Title: Autonomous Vehicles and Instrumentation for Oceanography

Autonomous Surface Vehicle (ROSS)

The small autonomous surface vehicle (ASV) automatically follows programmed mission transects, while measuring sensor outputs along the tracks. This website gives you a clear picture of the mechanical construction of the ASV, the distributed architecture of controller area network (CAN)-based nodes for science and vehicle payloads, highspeed radio-frequency (RF) communications, the performance of the heading autopilot, global positioning system (GPS)-based guidance algorithm, and the mission programming technique. The field trials of the ASV, performed off the coast of Goa, India, are focused on retrieving the 2-D spatial distribution of surface chlorophyll, which is one of the useful parameters in characterizing the nature of calibration–validation (CALVAL) sites for ocean remote sensing needs. A further benefit of ASVs is that they can be built at a low cost and used in monitoring applications of diverse coastal ecosystems.

Mechanical design of ROSS

The hull is a free flooding high-density polyethylene (HDPE) cylinder sliced in two halves to produce two open shells. The nose section of ROSS is a modified reducer section commonly used in agricultural irrigation equipment. This is mirror-welded to the lower half HDPE shell and affords sufficient nose volume to accommodate a miniature in situ chlorophyll sensor. The HDPE shells enclose a cylindrical HDPE battery bin containing a bank of 18 lithium polymer cells, and a polyvinyl chloride (PVC) casing that accommodates the electronics, embedded controllers, and the inertial measurement unit (IMU) as seen in the longitudinal section (Fig. 3). Each bin has O-ring seals on the face of the endcaps to prevent the entry of water into these bins. A short vertically oriented PVC cylinder which houses the GPS and RF modem electronics, and their antennas is mounted near the nose end of the hull (see Figs. 1 and 2). It is the only part of the sea skimmer that protrudes above the water line. The skimmer is propelled by two brushless dc motors fixed on a detachable aluminum framework attached longitudinally to a welded rib on the lower HDPE shell as in Figs. 1 and 2. The total weight of the sea skimmer in air averages 95 kg. When placed in water, the entire bulk of ROSS is 95% submerged below the sea surface, thus minimizing its profile to surface wind forces, reducing pitching motion from waves, and showing excellent wave transparency particularly in rough weather. Streamlining of the body structure could significantly reduce viscous drag forces when it moves in the water.

Specifications of ROSS

1) Hydrostatic Considerations:ROSS was designed to be positively buoyant, with principal buoyancy contributions from the battery bin and the electronics casing providing a total buoyancy of 108 kg to offset the total dead weight in air of 95.5 kg. For the purpose of locating the positions of the center of buoyancy (CB) and the center of gravity (CG) of the complete structure, the origin of the reference axes was located at the nose tip. Computations on AutoCAD™ show that the separation between CB and CG in the vertical plane is about 51 mm with CG coordinates (Xg, Zg) at (1112, 24) mm and CB coordinates (Xb, Zb) at (1210, +27) mm. The hydrostatic restoring moment prevents ROSS from rolling over under most conditions. As a safety measure, two slim line empty PVC floats were secured to the sides of the main hull to reduce roll and pitch to within 1. This was not used in all our tests, but if integrated with the main hull, it results in a highly stable platform.

2) Towing Resistance, Propulsion, and Endurance: At a typical forward speed of n = 1.4 m/s, which was captured from GPS field data, and an associated Reynolds’ number Re = 2.45 x 10⁶, the towing resistance  R (or axial drag) of ROSS can be estimated R = 0.5rACdv²  from to be ~40 N where r =1000 kg/m³ is the density of seawater and Cd = 0.4 is the drag coefficient of the hull taken to be a finite cylinder with fineness ratio ~5. The hull is attached to a motor frame which presents an additional estimated drag of 8 N, resulting in an approximate total towing resistance of 48 N and a towing power equal Pt = R x v to 66 W. The vehicle is propelled by two Tecnadyne 520 brushless dc motors each capable of a maximum forward thrust of 70 N at 940 r/min in water. In practice, input power of 304 W was supplied to the thrusters from a bank of lithium polymer batteries at a working current of 1.4 A per thruster. This gives a total propulsion system efficiency h = 0.21 . The endurance of ROSS is approximately 7 h assuming that the 12-Ah bank of batteries is derated by 20%, and that the thrusters require an average current drain of 1.4 A. The hotel payloads on ROSS, which include navigational and communications hardware, chlorophyll, altimeter sensors, and control electronics, have a current drain of <1.5 A from a separate bank with a similar discharge rate of 0.1 C (C-rate). The specifications of the experimental ROSS craft are summarized in Table I.

Table 1 Main specifications of the ROSS vehicle

In Situ CALIBRATION OF CHLOROPHYLL SENSORA

low-cost miniature submersible fluorometer (MiniTracker II, Chelsea Instruments, U.K.) was used in experiments to measure in situ chlorophyll concentrations along the mission tracks followed by ROSS. The fluorometer has a concentration range from 0.03 to 100 mg/m which varies linearly with output voltage in the range 0 to 4 V dc . It uses a high-intensity blue (430 nm) light-emitting diode (LED) source as the excitation source and receives fluorescence from chlorophyll cells at a center wavelength of 685 nm. The in situ sensor is mounted below the nose volume on ROSS, thus avoiding spurious signals arising from air bubbles released from the turbulent wake of the propeller motors which are located further down in the aft section. The motion of the craft causes seawater to flow past an open cowled enclosure on the sensor endcap. This arrangement emulates a dark chamber and does not require seawater to be pumped through it. The recommended method of calibrating chlorophyll sensors is by the in vivo chlorophyll method. The reason for this is because the calibration graph depends on the plankton species in seawater. For this determination, a Perspex box was filled with 3 L of freshly filtered seawater whose concentration was measured previously by the standard method using a Turner Design fluorometer [see (1)]. Measurements of the sensor output were made over a longer 15-s interval for different seawater samples, including a blank sample using distilled water. The in vivo graph shows a slope of 5.084 mg/m /V with a fitted linear regression line to measured chlorophyll concentration of in situ water samples in Fig. 4. We took adequate care in suspending the sensor inside the Perspex box of seawater, and in lining the sides of the box with black paper so as to cut down on possible spurious reflection and fluorescence from Perspex. The calibration equation used in converting sensor volts to chlorophyll is áChl ─ añ = (5.084) X sensor volts ─ D.147