M. Zamurad Shah

Lateral Control for UAVs using Sliding Mode Technique

Abstract This paper presents sliding mode based lateral control for UAVs using a nonlinear sliding approach. The control is shown to perform well in different flight conditions including straight and turning flight and can recover gracefully from large track errors. Saturation constraints on the control input are met through the nonlinear sliding surface, while maintaining high performance for small track errors. Stability of the nonlinear sliding surface is proved using an appropriate Lyapunov function. The main contribution of this work is to develop a robust lateral control scheme that uses readily available sensor information and keeps the track error as small as possible without violating control constraints. In the proposed scheme the only information used in the control law is the lateral track error and the heading error angle. No information is required about the desired path/mission, which therefore can be changed online during run-time. This scheme is implemented on a high fidelity nonlinear 6-degrees-of-freedom (6-dof) simulation and different scenarios are simulated with large and small track errors in windy and calm conditions. Simulation results illustrate the robustness of the proposed scheme for straight and turning flight, in the presence of disturbances, both for large and small track errors. Furthermore it is shown that the saturation limits of the control input are not exceeded in all cases.

Lateral track control of UAVs using the sliding mode approach: from design to flight testing

This paper develops sliding-mode-based nonlinear logic for guidance of unmanned aerial vehicles (UAVs) for curved and straight path following. UAV trajectories generally consist of straight path segments, curved arcs, circular loiters and other manoeuvres; tight ground track control is desired throughout the trajectory. This is achieved by controlling the lateral (cross-track) deviation of the vehicle in flight. The main objective of the guidance algorithm is to keep the lateral track error of the vehicle as small as possible while performing graceful and stable manoeuvres despite the presence of uncertainties and disturbing winds. Lateral track control is usually achieved by banking the vehicle, that is, by executing roll manoeuvres. The scheme must perform well without saturating the roll angle of the vehicle, which serves as the control input for the guidance algorithm. The algorithm proposed here is shown to perform well for both straight and circular path tracking while ensuring control boundedness, and hence no saturation. Crosswinds are a major source of disturbance for the guidance problem. This is incorporated into the design formulation and guidance gains are selected to provide the desired performance despite the presence of disturbing winds. A sliding-mode-based scheme is developed which includes a feedforward component related to the rate of change of the desired path heading. Stability of the algorithm is proved using an appropriate Lyapunov function. The algorithm is implemented in the flight control computer of a scaled YAK-54 research aircraft; flight test results are presented and compared with those from other guidance algorithms. Flight results demonstrate the effectiveness and performance of the proposed guidance scheme. The algorithm considers guidance in the 2D lateral plane only and minimizes deviations from the desired ground track of the vehicle.

Higher-order sliding mode based lateral guidance for unmanned aerial vehicles

A nonlinear sliding mode based scheme is developed for lateral guidance of unmanned aerial vehicles. The guidance and control system is considered as an inner and outer loop design problem, the outer guidance loop generates commands for the inner control loop to follow. Control loop dynamics is considered during derivation of the guidance logic, along with saturation constraints on the guidance commands. A nonlinear sliding manifold is selected for guidance logic design, the guidance loop generates bank angle commands for the inner roll control loop to follow. The real twisting algorithm, a higher order sliding mode algorithm is used for guidance logic design. Existence of the sliding mode along with boundedness of the guidance command is proved to ensure that controls are not saturated for large track errors. The proposed logic also contains an element of anticipatory or feed-forward control, which enables tight tracking for sharply curving paths. Efficacy of the proposed method is verified by flight testing on a scaled YAK-54 unmanned aerial vehicle. Flight results demonstrate robustness and effectiveness of the proposed guidance scheme in the presence of disturbances.

Sliding mode based roll control law design for a research UAV and its flight validation

Research unmanned air vehicles (UAVs) provide a cost effective solution for rapid development and testing of new algorithms in real system flight testing. The sliding mode control architecture is applied by the authors to the lateral dynamics of scaled YAK-54 UAV. Flight dynamic analysis and control system design is complicated by the problem of uncertain measurements, external disturbances and modeling uncertainties. Using the powerful tools of sliding mode theory we present an innovative robust lateral control algorithm. For robust stabilization of lateral dynamics, 1st-order sliding mode control was designed. Due to its limitation of chattering in the control channel and extra information demand, a 2nd-order super twisting algorithm was designed to provide chattering free improved robust control. The designed frameworks effectiveness is tested by applying the developed scheme on scaled YAK-54 UAV. The results of the flight tests are included in the paper to demonstrate the benefits of the adopted framework for the control design and further it focuses on bridging the gap between theory and experimental practice.

Lateral control of UAVs: Trajectory tracking via Higher-Order Sliding Modes

Nonlinear sliding mode approach is developed in this paper for lateral control of UAVs. The enabling guidance and control has achieved good performance with different flight conditions and evasive maneuvers. The proposed strategy can recover from large track errors without effecting the saturation constraints on the control input. The structure of guidance and flight control system is designed in a two loop configuration. The main contribution of this work is the development of new guidance scheme in which inner loop dynamics are also considered during the derivation of outer guidance loop for robust lateral control and never forcing unsuitable commands. HOSM (Higher-Order Sliding Mode) Real Twisting Algorithm is used because of relative degree 2 constraint, which maintains S and S = 0. The outer loop for guidance uses heading error angle, lateral track error and bank (roll) angle φ for the control law and PD controller is used in the inner loop. The designed guidance control systems robustness and performance is verified via computer simulations using high fidelity nonlinear 6-degrees-of-freedom (6-dof) Yak-54 UAV model under different scenarios, with small and large track errors and in the presence of wind disturbances.

