Heterogeneous string stability for vehicle formation control

This thesis considers the problems related to the formation control of Unmanned Aerial Vehicles (UAVs). Groups of small, inexpensive, low maintenance, fully autonomous UAVs will eventually replace traditional UAVs, such as the Global Hawk and the Predator, which are very expensive to build, operate, and manage. Since small UAVs can only carry a limited number of sensor suites, various different types of UAVs, each type with a different set of sensor suites, will need to cooperate together to accomplish a mission. Formation flight can be used to transport the UAVs to and from their missions and to perform tasks such as coordinated mapping and surveillance. Decentralized formation control is favored in practice over centralized control because fewer pieces of state information need to be communicated. The disadvantage of decentralized control is the propagation of errors along the vehicle formation. Vehicle formations can be designed to exhibit the string stability property. String stability refers to the attenuation of errors as they propagate through a vehicle string.;The focus of this thesis is on extending the homogeneous string stability results to heterogeneous systems. Much research has been done on the string stability of homogeneous vehicle strings. However, there exist very few works on heterogeneous vehicle strings. Unmanned helicopter formation flight experiments are performed as a part of this study. The experiments show that the homogeneous results do not work well with heterogeneous systems, and there is a need for heterogeneous string stability analysis. Heterogeneous vehicle strings under simple decentralized control laws are analyzed, and a definition for heterogeneous string stability is proposed. Heterogeneous string stability conditions are given for the constant spacing leader and predecessor following control strategy. A design procedure for controllers that give string stability and robustness to external disturbances is given. The resulting controller is simulated on heterogeneous vehicle strings to demonstrate its effectiveness. It is shown that string stability can be achieved for heterogeneous vehicle strings of arbitrary length and arbitrary vehicle type ordering.