2.019: Design of Ocean Systems I Fall 2005

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2.019 Design of Ocean Systems I Fall 2005

• Design of a small, autonomous surface vessel
  capable of tracking a subsurface acoustic source

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Challenge Statement

• Your goal is to create a new capability in
  ocean observation, by constructing a small,
  autonomous surface vessel system capable of
  tracking a subsurface acoustic source
• PROJECT TECHNICAL GOALS (descending priority)
• Demonstrate a small, autonomous surface vessel
  homing to a
• subsurface acoustic beacon.
• Demonstrate this in Sea State 3 conditions.
• Demonstrate navigation in a global frame, e.g.,
  GPS, compass.
• Demonstrate waypoint autopilot capability.

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A real-world problem
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Prioritized Objectives

• Develop a stable surface vessel
• Create an acoustic tracking system
• Maintain remote control of vessel
• Develop control system to allow for autonomous
  operation
• Create a user-friendly interface
• Develop waypoint tracking

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Trolling Motor
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Azimuth Motor
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Transducer
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Hydrophones
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GPS, Compass, Amp, Electronics
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System Requirements

• Platform
• Controls
• Acoustic Sensing

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System Requirements

• Platform

Small, surface vesselcapable of operating in
sea state 3
Sea State 3 Mean Wave Height is 0.5 - 1.25m,
Modal Wave Period is 5-14.8 seconds
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Hull

• Wilderness Systems Pungo 120, 3.7 m kayak
• Full length keel line
• Waterproof compartment in stern

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Hull Experiments

• Inclining test and initial condition response

We used a digital level for inclining
tests Initial condition response tests were
recorded with a high data rate Analog Devices
ADXL203 two axis accelerometer
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Results of Experiments
At ballast weight of 100 kg, metacentric height
is 26 cm
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These results show an optimal performance point
at 100 kg payload.
Damped Natural Frequency 1.2 Hz Damping Ratio
0.21
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Hull Wave Response

• Wave response at target weight

This gives a significant roll of 9 in average
sea state 3 conditions 0.88m significant wave
height and 7.5 second period
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Propulsion

• Minn-Kota Riptide trolling motor
• Seawater rated, has sacrificial anode
• 80 pounds bollard thrust
• Cable/pulley system to steer

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Power Systems

• All power provided by sealed lead acid batteries
• 2 80Ah batteries provide propulsion and amplifier
  power
• 2 18Ah batteries provide acoustic electronics
  power
• Batteries connect with keyed Anderson PowerPole
  connectors

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Safety

• 2 emergency stop buttons remove power from
  propulsion systems
• Warning light indicates propulsion system power
• All circuits are appropriately fused

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Integration

• Removable deck holds batteries, main and motor
  controller TT8 enclosures
• Emergency stop, main power distribution and bilge
  pump are hard wired into kayak
• All electrical connectors are keyed and labeled

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Packaging

• Electronics are in waterproof enclosures
• All connections made with environmentally sealed
  Switchcraft connectors
• All pass-throughs made with waterproof cord grips.

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Azimuth Motor

• 12v Servo motor
• Attached encoder
• Watertight housing
• Self-homing via hall sensor
• Proportional control with 360 rpm max slew rate
• Power consumption
• 2 A _at_ max slew

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Azimuth Motor Controller
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Directional Stability

• Bare hull is unstable (confirmed in first sea
  trials)
• Hydrodynamic center near center of mass
• Destabilizing spring term
• Addition of foil-shaped struts at stern drives
  hydrodynamic center further aft
• Provides stabilizing spring term

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Directional Stability

• Hydrophone struts have linear lift profile for
  AOA
• Hydrodynamic center at 3.02m aft of bow
• 1.02m aft of center of mass

CL
Source JavaFoil
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Trolling motor placement

• Forward mounted propulsor gives increased control
  stability.
• Places motor and associated electric noise and
  magnetic fields far away from compass and
  computers.

