Background of the Invention

This invention relates to underwater sonar communication that can be implanted in a sub-compact wearable or other device using micro transmitter, micro receiver and low overhead modulation algorithms

1. Background

Ultrasound Sonar has been used widely for underwater communication of voice and data. For voice communications, the audio frequencies are up-converted to ultrasound frequencies, transmitted underwater and finally down-converted back to audio frequencies at the destination. For data communication, the data bits are modulated to ultrasound frequencies using modulation techniques such as FSK (frequency shift keying) and PSK (phase shift keying). The modulated ultrasound frequencies are transmitted underwater and demodulated into data bits at the destination. These communication systems are typically medium to large in size, with big transmitters, big receiver and complex processing hardware. These large systems are not convenient for individual divers who want to minimize the load on their diving gear. They are also expensive due to complex hardware needed for extensive processing of signals.

A sub-compact underwater Sonar communication system is proposed with the use of micro transmitter, a micro receiver and light processing hardware. Using the proposed system architecture, the users will be able communicate underwater with a small footprint device, as small as a wearable diving wristwatch or a diving mask. The proposed solution uses smart signaling algorithms to minimize the need for expensive processing hardware. The compact transmitter and receiver are implemented using low-cost off the shelf components. The resulting communication device is not only extremely small and convenient to use, but also low-cost

2. System Overview

The sonar ultrasound communication system includes modulation hardware to convert voice or data to ultrasound, transmitter to broadcast ultrasound, receiver to detect transmitted ultrasound and demodulation hardware to convert back the signals into voice or data.

On the transmitter side, voice or digital data is modulated using a carrier ultrasound frequency. This signal if further transmitted underwater using a piezo bender/transducer attached to body of the device. This low-cost piezo bender, with typical thickness of 0.5mm, are embedded into a slot within the plastic surface, occupying zero added volume within the device.

On the receiver side, the system includes a micro-MEMS piezo microphone receiver with a waterproof membrane. This MEMS receiver is designed to detect ultrasound frequencies used for modulation, which are typically in 20khz to lOOkhz range. The receiver converts the ultrasound frequencies into electrical

SUBSTITUTE SHEET (RULE 26) signals. This signal is filtered and further de-modulated to convert it into voice or digital data. The MEMS microphone receivers are low-cost devices manufactured by semiconductor process and are ultra-small, in the range of 3x2xlmm.

For a low cost and low bandwidth communication a dual tone multi frequency (DTMF) type modulation is proposed. This technique, widely used for telephone key press, can work with low-cost hardware modulators. A two audio tone system used to generate 16 different digits in telephone system, is adopted with two ultrasound tones to represent 16 different digits. When higher bandwidth and performance is needed, the standard technique of frequency modulation such as FSK modulation and phase shift modulation such as PSK modulation can be used (fig.7).

3. System Detail

This section describes in detail, the three main components of the proposed sonar communication system: the transmitter, receiver and the modulator

3.1 Piezo surface vibrator transmitter

The traditional sonar transmitters use piezo transducers that cannot be integrated in a small enclosure such as a wearable wrist watch. In these transducers, the stacked piezo layers and the vibration surfaces that generate ultrasound signal take up internal volume. Another requirement for a diving communication system is that signal needs to be omnidirectional, where it is transmitted in all directions. This additional requirement needs additional reflector and thus adds further bulk to the transmitter.

The proposed design for transmitter uses a thin piezo plate (also called piezo bender) is attached to the body of the enclosure. The piezo bender is designed to have resonating frequency in the ultrasound range. The vibrations from the piezo bender are transferred to the surface of the enclosure, further vibrating the plastic or metal body of the enclosure. The vibrations on the larger surface of the enclosure generate and broadcast the ultrasound signals into the water. This concept is akin to vibration speakers, where a small transducer attaches to large surface to create audio from surface vibrations.

Piezo bender

These piezo plates can be as small as 10mm diameter with a thickness of 0.5mm, taking up negligible space (fig.8). By embedding plate this into slots on the inner surface of the enclosure, they are designed to take zero volume inside the device. By placing these plates in multiple surfaces of the enclosure, the ultrasound can be emitted in multiple directions. The enclosure surface thickness is designed so that it can transfer the ultrasound vibrations into the water.

Piezo bender on the inner surface of enclosure (fig.9)

3.2 MEMS microphone receiver

The traditional ultrasound receiver microphones (also called hydrophone) use piezo transducers. They convert ultrasound vibrations from environment into electrical pulse. These are typically medium to large size devices. They cannot be integrated in sub-compact enclosures such as wrist watch due to their bulk.

