Abstract:To obtain the explicit expression of Cramér-Rao lower bound (CRLB) and simplify the inversion process of higher-order Fisher information matrices (FIM) for undirected network navigation of multi-agent systems (MAS) with arbitrary information coupling, we proposed a distributed inversion method named block diagonalization method. Firstly, a high-order FIM was constructed for the arbitrary coupling undirected network navigation and positioning model, and then on the basis of its symmetry characteristics and the relationship between submatrices and network topology, the FIM was expressed as a linear combination of two block diagonal matrices, which contained the Fisher information of all nodes and edges respectively. Secondly, the matrix inversion lemma was used twice to derive the explicit expression of the inverse of FIM, i.e. CRLB and the equivalent FIM for network nodes. An iterative process was also introduced to decompose the whole inversion process into computations between several lower-order matrices, so as to mitigate the computational burden at every step. Finally, the proposed algorithm was verified by numerical experiments, and compared with the matrix block iterative inversion method. Results show that both methods had high computational efficiency and accuracy, but the computational burden of the proposed method was lower and the computational speed was faster, which verifies its accuracy and effectiveness. This algorithm can be used to analyze the Fisher information fusion process for every node or edge in arbitrary coupling undirected network navigation and positioning model.

Abstract:To realize the accurate 3D shape real-time estimation of soft manipulator and lay the foundation for deformation control and application, a real-time 3D shape recognition method based on the SOM algorithm is proposed for three-section soft manipulator. Firstly, the left and right frame data captured by ZED binocular camera are preprocessed to obtain the left and right binary images, with the 2D contour data of the soft manipulator extracted in real time. Then, the self-organizing map (SOM) algorithm is run to cluster the contour data to obtain the 2D centerline of the soft manipulator in order, and compared with K-means, Gaussian mixture model and thinning of three centerline extraction algorithms, revealing that SOM algorithm is more suitable for solving the centerline identification of complex shape of the soft manipulator. Finally, the 3D shape reconstruction of the soft manipulator is completed by using the triangulation model based on the disparity. Furthermore, the algorithm framework adapts data downsampling, SOM parameter optimization and other methods to improve real-time performance. The real-time shape sensing and comparative verification are carried out during the continuous deformation of the soft manipulator. Experimental results show that the algorithm presents a high shape sensing accuracy and a superior real-time tracking effect. Compared with other shape detection algorithms locally and globally, the proposed algorithm displays a better performance.

Abstract:In view of the problem of standoff target tracking and simultaneous arrival of multiple unmanned aerial vehicles (multi-UAVs), a new ratio Lyapunov vector field (RLVF) and a cooperative speed controller based on τ coupling theory were proposed. First, compared with other Lyapunov vector fields, a settable parameter was added to the new RLVF, so as to balance the convergence speed and heading rate limitation in the tracking process and thus maximize the performance of UAVs. Then, theoretical analysis showed that with smaller parameter, the convergence speed was faster, but the heading angular rate was higher. Besides, the parameter selection method under the limitation of maximum heading angular rate was proposed to realize a faster convergence speed. Finally, in order to ensure that the UAVs can reach the target point from different initial positions at the same time, a cooperative speed controller based on τ coupling theory was designed. Without coping with the actual irregular flight paths, the proposed controller only needs to calculate the straight-line distance from the current point to the target point, avoiding the complicated path integration problem in existing 4D path planning methods, and the algorithm is simpler, which is more conducive to engineering implementation. Results of digital simulation and hardware-in-loop experiment showed that by the proposed method, UAVs could achieve simultaneous arrival and fast convergence to the standoff radius, indicating the effectiveness and superiority of the method as well as its engineering implementation value.

Abstract:The rapid development and extensive application of ships highlight the vital role of the analysis and control of ship deck flow field. To improve the flow field of ship deck, a novel active flow control method based on jet is proposed, and by taking the position of helicopter rotor disk as an example, the effect of different jet device parameters on the optimization of helicopter rotor disk flow field is analyzed. First, the numerical simulation model of the flow field of the ship deck was established to examine the influence of active flow control on the ship deck flow field based on the Navier-Stokes equation. Then, the k-ε turbulence model was chosen and the effectiveness of the method was validated. Finally, the streamline and velocity distribution of ship deck flow field with jet device were simulated. Combined with the influence of flow field information on rotor force, the flow control effect of jet device on ship deck flow field was compared and analyzed. The results show that the addition of upper jet can reduce the influence range of reflux zone in the deck flow field and the velocity gradient of rotor disk flow field accordingly. The reduction of the velocity gradient of the rotor disk flow field tends to effectively reduce the aerodynamic variation and the response of the rotor. Adding jet devices under different inflow angles may reduce the response and improve the safety of the helicopter by controlling the deck flow field. As the jet velocity exerts a significant influence on the flow field control effect, the optimal jet velocity should be selected with reference to the installation position of the jet device to achieve better control effect.

