Study on the Characteristics of A New Hybrid Mooring System for DualPlatform Joint Operations

As the sustainable exploitation of marine resources develops,dual-platform joint operation has caught increasing attention.Dual-platform joint operation requires smaller relative motion between the two sub-platforms,which is normally difficult to be satisfied by the traditional mooring system.Therefore,a new hybrid mooring system is developed and studied in this article.To ensure safety during platform movements,both the number of anchor chains and the relative motion between the two sub-platforms are reduced in the new hybrid mooring system.By performing numerical simulations based on three-dimensional potential flow theory in AQWA and physical experiments,the performances of both the new hybrid and traditional mooring systems under two different wave conditions(i.e.,working wave and freak wave conditions) are systematically investigated.Regarding the new hybrid mooring system,the relative stability between the two sub-platforms of the new system is better,and the platforms can restore stability faster when affected by freak waves.

Structure and Dynamic Characters of New Radar Stabilized Platform

Aimed at requirements, a kind of radar stabilized platform with external rotor and orthogonal shafts is designed. Its dynamic characters are analyzed by using finite element theory and ANSYS software. Also, a traditional stabilized platform with inner rotor is designed for comparison. It is shown that the new platform reduces two transmission links and 20% of precise components, its weight decreases by 20%, its natural frequency and rigidity are enlarged, the operating accuracy and stationary are increased. The new stabilized platform is more superiority and practicability. Its design method and analysis results have already been used in development mission. It provides helpful reference for similar structure designs.

Orientation density and workspace analysis of a parallel stabilized platform testing system

An optimized workspace calculation method is proposed for parallel stabilized platform testing systems.This method refines the searched space progressively in order to approach the boundary of the workspace from both the inside and the outside of it.The orientation density is defined and used as an evaluation index to calculate the orientation workspace.The algorithm of the orientation density is embedded into the computer program of the workspace calculation.Then the workspaces of the testing system are solved.In the solution,the orientation density is regarded as a discrete function of the reachable workspace.As a result,the reachable workspace and the orientation workspace are represented in the same multidimensional graphs.Finally the useful workspace of the testing system is determined based on these results.This case study indicates that the calculation efficiency is enhanced by adopting the optimized method and the practicability of workspace study is improved by proposing the orientation density.

Reduced-order design of Robust H_∞ Controller for an Inertial Stabilized Aerial Platform

The uncertainty disturbance is one of the main disturbances that seriously influences the stabilization precision of an aerial inertially stabilized platform(ISP)system.In this paper,to improve the stabilization precision of the ISP under disturbance uncertainty,a robust H∞controller is designed in this paper.Then,the reduction order is carried out for high-order controllers generated by the robust H∞loop shaping control method.The application of the minimum implementation and balanced truncation algorithm in controller reduction is discussed.First,the principle of reduced order of minimum implementation and balanced truncation are analyzed.Then,the method is used to reduce the order of the high-order robust H∞loop shaping controller.Finally,the method is analyzed and verified by the simulations and experiments.The results show that by the reduced-order method of minimum implementation and balanced truncation,the stabilization precision of the robust H∞loop shaping controller is increased by about 10%.

Dynamic Simulation and Disturbance Torque Analyzing of Motional Cable Harness Based on Kirchhoff Rod Model

The motional payloads on stabilized platform must be linked by some cable harnesses with other immobile apparatus.During the operation of stabilized platform,these cable harnesses can create spring disturbance torque which is exerted on the stabilized platform and then reduce the stabilizing precision.None of current studies can deal with the spring disturbance torque problem.To analyze the spring disturbance toque,a dynamic thin rod model is presented for simulating the motional cable harness which is based on the Kirchhoff rod theorem and can consider the geometrically non-linear effects.The internal bending and torsion restoring torques are simulated and then a predictive analysis of the disturbance torque can be performed in motional cable routing design.This model is solved with differential quadrature method（DQM）.By using zeros of the Chebyshev polynomial as the grid points,the arc-coordinate is discretized to obtain a set of ordinary differential equations in time domain which is solved by implied method to obtain the profile and internal force of cable harness.The accuracy of this model is validated by comparing the simulation results and the experiment results（both the spring force and the deformed profile of the motional cable harness）.In the experiment,a special optical measuring instrument based on binocular vision is developed.The comparison of experimental and simulated results shows that the simulation model can represent the real motional cable harness well,and the spring disturbance force simulation results are precise enough for spring disturbance torque analysis.This study will be helpful to obtain an optimized motional cable harness layout design with small spring disturbance torque.

