Case Study P500
British sensors feedback position of water jets for USA support vessel
The first support vessel in the USA to be powered by waterjets is using a Rotary Inductive Position Transducer (P500) from British company Positek. New Zealand manufacturer Hamilton Jet supplied the quadruple HM571 waterjet systems for the 140ft vessel, which can reach speeds of up to 28 knots. The many benefits of waterjet propulsion, including outstanding manoeuvring capabilities - necessary for crew and equipment transfer - place high performance demands on the sensors within the steering and reverse control system.
The Hamilton waterjets provide manoeuvring by using a reverse duct that can deflect the jet stream anywhere between maximum forward and full reverse thrust. In addition a steering deflector controls the lateral direction of the jet stream. By combining reverse duct and steering positions, full 360 degree thrust vectoring is obtained.
Two P500 sensors are used in each hydraulic system to sense the position of the reverse duct and steering cylinders. Making very small adjustments of the reverse duct and steering deflector allows very precise manoeuvring and the ability to 'hover' the vessel in a given position.
Initially, Hamilton Jet incorporated two potentiometers into the system but soon found them to be completely inadequate, as Dick Borrett - Chief Engineer, Control Systems at Hamilton Jet explains: "In this installation, the potentiometers were failing after just two weeks in operation as they weren't robust enough to stand up to the vibration effects. This was causing mechanical wear on the conductive plastic tracks."
This led him to search for a non-contact position sensor, and Borrett discovered the Positek P500 product, which offers increased operational durability compared with potentiometers and better performance and at lower cost than other inductive sensors. The P500 is supplied with an internal micro electronic interface incorporating Positek's unique single chip integrated circuit design.
"We needed to move quickly and soon bought four of the devices. Subsequent testing proved them to be extremely satisfactory in terms of performance and durability. We decided to move over to the P500 as standard and will continue to do so for future projects." The accuracy levels required were within 1mm in terms of actual hydraulic cylinder position which was well within the capabilities of the P500.
The other problem with potentiometers was that we had to construct a housing that was oil and water proof to protect them , in what can be a fairly dirty environment" added Borrett.
"Positek provides a complete 'ready to go' unit. The P500 is totally sealed and enclosed which makes our manufacturing process a lot easier."
Hamilton Jet was one of the originators of the waterjet system and has now developed the more powerful HM range of waterjets for larger sized vessels.
The success of the 140ft support vessel is set to become a significant landmark for the company, and result in further business for UK based Positek which has already supplied 650 P500 products to Hamilton.
"We are going through a phase where work is increasing dramatically, with orders for similar waterjet propulsion systems coming in from the USA and many other countries around the world." concludes Borrett.