Design and Application of a Motion Platform in Three Degrees of Freedom

Aalesund University College is building the first full-size high-speed craft simulator in Norway. In preparation of the construction of the simulator there has been built a small-scale motion simulator to test the possibilities to imitate the movements of the ship. The model is based on a Stewart platform in three degrees of freedom (roll, pitch and heave). The platform deck is moved by three DCservomotors through three legs situated at the corners of an equal sided triangle with its center aligned with the mass center of the deck. The ship movements are generated by a simulator program on a PC, giving corresponding values for roll, pitch and heave at each instant. These values are transformed to equivalent values for the position of the three servo drives, which again are converted to analog voltages and applied as the reference signals to a Pulse Width Modulation card controlling each motor. Comprehensive testing have been done on the motion simulator, and it seems like the platform will imitate the movements of a ship in a satisfactory way. As part of the testing there has also been developed a control loop for manipulating the path of a moving object, e. g. a ball or a wheel, on the platform. For this purpose a camera was used to measure the position and velocity of the object in the xand y-directions, and these state variables were used in a feedback loop to control the path of the moving object. Different regulator strategies have been tried, like pole placement, optimum control, fuzzy logic and neural networks. The results have been satisfactory and the object is controlled to move in a circle or an 8-curve or from point to point as decided.