Abstract:The vertical and oblique descend states of Tiltrotor involve extremely dangerous vortex ring state (VRS). To master the aerodynamic load and flow field changes of Tiltrotor in the VRS, the vortex ring boundary mathematical model was established, targeted for corresponding flight simulation and flight test. The CFD method based on slip grid technology was implemented to simulate the VRS of the Tiltrotor, the evolution of the flow field, force and moment of the VRS during the vertical descent flight of the Tiltrotor, with the calculation method of the VRS boundary proposed and the analytical expression of the risk zone of the VRS presented. The results show that the wing of the Tiltrotor reduces the influence of the VRS of single rotor, yet the symmetry of the Tiltrotor rotor increases the risk of unbalanced rolling moments; the Tiltrotor tends to enter the mature VRS when the descent rate vH/v_h is 0.9, with the rotor-lift loss reaching the maximum value of 18%. Given that the descent rate corresponding to the VRS is 0.583≤vH/v_h≤1.516 during the vertical descent state, risk may arise from the overturning caused by the unbalanced lift of the left and right rotors of the Tiltrotor, and from the crash caused by the excessive rotor-lift loss of the Tiltrotor. In view of the dangerous degree of VRS, the vortex ring boundary mathematical models of vertical and oblique descent states are attempted, providing a basis for the design and safe flight of the Tiltrotor.

Abstract:To improve the speed and accuracy of the reentry trajectory optimization problem of the hypersonic glide vehicle, a reentry trajectory method combining improved sparrow intelligent optimization with parametric design is proposed. Firstly, the population is initialized by the Tent chaotic mapping and the elite reverse population method, and the position of the population is updated by the golden sine strategy. The number of scouts is reduced by the sine strategy, and the optimal solution for the population is selected and updated by the greedy strategy. The global search ability of the algorithm is enhanced without affecting the convergence rate. Then, the hypersonic reentry trajectory optimization problem is transformed into the parametric design problem of the attack angle profile and the bank angle profile, and the path constraint is transformed into the drag acceleration reentry flight corridor to ensure that the path constraint is always satisfied in the reentry process, and the penalty function method is used to ensure that the aircraft can accurately hit the target. Finally, the improved sparrow intelligent optimization algorithm is used to optimize the design parameters to make the objective function optimal. The simulation results show that the proposed improved sparrow algorithm has a faster convergence speed than the original sparrow algorithm, whale algorithm, and particle swarm algorithm, and the accuracy of the hypersonic glide vehicle reentry trajectory is further improved. The Monte Carlo simulation results show that the reentry trajectory optimization algorithm of the hypersonic glide vehicle proposed in this paper has certain robustness.

Abstract:To realize a stable and controllable catalytic inerting process in the aircraft fuel tank, based on the process of the oxygen-consuming catalytic inerting system, using the gas composition in the ullage of the fuel tank as the benchmark, a mathematical model of the system process is established with reference to the mass conservation and energy conservation equations. By figuring out the relationship between stable oxygen concentration and flow rate, a stable oxygen concentration characteristic model is proposed, showing the internal relationship between stable oxygen concentration and different performance parameters like flow rate, heat generation rate, and water production rate. The results reveal that there is a flow ratio interval where the stable oxygen concentration remains steady at 0%, and the stable oxygen concentration in this interval does not change when the flow ratio changes. Furthermore, the interval grows as the catalytic reactor efficiency increases. The increase of the stable oxygen concentration can effectively reduce the catalytic reactors heat generation and water production rates. Under the present calculation conditions, the stable oxygen concentration tends to increase from 1% to 9%, and the heat generation and water production rates decrease by 36.1%. The relationship between the stable oxygen concentration and the flow ratio is affected by the fuel type: With the higher vapor pressure, greater flow ratio is needed to maintain the same stable oxygen concentration. The altitude and aircraft climbing rate may affect the stable oxygen concentration. In the case of a constant flow ratio, stable oxygen concentration decreases with the increase of altitude, and a greater climbing rate may lead to a faster decline rate of stable oxygen concentration.

