End-effector In Space Research Articles

Robustness issues for kinematically redundant manipulator control

Kinematic redundancy of robotic manipulators has been known to be useful in accomplishing additional performance on top of the main task of tracking desired end effector trajectories. Robustness has been a major concern in manipulator control where exact physical properties of manipulators are unobtainable. In this paper, we review a method of analyzing robustness of joint space controllers for manipulators, and extend the analysis to end effector space control for the first time. We then generalize the approach to the case of kinematically redundant manipulators by introducing the redundancy as part of the manipulator states, and show that kinematic redundancy is useful in making manipulator controllers more robust. The final result of this paper is an optimization problem whose solution determines the self-motion which optimizes the system robustness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Review of the damped least-squares inverse kinematics with experiments on an industrial robot manipulator

The goal of this paper is to present experimental results on the implementation of the damped least-squares method for the six-joint ABB IRb2000 industrial robot manipulator. A number of inverse kinematics schemes are reviewed which allow robot control through kinematic singularities. The basic scheme adopts a damped least-squares inverse of the manipulator Jacobian with a varying damping factor acting in the neighborhood of singularities. The effect of a weighted damped least-squares solution is investigated to provide user-defined accuracy capabilities along prescribed end-effector space directions. An online estimation algorithm is employed to measure closeness to singular configurations. A feedback correction error term is introduced to ensure algorithm tracking convergence and its effect on the joint velocity solution is discussed. Computational aspects are discussed in view of real-time implementation of the proposed schemes. Experimental case studies are developed to investigate manipulator performance in the case of critical end-effector trajectories passing through and near the shoulder and wrist singularities of the structure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Kinematic Synthesis of Tendon-Driven Manipulators with Isotropic Transmission Characteristics

This paper presents a methodology for kinematic synthesis of tendon-driven manipulators with isotropic transmission characteristics. The force transmission characteristics, from the end-effector space to the actuator space, has been investigated. It is shown that tendon forces required to act against externally applied forces are functions of the structure matrix, its null vector, and the manipulator Jacobian matrix. Design equations for synthesizing a manipulator to possess isotropic transmission characteristics are derived. It is shown that manipulators which possess isotropic transmission characteristics have much better force distribution among their tendons.

Identification and Analysis of Robot Manipulator Singularities

The existence of singular positions inside the robot workspace is an inherent problem for task planning and robot control. At manipulator configurations near singular positions, very large joint motions are required to produce relatively small end-effector displacements. In the limit, when a singularity is encountered, the required joint motions become unbounded and the manipulator loses at least one degree of freedom. Identify ing the singularities in the robot workspace is not always an analytically tractable problem, except for manipulators with op portune placement of the joint axes. Typically, it is easier and more informative to evaluate the robot "arm" and "wrist" sin gularities, which reflect the limitations of the lower-order robot subsystems responsible for positioning and orienting, respec tively, the end effector in space. The objective of this article is to define analytically the interplay between the individual "arm" and "wrist" singularities and the global manipulator singularities. Specifically, a general formula that defines ma nipulator singularities in terms of the subsystem singularities is developed. The analysis leads to a novel, efficient method for identifying the singularities of six-axis manipulators. This is then illustrated through an example.

宇宙用マニピュレータによる浮遊物体収納時の制御

This paper describes a retraction control scheme for a space manipulator after grasping a floating object in space. Many control methods for a pre-retracting phase have been proposed by other authors. However those for a retracting phase have been very few, and the essential problem which an object is moving had not been treated so far. Firstly we divide a retracting process into four phases, and clearly show the problems to be resolved. Secondly we derive a new kinematic equation which relates angular velocities of joints with the integral of force and moment at an end-effector in space. Thirdly a force control method is proposed by using that important equation, and the possibility to grasp within the admissible error is proved effective: This control method corresponds to a velocity control for an end-effector feedbacking the integral of force and moment considering the derived kinematic equation. Fourthly we apply that control method to a retraction control, and demonstrate the stability of the proposed control scheme. Finally the effectiveness of the proposed retraction control method is shown by computer simulations.

Dynamic control of flexible, kinematically redundant robot manipulators

It is shown how choosing the self-motion inherent in redundant arms is crucially important when flexibility is present. The self-motion, by exciting or damping the flexural modes, alters the dynamic response of the arms. Kinematic redundancy can also be used in many cases to help damp out vibrations. This issue is examined, and control algorithms designed to regulate the flexibility while maintaining precise tracking of the end-effector trajectory are introduced. The controllers are of the computed torque type in end-effector space, and use self-motion of the links to reduce the flexible effects.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Use of Robots in Surgery: Development of a Frame for Prostatectomy

The potential contribution of robots in surgery is considerable, as they are capable of precise placement of their end effector in space with an accuracy that is impossible for a surgeon. One of th...

An ultrasonic ranging system for robot position sensing

Currently used robot position control systems consider only the joint angles when positioning an end effector in space. This approach can lead to serious errors due to structural deflection under load, backlash in joints, manufacturing tolerances, etc. A method is needed to directly measure the absolute position and orientation of the end effector in three-dimensional space in order to achieve positioning accuracy. An ultrasonic ranging system is proposed to accomplish this task. It consists of: (1) one or more transmitters mounted on the robot's end effector; (2) receivers (four or more required), mounted adjacent to the robot's work envelope; and (3) analog and digital electronics to drive the transducers, process the received signals, and calculate the position of the end effector. The end effector's position is calculated using the time of flight measured from the emitter to each receiver, by an intersection of spheres algorithm. The performance objectives of this system are: (a) measure position with accuracy of ± 0.001 in. (b) up to a range of 3 m. and (c) with at least 1-Hz update rate. A theoretical error analysis of the system is presented. Types of emitter signals are compared, operating frequency considerations are discussed, and hardware recommendations are made. [Work supported by NSF.]

Minimum distance collision-free path planning for industrial robots with a prismatic joint

A collision-free path is a path which an industrial robot can physically take while traveling from one location to another in an environment containing obstacles. Usually the obstacles are expanded to compensate for the body width of the robot. For robots with a prismatic joint, which allows only a translational motion along its axis, additional problems created by the long boom are handled by means of pseudoobstacles which are generated by real obstacle's edges and faces. The environment is then modified by the inclusion of pseudoobstacles which contribute to the forbidden regions. This process allows the robot itself again to be represented by a point specifying the location of its end effector in space. An algorithm for determining the shortest distance collision-free path given a sequence of edges to be traversed has been developed for the case of stationary obstacles.