A survey of control allocation methods for underwater vehicles

A control allocation system implements a function that maps the desired control forces generated by the vehicle motion controller into the commands of the different actuators. In this article, a survey of control allocation methods for over-actuated underwater vehicles is presented. The methods are applicable for both surface vessels and underwater vehicles. The paper presents a survey of control allocation methods with focus on mathematical representation and solvability of thruster allocation problems. The paper is useful for university students and engineers who want to get an overview of state-of-the art control allocation methods as well as advance methods to solve more complex problems.

Antispin thrust allocation for marine vessels

In extreme weather conditions, thrusters on ships and rigs may be subject to severe thrust losses caused by ventilation and in-and-out-of-water events. When a thruster ventilates, air is sucked down from the surface and into the propeller. In more severe cases, parts of or even the whole propeller can be out of the water. These losses vary rapidly with time and cause increased wear and tear in addition to reduced thruster performance. In this paper, a thrust allocation strategy is proposed to reduce the effects of thrust losses and to reduce the possibility of multiple ventilation events. This thrust allocation strategy is named antispin thrust allocation, based on the analogous behavior of antispin wheel control of cars. The proposed thrust allocation strategy is important for improving the life span of the propulsion system and the accuracy of positioning for vessels conducting station keeping in terms of dynamic positioning or thruster-assisted position mooring. Application of this strategy can result in an increase of operational time and, thus, increased profitability. The performance of the proposed allocation strategy is demonstrated with experiments on a model ship.

Thrust allocation with linear constrained quadratic cost function

In this paper, a method of thrust allocation based on a linearly constrained quadratic cost function capable of handling rotating azimuths is presented. The problem formulation accounts for magnitude and rate constraints on both thruster forces and azimuth angles. The advantage of this formulation is that the solution can be found with a finite number of iterations for each time step. Experiments with a model ship are used to validate the thrust allocation system.

Structure of 1,2,4-oxadiazolidine-3,5-dione monohydrate

C2H2N203.H20, Mr= 120.07, monoclinic,P21/c, a= 5.011 (1), b= 11.796(2), c= 7.689 (2)A,fl= 95.22 (2) ° , V= 452.61 A 3, Z= 4, Dx= 1.76, D m = 1.75 gcm -3, /].(Cu Ks) = 1.5418 A, g = 14-0 cm -l,F(000) = 248, T = 293 K, crystal quality was poor and the final R =0.107, wR =0.090 for 881 observed reflections. The compound is derived from a novel form of the monopropellant oxalohydroxamic acid. The two exocyclic C-O bond lengths of 1.240 (3) and 1.228 (4)A indicate double bonds. The C-N bond lengths of 1.334 (4), 1.390 (4) and 1.359 (4) A are characteristic of the amide bond. The N atom covalently bonded to the two carbonyl C atoms acts as a proton donor in an intermolecular hydrogen bond to the ring O atom: N1...O3i = 2.854 ]k (i =x-- 1,y, z), H...O = 2.15 A, N-H...O = 159 °.

MHD arcjet onset boundary of high specific impulse Plasma Thrusters

Magnetoplasmadynamic thrusters are known to enter a strongly unstable regime, calledas onset in the literature, under high specific impulse operation. This paper probes the early signs of onset in relatively moderate specific impulse operation by a single fluid plasma thruster simulation. The procedure involves solving the combined Maxwell’s-Navier-Stokes equation, with an onset criterion of radial current reaching close to zero values near the electrodes. Thruster parameters are varied starting from voltage potential, plasma temperature and cathodic radius. Onset curves are plotted which can provide important engine-specific information in order to understand the onset performance of the plasma thruster.

Iridium-decorated multiwall carbon nanotubes and its catalytic activity with Shell 405 in hydrazine decomposition

Iridium-functionalized multiwalled carbon nanotubes (Ir-MWNT) are the future catalyst support material for hydrazine fuel decomposition. The present work demonstrates decoration of iridium particle on iron-encapsulated multiwalled carbon nanotubes (MWNT) by wet impregnation method in the absence of any stabilizer. Electron microscopy studies reveal the coated iridium particle size in the range of 5-10 nm. Elemental analysis by energy dispersive X-ray diffraction confirms 21 wt% of Ir coated over MWNT. X-ray photoelectron spectroscopy (XPS) shows 4f(5/2) and 4f(7/2) lines of iridium and confirms the metallic nature. The catalytic activity of Ir-MWNT/Shell 405 combination is performed in 1 N hydrazine micro-thrusters. The thruster performance shows increase in chamber pressure and decrease in chamber temperature when compared to Shell 405 alone. This enhanced performance is due to high thermal conducting nature of MWNTs and the presence of Ir active sites over MWNTs.

