Time-scale Separation Principle Research Articles

Modeling and Transition Flight Control of Distributed Propulsion–Wing VTOL UAV with Induced Wing Configuration

The integration of propulsion and wing in distributed propulsion–wing UAVs (DPW UAVs) introduces significant propulsion-aerodynamic coupling, complicating dynamic modeling and flight control. This complexity is heightened by using induced wing surfaces for vertical takeoff and landing, requiring controllers to adapt to configuration changes and disturbances during transition flight. This paper develops a propulsion-aerodynamic coupling model for a medium-sized DPW UAV with induced wings (DPW-IW), enabling real-time aerodynamic performance calculations. Furthermore, a unified flight-control framework is proposed to avoid controller scheduling and switching during flight mode transitions. The proposed control framework employs the time-scale separation principle, divided into an outer loop and an inner loop. The outer loop uses a fuzzy controller to adjust allocation parameters, while the inner loop applies incremental nonlinear dynamic inversion (INDI) and control allocation (INCA) methods, providing robustness to nonlinear changes during flight transitions. Finally, simulations under various conditions demonstrate the controller’s effectiveness in ensuring smooth and robust transitions.

Guidance and Control of Spin-Stabilized Projectiles Based on Super Twisting Algorithm

In this paper, a hybrid integrated guidance and control design exploiting the time-scale separation principle for a canard-guided dual-spin projectile (DSP) is proposed. The DSP consists of two parts: i) the aft part, which has a high spin rate for providing gyroscopic and aerodynamic stability, and ii) the forward part, which contains a course correction fuze that houses two pairs of movable canards to ensure precise interception. The guidance and control algorithms operate in a two-loop structure where the outer guidance loop generates required pitch and yaw rate commands, whereas the inner loop tracks these commands by generating canard deflections. The rolling motion of the forward part is controlled using a coaxial motor. The multibody dynamic model of DSP has seven degrees of freedom and is highly coupled and nonlinear. For the guidance, a finite horizon optimal control problem is formulated as an output regulation problem with output vector taken as pitch and yaw components of zero effort miss vector. A coupled pitch–yaw autopilot for pitch and yaw rate command tracking and a roll autopilot for roll stabilization of the forward part are designed based on super twisting algorithm. The efficacy of the control design is shown through numerical simulations.

Adaptive integral-sliding-mode control strategy for maneuvering control of F16 aircraft subject to aerodynamic uncertainty

Aircraft dynamics are highly nonlinear and the traditional linear flight controllers cannot fully utilize the real-time performance ability of aircraft. This paper investigates a nonlinear control approach for high maneuvering fighter aircraft that deployed baseline nonlinear-dynamics-inversion (NDI) control and high-level robust adaptive integral-sliding-mode (ISM) control. The design objective is to maintain the acceptable flight quality at a high angle-of-attack under the influence of unknown disturbance and aerodynamics uncertainties. The nonlinear dynamics of F16 aircraft are divided into two loops by utilizing time-scale separation principle and adaptive ISM/NDI controller is designed for each loop. The aerodynamics coefficients are estimated using the iterative reweighted least square (IRLS) algorithm based on the wind tunnel real-time flight data available at NASA Langlet and Ame Research center. The internal dynamics stability is proven to validate the complete system stability. Simulations are conducted on F16 aircraft and compared the results with the NDI controller and ISM/NDI controller (without adaptive strategy).

Robust Constrained Trajectory Tracking Control for Quadrotor Unmanned Aerial Vehicle Based on Disturbance Observers

Abstract In this paper, a trajectory tracking control scheme for a quadrotor unmanned aerial vehicle (UAV) under unknown external disturbance and input saturation is developed. This scheme includes the position control system and attitude control one, in which the attitude control system is further divided into the fast loop for angular velocity and the slow one for attitude angle based on time-scale separation principle. Then, an input constrained dynamic surface control scheme combined with a disturbance observer is designed to achieve the total thrust, desired roll, and pitch angle in the position control system. For the coupled attitude system, a dynamic surface control scheme together with generalized model predictive controller (GMPC) is proposed to tackle both the fast loop system and the slow one. Since the unknown external disturbance and input saturation are considered, a sliding mode disturbance observer (SMDO) is further designed to achieve the strong robustness. Finally, some simulation results are presented to show robustness and effectiveness of our proposed tracking scheme.

