Robust Control Based on Feedback Linearization for Roll Stabilizing of Autonomous Underwater Vehicle Under Wave Disturbances

In the case of Autonomous Underwater Vehicle （AUV） navigating with low speed near water surface, a new method for design of roll motion controller is proposed in order to restrain wave disturbance effectively and improve roll stabilizing performance. Robust control is applied, which is based on uncertain nonlinear horizontal motion model of AUV and the principle of zero speed fin stabilizer. Feedback linearization approach is used to transform the complex nonlinear system into a comparatively simple linear system. For parameter uncertainty of motion model, the controller is designed with mixed-sensitivity method based on H-infinity robust control theory. Simulation results show better robustness improved by this control method for roll stabilizing of AUV navigating near water surface.

Approximate Optimal Tracking Control for Near-Surface AUVs with Wave Disturbances

This paper considers the optimal trajectory tracking control problem for near-surface autonomous underwater vehicles(AUVs) in the presence of wave disturbances. An approximate optimal tracking control(AOTC) approach is proposed. Firstly, a six-degrees-of-freedom(six-DOF) AUV model with its body-fixed coordinate system is decoupled and simplified and then a nonlinear control model of AUVs in the vertical plane is given. Also, an exosystem model of wave disturbances is constructed based on Hirom approximation formula. Secondly, the time-parameterized desired trajectory which is tracked by the AUV's system is represented by the exosystem. Then, the coupled two-point boundary value(TPBV) problem of optimal tracking control for AUVs is derived from the theory of quadratic optimal control. By using a recently developed successive approximation approach to construct sequences, the coupled TPBV problem is transformed into a problem of solving two decoupled linear differential sequences of state vectors and adjoint vectors. By iteratively solving the two equation sequences, the AOTC law is obtained, which consists of a nonlinear optimal feedback item, an expected output tracking item, a feedforward disturbances rejection item, and a nonlinear compensatory term. Furthermore, a wave disturbances observer model is designed in order to solve the physically realizable problem. Simulation is carried out by using the Remote Environmental Unit(REMUS) AUV model to demonstrate the effectiveness of the proposed algorithm.

NONLINEAR EVOLUTION ANALYSIS OF T-S DISTURBANCE WAVE AT FINITE AMPLITUDE IN NONPARALLEL BOUNDARY LAYERS

The nonlinear evolution problem in nonparallel boundary layer stability was studied. The relative parabolized stability equations of nonlinear nonparallel boundary layer were derived. The developed numerical method, which is very effective, was used to study the nonlinear evolution. of T-S disturbance wave at finite amplitudes. Solving nonlinear equations of different modes by using predictor-corrector and iterative approach, which is uncoupled between modes, improving computational accuracy by using high order compact differential scheme, satisfying normalization condition I determining tables of nonlinear terms at different modes, and implementing stably the spatial marching, were included in this method. With different initial amplitudes, the nonlinear evolution of T-S wave was studied. The nonlinear nonparallel results of examples compare with data of direct numerical simulations (DNS) using full Navier-Stokes equations.

Robust autopilot with wave filter for ship steering

Research was done to overcome traditional problems associated with automatic steering systems of a ship in a seaway. A ship’s dynamic model with wave disturbances was built and a wave filter was designed by means of an extended states observer (ESO). The model estimated the low frequency motion component from a heading measurement corrupted by colorednoise, so back-and-forth steering caused by high frequency wave disturbances was avoided. At the same time, a robust autopilot system, designed by variable structure control theory, was presented. Simulation results achieved in dangerous sea status show that the wave filter works very well and the autopilot has strong robustness to environmental disturbances and model perturbation, and more importantly, the frequency of rudder adjustments is reduced noticeably.

Influence of pressure disturbance wave on dynamic response characteristics of liquid film seal for multiphase pump

Slug flow or high GVF(Gas Volume Fraction)conditions can cause pressure disturbance waves and alternating loads at the boundary of mechanical seals for multiphase pumps,endangering the safety of multiphase pump units.The mechanical seal model is simplified by using periodic boundary conditions and numerical calculations are carried out based on the Zwart-Gerber-Belamri cavitation model.UDF(User Define Function)programs such as structural dynamics equations,alternating load equations,and pressure disturbance equations are embedded in numerical calculations,and the dynamic response characteristics of mechanical seal are studied using layered dynamic mesh technology.The results show that when the pressure disturbance occurs at the inlet,as the amplitude and period of the disturbance increase,the film thickness gradually decreases.And the fundamental reason for the hysteresis of the film thickness change is that the pressure in the high-pressure area cannot be restored in a timely manner.The maximum value of leakage and the minimum value of axial velocity are independent of the disturbance period and determined by the disturbance amplitude.The mutual interference between enhanced waves does not have a significant impact on the film thickness,while the front wave in the attenuated wave has a promoting effect on the subsequent film thickness changes,and the fluctuation of the liquid film cavitation rate and axial velocity under the attenuated wave condition deviates from the initial values.Compared with pressure disturbance conditions,alternating load conditions have a more significant impact on film thickness and leakage.During actual operation,it is necessary to avoid alternating load conditions in multiphase pump mechanical seals.

不同液压条件下锥形液膜破碎的实验研究(英文)

An experimental study on the breakup of a conical liquid sheet from a pressure swirl atomizer was conducted by using a Particle Image Velocimetry (PIV) system. The variation of wavelength, wave speed and amplitude of wave versus liquid pressure was obtained. The results indicate that the cone half angle increases with liquid pressure. The stripped half wavelength liquid fragment seems to break into a series of drops immediately and no obvious contraction from liquid fragment to ligament is seen. The recorded images also show that both short and long wavelength waves exist simultaneously, and influence the breakup of the conical liquid sheet. The result of this study is useful for the purpose of developing and verifying the atomization model of spray produced by a pressure swirl atomizer.

Theoretical analyses on bed topography responses in large depth-to-width ratio river bends with constant curvatures

Bed morphology is the result of a dynamic response to a complex meandering river system. It is an important factor for the further development of river. Based on the meandering river characterized by a large depth-to-width ratio, a theoretical model is established with the coupling of Navier-Stokes （N-S）~ sediment transport, and bed deformation equations. The flow characteristics and bed response of river are obtained with the perturbation method. The research results show that, under the effect of two- dimensional flow disturbance, the bars and pools present the regular response. For a given sinuousness, the amplitude of the bed response can be used as a criterion to judge the bedform stability. The effects of the Reynolds number, disturbance wavenumber, sinuousness, and bed morphology gradient on the bed response development are described.