Robust Control Of A Small-Scale Unmanned Helicopter

Miniature helicopters are highly unstable,agile,nonlinear under-actuated systems with significant inter-axis dynamic coupling.They are considered to be much more unstable than fixed-wing unmanned air vehicles,and constant control action is required at all times.However,helicopters are highly flexible aircraft,having the ability to hover,maneuver accurately and carry heavy loads relative to their own weight.Fixed-wing aircraft are used for application in favorable non-hostile conditions but in adverse conditions,agile miniature helicopters become a necessity.The conditions where a helicopter can perform better than fixed-wing UAVs include military investigation,bad weather,firefighting,search and rescue,accessing remote locations and ship operations.In such conditions,helicopters are subjected to unknown external disturbances such as wind and ground effect.These external disturbances have a significant opposing effect on the helicopter stability and can have disastrous results in extreme cases.So,designing and development of a controller for the miniature unmanned helicopter which can effectively reject the effect of these unknown external disturbances are of great theoretical significance and practical utility.The principal part of this dissertation is the development of a simplified nonlinear model of the miniature unmanned helicopter.Then research on the development of the robust nonlinear control techniques for it based on the linearized model of the miniature helicopter.Firstly,a simplified 11^th order nonlinear model of the miniature unmanned helicopter is derived based on the previously published research work in international journals.Then the model is linearized at hover condition and divided into two separate subsystems?longitudinal-lateral and heading-heave subsystem?.At hover condition,these two subsystems are weakly coupled and are suitable for separate controller design which makes the controller design procedure very simple.But during forward flight and aggressive maneuvers helicopter states are far from hover condition which deteriorates its control performance.Disturbance observer is used for this model mismatch and external disturbances approximation.Combining disturbance observer with sliding mode control,superior tracking performances are achieved for the miniature unmanned helicopter.The core contributions and main innovations of this dissertation are the following.1.Disturbance observer based sliding mode control for the small-scale unmanned helicopter hover operation.2.Fixed time disturbance observer based fixed time sliding mode control for the small-scale unmanned helicopter hover operation.3.Extended state disturbance observer based sliding mode control for the small-scale unmanned helicopter path tracking in complex wind conditions.