Nonlinear Analysis Methods Applied on Grid-Connected Photovoltaic Systems Driven by Power Electronic Converters

A novel nonlinear method of analysis is deployed in detail with the aim to design suitable controllers with guaranteed stability for grid-connected photovoltaic (PV) systems driven by power converters. By this method, all the system nonlinearities are considered in order to allow a reliable analysis in a wide range of operation and to avoid instabilities that as shown in this article can be occurred by inappropriate design. To this end, first, a detailed accurate nonlinear dynamic model is presented for the PV system by including a cascade-mode control scheme. Then, considering the closed-loop system and incorporating the concept of input-to-state stability (ISS), a rigorous novel stability analysis is developed, which achieves to prove asymptotic stability of the desired equilibrium point. A multistep process is deployed, in which the control-loops operating in cascade-mode are involved. However, since the cascaded-mode control scheme and analysis is based on the time-scale separation principle, a systematic tuning method is conducted for the accurate gain selection of both inner-loop and outer-loop controllers, which is based on the construction of suitable Lyapunov functions. To further validate the overall analysis, experimental results are carried out.

Power Electronic Control Design for Stable EV Motor and Battery Operation during a Route

Electric vehicles (EVs), during a route, should normally operate at the desired speed by effectively controlling the power that flows between their batteries and the electric motor/generator. To implement this task, in this paper, the voltage source AC/DC converter is considered as a controlled power interface between the electric machine and the output of the DC storage device; the DC/DC converter is used to automatically regulate the battery operating condition in accordance to the profile of the acting on the vehicle wheels, unknown external torque. Particularly, the speed is continuously regulated by the vehicle driver via the pedal while all other regulations for absorbing or regenerating energy are internally controlled. The driver command is acting as speed reference input on a PI outer-loop motor speed controller which, in its turn, drives a fast P inner-loop current controller operating in cascaded mode. In a similar manner, the machine and the battery performance are self-regulated by a pure PI current controller that achieves maximum electric torque per ampere operation of the motor and by a PI/P cascaded scheme for the DC-voltage/battery–current regulation, respectively. In order to exclude any possibility of instabilities and adverse impacts between the different parts, a rigorous analysis is deployed on the complete electromechanical system that involves the motor, the batteries, the converter dynamic models and the proposed controllers. Modeling the system in Euler–Lagrange nonlinear form and applying sequentially suitable Lyapunov techniques and the time-scale separation principle, a systematic method for tuning the gains of the inner- and outer-loop controllers is derived. Therefore, the proposed controller design procedure guarantees asymptotic stability by considering the accurate system model as a whole. Finally, the proposed approach is validated by simulating realistic route conditions, performed under unknown external torque variations.

Stability Analysis for Incremental Nonlinear Dynamic Inversion Control

As a sensor-based control method, incremental nonlinear dynamic inversion (INDI) has been applied to various aerospace systems and has shown desirable robust performance against aerodynamic model uncertainties. However, its previous derivation based on the time scale separation principle has some limitations. There is also a need for stability and robustness analysis for INDI. Therefore, this paper reformulates the INDI control law without using the time scale separation principle and generalizes it for systems with arbitrary relative degree, with consideration of the internal dynamics. The stability of the closed-loop system in the presence of external disturbances is analyzed using Lyapunov methods and nonlinear system perturbation theory. Moreover, the robustness of the closed-loop system against regular and singular perturbations is analyzed. Finally, this reformulated INDI control law is verified by a Monte Carlo simulation for an aircraft command tracking problem in the presence of external disturbances and model uncertainties.

