The Compass

by Web FishJan 6, 2013 @ 02:03pm

Provides information about about momentary vessel orientation with respect to the earth's magnetic field (compass), as well as earth's gravitational field (accelerometer). Basic navigation instrument.

  • Current selection: LSM303DLHC 3D Compass and Accelerometer
  • Status: Finalized

  • Criteria: Sensitivity, standard I/O, small physical footprint, cost 
  • Finalists: Sparkfun HMC6352, Sparkfun LSM303 Breakout Board, Pololu LSM303DLHC 3D Compass and Accelerometer
  • Main decision factors: Cost, standard I2C bus interface

 (click image for larger view) 

Spec highlights:

  • Dimensions: 0.5" × 0.8" × 0.1" (13 × 20 × 3 mm)
  • Weight without header pins: 0.6 g (0.02 oz)
  • Operating voltage: 2.5 to 5.5 V
  • Supply current: 10 mA
  • Sensitivity range (configurable):
    • Accelerometer: ±2, ±4, ±8, or ±16 g
    • Magnetometer: ±1.3, ±1.9, ±2.5, ±4.0, ±4.7, ±5.6, or ±8.1 gauss

 

Caveats:

  • Still unclear how well (if at all) the tilt compensation will work in a constant motion (sea waves) environment;
  • Unclear whether we'll be able to use built-in accelerometer to determine vessel orientation up/down (current flip status) - again, due to constant acceleration changes from riding surface waves;

 

PilotFish Navionics - Block Diagram

by Web FishJan 1, 2013 @ 10:55am

In a series of articles over the next few weeks we'll cover the various components of the on-board electronics driving the PilotFish vessel. Many components are still in flux, so expect changes as the project develops. As sub-modules and connection paths / interface points get solidified, those will be marked as finalized in future revisions of the diagram.

Here is a quick summary of the navionics sub-systems (click image for full size):

 

 

  • Central Processor: The main control unit. Runs the custom software which makes things move (and find their way)
  • Comm uplink: Telemetry uplink (current position, system status and sensor data reporting)
  • GPS receiver: Current position tracking
  • Compass: Current bearing and acceleration;
  • Aux navigation sensors: additional sensors needed for proper orientation and navigation
  • Additional sensors: environmental and other sensors not directly related to navigation;
  • Image acquisition: live video / snapshots of surroundings
  • Motor controller: accepting control input from CPU and converting it to high-power electric current control
  • Propulsion motors: electric drive motors and propellers
  • Rudder control: directional control
  • Propulsion solar panels: high-power solar harvesting, dedicated to powering main drive motors
  • Propulsion battery bank: high-capacity energy storage dedicated to powering main drive motors in the absence of sufficient solar power
  • Aux solar panels: additional solar harvesting capacity dedicated to powering navionics 
  • Navionics battery bank: navionics battery back-up