Low-frequency noise has long been a challenge in noise control, and acoustic metasurface has emerged as one of the most viable solutions for low-frequency acoustic control by virtue of its ultra-thin geometry. Acoustic metasurface is generally controlled by several structural parameters. Normally, traditional metasurface design methods typically involve extensive theoretical analysis and numerical simulations to achieve desired performance. This process requires frequent adjustment of structural parameters and a lot of computing resources. In this paper, we introduce a method to rapidly achieve on-demand design of low-frequency acoustic absorbing metasurface using an autoencoder-like neural network (ALNN), which can significantly accelerate the inverse design process after the training of ALNN. We have successfully applied the method to achieve on-demand design of acoustic metasurface in a very low frequency range (50Hz-100Hz) and can optimize the design according to the maximum bandwidth, minimum thickness and minimum weight with limited time. The effectiveness of the method is validated through numerical simulation and experiment. Our work contributes to accelerating the design of metasurface absorbers, especially providing new avenues for low-frequency acoustic absorption applications and research.

This paper investigates a cooperative fixed-time control for networked-delayed and disturbed quadrotors in payload transportation missions. First, the input time-delayed tracking error system is adequately transformed into a delay-free system via Artstein's reducing transformation. Second, a novel Distributed Cooperative Fixed-Time Stable Control Protocol (DCFTSCP) is proposed for the multi-quadrotor system to stabilize the geometrical spatial formation while carrying the payload. Furthermore, a disturbance observer is incorporated into the control framework to counteract the lumped disturbances and achieve the transportation task with a minimal swing. Overall, the designed DCFTSCP is able to stabilize the delayed tracking states at the origin in fixed-time uniformly to the Initial Conditions (ICs). It is worth mentioning that the proposed controller belongs to the homogeneous sliding mode control class. The reported related works rely on the bi-limit approximation method to prove the fixed-time convergence of the states. Nevertheless, this method suffers from a crucial shortcoming where it is unable mathematically to provide an explicit expression on the settling (convergence) time. In contrast, in the present study, the stability analysis is thoroughly investigated based on the algebraic Lyapunov tools, namely, Algebraic Lyapunov Equation (ALE) and Lyapunov Quadratic Function (LQF). Consequently, the upper-bound on the convergence-time is explicitly derived for the first time for time-delayed quadrotors aircraft used in cooperative payload transportation. MATLAB® simulations and real flight experiments are conducted to corroborate the theoretical studies of the paper. Overall, the results show that the proposed control protocol yields performance improvement regarding fixed-time tracking stability featuring fast transient, strong robustness, and high steady-state precision. In addition, the undesirable chattering problem of regular linear sliding mode control is noticeably attenuated. Furthermore, unlike conventional terminal sliding mode control techniques, the control input is singularity-free. Finally, several challenging issues in cooperative fixed-time control for networked-delayed quadrotors are highlighted for future research.

For UAV swarm systems, cooperative operation and mission performance of UAV nodes strongly depend on the network topology. In this paper, we propose a network topology construction method for UAV swarm network with the maximum topology duration as the optimal objective and network communication requirements, including the throughput and end-to-end transmission delay as the constraints. Simulation results show that the proposed method can improve the average network topology duration by more than 128% compared to the traditional topology construction method that does not consider the network topology duration.

Network topology construction plays an important role in the application of large-scale unmanned aerial vehicle (UAV) swarm. Current researches usually perform the topology construction in terms of criteria of nodes energy consumption, transmission delay and network throughput, etc. However, another important criterion, the stability of swarm network topology, which is much critical for dynamic scenarios, has not been fully considered. In this paper, a novel topology construction method for UAV swarm network based on the criterion of topology duration is proposed. Specially, the topology construction of swarm network is formulated as an optimization problem of maximizing the topology duration while satisfying the constraints of certain network throughput, end-to-end delay, and nodes energy consumption. Then, a novel Group Trend Similarity based double-head Clustering method(GTSC) is employed to solve this problem, in which group similarity of movement, intra- and inter-cluster distance, node forwarding delay, and energy strategy are comprehensively taken into account. The proposed method is effective when used to perform the network topology construction for UAV swarm, which is verified by the simulation results. Furthermore, in comparison with representative algorithms, the proposed GTSC method exhibits better performance on topology duration, network throughput, end-to-end delay and energy consumption balance especially in a large-scale swarm scenarios.

