Technology to predict acoustic far-field disturbances in the case of calculations in a rotating reference frame

The paper discusses the use of the Ffowcs Williams–Hawkings method to predict far-field acoustic disturbances in the case of near-field calculations in a rotating reference frame. This formulation is particularly convenient for the simulation of a single rotor. If the rotor velocity is sufficiently high, in Farassat’s classical 1A formulation, when using the control surface associated with the grid, singularities arise in the expression under the integral sign. In order to avoid them, we use an axisymmetric control surface and consider it fixed in the absolute reference frame. This makes it possible to reduce the additional difficulties caused by rotation and interpolate and calculate the derivatives with respect to the angle.     

Spacecraft relative rotation tracking without angular velocity measurements

We present a solution to the problem of tracking relative rotation in a leader-follower spacecraft formation using feedback from relative attitude only. The controller incorporates an approximate-differentiation filter to account for the unmeasured angular velocity. We show uniform practical asymptotic stability (UPAS) of the closed-loop system. For simplicity, we assume that the leader is controlled and that we know orbital perturbations; however, this assumption can be easily relaxed to boundedness without degrading the stability property. We also assume that angular velocities of spacecraft relative to an inertial frame are bounded. Simulation results of a leader-follower spacecraft formation using the proposed controller structure are also presented.     

基于C1连续的NURBS边界Gregory曲面片实现曲面拼接

该文通过使用C1连续的NUR BS边界Gregory(N BG)曲面片进行插值,实现曲面拼接,使得在曲面连接处达到G1连续。实现了插值曲面的高阶连续,解决了用平滑的NUR BS曲面对曲线网格区域进行插值这一难以实现的问题,使用户在设计曲线网格时,只需考虑形状设计而不必关心曲线的类型及插值到曲线网格区域的曲面方程。     

Distributed Formation Control of Quadrotor UAVs Based on Rotation Matrices without Linear Velocity Feedback

This paper considers the problem of the formation control for multiple underactuated quadrotor UAVs without linear velocity measurements. The objective of this paper is expected to realize smooth formation performances for a quadrotor group. A distributed formation controller is designed directly using rotation matrices to obviate the singularities associated with the Euler-angles or the ambiguity of quaternions. A two-step procedure is adopted for the control development after transforming the dynamics of each quadrotor into a new form. At first, an intermediary control variable is introduced to achieve the formation control, where some auxiliary systems are designed in the presence of lacking linear velocity measurements. Then, based on the thrust force and reference angular velocity decoded from the intermediary control variable, the torque input of each quadrotor is proposed for angular velocity tracking. The case of formation control with a virtual leader is also investigated, where at least one quadrotor has the information of the leader. The asymptotic stability of the proposed control system is analyzed by Lyapunov-like tools, and the effectiveness of the proposed control method is verified by simulation studies. The simulation results show that the proposed method can guide the quadrotor group to form the desired formation smoothly without linear velocity feedback, which has great potentials for the UAV group to execute the challenging mission in hostile environments.

     

Approximate conversion of a rational boundary gregory patch to a nonuniform B-spline surface

A rational boundary Gregory patch is characterized by the facts that anyn-sided loop can be smoothly interpolated and that it can be smoothly connected to an adjacent patch. Thus, it is well-suited to interpolate complicated wire frames in shape modeling. Although a rational boundary Gregory patch can be exactly converted to a rational Bézier patch to enable the exchange of data, problems of high degree and singularity tend to arise as a result of conversion. This paper presents an algorithm that can approximately convert a rational boundary Gregory patch to a bicubic nonuniform B-spline surface. The approximating surface hasC ¹ continuity between its inner patches.     

