A large deformation,rotation-free,isogeometric shell

Conventional finite shell element formulations use rotational degrees of freedom to describe the motion of the fiber in the Reissner–Mindlin shear deformable shell theory, resulting in an element with five or six degrees of freedom per node. These additional degrees of freedom are frequently the source of convergence difficulties in implicit structural analyses, and, unless the rotational inertias are scaled, control the time step size in explicit analyses. Structural formulations that are based on only the translational degrees of freedom are therefore attractive. Although rotation-free formulations using C⁰ basis functions are possible, they are complicated in comparison to their C¹ counterparts. A C^k-continuous, k ? 1, NURBS-based isogeometric shell for large deformations formulated without rotational degrees of freedom is presented here. The effect of different choices for defining the shell normal vector is demonstrated using a simple eigenvalue problem, and a simple lifting operator is shown to provide the most accurate solution. Higher order elements are commonly regarded as inefficient for large deformation analyses, but a traditional shell benchmark problem demonstrates the contrary for isogeometric analysis. The rapid convergence of the quadratic element is demonstrated for the NUMISHEET S-rail benchmark metal stamping problem.     

ON THE DESIGN OF STABLE AND RELIABLE INSTITUTIONS

We enunciate, as a general system-theoretic principle, that any functional activity of a system typically involves only a few of the underlying structural degrees of freedom of the system. This principle has a number of important consequences, some of which are explored in this note. In particular, it is suggested that the unreliability we observe in functional organizations, and their breakdown with time, lies primarily in the fact that structural degrees of freedom, not involved directly in a particular functional activity of a system, are free for other interactions. Since the structural degrees of freedom of a system are “linked” to each other through the equations of motion of the system, such nonfunctional interactions tend to progressively interfere with the original functional activity of the system. A variety of consequences of this behavior, for biological and human-designed organizations, are explored, and a variety of mechanisms for avoiding this intrinsic dialectic in social structures are indicated.     

Vehicle-type motion estimation by the fusion of image point and line features

Three-dimensional (3D) motion estimation is a very important topic in machine vision. However, reliability of the estimated 3D motion seems to be the most challenging problem, especially to the linear algorithms developed for solving a general 3D motion problem (six degrees of freedom). In real applications such as the traffic surveillance and auto-vehicle systems, the observed 3D motion has only three degrees of freedom because of the ground plane constraint (GPC). In this paper, a new iterative method is proposed for solving the above problem. Our method has several advantages: (1) It can handle both the point and line features as its input image data. (2) It is very suitable for parallel processing. (3) Its cost function is so well-conditioned that the final 3D motion estimation is robust and insensitive to noise, which is proved by experiments. (4) It can handle the case of missing data to a certain degree. The above benefits make our method suitable for a real application. Experiments including simulated and real-world images show satisfactory results.     

Forward dynamical analysis of flexible multibody systems using system-level model reduction

Fast simulation (e.g., real-time) of flexible multibody systems is typically restricted by the presence of both differential and algebraic equations in the model equations, and the number of degrees of freedom required to accurately model flexibility. Model reduction techniques can alleviate the problem, although the classically used body-level model reduction and general-purpose system-level techniques do not eliminate the algebraic equations and do not necessarily result in optimal dimension reduction. In this research, Global Modal Parametrization, a model reduction technique for flexible multibody systems is further developed to speed up simulation of flexible multibody systems. The reduction of the model is achieved by projection on a curvilinear subspace instead of the classically used fixed vector space, requiring significantly less degrees of freedom to represent the system dynamics with the same level of accuracy. The numerical experiment in this paper illustrates previously unexposed sources of approximation error: (1) the rigid body motion is computed in a forward dynamical analysis resulting in a small divergence of the rigid body motion, and (2) the errors resulting from the transformation from the modal degrees of freedom of the reduced model back to the original degrees of freedom. The effect of the configuration space discretization coarseness on the different approximation error sources is investigated. The trade-offs to be defined by the user to control these approximation errors are explained.     

ALE beam using reference dynamics

We present a small strain beam model based on the Arbitrary Lagrangian Eulerian setting for use in multibody dynamics. The key contribution of the present paper is to provide a formulation with large flexible reference motion and small overlaid deflections. We point out that the reference motion is described by actual degrees of freedom of the model. Therefore, we use a vector of generalized positions and an Eulerian coordinate, which itself is a degree of freedom and in which the flow of the beam material through an arbitrary volume is represented. The additional displacements describe small fluctuations around the reference motion. With this idea it is easy to separate the motion of belt drives, cable and rope ways or strings. In particular, the overlaid deflections are described for efficient numeric computation and may be analyzed in an easy way for vibrational behavior. The guiding reference motion is arbitrary, i.e., the transmission ratios are degrees of freedom and may change dynamically affecting also the fluctuations. Contacts with dry friction are foreseen and represented in the present model. It is validated and proven to be efficient in comparison with classic co-rotational and absolute nodal coordinate formulations in our application. The simulation of pushbelt continuously variable transmissions is taken as a high-dimensional industrial example.

