Research On Inertial Measurement Method Of Ship Heave Motion Based On Digital Filtering And Phase Compensation

Ships are in a complicated marine environment during sea operations and passively generate heave motion along the vertical axis of the semi-fixed coordinate system under the action of sea waves.High-precision measurement of heave motion is of great significance in military and civilian fields.This paper starts from meeting the actual needs of high-precision heave measurement of ships,explores the heave motion measurement method with strong adaptive ability,good real-time performance and easy to implement,and provides alternative solutions for engineering applications.The paper specifically studies the following problems:Firstly,the basic mathematical model of heave motion is established on the basis of the ocean wave model.The working principle of strapdown inertial navigation system is introduced and the main factors affecting heave motion measurement is analyzed on this basis.It is found that the instability of the height channel of the inertial navigation system,the zero offset of the accelerometer,and the low-frequency oscillation interference signal dominated by the Schuler period cause the vertical information obtained by inertial navigation solution to diverge,which cannot be used directly.Secondly,aiming at the problems of vertical information divergence obtained by inertial navigation solution,the IIR digital high-pass filter is introduced to perform high-pass filtering on the inertial navigation solution results,and extract the heave motion from the divergence information.After comprehensive analysis,the heave measurement method based on inertial technology and digital filtering is proposed.The whole process is realized through twice integrations and twice digital filtering.Thirdly,the basic principle of the digital filter is introduced,and the IIR digital high-pass filter for ship heave measurement is designed,and the good filtering performance of the designed filter is verified by the heave motion filtering simulation.Also,the inherent problems such as phase advance and amplitude attenuation introduced by the IIR digital high-pass filter are analyzed and verified.Fourthly,the Band-limited Multiple Fourier Linear Combiner(BMFLC)algorithm is introduced to estimate each frequency of the filter output signal in real time and with high accuracy.Based on the changing sea conditions,the Modified-BMFLC algorithm is further proposed which makes full use of the wave spectrum analysis results in each sliding monitoring window as a priori information,using unequal interval partitioning for the entire heave band instead of equal interval partitioning,which improves the algorithm’s frequency estimation efficiency of the filter output signal.Aiming at the problems of phase advance and amplitude attenuation of the filter output signal,a method of amplitude and phase compensation based on Modified-BMFLC algorithm is proposed to compensate both amplitude and phase for filtered vertical velocity and heave displacement respectively which is verified by simulation.After the summary,the heave measurement method based on the Modified-BMFLC algorithm is finally obtained.At last,simulation and experimental results verify the feasibility of the new method,at the same time,compared to the BMFLC algorithm,the measurement precision of the vertical velocity is at least about 20% higher and the measurement precision of the heave displacement is about 15% higher after using the Modified-BMFLC algorithm.