Path following presents a pivotal challenge within the realm of small fixed-wing unmanned aerial vehicles. Firstly, a Lyapunov-stable path guidance law was formulated to follow specific planar curved paths. To ensure differentiability of the guidance law, a modified, smooth saturation function was derived. Secondly, an analysis was conducted to ascertain the interrelationship between control parameters and input constraints, thereby identifying the relevant parameter domains. Thirdly, the nonlinear model predictive control technique was harnessed to optimize both guidance law parameters, enhancing the unmanned aerial vehicle’s capacity to achieve optimal performance in both straight-line and circular path following, hereafter referred to as PFC_NMPC. By leveraging Lyapunov stability arguments for switched systems, the stability of the corresponding nonlinear switched system was guaranteed. In this study, square and circular paths were generated to assess the path-following control of a simulated fixed-wing unmanned aerial vehicle. The performance of various guidance laws, including those with fixed parameters (PFC), those with parameters tuned using fuzzy logic (PFC_FL), PFC_NMPC, vector field, and pure pursuit with line-of-sight, was compared. Notably, the proposed PFC_NMPC method exhibited the ability to expedite the unmanned aerial vehicle’s convergence to the desired path while maximizing the effective flight path length.

Cardiovascular diseases are increasingly threating the global human health, hypertension is the most important risk factor for cardiovascular and cerebrovascular diseases. To improve the antihypertensive activity and cardiovascular protective effect of natural product (±)-7,8-dihydroxy-3-methyl-isochroman-4-one [(±)-XJP], a series of novel H2S-releasing isochroman-4-one derivatives were designed and synthesized by coupling hydrogen sulfide (H2S)-releasing donors with the analogs of (±)-XJP. Further, the H2S-releasing assay indicated that some target compounds showed excellent H2S generating ability. Moreover, these novel hybrids exhibited moderate to good in vitro vasodilation efficacy. Among them, the most potent compound exhibited potent in vivo antihypertensive activity with the maximum antihypertensive amplitude about 27%, which was more potent than that of the lead compound (±)-XJP. These results suggested that the hybridization of H2S-donors and (±)-XJP analogs may provide a promising approach for the discovery of novel antihypertensive agents.

In this study, the problem of guiding a small fixed-wing unmanned aerial vehicle (UAV) toward a predefined horizontal path is studied. A stable nonlinear guidance law, which is a function of the inertial positions and velocities of the UAV and the predefined path, is designed using Lyapunov stability arguments. The concept of the nonlinear model predictive control (NMPC) technique was applied to optimize a key parameter of the guidance law to improve the performance of the controller (PFC_NMPC), where the stability of the relative nonlinear system is maintained. The proposed method was verified in the MATLAB/Simulink environment to realize following the straight-line, square and circular paths. The path- following performance of the proposed method is compared with those of the guidance laws with parameter fixed (PFC) or tuned by fuzzy logic (PFC_FL). With the predictive ability, the proposed method can make the UAV fly more on the desired square and circular paths than the other two methods, PFC and PFC_FL. The error overshoot by using PFC_NMPC is much smaller than those by using the PFC and PFC_FL methods in the presence of wind at 8m/s.

We present a real-time monocular simultaneous localization and mapping (SLAM) system with a new distributed structure for multi-UAV collaboration tasks. The system is different from other general SLAM systems in two aspects: First, it does not aim to build a global map, but to estimate the latest relative position between nearby vehicles; Second, there is no centralized structure in the proposed system, and each vehicle owns an individual metric map and an ego-motion estimator to obtain the relative position between its own map and the neighboring vehicles’. To realize the above characteristics in real time, we demonstrate an innovative feature description and matching algorithm to avoid catastrophic expansion of feature point matching workload due to the increased number of UAVs. Based on the hash and principal component analysis, the matching time complexity of this algorithm can be reduced from O(log N) to O(1). To evaluate the performance, the algorithm is verified on the acknowledged multi-view stereo benchmark dataset, and excellent results are obtained. Finally, through the simulation and real flight experiments, this improved SLAM system with the proposed algorithm is validated.

