Yoshisada Murotsu

Reliability-based optimum design of a symmetric laminated plate subject to buckling

This study is concerned with the buckling reliability maximization of a symmetric laminated composite plate with respect to the mean ply orientation angle. The reliability is evaluated by modelling the buckling failure as a series system consisting of potential eigenmodes. The mode reliability is obtained by the first-order reliability theory (FORM), where material constants and orientation angles of individual layers, as well as the applied loads are treated as random variables. In order to keep track of the intended buckling mode during the reliability analysis, the mode tracking method is utilized. Then, the failure probability of the series system is approximated by Ditlevsens upper bound. The reliability maximization problem is formulated as a nested problem with two levels of optimization. Through numerical calculations, the reliability-based design is demonstrated to be important for the structural safety in comparison with the deterministic buckling load maximization design.

Theoretical and experimental studies for continuous path control of flexible manipulator mounted on a free-flying space robot

Some continuous path tracking control schemes for flexible manipulators are proposed by using a concept of virtual rigid manipulator. An extended local PD-control (extended LPDC), pseudo resolved motion rate control (pseudo RMRC), and pseudo resolved acceleration control (pseudo RAC) for flexible manipulators are presented. To suppress vibrating motion, a composite control with a reduced-order modal control is developed. To verify the stability, a sufficient condition for asymptotic stability of PD-control is derived. It is also clarified that the PD-control damps all the vibration modes irrespective of their natural damping. The validity of the proposed control schemes is explained through the singular perturbation method. The robust stability of the proposed control schemes is discussed. The effectiveness of the proposed control schemes is demonstrated through hardware experiments using a flexible manipulator mounted on a freeflying satellite.

HARDWARE EXPERIMENTS OF SPACE TRUSS ASSEMBLY BY AUTONOMOUS SPACE ROBOT

This paper addresses an experimental system simulating a free-flying space robot, which has been constructed to study autonomous space robots. The experimental system consists of a space robot model, a frictionless table system, a computer system, and a vision sensor system. The robot model composed of two manipulators and a satellite vehicle can move freely on a two-dimensional planar table without friction by using air-bearings. The robot model has successfully performed the automatic truss structure construction including many jobs, e.g., manipulator berthing, component manipulation, arm trajectory control avoiding collision, assembly considering contact with the environment, etc. Moreover, even if the robot fails in a task planned in advance, the robot accomplishes it by re-planning task-sequence using reinforcement learning. The experiment demonstrates the possibility of the automatic construction and the usefulness of space robots.

System identification and resolved acceleration control of space robots by using experimental system

An experimental system is constructed to evaluate performance and feasibility of modeling, identification and control methods for space robots. The experimental system achieves a two-dimensional weightless condition by means of a horizontal table and air-pads. As a preliminary experimental study, two identification methods for space robots are discussed. The first identification method is a conventional method for manipulators on an inertially fixed base. The second identification method can determine dynamic parameters of space robots under the condition that the satellite base is free to both translate and rotate. This method is based upon the conservation laws of linear and angular momentum of a space robot. Finally implementation schemes of resolved acceleration control are considered for the experimental system.<<ETX>>

Efficient formulation of inverse dynamics and control application to a planar variable geometry truss

This paper addresses an efficient formulation of inverse dynamics and an application to the control of a planar variable geometry truss (VGT). The VGT is composed of many members in comparison to its degree of freedom (N), i.e., the number of generalized coordinates after constraints are considered, since the VGT has many topological closed loops. By using a Lagrangian method, an efficient order N formulation of the inverse dynamics is realized in this paper because the derived equations of motion have a recursive form. Furthermore, the derived equations contain geometric constraints due to the topological closed loops and we do not have to add calculations for those constraints. The proposed method needs fewer calculations in comparison with the Newton-Euler method with constraint equations. The effectiveness of the proposed method for inverse dynamics is examined by a numerical simulation as well as a hardware experiment of a resolved acceleration control. The experimental result successively shows that the controller using the proposed method is well implemented in the hardware system with one millisecond sampling time, and it achieves a good control performance comparable to that shown by a numerical simulation.