Abstract:In view of the problem of adaptive weight mismatch caused by disturbance and steering vector mismatch in traditional beamformers, which leads to sharp decline of algorithm performance and even cancellation of expected signal, a robust beamforming method combining covariance matrix reconstruction and alternating direction method of multipliers (ADMM) was proposed. Firstly, on the basis of the maximum output power criterion of beamformer, an optimization model was designed to solve the optimal steering vector. Then, according to the spatial power spectrum function of the Capon algorithm, the covariance matrix was reconstructed with the defined interference range to widen the null and enhance the anti-motion interference ability of the system. Finally, for the quadratic inequality constraint problem of the steering vector, the essence was to estimate the difference between the steering vector and the expected steering vector. In this method, ADMM was adopted to solve the quadratic programming problem iteratively, and the specific solution of the steering vector in each iteration was obtained. In addition, the complexity of the algorithm was analyzed. Experimental results showed that compared with the existing beamforming algorithms, the proposed method widened the null at the interference point and improved the anti-jamming performance of the beam. Combined with complexity, it was proved that the algorithm was faster than the existing algorithm and could correct the mismatched steering vector well. This paper also provides a way to solve the quadratic inequality constraint problem and improve the performance of beamforming algorithm.

Abstract:In low frequency analog signal acquisition and processing circuit, signal measurement is often affected by noise and direct current (DC) drift. To remove DC drift and the existing noise from original signal accurately and obtain useful signal, a method of DC drift elimination based on segmented Empirical Mode Decomposition (EMD) is proposed. First, the signal is decomposed by EMD, and the local extremum points of the intrinsic mode function (IMF) components are identified for interval segmentation. Then, each segment of signal is decomposed by EMD, and the low-frequency components of each signal segment are selected to reconstruct the DC drift signal. In the end, the IMF components with noise as the main ingredients are screened by autocorrelation function for energy analysis, with all segments integrated and the signal after removing DC drift and noise reduction obtained. The simulation in the study shows the obvious effectiveness of the proposed method, compared with polynomial fitting, wavelet analysis, high-pass filtering and other methods. The strain signal of the robot force sensor in minimally invasive surgery is processed. The experimental results demonstrate that the Signal-Noise Ratio (SNR) is improved, specifically more than 6.39 dB, with the root mean square error (RMSE) significantly reduced. This method proves to be effective to eliminate the DC drift in the strain signal and achieve the purpose of noise reduction.

Abstract:To enhance the stability and speed of surface crack detection, a detection algorithm combining PeleeNet and YOLOv3 is proposed. PeleeNet framework is used to replace Darknet-53 framework of YOLOv3 so as to effectively integrate different local features and improve the detection rate. The feature attention module is integrated into the PeleeNet’s framework to highlight the saliency of the crack detection in the image, and through the receptive field module RFB broaden the effective field of view of the network, and increase the detection accuracy of small targets. Instead of the standard convolution, the depth separable convolution is employed to reduce the amount of parameter calculation in the feature pyramid network. Then, the CIoU loss function is introduced to strengthen the classification and regression accuracy of the model. The experimental results on the fracture data set show that AP₅₀ and AP₇₅ reach 97.68% and 77.87% respectively, 8.4% and 12.4% higher than the original YOLOv3. Meanwhile, the detection speed reaches 30 frames per second with the size of model parameters being only 30% of YOLOv3. As can be seen, the PeleeNet_yolov3 lightweight model proposed has produced an obvious effect on the detection of crack targets, with a small amount of calculations and parameters involved. Suitable for mobile terminal system, the study presents a great value of application especially for small volume, low power consumption and low computing power computing platforms.

Abstract:To deal with the asynchronous problem of system modal and controller modal of heterogeneous multi-agent systems caused by complex network attacks, physical limitations and other factors, ensure the consistency of multi-agent systems, and improve the security of system operation. This paper proposes a design method of output feedback controller based on dissipative performance. Aiming at the situation that the state cannot be measured in the actual project, a more practical output feedback control is adopted, and a distributed dynamic compensator is designed. Combined with the output adjustment technology, the heterogeneous multi-agent system is constructed as a closed-loop error system. The hidden Markov model is used to describe the asynchronous phenomenon between multi-agent system and controller, and a double-chain Markov jump model is formed. The stochastic stability and strictly (D,E,F)-α dissipativity conditions of closed-loop error system are given by the Lyapunov stability analysis method, and the output consistency of heterogeneous multi-agent system is realized. Furthermore, a gain design method is obtained by transforming the gain design of asynchronous controller with dissipative properties into a set of feasible solutions of linear matrix inequalities. Finally, a simulation example verifies the effectiveness of the control algorithm proposed in this paper. The results show that compared with the existing results, the dissipative asynchronous controller based on output feedback designed in this paper has good compatibility with various multi-agent systems. At the same time, there are still many technical problems that need to be solved urgently. With the development of science and technology, smoother network communication methods and sensitive sensor networks can be used as references for breakthroughs of key issues in the field of multi-agent collaborative control in the future.

