A New Remotely Operated Underwater Vehiclefor Dynamics and Control Research

David Smallwood1, Ralf Bachmayer, and Louis Whitcomb2

Department of Mechanical EngineeringJohns Hopkins University

1 IntroductionThis paper reports the development of a new remotely operated underwater vehicle (ROV) designed to serve as aplatform for rapid development and deployment of novel underwater vehicle systems. The goal is to enhance ourability develop new underwater vehicle subsystems in the laboratory, and rapidly field-test these new systems.Although a significant fraction of ONR and NSF sponsored underwater vehicle research is now directed towardsAUVs, we argue that ROVs continue to provide a highly efficient platform for the research and development ofadvanced underwater technology. Once developed and validated on ROVs, numerous technologies have beenreadily transitioned for use in autonomous underwater vehicles (AUVs). Section 1.1 examines several contexts inwhich ROVs have served as development platforms for critical AUV technology. Section 1.2 reviews the desiredperformance specifications for the new vehicle.

1.1 Historical Role of ROVs in UUV ResearchA decade of operational experience by numerous research groups has demonstrated ROVs to be ideal platforms for“rapid prototyping” and “rapid field deployment” of UUV subsystems. Recent examples include, sonar imaging andsurvey, optical imaging and survey, navigation, control, oceanographic sampling, and subsea manipulation.Numerous research results first pioneered with ROVs and towed vehicles are now commonly employed forAutonomous Underwater Vehicles (AUVs) and sea-floor observatories. Examples include the following:1. LBL Acoustic Navigation: Long baseline acoustic navigation (LBL) remains the most commonly used method

for acoustic underwater navigation. LBL was originally developed in the 1970s at WHOI for the AlvinSubmersible [9], was adopted for use on the Jason ROV in the 1980s [20], and transitioned in the 1990s for usein AUVs including ABE [21] and Odyssey [13].

2. Closed Loop Vehicle Control: The closed-loop control (dynamic positioning) systems originally developed forJason [23, 20] in the 1980s and 1990s have been transitioned for use in AUVs including ABE [22] and Odyssey[13].

3. Acoustic Survey: Precision quantitative acoustic benthic survey and mensuration techniques originallydeveloped with Jason [14] have recently been successfully transitioned for use in the ABE AUV [16]. Recentreports show that sonar features can be utilized for vehicle navigation, e.g. [10].

4. Optical Survey: Optical benthic survey and mensuration. Deep-ocean optical survey was pioneered in the1970s and 1980 with the Angus and Argo towed systems at WHOI. Early successes include finding the wreckof the R.M.S. Titanic during an optical search in 1985 [2]. These techniques have been subject to ongoingdevelopment with the Jason and Argo II ROV system for precision optical survey [14] and have recently beentransitioned for use in the ABE AUV system.

1 Corresponding author is David Smallwood.2 The authors are with the Department of Mechanical Engineering, 200 Latrobe Hall, Johns Hopkins University,3400 North Charles Street, Baltimore, Maryland, 21218 USA, email: d_smallwood@jhu.edu, ralf@jhu.edu,llw@jhu.edu. The authors gratefully acknowledge the support of the Office of Naval Research and the NationalScience Foundation under ONRYI grant #N00014-97-1-0487 and Career grant #BES-9625143 held by LouisWhitcomb.

In Proceedings of the 11th International Symposium on Unmanned UntetheredSubmersible Technology, Durham, NH, September 19-22, 1999, pages 370-377.

5. Low-Power Robot Arms: Low-power electrically actuated underwater robot manipulator arms were developedfor use on the Jason ROV as a highly efficient alternative to hydraulically actuated arms [24, 15]. Thismanipulator has proven to be highly effective at subsea sampling and manipulation tasks [1]. Electricallyactuated arms are a fundamental an enabling technology for future AUV missions requiring roboticmanipulation [12].

6. Electric Thrusters: DC electric thrusters provide dramatically improved propulsion efficiencies in comparisonto hydraulic thrusters. Electric thruster design originally pioneered for manned submersibles and ROVs (e.g.Jason) have served as the basis for the small highly efficient electric propulsors now universally employed inAUVs such as ABE, Odyssey, NPS, and FAU vehicles [3, 8, 19].

Based on the above, we argue that ROVs provide an effective development platform for underwater vehicleresearch, laboratory, testing, and field-trials of novel underwater vehicle sub-systems. This paper describes arelatively low-cost ROV, presently nearing completion, to serve as such a developmental platform.