Investigation on Cutting Stability of Jacket in Decommissioning Process

Jacket cutting operation is one of the most complicated and highest risk operations in the process of decommissioning offshore piled platform, the security and stability of which must be assured. In this paper, the current research on offshore structure removal and jacket cutting is introduced, on the basis of which the types of load along with the load calculation method are determined. The main influences on the stability of a jacket in cutting are analyzed. The experiment test plan is drawn by using orthogonal testing method, and the formula of critical load during the cutting procedure is deduced by differential evolution algorithm. To verify the method and results of this paper, an offshore piled platform to be decommissioned in the South China Sea is taken for an example, and the detailed schedule for jacket cutting is made with the three-dimensional finite element model of the jacket established. The natural frequency, stress, strain and stability of the jacket during cutting process are calculated, which indicates that the results of finite element analysis agree well with that of the deduced formula. The result provides the scientific reference for guaranteeing the safety of jacket in cutting operation.

Improved Angular Velocity Estimation Using MEMS Sensors with Applications in Miniature Inertially Stabilized Platforms

The performance of any inertially stabilized platform （ISP） is strongly related to the bandwidth and accuracy of the angular velocity signals. This paper discusses the development of an optimal state estimator for sensing inertial velocity using low-cost micro-electro-mechanical systems （MEMS） sensors. A low-bandwidth gyroscope is used alone with two low-performance accelerometers to obtain the estimation. The gyroscope has its own limited dynamics and mainly contributes to the low-frequency components of the estimation. The accelerometers have inherent biases and mainly contribute to the high-frequency components of the estimation. Extensive experimental results show that the state estimator can achieve high-performance signals over a wide range of velocities without drifts in both the t- and s-domains. Furthermore, with applications in miniature inertially stabilized platforms, the control characteristic presents a significantly improvement over the existing methods. The method can be also applied to robotics, attitude estimation, and friction compensation.

Application in prestiction friction compensation for angular velocity loop of inertially stabilized platforms

Abstract To overcome the influence of the nonlinear friction on the gimbaled servo-system of an inertial stabilized platforms （ISPs） with DC motor direct-drive, the methods of modeling and compensation of the nonlinear friction are proposed. Firstly, the inapplicability of LuGre model when trying to interpret the backward angular displacement in the prestiction regime is observed experimentally and the reason is deduced theoretically. Then, based on the dynamic model of direct-drive ISPs, a modified LuGre model is proposed to describe the characteristic of the friction in the prestiction regime. Furthermore, the state switch condition of the three friction regimes including presliding, gross sliding and prestiction is presented. Finally, a composite compensation controller including a nonlinear friction observer and a feedforward compensator based on the novel LuGre model is designed to restrain the nonlinear friction and to improve the control precision. Experimental results indicate that compared with those of the conventional proportion-integrationdifferentiation （PID） control method and the PID plus LuGre model-based friction compensation method, the dwell-time has decreased from 0.2 s to almost 0 s, the position error decreased to 86.7% and the peak-to-peak value of position error decreased to 80% after the novel compensation controller is added. It concludes that the composite compensation controller can greatly improve the control precision of the dynamic sealed ISPs.

High-precision gyro-stabilized control of a gear-driven platform with a floating gear tension device

This study presents an improved compound control algorithm that substantially enhances the antidisturbance performance of a gear-drive gyro-stabilized platform with a floating gear tension device.The tension device can provide a self-adjustable preload to eliminate the gap in the meshing process.However,the weaker gear support stiffness and more complex meshing friction are also induced by the tension device,which deteriorates the control accuracy and the ability to keep the aim point of the optical sensors isolated from the platform motion.The modeling and compensation of the induced complex nonlinearities are technically challenging,especially when base motion exists.The aim of this research is to cope with the unmeasured disturbances as well as the uncertainties caused by the base lateral motion.First,the structural properties of the gear transmission and the friction-generating mechanism are analyzed,which classify the disturbances into two categories:Time-invariant and time-varying parts.Then,a proportional-integral controller is designed to eliminate the steady-state error caused by the time-invariant disturbance.A proportional multiple-integral-based state augmented Kalman filter is proposed to estimate and compensate for the time-varying disturbance that can be approximated as a polynomial function.The effectiveness of the proposed compound algorithm is demonstrated by comparative experiments on a gear-drive pointing system with a floating gear tension device,which shows a maximum 76%improvement in stabilization precision.

Generation of Accelerated Stability Experiment Profile of Inertial Platform Based on Finite Element

The residual stress generated in the manufacturing process of inertial platform causes the drift of inertial platform parameters in long-term storage condition.However,the existing temperature cycling experiment could not meet the increased repeatability technical requirements of inertial platform parameters.In order to solve this problem,in this paper,firstly the Unigraphics（UG） software and the interface compatibility of ANSYS software are used to establish the inertial platform finite element model.Secondly,the residual stress is loaded into finite element model by ANSYS function editor in the form of surface loads to analyze the efficiency.And then,the generation based on ANSYS simulation inertial platform to accelerate the stability of experiment profile is achieved by the application of the analysis method of orthogonal experimental design and ANSYS thermal-structural coupling.The optimum accelerated stability experiment profile is determined finally,which realizes the rapid,effective release of inertial platform residual stress.The research methodology and conclusion of this paper have great theoretical and practical significance to the production technology of inertial platform.