Uses of electrodynamic forces in tethered satellite systems. II.Orbit-transfer

Part II of this work presents electrodynamic forces used for orbit transfer. The system presented in Part I is to be transferred to a new orbit of a different radius while maintaining a nearly Earth-pointing orientation at all times. The maneuver is to be performed with electrodynamic forces without the use of thrusters. To accomplish this mission, the notion of synchronized impulses, that is, running electric current through the tether at precalculated instants and amplitudes, is presented     

空间电动力绳系电流峰值点变化规律研究

为研究电流峰值点在几种影响因素下的变化规律,建立一种能够捕捉绳系电子发射与收集自洽平衡过程的新算法——电路空间耦合算法。为验证该方法的计算精度,以1.35kW霍尔推力器为等离子体源,在真空舱内开展绳系的电荷收集试验,在电路参数方面,计算精度约为12.8%,在场参数方面,计算精度约为3.6%。在此基础上,针对不同偏置电压、绳系长度以及绳系直径,对绳系的绳上电流分布、电势分布以及空间电势分布等参数进行数值计算。结果显示:电动力绳系的电流峰值点会随着偏置电压升高、绳系长度增加及绳系直径增大而发生比例上的向阳极端漂移,揭示了电子轨道运动限制的机制在各类壁面电荷输运机制中占优,导致电流峰值点漂移的产生。     

电动力绳系离轨系统电流与拉力混合展开控制

电动力绳系离轨系统是一种典型的空间碎片主动消除技术,本文针对电动力绳离轨系统在初始阶段展开的控制问题,仅通过调节有限的系绳张力与电流,不依赖其他推进器,将导电系绳释放至期望长度并抑制系绳的摆动。首先,采用倾斜偶极子地磁场模型,建立考虑系绳质量的近地轨道二体电动力绳系卫星系统动力学模型;其次,在系绳张力与电流存在约束的条件下,采用反步法结合抗饱和辅助函数解决控制输入受限问题,并引入动态尺度广义逆实现了欠驱动电动力绳系统展开稳定控制问题;然后,应用Lyapunov稳定性定理证明了其闭环系统的稳定性;最后,在MATLAB/SIMULINK平台上进行了仿真验证。结果表明：通过调节系绳张力与电流,可以将系绳释放到指定位置,同时控制输入满足约束条件。     

Study on excitation force characteristics in a coupled shaker-structure system considering structure modes coupling

The interaction between an elastic structure and electrodynamic shakers commonly exists in Ground Flutter Simulation Tests(GFST) with multi-point excitations, causing a considerable discrepancy between the practical excitation forces and desired ones. To investigate the excitation force characteristics on a cantilever beam excited by a voltage-sourced electrodynamic shaker,the coupled shaker-beam system is modeled to derive the excitation force formula using Hamilton’s principle and Galerkin’s a...     

Experiences with dynamic load simulation by means of modal forces in the presence of structural non-linearities

The dynamic qualification of space structures today is performed by means of driven base tests in order to simulate the dynamic environment during launch. This method is limited to structures that fit onto the shaker facility. For large and unwieldy structures an alternative approach can be utilised which applies modal forces for the qualification. Modal forces are patterns of single forces that are able to excite single resonances. These force patterns can be applied by movable electrodynamic shakers. This article reviews the background of the method. A test strategy is proposed which is based on narrow-banded swept sine excitation and considers structural non-linearities observed frequently. The qualification method was utilised during the qualification of the European Robotic Arm (ERA). The experiences gained during the test campaign are compiled in the paper. The advantages and disadvantages of dynamic qualification by means of modal forces are discussed.     

