The principle advantages of Autonomous Under Water Vehicle Gliders (AUVGs) are:
1) Very suitable for long-range and endurance, if low to moderate speed is acceptable.
2) The sawtooth profile is optimal for both vertical and horizontal observations in the water column.
3) Regular surfacing is excellent for capturing GPS and two-way communication, no other navigational aids are required and the system is very portable.
1.1 Forward Propulsion
Gliders are unique in the AUV world, in that the forward propulsion is created by varying vehicle buoyancy. Wings and control surfaces convert the vertical velocity into forward velocity so that the vehicle glides downward when denser than water and glides upward when buoyant (Fig 1). Gliders require no propeller and operate in a vertical sawtooth trajectory.
Fig 1. Force balance diagram of forces acting on Glider, angle of attack not included.
1.2 Navigation and Flight
The Slocum Battery Glider dead reckons to waypoints, inflecting at set depths and altitudes based on a mission text file. As set by the mission, the Glider periodically surfaces to communicate data and instructions and to obtain a GPS fix for location. Any difference in dead reckoning and position is attributed to current and that knowledge is used on the subsequent segment.
Vehicle Description
2.1 Architecture
The Slocum Battery Glider is comprised of three main separate hull sections in addition to two wet sections located fore and aft. The cylindrical hull sections are 21 cm OD 6061 T6 aluminum alloy chosen for simplicity, economy, and expandability. The nose end cap is a machined pressure resistant elliptical shape, and the tail cap a truncated cone to allow for penetrator surface. Composite wings are swept at 45 degrees and are easily replaced.
2.1.1 Nose Dome
A penetrator is located thru the front end-cap to the wet section that houses a 9 – 14 kHz transducer for Pinger use or Telesonar Modem, and a 200 kHz transducer for altimeter use. In addition, the nose dome has a hole on the centerline for large bore movement of water as is created by the displacement piston pump. Although not of substantial volume and desire to keep reflectors away from the transducer, external trim weights can be added inside the nose dome for ballast trimming.
2.1.2 Forward Hull Section
This section houses the Displacement Piston Pump, Pitch Vernier Mechanism, Altimeter Electronics, Batteries, and provisions for ballast weights. Internal wiring connectors are mounted on the pump endplate. The large battery pack also serves as the mass moved by the pitch control.
2.1.3 Payload Bay Mid Hull Section
The payload bay is 8 3/8” diameter and 12” long with a nominal capacity of 3 to 4 kg. Designed to be easily removed and replaced for calibration needs or sensor type changes this gives great ease and flexibility to the user. It consists of two rings and a hull section. The front ring is typically ported for the CTD sensor assembly. To complement this section, a software interface exists to allow a payload or science bay computer to be installed which can control the sensor packages and collect and store data. Any control system is applicable i.e. embedded processors, Persistor, PC104, etc. There are also provisions for ballast weight attachment points. When available the Pinger Magnetics are housed within the Payload Bay. With the exception of the wire harness and the tie rod that must run through the bay for connection from the aft to the forward section, this volume is set aside for more energy, science or other payloads.
2.1.4 Aft Hull Section
This section houses the strong back chassis that ties the Glider together. On the bottom of the strong back chassis are the ARGOS PTT, a Catalyst, and Air Pump System. In addition, the battery and internal wiring connector are located on the upper side of the strong back. An upper electronics chassis holds the Vehicle Controller, Hardware Interface Board, and the Attitude Sensor. GPS, Iridium, and RF modem engines are located on the lower electronics chassis tray along with a Science/ Payload Computer Switch Board. The Micron pressure transducer is ported thru the aft endcap and positioned remotely. The aft battery is located under the strong back and can be manually rotated for static roll offsets.
2.1.5 Aft Tail Cone
A faired wet area that houses the Air Bladder, Steering Assembly, Burn Wire, Jettison Weight, Power Umbilical, and has provisions for external trim weight and wet sensors. Protruding from the aft end cap through the Tail Cone is the Antenna Fin Support. This boom is a pressure proof conduit for the antenna leads and a low noise amplifier for GPS. Socketed into the support is the Antenna Fin. Below the support is a protected conduit for the Steering Motor Linkage.
2.1.6 Wings
In all operations, particularly coastal work, there is a risk of entraining weed or debris on the wings or tail causing major degradation in gliding performance and for littoral gliders a sweep angle of 45 degrees or more is recommended. Horizontal tail planes are not required, pitch stability is provided by the wings which are mounted aft of the center of buoyancy. In the low Reynolds number regime in which the glider operates (approximately 30,000) their un-cambered (“razor blade”) wings are very suitable.
