Real-time zero phase filtering for heave measurement

Heave measurement is an important technology for many marine works. Inertial Measurement Unit (IMU), which is able to sense the accelerations of the ship motion and integrate them to compute heave velocity and displacement, is the most widely used device for heave measurement. High-pass filters are always used in heave measurement to separate the heave signals from integration drifts. However, linear filters inherently alter amplitude and phase characteristics of the desired signals and incur attenuation and phase-shift. A new method that performs real-time zero phase filtering to improve the accuracy of measurement is presented in this paper. The basic concept of the proposed zero phase filter is to design a pair of cascaded high-pass and all-pass IIR filters, such that the phase lead caused by the high-pass filter is balanced by the phase lag introduced by the all-pass filter. The weighted-frequency Fourier linear combiner (WFLC) algorithm is adopted as an adaptive frequency estimator to provide the instantaneous dominant frequency of the signal of interest to adjust the coefficients of all-pass filter to yield the required performance. The proposed filter is implemented in an experiment to measure the heave motion of a parallel motion simulator.