Fiber optic control of an undulating platform

Abstract

The undulating vehicle SeaSoar is towed behind research vessels at speeds of approximately eight knots. The Woods Hole Oceanographic Institution (WHOI) sensor suite typically measures conductivity, temperature, depth (CTD), light attenuation, chlorophyll, PAR, bioluminescence, and acoustic backscatter. When towed with 1000 meters of cable (500 meters faired), the vertical range extends from the surface down to 350 meters with horizontal cycle spacing of 3 km. A new winch with 500 meters of cable (40 faired) is used in the shallow water (<100 meters depth) mode. The profiling range is typically within 2 meters of the surface down to 130 meters. A bottom avoidance system allows flight to less than 10 meters off the bottom. A GLOBEC requirement was to increase the data bandwidth to include two video telemetry channels. The earlier seven conductor copper-based load bearing cable used two conductors for control and three for telemetry. A new cable uses a novel design that contains three single-mode optical fibers, each surrounded by a copper conductor, enclosed within a two layered steel jacket. An onboard Engineering Unit (EU) which monitors dynamic engineering parameters has been modified to operate via a fiber optic transceiver. The vehicle sets its depth by rotating wings hydraulically powered and controlled by a current operated Moog valve. The PC-based WHOI SeaSoar Controller, which previously required two copper conductors to transmit the valve current down the sea cable, has been modified to send digital control data to the vehicle via a fiber optic transceiver. The current required to operate the wings is now developed in the EU. In the event of flight control telemetry loss, a hard-wired watchdog circuit forces the wings into an up position. Electronics located in the winch hub convert fiber optic signals to baseband video without the need for a fiber optic or coaxial slip rings.