show that a singular configuration is obtained when the variety spanned by the lines associated to the robot links has a rank less than 6. An important feature of the varieties of this geometry is that they can be described by simple geometric rules. Thus to find the singular configurations of parallel manipulators we have to find the configuration where the robot matches these rules. Such an analysis is performed on a special parallel manipulator and we show that we find all the well-known singular configurations but also new ones.

Multirobot plan generation in a continuous domain: planning by use of plan graph and avoiding collisions among robots

Abstract: A robot plan-generation system is described that treats continuous state changes over time for multiple robots. A model for a continuous domain is represented, and a parallel plant-generation system, based on production rules for multiple robots in this domain, is proposed. The system consists of a fundamental planning subsystem for multiple robots and a subsystem for detecting and avoiding mutual collisions of cylindrical-type robots, called PLAMAT and SYDAMUC, respectively. In addition to examples for each subsystem, an assembly problem is solved as an example for the total plan-generation system, and the usefulness of the system is confirmed. >

Instruction system of remote-control robot

Abstract: An instruction system of a remote-control robot having a work manipulator. The system comprises an instruction manipulator for providing work instructions to the work manipulator; a solid geometrical model forming device for forming solid geometrical models of the work manipulator and work circumstances; a simulating device for operating the positions and attitude of the geometrical models according the information of motions of the instruction manipulator and information from various sensors of the work manipulator to simulate geometrical models, as well as driving the work manipulator according to the information from the instruction manipulator, etc.; and a graphic display for displaying motions of the geometrical models. The solid geometrical model forming device forms geometrical models of the work manipulator and work circumstances, the simulating device simulating actual working positions of the geometrical models, and the graphic display displaying the simulated geometrical models. After that, the work manipulator is driven according to the instruction information. Thus, an operator can give work instructions to the work manipulator through the instruction manipulator while monitoring, at a real time, motions of the geometrical models displayed on the graphic display.

An efficient algorithm for the adaptive control of a manipulator

Abstract: A novel algorithm is described for the adaptive control of a robot manipulator which may contain closed kinematic loops. The algorithm identifies the mass properties of each link and the viscous friction coefficients for each joint of the manipulator. It is similar to the Newton-Euler inverse dynamics algorithm and hence obtains its computational efficiency through the recursive nature of the algorithm. >

A parallel solution to robot inverse kinematics

Abstract: Introduces an algorithm by which the inverse kinematics of a robot manipulator with closed-form solution can be computed in parallel to reduce the computational complexity roughly by a factor of n, the number of joints of the manipulator. The authors study the errors introduced by the algorithm by constructing statistical models of the errors. They evaluate the algorithm on a PUMA 260 manipulator, which has six degrees of freedom with six revolute joints and a reach of approximately 0.5 m. >

On-line generation of collision-free trajectories for multiple robots

Abstract: An online algorithm is proposed to test interferences and generate collision-free trajectories for the motions of loosely coordinated multiple robot manipulators controlled independently by means of PD (proportional differential) controllers. A hierarchical multiple-robot-control architecture is proposed. The approach is demonstrated by simulation studied. >

Optimal control of a single link flexible manipulator

Abstract: An exact state-space dynamic model for manipulator with flexible links is considered. The Bernoulli-Euler beam equations are used to derive a frequency domain matrix transfer function. This transfer function is then used to computer the Laplace transform of the state vector as a function of lateral position along a single link manipulator. The problem of optimal end-point control of the beam is addressed. A sixth-order state-space model is derived for the manipulator and the controller is based on this model. Several control laws are studied for this model. The manipulator is modeled as eighth-order, but the control law based on the sixth-order model is retained. The six states are estimated from the output of the eighth-order model. These states are fed back to the controller to derive the control torque used to drive the manipulator. A filter is introduced to compensate for spillover. The results are satisfactory, and are illustrated by simulated case studies. >

Analysis of kinematic singularities for robot manipulators in Cartesian coordinate parameters

Abstract: The authors analyze the singularity states of robot manipulators; only the manipulator inverse kinematic relationships are considered. In such relationships, three types of triangular equations arise for the kinematically general manipulator with the nominal decoupling conditions as found in conventional robots. These equations reveal the manipulator structural properties and the Cartesian configurations of the end effector in which the manipulator is singular; they may be used to determine the singularity states of a given trajectory. The method of singularity analysis presented is applicable to robots with six or less degrees of freedom. >

The GEC Tetrabot-a new serial-parallel assembly robot

Abstract: An assembly robot, the GEC Tetrabot (tetrahedral robot) is presented. It includes a parallel structure consisting of 3 threaded actuator rods which form the edges of a tetrahedron. The equilateral triangle base is part of the support structure for the manipulator. Each threaded rod is free to pivot at its attachment point about 2 axes which are perpendicular to the rod axis. The rods are driven in axial translation via a recirculating ball screw assembly, which minimises friction. a 3 axis wrist, attached to the lower end of the radial arm, provides the remaining 3 degrees of freedom. The wrist mechanism is conventional, comprising three further links arranged serially. The design thus combines the advantages of serial and parallel mechanisms, leading to a stiffer, faster, more accurate robot. >