Abstract:To improve the overall comfort of manned closed cabin, enhance the physiological and psychological comfort and work efficiency of operators, an in-depth study on evaluating the comfort level of manned closed cabin is conducted. Based on the comprehensive comfort evaluation of the cabin, the basic events affecting the comprehensive comfort of the cabin are identified with their values ranked in the order of importance so as guide the design of the manned closed cabin in a highly specific way. A fault tree analysis (FTA) model was advanced for the comfort of the manned closed cabin. The influencing factors of the comfort of the manned closed cabin were divided into three intermediate grades: physiological environment, physical factors and subjective feelings, with 17 basic events were extracted. The fuzzy Bayesian networks (FBN) was used for forward diagnosis to evaluate the overall comfort of manned closed cabin, and reverse causal inference was performed to identify the main causes for the discomfort of the cabin. In the case study, the overall comfort level of the manned closed laboratory of Northwestern Polytechnical University was evaluated, the validity and reliability of the FTA-FBN comfort evaluation approach for manned closed cabin was verified by the subjective evaluation of 16 participants. The influence probability of each basic event on the overall comfort of the cabin was calculated by backward reasoning and sorted. The improvement direction of the future closed cabin design was suggested, providing a fresh insight into the comfort evaluation of manned closed cabin.

Abstract:To solve the problem of overfitting with limited labeled samples for surface object detection based on semi-supervised learning and enhance the effectiveness of extracting objects from unlabeled samples, a semi-supervised surface object detection algorithm based on multi-view cross-consistency learning is proposed. First, this algorithm generates different views for training samples through data augmentations to enrich the diversity of the dataset. Then a multi-view target discriminator is advanced to generate pseudo-labels online for the unlabeled samples, extracting useful information from unlabeled samples. Finally, the multi-view cross-consistency learning is implemented to achieve cross-consistency regularization between the outputs of different views of the same instance, prompting the detection model to learn discriminant features and mitigate the risk of overfitting. The experimental results at maritime and inland rivers show that the proposed algorithm improves the discrimination of feature extraction. The detection accuracy of multi-category surface objects reaches 90.1%, 18.7% higher than the full supervised detection algorithm and over 3.8% higher than other semi-supervised detection algorithms. Regarding detection speed, the algorithm reaches 13.1 frames per second, basically meeting the real-time requirements. The algorithm through multi-view cross-consistency learning tends to improve the discrimination of features and reduce the overfitting risk of the detection model, with the performance of semi-supervised surface object detection optimized.

Abstract:To evaluate the reliability of space reusable locking device performing locking separation actions on orbit, aiming at the combined effect of spacecraft rendezvous and docking shock and performance degradation in the space environment, a reliability evaluation model considering interdependent competitive failure was established. Firstly, based on the function, structure and working principle of the device, the reliability characteristics were determined by the main fault modes on orbit. Secondly, considering both extreme and cumulative shock, a performance degradation model under shock was constructed. In view of the characteristics of small sample, high reliability and long life, accelerated degradation test and Bayesian method were implemented to estimate model parameters. Then, a reliability evaluation model was attempted based on the interdependent competitive failure theory of shock and degradation processes. Finally, taking a space reusable locking device as an example, the on-orbit shock simulation test and acceleration performance degradation test of the device were carried out. The model parameters were estimated by integrating the historical test data of similar products according to Bayesian formula, and the reliability function curve of the device was obtained. The result shows that in line with engineering practice, the device can still operate with a high reliability of 0.973 after 25 years on orbit. The proposed method and model proves to be capable of following closely the working conditions, and conducting a comprehensive reliability evaluation on the basis of the test and data available, providing an effective technical approach for mission reliability analysis and a theoretical basis for the design improvement and formulation of use strategy.

