Outer Loop Structure Research Articles

Adaptive Sliding Mode Control for Trajectory Tracking of Quadrotor Unmanned Aerial Vehicles Under Input Saturation and Disturbances

This paper addresses the challenging issue of trajectory tracking for uncertain quadrotor unmanned aerial vehicles (UAVs), particularly under the constraints of input saturation and external disturbances. It introduces adaptive laws for managing uncertainties in mass and inertia moments without requiring prior knowledge, ensuring effective control even with varying system parameters. To counteract the effects of input saturation, the study incorporates an auxiliary system designed to compensate for these limitations. Additionally, a disturbance observer (DO) is utilized to manage and mitigate the impact of time-varying external disturbances. The proposed control strategy integrates a sliding mode adaptive control approach with an inner–outer loop structure, enhancing robustness and adaptability. Numerical simulations demonstrate the effectiveness of the designed control strategy.

A Lyapunov Theory-Based SEIG–STATCOM Voltage Regulation Control Strategy

To improve the voltage regulation of asynchronous generators during load switching, a Lyapunov-based control strategy has been proposed to stabilize the generator’s voltage by connecting a static synchronous compensator. By constructing a Lyapunov function from the mathematical model, the error tracking problem is transformed into a global asymptotic stability problem of the Lyapunov function at the equilibrium point. The outer loop linearizes the direct current (DC) voltage control process, while the inner loop replaces integral terms with differential terms. The proposed Lyapunov method achieves linearized voltage control with a quadratic outer loop structure and the inner loop differential structure exhibits a shorter transient process, outperforming traditional methods. Simulation and experimental tests were then used, where the latter was a down-scale laboratory prototype experiment. Compared to traditional (voltage-oriented control) VOC, the outer loop (Lyapunov-function-based control) LBC reduces the DC voltage transient processes by approximately 9.4 milliseconds, while the inner loop LBC reduces both alternating current (AC) and DC voltage transient processes by approximately 2.6 ms and 8.7 ms, respectively.

Research on fault-tolerant control methods for Bi-rotor vehicles under actuator failure

A PID-self-impedance fault-tolerant controller based on the inner and outer loop structure is developed for a new form of bi-rotor vehicle, taking into account the fault-tolerant control problem when the actuator fails. First, the dynamics model and actuator fault model of the bi-rotor vehicle are established, the PID active fault-tolerant controller with control parameter switching is selected for the outer loop, and the self-immune controller is selected for the inner loop. Second, when a fault occurs, the outer loop PID controller’s settings are switched to the controller parameters of the relevant fault state based on the extent of the fault, and the parameters are matched with the robust controller of the inner loop to achieve fault-tolerant control. Finally, simulation experiments show that the designed fault-tolerant controller can still achieve stable tracking of position and attitude in the presence of actuator faults. This indicates that the flight control system is robust to fault-induced disturbances and effectively improves the vehicle’s fault-tolerant performance.

An Autonomous Tracking and Landing Method for Unmanned Aerial Vehicles Based on Visual Navigation

In this paper, we examine potential methods for autonomously tracking and landing multi-rotor unmanned aerial vehicles (UAVs), a complex yet essential problem. Autonomous tracking and landing control technology utilizes visual navigation, relying solely on vision and landmarks to track targets and achieve autonomous landing. This technology improves the UAV’s environment perception and autonomous flight capabilities in GPS-free scenarios. In particular, we are researching tracking and landing as a cohesive unit, devising a switching plan for various UAV tracking and landing modes, and creating a flight controller that has an inner and outer loop structure based on relative position estimation. The inner and outer nested markers aid in the autonomous tracking and landing of UAVs. Optimal parameters are determined via optimized experiments on the measurements of the inner and outer markers. An indoor experimental platform for tracking and landing UAVs was established. Tracking performance was verified by tracking three trajectories of an unmanned ground vehicle (UGV) at varying speeds, and landing accuracy was confirmed through static and dynamic landing experiments. The experimental results show that the proposed scheme has good dynamic tracking and landing performance.