A detailed theoretical design and technological implementation aspects are presented in this paper to address the aerial formation control problem of networked quadrotors with a fixed-time stability property. The control algorithm is embedded in a distributed fashion onboard each quadrotor along with a leader–follower scheme. The dynamics of the vehicles are subject to disturbances and nonlinearities. Given the communication topology within the graph, a Distributed Fixed-Time Consensus Observer (DFCO) is designed for the followers that are not all directly informed of the leader’s states. An output-feedback control is employed in the position-loop to ensure robust and velocity-free control where only the position of the quadrotors is measurable. Then, inspired by the homogeneity theory, a novel Homogeneous Nonsingular Terminal Sliding Function (HNTSF) that guarantees Global Asymptotic Stability (GAS) in the bi-limit approximation is designed for the position states. Subsequently, a Fixed-time Distributed Non-switching Nonsingular Formation Control Protocol (FDNNFCP) is proposed for the position-loop of each follower aircraft. Also, a Trajectory Tracking Controller (TTC) is designed for the leader. A Modified Robust Homogeneous-based Continuous Twisting Control (MRHCTC) with improved performance is developed to stabilize the attitude-loop. Considering the overall feedback system, the presented work provides a rigorous stability analysis through the bi-limit homogeneity theory and Lyapunov theorem. Moreover, Processor-In-the-Loop (PIL) simulations, ROS/Gazebo implementation, and outdoor flight experiments are conducted to characterize the control performance. Compared with relevant and recent literature on finite-time/fixed-time controllers besides the well-known Proportional–Integral–Derivative (PID) controller, the proposed control approach achieves superior performance in practice since: (i) Convergence-time of the quadrotors to the formation does not depend on their initial positions; (ii) Chattering issue of switching and discontinuous control approaches is mitigated; (iii) Null steady-state error is ensured along with improved robustness.

A formation flight control algorithm for a multi-agent system composed of a group of perturbated quadrotors is thoroughly investigated in this paper. Both theoretical and practical aspects are addressed in detail to design and implement the control algorithm in a distributed fashion among the networked agents. By adopting the sliding mode framework: (i) A novel Nonlinear Homogeneous Nonsingular Terminal Sliding Surface (NHNTSS) is designed. To ensure Global Asymptotic Stability (GAS) as well as fixed-time convergence of the states, the sliding surface design skillfully employs the generalized weighted homogeneity theory. (ii) A novel Distributed Fixed-Time Continuous Nonsingular Control Protocol (DFCNCP) is proposed for the position-loop of each quadrotor agent by utilizing the designed NHNTSS. Moreover, the control scheme comprises a disturbance observer to compensate for the lumped disturbances affecting the position dynamics and estimate the unmeasurable linear velocities of the quadrotors. Overall, the synthesized controller can drive the formation tracking errors into close vicinity of the origin in fixed-time uniformly to the values of the Initial Conditions (ICs), i.e., initial positions of the quadrotors in the 3D state-space. The mathematical proofs based on the bi-limit approximation in the existing related works can only indicate that the system is guaranteed to be fixed-time stable without any estimation of the settling (convergence) time. In contrast, rigorous stability analyses are conducted in the present work by virtue of algebraic Lyapunov tools, i.e., Algebraic Lyapunov Equation (ALE) and Lyapunov Quadratic Function (LQF). Thus, an expression of the upper-bound on the settling-time is given explicitly. Numerical simulations and real experiments in the form of exhaustive comparative studies are carried out to validate the findings of this research work. Overall, the obtained results confirm the superiority of the proposed control strategy in terms of uniform fast fixed-time convergence, strong disturbance rejection, chattering alleviation, and control precision.

This paper presents a robust scheme for fixed-time tracking control of a multirotor system. The aircraft is subjected to matched lumped disturbances, i.e., unmodeled dynamics, parameters uncertainties, and external perturbations besides measurement noise. Firstly, a novel Nonlinear Homogeneous Continuous Terminal Sliding Manifold (NHCTSM) based on the weighted homogeneity theory is presented. The sliding manifold is designed with prescribed dynamics featuring Global Asymptotic Stability (GAS) and fixed-time convergence. Then, a novel Fixed-time Non-switching Homogeneous Nonsingular Terminal Sliding Mode Control (FNHNTSMC) is proposed for the position and attitude loops by employing the developed NHCTSM and an appropriate reaching law. Moreover, the control framework incorporates a disturbance observer to feedforward and compensate for the disturbances. The designed control scheme can drive the states of the system to the desired references in fixed-time irrespective of the values of the Initial Conditions (ICs). Since the existing works on homogeneous controllers rely on the bi-limit homogeneity concept in the convergence proofs, the estimate of the settling-time or its upper-bound cannot be given explicitly. In contrast, this study employs Lyapunov Quadratic Function (LQF) and Algebraic Lyapunov Equation (ALE) in the stability analysis of both controller and observer. Following this method, an expression of the upper-bound of the settling-time is explicitly derived. Furthermore, to assure the Uniform Ultimate Boundedness (UUB) of all signals in the feedback system, the dynamics of the observer and controller are jointly analyzed. Simulations and experiments are conducted to quantify the control performance. The proposed approach achieves superior performance compared with recent literature on fixed-time/finite-time control and a commercially available PID controller. The comparative results witness that the developed control scheme improves the convergence-time, accuracy, and robustness while overcoming the singularity issue and mitigating the chattering effect of conventional SMC. (C) 2022 The Franklin Institute. Published by Elsevier Ltd. All rights reserved.