1.2 Vehicle Design GoalsThe principal objective of the new vehicle is to serve as a convenient, cost-effective platform for research,development, and experimental validation of vehicle control systems, vehicle navigation techniques, and vehiclecontrol algorithms. To achieve this goal, we selected the vehicle design goals listed in Table 1.

PARAMETER SPECIFICATION PURPOSE

Size: 1.5m x 1m x 1m Ease of handling and deploymentMass: 140Kg Ease of handling and deploymentStability: Passively stable in roll and pitch, re-configurable for

dynamic roll/pitch control.Provides both 4-DOF and 6-DOFvehicle dynamics.

Propulsion: Electric thrusters, 300 N each axis Control bandwidth and authorityPropulsionInstrumentation

Current mode amplifiers, instrumented for propellershaft position, 1000Hz Sample Rate

Thruster dynamics and controlresearch.

PositionInstrumentation

Six Degree of Freedom (6-DOF), 5Hz min. Vehicle navigation, dynamics andcontrol research.

Computer controlsystem

Easily re-programmed while at depth, providing upto 1000Hz sample rates.

Vehicle navigation, dynamics andcontrol research.

Video Standard NTSC Closed-loop optical servo research.Manipulator Arm Capable of supporting future pair of 6-DOF electric

arms.Development of manipulationtechniques for AUVs.

Payload Support Generic “payload port” providing power, RS422 andEthernet telemetry.

Development and testing of novelsubsystems.

Tether 10KVA DC power, real-time data and videotelemetry.

Ease of development andexperimentation

Table 1: Vehicle Design Goals

2 Vehicle DescriptionThe vehicle is presently under construction, but will look similar to the concept drawing in Figure 1. This sectionoutlines the design choices made by the authors to achieve the goals outlined in Table 1. Section 2.1 reviews thevehicle propulsion system. Section 2.2 reviews the vehicle navigation suite. Section 2.3 outlines the vehicle controlsystem architecture.

Figure 1: JHU ROV #1 Design Concept

2.1 PropulsionTo achieve the disparate goals of high thrust, small size, and precise propeller position instrumentation, we havedeveloped a compact 3-Phase DC electric thruster. The thruster is internally pressure compensated with mineral oil,and rated for full ocean depth operation. The thruster specifications are shown in Table 2: Thruster Specifications.The new thruster is pictured in Figure 2 and an assembly drawing is shown in Figure 3.

Motor Type 3 Phase DC Permanent Magnet Brushless Motors

Torque 6.5 N-m Maximum, 2.16 N-m Continuous

Thrust 150 N Peak

Power 1.5 kW

Control Current Mode Amplifiers providing 2mS current response to +/- 15 amps at150 bus voltage. Amplifiers housed external to thruster.

Feedback Resolver Shaft Position, 4096 count/rev (0.088°) Angular Resolution

Depth Full ocean depth. Internally pressure compensated.

Table 2: Thruster Specifications

Figure 2: New 3-Phase DC Brushless Thruster

Figure 3: Exploded View of New 3-Phase DC Brushless Thruster

The initial vehicle configuration will use five DC brushless thrusters for propulsion, with built-in support for anadditional sixth thruster. The initial configuration will employ two thrusters for forward/reverse thrust, two forlateral thrust, and one for vertical thrust. The current control amplifiers will be housed one of two AL7075 pressurehousings along with a PC/104 CPU dedicated to thruster control. The PC/104 thruster controller is capable of1000Hz closed-loop thruster control, and is provided with a direct high-speed telemetry link to the surface controlcomputer.

2.2 NavigationPrecision vehicle position sensing is an often overlooked and essential element of precision control of underwaterrobotic vehicles. The analytical and experimental development of undersea robotic vehicle tracking controllers israpidly developing, e.g. [20, 5, 7, 8, 4, 6], however few experimental implementations have been reported other thanfor heading, altitude, depth, or attitude control. Conspicuously rare are experimental results for X-Y control ofvehicles in the horizontal plane. This lacuna is a direct result of the fact that at present, few techniques exist forreliable three-dimensional position sensing of underwater vehicles.The new ROV will be equipped for full 6-DOF position measurement. Vehicle heading, roll, and pitch (and theirtime derivatives) are instrumented with a KVH ADGC gyro-stabilized magnetic compass system. Depth isinstrumented with a standard analog strain-gage pressure transducer. Depth and attitude sensors are housed in anAL7075 pressure housing. Vehicle XYZ position will initially be instrumented with a 300kHz Sharps time-of-flighthard-wired acoustic navigation. We hope to also add a 1200 kHz bottom-lock doppler navigation system. Thenavigation sensor specifications are listed in Table 3.