Trigger-Based K-Band Microwave Ranging System Thermal Control with Model-Free Learning Process

Micron-level accuracy K-band microwave ranging in space relies on the stability of the payload thermal control on-board; however, large quantities of thermal sensors and heating devices around the deployed instruments consume the precious inner communication resources of the central computer. Another problem arises, which is that the payload thermal protection environment can deteriorate gradually through years operating. In this paper, a new trigger-based thermal system controller design is proposed, with consideration of spaceborne communication burden reduction and actuator saturation, which guarantees stable temperature fluctuations of microwave payloads in space missions. The controller combines a nominal constant sampling PID inner loop and a trigger-based outer loop structure under constraints of heating device saturation. Moreover, an iterative model-free reinforcement learning process is adopted that can approximate the estimation of thermal dynamic modeling uncertainty online. Via extensive experiment in a laboratory environment, the performance of the proposed trigger thermal control is verified, with smaller temperature fluctuations compared to the nominal control, and obvious efficiency in system communications. The online learning algorithm is also tested with deliberate thermal conditions that deviate from the original system—the results can quickly converge to normal when the thermal disturbance is removed. Finally, the ranging accuracy is tested for the whole system, and a 25% (RMS) performance improvement can be realized by using a trigger-based control strategy—about 2.2 µm, compared to the nominal control method.

Adaptive event-triggered global asymptotic full-state stabilization of under-actuated surface vessels with unknown model parameters

This paper studies an adaptive event-triggered controller (ETC) for global asymptotic full-state stabilization of under-actuated surface vessels (USVs) with unknown model parameters. Applying an inner–outer loop structure, desired linear and angular velocities regarded as control inputs of the outer loop subsystem are designed free of any model parameters, such that the outer loop subsystem is globally asymptotically stabilized. The generated desired velocities are assigned as the reference signals for the inner loop subsystem, thereby the actual adaptive ETC is designed to track the reference velocities and to compensate for the effect of the unknown model parameters. The proposed scheme guarantees global asymptotic stability of the closed-loop system proved by the Lyapunov-based analysis. The switching threshold event-triggering mechanism (SWT-ETM) is introduced to alleviate the system’s burden requiring less resources for communication and actuator execution. Simulation results show the effectiveness of the proposed method.

Outer Loop Structure of a Spacecraft Thermal Control System with Heat Stores

Outer Loop Structure of a Spacecraft Thermal Control System with Heat Stores

Global event-triggered inner–outer loop stabilization of under-actuated surface vessels

An event-triggered controller (ETC) for global full-state stabilization of under-actuated surface vessels (USVs) is proposed. Based on an inner–outer loop structure with Lyapunov control design technique, the USV dynamics are separated into two subsystems to facilitate the design and stability analysis. A time-varying variable is constructed to design the desired velocities used as virtual control inputs to stabilize the under-actuated outer loop subsystem. For the fully-actuated inner loop subsystem, the actual control force and moment inputs are designed using a switching threshold event-triggering mechanism (SWT-ETM) to drive the actual velocities to the desired ones, thereby the inner loop subsystem and the closed-loop system are stabilized. The proposed ETC guarantees global asymptotical convergence of the stabilization error by choosing proper control parameters. It also significantly reduces the times for communication and the actuator burden while provides stable and accurate control performance. Simulation results are carried out to illustrate the effectiveness of the proposed scheme.

Quaternion-based position control of a quadrotor unmanned aerial vehicle using robust nonlinear third-order sliding mode control with disturbance cancellation

In this paper, we propose a robust nonlinear position and attitude control method for quadrotor using higher order sliding mode control concept. The control of quadrotor is realized in an inner- and outer-loop structure. Both inner- (attitude control) and outer (position control)-loop controllers are synthesized using third-order sliding mode control. The attitude control is designed in a quaternion framework to avoid gimbal lock and for better computational efficiency. A low-pass filter is used to reduce the effect of chattering in higher order sliding mode. A disturbance observer is designed for disturbance estimation. The robustness of proposed control method is ensured by providing the disturbance compensation term in the control law. Lyapunov stability analysis is provided for both inner- and outer-loop controls. Numerical results show that the proposed method provides effective control solution under continuous disturbance/uncertainties due to unmodeled dynamics, parameter variations, and external disturbance with high position accuracy.

