Patent ID: 8296053

Claim:
A system for determining relative motion between combat system elements on a ship having a hull, comprising: first and second accelerometers associated with first and second combat system elements, respectively for generating first and second acceleration signals; first and second inertial navigation systems (INS) sensors positioned at different locations on-board the ship, the INS sensors configured to generate ship attitude signals; first and second filter modules coupled to said first and second accelerometers for receiving and conditioning said first and second acceleration signals, respectively for thereby generating conditioned accelerometer signals; and first and second displacement control modules coupled to said first and second filter modules for receiving said conditioned accelerometer signals from said first and second filter modules, respectively, and first and second ship attitude signals, and generating first and second signals representative of rotational and translational displacements of said combat system elements and calculating relative motion between the first and second combat system elements; wherein said first and second displacement control modules: determine the translational and rotational displacements of the first and second combat system elements due to hull modal vibration at times T 1 and T 2 ; determine the translational and rotational displacements of the first and second combat system elements due to forced vibration at times T 1 and T 2 by receiving first and second INS attitude signals from first and second INS sensors, differencing the first and second INS attitude signals to remove rigid body motion, and comparing the differenced attitude data to forced vibration deflection shapes determined previously from at-sea vibration data collection; and determine relative motion between the first and second combat system elements by differencing the translational and rotational displacements at each of the first and second combat system elements at times T 1 and T 2 ; and wherein the rotational displacements of the first and second combat system elements due to hull modal vibration at times T 1 and T 2 are determined by analyzing the conditioned accelerometer signals in the frequency domain to identify the first three ship's hull vibration mode frequencies, calculating a contribution factor for each of the three modes based on the fast-Fourier transform (FFT) magnitude of the signal, and calculating the rotational displacements using mode shapes derived from a hull finite element model and the calculated mode shape contribution factors.