Shuangshuang Fan

Underwater vehicle control and estimation in nonuniform currents

While ocean and atmospheric vehicles often operate in time-varying, nonuniform currents, the effects of flow accelerations on vehicle dynamics are typically ignored in motion models used for control and estimation. Vehicle dynamics are either ignored entirely (the kinematic particle model), the vehicle is treated as a point mass (the dynamic particle or “performance” model), or the flow is assumed to be uniform. As applications for autonomous ocean and atmospheric vehicles expand into more constrained, dynamic environments, such as shallow water or urban airspace, the benefits of using more precise motion models for control and estimation become more compelling. Forces and moments due to flow gradients are strongest when apparent mass effects are important and flight paths are most sensitive to these disturbances when flow-relative velocities are small. The paper presents a dynamic model for a streamlined underwater vehicle in a nonuniform flow. To illustrate the utility of the full dynamic model, open- and closed-loop numerical motion predictions are compared with those of simpler models for a variety of nonuniform flow fields. We also demonstrate the application of the full dynamic model for flow estimation using an observer.

Terminal navigation and control for docking an underactuated autonomous underwater vehicle

Robust and accurate vehicle guidance and control are critically important for docking an underactuated Autonomous Underwater Vehicle (AUV) to a small and simple docking structure. In this work, only one high-power LED light is used for AUV visual guidance. Without the distance information, we estimate the relative heading, and control the AUV to track the docking station axis line with constant heading. The method is then modified to cope with the environment with current. Finally, the traditional PID control is used for yaw control. Through simulation and preliminary pool experiment, the performance of the navigation and control system are verified.

Underwater glider design based on dynamic model analysis and prototype development

Underwater gliders are efficient mobile sensor platforms that can be deployed for months at a time, traveling thousands of kilometers. Here, we describe our development of a coastal 200 m deep underwater glider, which can serve as an ocean observatory platform operating in the East China Sea. Our glider is developed based on dynamic model analysis: steady flight equilibrium analysis gives the varied range of moving mass location for pitch control and the varied vehicle volume for buoyancy control; a stability analysis is made to discuss the relationship between the stability of glider motion and the location of glider wings and rudder by root locus investigation of glider longitudinal- and lateral-directional dynamics, respectively. There is a tradeoff between glider motion stability and control authority according to the specific glider mission requirements. The theoretical analysis provides guidelines for vehicle design, based on which we present the development progress of the Zhejiang University (ZJU) glider. The mechanical, electrical, and software design of the glider is discussed in detail. The performances of glider key functional modules are validated by pressure tests individually; preliminary pool trials of the ZJU glider are also introduced, indicating that our glider functions well in water and can serve as a sensor platform for ocean sampling.

Stability and performance of underwater gliders

Underwater gliders are efficient mobile sensor platforms that can be deployed for months at a time, traveling thousands of kilometers. As with any vehicle, different applications impose different mission requirements which impact vehicle design. In this paper, we consider a conventional glider configuration and investigate the relationship between geometry and the stability and performance characteristics. We consider two specific flight conditions: minimum drag and maximum horizontal speed. Configuration parameters of interest include the fineness ratio of the hull; the wing position, wingspan, and aspect ratio; and the area and position of the vertical stabilizer.

Performance and Stability Analysis for ZJU Glider

Underwater gliders provide an effective, low-cost method for sampling the ocean over large spatial and temporal scales. In this paper, we present a series of theoretical analyses to provide guidelines for vehicle design, which are used to develop a coastal 200-m-depth underwater glider known as the Zhejiang University (ZJU) glider. The ZJU glider uses a longitudinally actuated moving mass for pitch control and a rudder for turning control. Computational methods and analytical approaches are chosen to solve the viscous and inviscid terms of glider hydrodynamics, respectively. Steady flight equilibrium analysis gives the varied range of moving mass location for pitch control and varied vehicle volume for buoyancy control. Size analysis investigates the effects of glider geometric parameters on motion performance. For wings-level flight, we describe the variation in the maximum lift-to-drag ratio corresponding to a given vehicle size and speed. For turning motion, we investigate the manner in which the turning performance varies with vertical rudder configuration. Stability analysis determines the relationship between the stability of glider motion and the locations of the glider wings and rudder. Pool trials indicate that the ZJU glider functions well in water and is capable of serving as a sensor platform for ocean sampling.

