A custom designed passive acoustic tracking system answers the competition requirements for locating the “X” and the breaching area.  When designing the acoustic system, we considered the competition environment and built our system accordingly.  Specifically, the system is able to detect pings at specified frequencies, allows the user to set the detection frequency, and determines the vehicle’s heading in respect to the acoustic beacon.  The system is designed for low power consumption consonant with battery powered devices.  Further, it relaxes the microcontroller load by converting azimuth and elevation angles to voltages for analog to digital conversion.  The acoustic tracking system relies on two primary components, the transducer array and the processing board.  The system was fully tested before vehicle integration by using our department’s acoustic tank and an ORE acoustic beacon. 

The transducer array was custom designed and fabricated with the help of our sponsor, Harris Acoustics.  The array consists of four piezoelectric elements placed on a cork backing.  They are spaced approximately one-half wavelength apart (at our highest frequency, 30 kHz) to avoid ambiguities during measurements.  One element serves as a reference channel.  Two other elements provide elevation and azimuth comparison.  A fourth element is kept as a spare. The elements were potted on top of a hollow brass housing used to store the pre-amplifier circuits.  A SubConn cable connects the hydrophone array to the processing board within the pressure bottle.  Amplifying the signal directly at the hydrophone ensures a low noise and reliable signal for the processing board circuitry.

The processing board conditions the signals, detects the acoustic beacon, and measures azimuth and elevation angle relative to the beacon.  The incoming channels are filtered using broadband, band pass filters that remove noise outside of the beacon frequencies (less than 22 kHz, and greater than 30 kHz).  Additionally, the amplitudes of each channel are calibrated with respect to the reference channel.  The azimuth and elevation signals are ultimately turned into variable duty-cycle square waves that correspond to the phase differences between the reference channel.  The variable duty cycle square waves are converted into DC values that are held by sample and hold circuitry triggered by a ping detection. 

To detect a frequency-specific acoustic beacon, one channel is also tied to a digitally controlled narrowband filter.  The narrowband filter uses a digitally controlled potentiometer, allowing the user to set the resistance.  This set resistance corresponds to the filter’s center frequency.  Both narrow and broadband signals are rectified and integrated, so that the envelope of each signal can be compared.  The gain of the narrowband filter is set slightly higher than the broadband filter.  When the desired beacon pings, the narrowband signal will be larger than the broadband signal.  This causes a detection, which triggers the sample and hold for angle

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The detection and voltages corresponding to angle values are sent to the 5213 microcontroller which uses the information to help find the “X” and surface in the breaching area.