subzero

Casual research in to underwater robotics powered by the Raspberry Pi Zero.

Sensors

• Depth (Pressure)
  • Measurement Specialties has a line of amazing MEMS-based pressure sensors that are tiny, accurate, and easy to use. SparkFun has a breakout board for the MS5803-14BA and The Cave Pearl Project goes in to detail on embedding external sensors in epoxy. An inexpensive breakout board is available on Taobao.
• Positioning
  • GPS is only really an option on the surface since water attenuates RF signals pretty quickly.
  • Many ROVs use an underwater acoustic positioning system such as Long Baseline Acoustic Positioning System or Short Baseline Acoustic Positioning System.
  • Inertial Navigation Systems are often used to augment other systems.
• Bottom (Depth sonar), sometiles called an Altimeter
  • If detecting bottom when 30cm to 3 meters is acceptable, an inexpensive option is available with an external sensor or with an embedded sensor on the DYP-ME007Y board. It should be possible to embed either type in epoxy.
  • Coconut Pi and Model_UBoatMC have used the weatherproof but not rated for underwater use Maxbotix MB7078 at depths of 1-3 meters.
  • Kongsberg, Valeport sell commercial versions but are larger than I'd like.
  • Oceantools and Tritech have smaller models.
  • In Development of a Small Sonar Altimeter and Constant Altitude Controller for a Miniature Autonomous Underwater Vehicle, Jessica Luan describes building her own altimeter from a waterproof transducer.
  • This Arduino forum thread makes me think that these inexpensive 40 KHz transducers could be used in lieu of making your own from scratch for Luan's approach.
  • There are several low-ish cost ($75-175) transducers for boat fish/bottom finders from Garmin and others. The challenge here would be interpreting raw data or using an NMEA 0183 or NMEA 2000 transducer and decoding that.
  • Mark Thompson shares in detail his home built side-scanning sonar.
  • The Ardupilot controlled transducer boat details a surface-based sonar vessel powered by Ardupilot.

Thrusters and other components

• Bue Robotics has a relatively inexpensive electrical thruster that they funded with Kickstarter. They also carry cast acrylic watertight enclosures and end-cap cable penetrators.
• Lots of DIY builds involve repurposing an electric bilge pump, removing the housing, and adding a propeller.

Communication

• Most ROVs use a tether to communicate. Tethers appear to be the only way to carry enough bandwidth from the ROV to base station to cover the requirements of live video and other sensor/telemetry requirements.
• I plan to experiment with RTL SDR inexpensive software defined radios at the lower end of their useful spectrum (20-100MHz) to see if there's a way to carry a little bit of bandwidth longer distances.
• UCSD have created an extremely low bandwidth (200 BPS) but long distance (2km) acoustical modem for $600. Other commercial options include LinkQuest and Teledyne Benthos in the $8k-$10k range.
• WHOI Micromodem is also in the $8k range but might be cheaper to construct these days.
• Ryan Kastner at UCSD talks about a custom transducer modem.
• Northeastern University has done some research on underwater sensor networks.
• The team behind Design and Implementation of an Omni-Directional Underwater Acoustic Micro-Modem Based on a Low-Power Micro-Controller Unit from the journal Sensors looks at transmission loss at various frequencies and builds their own underwater modem with a transducer. They reach 5kbps at 30m using a 70MHz transducer.
• Aquasent is a commercial underwater modem provider.
• Mike's Sub Works uses 75 MHz RC gear to communicate with small ROVs.

Other projects

• ROVs in a Bucket walks through an inexpsneisve ROV build with PVC pipe and modified bilge pumps as thrusters.
• OpenROV have been working on ROVs in the open for awhile and also sell kits.
• Cornell University Autonomous Underwater Vehicle contains lots of information about their builds and lots of open source software. They published a paper about their most recent vehicle, Argo.
• ArduPilot is a long running successful project that encompasses hardware and mission planning mostly for airborne UAVs. No direct applications to AUVs but lots of things were learned the hard way here.
• Coconut Pi a Raspberry Pi and ArduPilot project from 2013.
• The Robot Operating System is used in many land-based robots and some UAVs and is worth further investigation.
• Collective Cognitive Robots is researching collaboration between swarms of underwater robots.
• Amador Valley High School Robotics Club Barracuda for the RoboSub competition.

Books and Articles

• The ROV Manual, Second Edition: A User Guide for Remotely Operated Vehicles: This focuses on observation-class ROVs and larger, and has a lot of information a small ROV/AUV can ignore. Overall it's a good into that filled in the state of the art of sensors and other things. It goes in to a ridiculous amount of detail of some things and glosses over others, but it's a helpful bootstrap.
• Underwater Robotics : Science, Design and Fabrication is the other big book on the subject. I haven't skimmed through this one.
• The American Practical Navigator, first published by Bowdich and updated by lots of people has lots of background on navigation.
• The Internet Underwater: An IP-compatible Protocol Stack for Commercial Undersea Modems a paper about running a full TCP/IP stack on the Teledyne Benthos SM-75.
• An Evaluation of Potential Operating Systems for Autonomous Underwater Vehicles [PDF] compares general and embedded operating systems as well as many robotics-oriented platforms.
• Achieving High Navigation Accuracy Using Inertial Navigation Systems in Autonomous Underwater Vehicles details the intertial navigation system of the Bluefin AUV.
• 6-DoF Navigation Systems for Autonomous Underwater Vehicles: A look at ded (deduced) reckoning in underwater vehicles.

Simulation

• UWSim is an open source dedicated ROV/AUV simulator for Linux.
• Gazebo is an open source cross-platform general robotics simulator. AUV plugins include gazebo-uav-sim and freefloating_gazebo.
• MARS the MArine Robotics Simulator is designed for hardware-in-the-loop testing of AUVs, integrating with ROS and several robots developed at Institut für Technische Informatik.

Websites

• Woods Hole Oceanographic Institution has an AUV page with links to several vehicles.
• The Cave Pearl Project has a lot of info on DIY encasing and sensors.
• The Autonomous Undersea Systems Institude
• Autonomous Undersea Vehicle Applications Center
• Homebuild ROVs forum
• RC-Sub-Workshop is aimed at functional RC scale models but has great prices on acrylic tubes, motors, and specialized electronics.
• The SubCommittee is a group of scale RC sub enthusiasts with lots of basic information and links to vendors.

Vehicle Design

Version 0: Sensors on a Line

The first task is to construct a platform for component validation. This will likely be a small watertight enclosure weighted for negative bouyancy suspended from the surface from a line. A minimal set of components:

• Raspberry Pi Zero
• Power source
  • The $6 2000 mAh Amazon Basics battery works great but the triangular portion of the cord blocks usage of the USB port. An extension cord or a different battery should solve that though.
  • Tzumi markets several low-cost mobile device rechargers under the Pocket Juice name. This 2200 mAh model can be found for as little as $5 in stores and comes with a micro USB cord that doesn't block the other port.
• Sensors
  • Pressure/temperature
  • Bottom/distance

This should also be able to serve as a communications test platform, but that's not strictly required for initial testing.