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Hydrodynamic/autopilot design of an AUV

The Navy is currently developing a small AUV that consists of a hull with counter rotating propellers on the front, a wing, and control fins arranged in an "X" configuration at the rear. This paper addresses the hydrodynamics as well as the autopilot design for this unique underwater vehicle. A high fidelity, six-degree-of-freedom (6-DoF) hydrodynamic and dynamics model was developed for use in the hydrodynamic and autopilot design. The performance analysis required frequency domain, linear time-domain, and nonlinear time-domain models. The vehicle model is a rigid body model consisting of individual geometry components that react to the local angle of attack/sideslip at each geometry component during nonlinear time domain simulations. All hydrodynamic models required for this simulation were developed by VCT. VCT utilized its generic vehicle modeling environment, referred to as VCT Toolstrade, to develop and integrate the hydrodynamics of the vehicle, IMU actuators and control system models that were required for the 6-DoF vehicle simulation. Production cost requirements limited the cost of the actuators and CPU to low frequency and low update rate COTS components. Since the speed of the AUV is equivalent to that of a 100 knot submarine, the challenge with the autopilot design was to control the vehicle high frequency dynamics with these low cost and low frequency components. The design approach used by VCF allowed the vehicle to be controlled with an off the shelf 4 Hz actuator and a 10 Hz update rate autopilot