WIP

Introduction#

Acoustics is the branch of physics related to the study of mechanical waves in gases, liquids, and solids. Underwater Acoustics is the scientific study of natural and man-made sounds underwater. Applications include sonars to locate submarines, underwater communications by whales, et Cetra. To make our AUVs of industrial level standards and perform properly among the different sounds produced underwater, various competitions all around the world incorporate acoustic localization tasks in the arena. For example in Singapore AUV Challenge, acoustic pingers located at different props are used for target acquisitions task and localization task). Similarly, the Robosub competition conducted by AUVSI also plants different frequency pingers to guide the vehicle around the arena. Check out Resources - RoboSub link for their configurations as they change the theme every year.

An acoustic localization system detects a frequency emitting source at a distance, using Time Difference of Arrival (TDOA) of the sound wave.

Goal#

The types of equipment used are comparatively costly and complicated to make this system fuse with the operations of the vehicle. For this reason, the points to accomplish the tasks including a pinger is higher as compared to other tasks. Check SAUVC Rulebook and mission and scoring of Robosub competitions.

Objective#

Our first objective is to use the optimum number of hydrophones to detect two pingers emitting 37.5 kHz and 40 kHz frequencies(Specifications), respectively.

The tasks:

1. Yellow Flare - localization task This task aims to localize a yellow flare. There is only ONE yellow flare is marked with an acoustic pinger. This flare could be located anywhere within the main arena. The AUV should locate and bump the flare causing the golf ball on the flare to drop out. The flares will be yellow in color.
   Points would be awarded as follows. If the AUV successfully causes the ball to drop from : flare with the pinger: 40 Points [This task from the rulebook of SAUVC 2018]

1. Red Drums - target acquisitions task There are 4 colored drums in the arena. All of them are on a green mat of size 8m x 2m. One of the drums, chosen at random, will be blue in color, while the rest are red in color. One of the red drums, chosen at random, will contain an acoustic pinger. The AUV needs to drop a ball in one of the drums to complete this task. The location of the red drum which contains the acoustic pinger may be randomized between attempts, as may be the order of the drums. Points will be awarded as follows. Drop the ball in the blue drum: 30 Points Drop the ball in the red drum with the pinger: 50 Points Dropping the ball in any other red drum: 10 Points

Source: SAUVC Rulebook

The survey#

We checked Robosub journals of previous years of different teams - (Past Programs - RoboSub). The brief survey of hydrophones used by different teams in Robosub in 2019 - AUV2k19/hydrophones_study

For a list of hydrophones, quantity, company & placement of hydrophones by other teams, check References.

Equipments#

1. H2C hydrophones from Aquarian Audio with 3.5 mm TRS output connection. Key features: Groves for easy mounting, 3.5mm connector & range of detection (10 kHz - 100 kHz)

2. NI myRIO embedded board. Key features: 1.Inbuilt FPGA for audio processing. 2.One 3.5mm along with three analog inputs for audio signals. 3.Inbuilt IMU that can be used further for movement of the vehicle. 4.WiFi connectivity.

Source: NI MyRIO User Guide and Specification

The process#

Placement of hydrophones#

They are placed at specific distances in an equilateral shape for accurate results. Reference: Design and analysis of air acoustic vector-sensor configurations for two-dimensional geometry The approximate distance of the placement of the arrays is calculated to be 3 inches apart. The distance is measured from the center of the hydrophones. It has to be in the form of an equilateral triangle, as seen in the figure.

1. Time Delay Of Arrival(TDOA): The figure for 2-D plane detection using 3 hydrophones. The analog signals from the frequency emitter are detected by the hydrophones placed on our vehicle, which is then passed through a signal processing board (For that we're using National Instruments MyRIO microcontroller board.) Then the signal is passed through the Fast Fourier transform algorithm, for detecting the exact frequency. The value thus obtained is then passed to the AUVs computer, which commands the thrusters to move towards the detected source.

Future Work#

The assembly of all the components at one place and its optimization is required: 1.Connecting all 3 hydrophones to a microcontroller to receive input without wire/connection interference 2.Processing of audio signals for higher frequencies (50 kHz) 3.Attaching to the vehicle at a certain angle 4.Testing to eradicate reflections of signals form props and sidewalls of swimming pool

References#

We looked in the 2012 paper (insert document link) of Istanbul Technical University 3 hydrophones - 2012 paper (insert document link) of FEFU University and DARYABIRD (insert document link) IIT Bombay Matsya 2.0, 2013 FEFU Malardalen 2012 - H2C aquarian 2016 REDEFIANCE, 2016 Leviathan 3 H1C 2016 DORY 4 Teledyne 2013 ZYRA - used GCCPHAT (Generalized Cross Co-relation PHase Transform) for finding Delay of Arrival (DOA) implementation Graphical Programming for Field Programmable Gate Arrays: Applications in Control and Mechatronics (PDF link)