Abstract
The design process for Variable Buoyancy System (VBS) is not known in full, and existing approaches are not scalable. Furthermore, almost all the small size Autonomous Underwater Vehicles/Gliders (AUVs/G’s) use very low capacity of buoyancy change (in the range of few milliliters) and the large size AUVs require large buoyancy change. Especially for adverse weather conditions, emergency recovery or defense-related applications, higher rate of rising/sinking (heave velocity) is needed along with an ability to hover at certain depth of operation. Depth of UVs can be controlled either by changing the displaced volume or by changing the overall weight and, herein, our focus is on the later. This article presents the problem of design and analysis of VBS for efficient hovering control of underwater vehicles at desired depth using the state feedback controller. We formulate and analyze the design and analysis approach of VBS using the fundamental of mechanics, system dynamics integration and control theory. Buoyancy is controlled by changing the overall weight of the vehicle using the ballasting/de-ballasting of water in ballast tanks through the use of Positive Displacement Pump (PDP) for control in heave velocity and hovering depth. Furthermore, detailed mass metric analysis of scalable design of VBS for different buoyancy capacities is performed to analyze the overall performance of the VBS. Also, the performances of AUVs integrated with VBS of different buoyancy capacities are investigated in both the open loop and closed loop with the LQR state feedback controller. Hovering performance results are presented for three Design Examples (DEs) of AUVs with 2.8 m, 4.0 m and 5.0 m length and they are integrated with various buoyancy capacities at 9 kg/min rate of change of buoyancy. Results indicate that the AUVs achieve the desired depth with almost negligible steady state error and when they reach the desired hovering depth of 400 m the maximum pitch angle achieved of 16.5 degree for all the Des is observed. Maximum heave velocity achieved during sinking is 0.44 m/s and it reduces to zero when the vehicle reaches the desired depth of hovering. The presented computer simulation results indicate good performance and demonstrate that the designed VBS is effective and efficient in changing the buoyancy, controlling and maintaining the depth, controlling the heave velocity and can be used in rescue/attack operations of both the civil and defense UVs.
Highlights
A large area of the planet earth is occupied by oceans and they cover approximately 71% of Earth’s surface and 90% of the Earth’s biosphere
Our conceptual design approach of Design Option 2 (DO2) is shown in Figure 5 and we focus on the design of large capacity pump-driven Variable Buoyancy System (VBS)
We focus on the design of VBS for Underwater Vehicles (UVs) operating at the depth of upTottoal4c0h0anmgeetienr.buWoeyaonbcsyerve that the external gear pump is β= Weight o f the VBS
Summary
A large area of the planet earth is occupied by oceans and they cover approximately 71% of Earth’s surface and 90% of the Earth’s biosphere. Even though their importance is undeniable, they largely remain unexplored, i.e., less than 20% of the World Ocean has been explored [1] This scene demands vehicles that can be used for surveying the oceans for high endurance and operable at high depths. Except the Remotely Operated Vehicles (ROVs) and other hybrid vehicles that have an umbilical cable connecting the vehicle and mother-ship, all other UVs either carry the source of power on-board or are designed with self-propulsion mode. This scenario demands that the power on-board is utilized efficiently so that the endurance in terms of either range or operational abilities is optimized. Alternate mechanisms for buoyancy control need to be explored
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