利用受限张力的拖曳变轨欠驱动姿态稳定策略

在空间绳系拖曳变轨中,目标和平台形成一种哑铃型绳系系统,且仅依靠有限的平台推力和系绳张力来抑制系绳的摆动。针对此类输入受限的欠驱动控制问题,提出了一种利用受限张力的姿态稳定策略。首先,推导了组合体姿态动力学模型。然后通过数值求解姿态平衡方程得出理论面内姿态指令,再采用高斯伪谱法对其优化获得实际指令。最后,基于分层滑模理论设计欠驱动张力控制律,并嵌入抗饱和模块以缓解张力饱和。仿真表明空间平台能在正向有限的张力控制下,平滑地收放系绳使面内角和绳长跟踪实际姿态指令。此外,所提策略对目标体摆动和传感器误差也具有良好的鲁棒性。     

Dynamics and de-spin control of massive target by single tethered space tug

This paper investigates the dynamics and de-spin control of a massive target by a single tethered space tug in the post-capture phase. The dynamic model of the tethered system is derived and simplified to a dimensionless form. Further, a decoupled PD controller is proposed, and the local stability of the controller is analyzed by linearization technique. Parametric studies of the dynamics and de-spin control of a massive target are conducted to characterize the dynamic process of de-spin with the proposed control law. It is shown that the massive target can be de-span by a single and small space tug with limited thrust within finite time. The thrust tangent with the tether de-spins the target while the thrust normal to the tether prevents the tether from winding up the target. The tether length has a positive contribution to the de-spin of a target. The longer tether leads to a faster de-spin process.     

空间绳系机器人捕获目标后的面内自适应回收方法

在空间绳系机器人(TSR)捕获目标星后,操作机构与目标星形成质量、惯量和系绳连接点位置等参数未知的组合体,且系绳长度、偏角与组合体姿态严重耦合,控制输入严格受限,回收控制十分困难。针对其回收难题,综合考虑系绳长度、系绳偏角与组合体姿态,利用拉格朗日法建立了轨道面内的动力学模型,并基于动态逆理论设计了一种自适应抗饱和回收控制方法。首先,在对组合体质量、惯量与系绳连接点进行在线估计的基础上,设计一种自适应动态逆回收控制器;然后,设计辅助变量对控制输入进行补偿,解决控制输入受限问题;最后进行仿真验证。仿真结果表明,在线估计器能够快速有效地估计组合体动力学参数,回收控制系统能够利用受限的控制输入克服抓捕时刻的系绳偏角和组合体姿态扰动,并沿设计的回收轨迹实现稳定有效回收。     

面内轨道转移过程中的绳系系统摆振特性研究

轨道转移过程中的绳系系统处于非开普勒轨道,导致系统呈现复杂的动力学行为并影响着星体的飞行安全,因此研究系统摆振特性具有重要的理论和实际意义。针对复杂的非线性和强耦合问题,利用动量矩定理建立绳系系统姿态动力学方程,以切向常值加速度轨道转移为任务背景,给出了系统质心运动轨迹;通过分析面内摆角的静态分岔现象,推导了面内、面外摆角的一阶摆动解析解;引入经典的珠点模型,研究系绳纵向和横向的振动特性,并分析了系绳摆动与系绳振动之间的耦合关系。仿真结果表明:面内轨道转移过程中,面内、面外摆角以固定的频率绕平衡位置做往返摆动,摆动频率大小以及平衡位置的变化均与系绳长度、推力加速度和所处轨道密切相关,面内摆角摆动频率接近轨道角频率时会引起共振现象,系绳在轨道转移过程中会出现大幅度横向振动等现象。     

Line-of-sight-pointing for satellites in high altitude inclined elliptic orbits

With a growing demand for space communications and resulting overcrowding of geostationary orbit (GEO), the importance of high altitude inclined elliptic orbits is gaining impetus. However, the satellites in these orbits suffer from a severe problem of apparent periodic angular drift around their line-of-sight. This paper addresses this problem and proposes a cost effective method based on tether to continually tilt the satellites in order to compensate for longitudinal and lateral drifts relative to the ground station. The proposed system comprises two satellites connected by a flexible tether at a point on each satellite with offsets. A control strategy is developed for tether offset variations that ensures judiciously controlled changes in the satellite orientations. The numerical simulation of the governing nonlinear equations of motion establishes the feasibility of the concept. A high degree of line-of-sight pointing of dual satellites as well as the simplicity of the proposed control mechanism makes the concept particularly attractive for future space applications.     

