Guidance of Aerospace Vehicles: A Sliding mode approach.
Recent advances in guidance technologies have enabled Unmanned Aerial Vehicles (UAVs) to execute the missions autonomously without the human interaction. A typical guidance problem is path-following, which is concerned with the design of control laws that force a vehicle to reach and follow a geometric path defined in 3-D space. A subset of 3-D path-following problem is the 2-D lateral track following; the objective is to ensure accurate ground track following of the vehicle. Path-following with bounded control input in the presence of uncertainties (e.g. in velocity) and input disturbances like wind is a challenging task. Another challenging task is good performance of guidance algorithm for both large and small track deviations without using gain scheduling. Addressing these challenges, this thesis presents novel nonlinear guidance schemes for 2-D and 3-D path-following of aerial vehicles.
This dissertation considers the development and application of sliding mode theory in the area of guidance law design for UAVs. For lateral path-following problem, the limitations of a linear sliding surface are indicated here and it has been shown that traditional linear sliding surface based design is not a viable solution. Nonlinear sliding surfaces for lateral and longitudinal planes are thereafter presented here which overcomes these limitations, and its stability is guaranteed using the Lyapunov theory. For the selection of optimum sliding parameters, work and energy principle based criterion is defined here considering the UAV maneuvering capabilities. The presented criterion for optimized sliding coefficients is related to the minimization of work done and hence it also minimize the reaching time to the desired path. Using numerical techniques, the sliding surface coefficients that corresponds to an optimal path can be selected; this working is demonstrated for a research UAV.
Based on nonlinear lateral sliding surface (that meets the criterion of a ‘good helmsman’), a novel lateral guidance scheme is thereafter presented here for straight and circular path following cases. The proposed guidance logic is derived from the sliding mode control technique, and is particularly suited for unmanned aerial vehicle (UAV) applications. Control boundedness is also proved to ensure that the controls are not saturated even for large track errors. To demonstrate the effectiveness of this algorithm, a test platform (scaled YAK-54 UAV) is developed that have a generic MPC-565 processor based flight control computer for programming of different guidance and control algorithms. The proposed guidance law is programmed in the flight control computer of the test vehicle and different scenarios of large and small cross track errors are generated in the flight. Flight results are presented here that confirm the effectiveness and robustness of the proposed guidance scheme. Moreover, the effect of wind is also analyzed and flight results in the presence of wind is presented to show the robustness of proposed algorithm. The flight test results are also compared with that of 6-dof simulation.
The work is then further extended for generalized 3-D path-following problem. Generalized 3-D kinematic equations are considered here during the design process to cater far the coupling between longitudinal and lateral motions. Using the presented optimized nonlinear manifolds, guidance scheme is then derived for the multiple-input multiple-output (MIMO) system considering the coupling between longitudinal and lateral planes. The Proposed scheme is implemented on a 6-degrees-of-freedom (6-dof) simulation of a UAV and simulation results are presented here for different 3D trajectories with and without disturbances.