Adaptive control of underwater vehicle-manipulator systems using radial basis function networks

This paper deals with a control scheme for underwater vehicle-manipulator systems with the dynamics of thrusters in the presence of uncertainties in system parameters. We have developed two controllers that overcome thruster nonlinearities, which cause an uncontrollable system: one is a regressor-based adaptive controller and the other is a robust controller. However, the structure of the adaptive controller is very complex due to the feedforward terms including the regressors of dynamic system models, and the error feedback gains of the robust controller with a good control performance are excessively high due to the lack of feedforward terms. In this paper we develop an adaptive controller that uses radial basis function networks instead of the feedforward terms. The replacement leads to a moderately high gain controller whose structure is simpler than that of the regressor-based adaptive controller.     

Coordinated control method of space-tethered robot system for tracking optimal trajectory

Space-tethered robot system is a new kind of space robot, which consists of a robot platform, space tether, and operation robot. This paper presents the coordinated control method in order to save thruster fuel of operation robot in the process of tracking the optimal approach trajectory. First, the optimal approach trajectory of an operation robot is designed using the Gauss pseudospectral method, which resulted in continuous optimal control force using the Lagrange interpolation scheme. The optimal control force is optimized and distributed to space tether and thrusters through simulated annealing algorithm in discrete points, which minimized fuel consumption of thrusters. The distributive continuous force is obtained via cubic polynomial fitting of optimal distributive force in 0.1s discrete time point. To tracking the optimal trajectory, Fuzzy Proportional-Derivative controller is designed with the help of optimal distribution force which come from optimization model. Simultaneously, the relative attitude of the operation robot is stabilized using attitude time-delay algorithm through the reaction wheels. Numerical results are presented, demonstrating the validity of saving thruster fuel and well performance in tracking the optimal trajectory.

     

An experimental investigation into the fault-tolerant control of an autonomous underwater vehicle

Autonomous underwater vehicles (AUVs) are expected to function in an unstructured underwater environment. One of the main requirements for effective operation in such an environment is to accommodate faults. In this paper, we have investigated a new approach to the allocation of thruster forces of an autonomous underwater vehicle under thruster faults. Generally, an AUV is equipped with more thrusters than what is minimally required to produce the desired motion. The proposed framework exploits the excess number of thrusters to accommodate thruster faults during operation. First, a redundancy resolution scheme is presented that considers the presence of excess number of thrusters along with any thruster faults and determines the reference thruster forces to produce the desired motion. These reference thruster forces are then utilized in the thruster controller to generate the required motion. This approach resolves the thruster redundancy in the Cartesian space and allows the AUV to track the task-space trajectories with asymptotic reduction of the task-space errors. Results from actual underwater experiments are provided to demonstrate the viability of the proposed scheme.     

Development of dam inspection robot with negative pressure effect plate

This paper describes the development of an underwater robot that performs visual inspection while making mechanical contact with a dam surface by a pulling force generated from thrusters with negative pressure effect plates. In general, small and lightweight remotely operated vehicles (ROVs) have low‐power thrusters. Their positioning performance is inferior with respect to external disturbances. Moreover, it is difficult for untrained operators to control the ROV position to check and inspect dams visually. A negative pressure effect plate attached to a thruster produces negative pressure that maintains mechanical contact between the robot and a dam wall surface and ensures stable robot motion on a dam surface to acquire clear continuous images. As described herein, we theoretically investigate the negative pressure effect plate characteristics and experimentally measure the force generated by the pressure difference. Results show that the force of a thruster with the effect plate is five times greater than the nominal thrust force provided by the thruster alone. Based on those results, we designed and developed the underwater robot with the negative pressure effect plates for dam inspection. Moreover, we conducted an experiment in a water tank and a field test at Shorenji Dam, Mie‐prefecture, Japan. The experimentally obtained results indicated that the negative pressure effective plate is effective at generating sufficient force and at realizing stable robot motion on the dam surface. Results demonstrate that the developed robot acquired clear images of the dam surface continuously with no sophisticated operator or controller.     

Motion/force control scheme for electrically driven cooperative multiple mobile manipulators

This paper presents the motion and force control problem of rigid-link electrically driven cooperative mobile manipulators handling a rigid object. Although, the motion/force control problem of cooperative mobile manipulators has been enthusiastically studied. But there is little research on the motion/force control of electrically driven cooperative mobile manipulators. Due to the inclusion of the actuator dynamics with the manipulator’s dynamics, the controller exhibits some important characteristics. For the electromechanical system, we have designed a novel controller at the dynamic level as well as at the actuator level. In the proposed control scheme, at the dynamic level, uncertain non-linear mechanical dynamics is approximated with a hybrid controller containing model-based control scheme combined with model-free neural network based control scheme together with an adaptive bound. The adaptive bound is used to suppress the effects of external disturbances, friction terms, and reconstruction error of the neural network. At the actuator level, for the approximation of the unknown electrical dynamics, the model-free neural network is utilized. The developed control scheme provides that the position tracking errors, as well as the internal force, converge to the desired levels. Additionally, direct current motors are also controlled in such a way that the desired currents and torques can be attained. In order to make the overall system to be asymptotically stable, online learning of the weights and the parameter adaptation of the parameters is utilized in the Lyapunov function. The superiority of the developed control method is carried out with the numerical simulation results and its superior robustness is shown in a comparative manner.     