     

A simple algorithm for intelligent manipulator collision-free motion

The collision-free planning of motion is a fundamental problem for artificial intelligence applications in robotics. The ability to compute a continuous safe path for a robot in a given environment will make possible the development of task-level robot planning systems so that the implementation details and the particular robot motion sequence will be ignored by the programmer.A new approach to planning collision-free motions for general real-life six degrees of freedom (d.o.f.) manipulators is presented. It is based on a simple object model previously developed. The complexity of the general collision detection problem is reduced, and realistic collision-free paths are efficiently found onCS planes. A heuristic evaluation function with a real physical sense is introduced, and computational cost is reduced to the strictly necessary by selecting the most adequate level of representation. A general algorithm is defined for 6 d.o.f. robots that yields good results for actual robot models with complex design structures with the aid of various heuristic techniques. The problem of adaptive motion is also considered.     

Synthesis of multi-DOF mechanisms by using connecting chains

A procedure for the synthesis of mechanisms with multiple degrees of freedom using connecting chains has been developed. The concepts of mobile and quasi-mobile spaces are defined with respect to the screw spaces formed by joint motors in the connecting chains. In a mechanism with multiple degrees of freedom such that a fixed link and an output link are connected by several chains, the output link can execute any motion belonging to the space which is given by the intersection of the screw spaces of the connecting chains. The condition that the screw spaces of the connecting chains are either mobile or quasi-mobile with respect to the motion of the output link is proved to be sufficient for this intersection of screw spaces to remain unchanged throughout the motion of the mechanism. Examples of the synthesis of mechanisms for translational motion are used to demonstrate that mechanisms with multiple degrees of freedom capable of executing prescribed motions can be synthesized by using this condition.     

Optimal path planning of redundant free-floating revolute-jointed space manipulators with seven links

The path planning of free-floating manipulators is of great interest in space operations. The manipulators in the free-floating mode exhibit nonholonomic characteristics due to the nonintegrability of the angular momentum, which makes the problem complicated. This paper analyzes the path planning of redundant, free-floating space manipulators with revolute joints and 7 degrees of freedom. The primary task of manipulators is to move the manipulator arms so that the desired end-effector position and orientation can be achieved. The motion of the manipulators can produce an attitude disturbance of the base, which has an adverse impact on the spacecraft operation. Thus, it is necessary to minimize the base attitude disturbance in order to reduce the fuel consumption for attitude maintenance. Practically, the path planning of redundant free-floating manipulators with higher degrees of freedom (7 degrees of freedom in this paper) in three-dimensional space is more complicated than path planning with fewer degrees of freedom, including planar or fixed base cases. This paper provides a tractable planning method to solve this problem, which could avoid the pseudo inverse of the Jacobian matrix. The sine functions, whose arguments are the polynomial functions with unknown coefficients, are used to specify the joint paths. The PSODE algorithm (particle swarm optimization combined with differential evolution) is applied to optimize the unknown coefficients of the polynomials in order to achieve the desired end-effector position and orientation and simultaneously minimize the base attitude disturbance. The simulations demonstrate that this method could provide satisfactory smooth paths for redundant free-floating space manipulators.     

Kinematics and the implementation of an elephant's trunk manipulator and other continuum style robots

Traditionally, robot manipulators have been a simple arrangement of a small number of serially connected links and actuated joints. Though these manipulators prove to be very effective for many tasks, they are not without their limitations, due mainly to their lack of maneuverability or total degrees of freedom. Continuum style (i.e., continuous "back-bone") robots, on the other hand, exhibit a wide range of maneuverability, and can have a large number of degrees of freedom. The motion of continuum style robots is generated through the bending of the robot over a given section; unlike traditional robots where the motion occurs in discrete locations, i.e., joints. The motion of continuum manipulators is often compared to that of biological manipulators such as trunks and tentacles. These continuum style robots can achieve motions that could only be obtainable by a conventionally designed robot with many more degrees of freedom. In this paper we present a detailed formulation and explanation of a novel kinematic model for continuum style robots. The design, construction, and implementation of our continuum style robot called the elephant trunk manipulator is presented. Experimental results are then provided to verify the legitimacy of our model when applied to our physical manipulator. We also provide a set of obstacle avoidance experiments that help to exhibit the practical implementation of both our manipulator and our kinematic model.     

