Applications of Motion Capture Technology in a Towing Tank
Model tracking is an essential element of vessel maneuvering tests, and motion capture has become the method of choice for tracking free running models in large maneuvering basins. The Boldrewood Towing Tank at the University of Southampton was designed in-house and built following high standards to be at the forefront of physical testing for many years to come. It features a combination of conventional proven systems as well as the latest technology.
The limited availability of large ocean basins for research and education has led the University of Southampton to purchase two Qualisys motion capture systems for its new Boldrewood towing tank: one for above water measurements and one for underwater measurements. The two systems can also be coupled for hybrid measurements.
Qualisys motion capture technology is versatile and has allowed the university to develop new experimental methods used for various education, research and commercial projects. The flexibility offered by the setup and various camera arrangements enables model dynamics to be measured in air, in water and across the water surface.
The above water system consists of four wide-angle cameras (70°) and four narrow angle cameras (49°). This offers an infinity of combinations, but experience shows that three different arrangements cover most of the work undertaken in the Boldrewood towing tank.
Arrangement A
For free running experiments with large or fast models, it is necessary to cover the largest possible tank length so sufficient data can be captured. By placing the wide-angle cameras on the side of the tank at a height of about 2.50m and the narrow angle cameras on the tank rails, a tank length between 8 and 10m is covered, which is enough to characterize the behavior of a fast model in open seas.
Arrangement B
The second arrangement is used for smaller and slower free running models or static tests when the model is moored in the tank. These tests only require a small volume to be covered. In this case, it was found that using four wide angle cameras works well. Two of them are placed on the side of the tank and two of them on the carriage
Arrangement C
The final arrangement is used when the model is connected to the carriage dynamometer. As the model is static relative to the carriage, the covered volume is very small. Using four wide angle cameras gives good results, but care has to be taken with regards to the obstructions present in the moonpool (various beams, platform, dynamometer post, etc).
Not only does the Qualisys motion capture system provide motion data for analysis, but it also can be used in real time as an input for a control algorithm. The system calculates real-time six degrees of freedom (6DOF) data from rigid bodies that can be accessed by using Qualisys’ SDK and translated into a NMEA compatible format. The obtained data can then be forwarded to a model computer via WiFi. The data is processed on the model computer with the appropriate algorithm depending on the experiment.
Sports Engineering
The recent application of these techniques in a towing tank has enabled new Sports Engineering research into aquatic sports.
The first project consisted in measuring the behavior of a free running kayak in which both the hull and the paddle handle were acquired as rigid bodies and therefore their respective motions measured. This allowed the relationship between the paddle motions and the kayak speed and acceleration to be characterized for various stroke rates and wave conditions.
Another project assessed the hydrodynamic performance of a rowing oar. For this, a rowing boat was attached to the side of the tank. Both the above water system and underwater system were installed and coupled to track the motions of the oar shaft in air and the blade in and out of the water. Interferences (mostly air bubbles) were observed near the water interphase, but the results allowed the motions and deflections of the oar to be assessed. These measurements, along with force measurements at the junction of the paddle and boat, and at the attachment of the boat by the tank side were used in a detailed analysis of the rowing performance of the blade.
Both motion capture systems have also been used within a swimming pool to capture the kinematics of a freestyle swimmer’s body and arms, both above and below the water. This allowed arm velocities and orientations to be determined over a typical stroke cycle.
Autonomous Surface Vehicle & Indoor Positioning
The ASV “SMARTY” is a three degrees of freedom (3DOF) surface vehicle designed for demonstration purposes at the University of Southampton. It is fitted with four brushless DC motors acting as thrusters. The motors are rotated 45° in the XY plane in order to provide vectorized thrust. Equipped with an onboard Linux powered computer, WiFi and Bluetooth connections, gyroscope and accelerometer sensors and a camera, the ASV can be operated either by a remote controller, or in autonomous mode using the information obtained from the motion capture system, via a host computer.
For the remote mode, the desired thrust input is obtained from a remote controller (RC) that connects to the vehicle computer via Bluetooth. The RC provides a normalized thrust reference described in a coordinated system fixed to the rigid body, which is internally converted to pulse width modulation references for the motor drivers. When operating in autonomous, the platform uses the real-time feedback from the QTM as a positioning system to follow a predefined collection of waypoints. Two empirically tuned proportional derivative controllers were included to control concurrently the heading and position of the ASV.
The follow-up of the work is to carry out dynamic positioning tests in various configurations of wind and waves. A twin shaft model fitted with both stern and bow thrusters is currently being built and will be used to develop a dynamic positioning algorithm.
Future work
There are many possibilities for future work offered by the motion capture systems in the Boldrewood towing tank. An experiment with a free running sailing yacht performed in the Boldrewood towing tank showed that it is possible to use small foam floaters fitted with reflective markers as wave probes.
As the wavemaker fitted in the towing tank is capable of generating oblique waves in the tank, it should be possible to use multiple small floaters as wave probes to generate a three dimensions map of the free surface of the tank when this type of waves is being generated. With a small enough model (due to the limited available width of 6 m in the tank), it will then be possible to perform self-propelled maneuvering experiments such as zig-zag tests.
It is planned to perform a similar precision study about the 6DOF given by the Qualisys system, comparing them to on-board inertial measurement units. Ideally, this will be performed when the carriage drive system has been installed so the self-propelled model can be tracked for the entire length of the tank and sufficient amounts of data collected.
In the application areas of industry and research, the motion capture systems provide an easy to install and use and versatile solution to a large range of problems. These systems have been extensively used by students, researchers and engineers at the Boldrewood towing tank. There is still some work to be done to fully explore the capacities, issues and benefits of the Qualisys systems, and we are very much looking forward to all the projects that will require their use.
The Qualisys technology is versatile and has allowed the University to develop new experimental methods used for various Education, Research and Commercial projects.
Bertrand MalasWant to know more?
To learn more about the projects pursued at the Boldrewood Towing Tank at the University of Southampton, refer to the following links.