带有饱和受限的挠性卫星变结构姿态容错控制

针对挠性卫星在飞行过程中存在推力器故障、控制输入饱和受限以及外部干扰的姿态控制问题,提出了一类基于变结构控制的鲁棒容错控制设计方法.该设计在继承变结构控制的优点的同时,显式地引入推力器输出的饱和幅值,以确保控制输出在其要求界的范围内.同时,充分利用星上推力器的硬件冗余以实现对部分推力器故障的容错,这种思想的引入使得所设计的控制器对故障具有很强的自适应能力,而且对设计者而言,推力器故障信息不需要进行在线的检测和分离.最后,将该控制器应用于某型卫星姿态调节控制,仿真结果表明该控制器能有效地抑制外部干扰、挠性结构的振动和推力器故障的约束,在实现姿态调节的同时,保证其控制输出满足饱和受限界的要求,具有较好的控制性能.     

Electrohydraulic force control design of a hardware-in-the-loop load emulator using a nonlinear QFT technique

This paper presents the design of a robust force control system for an electrohydraulic load emulator utilized as part of a hardware-in-the-loop flight simulation experiment. In this application, the force controlled hydraulic actuator is used to artificially recreate in-service loads upon a second hydraulic flight actuator operated in closed-loop position control. Electrohydraulic force control is more difficult than electrohydraulic position tracking because the load dynamics influence the force transfer function in a way that makes it challenging to develop an accurate force tracking system using simple feedback control. Nonlinear quantitative feedback theory (QFT) is applied in this paper to address this issue. First, an effective and robust feedback controller is designed by nonlinear QFT to desensitize the force control loop to nonlinear servovalve flow/pressure effects and typical system uncertainties. A secondary compensator is also designed within the QFT framework to extend the force tracking bandwidth with respect to the load motion. Experiments demonstrate acceptable force tracking performance within the scope of a representative flight-simulation experiment.     

Velocity-free fault-tolerant control allocation for flexible spacecraft with redundant thrusters

This paper proposes a novel velocity-free nonlinear proportional-integral (PI) control allocation scheme for fault-tolerant attitude control of flexible spacecraft under thruster redundancy. More specifically, the nonlinear PI controller for attitude stabilisation without using body angular velocity measurements is first designed as a virtual control of the control allocator to produce the three-axis moments, and can ultimately guarantee uniform boundedness of the closed-loop system in the presence of external disturbances and possible faults. The associated stability proof is constructive and accomplished by the development of passivity filter formulations together with the choice of a Lyapunov function containing mixed terms involving the various states. Then, a robust least-squares-based control allocation is employed to deal with the problem of distributing the three-axis moments over the available thrusters under redundancy, in which the focus of this control allocation is to find the optimal control vector of the actuator by minimising the worst-case residual, under the condition of thruster faults and control constraints like saturation. Simulation results using the orbiting flexible spacecraft model show good performance under external disturbances and even in different thruster fault scenarios, which validates the effectiveness and feasibility of the proposed scheme.     

Experimental study on fine motion control of underwater robots

This paper considers thruster dead zones and saturation limits, which are nonlinear elements that complicate fine motion control of underwater robots. If the vehicle is configured with redundant thrusters, the respective dead zones and their surrounding nonlinear regions could be avoided by implementing a null motion solution for the command input of the vehicle. This solution is derived from the vehicle's geometry and is realized before the application of the motion control algorithm. The result is an improvement in system performance exclusive of the implemented controller type. The approach is illustrated through simulation and experiment with an underwater robot, ODIN.     

基于干扰特性指标与推力分配的载人潜水器容错控制

载人潜水器在深海复杂恶劣的环境中运行, 容易受到外部环境干扰和推进器故障的困扰, 本文将干扰特性指标与基于推力分配的主动容错控制方法相结合, 提出了一种新的主动容错控制方法. 首先, 建立了载人潜水器系统的数学模型; 其次, 设计非线性干扰观测器估计系统所受的集总干扰, 并设计无推进器故障时的控制器; 然后, 针对推进器故障问题, 引入权重矩阵进行推力分配以保护故障推进器, 并设计了干扰特性指标表示干扰的性质对跟踪误差和推力损失的影响; 在此基础上, 给出了一种基于干扰特性指标与推力分配的新型容错控制方法, 李雅普诺夫方法保证了该控制算法的理论稳定性; 最后, 通过数值仿真, 验证所设计的控制方法的优越性. 其优越性主要体现在: 能够利用可用干扰与补偿有害干扰, 并将可用的干扰能量用于补偿故障推进器的推力缺失.     