Fault-tolerant control for over-actuated hypersonic reentry vehicle subject to multiple disturbances and actuator faults

In this paper, a fault-tolerant control scheme is proposed for attitude tracking problem of hypersonic reentry vehicle subjected to multiple disturbances and time-varying actuator faults. For convenience of fault-tolerant controller design, conventional hypersonic reentry vehicle model is transformed into control-oriented model by incorporating actuator faults into lumped disturbance. Based on the lumped disturbance estimate provided by high order sliding mode observer, fault-tolerant controllers are developed within the frame of active disturbance rejection control in attitude loop and angular rate loop according to time-scale separation and singular perturbation principle. Then, the desired control moment is allocated to actuators based on recurrent neural network. With the well-designed fault-tolerant controllers and control allocation, a novel meta-heuristic dynamic adaptation salp swarm algorithm is employed to optimize control parameters to achieve minimum attitude tracking error. Comparative simulations are conducted to verify the effectiveness of the developed dynamic adaptation salp swarm algorithm and the investigated fault-tolerant control scheme.

Immersion and invariance adaptive finite-time control of air-breathing hypersonic vehicles

In this paper, a novel adaptive finite-time control of air-breathing hypersonic vehicles is proposed. Based on the immersion and invariance theory, an adaptive finite-time control method for general second-order systems is first derived, using nonsingular terminal sliding mode scheme. Then the method is applied to the control system design of a flexible air-breathing vehicle model, whose dynamics can be decoupled into first-order and second-order subsystems by time-scale separation principle. The main features of this hypersonic vehicle control system lie in the design flexibility of the parameter adaptive laws and the rapid convergence to the equilibrium point. Finally, simulations are conducted, which demonstrate that the control system has the features of fast and accurate tracking to command trajectories and strong robustness to parametric and non-parametric uncertainties.

The Control of Asymmetric Rolling Missiles Based on Improved Trajectory Linearization Control Method

According to motion characteristic of an asymmetric rolling missile with damage fin, a three-channel controlled model is established. The controller which is used to realize non-linear tracking and decoupling control of the roll and angle motion is introduced based on an improved trajectory linearization control method. The improved method is composed of the classic trajectory linearization control method and a compensation control law. The classic trajectory linearization control method is implemented in the time-scale separation principle. The Lipschitz non-linear state observer systematically obtained by solving the linear matrix inequality approach is provided to estimate state variables and unknown parameters, and then the compensation control law utilizing the estimated unknown parameters improves the TLC method. Simulation experiments show that the adaptive decoupling control ensure tracking performance, and the robustness and accuracy of missile attitude control are ensured under the condition of the system parameters uncertainty, random observation noise and external disturbance caused by damage fin.

Automatic Aircraft Loss-of-Control Prevention by Bandwidth Adaptation

An automatic aircraft loss-of-control prevention system by bandwidth adaptation is presented in this paper. The adaptation law employs the time-varying parallel differential eigenvalues for a tradeoff between tracking performance and severe wind tolerance capability in real time. It is implemented as an augmentation to a 6-DOF trajectory tracking controller designed with constant gains by the singular perturbation (timescale separation) principle in a multiple-timescale nested-loop architecture. Theoretical analysis is presented to justify the design rationale. It relates the ratio of timescale separation between the inner and outer loop (singular perturbation) to the phase margin of the outer-loop (perturbed) system, which is important in its own right. Simulation studies on tailwind, headwind, crosswind, downdraft, and updraft are presented to demonstrate the effectiveness of the proposed loss-of-control prevention strategy.

Neuro-adaptive Augmented Dynamic Inversion Controller for Quadrotors

Abstract In this paper, an adaptive nonlinear control design for quadrotors is presented using nonlinear dynamic inversion and model-following neuro-adaptive techniques. The baseline controller is designed using dynamic inversion approach that exploits the time scale separation principle. The design works well if the model is perfectly known. However, the quadrotor system can have uncertainty in parameters. To tackle this, a neuro-adaptive controller is augmented with the baseline controller. The approach uses a single layer neural network to learn unknown dynamics and an adaptive law is employed to ensure that the quadrotor behave in the desired manner. A Lyapunov approach is used to show that the approximated dynamics remains bounded.