Feedforward dynamics for the control of articulated multi-limb robots

We describe a general approach for using linearizing feedforward control inputs for large degree of freedom (dof) multi-limb robots operating in scenarios involving motion and force constraints, and under-actuated degrees of freedom arising from the task and the environment. Our solution is general and has low computational cost needed for real-time control loops. It supports the tuning of the feedforward term to meet multiple task objectives. Being structure-based, it is able to easily accommodate changes in motion and force constraints that often occur in robotics scenarios.     

Autonomous motion planning of a hand-arm robotic system based on captured human-like hand postures

The paper deals with the problem of motion planning of anthropomorphic mechanical hands avoiding collisions and trying to mimic real human hand postures. The approach uses the concept of “principal motion directions” to reduce the dimension of the search space in order to obtain results with a compromise between motion optimality and planning complexity (time). Basically, the work includes the following phases: capturing the human hand workspace using a sensorized glove and mapping it to the mechanical hand workspace, reducing the space dimension by looking for the most relevant principal motion directions, and planning the hand movements using a probabilistic roadmap planner. The approach has been implemented for a four finger anthropomorphic mechanical hand (17 joints with 13 independent degrees of freedom) assembled on an industrial robot (6 independent degrees of freedom), and experimental examples are included to illustrate its validity.     

Geometric Motion Control for a Kinematically Redundant Robotic Chain: Application to a Holonomic Mobile Manipulator

For kinematically redundant robotic manipulators, the extra degrees of freedom available allows freedom in the generation of the trajectories of the end‐effector. In this paper, for this scope, we use techniques for motion control of rigid bodies on Riemannian manifolds (and Lie groups in particular) to design workspace control algorithms for the end‐effector of the robotic chain and then to pull them back to joint space, all respecting the different geometric structures of the two underlying model spaces. The trajectory planner makes use of geometric splines. Examples of the different kinds of curves that are obtained via the De Casteljau algorithm in correspondence of different metric structures in SE(3) are reported. The feedback module, instead, consists of a Lyapunov based PD controller defined from a suitable notion of error distance on the Lie group. The motivating application of our work is a holonomic mobile manipulator for which simulation results are described in detail. © 2003 Wiley Periodicals, Inc.     

Cluster computing of mechanisms dynamics using recursive formulation

This paper presents the O(n) recursive algorithm for forward dynamics of closed loop kinematic chains adapted to parallel computations on a cluster of workstations. The Newton–Euler equations of motion are formulated in terms of relative coordinates. Closed loop kinematic chains are transformed into open loop chains by cut joint technique. Cut joint constraint and Lagrange multipliers are introduced to complete the equations of motion. Constraint stabilization is performed using the Baumgarte stabilization technique with application to multibody systems with large number of degrees of freedom. Numerical simulations are carried out to study the influence of the degrees of freedom of the multibody system on computational efficiency of the algorithm using the Message Passing Interface (MPI). We also consider the ways of minimization of communication overhead which has significant impact on efficiency in case of cluster computing.     

Error Analysis for a Hybridizable Discontinuous Galerkin Method for the Helmholtz Equation

Finite element methods for acoustic wave propagation problems at higher frequency result in very large matrices due to the need to resolve the wave. This problem is made worse by discontinuous Galerkin methods that typically have more degrees of freedom than similar conforming methods. However hybridizable discontinuous Galerkin methods offer an attractive alternative because degrees of freedom in each triangle can be cheaply removed from the global computation and the method reduces to solving only for degrees of freedom on the skeleton of the mesh. In this paper we derive new error estimates for a hybridizable discontinuous Galerkin scheme applied to the Helmholtz equation. We also provide extensive numerical results that probe the optimality of these results. An interesting observation is that, after eliminating the internal element degrees of freedom, the condition number of the condensed hybridized system is seen to be almost independent of the wave number.     