Gait transition and modulation in a quadruped robot: A brainstem-like modulation approach

In this article, we propose a bio-inspired architecture for a quadruped robot that is able to initiate/stop locomotion; generate different gaits, and to easily select and switch between the different gaits according to the speed and/or the behavioral context. This improves the robot stability and smoothness while locomoting.We apply nonlinear oscillators to model Central Pattern Generators (CPGs). These generate the rhythmic locomotor movements for a quadruped robot. The generated trajectories are modulated by a tonic signal, that encodes the required activity and/or modulation. This drive signal strength is mapped onto sets of CPG parameters. By increasing the drive signal, locomotion can be elicited and velocity increased while switching to the appropriate gaits. This drive signal can be specified according to sensory information or set a priori.The system is implemented in a simulated and real AIBO robot. Results demonstrate the adequacy of the architecture to generate and modulate the required coordinated trajectories according to a velocity increase; and to smoothly and easily switch among the different motor behaviors.     

Trajectory for saving energy of a direct-drive manipulator in throwing motion

In this article, equations of motion of a closed-type manipulator, whose mechanism can easily be made lighter, are derived in consideration of the characteristics of the driving source. Considering the final condition of angular displacement and angular velocity in throwing motion, trajectories of velocity for saving energy are calculated by iterative dynamic programming, and the dynamic characteristics of manipulator control based on the trajectory for saving energy are analyzed theoretically and investigated experimentally. This work was presented in part at the 11th International Symposium on Artificial Life and Robotics, Oita, Japan, January 23–25, 2006     

一种插值与逼近运动物体活动标架的新方法

插值运动物体给定活动标架(原点位置和3个坐标轴朝向)是计算机图形学、机器人等领域的一个重要问题.该文提出一种采用B样条曲线插值与逼近运动物体活动标架的新方法.采用4个欧拉参数对正交活动标架的旋转变换矩阵进行参数化,得到了一个简单的优化方程.采用迭代法求解该优化方程来逼近运动轨迹任意处伪活动标架的旋转变换矩阵.还证明了插值和逼近所引起的误差是可控的.由于活动标架的计算只涉及2阶或3阶线性方程组,所以此方法具有很高的运行效率.     

基于水平线插值的图像修复算法

介绍用水平线插值的方法进行图像修复。讨论了基于Euler弹性的图像水平线模型,并用这个模型作为图像水平线的局部插值核。其次用星型插值的方法,从破损区域边界处出发,由外向内、一环一环地对水平线进行插值,把水平线逐步地延伸进去,让修复区域逐步缝合起来。该文的算法速度快,能保持强边缘,而且结构更合理,重构的水平线不再只是直线,而是光滑曲线。     

Global set stabilization of the spacecraft attitude control problem based on quaternion

In this paper, we develop a global set stabilization method for the attitude control problem of spacecraft system based on quaternion. The control law that uses both optimal control and finite‐time control techniques can globally stabilize the attitude of spacecraft system to a set of equilibria. First, for the kinematic subsystem, we design a virtual optimal angular velocity. To obtain the global minimum of the performance index, this optimal angular velocity is only discontinuous in initial values. It can be regarded as a combination of open loop control and closed loop control. Then for the dynamic subsystem, we design a finite‐time control law that can force the angular velocity to track the virtual optimal angular velocity. It is proved that the closed loop system satisfies global set stability in the absence of disturbances. In the presence of disturbances, the system trajectory will converge to a neighborhood of the equilibrium set. Rigorous analysis shows that by introducing finite‐time control techniques, the closed loop system possesses a better disturbance rejection property. The control method is more natural and energy‐efficient. The effectiveness of the proposed method is demonstrated by simulation results. Copyright © 2009 John Wiley & Sons, Ltd.     

Technical note: piecewise continuous quadratic interpolation

A piecewise continuous quadratic polynomial is derived from the Taylor expansion to interpolate a given set of data in a way that the polynomial and its first derivatives are continuous at the data points. Comparison with a previously published work indicate that the present method is more systematic, simpler to program, and it has the advantage that it can easily be used for higher order interpolation.     