Hybrid control of orbit normal and along-track two-craft Coulomb tethers

The dynamics and stability of a charged two craft formation with nominal fixed separation distance (Coulomb tethers) is studied where the cluster is aligned with either the along-track or orbit normal direction. Unlike the charged two-craft formation scenario aligned along the orbit radial direction, a feedback control law using inter-spacecraft electrostatic Coulomb forces and the differential gravitational accelerations is not sufficient to stabilize the Coulomb tether length and the formation attitude. Therefore, a hybrid feedback control law is presented which combines conventional thrusters and Coulomb forces. The Coulomb force feedback requires measurements of separation distance error and error rate, while the thruster feedback is in terms of Euler angles and their rates. This hybrid feedback control is designed to asymptotically stabilize the satellite formation shape and attitude while avoiding plume impingement issues. The effects of differential solar drag on the formation and the ability of the controller to withstand this disturbance is also studied.     

Tethered dual spacecraft configuration: a solution to attitude control problems

A new dual spacecraft configuration comprising of two spacecraft halves judiciously connected through extremely short tethers is proposed. The simple tethered configuration induces stabilizing torques when subjected to attitude disturbances, thus ensuring 3-d pointing stability of both the satellite platforms. The enhanced system performance obtained using tethers enables a much greater flexibility in the choice of satellite mass distribution. Three particular TSS models involving parallel tethers, a `parachute-like' conical tether layout as well as a single tether connection have been considered. A detailed numerical response simulation shows that these modes of tether attachments bring about a radical change in the satellite attitude behavior from the nominal one involving relatively large librational amplitudes or instability to a virtually fixed desired orientation. Of these, the parachute configuration appears to be superior. The passive nature of the proposed mechanisms using tether lengths on the order of merely a couple of meters needed for small and medium size systems makes the concept particularly attractive for future space missions. Finally, it is felt that the proposed tethered dual satellite systems may offer a simple answer to three-axis attitude control problems.     

Real time estimator for control of an orbiting single tether system

A real-time estimator is developed for the control of the Tether Dynamics Explorer (TDE) system. TDE is being used in a series of tethered satellite flight experiments whose purpose is to validate existing system models and test proposed control laws. Each experiment consists of an orbiting Delta II second stage which deploys toward the Earth a small box-shaped passive endbody at the end of a flexible tether. A discrete extended Kalman filter (DEKF) is presented which can operate in real time and in conjunction with control laws. This filter estimates the in-plane and out-of-plane tether libration angles and their rates from a proposed three-axis tether tension measurement device in the Delta second stage. The simulation results indicate that the DEKF can estimate the libration angle and their rates from a three-axis tension and length measurement with an acceptable error. The relatively simple computations required make this algorithm particularly well suited for real-time operation.<>     

Intelligent UUVs: Some issues on ROV dynamic positioning

Intelligent unmanned underwater vehicles (UUVs) fall under two main group categories: the remotely operated vehicles (ROVs), which are characterized by remote operation and presence of a tether cable; and the autonomous underwater vehicles (AUVs), which are characterized by their autonomous behavior and absence of a tether cable. One fundamental issue of the UUV design is the dynamic position control system. This system plays a crucial role together with the sensor architecture in the degree of system autonomy that can be achieved. This paper is concerned with a few issues when dynamically positioning remotely operated underwater vehicles (ROVs). By restricting the operating regime of ROVs to slow velocity requirements the paper investigates the implementation of a few decentralized control strategies and compare their performance measures, which are assessed by simulating a nonlinear ROV system model for each control strategy. Issues concerning input tracking, disturbance rejection, and plant variations are discussed. The evaluations consider the use of linear PID feedback and feedforward variants, and a robust nonlinear control strategies applied to a full order, fully coupled, and nonlinear vehicle model. These evaluations consider a vehicle undertaking standard mission activities where the tether cable dynamics, with load estimates obtained from a lumped mass cable model, and the vehicle actuator system are present. The paper shows that much of the performance deterioration may be attributed mainly due to cable inertia. The authors also verify that the nonlinear robust control strategy does not necessarily allow for better performance over the linear feedback control strategies implemented when vehicle motions are confined to slow velocity profiles. These and other partial results will aid the design of the control system for an underwater vehicle currently under construction     

