Inner-loop Control Law Research Articles

A hybrid INDI control for ensuring flying qualities in failures of Xcg measurement subsystem

Modern version of fighter aircraft has a wide range of longitudinal center-of-gravity (Xcg) travel due to wide-range fuel tanks layout, various weapons, many payload stations, and so on. The wide Xcg travel degrades flying qualities of a closed-loop control system; moreover, a catastrophic accident can be caused by loss of stability due to the wide Xcg travel in severe cases. For this reason, the inner-loop control law generally employs additional feedback variables such as weight and Xcg position to guarantee the flying qualities and satisfy stability requirements. Nevertheless, this design approach can seriously deteriorate the flight safety and stability of the aircraft in the event of fuel mismanagement and Xcg measurement subsystem failures. Therefore, additional design technique should be considered to ensure the flight safety in response to the situation of the failure conditions. This paper proposes two types of a hybrid Incremental Nonlinear Dynamic Inversion control to guarantee the minimum flight safety for return to base in case of fuel mismanagement and Xcg measurement subsystem failures. The frequency-domain linear analysis and time-domain simulations were performed based on a supersonic trainer mathematical model to evaluate the performances of the proposed control methods. The evaluation results confirm that the proposed control method satisfies the level of the required flying qualities and ensure flight safety even in the event of the subsystem failures for the Xcg measurement.

Applying Eigenstructure Assignment to Inner-Loop Flight Control Laws for a Multibody Aircraft

Unmanned aircraft used as high-altitude platform system has been studied in research and industry as alternative technologies to satellites. Regarding actual operation and flight performance of such systems, multibody aircraft seems to be a promising aircraft configuration. In terms of flight dynamics, this aircraft strongly differs from classical rigid-body and flexible aircraft, because a strong interference between flight mechanic and formation modes occurs. For unmanned operation in the stratosphere, flight control laws are required. While control theory generally provides a number of approaches, the specific flight physics characteristics can be only partially considered. This paper addresses a flight control law approach based on a physically exact target model of the multibody aircraft dynamics rather than conventionally considering the system dynamics only. In the target model, hypothetical spring and damping elements at the joints are included into the equations of motion to transfer the configuration of a highly flexible multibody aircraft into one similar to a classical rigid-body aircraft. The differences between both types of aircraft are reflected in the eigenvalues and eigenvectors. Using the eigenstructure assignment, the desired damping and stiffness are established by the inner-loop flight control law. In contrast to other methods, this procedure allows a straightforward control law design for a multibody aircraft based on a physical reference model.

Research on Guidance and Control Law Design of Decelerating Transition and Vertical Landing for a STOVL UAV

In this paper, an integrated design method for guidance and control laws for a short takeoff and vertical landing (STOVL) unmanned aerial vehicle (UAV) during decelerating transition and vertical landing phases under thrust vector control is studied. Based on the principle of time-scale separation, the outer-loop guidance law adopts an implicit dynamic inverse method to provide attainable overload vector control instructions for decelerating transition and vertical landing phases. Meanwhile, the inner-loop control law adopts an improved eigen-structure assignment method, which tracks guidance instructions and maintains a stable attitude. The attitude nozzles are joined to the attitude control with the dropping of dynamic pressure, thereby assisting aerodynamic surfaces to stabilize attitudes. Subsequently, the feasibility and effect of the control methods for a STOVL UAV during decelerating transition and vertical landing phases are verified by a 6-DOF flight simulation platform. The simulation results demonstrate that the integrated design method is beneficial for a quick evaluation of the overall scheme.

High-Accuracy Position Control of a Rotary Pneumatic Actuator

Rotary pneumatic actuators provide several advantages over electromechanical actuators (e.g., higher power-to-weight ratio, lower cost, and inherent safety) but are generally inferior in terms of accuracy and robustness when closed-loop position controlled. This paper presents the modeling, controller design, and experimental verification of a high-accuracy position-controlled rotary pneumatic actuator. A novel inverse valve model enables the development of a fast and precise inner-loop pressure control law. The outer-loop position control law combines feedback with model-based feedforward terms, including an adaptive friction compensator. The robust stability of the inner and outer subsystems is analyzed. Experimental results are included for rotating an arm in the vertical plane. The hardware features low-cost ON / OFF solenoid valves and low-cost pressure sensors. For a multiple cycloidal reference trajectory covering a 90° range, the root-mean-square error (RMSE) averaged over five tests was 0.156°. Steady-state errors less than or equal to 0.0045° were achieved for these moves and for moves as small as 0.045°. Robustness to unknown payloads was achieved using an improved payload estimator. For example, for a payload 53% less than nominal, the RMSE with the estimator was 75% less than without it. The experimental results are superior to those reported in the prior literature.