Experiments on estimating egomotion and structure parameters using long monocular image sequences

This paper presents a simple but robust model based approach to estimating the kinematics of a moving camera and the structure of the objects in a stationary environment using long, noisy, monocular image sequences. Both batch and recursive algorithms are presented and the problem due to occlusion is addressed. The approach is based on representing the constant translational velocity and constant angular velocity of the camera motion using nine rectilinear motion parameters, which are 3-D vectors of the position of the rotation center, linear and angular velocities. The structure parameters are 3-D coordinates of the salient feature points in the inertial coordinate system. Due to redundancies in parameterization, the total number of independent parameters to be estimated is 3M+7, whereM is the number of feature points. The image plane coordinates of these feature points in each frame are first detected and matched over the frames. These noisy image coordinates serve as the input to our algorithms. Due to the nonlinear nature of perspective projection, a nonlinear least squares method is formulated for the batch algorithm, and a conjugate gradient method is then applied to find the solution. A recursive method using an Iterated Extended Kalman Filter (IEKF) for incremental estimation of motion and structure is also presented. Since the plant model is simple in our formulation, closed form solutions for the state and covariance transition equations are easily derived. Experimental results for simulated imagery as well as several real image sequences are included.The support of the Advanced Research Projects Agency (ARPA order No. 8459), the U.S. Army Topographic Engineering Center under contract DACA 76-92-C-0009, and the Department of Electrical Engineering at the University of Maryland is gratefully acknowledged.     

Rigid body attitude coordination without inertial frame information

We study a motion coordination problem where the objective is to achieve identical orientation and synchronous rotation for a group of rigid bodies. Unlike existing designs which assume that the inertial frame is available to each agent, we develop a passivity-based design which relies only on relative attitude information with respect to neighboring agents. The desired equilibria, where all the rigid bodies possess the same attitude and rotate at a desired angular velocity, are shown to be locally asymptotically stable and a manifold of undesired equilibria may exist. We then consider the situation where the reference angular velocity is available only to the leader, and propose a distributed adaptive controller with which the other agents reconstruct this reference angular velocity.     

高速高机动目标自适应IMM跟踪算法

针对目标运动过程中有转弯机动等复杂运动模式的高速高机动目标,设计了自适应两层IMM跟踪算法。该算法内层由改进的机动目标当前统计模型构成,把目标速度方向角作为伪测量值进行滤波,实时获得目标的角速度和角加速度;外层模型由常速模型和曲线模型构成,把内层模型得到的切向加速度和转弯角速度作为曲线模型参数,利用IMM算法进行滤波。仿真结果表明,该算法对高速高机动目标具有较高的跟踪精度,算法实现简单,具有一定的实际应用价值。     

Frame loads accuracy assessment of semianalytical multibody dynamic simulation methods of a recreational vehicle

The design of a vehicle frame is largely dependent on the loads applied on the suspension and heavy parts mounting points. These loads can either be estimated through full analytical multibody dynamic simulations, or from semi-analytical simulations in which tire and road sub-models are not included and external vehicle loads, recorded during field testing, are used as inputs to the wheel hubs. Several semi-analytical methods exist, with various modeling architectures, yet, it is unclear how one method over another improves frame loads prediction accuracy.

This study shows that a semi-analytical method that constrains the vehicle frame center of gravity movement along a recorded trajectory, using a control algorithm, leads to an accuracy within 1% for predicting frame loads, when compared to reference loads from a full analytical model. The control algorithm computes six degrees of freedom forces and moments applied at the vehicle center of gravity to closely follow the recorded vehicle trajectory. It is also shown that modeling the flexibility of the suspension arms and controlling wheel hub angular velocity both contribute in improving frame loads accuracy, while an acquisition frequency of 200 Hz appears to be sufficient to capture load dynamics for several maneuvers. Knowledge of these loads helps engineers perform appropriate dimensioning of vehicle structural components therefore ensuring their reliability under various driving conditions.

