A New Remotely Operated Underwater Vehicle for Dynamics and Control Research

Transcription

1 In Proceedings of the 11 th International Symposium on Unmanned Untethered Submersible Technology, Durham, NH, September 19-22, 1999, pages A New Remotely Operated Underwater Vehicle for Dynamics and Control Research David Smallwood 1, Ralf Bachmayer, and Louis Whitcomb 2 Department of Mechanical Engineering Johns Hopkins University 1 Introduction This paper reports the development of a new remotely operated underwater vehicle (ROV) designed to serve as a platform for rapid development and deployment of novel underwater vehicle systems. The goal is to enhance our ability 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 towards AUVs, we argue that ROVs continue to provide a highly efficient platform for the research and development of advanced underwater technology. Once developed and validated on ROVs, numerous technologies have been readily transitioned for use in autonomous underwater vehicles (AUVs). Section 1.1 examines several contexts in which ROVs have served as development platforms for critical AUV technology. Section 1.2 reviews the desired performance specifications for the new vehicle. 1.1 Historical Role of ROVs in UUV Research A 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 and survey, 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 for Autonomous 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 Alvin Submersible [9], was adopted for use on the Jason ROV in the 1980s [20], and transitioned in the 1990s for use in AUVs including ABE [21] and Odyssey [13]. 2. Closed Loop Vehicle Control: The closed-loop control (dynamic positioning) systems originally developed for Jason [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 originally developed with Jason [14] have recently been successfully transitioned for use in the ABE AUV [16]. Recent reports 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 the 1970s and 1980 with the Angus and Argo towed systems at WHOI. Early successes include finding the wreck of the R.M.S. Titanic during an optical search in 1985 [2]. These techniques have been subject to ongoing development with the Jason and Argo II ROV system for precision optical survey [14] and have recently been transitioned 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, USA, d_smallwood@jhu.edu, ralf@jhu.edu, llw@jhu.edu. The authors gratefully acknowledge the support of the Office of Naval Research and the National Science Foundation under ONRYI grant #N and Career grant #BES held by Louis Whitcomb.

2 5. Low-Power Robot Arms: Low-power electrically actuated underwater robot manipulator arms were developed for use on the Jason ROV as a highly efficient alternative to hydraulically actuated arms [24, 15]. This manipulator has proven to be highly effective at subsea sampling and manipulation tasks [1]. Electrically actuated arms are a fundamental an enabling technology for future AUV missions requiring robotic manipulation [12]. 6. Electric Thrusters: DC electric thrusters provide dramatically improved propulsion efficiencies in comparison to 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 in AUVs 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 vehicle research, laboratory, testing, and field-trials of novel underwater vehicle sub-systems. This paper describes a relatively low-cost ROV, presently nearing completion, to serve as such a developmental platform. 1.2 Vehicle Design Goals The 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 vehicle control 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 deployment Mass: 140Kg Ease of handling and deployment Stability: Passively stable in roll and pitch, re-configurable for dynamic roll/pitch control. Provides both 4-DOF and 6-DOF vehicle dynamics. Propulsion: Electric thrusters, 300 N each axis Control bandwidth and authority Propulsion Instrumentation Position Instrumentation Computer control system Current mode amplifiers, instrumented for propeller shaft position, 1000Hz Sample Rate Six Degree of Freedom (6-DOF), 5Hz min. Easily re-programmed while at depth, providing up to 1000Hz sample rates. Thruster dynamics and control research. Vehicle navigation, dynamics and control research. Vehicle navigation, dynamics and control research. Video Standard NTSC Closed-loop optical servo research. Manipulator Arm Capable of supporting future pair of 6-DOF electric Development of manipulation arms. techniques for AUVs. Payload Support Tether Generic payload port providing power, RS422 and Ethernet telemetry. 10KVA DC power, real-time data and video telemetry. Table 1: Vehicle Design Goals Development and testing of novel subsystems. Ease of development and experimentation

3 2 Vehicle Description The vehicle is presently under construction, but will look similar to the concept drawing in Figure 1. This section outlines the design choices made by the authors to achieve the goals outlined in Table 1. Section 2.1 reviews the vehicle propulsion system. Section 2.2 reviews the vehicle navigation suite. Section 2.3 outlines the vehicle control system architecture. Figure 1: JHU ROV #1 Design Concept 2.1 Propulsion To achieve the disparate goals of high thrust, small size, and precise propeller position instrumentation, we have developed 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 Torque Thrust Power Control Feedback Depth 3 Phase DC Permanent Magnet Brushless Motors 6.5 N-m Maximum, 2.16 N-m Continuous 150 N Peak 1.5 kw Current Mode Amplifiers providing 2mS current response to +/- 15 amps at 150 bus voltage. Amplifiers housed external to thruster. Resolver Shaft Position, 4096 count/rev (0.088 ) Angular Resolution Full ocean depth. Internally pressure compensated. Table 2: Thruster Specifications

