Deployment of a towed, flyable fish for the Video Plankton Recorder

Abstract

We have developed a towed, flyable fish for the Video Plankton Recorder that can fly vertically, horizontally, and hold a constant depth. The Video Plankton Recorder (VPR) has been developed to sample plankton taxa distributions over a broad range of scales. This new fish, or instrument platform, is part of a project to develop a second-generation towed VPR. Associated upgrades to camera and image processing will be discussed elsewhere. The previous towed VPR was mounted on a depressor fin and was winched up and down from a tow-ship and could automatically determine plankton taxa underway. The needs of the new VPR are that the fish to be towable at a ship speed of 5 m/s with the fish flying at 200 m depth, fly rapidly between different depths to allow denser horizontal sampling, have a horizontal optical path, fly out of the ship's wake into undisturbed water, and have minimal disturbance of the water in the optically sampled volume by the cable, bridle, or two-fish. The new tow-fish system has been towed at up to 6 m/s and has sample from the surface down to 200 m depth. The fish has three servos controlling three fins to control pitch, yaw, and roll. The yaw control is needed to fly the fish horizontally out of the tow-ship's wake when the fish is near the surface. At a 5 m/s design tow speed, dynamic roll control is needed because the restoring moment, from a separation of the center of buoyancy and center of gravity, is swamped by hydrodynamic moment, from the separation of the center of buoyancy and center of gravity, is swamped by hydrodynamic moments. For the fish to be stable at a constant shallow depth, the tow bridle does not make the system stable in roll as it can when the fish is pulling downwards. The challenges of the control system are to control the fast response to roll and pitch in the presence of disturbances from cable strumming and surface waves. The fish has a PC-104 format computer controller sampling an inertial measurement unit consisting of three-axis rate gyros and three-axis accelerometers. The fish system has a PC on the surface in the two-ship to display and record engineering data and to communicate supervisory instructions to the PC-104 computer in the fish. The new fish uses a smaller 8.18 mm (0.322 inch) diameter cable allowing the use of a smaller, lighter winch and has been towed from a 14 metre boat.