Tip driven propellor system needs no drive shaft or gearbox

Thrusters developed first for underwater robot vehicles, and now being extended to submersibles, are unique in that their propellers are driven from their tips, and not by a central drive shaft. As well as making the devices much more compact, they do away with all the problems associated with having to have a motor, gearbox or pulley in front of the propeller, including any need for stuffing boxes or shaft seals. Larger sizes also do away with the need for heavy hydraulic systems. They thus have great potential as immersed pumps, thrusters for ships, fans possibly, and as a means of producing small and perhaps not so small scale hydroelectric power from streams. The devices are the invention of Dr Suleiman Abu Sharkh, a lecturer in the Electromechanical Research Group in the School of Engineering Sciences at the University of Southampton. They are being developed for commercial use by TSL Technology in Alresford, whose managing director is Dr Michael Yuratich. Encountered at the just held Underwater Vehicle Showcase at Southampton, Dr Yuratich told Eureka that the idea goes back to the 1970s, but that getting the devices to function efficiently has been more than a little difficult. The propellers work equally well in both directions, making use of a unique profile that we have been asked not to describe, and are the result of much computer modelling at the university. This has included the application of special programs developed at the university to model propeller blades and tips, and the behaviour of induced vortices. The blades are attached to a ring to which are attached permanent magnets. The ring and magnets form the rotor of a brushless motor. Brushless motors normally require the use of Hall effect sensors to detect rotor position in order to turn the stator electromagnets on and off at the right time. In this case, the Hall sensors would have to be mounted in the same housing as the electromagnets which would be difficult, as well as posing the need for an extra cable from the housing to the controller, never a welcome idea underwater. Despite being essentially brushless servos, the motors are therefore run sensorless, detecting rotor position from motor current and voltage current relationships, which Dr Yuratich says is "Easy enough to achieve at constant speed". However, these motors have to be able to achieve fast starts, kick starts, fast reverses, braking and to recover from a stall. Nonetheless, suitable control algorithms have been developed which, "Seem to work well enough", and the motors shown to Eureka certainly appeared to perform as specified. Dr Yuratich declined to reveal how this had been achieved, except to say that research at the start of the project had produced details of some 70 patented techniques, and that TSL's method was embedded in a high speed gate array. The company currently has three products. The first is a 50mm diameter thruster which apparently took six weeks to be developed, which was followed by a 70mm diameter thruster which took six months. Currently, a 300mm diameter unit and controller is being tested, capable of delivering 2.5kN of thrust for a "Major ROV (Remotely Operated Vehicle) manufacturer. The smaller units are used are smaller ROVs and AUVs (Autonomous Underwater Vehicles) and for controlling the working depth of other equipment. The thrusters are run flooded with sea water. They have no seals and require no lubrication. While they have been only tested down to 750m, they can in theory be used at any depth. The 70mm diameter model delivers 50N of continuous thrust. In its standard version, it is run at 28V, although all the company products can be wound to be run off any voltage. Because of the good inherent cooling, it is possible to run the thrusters at much higher powers than they are normally designed to be used at, although life is likely to be reduced and efficiency diminished. To complement the small thrusters, the driver boards come in two versions, one with one 200W channel and one with eight channels, up to 300W per channel, total 1000W, with RS232/RS485/CANbus serial port control and monitoring, torque speed and speed control loops. For larger units, the company is testing 80kW underwater power supplies capable of converting 3kV AC ship supplies to 600V without requiring the use of heavy transformers. While the small thrusters and their control boards are well proven underwater, and the larger models soon will be, the fundamental design of the thruster units makes them suitable for many other applications. One application already being considered is for recreational submarines, such as the Gemini "Underwater sports car" being developed by Subeo (more information at www.subeo.com). Dr Yuratich believes that really large versions would be of great benefit as manoeuvring thrusters on ships, especially ferries, because they are so much shorter than conventional units and could therefore be mounted around the periphery of the hull. Thrusters are inherently pumps, and as such, would offer the supreme advantage that they could be immersed in whatever they were pumping without the need to take shafts out of the vessel or area they were working in to connect them to external motors and gearboxes. This would make mounting them completely flexible, because they could work deep inside, and avoid all the problems of wear, energy loss and possible leaks associated with shaft seals. As fans, they would require some redesign because air does not behave quite like water, and provision would have to be made to improve cooling. But for use as power generators, they require little redesign, because like most electric motors, they are inherent generators when run in reverse. Highest efficiencies for turbines in streams are in theory achieved without ducts, but it is usually essential to direct fluid into a duct, in order than flow speed can be increased through the turbine. Dr Yuratich sees even his small units having possible application for trickle charging outdoor and military equipment. Larger units may also prove useful for the same reasons as the thrusters, that is they can be mounted right inside the flow, and require no shafts or seals. To access this and other related articles go to TSL Technology (site in preparation at time of writing) Email Dr Michael Yuratich Subeo Eureka says: Thrusters without separate motors, shafts, seals or lubrication are a major breakthrough, not only for the underwater vehicles for which they have been developed but also for surface craft, ships, pumps and small and perhaps not so small scale hydroelectric generators Pointers *The propellers are driven from their tips so there are no shafts, seals, or gearboxes * The units are much shorter than conventional designs, and can run at any depth without lubrication * Present products include a 50mm model, a 70mm model producing 50N of continuous thrust and a 300mm model being tested development capable of producing 2.5kN of continuous thrust