Hybrid Underwater Glider for Underwater Docking: Modeling and Performance Evaluation

The development of a novel type of hybrid underwater glider that combines the advantages of buoyancy-driven gliders and propeller-driven autonomous underwater vehicles has recently received considerable interest. However, few studies have considered a hybrid glider with docking capability, which would expand the gliders applications. This study presents a hybrid glider with a rotatable thruster for realizing underwater docking. A tailored dynamic model of the hybrid glider is derived, and the motion performance is evaluated by simulations and experimental tests. A comparison between the experiments and simulations shows that results are in agreement, thus indicating the feasibility of the dynamic model and the accuracy of the hydrodynamic coefficients. In addition, the hybrid glider open-loop docking tests validate the feasibility of the mechanical docking system. Moreover, the experimental tests also validate the gliders different functions and indicate that the hybrid glider with rotatable thruster has high maneuverability even at low speeds. Thus, this type of hybrid glider can be used for underwater docking.

Improving robustness of passive source localization via convex optimization based mode filtering

Model-based passive source localization is known to be quite sensitive to model mismatch. In this paper, we present a new implementation based on matched-mode processing. Specifically we formulate mode filtering as a convex optimization problem, and ℓ1-regularized least squares is applied, which is robust in terms of the numerical implementation. Simulations with respective full-depth and partially-spanned apertures show that with sound speed mismatch in water column, the proposed method has a better or equal tolerance over traditional matched-field processing; with sound speed mismatch in sediment layer, combining with mode selection, where higher order modes strongly interacting with the bottom are removed, our approach has a slightly decreased bias. Overall the new method demonstrates improved robustness of passive source localization for the examples we studied.

Impact of Current Disturbances on AUV Docking: Model-Based Motion Prediction and Countering Approaches

Underwater docking enables autonomous underwater vehicles (AUVs) to operate independently of a surface vessel for extended periods. To perform the docking mission, special attention has to be paid to the navigation, guidance, and control issues of the vehicle, especially under current disturbances. Based on a full dynamic model of an AUV in currents, this paper studies the influences of current disturbances on AUV docking motion. A comprehensive docking scheme is introduced, which combines Kalman filter type navigation, guidance with current compensation, and proportional-integral-derivative (PID) controllers for both cross-track and heading control, to ensure successful docking operations. To counter the current effects, the proposed guidance algorithm applies current estimation and attitude compensation for motion correction online; in addition, an upstream control strategy in the case of strong current is also discussed. The proposed algorithms are initially validated through model-based numerical simulations, which provide effective guidance for the succeeding docking experiments conducted in a current generating pool. The feasibility and effectiveness of the countering approaches for AUV docking under current disturbances are demonstrated during the pool trials.

Path planning for autonomous underwater vehicle docking in stationary obstacle environment

In this paper, we study the curvature-constrained path planning problem for AUV docking in stationary obstacle environment. Expanded obstacle circles are used to redefine the polygonal obstacles, thus the curvature constraint is incorporated into the path planning algorithm. Unlike the existing algorithms that try to avoid the obstacles when the planning path collides with them, the novel algorithm proposed in this paper takes the overall situation into consideration from the very beginning. Modeling the path planning problem using graph theory, the algorithm guarantees to generate the optimal Dubins path in polynomial complexity. The simulation results show that this algorithm outperforms the existing ones in terms of the path length.

Nonlinear observer design for current estimation based on underwater vehicle dynamic model

As applications for autonomous ocean vehicles expand into more dynamic and constrained environments, such as shallow, coastal areas, the benefits of using more precise dynamic model for control and estimation become more compelling. This paper presents a nonlinear observer for current estimation based on AUV dynamic model. Here, AUV dynamic model in currents is taken into consideration. Motivated by the design method of high-gain observer, we take the current disturbances as the uncertainties of the vehicle dynamic system and design the observer gain matrix with the goal of making the observer robust to the effect of current disturbances. The nonlinear observer estimates vehicles relative velocity firstly; current velocity is further calculated in an indirect way. The proposed current estimation method is validated by numerical simulation.