Nonlinear Flight Control Design Research Articles

Full envelope nonlinear flight controller design for a novel electric VTOL (eVTOL) air taxi

AbstractOn-demand urban air transportation gains popularity in recent years with the introduction of the electric VTOL (eVTOL) aircraft concept. There is an emerging interest in short/medium range eVTOL air taxi considering the critical advantages of electric propulsion (i.e. low noise and carbon emission). Using several electric propulsion systems (distributed electric propulsion (DEP)) has further advantages such as improved redundancy. However, flight controller design becomes more challenging due to highly over-actuated and coupled dynamics. This study defines and resolves flight control problems of a novel DEP eVTOL air taxi. The aircraft has a fixed-wing surface to have aerodynamically efficient cruise flight, and uses only tilting electric propulsion units to achieve full envelope flight control via pure thrust vector control. The aircraft does not have conventional control surfaces such as aileron, rudder or elevator. Using pure thrust vector control has some design benefits, but the control problem becomes more challenging due to the over-actuated and highly coupled dynamics (especially in transition flight). A preliminary flight dynamics model is obtained considering the dominant effects at hover and high-speed forward flight. Hover and forward flight models are blended to simulate the transition dynamics. Two central challenges regarding the flight control are significant nonlinearities in aircraft dynamics during the transition and proper allocation of the thrust vector control specifically in limited control authority (actuator saturation). The former challenge is resolved via designing a sensor-based incremental nonlinear dynamic inversion (INDI) controller to have a single/unified controller covering the wide flight envelope. For the latter one, an optimisation-based control allocation (CA) approach is integrated into the INDI controller. CA requires special attention due to the pure thrust vector control’s highly coupled dynamics. The controller shows satisfactory performance and disturbance rejection characteristics. Moreover, the CA plays a vital role in guaranteeing stable flight in case of severe actuator saturation.

Output feedback based adaptive composite nonlinear flight control design for a small-scale un-crewed helicopter

This correspondence deals with the trajectory tracking control of an un-crewed helicopter during hover/low-speed flights. A multi-loop architecture is used in which the inner-loop holds the fast changing dynamics and the outer-loop establishes the trajectory tracking. The inner-loop is closed with a constrained H∞ based controller which is cautiously designed to address actuator saturation, atmospheric wind disturbance, and parametric uncertainty. The outer-loop adaptive composite nonlinear control comprises of a linear part and a nonlinear part. A novel adaptive controller is proposed as the nonlinear part which improves the tracking performance by adaptively adjusting the damping ratio. The linear control element of the outer-loop is framed similar to inner-loop. Utilization of output feedback instead of full state feedback makes the flight control design simple and practically feasible. The closed loop stability and robustness property of the proposed scheme are analyzed. Simulation studies are performed to establish the hovering, station keeping, and trajectory tracking performance of the suggested control structure. Further, the performance of the proposed scheme is compared with a constrained static output feedback controller and a model reference adaptive proportional integral controller to confirm its superiority.

Autonomous Mission Management Based Nonlinear Flight Control Design for a Class of Hybrid Unmanned Aerial Vehicles

In this paper, a nonlinear flight control law is designed for a hybrid unmanned aerial vehicle (UAV) to achieve the advanced flight performances with the autonomous mission management (AMM). The hybrid UAV is capable of hovering like quadrotors and maneuvering as fixed-wing aircraft. The main idea is to design the flight control laws in modules. Those modules are organized online by the autonomous mission management. Such online organization will improve the UAV autonomy. One of the challenges is to execute the transition flight between the rotary-wing and fixed-wing modes. The resulting closed-loop system with the designed flight control law is verified in simulation and the simulation results demonstrate that the resulting closed-loop system can successfully complete the designated flight missions including the transition flight between the rotary-wing and fixed-wing modes.