4 Figure 2: New 3-Phase DC Brushless Thruster Figure 3: Exploded View of New 3-Phase DC Brushless Thruster

5 The initial vehicle configuration will use five DC brushless thrusters for propulsion, with built-in support for an additional sixth thruster. The initial configuration will employ two thrusters for forward/reverse thrust, two for lateral thrust, and one for vertical thrust. The current control amplifiers will be housed one of two AL7075 pressure housings along with a PC/104 CPU dedicated to thruster control. The PC/104 thruster controller is capable of 1000Hz closed-loop thruster control, and is provided with a direct high-speed telemetry link to the surface control computer. 2.2 Navigation Precision vehicle position sensing is an often overlooked and essential element of precision control of underwater robotic vehicles. The analytical and experimental development of undersea robotic vehicle tracking controllers is rapidly developing, e.g. [20, 5, 7, 8, 4, 6], however few experimental implementations have been reported other than for heading, altitude, depth, or attitude control. Conspicuously rare are experimental results for X-Y control of vehicles in the horizontal plane. This lacuna is a direct result of the fact that at present, few techniques exist for reliable 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 their time derivatives) are instrumented with a KVH ADGC gyro-stabilized magnetic compass system. Depth is instrumented with a standard analog strain-gage pressure transducer. Depth and attitude sensors are housed in an AL7075 pressure housing. Vehicle XYZ position will initially be instrumented with a 300kHz Sharps time-of-flight hard-wired acoustic navigation. We hope to also add a 1200 khz bottom-lock doppler navigation system. The navigation sensor specifications are listed in Table 3. Variable Sensor Precision Update Rate Heading KVH ADGC 1 10 Hz Roll and Pitch KVH ADGC Hz Depth Entran EXPO-X73-300P 0.75% Analog XYZ Position XYZ Velocity 300 khz Sharps Acoustic Transponder System 1200 khz RDI Workhorse Doppler 0.5 cm 5 Hz 1 mm/sec 5 Hz Table 3: Vehicle Navigation Sensors 2.3 Control System Architecture The new vehicle control system architecture will enable rapid development and field-testing of advanced UUV systems. The proposed control system will be structured for ROV control, as depicted in Figure 4, but will contain a variety 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].

6 Surface Systems Sy stem GMT Clock Ship Dy namic Positioning Sy stem Surface Core Control System Ship LBL Acosutic Nav igation System GPS Satellite Nav Sy stem Vehicle Control Computer LBL-Doppler Nav igation Process Nav igation Process Pilot s user Interface, joystick, Instruments Engineer user Interface Real-Time Data Logging Sy stem Power Management and Hotel Process Control Process Manipulator user interf ace, instruments Science Pay load Interf ace, instruments Work package interf ace, instruments Video Distribution and Recording Subsy stem Surf ace Telemetry Pilot Video Display Data/Video Telemetry f rom Vehicle to Surf ace Sonar DCON Manipulator DCON LBL Nav igation DCON Vehicle Telemetry Work Package DCON Doppler Nav igation DCON Thruster DCON Science Pay load DCON Nav igation DCON Video DCON Power Mgmt and Hotel DCON Vehicle On-Board Core Control System Figure 4: New ROV Control System: Initial configuration shown with solid lines. Proposed future enhancements shown with dashed lines 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-time commands from a surface control system. We will adopt a data concentrator (DCON) type of architecture, similar

7 to the MBARI Tiburon vehicle [11]. Each data concentrator (DCON) module will independently control the power and data telemetry for an entire vehicle subsystem or scientific payload. The DCONs will receive commands from the 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 high bandwidth 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. The simplicity of the data concentrator design will render them both highly reliable and easily re-configurable Surface Control System The surface control system is the central brain of the ROV control system. It is comprised of a core system of safety-critical systems that are essential for safety and control of the ROV, and an extended system providing non safety-critical systems such as data logging and video recording. The structure of this system is depicted in The safety-critical core system is comprised of the vehicle control computer and user interfaces for the vehicle pilot and engineer. The pilot station provides real-time video, navigation instruments, has joystick controls for closedloop control of the vehicle reference trajectories and navigation way-points, and has controls for the vehicle s manipulator arms. The engineer station has a more comprehensive set of real-time vehicle status indicators, and enables 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 accelerates development reprogramming an on-ship computer is significantly easier than reprogramming an embedded vehicle control computer. 3 Current Status and Future Work The vehicle design was completed in December It is currently under construction, scheduled for completion in May Starting in Summer 1999, wet trials are scheduled to begin with full use in thrust control algorithm experiments scheduled for the Fall of An initial goal will be the experimental evaluation of the effect of thrust control algorithms, e.g. [17], on closed-loop vehicle maneuvering. References [1] R. Bachmayer, S. Humphris, D. Fornari, C. V. Dover, J. Howland, A. Bowen, R. Elder, T.Crook, D. Gleason, W. Sellers and S. Lerner, Oceanographic Research Using Remotely Operated Underwater Robotic Vehicles: Exploration of Hydrothermal Vent Sites On The Mid-Atlantic Ridge At 37 North 32 West, Marine Technology Society Journal, 32 (1998), pp [2] R. D. Ballard, The Discovery of the Titanic, Warner/Madison Press Books, New York, NY, USA, [3] A. M. Bradley and D. R. Yoerger, Design and Testing of the Autonomous Benthic Explorer,, Proceedings AUVS '93, [4] S. K. Choi and J. Yuh, Experimental Study on a Lerning Control System with Bound Estimation for Underwater Robots, Proc. IEEE Int. Conf. Robt. Aut. (1996), pp [5] R. Cristi, F. A. Papoulis and A. J. Healey, Adaptive Sliding Mode Control of Autonomous Underwater Vehicles in the Dive Plane, IEEE Journal of Oceanic Engineering, 15 (1990), pp [6] T. I. Fossen, Guidance and Control of Ocean Vehicles, John Wiley and Sons, New York, [7] K. R. Goheen and E. R. Jeffereys, Multivariable Self-Tuning Autopilots for Autonomously and Remotly Operate Underwater Vehicles, IEEE Journal of Oceanic Engineering, 15 (1990), pp [8] A. J. Healey and M. R. Good, The NPS AUV II autonomous underwater vehicle test-tube: design and experimental verification, Naval Engineers Journal (1992), pp

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