2.2 Specific Components
2.2.1 Displacement Piston Pump
A single-stroke piston design, using a 90 watt motor and a rolling diaphragm seal, moves 504 cc of sea water directly into and out of a short 12 mm diameter port on the nose centerline (the stagnation point). The pumps are rated for different pressures based on the gearbox associated with the motor. The mechanical gear drive is not the limiting factor; it is the maximum amount of energy that is desired to pull from the battery source. The selection of gearbox/motor assembly should be optimized for the working depth to allow for quick inflections (more important in shallow water) and to minimize energy used on the return stroke. It is important to note that the pump should not be run without either external pressure or internal vacuum on the rolling diaphragm. Restated, the pump should be installed in the hull section and a vacuum drawn to minimum 1 inHg lower than external atmosphere. This ensures that the diaphragm folds smoothly as it rolls, otherwise damage may result. To eliminate back drive of the pump at pressure a latching brake is used to hold the motor when at rest.
2.2.2 Pitch Vernier
Provided that the h moment is 3 to 5 mm, the fluid movement from the Displacement Piston Pump provides the moment for changing pitch (water moves into the nose making the vehicle nose heavy when diving, similarly making the nose buoyant when rising). To trim to the desired dive and climb angles a lead screw drives the forward 8.4 Kg battery pack fore or aft as a vernier. The battery pack is put full forward during surfacing to better raise the tail out of the water for communications.
2.2.3 Altimeter
The Airmar altimeter, 0-100 m range electronics are supported on the Displacement Piston Pump Cylinder. The transducer leads feed through a bulkhead connector on the Front Endcap. The transducer is mounted such that it is parallel to a flat sea bottom at a dive angle of nominally 26 degrees.
2.2.4 CTD
A typical sensor package on the Glider is a Sea Bird non-pumped, low drag conductivity, temperature, and depth package. An appendage to the side of the payload bay, the CT sensor is delicate and should be protected from abuse. A 500 PSI pressure transducer is used for the depth measurement. The SBE electronics and sensors are calibrated as a single unit.
2.2.5 ARGOS
The Seimac Smartcat PTT with the extended voltage option is used for recovery situations reporting GPS position when available. See Appendix ARGOS Data Format and C:\CVS_glider_code\glider\code\prntargos.c for a conversion program.
2.2.6 Catalyst
A catalyst is used to recombine Hydrogen and Oxygen into H2O to reduce the risk of explosion. The reaction is exothermic and the catalyst may become hot. This item does not need periodic replacement. See Disclaimer.
2.2.7 Air Pump System
An air bladder in the flooded tail cone is used to provide additional buoyancy on the surface for bettering communications. It is inflated, using air from the hull interior, providing 1400 ml of reserve buoyancy. The air pump is mechanically switched off when the differential pressure (between the air bladder and the internal hull pressure) becomes 6.25 PSI. This has been factory set. When surfaced, the Glider equilibrates with the tail elevated, and the boom holds the antenna clear of the water. This air is vented inward via a latching valve for descent.
2.2.8 Vehicle Controller
A Persistor CF1, based on a Motorola 68338 processor is used to control the functions of the Glider. This board has low current consumption capability and supports the use of Compact Flash cards and miniature hard drives enabling large amounts of data to be stored. Controller code is written in C and architecturally is based on a layered single thread approach where each task is coded into a behavior and behaviors can be combined in almost any order to achieve flexible and unique missions. Each device is labeled as a sensor and is logged every time that the value changes during a mission. This data is retrieved as a binary file and is post-parsed into a matrix that allows the user to easily construct graphical views of vehicle performance or scientific data. A subset of the sensors can be chosen as a science data package so as to reduce surface radio transmission time. The Persistor can have in memory any number of pre-written missions (text files) that can be called or a new mission can be created, downloaded to the Glider via the RF LAN and run. Mission changes might include different inflect depths, new GPS waypoints, or turning a behavior on or off such as current correction.
2.2.9 Hardware Interface Board
The Persistor is mated to this driver board that interfaces to all of the sensors, communications, and drive mechanisms. See Appendix Schematic and Wiring Diagram. The board runs on a nominal 15 volts DC. A section of the board is dedicated to a hardware abort mechanism. As a recovery precaution for errant events, a timer (set to either 2 or 16 hours) is reset (COP_tickled) every time there is a GPS fix or a keystroke while in Glider Dos. Both of these situations indicate that the Glider is safely on the surface. If the timer elapses, however, the following items will come alive: Air Pump, ARGOS PTT, Pinger (if available), and the Burn Wire for the Jettison Weight. The 10 kHz Pinger (if available) will change to an 8 second duty cycle and at ~4.2ma (10ms/8 sec rate x 50watts/15v) will emit sound on a single 10 C-cell battery pack for ~ 60 days.