多电发动机分布式控制系统总体方案研究

多电发动机控制系统十分复杂,需要采用分布式控制。分布式结构易于从部件级到子系统级再到系统级进行试验,同时大多数试验可通过仿真程序同步进行。基于某型发动机技术平台的分布式控制系统总体方案,在保留原平台的控制功能和控制规律基本不变的情况下,按多电发动机控制要求,大量采用电介质替代燃油介质实现控制功能,并按分布式控制方式进行系统总体方案设计。重点验证了新增控制功能、新原理控制元件和控制方法以及分布式控制总体运行模式,使其能在现有发动机平台上进行多电发动机分布式控制关键技术验证。     

低频电动振动台的关键技术

简要介绍了低频电动振动台校准系统的结构和工作原理，分析了其存在非线性的原因。针对这些因素提出了在整个系统特别是对其核心——低频电动振动台的研制设计过程中应采取的措施、改进的方法和关键技术。     

空间绳系机器人逼近目标协调控制方法

 为了节省空间绳系机器人的末端执行装置在逼近目标卫星过程中推力器所使用的燃料,本文提出一种利用推力器、反作用轮及空间系绳的协调控制方法。首先利用二次型最优控制器(LQR)算法计算出末端执行装置逼近目标所需的理想轨道控制力,然后利用模拟退火算法将所需轨道控制力优化分配到推力器及空间系绳,同时利用时间延迟算法通过反作用轮补偿空间系绳产生的姿态干扰力矩。仿真结果表明,利用该协调控制方法能显著节省末端执行装置上推力器的燃料消耗,有效抑制空间系绳协调控制力产生的姿态干扰,使末端执行装置保持相对稳定的姿态。     

Optimization of the tether-assisted return mission of a guided re-entry capsule

This paper presents the mission analysis of a tether-assisted payload retrieval from the International Space Station (ISS). The objective is to assess all relevant phases of such a mission in order to allow a comparison with a conventional mission employing a propulsive deorbitation. The controlled tether deployment procedure and the guided return flight of the released re-entry capsule are optimized. A preferable deployment strategy is identified that allows for favorable entry conditions and low flight loads. The optimal deployment trajectories serve as a basis for an optimal dynamic regulator. This approach is extended towards an adaptive concept, where artificial neural networks are applied to deployment control. For the guidance of the capsule a predictive concept is proposed that is based on the optimal re-entry trajectories identified previously. By applying these concepts, the attainable landing accuracy during return amounts to an average of 5 km, and the application of the tether system exhibits overall system mass advantages. This demonstrates that the tether-assisted return mission is a competitive alternative.     

Optical fiber geostationary tether satellite (F-GTS) system design

The geostationary satellite orbit (GSO) is a limited natural resource and its efficient utilization is very important. The geostationary tether satellite (GTS) system has a number of satellites aligned along the local vertical on either side of the nominal geostationary position. The system is synchronized with the Earth's rotation and all the various altitudes are geostationary, Furthermore, optical-fiber geostationary tether satellite (F-GTS) system has been introduced to improve the GTS system, with regard to increment of communication capacity, simplification of interference paths and intersatellite link (ISL) capability. The F-GTS system design is discussed with the purpose of achieving a realistic satellite network. Three frequency bands, i.e., the 14/11, 30/20, and 50/40 GHz bands, are examined for selection of the optimum frequency band. The F-GTS system example for covering the service areas in Japan is discussed with regard to satellite antenna diameter, communication capacities, etc. To apply the F-GTS system to the whole GSO, the diagonal azimuth orbit arrangement method is proposed for low latitude service areas. Moreover, the F-GTS communication capacity and total communication capacity, when the F-GTS systems are applied to the whole GSO, are also examined.     

Materials samples face rigors of space

The Materials International Space Station Experiment (MISSE) is described. This project is designed to conduct long duration materials tests on samples attached to the ISS. A batch of 750 material samples were delivered on STS-105 and attached to the ISS airlock. They will be exposed to the space environment for 18 months and are slated to return on STS-114. A second batch of 750 samples is being prepared. The experiment containers were used originally for the Mir Environmental Effects Payload, which tested a variety of substances, including some slated for use on the ISS. Researchers are particularly interested in the effects of atomic oxygen on the samples. Some samples are being tested to determine their use in radiation protection. As part of the MISSE project, ultrathin tether materials are being tested for use on the Propulsive Small Expendable Depoloyer System (ProSEDS), which will use a tether system to change a satellite's orbital altitude.