Modeling and Incremental Nonlinear Dynamic Inversion Control of a Novel Unmanned Tiltrotor

A novel, tiltrotor, unmanned aerial vehicle configuration has been designed, and a preliminary dynamic model has been created for control design purposes based on a component buildup approach. This method has been preferred over a conventional stability derivative approach because it models the nonlinear aircraft dynamics in the entire flight envelope, ranging from hover to forward-flight conditions at any airflow angles. The flight control system has then been synthesized based on an incremental nonlinear dynamic inversion technique. The incremental strategy was adopted to solve the problem of the nonlinear dynamic model not being control affine due to effector redundancy, dramatic nonlinearities, and cross-coupling effects introduced by the use of thrust vectoring during hover and transition phases. Local control derivatives were calculated online from the dynamic model. Effector redundancy was managed, developing a control allocation module for distributing the control effort according to a daisy-chaining logic and according to the actuator availability and effectiveness. The timescale separation principle was applied, dividing control laws into two loops: an outer loop controlling slower dynamics, and outputting virtual controls for an inner loop controlling faster dynamics. Different piloting logics were identified for hover and forward-flight conditions, although the largest possible degree of commonality was sought. A command blending strategy was devised to control the aircraft during transition phases. Simulations confirmed the effectiveness of the proposed solutions, showing satisfactory tracking of reference inputs with moderate control effort.

Nonlinear Autopilot Design and Guidance-in-Loop Validation for a Tactical Air-to-Air Flight Vehicle

In this paper, design and analysis of nonlinear autopilot of a tactical flight vehicle has been carried out for surface to air application. The lateral autopilot design is based on dynamic inversion and time scale separation principle. The roll autopilot design is based on back stepping technique. At first, better tracking performance of present autopilot with respect to linear design at higher angles of attack in presence of side force and aerodynamic nonlinearity and its robustness study with respect to plant parameter variation has been carried out through six degrees of freedom simulation. Later, successful tracking performance of the pursuer nonlinear autopilot while tracking guidance command has also been demonstrated through realistic six degrees of freedom simulation of a pursuer evader engagement.

Nonlinear [formula omitted] control of a laboratory helicopter with variable speed rotors

This paper considers the problem of a nonlinear L 2 -disturbance rejection design for a laboratory twin-rotor system. Since the rotor blades present fixed angle of attack, control is achieved by using the rotor speeds as control variables. This mechanical device features highly nonlinear strongly coupled dynamics. The control is developed considering a reduced order model of the rotors obtained by application of a time-scale separation principle, including integral terms on the tracking error to cope with persistent disturbances. An explicit suboptimal solution to the associated partial differential (HJBI) equation is applied. This yields global asymptotical stability for the reduced system. The controller exhibits the structure of a partial feedback linearization with an external nonlinear PID. The paper proposes systematic tuning procedure allowing independent weights for each degree of freedom. The methodology has been tested by experimental results using a laboratory helicopter.

A Hierarchical Controller Design for a Reactor/Separator System with Recycle

A hierarchical control structure that comprises regulatory and coordination control layers is designed for a reactor/separator process with recycle. At the regulatory control layer, the process is divided into subunits on the basis of the controlled group unit approach. In defining inventory controls for the subunits, Luyben's rule (fixing one of the flows on the recycle path) is employed to reduce dynamic interaction between subunits. The higher level coordination control layer is designed on the basis of the time-scale separation principle, which slowly manipulates the recycle flow between the subunits. A control structure that results in a wider operation range and small dynamic interaction is proposed, and its control performance is compared with that of a conventional control structure through simulations.

Reconfigurable Flight Control System Design Using Direct Adaptive Method

A reconfigurable flight control system provides better survivability through the automatic reconfiguration of control system when faults occur during flight. The adaptive control method has been effectively applied to the reconfigurable flight control system design. However, reconfigurable flight control systems based on the indirect adaptive control method require persistent input excitation and smooth input-output data. To deal with the persistent input excitation problem and to obtain smooth control input, the system identification algorithm in reconfiguration flight control systems usually imposes some constraints on past input-output data, which may deteriorate the reconfiguration performance. Thus, a new reconfigurable flight control system based on the direct adaptive method is proposed to achieve better reconfiguration performance without the system identification process. The proposed control method uses a model following controller with direct adaptive update rules. To control the inner-loop states and the outer-loop states of the flight system simultaneously,the timescale separation principle is applied. The reconfiguration performance of the proposed control method is evaluated through numerical simulations using a six-degree-of-freedom nonlinear aircraft model.