Abstract:A differential game guidance law for bearings-only measurements is proposed to improve the estimation performance of the missile guidance information. The situation of line of sight angles only or bearings-only measurements can be generated from disturbed or passive strapdown seeker of missile. First, through the analysis of the game space of the norm differential game guidance law, it is clear that the characters of the capture zone can be used to improve the observability of bearings-only measurements. Secondly, on the basis of completing the design of bearings-only estimator, the relationship between missile commands and observability is established by using the character of estimation mean square error reflecting the observability. Then, in the capture zone of the differential game space, guidance control commands are selected to guarantee the miss distance and improve the observability. Lastly, the execution process of the proposed differential game guidance law for enhancing observability is presented, and the zero-effort miss distance parameter and the time-to-go parameter are designed for the consideration of the control system delay, principle of miss distance control priority and other external uncertainties. This approach of guidance law design breaks through the frame of certainty equivalence principle, and realizes the integrated design of filter and guidance law. The nonlinear simulation comparison shows that the proposed guidance law can improve estimation performance of the target information and has higher single-shot kill probability.

Abstract:To improve the surface tribological properties of TC4 titanium alloy, three kinds of micro textures have been processed on the surface of TC4 titanium alloy by picosecond ultraviolet laser technology. The tribological properties of the textured TC4 titanium alloy under multi-contact conditions were investigated by multifunctional friction and wear testing machine. The surface hardness, surface roughness, three-dimensional profile and morphology of wear marks of textured TC4 titanium alloy were analyzed by microhardness tester, scanning electron microscope and laser confocal microscope. The results reveal that the surface hardness of textured TC4 surface increases by about 60%, and the triangular textured surface shows the highest hardness. Micro texture effectively reduces the friction coefficient of the TC4 surface. The circular and rectangular textured surfaces indicate the lowest friction coefficient, about 10% lower than that of non-textured surface. Capable of capturing the wear debris, the micro texture can reduce abrasive wear and improve wear resistance. Under the same contact conditions, the wear volume of textured samples is reduced by about 50% compared with no-textured surface. When the load is constant, the friction coefficient of textured TC4 surface decreases with the increase of velocity. Under the same wear speed, the lower load leads to a decrease of friction coefficient on the textured TC4 surface. This study presents an effective way to improve the tribological properties of titanium alloy, providing an alternative way to reduce the loss and failure caused by the friction and wear on titanium alloy.

Abstract:Water column separation occurs when the water pressure increases to vapor pressure during hydraulic transients in long distance water conveyance pipelines. Abnormal high pressure caused by water columns rejoining can lead to pipe vibration, deformation, and even explosion accidents. The existing mathematical models of water column separation and rejoining water hammer are mostly solved by the method of characteristics (MOC), and rarely consider the energy attenuation caused by unsteady friction factors. In order to improve the computational accuracy and stability of water column separation and rejoining, second-order Godunov scheme of finite volume method (FVM) was introduced to solve the discrete gas cavity model (DGCM) with unsteady friction factor. The virtual boundary method was proposed to realize the unified calculation of pipe boundary and internal nodes. The simulation results of the proposed model were compared with the experimental data and the calculated results from the existing steady friction model. The sensitivity of parameters including mesh number and pressure correction coefficient was analyzed. Results show that the proposed model was capable of accurately simulating the transient pressure in the cases of both pure water hammer and water column separation and rejoining water hammer, which was basically identical with the experimental data. The calculated transient pressure considering the unsteady friction factor was more consistent with the experimental data. Compared with MOC, when the Courant number was less than 1.0, the transient pressure calculated from the proposed model was more accurate and stable. In particular, the mathematical model could more accurately reproduce the experimental results when the pressure correction coefficient was 0.9 and finer mesh was used.

Abstract:Light transmission efficiency (LTE) is the most important parameter to evaluate the daylighting performance of light pipes. In the calculations of LTEs of light pipes, the cumulative effect of light polarization caused by several specular reflections in pipes is often neglected. Therefore, in order to make clear the cumulative effect of light polarization generated by continuous specular reflection, through optical theoretical analysis and formula derivations, after determining the influence of light polarization on the reflectivity of specular reflection materials, this article for the first time clarified the change rule of the reflectivity of interior surface of a light pipe with the number of reflections and the influence of the variation of reflectivity on the LTE of the light pipes. The results indicated that each time a light beam projects on the pipes inner surface, there will be a light polarization that produces different reflectivity on the two orthogonal components, i.e ρ⊥ and ρ‖ and ρ⊥≥ρ‖; In every reflection, the values of ρ⊥ and ρ‖ remains fixed, but the overall reflectivity will gradually rise with the increase of reflection times, so the light loss rate will continue to slow down. Finally, the LTE expression of a single beam in a light pipe is derived, which lays a theoretical foundation for the establishment of the LTE mathematical calculation models under complex light sources.