Adaptive sliding mode roll control of a canard-controlled missile

This work is meant to design a roll controller for a short range missile with a free-spinning tail using the sliding mode control technique. Four canards located near the nose of the missile provide aerodynamic control. Pitch/yaw stability is achieved through four rotating fins located at the tail of the missile. Range of the missile dictates the whole trajectory in the atmosphere where dynamic pressure varies between 0 and 7 bars. Also the missile can be fired at different launch angles, which implies that the profile of dynamic pressure is not fixed. Control strategy strongly depends on dynamic pressure in the case of canard-controlled missile. In the case of varying dynamic pressure, design of a robust roll controller that can keep roll angle near zero is a challenging task. In this paper, a roll controller is designed for a canard-controlled short range missile using sliding mode control technique. Adaption is done in some control parameters to cater the variation in dynamic pressure and different launch angle. The robustness of the roll controller is tested in the presence of aerodynamic and other disturbances. It is established that the proposed roll controller is capable of rejecting aerodynamic and other disturbances and keep the roll angle close to zero.

Sliding mode based lateral control for UAVs using piecewise linear sliding surface

This paper presents sliding mode based lateral control for UAVs using piecewise linear sliding surface. A single linear surface is replaced by piecewise linear sliding surface to ensure the performance during small track errors or divergence and a bounded heading error in the case of large track error, thus avoiding the actuator saturations. Stability of the proposed sliding surface is analyzed with piecewise analytical solution of the proposed sliding surface. The proposed scheme uses only the readily available sensor information that is lateral track error and heading error angle and no succeeding mission information are required here. The proposed scheme is implemented on the 6-degrees-of-freedom (6-dof) simulation and different cases of large and small track errors are simulated in the presence/absence of wind to successfully validate the proposed control scheme.

Wind estimation for lateral path following of UAVs using higher order sliding mode

In design of guidance algorithms wind is often ignored or only considered implicitly. Persistent winds has a very significant nonlinear effect on the guidance scheme as for small UAVs these disturbances can strongly affect their spatial orientation. This research work extends the idea of sliding mode control for parameter estimation of UAV nonlinear dynamics. The uncertain parameter estimation scheme is designed to estimate wind based on the higher order sliding mode robust differentiator (HOSMD) using rate of change of heading of the vehicle. Further these estimates are then included in the guidance algorithm. The UAVs guidance algorithms objective is to derive the lateral track error towards zero with graceful and stable manoeuvres and then to keep it as minimum as possible while subject to disturbing winds. In this scheme a second order sliding motion is established along designed sliding manifold and outputs the reference bank commands for improved tracking performance using estimates during curved arcs. The estimation is combined seamlessly with robust guidance algorithm to produce integrated identification and guidance scheme for lateral path following application. The combined framework is capable of robust accurate path following in the presence of wind disturbance. The algorithm is implemented in the flight control simulation of scaled YAK-54 research UAV; simulation test results are presented. These results demonstrate the effectiveness and performance of the proposed lateral guidance scheme.

Linear control techniques application and comparison for a research UAV altitude control

Research UAVs provide an efficient and cost effective platform to test newly designed guidance and control algorithms before implementing on industrial systems. This paper aims to present the performance comparison of traditional linear lead-lag, H∞ and LQR techniques for a research UAV. In lead-lag based design, the concept of stability margin approach is exploited. Selection of weighting function for H∞ based control design is explained along with the selection of suitable gain and phase margins to ensure system stability. For LQR based design, procedure to derive the optimal state feedback gain matrix is elaborated. Based on frequency response of H∞, lead-lag based design is re-iterated to achieve robustness. The designed controller performance is tested through high fidelity 6-degrees-of-freedom (6-dof) nonlinear simulations of scaled Yak-54 UAV model in the presence and absence of disturbances.

Deep sea motion under higher sea states

Surface waves, induced due to different sea states, greatly affect the dynamics and control of the vehicles operating undersea or on the sea surface. In certain missions involving longer operation time, vehicle may encounter disturbances that are induced by higher sea states, forcing to guide the vehicle to safer depths. In the moderate sea states, taken up to sea state 3, these disturbances are accounted for the motion closer to the sea surface and considered negligible as vehicle moves down to few meters. In case of a rough and higher sea states, disturbances may be experienced even down to hundred meters. This paper attempts to provide a three-dimensional generalization of disturbances in deep sea operating vehicles, by simulating their 6dof motion under higher sea states. A sea state model is realized in terms of inertial forces and moments that vehicle would experience during the motion. An analytical formalism is derived to estimate the induced forces and moments integrated over a given vehicle arbitrarily oriented in the motion. Three limiting cases are considered in this work: (1) the deep water waves: 0.5 <; ho/λ <;∞ (2) the intermediate depth waves: 0.05 <; ho/λ <; 0.5 and (3) the shallow water waves: 0 <; ho/λ <; 0.05. For illustration, derived forces and moments are applied to a well known autonomous underwater vehicle (AUV) known as REMUS (Remote Environmental Monitoring Unit) taken as reference vehicle for the analysis. Slender shape of REMUS closely approximates the known vehicles like submarine, remotely operated vehicle (ROV) and unmanned ocean vehicle (UOV) to which these results are applicable. Numerical results show that in case of deep water wave at lower sea state, the disturbance no longer remains significant after certain depth. On the other hand in shallow and deep water wave case at higher sea states, the disturbance is found significant, affecting the dynamics of the underwater vehicles, down to larger depths of operation. Deep water wave case is further taken up for detailed study of vehicle motion in three dimensions.