Singularity, uncertainty and compliance of robot manipulators

Abstract: Kinematic singularities of the manipulator Jacobian are studied. For generic kinematic maps it is shown that singular points form smooth manifolds of prescribed dimension in the joint space of the manipulator. For three-joint robots, a condition for genericity using determinants is derived. The condition lends itself to symbolic computation and is sufficient for the study of decoupled manipulators. The decoupled problems, viz., orientation singularity and translation singularity, are studied in detail. A complete characterization of orientation singularities of robots with any number of joints is given. The translation singularities of the eight possible topologies of three-joint robots are also studied. A complete description of the singularities is given in the simpler cases, and the conditions on the link parameters for genericity are determined. For the more complicated cases, the singularities are studied after making certain assumptions about the link parameters. The uncertainty of a robot manipulator which is used to perform a task with a specified tolerance is considered. A formula is derived for the efficient computation of a tight bound on the uncertainty of the end-effector, given the uncertainty in the kinematic parameters of the robot. It is shown that the total uncertainty is the Minkowski difference of the manipulator uncertainty and the task position uncertainty. Simulations are performed in which the results are used to determine configurations of a robot for which the total uncertainty is within a specified tolerance. The suitability of the compliance of a manipulator for performing a planar peg-in-hole type assembly task is studied. Manipulators are modeled as having rigid links and compliant joints, following experimental results. It is shown that any symmetric positive semi-definite compliance at the end-effector can be realized by a manipulator of the above type. A new condition on the stiffness is proposed, for preventing jamming. If the peg is supported by the end-effector of a robot, we can determine configurations of the robot at which jamming can be avoided. Simulations are performed to compute the no-jam configurations of a manipulator.

Structure design and kinematics of a robot manipulator

Abstract: In this paper we show that a robot manipulator with 6 degrees of freedom can be separated into two parts: arm with the first three joints for major positioning and wrist with the last three joints for major orienting. We propose 5 arms and 2 wrists as basic construction for commercially robot manipulators. This kind of simplification can lead to a general algorithm of inverse kinematics for the corresponding configuration of different combinations of arm and wrist. The approaches for numerical solution and closed form solution presented in this paper are very efficient and easy for calculating the inverse kinematics of robot manipulator.

Dynamic model of two-link robot manipulator with both structural and joint flexibility

Abstract: Equations were developed which describe the motion of a robot manipulator which incorporates both structural and joint flexibility. The emphasis was on the dynamic model of a one-link flexible manipulator to show how critical it is to incorporate joint flexibility and to demonstrate the significance of cross coupling among state variables for small deflection. A set of decoupled dynamic equations was obtained based on the assumed-mode method and on orthogonality relations. By comparison to a model which incorporates cross coupling, it was shown that it is sufficiently accurate to use the decoupled dynamic equations for small joint and structural deflections. Simulations of varying joint stiffness characteristics indicates that there is no rigid mode for the flexible joint system. As such, both structural and joint flexibility must be considered in the analysis and control of such systems.<>

Robot Manipulator Control and Computational Cost

Abstract: In this paper robot control algorithms are considered from the point of view of computational complexity. All the algorithms provide for hybrid position and force control which is appropriate to unstructured environments. Both joint coordinate and generalized coordinate motion schemes are considered. The findings are interesting in that the peripheral processing involved in all methods, sometimes, swamps the orders of magnitude differences in computation involved in the inner loops. It is also interesting to note the similarity in complexity between direct methods and differential. Between joint coordinate methods and generalized coordinate methods there is a general two to one ratio in complexity. In general a 0.75 M Flop, 1.25 Mip machine is required to provide for a control rate of 250 hz. Comments University of Pennsylvania Department of Computer and Information Science Technical Report No. MSCIS-88-10. This technical report is available at ScholarlyCommons: http://repository.upenn.edu/cis_reports/621 ROBOT MANIPULATOR CONTROL AND COMPUTATIONAL COST YehongZhang Richard P. Paul MS-CIS-88-10 GRASP LAB 134 Department of Computer and Information Science School of Engineering and Applied Science University of Pennsylvania Philadelphia, PA 191 04

A Cartesian tensor approach for fast computation of manipulator dynamics

Abstract: Orthogonal second-order Cartesian tensors are used to formulated the Newton-Euler dynamic equations for a robot manipulator. Based on this formulation, an efficient recursive procedure is developed to evaluate the joint torques. The procedure is applicable to all rigid-link manipulators with open-chain kinematic structures with revolute and/or prismatic joints. For simplicity of presentation, only manipulators with (kinematically more complex) revolute joints are considered. An efficient implementation of the proposed method shows that the joint torques for a six-degree-of-freedom manipulator with revolute joint, can be computed in approximately 500 multiplications and 420 additions. For manipulators with 0 degrees or 90 degrees twist angles, the required computations are reduced to 380 multiplications and 315 additions. >

Fuzzy Control of Robot Manipulator

Abstract: Common fuzzy control regulation causes that the system of robot controlled produces oscillation and static error . In order to sovle the problem , this paper proposes a new mixed fuzzy control method of robot manipulator Simulation and control experiment to a robot manipulator have shown that this kind of mixed fuzzy control system has move excellent dynamic performances than PID regulation control and it makes robot obtain satisfactory static behavious.