Variable Sensor Precision Update RateHeading KVH ADGC 1° 10 Hz

Roll and Pitch KVH ADGC 0.1° 10 Hz

Depth Entran EXPO-X73-300P 0.75% AnalogXYZ Position 300 kHz Sharps Acoustic

Transponder System0.5 cm 5 Hz

XYZ Velocity 1200 kHz RDI WorkhorseDoppler

1 mm/sec 5 Hz

Table 3: Vehicle Navigation Sensors

2.3 Control System ArchitectureThe new vehicle control system architecture will enable rapid development and field-testing of advanced UUVsystems. The proposed control system will be structured for ROV control, as depicted in Figure 4, but will contain avariety of modules, which can be adapted for AUV control. The control system uses a two-part system design,partitioning safety-critical from non safety-critical subsystems for cost-effective implementation and enhancement –a strategy employed successfully on the original Jason and Argo II control systems [18].

Surface Systems

Surface Core Control System

Vehicle On-Board Core Control System

Vehicle Telemetry

Thruster DCON

Nav igation DCON

Power Mgmt and Hotel DCON

Video DCON

Work Package DCON

Science Pay load DCON

Surf ace Telemetry

Sonar DCON

LBL Nav igation DCON

Doppler Nav igation DCON

Pilot’s user Interf ace, joy stick, Instruments

Engineer user Interf ace

Manipulator user interf ace, instruments

Work package interf ace, instruments

Vehicle Control Computer

Control Process

Nav igation Process LBL-Doppler Nav igation Process

Power Management and Hotel Process

Video Distribution and Recording Subsy stem

Ship LBL Acosutic Nav igation System

GPS Satellite Nav Sy stem

Ship Dy namic Positioning Sy stem

Sy stem GMT Clock

Science Pay load Interf ace, instruments

Real-Time Data Logging Sy stem

Data/Video Telemetry f rom Vehicle to Surf ace

Manipulator DCON

Pilot Video Display

Figure 4: New ROV Control System: Initial configuration shown with solid lines. Proposedfuture enhancements shown with dashed lines.

2.3.1 On-Board Vehicle Control System: Data Concentrator Architecture.The on-board vehicle control system controls and monitors all vehicle sensors and actuators in response to real-timecommands from a surface control system. We will adopt a “data concentrator” (DCON) type of architecture, similar

to the MBARI Tiburon vehicle [11]. Each data concentrator (DCON) module will independently control the powerand data telemetry for an entire vehicle subsystem or scientific payload. The DCONs will receive commands fromthe surface control computer, monitor the status, and report data from on-board vehicle subsystems and instruments.Each data concentrator operates asynchronously, and will communicate to the surface control computer via a highbandwidth fiber-optic telemetry-link.The data concentrator vehicle control system architecture employs relatively simple on-vehicle computer systems.We anticipate employing commercial-off-the-shelf (COTS) embedded computers for the data concentrators. Thesimplicity of the data concentrator design will render them both highly reliable and easily re-configurable.

2.3.2 Surface Control SystemThe surface control system is the central “brain” of the ROV control system. It is comprised of a “core” system ofsafety-critical systems that are essential for safety and control of the ROV, and an “extended” system providing nonsafety-critical systems such as data logging and video recording. The structure of this system is depicted inThe safety-critical core system is comprised of the vehicle control computer and user interfaces for the vehicle pilotand engineer. The pilot station provides real-time video, navigation instruments, has joystick controls for closed-loop control of the vehicle reference trajectories and navigation way-points, and has controls for the vehicle’smanipulator arms. The engineer station has a more comprehensive set of real-time vehicle status indicators, andenables the engineer to control all vehicle subsystems. The modules are depicted in Figure 4.Concentrating the vehicle intelligence in the ship-board control computer dramatically simplifies and acceleratesdevelopment – reprogramming an on-ship computer is significantly easier than reprogramming an embedded vehiclecontrol computer.

3 Current Status and Future WorkThe vehicle design was completed in December 1998. It is currently under construction, scheduled for completionin May 1999. Starting in Summer 1999, wet trials are scheduled to begin with full use in thrust control algorithmexperiments scheduled for the Fall of 1999. An initial goal will be the experimental evaluation of the effect of thrustcontrol algorithms, e.g. [17], on closed-loop vehicle maneuvering.

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