Development of Active Heave Compensator

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Heave compensator is used to isolate instruments and structures, which are suspended from the floating body, from heave motion. In deepsea, heave motion detection is very difficult because the direct and realtime measurement by ultra sound and so on are impossible. In this research an active motion compensator system, which consists of accelerometer and actuator, has been developed. In this system, drift is removed by using the knowledge that average pisition of floating body stays mean water level.Firstly FIR digital filter was designed to double integrate the acceleration and remove the drift, high frequency and low frequency noise. The calculation is done numerically in a microprocessor. It became obvious during testing that the calculation of floating body position is extremely sesitive to any false fluctuation of signal, such as low-amplitude steady-state and very low frequency offsets from the true signal. With bypassed Low Pass Filter, low frequency component of noise is detected and successfully removed from the signal. This filter can detect heave motion of relatively narrow bandwidth. Simulation and experiment were performed and the function of the filter was confirmed.Secondly aiming more precise filter, another type of filter with sequential least square algorithm was developed. This algorithm enable to identify noise component without loss of realtimeness. Periodical reset is also employed to ensure the algorithm works permanently and to prevent divergence of parameters. With this algorithm heave motion is accurately detected without drift and phase delay. This algorithm also stable when initiated during the heaving. This algorithm was successfully tested by experiment with heave compensator on the servo-controlled oscillating stage.