Heave Motion Estimation on a Craft Using a Strapdown Inertial Measurement Unit

Abstract

This paper deals with heave position and heave velocity estimation on a craft. The estimation is done without any knowledge of the specified craft. An accurate estimation of these signals is useful when one wants to control or monitor the heave motions on a platform or a ship such as in a active heave compensated systems. The necessary sensor input for the proposed guidance system is a strapdown inertial measurement unit (IMU) which consists of three gyroscopes and three accelerometers. In this case study, the heave motion estimation is required as input for a control system on a Surface Effect Ship (SES) where it is desired to control the air cushion pressure in order to damp vertical motions. The motions are induced by sea wave propagations. A SES will experience high frequency accelerations on the hull compared to other vessels. A lift fan sets up these accelerations or process disturbances and complicates the performance of the estimation. The estimation is performed using an observer. The observer model is based on a set of superimposed sinusoidals, each with a different excitation frequency. The sum of these denotes the actual heave motion. The estimation algorithm is adaptive in terms of changes in the sea states. Results will be given using real experimental data from model tests of a 3 meter long SES.