     

Stabilization of a 3D axially symmetric pendulum

Stabilizing controllers are developed for a 3D pendulum assuming that the pendulum has a single axis of symmetry and that the center of mass lies on the axis of symmetry. This assumption allows development of a reduced model that forms the basis for controller design and global closed-loop analysis; this reduced model is parameterized by the constant angular velocity component of the 3D pendulum about its axis of symmetry. Several different controllers are proposed. Controllers based on angular velocity feedback only, asymptotically stabilize the hanging equilibrium. Then controllers are introduced, based on angular velocity and reduced attitude feedback, that asymptotically stabilize either the hanging equilibrium or the inverted equilibrium. These problems can be viewed as stabilization of a Lagrange top. Finally, if the angular velocity about the axis of symmetry is assumed to be zero, controllers are introduced, based on angular velocity and reduced attitude feedback, that asymptotically stabilize either the hanging equilibrium or the inverted equilibrium. This problem can be viewed as stabilization of a spherical pendulum.     

Transfinite surface interpolation with interior control

There are various techniques to design complex free-form shapes with general topology. In contrast to the approaches based on trimmed surfaces and control polyhedra, in curve network-based design feature curves can be directly created and edited in 3D. Multi-sided patches interpolate this curve network with slopes given by associated tangent ribbons. The patches are smoothly connected and yield a natural and predictable surface model. This paper focuses on special design techniques to adjust the interior of transfinite patches when further shape control is needed. While the boundary constraints are retained, additional vertices, curves and even interior control surfaces are supplemented to gain more design freedom. The main idea is to apply different distance-based blending functions with special parameterizations over non-regular, n-sided domains. This concept can be naturally extended to create one- and two-sided patches as well. Shape variations will be demonstrated by a few simple examples.     

结合图像融合算法与人眼跟踪的立体视频

由于多视点立体视频合成具有数据量大,图像处理速度要求较高,支持的立体视角有限等特点,这些问题一直没有很好的解决并已成为多视点立体视频产业化的瓶颈。针对这种情况,提出了一种基于立体图像融合算法与人眼跟踪算法的立体视频处理系统。首先,按顺序循环读取立体视频中的每一帧,然后用立体图像融合算法对每一帧进行合成运算,接下来将融合后的图像依原有顺序进行显示与播放。同时加入人眼跟踪算法,根据观看者眼部所处的位置实时投放对应视区的图像。图像融合算法与人眼跟踪的结合有效地扩大了立体视角。实验结果表明,该方法实现了将多视点视频在立体显示器中以自由立体显示的方式展现出来,使观看者在屏幕前可以自由移动而不影响立体观看效果,同时播放速度流畅,能给观众带来比较真实的立体感受。     

Algorithms for the synthesis of observers for nonlinear dynamic systems

For nonlinear dynamic systems, an approach based on cycle-by-cycle linearization is proposed to construct algorithms for state observers based on linear estimation theory. This approach makes it possible to analytically synthesize a full-rank and a reduced observer of the angular velocity of a spacecraft rotation relative to the body reference frame given the measurements of three angles determining the position of this reference frame in the inertial space. Simulation results are presented.     

速度分段的增量动态逆小型无人机控制律设计

针对小型无人机模型参数精度不高的问题,提出一种速度分段的增量非线性动态逆姿态控制方法。该方法根据无人机力矩方程,用非线性动态逆构建出角加速度为隐式输入,舵面偏转量为输出的关系式;运用小扰动原理计算角加速度,并通过在不同的速度区间内计算机体力矩对角速度的偏导,并将该结果作为角加速度修正因子的方法,得到舵面偏转增量与角加速度的直接关系式;基于新发现的关系式构建了内、外环控制律,角加速度作为反馈降低了对模型参数的敏感性,修正因子的引入提高了控制精度。仿真实验表明了该方法的有效性。