推进器饱和约束的水面无人艇固定时间精准跟踪控制

针对复杂扰动、完全未知系统动态以及推进器饱和约束的水面无人艇高精度跟踪控制问题,提出一种基于固定时间非奇异终端滑模的无模型固定时间精准跟踪控制(MFPTC)方案.首先,设计有限时间集总观测器,精确重构和补偿集总未知项;其次,引入自适应辅助系统消除推进器饱和特性,使得MFPTC方案在饱和约束下实现期望时间内对预定轨迹的精准跟踪;进而,基于反正切函数构造固定时间幂次趋近律,加快滑模变量收敛速度且有效削弱控制抖振;最后,采用CyberShip Ⅱ实验模型进行仿真研究,结果验证所提出MFPTC方案的有效性与优越性.     

Thruster assisted position mooring based on structural reliability

This paper addresses dynamic positioning of surface vessels moored to the seabed via a spread mooring system, referred to as position mooring. In normal weather conditions, the mooring system constrains the vessel and the controller applies thruster force for motion damping and heading control, only. When environmental loads due to wind, waves, and current increase, thruster assistance is required also for positioning in order to avoid damage to the mooring lines. While traditional position mooring systems apply thruster force based on constraining the vessel to lie within a predefined geographical region, the controller designed in this paper instead employs structural reliability measures for the mooring lines to restrict movement. These structural reliability measures become an intrinsic part of the controller, enabling it to automatically adjust the admissible region for the vessel in the presence of changing winds, currents, and waves.     

Composite disturbance attenuation based saturated control for maintenance of low Earth orbit (LEO) formations

Maintenance of high performance formation control is important for low Earth orbit (LEO) formation missions of small spacecraft.In this paper,a model of nonlinear relative motion dynamics is built,and then nonlinear and important perturbations affecting the formation configuration,such as J 2 and atmospheric drag,are analyzed as disturbances.Global navigation satellite system based relative positioning with nonlinear filtering is adopted to provide state information associated with the perturbations.By combining disturbance observer based control with H ∞ state feedback,a composite disturbance attenuation controller is proposed for maintenance of continuous and accurate formation.With consideration of precise control relying on micro thrusters,a composite disturbance attenuation based saturated controller is designed and its stability is proved.Finally,through numerical simulations,we demonstrate that control accuracy is improved after effectively avoiding perturbations and that stabilization can be satisfied using this method.     

Thruster fault-tolerant control of a hovering AUV with four horizontal and two vertical thrusters

This paper deals with the motion control of a hovering autonomous underwater vehicle (AUV) with four horizontal and two vertical thrusters, when one or more thrusters are completely malfunctioned. Three thruster fault cases are considered: one horizontal thruster is faulty; two horizontal thrusters are faulty; and one vertical thruster is faulty. Through a series of simulations and an experiment, it is validated that the AUV can track a planned path in a 3-D space with minimally three thrusters (two are horizontal ones); however, some cases require the changes of the vehicle’s preferred direction of motion and its movement manner. Additionally, when the number of active horizontal thrusters is less than the required degree-of-freedom, a continuous state feedback control law does not exist due to the non-holonomic constraint. This paper highlights that the hovering AUV can overcome the non-holonomic constraint if using the feature that their translational and rotational motions can be controlled independently.     

Modeling and control of an atomic force microscope using a piezoelectric tuning fork for force sensing

This paper investigates modeling and control issues associated with an atomic force microscope which uses a piezoelectric tuning fork for atomic force sensing. In the modeling part, the dynamics of piezoelectric tuning fork and its atomic interaction with the test sample via the scanning tip are physically characterized. The modeling results explain not only the atomic force sensing mechanism but also the important characteristics observed in experimental frequency responses. In the control part, an LTR controller is designed to maximize the controller bandwidth and yet maintain robustness against unmodeled dynamics and different operating conditions. Scanning results indicate that the LTR controller exhibits superior performance than a conventional PI controller.     

Motion/force tracking control of nonholonomic mechanical systems via combining cascaded design and backstepping

This paper considers motion/force tracking control of a class of Lagrange mechanical systems with classical nonholonomic constraints. A tracking control method is proposed by combining cascaded methods and backstepping techniques. The main results of this paper include three parts: (1) error dynamics between the kinematic system and the desired paths are transformed into a cascaded system consisting of two subsystems and an interconnection function; (2) under the framework of cascaded methods, virtual controllers for the subsystems are designed to stabilize the error dynamics; (3) the tracking controller is designed for the overall mechanical systems using backstepping techniques.     

