The specific requirements of a personal sonar safety system for free swimming divers have been examined. Using the characteristics of the piezo plastic material PVdF, a system has been developed to fulfill the need at a cost only available from a full scale production process
Diving is an exhilarating and dangerous pastime even in the UK where safe diving is seriously practised, however the accident statistics represent the high risk content.
The first requirement in all accident alerting systems is that they buy rime for the hapless adventurer Divaid is a tracking and alarm sonar carried by the swimmer, so that the safety officer knows his position at all times and can be alerted to any mishaps
The basis of the design is the exploitation of some of the less obvious features of the piezo-plastic material PVdF
The system must be capable of operation from the simplest of vessels or stations. Hence it must be self powered, rugged, directional, multi diver operational and capable of giving distance bearing and alarms. A diver recall system is desirable but not mandatory. The operational range should be in the region of several hundred metres, with the possibility of longer range detection in the event of drift away on the surface Power will have to be derived from self contained batteries
The coding method chosen is a transmitted pulse train forming a modified Telex code of 9 bits which carries a unique noise sensing partition to reduce spurious readings under high noise conditions- An alarm signal contains all the normal data but also includes a prolonged tone identifiable in preference to other signals The implication of a digitally coded signal is that a higher transmitted bandwidth is necessary; that is a transducer of low but controlled "Q"
The coding uniquely identifies 32 divers
The derived system comprises digitally coded beacons for each individual swimmer, synchronised to a central unit carried on the boat The battery operated transmitters have a throw away release that can be operated if the carrier gets into difficulty. The digital coding is then modified, producing an alarm signal which alerts the control boat and gives the signal processing priority The received signal contains distance, azimuth identity and alarm data, which is processed and displayed sequentially
Azimuth information is derived from a contiguous PVdF shaded array, which also contains an omni directional array for normal mode monitoring
The electronics are an implementation of a classic synchronised acoustic system with a time vaned gain element, but carefully developed to match the characteristics of PVdF.
The key to high performance sonar is the correct design of the transducers
The transmit transducer is a breathing mode transducer constructed from PVdF representing a unique patented design The main advantage being that the bandwidth of the transducer can be accurately adjusted to suit that required for digital data transmission without compromising the optimal theoretical efficiency.
In principle the breathing mode transducer has a very simple operation and could be implemented in many piezo sensitive materials.