2.2.10 Attitude Sensor
The Precision Navigation TCM3 provides the bearing, pitch, and roll indications of the Glider. These inputs are used for dead reckoning the vehicle while under water. Recalibrating the compass, depending on the magnetic anomalies of the usage area, may at times be necessary. See Compass Calibration.
2.2.11 GPS
The output used is NMEA string RMC delivered every 5 seconds.
2.2.12 Iridium
The Iridium bi-directional satellite modem is located on the lower electronics tray with a separate switching power supply and a Low Noise Amplifying (LNA) switching board for the antenna.
2.2.13 RF modem
FreeWave 900 MHz radio modem is used for the local high speed communications link to the Glider. It presently is ported to the console on the Persistor, permitting code load changes. See Appendix Freewave Manual.
2.2.14 Science/Payload Computer Switch Board
A hardware relay is used to switch the RF modem communications to the Science/Payload Computer to allow direct access through the software application consci.pxe on the Persistor. A disconnect of carrier detect of a few seconds will revert the RF communications back to the Glider Controller Persistor. In the field, disconnecting power to the host side RF modem for a few seconds will accomplish this.
2.2.15 Pressure Transducer
Micron strain gage transducers are used for vehicle control and dead reckoning. Ported through the aft cap, the transducer is isolated by oil filled stainless tubing to prevent thermal shock.
2.2.16 Air Bladder
A 1400 cc bladder provides buoyancy and stability while the Glider is surfaced, lifting the antenna support out of the water. The bladder is filled via the Air Pump System. Although the bladder is rugged, care should be taken to have the Aft Tail Cowling in place when the bladder is filling. The bladder is then supported as it inflates until shut off by the pressure switch. Likewise, when removing the Aft Tail Cowling it is important to deflate the Air Bladder, as it will be hard up against the Cowling. See Opening Procedures for more details.
2.2.17 Steering Assembly
A tail fin is moved as a controlled plane acting as a rudder. The steering motor and rotary potentiometer are located external to the pressure hull and are oil filled and pressure compensated. The maximum tail fin angle spans ± 45 degrees. The hinge is synthetic as the fixed portion of the fin contains antenna structures.
2.2.18 Burn Wire
An emergency abort system, a replaceable/rebuildable battery activated corrosive link that after approximately 15 minutes in salt water and 4 hours in fresh water will release the spring ejected Jettison Weight. The wire is 20 AWG Inconel held in a delrin bushing and mated and sealed to a single pin Mecca connector. Note: activating the Burn Wire in air will have no effect, as it takes ions in the water to complete the return path to ground. See Maintenance for more details.
2.2.19 Jettison Weight
A slug of machined antimony lead weighing 470 grams that is attached to the Burn Wire Assembly by 300 lb test monofilament. When the Burn Wire is electrically corroded the Jettison Weight is forcibly ejected by a spring forcing the Glider to surface (within in the limits of the mass lost).
2.2.20 Power Umbilical
An Impulse cable is used to switch or supply power to the Glider. When the (red band) Dummy Plug is inserted or the connector end is empty there is no power applied to the vehicle. This is done so that any person can be instructed to easily remove power from the system without special tooling. Further, for safety reasons no internal spark is generated as could be with an internal switch. To power the Glider on either use an external power cable (15 volts DC) or insert the (green band) Shorting Plug. The Umbilical is accessible external to the Glider Aft Tail Cone. See Appendix Wiring Diagram and Section Maintenance for Plug care.
2.2.21 Antenna Fin
The tail fin presently houses three antennas: ARGOS 401 MHz, RF modem 900 MHz, and a patch with combined GPS 1575 MHz and Iridium 1626 MHz. The fin is bonded to a socket that is installed into the antenna support with dual radial O-ring seals. This provides a passage for the antenna cables and allows for easy replacement/upgrade of the Antenna Tail Fin module.
2.2.22 Batteries
Battery packs consist of 10 Duracell C-cells in series, diode protected, nominally at 15 volts. As indicated below, the number of packs can be adjusted depending on reserve buoyancy after Payload considerations. Given 26 packs (260 C-cells) the battery weight is 18.2 kg and energy available 7,800 kjoules.
Location # of packs
Pitch Battery 12
Aft Battery 10 - 12
Nose Batteries 1- 2
For power management typically all of the packs except one of the Aft Battery Packs are tied into Battery Main. The one separate pack is tied to Battery Backup that is an OR to the main and runs the Abort Timer, Burn Wire, ARGOS, and Pinger (if available) in the event of Main power loss.
