aurora towfish

8/12/2019 Aurora Towfish

http://slidepdf.com/reader/full/aurora-towfish 1/4

tions such as mine counter-measures and seabed surveys.In addition, the applicability

of both the design approachand ISER’s modeling capabilities tonew vehicle designs for deeper tows orlarger vehicles has been clearlydemonstrated.

This article describes: the prototypetowfish and its trials to date, a summa-ry of ISER’s dynamic modeling capa-bilities and a summary of the status of the Aurora towfish program.

Vehicle DesignThe Aurora towfish is broken down

into five subsystems that are discussedbelow: Hull. The Aurora hull consists of a

hemispherical free flooded nose fol-

were validated through a systematicseries of sea trials.

The Aurora development programproduced a functional active towfishthat can meet many commercial andmilitary survey platform requirements.

The deep depth capability, enhancedattitude stability (pitch, roll and yaw)and adaptable wet/dry payload capaci-ties make Aurora suitable for applica-

By Jesse Houle

Mechanical Engineer

and

Mae Seto

Mechanical Engineer

International Submarine

Engineering Ltd.

Port Coquitlam, British Columbia,

Canada

International Submarine EngineeringResearch Ltd. (ISER) has been

developing Autonomous UnderwaterVehicles (AUVs) since 1981. In 1996the conceptual design of a modular,

affordable, small, marine scienceAUV, Aurora, was initiated. Before thedesign was complete a need for anactively stabilized towfish was identi-fied. This requirement was, in part,driven by the need for a stable modu-lar platform to mount variable pay-loads such as multibeam sonars andwet or dry science payloads. Anotherfactor that motivated the design wasscope reduction, minimizing the scope(deployed cable length) required toachieve an operating depth. Scope

reduction not only allows deeper depthoperations (for a given scope) but italso reduces the percentage of surveytime spent in turns.

Preliminary analysis indicated that amodified Aurora, fitted with a largeactive wing, was a suitable candidatefor towfish development. The analysisutilized the existing ISER AUVdynamics database. This database, inconjunction with dynamics modelsdeveloped by ISER and the DefenseResearch Establishment Atlantic

(DREA), were used to predict theAurora and Dolphin/Aurora systemperformance. The results of the modelpredictions and the models themselves

AURORA—An Active Tow Body

For InstrumentationThe Aurora Development Program Produced a Functional Active Towfish That can Meet Many Commercial and MilitarySurvey Platform Requirements

ST Instrumentation Feature

Schematic representation shows Dol- phin semi-submersible towing Aurora towfish. Inset photo is Aurora towfish and Klein 5500 multibeam sonar during integration trials—Vancouver.

Reprinted from Sea Technology



Klein 5500 sonar image of an anchor scarred seabed recorded during Aurora/Klein integration trials.

the vehicle to accommodate differentpayload sections and stability require-ments. The overall physical specifica-

tions of the prototype towfish are asfollows:

• LOA: 3.3 m• Main wing span: 2.5 m• Hull diameter: 40 cm• Total height: 71 cm• Displacement: 465 kg

Control. The prototype towfish con-troller is based on a 68030 processorinstalled in a Gespac card/rack system.This hardware was chosen for the pro-totype to achieve commonality with

other ISER AUVs. Production towfishwill be retrofitted with more compact,expandable hardware.

The following onboard instrumentsmeasure towfish attitude (pitch, rolland yaw) rates, depth, altitude andheading:

• Attitude, Rates, Heading: WatsonAHRS BA303 solid state gyro

• Altitude and Speed: EDO 3050Doppler log

• Depth: Paroscientific QuartzDepth sensor

The EDO doppler can be replacedwith a more cost-effective verticalecho sounder for applications whereconstant altitude runs are requiredwith less stringent positioning require-ments. With the current instrumenta-tion package, Aurora is capable of per-forming constant depth, constant alti-tude and line following runs.

Communications. The tow cablesare armored electro-mechanicalcables. They provide towfish power,telemetry and payload communica-

tions. The towfish is easily adaptableto mission specific tow cables wherepayload dictates special power orcommunications requirements.

lowed by a pressure hull, then a freeflooded glass reinforced plastic (GRP)payload compartment, and a tail sec-

tion. Mounted to the pressure hull isthe fully moveable main wing depres-sor and tow point assembly. The tailsection houses the active hydroplanes’assemblies.

The towfish pressure vessel is amodular, ring stiffened, cylindricalstructure with circular plate end end-caps and is similar other ISE AUV hulldesigns. Adjacent hull sections arebolted together with external fasten-ers. A removable stiffening ring issandwiched between each hull section.

