Design and Modeling of a Two-Stage Towed Sensor Platform

Sea Technology, Jul 2006 by Schuch, Eric M, Linklater, Amy C, Woolsey, Craig A

A Passively and Actively Stabilized Towfish for Experimental Ocean Science

Advances in sensor technology have enabled new measurement techniques for ocean science. It has been proposed, for example, that a specially constructed, five-beam acoustic Doppler current profiler (VADCP) could be used to search for and measure small-scale ocean turbulence.1 Measuring small-scale ocean turbulence using a VADCP requires that a sensor's tilt attitude be precisely regulated. Moreover, the device should move quickly enough to search for turbulent hot spots, while providing real-time data to the ocean scientist, who can then adjust the survey plan according to the incoming data. These performance requirements and other concerns suggest the use of a towed sensor platform or towfish.

This article describes a towfish which was designed to house a Teledyne RD Instruments (San Diego, California) VADCP that is used by Dr. Ann Gargett of Old Dominion University's Center for Coastal Physical Oceanography.2 Towfish have long been used in ocean science and military applications. Some are even available commercially. The VADCP has a large, irregularly shaped housing and requires a custom platform. The towing system that was designed to house the VADCP incorporates passive and active stabilization in order to maintain the sensor's tilt angle within 1° of zero at towing speeds ranging from one to three meters per second.

The strict attitude regulation requirement presents a considerable challenge, even in light sea states. Wave-induced motion of the towing vessel is transmitted through the tether to the towfish. However, a clever towing arrangement can significantly reduce the effect of wave disturbances. The two-stage tow is an effective way to passively attenuate disturbances due to a towing vessel's motion. In the simplest arrangement, a depressor weight is attached to the umbilical towing cable some distance ahead of the towfish. The portion of cable between the towing vessel and the depressor is called the main catenary, while the portion between the depressor and the towfish is called the pigtail. Although the depressor weight experiences disturbance forces that are transmitted from the towing vessel, the pigtail length can be adjusted to attenuate the effect of these disturbances on the towfish motion.

Using a simple metal frame as the towfish, Gargett demonstrated a two-stage towing arrangement which maintained a tilt angle of less than 2°, with a root-mean-square tilt motion of less than 1°. However, with no active stabilization system, it was necessary to iteratively trim, launch and recover the platform in order to minimize pitch and roll biases. With tether lengths of several hundred meters, deployment and recovery is a tedious process. Using two independent servo-actuated stern planes, a towfish can actively cancel attitude biases and reject time-varying disturbances4,5.

System Design and Operation

The primary performance requirement is to regulate the VADCP tilt angle within 1° of zero at towing speeds ranging from one to three meters per second in sea state 3 conditions. In addition, the towfish must operate to depths of 200 meters and must pass data continuously from the VADCP to a supervisory computer aboard the towing vessel. The towfish is designed to carry the VADCP in either an upward or downward-looking configuration, although changing the sensor's orientation requires recovering the system.

Besides the VADCP, the system includes two servo-actuators, a PC-104 computer and a sensor suite comprising a vertical gyroscope (VG), a depth sensor and an altimeter. Power is provided From the towing vessel through the towing cable.

The towfish frame houses three large pressure vessels: the VADCP and two cylindrical housings that contain power converters, an onboard computer and a VG. The computer runs a LabVIHW program which collects serial data from the VADCP, VG, altimeter and depth gauge. The LabVIEW program also implements two proportional-integral-derivative (PID) control loops to regulate roll and pitch attitude. The controller outputs aileron and elevator commands, which are then mixed to provide appropriate individual commands for the two stern planes.

The LabVIEW-based control system allows an operator aboard the towing vessel to monitor towfish depth, altitude and tilt angle while the system is deployed by using the same serial link which transmits the VADCP data to the ship. The operator may toggle the control mode between manual and automatic. When the towfish is first deployed, the operator manually adjusts the stern plane angles until the mean roll and pitch angles are zero. The operator may then switch to automatic (feedback control) mode and may tune the PID control gains while underway.

To aid emergency recovery, the towfish is trimmed to be five percent buoyant and carries a xenon strobe/radio pinger in the topside vertical (In. Should the tether break between the depressor weight and the towfish nose, the towfish would float to the surface where the strobe/pinger would provide a recovery signal. As an additional safety precaution, the depressor weight is secured to the towing cable through a breakaway device. If the depressor were to inadvertently run aground, the weight of it would break away allowing the system to ascend safely away from the bottom.