7,8-Dihydroxy-3-methyl-isochromanone-4 (XJP), is a polyphenolic natural product with moderate antihypertensive activity. To obtain new agents with stronger potency and safer profile, we employed XJP and naftopidil as the lead compounds to design and synthesize a novel class of hybrids as antihypertensive agent candidates. In the present study, a series of hybrids (6a–r) of XJP bearing arylpiperazine moiety, which is identified as the pharmacophore of naftopidil, were designed and synthesized as novel α1-adrenergic receptor antagonists. The biological evaluation showed that target compounds 6c, 6e, 6f, 6g, 6h, 6m and 6q possessed potent in vitro vasodilation potency and α1-adrenergic receptor antagonistic activity. Furthermore, the most potent compound 6e significantly reduced the systolic and diastolic blood pressure in spontaneously hypertensive rats (SHRs), which was comparable to that of naftopidil, and it had no observable effects on the basal heart rate, suggesting that 6e deserves to be further investigated as a potential clinical candidate for the treatment of hypertension.

A series of novel 4-isochromanone compounds bearing N-benzyl pyridinium moiety were designed and synthesized as acetylcholinesterase (AChE) inhibitors. The biological evaluation showed that most of the target compounds exhibited potent inhibitory activities against AChE. Among them, compound 1q possessed the strongest anti-AChE activity with an IC50 value of 0.15nm and high AChE/BuChE selectivity (SI>5,000). Moreover, compound 1q had low toxicity in normal nerve cells and was relatively stable in rat plasma. Together, the current finding may provide a new approach for the discovery of novel anti-Alzheimer's disease agents.

A guidance law has been designed to guide the small unmanned aerial vehicle towards the predefined horizontal smooth path. The guidance law only needs the mathematical expression for the predefined path, the positions, and the velocities of the vehicle in the horizontal inertial frame. The stability of the guidance law has been demonstrated by the Lyapunov stability arguments. In order to improve the path following performance, one of the parameters of the guidance law is tuned by using the fuzzy logic which will still keep its stability. The simulation experiments in the Matlab/Simulink environment to realize the square-,circular-, and the athletics track-style paths following are given to verify the effectiveness of the proposed method. The simulation results show that the path following performance will be improved with smaller overshoot and oscillation amplitude and shorter arrival time with the parameter tuned.

This article proposes a composite path following controller that allows the small fixed-wing unmanned aerial vehicle to follow a predefined path. Assuming that the vehicle is equipped with an autopilot for altitude and airspeed maintained well, the controller design adopts the hierarchical control structure. With the inner-loop controller design based on the notion of active disturbance rejection control which will respond to the desired roll angle command, the core part of the outer-loop controller is designed based on Lyapunov stability theorem to generate the desired course rate for the straight-line paths. The bank to turn maneuver is used to transform the desired course rate to the desired roll angle command. Both the hardware-in-the-loop simulation in the X-Plane simulator and actual experimental flight tests have been successfully achieved, which verified the effectiveness of the proposed method.

To ensure the effectiveness of the RF(Radio Frequency) guided HILS(Hardware-In-the-Loop Simulation) system, the credibility evaluation was necessary. At present, most of the methods were applied in the case of the simulation system input being consistent with the real system input, which is difficult to achieve in some subsystem of the RF guided HILS system. In this paper, the relative complete system reliability evaluation index system was established. Subsystems were classified, for different categories of subsystem, corresponding reliability assessment methods were studied. In particular, reliability evaluation methods based on performance level were put forward, when certain subsystems cannot be evaluated by consistency analysis methods. A simple example was given to show the evaluation process.

