Mechanical Design of a Sonar Mount for an Unmanned Surface Vehicle

Abstract

Trends in USV research will continue on the path toward a fully autonomous USV capable of troop transport or enemy engagement. Imaging sonar will be an integral part of this development. However, due in part to sonar's inherent physical limitations, as well as its sensitivity to environmental factors, sonar technology represents a bottleneck to the development of situationally aware USVs capable of high-speed maneuvers. The work presented in this thesis is intended to provide a platform to bridge this gap, which is the design, analysis, and field testing of a mount for an imaging sonar intended as a retrofit for an existing vessel. The result of this work represents a step toward the ultimate goal of a fully autonomous USV, and will enable the advancement of research in the use of imaging sonar on surface vehicles. This thesis examines the problem of mounting a sonar on a surface vehicle from a fundamental perspective. It describes the development of a list of customer needs, presents a prototype design, and presents the important analyses for the prototype. The prototype mount was built, and field testing for proof of concept was carried out on the Virginia Tech USV, which is a Rigid Hull Inflatable Boat (RHIB), and the Navy Special Operations Craft - Riverine (SOC-R) on the Pearl River at Stennis Space Center. Testing showed the mount to be highly effective at limiting risk to personnel and equipment while operating in difficult environments like swamps. However, it also exposed some limitations associated with the mount's breakaway device, and the mounting location at the side in 2012, and at the stern in 2013. Based on experience gained from testing, a new mount design is presented for use at the bow. The bow location offers better impact protection to the sonar as long as the sonar can be positioned above the boat's draft. Field tests also exposed the need for an omnidirectional breakaway device which limits impact loads on the sonar during collisions. The Ball and Socket Breakaway (BSB) device was designed to satisfy this need. The BSB is acts as a "mechanical fuse," which holds the sonar rigidly under normal operating conditions, but will slip and rotate when the sonar strikes an object. It is designed to respond to impact loads on the sonar from the front, sides, or back, resulting in improved sonar protection during the varied maneuvers necessary for operation in shallow, narrow passageways. The expected moment holding capacity of the BSB as it is currently designed is 300 N-m (2650 lb-in), which should allow for speeds up to 3 m/s (6 kt) before drag-induced breakaway.