Fault Tolerant Design and Reliable Control of High Precision Under- Actuated Satellite Tracking Antenna

The ability to maintain communication over large distances has always been an important issue. Communication can be maintained through satellite communication as the constellation of the communication satellites ensures that it is always possible to make contact with a satellite, regardless of the actual position on Earth. The important factor in this case is to track a satellite and sustain contact with it. For stationary antennas this is simple: once the satellite has been tracked, the communication antenna on the ground will remain fixed on it. The challenge arises when the station is not stationary, which is the case with e.g. marine communication. Once the antenna on the ship has tracked a communication satellite, movements of the ship, partly due to waves, will force the antenna to point away from the satellite and thereby break the communication. In this case it is of great importance that the antenna has a control system that ensures that antenna compensates for these disturbances and remains locked on the satellite.

The Danish company SpaceCom is specialized in developing antenna systems with full hemispheric coverage for satellite communication. The antenna is intended for marine use where ship motions due to waves are greatest challenge. If the antenna system is not capable of compensating for the disturbances in an appropriate way, the antenna system will not be able to maintain contact with a satellite.

• As mentioned, in this project the configuration of the system is changed to an under actuated system.
• The model of the system is considered to be nonlinear as it is in real world.
• The controlled system has some singularities in certain operational points- the most important case is when the satellite is in its nearest distant to the antenna.

An elaboration of above mentioned approaches is described in the following: The problem of controlling the output of a system so as to achieve asymptotic tracking of prescribed trajectories and asymptotic rejection of disturbances is a central problem in control theory. There are essentially three different possibilities to approach the problem: tracking by dynamic inversion, adaptive tracking, and tracking via internal model. Internal-model-based tracking is able to handle simultaneously uncertaintities in the plant parameters as well as in the trajectory which is to be tracked. A fault tolerant control design approach will be pursued in order to retain some portion of antenna's control integrity in the event of the specified set of possible component faults or large changes in the system operation conditions that resemble these faults. This can only be done if the control system has built in an element of automatic reconfiguration, once a malfunction has been detected and isolated. Facing with some conditions, it is possible to change the mode of the control system. In fact, there could be two levels of control system. The lowest level is usually characterized by continuous variable dynamics and the highest by a logical decision-making mechanism. The interaction of these different levels, with their different types of information, leads to a hybrid system formulation.

To verify the employed approach, it is possible to follow two steps.

• The performance of the designed control system can be verified through simulation using Matlab and Simulink.
• The accept test will be performed at SpaceCom premises- These premises are established based on requirement that are defined by Inmarsat- where the antenna system will be mounted on a wave simulator that is able to simulate the disturbances in Roll, Pitch, and Turn axis.