Path following is a fundamental problem for unmanned air vehicles (UAVs). In this paper, a Lyapunov stable path guidance law is designed to generate the course control command for a small UAV. The control parameters of the guidance law is optimized by the nonlinear model predictive control (NMPC) techinque. The relative simulations with the three different approaches: Lyapunov stable controller with parameters optimizated (LC_NMPC), the normal NMPC and the Lyapunov stable controller with constant parameters (LC) are compared with each other. The simulation results show that the LC_NMPC can reach good path following performance with shorter rise time and zero steady error.

The synthesis of 4-isochromanones via Parham-type cyclization with Weinreb amide as the internal electrophilic group, t-BuLi as the lithium reagent was described. The reaction was efficient and could be completed in one minute. The application scope of this new protocol was investigated and the desired products could be obtained in good to excellent yields. Besides, the synthetic potential of this method was further demonstrated by the synthesis of natural product (±)-XJP, which was obtained in six steps with overall yield up to 54%.

This paper proposes a nonlinear controller that allows the unmanned air vehicle (UAV) to follow a predefined path. The controller only needs the information of the inertial speed of the UAV and the distance between the vehicle and the desired path. In order to improve the performance of the controller, a parameter of the controller is optimized by using the concept of model predictive control (MPC). Lyapunov stability arguments are used to demonstrate that the path following error will be regulated to zero, even in the presence of wind disturbances. Simulation results in Matlab are given to show the effectiveness of the controller. The simulation results show that the proposed path following controller with parameter optimized by MPC (PLC_MPC) which takes the advantages of the predictive ability of the MPC and the simple structure of the normal path following controller (PLC) has better path following performance than that by using the PLC method.

A series of novel 4-isochromanone hybrids bearing N-benzyl pyridinium moiety as dual binding site acetylcholinesterase inhibitors have been designed and synthesized. The screening results showed that most of the compounds exhibited potent anti-AChE activity in the range of nM concentrations. The 1-(4-fluorobenzyl) substituted derivative 9d exhibited the most potent anti-AChE activity with IC50 value of 8.9 nM and high AChE/BuChE selectivity (SI >230). Kinetic and molecular modeling studies suggested that compound 9d was mixed-type inhibitor, binding simultaneously to CAS and PAS of AChE. Besides, the preliminary structure–activity relationships were discussed.

The first asymmetric total synthesis of antihypertensive natural products S-(+)-XJP and R-(-)-XJP has been achieved in 8 steps starting from commercially available 6-bromo-2-hydroxy-3-methoxybenzaldehyde. Key steps included intramolecular Heck reaction and oxidative ozonolysis reaction with the retention of stereochemistry. A latent functionality strategy was implemented to circumvent the racemization in this endeavor. The protocol described here provided a fast and easily accessible synthetic method to obtain optically pure isochroman-4-one derivatives. Furthermore, the in vivo antihypertensive effects of (±)-XJP, S-(+)-XJP and R-(-)-XJP were investigated on spontaneously hypertensive rats. The obtained results could provide valuable information to identify a promising lead for further chemical modification research.

Purpose – The purpose of this paper is to present a Rao–Blackwellized particle filter (RBPF) approach for the visual simultaneous localization and mapping (SLAM) of small unmanned aerial vehicles (UAVs). Design/methodology/approach – Measurements from inertial measurement unit, barometric altimeter and monocular camera are fused to estimate the state of the vehicle while building a feature map. In this SLAM framework, an extra factorization method is proposed to partition the vehicle model into subspaces as the internal and external states. The internal state is estimated by an extended Kalman filter (EKF). A particle filter is employed for the external state estimation and parallel EKFs are for the map management. Findings – Simulation results indicate that the proposed approach is more stable and accurate than other existing marginalized particle filter-based SLAM algorithms. Experiments are also carried out to verify the effectiveness of this SLAM method by comparing with a referential global positioning system/inertial navigation system. Originality/value – The main contribution of this paper is the theoretical derivation and experimental application of the Rao–Blackwellized visual SLAM algorithm with vehicle model partition for small UAVs.