A formation control with fixed-time stability under network delay is investigated in this paper for a team of quadcopters cooperatively carrying a slung payload under a leader-follower paradigm. The vehicles’ dynamics are subject to nonlinearities, matched internal/external perturbation, and load disturbances. First, an Artstein’s reducing transformation is skillfully utilized to transform the input time-delayed double integrator error system into a delay-free system. Then, a novel Distributed Fixed-Time Homogeneous Sliding Mode Control Protocol (DFHSMC) is proposed for the multi-quadrotor system. Moreover, the control scheme comprises a disturbance observer to compensate for the lumped disturbances affecting the position dynamics. Thus, achieving the transportation task with a minimal swing. Overall, the synthesized controller can stabilize the delayed formation tracking errors at the origin in fixed-time uniformly to the values of the Initial Conditions (ICs). Besides, an explicit expression for the upper-bound on the settling-time is given. Finally, real experiments are carried out to corroborate the findings of this research work.

This paper presents an accurate solution of finite-time Cartesian trajectory tracking control problem of a quadrotor system by designing and implementing a novel robust flight-control algorithm. The quadrotor is subject to nonlinearities, unmodeled dynamics, parameters’ uncertainties, and external time-varying disturbances. To reject the disturbances and enhance the control system’s robustness, a terminal sliding mode-based active antidisturbance control (TSMBAADC) approach is proposed for rotational and translational subsystems. To improve the tracking performance, a nonlinear continuous terminal sliding manifold and a fast reaching law are proposed in this work to quickly drive the systems’ states to the equilibrium point even in the presence of lumped disturbances. The convergence time of the states can be pretuned based on the parameters of the sliding manifold and the reaching law. Lyapunov theorem is used to provide a rigorous stability proof for the feedback control system. Numerical simulations and processor-in-the-loop (PIL) experiments are conducted to validate and implement the designed flight control algorithm on real autopilot hardware. The novelty of the proposed research lies in hardware implementation of a sophisticated version of modern control technique that exhibits a multitude of distinguishing features including but not limited to (i) finite-time tracking stability featuring fast convergence is ensured, (ii) chattering and singularity problems in sliding mode control (SMC) are avoided, and (iii) null steady-state error is achieved along with enhanced robustness. Finally, the proposed control law is compared with two recently reported research works. Results of performance comparison in term of the integral of square error (ISE) and the absolute value of the derivative of the input u t (IADU) dictate that the proposed technique overperforms by precision and chattering alleviation.

This paper solves an accurate fixed-time attitude and position control problems of a quadrotor UAV system. The aircraft system is subject to nonlinearities, parameter uncertainties, unmodeled dynamics, and external time-varying disturbances. To deal with the under-actuation problem of the quadrotor’s dynamics, a hierarchical control structure with an inner–outer loop framework is adopted for the flight control system design. Robust nonlinear control strategies for attitude and position control are innovatively proposed based on a new continuous nonsingular terminal sliding mode control (CNTSMC) scheme. A full-order homogeneous terminal sliding surface is designed for the attitude and position states in such a way that the sliding motion is fixed-time stable independently of the system’s initial condition. Hence, this contributes to enhancing the control system robustness. A disturbance observer-based control (DOBC) approach is developed to stabilize the inner rotational subsystem (attitude-loop). This compounded control structure integrates a finite-time observer (FTO) and the CNTSMC scheme. The FTO observer is incorporated into the control framework to cope with the strong perturbations. An output-feedback control approach is adopted for the outer translational subsystem (position-loop) to ensure a velocity-free control. In this context, the CNTSMC scheme is combined with a fixed-time extended state observer (FXESO) to achieve an active disturbance rejection control (ADRC) by estimating and canceling the lumped disturbances. Therefore, within the developed control approach including the robust CNTSMC scheme, DOBC, and ADRC strategies, robust and accurate trajectory tracking control can be achieved despite uncertainties and disturbances. Stability analysis of the closed-loop system is rigorously investigated by using the Lyapunov theorem, bi-limit homogeneous theory, and the notion of input-to-state stability (ISS). Extensive experimental tests under the influence of various disturbances are conducted to corroborate the theoretical findings. To this end, an effective model-based design (MBD) framework is established to implement the developed control algorithms in real autopilot hardware. Furthermore, processor-in-the-loop (PIL) experiments are also carried out within the MBD framework. A comparative study is made involving our control algorithms and other control strategies. Overall, the obtained results show that the synthesized control system yields performance improvement regarding fixed-time tracking stability featuring fast transient, strong robustness, and high steady-state precision. Besides, the chattering effect of regular linear sliding mode control (LSMC) is significantly alleviated. Moreover, unlike conventional TSMC, the control input shows no singularity.