Iterative Gaussian-Approximated Message Passing Receiver for MIMO-SCMA System

Sparse code multiple access (SCMA) and multiple-input multiple-output (MIMO) are viewed as two key techniques to address the high spectral efficiency requirements for the 5G wireless system. However, the complexity of standard message passing algorithm (MPA) for the MIMO-SCMA detection yet increases exponentially with the degree of resource nodes $d_f$ and the number of transmit antennas $N_t$ . In this paper, to tackle this problem, we design two types of low-complexity MPA over the MIMO-SCMA extended factor graph to perform joint multiuser and MIMO detection. The main idea of the proposed algorithms is approximating the discrete interferences at function nodes (FNs) as the continuous Gaussian messages so that the traversal enumeration at the FN update can be avoided. The symbol level Gaussian approximated MPA (GAMPA-S) updates the symbol likelihood during the detector's inner-loop iterations which is similar to the standard MPA, and this algorithm is compatible to any SCMA codebooks. The bit level Gaussian approximated MPA (GAMPA-B) updates the bit likelihood ratio which shows lower complexity compared to the GAMPA-S. However, the GAMPA-B requires that the SCMA codebooks can satisfy the bit significance expansion property. In the receiver, the proposed detectors are combined with users’ decoders to form the outer-loop structure, in which a critical damping factor (DF) is designed to compress the amplitude of extrinsic information exchanged between the detector and decoders. The extrinsic information transform chart is introduced to optimize the DF. Consequently, the complexity of the proposed receiver just increases linearly with $d_f$ and $N_t$ while its performance is comparable or even better than the conventional receiver. This would effectively boost the practical use of MIMO-SCMA techniques.

Integral Sliding Mode Flight Controller Design for a Quadrotor and the Application in a Heterogeneous Multi-Agent System

This paper investigates a novel integral sliding mode control (ISMC) strategy for the waypoint tracking control of a quadrotor in the presence of model uncertainties and external disturbances. The proposed controller has the inner–outer loop structure: The outer loop is to generate the reference signals of the roll and pitch angles, while the inner loop is designed by using the ISMC technique for the quadrotor to track the desired $x$ , $y$ positions and roll and pitch angles. The Lyapunov stability analysis is provided to show that the negative effects of the bounded model uncertainties and external disturbances can be significantly decreased. The designed controller is then applied to a heterogeneous multi-agent system (MAS) consisting of quadrotors and two-wheeled mobile robots (2WMRs) to solve the consensus problem. We present the control algorithms for the 2WMRs and quadrotors. Consensus of the heterogeneous MAS can be reached if the switching graphs always have a spanning tree. Finally, the experimental tests are conducted to verify the effectiveness of the proposed control methods.

Position and Attitude control of a Quadrotor UAV using Super Twisting Sliding Mode

Abstract This paper discusses a super twisting sliding mode controller designed for a quadrotor Unmanned Aerial Vehicle by utilising a cascaded inner outer loop structure. The sliding mode controller helps reject disturbances and model uncertainties, whereas the super twisting algorithm helps reduce the chattering effect. The controller is tested against wind turbulence conditions and modelling uncertainties. Results are compared with popularly used linear LQR-PD controller and a non-linear feedback linearization based controller. It's robustness is also compared against a conventional first order sliding mode controller and the results are presented.

Design of Autopilot Control Laws for MAV using Super Twisting Control∗∗Authors acknowledges the financial support from National Program on Micro Aerial Vehicle, DRDO-ADE, India

Autopilot control law has been designed for Micro Aerial Vehicle (MAV) using the second order sliding mode (SOSM) approach. The control implementation has an inner loop and an outer loop structure, outer loop converts the guidance commands to desired attitudes. Inner loop attitude tracking control is done using output feedback based higher order sliding mode technique. Finite time convergence properties of second order sliding mode technique has been used for designing tracking controller. Lyapunov based stability proof has been given to analyze the zero dynamics stability of the MAV longitudinal dynamics during tracking. Nonlinear 6DoF model of the Blackkite 300 mm wingspan fixed wing MAV, is used both for control design and as well as to verify controller performance against the classical PID control methods.

Trajectory Tracking Controller Design for an Unmanned Air Vehicle

This paper presents a complete trajectory tracking controller design for an Unmanned Air Vehicle using Linear Parameter Varying design methods. The longitudinal and lateral controllers are designed using an inner loop – outer loop structure with the inner loop LPV controller designed using µ- synthesis. The inner-loop is then approximated with a reference model and the outer loop is designed using loop-shaping techniques. Full scale nonlinear simulations are used to test the efficiency of the designed controller and of the proposed design approach.

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