Obstacle detection by a forward looking sonar integrated in an autonomous underwater vehicle

摘要

Autonomous underwater vehicles (AUV) by nature operate in partially unknown environments. Any obstacle lying in the path of the vehicle is a potential mission-terminating threat. Inclusion of a forward looking sensor would provide valuable information to the AUV. Threat assessment and navigation plans would use this information in order to avoid obstacles. Any such system should meet the requirements of an embedded autonomous system, that is small size and low power consumption. The obstacle detection system is to be integrated in the AUV via a common interface protocol. It is the intent of this paper to explore one possible solution to implementing such an obstacle detection system. The system used in this project is a forward looking sonar (FLS). This sonar system is a commercially available unit modified for performing obstacle detection tasks. With less than 4 W total power consumption this sonar can be integrated in an AUV. The small volume of the system allows easy placement in existing small AUV designs. The sonar control software is implementing in DOS on a PC/104 486 CPU. Filtered decision information is presented to the control logic of the existing AUV through a standard interface type (Lontalk network). A grid occupancy search method is used to detect the closest object in the vehicle's path. The region forward of the FLS is sub-divided into various cells. The cells are filled with the raw intensity data collected from the FLS sensor. For each filled cell, a cell signature is computed. The maximum signature cell is extracted from the grid. This cell contains transformed target information such as, range, bearing to target, and cell signature. The scanning scheme performs a first sweep at a short range for a quick detection of close targets, followed by a second sweep at a medium range. Cell signature definition and cell mapping are the research efforts associated with this paper. Experiments are performed on a moving platform, with the ultimate goal of testing the detection system integrated in a small AUV. This low-cost obstacle detection system can be easily attached to the existing FAU platforms, greatly improving the system safety and reliability of an AUV operation in high threat areas.