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Resistance (_at_ 1.25 m/s)

• Cylinder struts
• 65.5 N
• Streamlined struts
• 35.3 N
• Thrust available _at_ full throttle
• 242.3 N
• Effective speed limit
• 2.4 m/s (4.6 kts)

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Resistance in Waves
Added resistance due to wave action found to be
negligible, even in sea state 3.
Source "Experimental Results of Non-linear
Seakeeping Motions, Wetted Surface and Sectional
Force Tests on a Ship with Large Bow Flare," by
S.B. Cohen (1995, University of Michigan
Department of Naval Architecture and Marine
Engineering)
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System Requirements

• Control System

Autonomous operation in acoustic tracking and
waypoint navigation mode
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Sensors

• Garmin eTrex Legend handheld GPS
• PNI Corp. TCM2 Compass
• Wireless

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Control System Layout
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System Requirements

• Acoustic Sensing

Demonstrate homing to a subsurface acoustic
beacon.
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Acoustic Layout and Interaction
Hi location of hydrophone i, i 1, 2, 3 T
location of transducer
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Acoustic Components

• International Transducer Corp. ITC-1001 Spherical
  Transducer
• Omnidirectional transmitting/receiving
• Sonardyne Type 7656 Transponder
• Interrogation frequency 20,492 Hz
• Reply frequency 29,762 Hz
• Sensor Technology Limited SQ03 Hydrophone

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Range and Bearing
Transponder
Bearing to source
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Acoustic Navigation

• Constrained by challenge statement
• Short baseline acoustic navigation
• Transponder at source
• Determine position of source relative to ASV
• Affords portability of system to different
  operating environments

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Acoustic System
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Signal Modulation

• Product of two sinusoids
• Low-pass filter to remove HF component
• Resultant output is 500 Hz, which TT8 can sample
  at 5X the Nyquist rate

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New Hardware Developments

• Expanded modulation setup to include hydrophones,
  programmable amplifiers, bandpass filters
• Constructed 3-channel setup on breadboard for
  testing purposes
• Printed circuit boards for packaging after
  circuit design was verified

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Discoveries/Fixes

• MAX038 is unstable as a carrier sine, therefore
  unfit for modulation XR2206 from pinger works
  better
• Breadboards are less than ideal, especially if
  you have to move them around a lot
• PCBs are hard to get right the first time

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Working hardware

• The Stack consists of 4 boards 3 hydrophone
  sense and modulation, and one for ping and
  carrier sine generation
• Stack and TT8 fit in 4x4x8 otter box with
  splash-proof connections to hydrophones,
  amplifier, data and power
• Modulation output is stable to 200 Hz

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Recorded Signals
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Autocorrelation
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Autocorrelation
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Autocorrelation
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Autocorrelation
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Acoustic Processing Program
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Accomplishments Constructed a Short Base Line
Acoustic Tracking System from scratch
Experimented with and designed circuits to
amplify and control a hydrophone array
Built a circuit to drive a transducer, simulating
the transponder's signal, from a TT8
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Learned Auto-Correlation and other Signal
Processing Techniques
Characterized the Roll Response of our hull to
Initial Conditions
Conducted an Incline Test
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Designed and Constructed Azimuth Motor Controller
and Watertight Container
Learned to process GPS data with a TT8 and C
programs
Tested the Transponder Hardware and determined
its response is in the noise and must be filtered
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Accomplishments
Testing wireless control and autonomous steering
Testing acoustic system hardware
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Next Steps

• Finish integrating acoustics hardware with
  hull/power plant
• Accurately calculate bearing from real acoustic
  data
• Integrate acoustic data output with control
  system
• AARGH!!! FOLLOW THAT SOUND!!!

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Thank you!
Mechanical Engineering Department Center for
Ocean Engineering MIT Sea Grant MIT Towing
Tank Edgerton Center Student Shop Pappalardo
Machine Shop MIT Sailing Pavilion MIT 13SEAS

• Dr. Franz Hover
• Professor Michael Triantafyllou
• Professor Chryssostomos Chryssostomidis
• Christiaan Adams
• Dr. Ethem Sozer
• Professor Arthur Baggeroer
• Professor Steven Leeb
• Matt Greytak

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Questions?