Design of a controllable variable buoyancy module and its performance analysis as a cascaded system for selective underwater deployment

Variable buoyancy systems (VBSs) are commonly used in underwater vehicles to achieve efficient heave motion. Most of the time, these systems are used as a part of conventional underwater vehicles with diverse methods to achieve the necessary buoyancy variation. The potential of VBSs as standalone systems for heave motion underwater has not been given due attention and is still an unexplored area. In this paper, the design of a VBS as a standalone module for heave control and the possibility of using multiple such modules in a cascaded structure for selective underwater deployment is presented. Such cascaded systems have a variety of applications ranging from coastal surveillance to selective deployment for ocean sampling, and so on. A metallic-bellows-based variable buoyancy module (VBM), actuated using linear actuators, is proposed here. The mathematical modelling of the VBM and its open-loop performance are analysed for suitability with regards to the system to be cascaded. Two prototypes have been developed based on the optimal design requirements and tested. The developed modules are simple and efficient for shallow water applications, and can be used as add-ons for existing underwater vehicles or can just be used as standalone systems. These two modules are then cascaded and the performance of the cascaded system is analysed with simulations and experiments. The analysis shows the suitability of the system for selective deployment at various depths.