Underwater Glider Control

Fig. 3. Spray's method of travel (Spray, 2008b)

The "glide control in Spray is achieved exclusively by axial translation and rotation of internal battery packs. Pitch is controlled simply by moving the center of gravity in the manner of a hang glider. Turning is initiated by rolling. This gives the lift vector a horizontal component and induces vehicle sideslip in the plane of the wing in the direction of the buoyant force. The horizontal component of lift provides the centripetal force for turning while sideslip acting on the vertical stabilizer produces the yaw moment needed to change vehicle heading. For example, to turn right during descent the right wing is dropped, like a conventional airplane, generating a lift component to the right that drives the vehicle to the right. Sideslips down and to the right acts on the vertical stabilizer causing the nose to yaw to the right. To turn right in ascent the glider is rolled oppositely by dropping the left wing" (Griffiths, 2002).

Additionally, the vehicle must rotate 90 degrees to present the GPS and satellite communication antennas housed in a wing. Payload is mounted either in the expandable aft flooded component section or on the hull. Seaglider & Deepglider

The Seaglider and Deepglider (see Figures 5, 6 and 7), commercially sold by iRobot (Bedford MA, USA), are similar to the Spray and Slocum Battery gliders. The Seaglider and Deepglider are identical in looks but the Deepglider is made out of a composite pressure hull of thermoset resin and carbon fiber making it capable of diving to a depth of 6000 meters (Osse & Lee, 2007)(0sse et al., 2007). The Seaglider using an efficient use of energy allows it to operate one-year 4600 km missions (Eriksen et al., 2001). Seaglider uses a hydrodynamic aluminum pressure hull that is contained within a free-flooded fiberglass fairing2 that supports the wings. The flooded aft section is used to carry self-contained instruments on both the Seaglider and Deepglider. Both vehicles have a trailing antenna rod and the fairing encloses the pressure hull. In weak currents the vehicles can maintain position by pitching vertically with minimal buoyancy. As with the Slocum series and Spray gliders, the Seaglider and Deepglider control their buoyancy with a hydraulic system similar to the ALACE system. In both the Seaglider and Deepglider it is the movements of internal masses (i.e., batteries) which control the pitch and yaw of the vehicle while gliding, and also raise the antenna for communication and GPS navigation. Another interesting aspect of these two vehicles is that due to the lifting wings being so far aft the turning method is opposite to what one would expect (i.e., opposite to the Slocum and Spray gliders). To turn right while descending the left wing is lowered so that the wing lift pushes the stern left, "overcoming lift off the vertical stabilizer, and initiating a turn to the right. Hydrodynamic lift on the side slipping hull produces the centripetal force to curve the course. Conversely, in ascent a roll to the left produces a left turn" (Griffiths, 2002). The Seaglider has made thousands of dives since its inception in 1999. Some of these dives can be seen on Seaglider's website: http://www.apl.washington.edu/projects/seaglider/ summary.html. The first Deepglider tests were made in November 2006 off the Washington state coast where it made test dives for 39 days with dives down to 2713 meters depth and a lateral distance of 220 km.

Seaglider (Griffiths, 2002)(APL, 2008) (Osse et al., 2007)

Weight: Hull Diameter: Vehicle Length: Wing Span: Depth Range: Payload: Speed, projected:

Energy: Energy:

Range: Navigation: Sensor Package:

Communication: Deepglider (Osse et al., 2007) Weight: Hull Diameter: Vehicle Length: Wing Span: Depth Range: Payload: Speed, projected:

Energy:

25 kg

0.25 m/sec (1/2 knot) horizontal

Lithium primary batteries

Lithium primary batteries

4600 km (3800 km proven mission)

GPS, and internal dead reckoning, altimeter

Sea-Bird temperature-conductivity-dissolved oxygen,

Wet Labs fluorometer-optical backscatter

Iridium satellite

25 kg

0.25 m/sec (1/2 knot) horizontal Lithium sulfuryl chloride batteries

2 A fairing is a structure whose primary function is to produce a smooth outline and reduce drag

Range: Navigation: Sensor Package:

Communication:

8500 km

GPS, and internal dead reckoning, altimeter Sea-Bird temperature-conductivity-dissolved oxygen, Wet Labs fluorometer-optical backscatter Iridium satellite

Fig. 4. Spray Schematics (Spray, 2008b)

Satellite Thermal Control

Fig. 4. Spray Schematics (Spray, 2008b)

