Research On The New Method Of Ship Captive Model Test Based On The Test Platform With Six Degree Of Freedom

Hydrodynamic coefficient is an important parameter for ship maneuverability and sea-keeping prediction.It is of great significance to explore an efficient,stable and accurate measurement method of hydrodynamic coefficient for the study of ship maneuverability and sea-keeping.Captive model test is an important test method for the measurement of hydrodynamic coefficient.Although the classical Rotating Arm Mechanism(RAM)and Plane Motion Mechanism(PMM)have been widely used in captive model test in the past decades,both the rotating arm mechanism and plane motion mechanism have motion limitations for the measurement of the physical model with complex motion and the coupled hydrodynamic system.In this paper,the ship captive model test method is taken as the research object.On the basis of discussing the contribution of different test mechanisms to the measurement of hydrodynamic coefficient of captive model test method and their advantages and disadvantages,a "new ship captive model test method based on a new six degree of freedom test platform" is proposed.This paper focuses on the following aspects:A new six degree of freedom test platform is proposed.The platform consists of a control system,a trailer,a six degree of freedom motion mechanism,a six dimensional force/moment sensor and an ultrasonic range finder,which can drive the ship model to move in six dimensions.The control algorithm for the desired motion of the test platform around any set working origin is derived.The mapping of the motion attitude of the upper platform to the length of the driving leg is established and then the relationship of displacement,velocity and acceleration between the six driving legs and the ship model is established.To further simplify the calculation,the vector expression is solved as a matrix expression.Two conversion matrixes,noted as J1 and J2,are directly established to quickly convert the displacement,velocity and acceleration relationship between the six driving legs and the ship model,which improves the calculation efficiency.The six degree of freedom motion mechanism and the trailer can move relative to each other,which can overcome the shortage that the traditional constraint model test can only carry out the "small drift angle" test in the straight-line towing motion.This method improves the range of test and the accuracy of test data.This paper presents a method to measure the uncoupled hydrodynamic coefficients under the three degree of freedom motion equation by using a new type of test mechanism.According to the characteristics of ship constraint model test,the whole model of Abokowitz is selected as the basic equation of ship motion.Through the analysis of the equation,the principle of solving the hydrodynamic coefficient of the ship under the constraint model test is determined,and the method of solving the hydrodynamic coefficient related to speed and acceleration is proposed.The uncoupled hydrodynamic coefficients of the three degree of freedom motion equations are measured by the experiments of the straight-line towing oblique motion,the straight-line towing rudder motion,the pure sway,the pure surge,the pure yaw and the circular motion.A new type of test platform is used to replace the water tank of the rotating arm.A six degree of freedom motion mechanism and a trailer is used to achieve a point of rotation around the body.Under the condition that the angular velocity is small and the time history is sufficient,the size limit of the mechanism itself is broken.This method improves the radius of motion,and solves the problems of high cost,large land occupation and high cost of traditional rotating arm pool test.Under the proposed scheme of uncoupled hydrodynamic coefficient measurement,the static drag test,dynamic test,rotation test and coupling motion test which can be completed by multiple platforms in the past can be completed by only one test platform,which has the characteristics of good integration,low cost and high efficiency.A new method for measuring the coupling hydrodynamic coefficients which can not be efficiently and accurately measured by the traditional constrained model test is proposed.Three new coupling test methods are used to measure the coupling hydrodynamic coefficients and derivatives of the ship under the three degree of freedom equation,i.e.the constant drift angle circular motion coupling test,the fixed rudder angle circular motion coupling test and the drift angle rudder angle coupling test.This method makes the number of hydrodynamic coefficients contained in the equations of single test method less,the difficulty of decoupling low and the error of fitting small.In order to improve the accuracy of test data,the traditional discrete Fourier series method and polynomial direct interpolation fitting method are improved by using the characteristic coefficient method and error correction method respectively.Secondly,real-time state data driving mode is used to improve the real-time performance of the test platform.Finally,based on the fact that the error of hydrodynamic coefficient or derivative static uncoupling test results is less than that of static coupling test,and the error of static coupling test results is less than that of dynamic test,the static test correction method is used to correct the dynamic test and coupling test results,so as to improve the accuracy of test data.At the same time,this paper makes innovative research in the following three aspects:(1)A new method for measuring hydrodynamic coefficients by constrained model test based on 6-DOF test platform is proposed.The control algorithm of the desired motion of the test platform around any set working origin is derived,which drives the ship model to carry out six-dimensional controllable motion.The static towing test,dynamic test,rotation test and coupling motion test which need more than one platform in the past have been completed in one test platform,which has the characteristics of good integration,low cost and high efficiency.(2)This paper presents a method to measure the coupling hydrodynamic coefficient of a ship under three degree of freedom equation by three new coupling motion test methods.According to the principle of "Minimum Coupling Degree",the coupling hydrodynamic coefficient which is difficult to be measured by traditional mechanism under the three degree of freedom equation of ship is completed by using the circular motion with fixed drift angle,circular motion with fixed rudder angle and coupling test with drift angle and rudder angle.The error of measurement data is less than 8%.(3)A data processing method based on static test data to correct dynamic test error is proposed to improve the measurement accuracy of hydrodynamic coefficient.In general,t,based on the fact that the error of hydrodynamic coefficient or derivative static uncoupling test results is less than that of static coupling test and the error of static coupling test results is less than that of dynamic test,some test data with high accuracy in static test are put into dynamic equation and then the test data are fitted in order to reduce the error of dynamic test data.The test results show that the method proposed in this paper can accurately measure the hydrodynamic coefficient of the ship under the three degree of freedom equation,and the test results meet the actual needs of the project,which has a certain reference value for the study of ship maneuverability.