A theoretical approach to facilitating transition phase motion in a positively buoyant autonomous underwater vehicle

Positively buoyant autonomous underwater vehicles (AUVs) operate at survey speeds with a pitch angle that is maintained through application of the control surfaces, sufficient to generate hydrodynamic forces to counteract the excess buoyancy. To facilitate lower forward speeds and the ability to hover requires some additional method of control. This paper reviews possible options and then indicates how control can be achieved using a single or pair of through-body tunnel thrusters. New equations appropriate to AUVs are proposed and experimental results are used to estimate the equation parameters. These equations are used within a simulation of the Autosub AUV to determine the response of the AUV during the transition between survey and low speed operation. The results obtained from the simulations are analysed in terms of the performance of the AUV and the demanded energy levels to assess the feasibility of using tunnel thrusters as a low speed control device.