Project Update: Acoustics
At RoboSub, pingers are placed underwater and can be used to locate the robot. A pinger is a type of underwater speaker releasing a periodic sound pulse at a frequency set at the beginning of the course run. In past competition years, pingers have been placed near one or two locations, giving the vehicles a clue where to find the next task. To take advantage of this the team created an external housing with four hydrophones (underwater microphones) to determine the pinger’s location
Our module takes advantage of differences in time it takes the signal to travel to different hydrophones. Because the microphones are in different positions, the sound must travel different distances; therefore, the microphones will hear the ping at different times. These time differences can be used to determine where the pinger is, as shown in example in the picture below.
With two microphones, the direction of the pinger can be determined on one axis, with three it can be determined on two axes, and with four the distance can also be determined. The team went with four with the goal to determine distance to the pinger.
To collect the microphone data (via a suggestion from a TI employee) an FPGA reads values from 4 ADCs. This data collection is done with a custom designed printed circuit board. Once that data is collected by our main computer, computation begins on determining time differences.
The time difference determination works by finding the start of the ping and then identifying the ping waveform in each of the other microphones. Once the waveforms are identified and fully matched up, the timing differences are recorded. Below is an example of matched up waveforms.
Currently the team is seeing about 1-2 degrees variation between simultaneous readings, but with continued testing this number is decreasing. The system can provide azimuth (heading) and altitude (pitch) of the pinger, and work is being done to determine the distance by purely acoustic means. If that fails, the team plans to use multiple acoustic readings to determine absolute position.
The project, started in late 2017, is now successfully returning data and increasing in reliability. It is sure to be a useful addition this year at RoboSub 2019.