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Electronics design specification for underwater Sound Velocity sensor
This design project involves the production of a working prototype of an underwater sound velocity sensor. The speed of sound is not constant underwater, it varies according to a number of factors including salinity, pressure and temperature. As the speed of sound varies, that means that acoustic positioning systems become less accurate as they rely on a ‘time of flight’ method of measurement where a sound pulse is sent from one transponder to another. Obviously if the time taken for the sound pulse to reach the target transponder is not known accurately then it becomes impossible to accurately measure the distance between them. For this reason it is common to obtain a value for the speed of sound in the local environment where the equipment is being used.
The speed of sound can be calculated mathematically by measuring the conductivity, temperature and depth of the sensors and then using a complex mathematical formula to compute the sound velocity. However, a far more accurate method is possible by utilising a direct reading sound velocity sensor. This sensor simply sends a sound pulse across a known distance (50mm is good enough) and makes a highly accurate timing of when the sound pulse arrives at the other end. Using the simple formula ‘Speed=Distance x Time’ the sound velocity can be calculated.
The project involves the selection of acoustic transducers suitable for use in deep water (probably piezoceramic). These transducers will be fitted on either end of a 50mm long rigid mount. The electronics will generate a sound pulse which will be fires from one transducer to the other. A very accurate timer is started when the pulse is fired and stopped when the pulse is received. The timer will give an output proportional to the sound velocity.
It is anticipated that very accurate timing will be required (around 1ns) in order to give an accuracy which is acceptable. We would be looking for an accuracy of better than 0.2 m/s.
The project will involve the following tasks:
Selection of suitable transducers (as small as possible)
Circuit design
Construction of a prototype frame (our company will do this work)
Construction of the electronic prototype board
Testing of the prototype
Design of the final PCB
Manufacturing of the final device (our company will do this work)
The final system is likely to be battery powered and needed for long periods of time underwater, therefore the use of power in the system should be kept to a minimum. The system should also be designed to be as small as possible. It is anticipated that the system will be ‘polled’ by an external device under serial control (RS232) ie a simple command structure will be used to request a sound velocity reading and the system will reply with a serial data telegram containing the Sound velocity value in ASCII format.
Our company will pay for all components used in the design; they can be delivered directly to the chosen designer. Any mechanical design for the housing or the frame will be done by our company and free issued to the designer.
Target project completion – 2 months from acceptance of bid.
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