P14251: Acoustic Underwater Communications

Subsystem Design

Table of Contents

Identified Subsystems

Computer Engineering:

Electrical Engineering:

Mechanical Engineering:

Concept Improvement (generation, improvement, selection)

Since our System Design, we found several different means that successfully accomplished our requirements while staying within all specifications and budget. These methods made the "Hybrid model" from our Pugh analysis obsolete.

Systems Architecture

Software Subsystem

The software architecture is structured around implementing the CSMA/CA wireless communication protocol, which will allow for very little noise on the channel, and functions well with swarm expansions to the project. A "Request-to-send" will be sent over the channel first, and if a "Clear-to-send" is received, then data will begin being transmitted.

A central control module is the primary program running on the microprocessor, and is responsible for managing the other necessary modules. The control module receives and transmits data from receiver and transmitter modules, respectively, and decides what operations need to be performed on the data before sending it off to the appropriate modules.

For error-handling, it was decided that a hybrid scheme of EECs and ARQs would be best for allowing reliable error correction. An EEC (error correcting code) consists of redundant bits that allow errors within the data stream to be corrected at the receiving end. The amount of redundancy to add is equal to double the number of errors desired to be corrected, so to correct a stream with a 10% error rate, 20% redundancy is needed. If the errors cannot be corrected, then an ARQ (Automatic Repeat Request) is sent back to the transmitter to request that the data is resent over the channel.

For data compression, numerous software packages exist that can losslessly compress at roughly a 2:1 ratio. Software packages also exist that can encrypt and at whichever security level we have time to encrypt for. The actual algorithms in use will be later investigated as part of the detailed design.

The following functions are each implemented as their own module: Controller, Data Receiver, Data Transmitter, Compressor/Decompressor, Encryptor/Decryptor, Data Framer, Error Encoder/Decoder/Handler, and DSP (if necessary). Additionally, there exists a User Interface on the PC, and a receiver and transmitter to interface between the PC and the microprocessor.

Hardware Subsystem

The Microprocessor selected for running the communications software is the Raspberry Pi, due to its high speed (700 MHz ARM11-Based processor), numerous resources (512 MiB SDRAM), and cost ($50 after shipping), along with its high versatility and wide-spread usage.

The Raspberry Pi will interface with either an ADC or an actual demodulation circuit for receiving incoming messages in a digital form, and will transmit outgoing messages to a modulation circuit, which will likely be the AD9835.

The 700 MHz clock will theoretically allow for any programming overhead to be completed within a few milliseconds.

Communication (Modulation/Demodulation) Subsystem

A QPSK system will be developed using a modulation and demodulation IC's. The QPSK system will use four different phases to represent 00, 01, 10, 11 that will be transmitted through the water using the speaker.

The modulation IC has the capability of storing two different frequencies and four different phases. The modulation IC will produce one of the four phases at a time based on the inputs received from the raspberry pi.

The demodulation IC will receive a band passed signal from the hydrophone. Based on a capacitor attached to one of the output pins on the demodulation IC, a different voltage will be passed to the receiving raspberry pi based on the incoming signals phase.

Power Systems

A 12V battery will be able to supply enough voltage to the speaker to produce approximately 10W in electrical power. For a 15W system, 1.25A of current is needed. To last an hour, we will need 1.25Ah of energy.

In order to produce the 5V and 3.3V required for the necessary electronics and microprocessor, two buck converters can be used. Adjusting the inductor and capacitor values can ensure constant current and low voltage drops on the output.

If a negative voltage is needed, a buck-boost converter can be used. This allows for a common ground between all voltages while still producing the negative voltage needed.

Amplification on the transmitting side will include an op-amp voltage gain and a class AB amplifier for current gain in order to provide the 10W to the speaker. On the receiving side, automatic gain control amplifier will be used to achieve the same magnitude signal independent of the transmission distance.

Finally, the metal part of the structure will be grounded through both a resistor and capacitor to remove any high and/or low frequency voltages induced on the structure.

Structural System

Leaving the System design review, our design moving forward was a steel mounting plate with a plastic enclosure for the other sides. The cables would interface through the mounting plate and also serve as a heat sink. However, upon receiving the quote for the rapid prototyped enclosure, this design had to be abandoned for cost reasons.

A new design was developed that was incorporated entirely out of sheet metal. This design would require welds to hold it together which would provide a point of weakness. However, the case could very easily withstand the pressure of saltwater. Several vendors were contacted regarding the bending and welds. At this time we haven't heard back from them with a quote.

Thermal System

For our thermal analysis, a worst case scenario was developed for a basic thermal conduction, steady-state model. Given the maximum temperature in the ambient water from our specifications, (85 deg F), we wanted to make sure that our electronics wouldn't overheat.

To do this, the air was assumed to be conducting as opposed to resisting through convection, due to the space in the box, and the lack of circulation. The model also had 5 sides of plastic, which is less thermally conductive than steel or copper, one side of steel, and that the external flow in the water was 0.05 m/s or 0.11 mph.

For the worst case, it was assumed that there was a steel heat sink, that there was a full inch of air in length on every side of the electronics and that the power being sinked was the full 15 watts from the power specification. The results from this model put the maximum surface temperature on the electronics around 63 degrees C, which is lower than the operating temperatures of the electronics so as long as we have some form of heat sink, just to the internal surface of our metal plate the electronics should be fine.

Risk Assessment

At this stage, we have a speaker ordered for experimenting with and are in the process of getting some material tests started so that we can confirm the validity of our system. The risk is that if the speaker doesn't function, then the next best speaker will be beyond our budget.

The next greatest cause of risk is the box, as we have yet to get a quote. To compensate for this, we have assigned a large portion of our budget to this item.

Subsystems Design Review

P14251 Subsystem Design Review

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