This paper has presented an implementation of visual based non-GNSS navigation method for small VTOL (Vertically Taking-off and Landing) UAV (Unmanned Aerial Vehicle). A novel method of constructing landmarks is described in detail. It uses a pair of landmarks instead of the general landmark which usually is a single geometry or coding image. The paired-landmark method inherits general landmark methods' advantages of low-cost, high-efficiency and the ability of absolute positioning. And it also solves the problem of requiring too much different characteristic landmarks when handling large area navigation. The paired-landmark method has been verified by a real parameter simulation environment. Real flight experiment is also carried out to test the effectiveness and performance of the paired-landmark navigation method. It can be concluded that, besides achieved the expected positioning precision, the proposed system also reduced the consumption of calculation and memory space.

For the autonomous flight of a small unmanned helicopter in a GPS-denied environment, a fast simultaneous localization and mapping (FastSLAM) algorithm based on Rao-Blackwellized particle filter (RBPF) was designed, and a monocular visual SLAM system for small unmanned helicopters in GPS-denied environments was implemented by using the Fast SLAM algorithm. The onboard monocular camera of the system uses the scale invariant feature transform (SIFT) to detect and match landmarks. The visual observation is fused with the inertial measurement to estimate the state of the vehicle and build the feature map simultaneously. An undelayed inverse depth parametrization method is applied to the landmark initialization. The stability and the effectiveness of this system were verified by simulations. The real flight experiments were also carried out to test the performance of the algorithm. The results show that the proposed system can estimate the state of the vehicle with higher accuracies in all items such as attitude, velocity and position, compared with the traditional GPS/INS navigation system. It can provide reliable navigation information for small unmanned helicopters in GPS-denied environments.

This paper presents a hierarchical simultaneous localization and mapping (SLAM) system for a small unmanned aerial vehicle (UAV) using the output of an inertial measurement unit (IMU) and the bearing-only observations from an onboard monocular camera. A homography based approach is used to calculate the motion of the vehicle in 6 degrees of freedom by image feature match. This visual measurement is fused with the inertial outputs by an indirect extended Kalman filter (EKF) for attitude and velocity estimation. Then, another EKF is employed to estimate the position of the vehicle and the locations of the features in the map. Both simulations and experiments are carried out to test the performance of the proposed system. The result of the comparison with the referential global positioning system/inertial navigation system (GPS/INS) navigation indicates that the proposed SLAM can provide reliable and stable state estimation for small UAVs in GPS-denied environments.

For controlling a small unmanned helicopter to reach its goal of hovering or cruise flight, two decoupled mathematical models for the small unmanned helicopter were achieved by system identification based on the building of the helicopter's linear model for hovering, and then an augmented linear quadratic Gaussian (LQG) controller was designed. The augmented LQG controller consists of a Kalman filter, a traditional linear quadratic integral (LQI) controller, and a feedforward term. The Kalman filter is used to estimate the unmeasured states. The integral action of the LQI is to reduce the steady state errors, and the feedforward term is used to speed up the tracking of reference signals. The simulations and the actual flight results indicate that the presented augmented LQG controller can not only stabilize the dynamics of unmanned helicopters, but also track the reference control signals well.

This paper presents a vision-aided inertial navigation system for small unmanned aerial vehicles (UAVs) in GPS-denied environments. During visual estimation, image features in consecutive frames are detected and matched to estimate the motion of the vehicle with a homography-based approach. Afterwards, the visual measurement is fused with the output of an inertial measurement unit (IMU) by an indirect extended Kalman filter (EKF). A delay-based approach for the measurement update is developed to introduce the visual measurement into the fusion without state augmentation. This method supposes that the estimated error state is stable and invariant during the second half of one visual calculation period. Simulation results indicate that delay-based navigation can reduce the computational complexity by about 20% compared with general augmented Vision/INS (inertial navigation system) navigation, with almost the same estimate accuracy. Real experiments were also carried out to test the performance of the proposed navigation system by comparison with the augmented filter method and a referential GPS/INS navigation.