Photo Gallery
Photo Credits: Pictures (1, 5, 6, 7, 9) to Webb Research. Pictures (2, 3, 4, 8, 10, 11) to Rutgers University. Pictures (12, 13, 14) to DRDC Canada
Videos:
Bottom Crash Test; 2.399K(Video Credit: Rutgers)
Glider Up at the Surface; 4.769K (Video Credit: Rutgers)
Glider Flying; 3.879K (Video Credit: Rutgers)
Click here to download Quicktime Player
Publications:
Curtin, T. B., J. G. Bellingham, J. Catipovic, and D. Webb (1993). Autonomous Oceanographic Sampling Networks. Oceanography, Vol. 6, No. 3, pp 86-94.
Webb, D. C., and P. J. Simonetti (1997). A Simplified Approach to the Prediction and Optimization of Performance of Underwater Gliders. Proceedings of the 10th International Symposium on Unmanned Untethered Submersible Technology, September 7-10, 1997, Document Number: 97-9-01, published by the Autonomous Undersea Systems Institute. pp 60-68.
Webb, D. C., and P. J. Simonetti (1999). The SLOCUM AUV: An Environmentally Propelled Underwater Glider. Proceedings of the llth International Symposium on Unmanned Untethered Submersible Technology, August 23-25, 1999. Published by the Autonomous Undersea Systems Institute, pp 75-85.
Webb, D. C., P. J. Simonetti, and C. P. Jones (2001) . SLOCUM, An Underwater Glider Propelled by Environmental Energy, IEEE Journal of Oceanic Engineering, Vol. 26, No. 4, October, 2001, pp. 447-452.
Blaha, John, P., G.H. Born, N.L. Guinasso, Jr., H.J. Herring, G.A. Jacobs, F. J. Kelly, R.R. Leben, R. D. Martin, Jr., G.L. Mellor, P. Peter Niiler, M.E. Parke, R.C. Patchen, K.Schaudt, N.W. Scheffner, C.K. Shum, C. Ohlmann, W. Sturges, G.L. Weatherly, D. Webb, H.J. White, (2000), Gulf of Mexico Ocean Monitoring System, Oceanography, Vol. 13, No. 2, pp10-17.
Jones, C. P. Slocum Gliders as Coastal Observations Remote Sensing Platforms, Oceanology International, March 2002. *
Creed, E. L. Mudgal, C., Gleen, S.M. Schofield, O.M., Jones, C.P. Webb, D.C., Using a Fleet of Slocum Battery Gliders in a Regional Scale Coastal Ocean Observatory, MTS/IEEE, Conference Proceedings, Biloxi, MS. Vol. 3, pp 1234-1238, Oceans 2002.
Simonetti, Paul, SLOCUM GLIDER: Design and 1991 Field Trials, WHOI, ONT contract N00014-90C-0098
Russ, E. Davis, Eriksen, Charles, Jones, Clayton, P., Autonomous Buoyancy-driven Underwater Gliders, Technology and Applications of AutonomousUnderwater Vehicles, edited by Gwyn Griffiths, Volume 2, ISBN 0-415-30154-8, 2003, 37- 58., Taylor & Francis, ISBN 0-415-30154-8, London, & New York
Creed, E., Kerfoot, J. Mudgal, C., Glenn, S., Schofield, O., Jones, C., Webb, D., Cambell, T., Twardowski, M., Kirkpatrick, G., J. Hillier (2003) Automated Control of a Fleet of Slocum Gliders within an Operational Coastal Observatory, Oceans 2003, MTS/IEEE Conference Proceedings, San Diego, CA, V1, pp 726-730 .*
Glenn, Scott, Schofield, Oscar, Jones, Clayton et al, The Expanding Role of Ocean Color and Optics in the Changing Field of Operational Oceanography, Oceanography, June 2004, 86-95.
Schofield, O., Glenn, S. Kirkpatrick, G., Jones, C., and Twardowski, M. (2004) Measuring Mesoscale in-situ Optics of the Continental Shelves with Autonomous Webb Gliders. Oceans 2004. *
Jones, C., Creed, E., Glenn, S., Kerfoot, J., Schofield, O., Slocum Gliders – A Component of Operational Oceanography, Proceedings of the UUST Conference, New Hampshire, 20-25 August 2005.
Griffiths, G., Jones, C. P., Ferguson, J., Bose, N., ECOR Specialist Panel on Underwater Vehicles, Undersea Gliders (not published as of 7/07) Draft
Schofield, O., Kohut, J., Aragon, D.,, Creed, L., Graver, Haldeman, C., Kerfoot, J., Roarty, H., Jones, C., Webb, D. C., Glenn, S., June 2007, SLOCUM Gliders: Robust and Ready, Journal of Field Robotics, June 2007, Vol. 24, Issue 6, pp 474-485.
Rutgers (RU-COOL) and Webb Research, Slocum Gliders - Advancing Oceanography, published Proceedings of the International Symposium on Unmanned Untethered Submersible Technology (UUST07).