稳态等离子体推进器羽流的粒子模拟

本文利用结合了particle-in-cell方法的直接模拟Monte Carlo方法模拟了稳态等离子体推进器的羽流场.为了更真实地再现实验设备中的流动,模拟中包含了背压气体,并在计算中直接模拟,改变了以往的中性粒子平衡态的处理方式.为了匹配试验结果,修正了推进器出口处气流偏转角分布,并建议最大偏转半角取为20°.计算结果与现有的实验结果符合较好.

氢-氮电弧加热推力器运行参数与性能

采用氢气和氮气的摩尔比r在1.0～2.0范围的混合气体作为推进剂,实时检测电弧加热推力器运行中的弧电流、弧电压、入口压力、真空室压力、质量流量和喷口温度等工作参数,研究了比功率、r和质量流量对推力器产生的推力、比冲和推力效率的影响。实验结果表明,同时增加混合气体的质量流量和r能有效提高推力器的推力;较高的r能获得高的比冲和效率,尤其是比冲随着r的提高明显地单调上升。

一种电弧加热推进器推力测试架

设计制作了采用悬臂梁结构的微小推力测试架。通过标定、真空测试、温漂检验和实测应用,证实其可用于电弧加热推进器推力性能的测量。

Development of arc root attachment in the nozzle of 1 kW N2 and H2-N2 arcjet thrusters

Arc root behavior affects the energy transfer and nozzle erosion in an arcjet thruster. To investigate the development of arc root attachment in 1 kW class N2 and H2-N2 arcjet thrusters from the time of ignition to the stably working condition, a kinetic series of end-on view images of the nozzle obtained by a high-speed video camera was analyzed. The addition of hydrogen leads to higher arc voltage levels and the determining factor for the mode of arc root attachment was found to be the nozzle temperature. At lower nozzle temperatures, constricted type attachment with unstable motions of the arc root was observed, while a fully diffused and stable arc root was observed at elevated nozzle temperatures.

Modeling study on the flow, heat transfer and energy conversion characteristics of low-power arc-heated hydrogen/nitrogen thrusters

A modeling study is conducted to investigate the effect of hydrogen content in propellants on the plasma flow, heat transfer and energy conversion characteristics of low-power (kW class) arc-heated hydrogen/nitrogen thrusters (arcjets). 1:0 (pure hydrogen), 3:1 (to simulate decomposed ammonia), 2:1 (to simulate decomposed hydrazine) and 0:1 (pure nitrogen) hydrogen/nitrogen mixtures are chosen as the propellants. Both the gas flow region inside the thruster nozzle and the anode-nozzle wall are included in the computational domain in order to better treat the conjugate heat transfer between the gas flow region and the solid wall region. The axial variations of the enthalpy flux, kinetic energy flux, directed kinetic-energy flux, and momentum flux, all normalized to the mass flow rate of the propellant, are used to investigate the energy conversion process inside the thruster nozzle. The modeling results show that the values of the arc voltage, the gas axial-velocity at the thruster exit, and the specific impulse of the arcjet thruster all increase with increasing hydrogen content in the propellant, but the gas temperature at the nitrogen thruster exit is significantly higher than that for other three propellants. The flow, heat transfer, and energy conversion processes taking place in the thruster nozzle have some common features for all the four propellants. The propellant is heated mainly in the near-cathode and constrictor region, accompanied with a rapid increase of the enthalpy flux, and after achieving its maximum value, the enthalpy flux decreases appreciably due to the conversion of gas internal energy into its kinetic energy in the divergent segment of the thruster nozzle. The kinetic energy flux, directed kinetic energy flux and momentum flux also increase at first due to the arc heating and the thermodynamic expansion, assume their maximum inside the nozzle and then decrease gradually as the propellant flows toward the thruster exit. It is found that a large energy loss (31-52%) occurs in the thruster nozzle due to the heat transfer to the nozzle wall and too long nozzle is not necessary. Modeling results for the NASA 1-kW class arcjet thruster with hydrogen or decomposed hydrazine as the propellant are found to compare favorably with available experimental data.