Underwater Glider

Fig. 5. Seaglider's method of travel (Seaglider, 2008)

Remote Control Helicopter Schematics
Fig. 6. Seaglider Schematic (Griffiths, 2002)
Remote Control Helicopter Schematics

^-Shear Control Sleeve

Fig. 7. Deepglider (Osse et al., 2007) Slocum Thermal Glider

The Slocum Thermal glider (see Figures 8 and 9) was developed and optimized for long duration missions with a well-developed thermocline. The propulsion of the vehicle is

^-Shear Control Sleeve

Fig. 7. Deepglider (Osse et al., 2007) Slocum Thermal Glider

The Slocum Thermal glider (see Figures 8 and 9) was developed and optimized for long duration missions with a well-developed thermocline. The propulsion of the vehicle is derived from harnessing the energy of the thermal gradient between the ocean's surface and bottom for use as the vehicle's propulsion. "In missions with electric-powered gliders, 60-85% of the energy consumed goes into propulsion, so a thermal-powered glider may have a range 3—4 times that of a similar electric-powered vehicle. Except for its thermal buoyancy system and using roll rather than a movable rudder to control turning, Slocum Thermal is nearly identical to Slocum Battery" (Griffiths, 2002).

The Slocum Thermal glider uses the change in volume from a material's (ethylene glycol) freezing and melting as the means of vehicle propulsion. The vehicle begins to descend by venting the external bladder into an internal bladder using the pressure difference between the two chambers (i.e., the hull/internal bladder, filled with Nitrogen, is slightly below atmospheric pressure). As the vehicle passes through the freezing point of the material during its descent the contraction of the material causes the fluid in the internal reservoir to be drawn out into a heat exchanger. To ascend the pressurized material in the heat exchanger is transferred to the external bladder causing the vehicle to switch from negative to positive buoyancy. As the vehicle ascends the warming of the ocean waters cause the material to melt and expand further increasing its buoyancy. The vehicle arrives at the surface with the same conditions it had at the start, i.e. in a stable thermal equilibrium with the external bladder inflated, the material expanded, and the internal bladder at a slightly negative pressure. The material and pressurized nitrogen is at a slightly greater pressure than the external ocean pressure. The thermodynamic stages of the system can be seen in Figure 10.

Slocum Thermal (Webbresearch, 2008b) Weight: 60 kg

Hull Diameter: 21.3 cm

Vehicle Length: 1.5 m

Wing Span: 120 cm

Payload: 2 kg

Speed: 0.4 m/sec horizontal (projected)

Energy: Thermal engine, Alkaline batteries for instruments, communication and navigation Endurance: 5 years

Range: 40,000 km

Navigation: GPS, internal dead reckoning, altimeter

Sensor Package: conductivity, temperature, depth

Communications: RF modem, Iridium satellite, ARGOS

The Spray, Slocum (Battery & Thermal), Seaglider and Deepglider are very similar in size and general characteristics. They were designed with the same objectives, specifically in being small and easily deployed and recovered by only a couple of people. The vehicles were to be slow and the propulsion using only buoyancy control envisioned by Douglas Webb and Henry Stommel. The vehicles are dependent on the energy efficiency and glide trajectory angle during each traverse to monitor the ocean. Currently, various institutions (e.g., the University of Southampton, Great Britain) are starting the investigation of long-duration, highly efficient, slow-speed, powered autonomous underwater vehicles. These investigations will lead to the development of new highly optimized efficient wings. The optimum vehicle to handle a saw-tooth method of data sampling, as well as a vertical and horizontal means of sampling will be some form of hybrid vehicle with a glide and a power mode that takes each sampling means into account.

Underwater Vehicle Thermal Power
Fig. 8. Slocum Glider Schematic (Webb et al., 2001)

Buoyancy 50.2 kg

Buoyancy 50.2 kg

Buoyancy Outline Picture

Total drag wing and body

Total controlled movement of center of gravity

Weight 50.0 kg

Total controlled movement of center of gravity

Component of . net buoyancy to overcome drag

. Separation of center of buoyancy and center of gravity, 5mm Wing

Total drag wing and body

Weight 50.0 kg

Net pitch moment=0

Fig. 9. Slocum Thermal - Gliding forces on the vehicle (Webb et al., 2001)

Dive Control Vans
Fig. 10. Slocum Thermal Cycle (Webb et al, 2001)
+1 0

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