Requirements and design of highly accurate position control system for underwater visual inspection

Remotely operated vehicles (ROVs) have been used for various underwater tasks such as mapping, inspection, welding, and maintenance of equipment. Owing to the strong demand for downsizing and cost-effective operation in relevant markets, the technologies developed for ROVs have improved rapidly. Nevertheless, several difficulties prevent achieving the requirements of a high-performance control system. In particular, the stability of a downsized ROV is a critical requirement to acquire high-quality mission sensor data such as underwater optical images or imaging sonar data. In this paper, we describe several requirements for a highly accurate positioning control system. We have designed a controller based on the assumption that an ROV could be modeled by a linear system in position keeping control. The proposed controller consists of a modelbased compensator called a robust internal-loop compensator (RIC) in an internal loop, and a conventional PID controller and a null motion controller in an external loop, where the model we used in the RIC was designed by using experimental data of position keeping motion. The proposed scheme was experimentally verified by an observation class ROV, iTurtle, developed by Korea Research Institute of Ships and Ocean Engineering (KRISO).