一类Lagrange系统的PSM问题

ＰＳＭ思想在「１」中第一次提出，得到了广泛的应用，本文根据ＰＳＭ思想，建立一个特殊模型，在非线性系统中拉格朗日系统的ＰＳＭ，民模型的ＰＳＭ控制。     

A search algorithm for motion planning with six degrees of freedom

The motion planning problem is of central importance to the fields of robotics, spatial planning, and automated design. In robotics we are interested in the automatic synthesis of robot motions, given high-level specifications of tasks and geometric models of the robot and obstacles. The “Movers'” problem is to find a continuous, collision-free path for a moving object through an environment containing obstacles. We present an implemented algorithm for the classical formulation of the three-dimensional Movers' problem: Given an arbitrary rigid polyhedral moving object P with three translational and three rotational degrees of freedom, find a continuous, collision-free path taking P from some initial configuration to a desired goal configuration.This paper describes an implementation of a complete algorithm (at a given resolution) for the full six degree of freedom Movers' problem. The algorithm transforms the six degree of freedom planning problem into a point navigation problem in a six-dimensional configuration space (called C-space). The C-space obstacles, which characterize the physically unachievable configurations, are directly represented by six-dimensional manifolds whose boundaries are five-dimensional C-surfaces. By characterizing these surfaces and their intersections, collision-free paths may be found by the closure of three operators which (i) slide along five-dimensional level C-surfaces parallel to C-space obstacles; (ii) slide along one- to four-dimensional intersections of level C-surfaces; and (iii) jump between six-dimensional obstacles. These operators are employed by a best-first search algorithm in C-space. We will discuss theoretical properties of the algorithm, including completeness (at a resolution). This paper describes the heuristic search, with particular emphasis on the heuristic strategies that evaluate local geometric information. At the heart of this paper lie the design and implementation of these strategies for planning paths along level C-surfaces and their intersection manifolds, and for reasoning about motions with three degrees of rotational freedom. The problems of controlling the interaction of these strategies, and of integrating diverse local experts for geometric reasoning provide an interesting application of search to a difficult domain with significant practical implications. The representations and algorithms we develop impact many geometric planning problems, and extend to Cartesian manipulators with six degrees of freedom.     

Fuzzy redundancy resolution and motion coordination for underwater vehicle-manipulator systems

The problem of redundancy resolution and motion coordination between the vehicle and the manipulator in underwater vehicle-manipulator systems (UVMSs) is addressed in this paper. UVMSs usually possess more degrees of freedom than those required to perform end-effector tasks; therefore, they are redundant systems and kinematic control techniques can be applied aimed at achieving additional control objectives besides tracking of the end-effector trajectory. In this paper, a task-priority inverse kinematics approach to redundancy resolution is merged with a fuzzy technique to manage the vehicle-arm coordination. The fuzzy technique is used both to distribute the motion between vehicle and manipulator and to handle multiple secondary tasks. Numerical case studies are developed to demonstrate effectiveness of the proposed technique.     

Finite element analysis of unbalance response of high-speed polygon mirror scanner motor considering the flexibility of the structure

This paper presents the finite element method and the mode superposition method to analyze the unbalance response of a high speed polygon mirror scanner motor supported by sintered bearing and flexible supporting structures. Finite element equations of each component of the polygon mirror scanner motor and the flexible supporting structures are consistently derived by satisfying the geometric compatibility in the internal boundary between each component. The rotating polygon mirror is modeled by annular sector element, and its rigid body motion is also considered. The rotating components except for the polygon mirror are modeled by Timoshenko beam element including the gyroscopic effect. The flexible supporting structures are modeled by using a 4-node tetrahedron element and 4-node shell element with rotational degrees of freedom. The rigid link constraints are imposed at the interface between sleeve and sintered bearing to describe the physical motion at this interface. A global matrix equation obtained by assembling the finite element equations of each substructure is transformed to a state-space matrix-vector equation, and both damped natural frequencies and modal damping ratios are calculated by solving the associated eigenvalue problem by using the restarted Arnoldi iteration method. Unbalance responses are calculated by superposing the eigenvalues and eigenvectors of the free vibration analysis. The validity of the proposed method is verified by comparing the simulated unbalance response with the experimental results. This research also shows that the flexibility of supporting structures plays an important role in determining the unbalance response of the polygon mirror scanner motor.     

基于点线特征的解耦视觉伺服控制方法

针对机器人的自动对准问题,提出一种基于点线特征的解耦视觉伺服控制方法。所提方法以点和直线作为图像特征,并利用图像特征的交互矩阵解耦姿态控制和位置控制,实现六自由度对准。首先利用直线及其交互矩阵设计姿态控制律,以消除旋转偏差;然后利用点及其交互矩阵设计位置控制律,以消除位置偏差;最后实现机器人末端目标的自动对准。在对准控制过程中,基于执行的相机运动量以及相机运动前后特征的变化量,可实现对深度的在线估计。另外,还设计了监督器对相机的运动速度进行调节,从而确保特征一直处于相机视野当中。在Eye-in-Hand机器人平台上,分别用所提方法和传统的基于图像的视觉伺服方法实现了机器人的六自由度对准。所提方法经过16步实现了机器人的自动对准,对准结束时机器人末端位姿的最大平移误差为3.26 mm,最大旋转误差为0.72°。相较于对比方法,该方法的控制过程更加高效,控制误差收敛更快,对准误差更小。实验结果表明,所提方法可以实现快速高精度的自动对准,能够提高机器人操作的自主性和智能化水平,有望应用于目标跟踪、拾取和定位、自动化装配、焊接、服务机器人等领域。