A study on control of planar variable geometry truss used for autonomous docking system

This paper is concerned with a control of a redundant variable geometry truss (VGT). A dynamics-based control cannot guarantee the asymptotic stability of closedloop system if the controller directly uses a pseudo inverse of Jacobian matrix. To avoid this problem, a resolved acceleration control is proposed with the redundancy considering. The control scheme can achieve the asymptotic stability in spite of the redundant system. To make the control be a real-time one, an efficient formulation of inverse dynamics for the VGT is used. Then an experimental docking system of VGT is constructed, which is a redundant system with many sensors and actuators. The experimental system is composed of a docking mechanism, a frictionless table system, a computer system, and a vision sensor system. Feasibility and effectiveness of the control method are examined by numerical simulations and experiments using the developed system.

Manipulation variable feedback control of flexible manipulators by using virtual rigid manipulator concept

A concept of virtual rigid manipulator (VRM) of flexible manipulators for stable manipulation variable feedback controls is defined. Several control schemes are presented for flexible manipulators by using VRM concept, Le., an extended local PD-control (extended LPDC), pseudo resolved motion rate control (pseudo RMRC) , pseudo resolved acceleration control (pseudo RAC), and a composite control with a reduced-order modal control. To show their stability, a sufficient condition for asymptotic stability of PD-control is derived. It is clarified tha t the resolved motion rate control using the VRM is equivalent t o the PD-control of joint variables. In the same manner, t he proposed control schemes are transformed into joint variable feedback by using the VRM. Therefore, the proposed control schemes are collocated controls of joint variable feedback, and their stability is guaranteed.

Inertia Parameter Identification of Unknown Objects Handled by a Space Robot

本論文では, マニピュレータと移動用衛星から構成されるフリー・フライング・スペースロボットが未知物体を把持している場合に, その慣性パラメータを同定する二つの方法について論じている.一方は運動量と角運動量の保存則に基づく方法で, 他方は, Newton-Eulerの運動方程式に基づく方法である.これらの同定手法の特長は, 力やトルクの情報を使わずに, 並進と角度に関する速度と加速度を使う点である.提案手法の有効性は, 数値シミュレーションおよび地上模擬実験によって検証される.

Structural Systems Reliability Assessment of Semi-Submerged Catamaran

Reliability-based design approaches are needed for novel ships whose design and operational experiences are few and which are subject to external loads of random nature such as wave-induced loads. This paper deals with reliability assessment of Semi-Submerged Catamaran (SSC), one of the novel ships. An SSC is modeled as a spatial framework to which side force under beam sea is applied. Structural systems reliability analysis is carried out by using the method in which probabilistic plastic collapse analysis and failure mode analysis are combined. Effects of some design alternations on systems reliability are also discussed.Main conclusions are summarized as follows;(1) Reliability assessment is necessary for a novel ship such as an SSC in order to realize a rational design of the structure.(2) Systems reliability is indispensable because component reliability can not necessarily predict structural safety of the vessel.(3) Important design parameters are clarified through parametric studies.(4) Importance factor of members under damaged conditions is proposed.

Contribution to the Identification of Dominant Failure Modes in Structural Systems

This paper is concerned with the extension and application of a multiplication factor method to the identification of dominant failure modes in structural systems. First, the multiplication factor method proposed for a simple limit state function consisting of two basic variables, i.e., a resistance and a load, is extended to estimate the failure probabilities of the general cases where the resistance and the load effect are expressed as linear combinations of basic random variables. Second, the proposed method is compared through numerical examples with the advanced first-order second-moment method, and its effectiveness is verified. Third, the multiplication factor method is implemented to the automatic selection of dominant failure modes in structural systems by using the branch-and-bound method. Finally, the validity of the proposed procedure is demonstrated by identifying the dominant failure modes which include the non-normal basic variables.