Development of a dual-shaft propeller thruster equipped with rotational speed sensor for UVMS control

Majority of underwater robots utilize single propeller thrusters for navigation. A disadvantage of using a single-shaft propeller thruster is that the thrust force generated from a single propeller for reverse and forward thrust is asymmetric due to the disturbed flow caused by the thruster’s body which may reduce thruster efficiency. Moreover, measurement procedures to precisely calculate propeller’s rotation speed were also not available. To address these problems, this paper proposes a dual-shaft magnetic coupling-driven propeller thruster for underwater robot, equipped with sensors for measuring propeller’s rotational speed. Numerical studies and experimental results on the position and orientation control of the proposed thruster are presented. Detail comparison of the rotational speed, thrust force and duty ratio between numerical calculation and actual experimental measurement results show the effectiveness of the proposed thruster. This paper also demonstrated that by using the proposed thruster, the control performance of an underwater robot can be improved significantly compared to commercially available thruster. The ability to determine propeller rotation directions is also a major advantage of this work.     

Semi-decentralized adaptive fuzzy control for cooperativemultirobot systems with H∞ motion/internal forcetracking performance

We present a semi-decentralized adaptive fuzzy control scheme for cooperative multirobot systems to achieve H(infinity) performance in motion and internal force tracking. First, we reformulate the overall system dynamics into a fully actuated system with constraints. To cope with both parametric and nonparametric uncertainties, the controller for each robot consists of two parts: 1) model-based adaptive controller; and 2) adaptive fuzzy logic controller (FLC). The model-based adaptive controller handles the nominal dynamics which results in both zero motion and internal force errors for a pure parametric uncertain system. The FLC part handles the unstructured dynamics and external disturbances. An H(infinity) tracking problem defined by a novel performance criterion is given and solved in the sequel. Hence, a robust controller satisfying the disturbance attenuation is derived being simple and singularity-free. Asymptotic convergence is obtained when the fuzzy approximation error is bounded with finite energy. Maintaining the same results, the proposed controller is further simplified for easier implementation. Finally, the numerical simulation results for two cooperative planar robots transporting an object illustrate the expected performance.     

In-orbit thruster calibration techniques and experiment results with UoSAT-12

In this paper, two practical thrusters calibration approaches are proposed, discussed and evaluated through in-orbit flight tests. In the single thruster pulse method, one thruster is commanded to fire for a fixed period to produce a pulsed external torque disturbance. Reaction wheels are used to stabilize the satellite subject to the thruster firings. Magnetorquer coils are switched off during the experiment to reduce the external magnetic disturbances. A preliminary test without thruster firing is used to record the unmodelled external disturbances. The resulting satellite attitude responses and reaction wheel status subject to the thruster firings are recorded for calibration analyses. A batch filter algorithm is developed to estimate the thruster torque coefficients from the recorded experimental data. In the autonomous method, a repetitive torque disturbance (by using a reaction wheel) is exerted to the stabilized satellite, the cold-gas thrusters are then applied in an on-board control system to stabilize the satellite attitude. By recording the known attitude disturbances and the thruster controller outputs, a recursive least-squares algorithm is used to process the recorded data and to estimate the thruster parameters. Both simulation and in-orbit test results are presented to evaluate the performance and design objectives. The in-orbit test results converge to the same values using the two different approaches.     

Adaptive control of nonlinear systems: A case study of underwater robotic systems

The problem of controlling underwater mobile robots in 6 degrees of freedom (DOF) is addressed. Underwater mobile robots where the number of thrusters and control surfaces exceeds the number of controllable DOF are considered in detail. Unlike robotic manipulators underwater mobile robots should include a velocity dependent thruster configuration matrix B( q ), which modifies the standard manipulator equation to: Mq + C( q ) q + g(x) = B( q ) u where x = J( x ) q . Uncertainties in the thruster configuration matrix due to unmodeled nonlinearities and partly known thruster characteristics are modeled as multiplicative input uncertainty. This article proposes two methods to compensate for the model uncertainties: (1) an adaptive passivity-based control scheme and (2) deriving a hybrid (adaptive and sliding) controller. The hybrid controller combines the adaptive scheme where M, C, and g are estimated on-line with a switching term added to the controller to compensate for uncertainties in the input matrix B. Global stability is ensured by applying Barbalat's Lyapunov-like lemma. The hybrid controller is simulated for the horizontal motion of the Norwegian Experimental Remotely Operated Vehicle (NEROV).