This paper proposes a novel control scheme for a group of quadrotors aircrafts that form a leader-follower configuration and are subjected to nonlinear behavior with lumped disturbances. For each aircraft, a distributed formation control law is designed. The desired geometrical pattern is achieved and the reference formation trajectory is tracked using the synthesized fixed-time position control robust law. Considering the overall feedback system, the presented work also presents a rigorous stability analysis of the system. Moreover, to characterize the control performance, simulations are conducted in a realistic ROS/Gazebo environment. Compared with the relevant literature, the proposed scheme demonstrates superior performance in practice because (i) convergence-time of the agents does not depend on their initial positions; (ii) chattering problem of switching control methods is avoided; (iii) zero error in steady-state is obtained while ensuring robustness.

This research addresses the problem of robust attitude control for a quadrotor operating in an environment polluted with lumped disturbances. A new continuous terminal sliding mode-based active anti-disturbance control (CTSMBAADC) is proposed by innovatively introducing a finite-time disturbance observer (FTDO) in homogeneous continuous nonsingular terminal sliding mode control (HCNTSMC) law. The HCNTSMC scheme drives the states of the system to the reference setpoint in finite-time. Rigorous stability analysis of the feedback loop system is based on input-to-state stability (ISS) concept and more importantly Lyapunov theory. Real-time experiments are performed to validate the designed control law. Results witness that the proposed control structure offers superior performance in terms of robustness and accuracy while avoiding the singularity problem and significantly alleviating the chattering phenomenon.

为减小工作于连续导电模式(continue conduction mode,CCM)的单电感双输出(single-inductor dual-output,SIDO)Buck变换器的输出交叉影响,提出了峰值电流-峰值电压(peak-current and peak-voltage,PCPV)控制方法.分析了PCPV控制SIDO Buck变换器的电路结构和工作原理,利用电感伏秒平衡和电容安秒平衡原理推导了输出电压与输入电压的增益表达式,并采用状态空间平均方法,建立了PCPV控制SIDO Buck变换器的状态空间平均模型;在此基础上,建立了PCPV控制SIDO Buck变换器的小信号模型,并与传统峰值电流(peak-current-mode,PCM)控制SIDO Buck变换器对比分析交叉影响.研究结果表明:PCM控制SIDO Buck变换器输出电压较大的输出支路对输出电压较小的输出支路的交叉影响为300 mV,而PCPV控制SIDO Buck变换器输出电压较大的输出支路对输出电压较小的输出支路几乎无交叉影响;PCM控制SIDO Buck变换器的负载瞬态调节时间为12.5 ms,而PCPV控制SIDO Buck变换器的负载瞬态调节时间最大为10 ms.相比PCM控制SIDO Buck变换器,PCPV控制SIDO Buck变换器有效地减小了交叉影响,且提高了瞬态性能.最后通过实验结果验证了理论分析的正确性.

This paper deals with robust attitude control problem of the quadrotor UAV in the presence of model uncertainties and external disturbances such as wind gusts. A Disturbance Observer-Based Control (DOBC) method is proposed by combining an Improved Twisting Control (ITC) algorithm and a Finite-Time Observer (FTO). Within the ITC scheme, the chattering problem of discontinuous Sliding Mode Control (SMC) techniques can be reduced due to the continuous control signal. Besides, finite-time convergence of the system states can be ensured to achieve accurate control. On the other hand, to reject external disturbances, the FTO observer is incorporated into the control framework. Stability analysis of the closed-loop system is rigorously investigated by using a homogeneous Lyapunov Function (LF). Experimental tests are conducted to validate the theoretical findings. A comparative study is made involving the proposed ITC-FTO strategy and three other controllers, including Continuous Twisting Controller (TC), Integral Backstepping Sliding Mode Controller (IBSMC), and a Nominal Backstepping Controller (NBSC). Overall, the obtained results show that the suggested control system yields performance improvement regarding accuracy and robustness. Meanwhile, the chattering effect of conventional SMC is remarkably alleviated.