비행 중 변화하는 종축 무게중심 대처 비행성 향상을 위한 비선형 동적 모델 역변환 제어 개발

The aim of designing the inner-loop flight control law is to satisfy the flying quality requirements which include the change of CG(Center of Gravity) position due to fuel consumption, payloads drop and/or failure occurrence in major subsystems such as fuel system, etc. while operating in overall flight envelope. Since the position of CG directly affects the aircraft stability, the change of its position must be managed strictly not only by arranging the fuel tanks layout optimally in configuration design stage, but also by controlling fuel burn sequence in FMS(Fuel Management System) during flight. Additionally, by utilizing the additional informations such as weight and CG position into the inner-loop control law, improved flying qualities and maneuvering performance can be achieved in CG travel situation. In this paper, the NDI(Nonlinear Dynamic Inversion) control method which uses the CG position and weight information feedback variables is proposed. Since NDI is a novel control methodology that directly uses the vehicle dynamics, the effects of CG travel can be efficiently applied. The performances of the proposed control method are evaluated by numerical simulations including CG travel using the advanced supersonic trainer model.

Optimal Trajectory Reconfiguration and Retargeting for Reusable Launch Vehicles

Reconfigurable inner-loop control laws improve the fault tolerance of a vehicle to control effector failures. The control effectors can produce significant perturbations to the nominal forces and can also affect the flight conditions over which the vehicle can be controlled. Three-degree-of-freedom dynamical models typically include wing-body aerodynamic force effects but ignore the aerodynamic forces produced by the control surfaces. These trim effects are a component of 6-degree-of-freedom models; however, such models are cumbersome for the purposes of trajectory optimization. In this work, a method for including the trim effects as well as control-induced trajectory constraints in a 3-degree-of-freedom model is presented. The method uses nonlinear control allocation to determine the control effector positions, required to rotationally balance the vehicle, and uses that information to compute force perturbations and new control-induced trajectory constraints. The contributions of the control effectors to the vehicle lift and drag are computed, as well as constraints that define regions of the flight envelope where the vehicle cannot be rotationally balanced. An example is presented that includes control failure effects in a maximum downrange trajectory optimization problem. Constrained trajectories are planned by solving an optimal control problem using a Legendre pseudospectral method and state-dependent constraints, resulting from a control failure, are included. Results show that these methods hold the potential to enhance safety margins even when control effectors have failed.

Vector Field Path Following for Miniature Air Vehicles

In this paper, a method for accurate path following for miniature air vehicles is developed. The method is based on the notion of vector fields, which are used to generate desired course inputs to inner-loop attitude control laws. Vector-field path-following control laws are developed for straight-line paths and circular arcs and orbits. Lyapunov stability arguments are used to demonstrate asymptotic decay of path-following errors in the presence of constant wind disturbances. Experimental flight tests have demonstrated mean path-following errors on less than one wingspan for straight-line and orbit paths and less than three wingspans for paths with frequent changes in direction.

Design and Flight Testing Evaluation of Formation Control Laws

This brief presents the results relative to the design and flight testing of formation control laws using a set of YF-22 research unmanned aerial vehicles (UAVs) designed, built, and instrumented at West Virginia University, Morgantown. In the formation flight configuration, a radio control pilot maintains ground control of the leader aircraft while the autonomous follower aircraft maintains a predefined position and orientation with respect to the leader aircraft. The formation controller was designed to have an inner and outerloop structure, where the outerloop guidance control laws minimized the forward, lateral, and vertical distance error by controlling the engine propulsion and generating the desired pitch and roll angles to be tracked by the innerloop controller. The horizontal components of the outerloop controller were designed using a nonlinear dynamic inversion (NLDI) approach, while the vertical channel of the outerloop controller and the innerloop control laws were instead linear. Flight testing results from two-aircraft formation flights confirmed the performance of the designed formation controller

A flight control design of a re-entry vehicle using a double-loop control system with fuzzy gain-scheduling

This paper presents a flight control design of an Automatic Landing FLight EXperiment (ALFLEX) vehicle using a double-loop control system (DLCS) with the fuzzy gain-scheduling (FGS) state feedback. The DLCS consists of an inner and an outer loop. The inner-loop control law is designed by the FGS state feedback to guarantee the stability over the entire operating range of the ALFLEX vehicle, while the outer loop control law is designed by a static gain to improve the tracking property. Futhermore, guidance laws for the longitudinal and the lateral directions are included in the control system to navigate the ALFLEX vehicle to a reference trajectory even if the initial condition is deviated from the nominal. The designed DLCS showed satisfactory performance in the numerical simulation of the ALFLEX vehicle.