Servo hydraulic system upgrades marine research rig

With marine technology advancing rapidly, there is a huge demand for model testing facilities capable of running highly accurate and reproducible tests. Marintek, Norwegian Marine Technology Research Institute, based in Trondheim, Norway, offers one of the largest ocean-basin laboratories in the world. At 80m long, 50m wide and with an adjustable depth of 0-10m, it recreates sea conditions using an array of flaps – two on the end and 144 single flaps along its length. In addition, the basin is equipped with a carriage system that caters for free-running models at speeds of up to 5m/s, at any angle to the waves.

Marintek chose Moog to upgrade the Ocean Basin with new technology, the aim being to not only make the system more robust and more reliable but also to provide enhanced capabilities - such as larger waves over a wider area of the basin and more directional flexibility. The most important driver for the upgrade was the need to increase overall performance. Indeed, an accuracy of 0.1 degrees for all flaps was requested to guarantee the creation of highly reproducible waves with a height of up to 0.4m as frequently as one wave every 1.6 seconds.

Another major requirement was the capability to self-monitor – taking into account not only the cost-per-hour, but also the impact of the test results obtained on human safety and the investments required to build the resulting ships and platforms. The system had to be integrated with the existing environment, especially the waveform computation system and measurement equipment, while also providing the flexibility to cater for future expansion. Due to the existing 700kW hydraulic infrastructure, a hydraulic system was the obvious choice.

System architecture

The system supplied by Moog comprises 144 hydraulic cylinders, each servo-controlled by a D636 Axis Control Valve (ACV). With the position sensor of each cylinder being connected directly to the servo valve, the D636 closes the control loop and offers additional features such as self-monitoring of the control loops and the position sensor.

Each group of 12 ACV servo valves is connected via CANopen to a Moog Servo Controller (MSC), a freely programmable motion controller with multiple interfaces such as CANopen, Ethernet and Profibus-DP. The MSC offers two independent CAN interfaces, each controlling six valves. Furthermore, the 12 MSCs are connected to the waveform computation system, distributing the set points and the actual position and status information from each valve. In addition to the hardware, the Moog Axis Control Software (MACS) – an IEC 61131-compliant development environment – was used to create the application programs.

Initially a test system - comprising one hydraulic cylinder, a position sensor, an ACV and an MSC controller - was installed to verify the calculated accuracy and dynamics. Subsequently, with the results fulfilling all requirements, the upgrade of the whole basin commenced.

In total, 12 cabinets were installed along the length of the basin, each containing one MSC connected via CANopen to 12 D636 ACVs. Due to the use of a fieldbus, the installation effort was surprisingly low considering the number of devices and the physical size of the system. Indeed, CANopen was selected because of its multi-master capability, its flexibility and integrity.

Frank Andersson, senior engineer at Marintek, explains: "The biggest surprise for me was how easy it was to program this system with all the 144 axes." Andersson was involved in the software development from the start of the project, as he wanted to be able to extend the system by himself in the future. As a result of the upgrade, the usable length of the basin for wave testing has been increased – thanks to the accuracy of the flapper movement – which is a major advance for testing high-speed ships and ferries, as the number of test sequences can be reduced.

With the advanced feedback capabilities now available from Moog's ACV range, designers and engineers are no longer faced with a trade-off between force and accuracy.