Direction-of-arrival (DOA) estimation using a linear co-prime microphone array has attracted tremendous attention in many acoustic applications. Spatial smoothing MUSIC (SS-MUSIC)-based methods and compressive sensing-based methods are two popular approaches for DOA estimation using co-prime linear arrays. However, these methods exhibit high computational complexity and suffer from performance degradation under low signal-to-noise ratio conditions. This paper proposes a particle filter (PF)-based DOA tracking algorithm using a linear co-prime array. The proposed PF employs the current spatial information obtained from the array measurement and temporal information obtained from a constant-velocity motion model to estimate the DOA values recursively, which is a more suitable approach for real-world applications. An improved likelihood function model derived from the SS-MUSIC pseudo-spectra is implemented in the PF-based algorithm. Theoretical analyses and simulation results and the results of real acoustic data processing experiments are presented to demonstrate the effectiveness of the proposed PF-based algorithm.

In this letter, the problem of direction of arrival (DOA) tracking using co-prime array is studied. A novel particle filter (PF)-based DOA tracking algorithm is proposed to recursively estimate the DOAs based on the measurement model and the state transition model. A modified likelihood function model based on the propagator method (PM) pseudo-spectrum is devised to update the involved particles under lower SNR or less samples condition. Compared to the existing techniques, such as the spatial smoothing multiple signal classification method (SS-MUSIC), compressive sensing (CS) and conventional likelihood PF (CL-PF) methods, the proposed algorithm can exhibit improved performance in term of estimating accuracy and computational cost. Simulation studies are carried out to show the superiority of the proposed PF algorithm.

A three-dimensional transducer array directivity formula is derived theoretically in this paper to solve the array pattern choice problem of acoustic directional device, based on which the three-dimensional directivity for polygon arrays is analyzed, and the results show: in three-dimensional space, the beam width of the polygon array varies periodically with the rotation angle, and the number of cycles varies with the array form; the mean value of the square array’s beam width is smallest, but its fluctuation is greatest; the fluctuation of the hexagon array’s beam width is smallest, but its mean value is largest; the mean value of the approximate octagonal array’s beam width is close to that of the square array, whose fluctuation is also relatively small, and it has the strongest ability of suppressing side-lobes, which makes it suitable to be the array pattern for acoustic directional device. The three-dimensional directivity analyzing method employed in this paper overcomes the local extremum problem of traditional two-dimensional directivity analyzing method, which is significant for transducer array’s directivity design.

为提高航天服下肢活动的灵活性,给出一种耦合硬质髋关节的机构模型、表达参数及约束条件.根据机构设计原理,建立大腿-髋关节耦合的混合机构模型,完成机构自由度的计算.采用矢量表达法,建立大腿-髋关节机构的平面运动方程,完成髋关节机构的运动计算与分析.结果表明:髋关节概念模型与人体下肢具有相同的自由度,建立的机构运动方程与实际人体大腿的运动形式相一致,髋关节机构模型是合理可行的.

Our paper presents a static obstacles avoidance and path planning method for Unmanned Aerial Vehicle (UAV) in outdoor three-dimension (3D) environment. We propose a rectified rapidly exploring random tree (RRT*) algorithm where a method is proposed for smoothing and rectifying the final path generated by RRT* in order to reduce the energy consumption during the flight. We also introduce an obstacle avoidance method which allows an efficient free-path collision generation in a complex 3D operating state space. This method ensures that the environment modeling is very close to the reality which allows a high accuracy obstacle avoidance with a secure distance between the UAV and the obstacles edges. Our algorithm was validated through the experiments including the Hardware-in-the-loop simulation (HIL) and real outdoor flights.

This paper proposes a unified system framework based on linear frequency modulation (LFM) and orthogonal frequency-division multiplexing (OFDM) as a solution for resource sharing, especially sharing space and hardware. The proposed OFDM-LFM framework can not only transmit data flow by OFDM but also make it possible to extract features from the signal by LFM. Further, the signal features are used to construct a pseudospectrum related to the target speed and distance, thereby estimating the speed and distance of the target. Unified means using signal features to estimate target speed and distance while ensuring data transmission including communication and remote control data information. Besides the traditional data transmission mode, the ability of estimating target speed and distance is an additional benefit. The proposed unified framework makes control of transmission easier and saves more hardware resources. The simulation results show that the proposed LFM-OFDM framework can not only transmit data information including communication and remote control data information, but also estimate the speed and distance of the target by its signal features.