SUBSTITUTE SHEET (RULE 26) The proposed solution uses a semiconductor MEMS (micro electromechanical system) receiver microphone. They are designed to be sensitive to ultrasound frequencies in the range of 20khz - lOOkhz. These devices can be designed to be as small as 3x2xlmm and can be easily integrated into sub-compact wearable type devices. Multiple receivers can be integrated in the enclosure to detect signals from multiple directions. Use of multiple receivers also help in detecting the direction of the ultrasound signal.

Using multiple receivers and the beamforming microphone technology, the system can detect the direction of the source signal. The time delay and phase shift of the ultrasound signal between different microphones can be used to perceive the direction of the sound. One application for this technology is boat finder feature where diver is directed towards the boat. The boat will be emitting ultrasound beacon towards the divers. The device worn by the diver will detect the direction of the signal coming from the boat and the divers are navigated towards the boat

The MEMS receiver is protected with a waterproof membrane that can transfer the ultrasound frequencies without loss. This protected MEMS receiver is exposed to the environment with an audio port inside the enclosure. The gasketing around the MEMS receiver seals the enclosure from the water.

MEMS ultrasound microphone receiver (fig.10)

3.3 DTMF based modulation

Traditional sonar communication systems use standard modulation techniques such as frequency shift key (FSK) to convert digital data into ultrasound signals. While these techniques provide good speed and bandwidth, they require complex and expensive hardware for modulation and environmental ultrasound noise filtering.

The proposed design uses dual tone multi frequency (DTMF) signaling for a simple, low bandwidth modulation of the data into ultrasound signals. The system is similar to the DTMF tones used in telephones where each digit pressed on the phone dial transmitted as two-tone frequency. The destination will detect the two frequencies in the signal and convert it to the transmitted digit. By using 4 frequencies for first tone and 4 other frequencies for second tone, a total of 16 digits can be represented for signaling. In our proposed solution, the speech frequencies used in telephone signaling is replaced with ultrasound frequencies for underwater signaling.

In one embodiment, 2O-21khz range is used for first frequency and 22-23khz range is used for second frequency. The 16 possible combinations of signals can be used to represent 10 digits (0-9), Start/End command and 4 additional commands. Using a 50msec tone duration, 20 bytes of data can be transmitted per second. Table 1:

Two tone frequencies to represent 16 signals

Dual tone frequency (fig.ll).

SUBSTITUTE SHEET (RULE 26) The use of DTMF technique in ultrasound communication eliminates the need for expensive hardware used in complex modulation and false signal filtering. While this method may not be not suitable for highspeed communication, it can suffice for low-speed communication. The solution can be implanted effectively with low-cost simple hardware.

The frequencies of operation could be in the range of 20khz to lOOkhz. The lower frequencies are more unidirectional with better transmission ranges. But the data rates are lower. You cannot go under 20khz, as it causes audible sound that divers can hear. The higher frequencies have better data rates, but they have more attenuation losses and shorter range. They are also more directional.

Due to this varied frequency transmission nature of sonar frequencies, the system could use 20-30khz range for boat to diver communication with distance range of few miles. The diver team communication could use 30-40khz range to cover intermediate distance range of lOOmeters. This higher frequency also provides faster data rates. The communication between instruments of the diver could use 40-50khz range for short distance range of 2M.

The present invention is shown in the following figures:

Fig.l shows a Piezo bender with pigtail leads.

Fig.2 and Fig.3 show a Piezo bender attached to plastic surface (front and back view).

Fig.4 shows a Piezo bender attached to plastic. Setup showing pulse frequency generation at 24V.

Fig.5 shows a MEMS microphone array

Fig.6 shows a MEMS microphone array powered by 3V and attached to audio card.

Beamforming microphone array to detect direction of sound

The sound (audible and ultrasound) beamforming is a technique used to detect the sound from one direction or detect the directivity of the sound using an array of microphones. At very basic level, the technique involves detection of which microphone receives the signal first. The distance between the microphones is known and thus the time (and the signal wave phase difference) it takes for the sound to reach from one microphone to next is known. This phase difference is used to detect the direction of sound. When you have an array of 2x2 microphones, using the phase difference between the 4 microphones, the direction of the sound is calculated by looking at the phase difference between all 4 microphones.

To detect sound from only from one direction, the signals are phase shifted in each microphone with estimated phase shift as per the required direction of the sound. After this shift, the signals from the direction of interest become additive and get amplified. Signals from any other direction gets negated and reduce in amplification. This process could be repeated in all directions to detect the peak sound direction.

One dimensional array is used to detect sound from the direction of the array. Two-dimension array can be used to detect sound in 2 axis directions. Three-dimension array can detect direction from all 3 axis.

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