These features allow for easy access tothe pressure hull interior and maxi-mize the amount of useable spacewithin the hull. An o-ring, located onthe face of each cylindrical hull sec-tion, provides the seal between sec-tions and the mating ring stiffener orend dome. The basic towfish configu-ration consists of four identical 46centimeter long hull sections. Addi-tional hull sections can be addeddepending on the mission scenario andpayload requirements.

Control Surfaces and Actuators.Four actively controlled stern planesand a main wing provide the maneu-vering, stability and depth control forthe towfish. The four aft planesmounted in a cruciform configurationin the tail control towfish pitch, roll,and yaw. The main wing providesheave and depth control and wasdesigned to generate 1590 kg (3500lbf) of depressive force at 5 meters persecond. The wing root is integral to thetow point assembly for efficient trans-

fer of depressive loads to the towcable. The wing root is mounted to theexternal rails on top of the hull and canbe easily relocated along the length of

Payload. Towfish is configurable toa variety of mission payloads by theaddition or removal of modular hullsections. Three main facilities areavailable for payload. Dry electronicscan be installed in the existing drypayload hull section. The GRP wetpayload section aft of the pressure hullcan accommodate atmospheric or

compensated housings or external sen-sors. The wet payload section can alsobe replaced with one that can accom-modate more payload if required.Finally, additional wet payload can befastened to the rails on the top and bot-tom of the pressure hull sections.

Sea Trials PlanAurora was first tested in July 1997.

Since then it has undergone a system-atic series of trials. In total, 52 trialsdays have been logged, three of which

were with the Klein 5500 multibeamsonar. Initially, trials were performedto evaluate the mechanical design, thepassive stability of the towed body andthe performance of the main wingdepressor. Following the passive trials,a series of fully active system trialswere performed under both Dolphintowing and the ISE test vessel,researcher, towing. The following is alist of towfish accomplishments todate.

Dolphin: To 130 Meters DepthA series of full scale Dolphin tow

trials were performed to evaluate theDolphin–towfish performance at 5meters per second and 130 meterstowfish depths (300 meters of cabledeployed). The parameters investigat-ed include towfish and Dolphin stabil-ity, maneuvering and towing abilities,and the interactions between Dolphinand the towfish.

Most tow trials were performed withthe towfish in depth-keeping mode.

Straight-line runs assessed the systemperformance under sonar scanningconditions while turning maneuversassessed the operational systemmaneuverability. The trials data col-lected is currently being used to vali-date dynamics models of this tow sys-tem.

Several sets of researcher towing tri-als were performed to verify towfishhardware and software configurations.In July 1998 a series of trials verifiedthe operation of an altitude keeping

(constant altitude) software algorithm.Test runs were performed at constantaltitudes of 30 meters off the seabed,in water depths up to 90 meters. The



control with no tow; 2) the cable/tow-fish system assuming a large tow vehi-cle (i.e., no vehicle/towfish interac-tions, and 3) the coupled Dolphin/

cable/towfish dynamics and controlfor configurations where the forcesgenerated by Dolphin and the towfishaffect one another. Systematic dataseries for a variety of maneuvers existfor all three scenarios making it possi-ble to do the full validation outlinedabove. The current focus is centeredon the coupled Dolphin/ cable/towfishstage with completion slated for April1999. This model is a research, devel-opment and design tool for many gen-eral types of tow configurations.

ConclusionThe Aurora towfish prototype is a

fully active towed body that can meet

the data that the ISEgeneric maneuver-ing simulator usesto customize itself to a specific vehicle.The maneuveringbehavior of thevehicle can then beassessed. In the case

of a towfish, stabili-ty, sonar angularrates, hydroplaneresponse, tow cabletension, and manyother operationallyoriented character-istics can be evalu-ated using only acomputer.

ISE and DREAhave also developeddynamic models for

remote minehuntingsystems where thetowfish is towed byan autonomous,semi-submersibledrone (like ISER’sDolphin vehicle).Here, drone andtowfish hydrody-namics and control,together with thetow cable joiningthem, are simulta-

neously modeled.The cable is mod-eled as a series of finite segments.This fully interactive two-body modelcaptures the complex many degree-of-freedom dynamics of this coupled sys-tem, allowing vehicle geometry andcontrol to be optimized under realisticconditions.

These models are validated againstfull scale drone and towfish trials at 10knot speeds for towfish depths up to

130 meters (300 meter cable scope).

Validation of TowfishThe RMS model simulates the

dynamics and control of the tow vehi-cle (Dolphin), cable, towfish (Aurora)and the interactions between the towvehicle and the towfish. The interac-tions are particularly relevant for aremote minehunting system where thetow forces at the tow vehicle can be asizable fraction of the vehicle buoyan-cy and thrust. This model was built on

existing work 1-7 and is currentlyundergoing intensive validation.