Modeling study to compare the flow and heat transfer characteristics of low-power hydrogen, nitrogen and argon arc-heated thrusters

A modelling study is performed to compare the plasma °ow and heat transfer char- acteristics of low-power arc-heated thrusters (arcjets) for three di®erent propellants: hydrogen, nitrogen and argon. The all-speed SIMPLE algorithm is employed to solve the governing equa- tions, which take into account the e®ects of compressibility, Lorentz force and Joule heating, as well as the temperature- and pressure-dependence of the gas properties. The temperature, veloc- ity and Mach number distributions calculated within the thruster nozzle obtained with di®erent propellant gases are compared for the same thruster structure, dimensions, inlet-gas stagnant pressure and arc currents. The temperature distributions in the solid region of the anode-nozzle wall are also given. It is found that the °ow and energy conversion processes in the thruster nozzle show many similar features for all three propellants. For example, the propellant is heated mainly in the near-cathode and constrictor region, with the highest plasma temperature appear- ing near the cathode tip; the °ow transition from the subsonic to supersonic regime occurs within the constrictor region; the highest axial velocity appears inside the nozzle; and most of the input propellant °ows towards the thruster exit through the cooler gas region near the anode-nozzle wall. However, since the properties of hydrogen, nitrogen and argon, especially their molecular weights, speci¯c enthalpies and thermal conductivities, are di®erent, there are appreciable di®er- ences in arcjet performance. For example, compared to the other two propellants, the hydrogen arcjet thruster shows a higher plasma temperature in the arc region, and higher axial velocity but lower temperature at the thruster exit. Correspondingly, the hydrogen arcjet thruster has the highest speci¯c impulse and arc voltage for the same inlet stagnant pressure and arc current. The predictions of the modelling are compared favourably with available experimental results.

低功率氩电弧加热发动机的数值模拟

采用可求解可压缩流动与传热的全速度SIMPLE算法, 对低功率氩电弧加热发动机内部的传热与流动进行了数值模拟, 获得了电弧加热发动机内的温度、速度、马赫数及流线分布. 计算结果表明: 电弧加热发动机内最高温度出现在阴极下游附近中心轴线处, 这是因为电弧在阴极表面收缩形成阴极弧点, 从而焦耳热成为该高温区的主要加热机制; 沿着发动机中心轴线, 气体温度和速度开始时随着距阴极距离的增加而迅速增加, 然后在等离子体流向喷管出口的过程中, 气体温度和速度逐渐下降. 此外还详细考察了弧电流变化对电弧加热发动机内部传热与流动特性的影响, 计算获得的发动机流量和比冲与实验结果基本一致.

基于复合喷气压电驱动器的柔性机械臂振动控制

空间机器人和大型柔性空间结构在航天器调姿、变轨、外部扰动的情况下将引起振动问题,其低频大幅值振动将持续很长时间,这将影响航天器系统的稳定性和控制精度。为了快速抑制低频大幅值振动及残余振动,提出采用复合可控反作用力幅值的喷气式驱动和压电陶瓷驱动方案进行振动控制。进行基于复合控制的柔性臂系统动力学建模并给出控制算法。设计并建立柔性机械臂试验平台,构建气动驱动控制回路及压电驱动控制回路。进行基于压电陶瓷驱动器、喷气式驱动器及复合喷气和压电驱动器的柔性臂大幅值低频模态振动控制的几种方法试验比较研究。试验结果表明,采用的控制方案和方法既可以快速地抑制柔性机械臂统的低频大幅值振动,又明显地同时抑制高频和低频小幅值残余振动。

小型自治水下机器人运动控制系统研究

以主推加舵控制的小型自治水下机器人为研究对象,建立了水下机器人的数学模型并进行了分析。根据机器人结构的特点,对模型进行了必要的简化。设计了机器人的运动控制系统。通过湖试验正控制器的性能。