Immersion and invariance control for Euler angles of a fixed‐wing unmanned aerial vehicle

AbstractIn this paper, an asymptotically stabilizing, nonlinear flight controller design via immersion and invariance (I&I) approach for a fixed‐wing unmanned aerial vehicle is presented. The objective is to adjust pitch, roll, and yaw angles with the control inputs aileron, elevator, and rudder deflections. The important issues in designing a controller for fixed‐wing unmanned aerial vehicles (UAVs) are to cope with its nonlinear dynamics and to obtain a controller that can deal with various disturbances. Existing linear controllers cannot deal well with the nonlinear dynamics of the UAVs and that can not guarantee stability outside of the considered operating point. On the other hand, widely used nonlinear controllers have some weaknesses. Such as sliding mode controllers are robust to disturbances but suffer from the chattering phenomenon, and the backstepping controllers are not very robust to disturbances. The proposed controller ensures global stability with smooth and precise reference tracking and also yields significant disturbance attenuation without the chattering phenomenon. I&I approach also enables us to directly assign closed‐loop dynamics. Thus, tuning of the proposed controller is straightforward. Most of the other nonlinear methods do not have straightforward tuning of control parameters to assign desired dynamics to a closed‐loop system. The superior performance of the proposed controller is shown via simulations for various disturbances in comparison with the sliding mode controllers; the one with standard control law and the one with modified control law that eliminate the chattering phenomenon.

An Improved Nonlinear Flight Controller Design Using Active Disturbances Rejection Control Method

An improved large envelope nonlinear flight control method using active disturbances rejection control (ADRC) method and wavelet neural network is approved in this paper. Wavelet neural network is used to realize the inversion of the 6-DOF nonlinear airplane model. The wavelet neural network is optimized using simulated annealing particle swarm optimization algorithm to improve the approach precision. In order to improve the robustness and control performance in all disturbances, ADRC is used to realize the high precision flight control. The simulation results show that the large envelope flight controller has excellent control performance.

Nonlinear Flight Control Design Using Constrained Adaptive Backstepping

The design of an adaptive backstepping flight control law for the F-16/MATV (multi-axis thrust vectoring) aircraft is discussed. The control law tracks reference trajectories with the angle of attack a, the stability-axes roll rate p s , and the total velocity V T . Furthermore, the sideslip angle β has to be kept at zero. B-spline neural networks are used inside the parameter update laws of the backstepping control law to approximate the uncertain aerodynamic forces and moments. Command filters are used to implement the constraints on the control surfaces and the virtual control states. The stability of the parameter estimation process during periods of saturation is guaranteed by using a modified tracking error definition, in which the effect of the saturation has been filtered out. The controller and its performance are evaluated on a nonlinear, six-degrees-of-freedom dynamic model of an F-16/MATV aircraft in a number of simulation scenarios.

Extended nonlinear flight controller design for aircraft

In this paper, we consider the problem of control of a high performance aircraft at large angle of attack using a new method of scheduling local nonlinear control laws designed at different equilibrium points. This approach, referred to here as the extended nonlinear flight control (ENFC), extends the well-known concept of gain scheduling and the methods of extended linearization and pseudo-linearization. First, local non-linear feedback controls are designed at a number of operating points, using a nonlinear optimal control design method. Then these local nonlinear control laws are scheduled using the proposed ENFC approach. The scheduled nonlinear control law is such that, at any equilibrium operating point, the Taylor-series-expansion of the control law matches exactly that of the local nonlinear control law designed for that operating point. Simulation results are presented to show the effectiveness of this design methodology.

A nonlinear flight controller design for aircraft

In this paper, a nonlinear flight controller design approach using the method of extended linearization is described and applied to an F-8 aircraft longitudinal model. Performances of both gain-scheduled and the nonlinear controllers are compared using simulation studies. The results indicate that the nonlinear controller is insensitive to the choice of scheduling variable, and returns all the state variables to their respective trim conditions under large initial deviations in angle of attack, while the gain-scheduled controllers fail to do so. In general, the proposed nonlinear control design gives better performance and removes some of the difficulties in the conventional gain-scheduling approach.

Nonlinear flight control design via sliding methods

Nonlinear inversion/sliding control techniques are applied to design a pitch axis control system for high-performance aircraft. The control objectives are to track pilot g commands while satisfying flying quality specifications. In the pitch axis problem, the dominant nonlinearities are the aerodynamic coefficients variation with angle of attack and saturation of the actuators position and rate response. Two design approaches are investigated; the first defines a single output to be controlled (pilot's normal acceleration) and coordinates the elevator and the flaperon as a single input. The nonminimum phase nature of the resulting input/output pair necessitates defining a modified output to avoid stability problems inherent in inversion methods. The second approach defines a two input/two output problem and directly incorporates the flying quality specifications into the output definition. These two methods are illustrated using a simulation model. The latter approach is shown to allow more freedom to avoid actuator saturation at high g commands.