Model validation has three compo-nents: 1) the Dolphin dynamics and

towfish followed a gradually changingbottom contour with towfish depthschanging from 30 to 60 meters over a1-kilometer course.

In preparation for trials scheduledearly in 1999, the Klein 5500 multi-beam side-scan sonar was integratedto the towfish and trialed in December1998. The objective of the trials was to

test the stability of the towfish and itssuitability as a side-scan sonar plat-form. In addition, a new communica-tions link from the sonar to the surfacewas tested.

Sonar power and digital data weretransmitted over a single coaxial cableembedded in the towfish tow cable.Klein electronics, independent of thetowfish processor and electronics, per-formed all of the sonar data acquisi-tion, processing and storage.

Towing speeds were from 3 to 5.5

meters per second, altitudes were from15 to 30 meters off the seabed andwater depths varied from 25 to 100meters. Good sonar returns of anchorscarred sandy and rocky sea bottomsat the end of Vancouver harbor wereobtained. In addition, ship wreckswere imaged in deeper waters in Van-couver’s Indian Arm. The trials weresuccessful and have provided a goodfoundation for fully integrated Dol-phin–Aurora–Klein trials scheduledfor early 1999.

Vehicle Dynamics ModelsAs part of the Dolphin towing trials,

data was collected in order to validateanalytical models used to predict sys-tem performance. Within the last threeyears ISE, in collaboration withDREA, have developed comprehen-sive dynamics models for all of itsvehicles, including the Aurora towfish.These models are important researchand development tools. They provideperformance assessment during design

and are testbeds on which new andinnovative designs, control algorithms,and maneuvering strategies are devel-oped

The dynamics models are based oncoefficients, which summarize most of the hydrodynamic characteristics of avehicle. These coefficients are deter-mined from vehicle geometry usingboth empirical and theoretical meth-ods that DREA has either collectedfrom the literature or developed in-house. These methods are also used in

determining hydroplane geometry andlocation based on stability and controlrequirements for a vehicle’s mission.

The hydrodynamic coefficients are

Klein 5500 sonar image of a submerged hull recorded during Aurora/Klein integration trials.



many commercial and military surveyapplications in its current state. Aurorastarted from the identification of aneed, in both commercial and militaryapplications, for a deep, stable, adapt-able towfish. ISER’s AUV designexperience, application of analyticalmodeling tools and a systematic seatrials schedule resulted in a functional,

reliable, and cost-effective towfish.Analytical tools applied to the tow-fish design process, and developedconcurrently with the Aurora programhave been proven as valid design tools.Some of these tools are currentlybeing validated with full-scale trialsdata. /st/

References1. Kamman, J.W., T.C. Nguyen,

“User’s Manual for DYNTO-CABS,” Technical MemorandumNCSC TM 550-90, 1990.

2. Kamman, J.W., S.D. Patek, S.A.Hoeckley, “Application of Multi-variable Linear Control Design toMarine Towed System,” Guidance,Control and Dynamics, 1996, Vol.19, No. 6, pp. 1246-1251.

3. Chapman, D.A., “Effects of ShipMotion on a Neutrally-StableTowed Fish,” Ocean Engineering,1982, Vol. 9, No. 3, pp. 189-220.

4. Chapman, D.A., “Towed CableBehaviour During Ship TurningManeuvers,” Ocean Engineering,1984, Vol. 11, No. 4., pp. 327-361.

5. Le Guerch, E., “The Deep Towingof Underwater Fish Behaviour Pat-terns During Half-Turn Maneu-vers,” Ocean Engineering, 1987,Vol. 14, No. 2, pp. 145-162.

6. Zhu, K., and Li., “Coupled MotionSimulation of Underwater Towedand Self-Propulsive Vehicle,” Pro-ceedings of the Seventh Interna-tional Offshore and Polar Engineer-ing Conference, Honolulu, USA,1997, Vol. II, pp. 38-43.

7. Seto, M., G.D. Watt, “The Interac-tion Dynamics of a Semi-Sub-mersible Towing a Large Towfish”,Proceedings of the Eighth Interna-tional Offshore and Polar Engineer-ing Conference, Montreal, Canada,1998, Vol. II, pp. 263-270.

Jesse Houle is amechanical engineer

and has been with ISE since 1993. Hehas been involved

with the design and project management aspects of AUV and

ROV projects at ISE. Houle is currently

project manager of the Aurora towfish pro-

gram.

Mae Seto has been amechanical engineer

with ISE since 1995.Prior to that she wasworking on a Ph.D.

in aerodynamics.Seto currently works

on vehicle dynamicsand control and has been involved with

several AUV projects including the Dol- phin semi-submersible and Aurora towfish programs.

aurora towfish

Documents