Bayesian Linear Regression with Cauchy Prior and Its Application in Sparse MIMO Radar

In this paper, a sparse signal recovery algorithm using Bayesian linear regression with Cauchy prior (BLRC) is proposed. Utilizing an approximate expectation maximization(AEM) scheme, a systematic hyper-parameter updating strategy is developed to make BLRC practical in highly dynamic scenarios. Remarkably, with a more compact latent space, BLRC not only possesses essential features of the well-known sparse Bayesian learning (SBL) and iterative reweighted l2 (IR-l2) algorithms but also outperforms them. Using sparse array (SPA) and coprime array (CPA), numerical analyses are first performed to show the superior performance of BLRC under various noise levels, array sizes, and sparsity levels. Applications of BLRC to sparse multiple-input and multiple-output (MIMO) radar array signal processing are then carried out to show that the proposed BLRC can efficiently produce high-resolution images of the targets.Comment: 22 page

Safe passing critical criterion for drawn top-coal on rear conveyor and accurate control approach for drawing opening dimension

Fully mechanized top coal caving technology has become the mainstream way of high yield and high efficiency mining in extra thick coal seams in China. The accurate control of the top-coal drawing mechanical parts is of significance to realize the automation and intellectualization of top-coal caving mining. Mastering the spatial motion law of the coal caving mechanism is the premise of accurate control. The immediate shape of the hydraulic support coal caving mechanism is jointly controlled by the support height, support attitude, extension length of the plug plate, and the relative position of the rear scraper, which has an important impact on the coal caving opening and the coal-passing height of the support. This study establishes a 3–D numerical model of four- legs top-coal caving hydraulic support (No. ZF15000/27.5/42) by using the finite element software ABAQUS. Hinge and translator connectors are used to simulate the rotation behavior and expansion-contraction behavior for hinge point and plug plate, respectively. Taking the support height (H), tail beam swing angle (α), and the plug plate extension length (l) as control variables, the spatial motion law of the hinge point between shield beam and tail beam and the end of the plug plate are modeled. The critical security equation for evaluating collision between top-coal drawing mechanical parts and rear scraper is obtained by using Levenberg-Marquardt fitting iteration method. A database for describe the calibration relationship, which contains the end coordination of plug plate, the dimension of the top-coal drawing opening, and H, α, and l, is established. The sensor type and installation position for sensing and controlling the attitude of the top-coal drawing mechanism are recommended, the approach for calculating the tail beam angle based on travel sensor is derived. Through field verification of top-coal drawing opening width, it is concluded that the relative error between measured value and calculated value meets the requirements for accurate control of top-coal drawing mechanism. The approach for controlling the top-coal drawing opening dimension is proposed, which has been successfully applied in the field

Numerical Simulation of Hydraulic Fracture Propagation in Coal Seams with Discontinuous Natural Fracture Networks

To investigate the mechanism of hydraulic fracture propagation in coal seams with discontinuous natural fractures, an innovative finite element meshing scheme for modeling hydraulic fracturing was proposed. Hydraulic fracture propagation and interaction with discontinuous natural fracture networks in coal seams were modeled based on the cohesive element method. The hydraulic fracture network characteristics, the growth process of the secondary hydraulic fractures, the pore pressure distribution and the variation of bottomhole pressure were analyzed. The improved cohesive element method, which considers the leak-off and seepage behaviors of fracturing liquid, is capable of modeling hydraulic fracturing in naturally fractured formations. The results indicate that under high stress difference conditions, the hydraulic fracture network is spindle-shaped, and shows a multi-level branch structure. The ratio of secondary fracture total length to main fracture total length was 2.11~3.62, suggesting that the secondary fractures are an important part of the hydraulic fracture network in coal seams. In deep coal seams, the break pressure of discontinuous natural fractures mainly depends on the in-situ stress field and the direction of natural fractures. The mechanism of hydraulic fracture propagation in deep coal seams is significantly different from that in hard and tight rock layers

Experimental Enrichment of Low-Concentration Ventilation Air Methane in Free Diffusion Conditions

The massive emission of low concentrations (≤0.5%) of methane (CH4) from ventilation roadways results in resource waste and environmental pollution. To mitigate these emissions, an enrichment tower for low-concentration methane is designed, and segregation and non-segregation experiments are conducted. The results reveal that stable concentrations of methane under segregation and non-segregation states in the enrichment tower gradually increase with height, with a maximum methane concentration of 0.64% and 0.54%, respectively. This shows that the methane enrichment effect in free diffusion conditions is more significant under the segregation state than under the non-segregation state. The stable concentration of methane in the middle and upper sections of the enrichment tower shows an increasing trend. However, the stable concentration of methane in the lower section of the enrichment tower has an increasing trend (less than 0.50%). According to the methane molecule Boltzmann distribution law, methane concentration enrichment decreases with height, and the conversion of the methane from the segregated to non-segregated is irreversible. Consequently, industrial applications of methane enrichment from buoyant forces are not feasible for low concentrations of methane

Strength and Failure Characteristics of Natural and Water-Saturated Coal Specimens under Static and Dynamic Loads

Rock bursts occur frequently in coal mines, and the mechanical properties of saturated coal specimens under coupled static-dynamic loading need to be studied in detail. Comparative tests of coal specimens having different water content under static and static-dynamic loading are conducted using the split Hopkinson pressure bar (SHPB) and RMT-150C test systems. The results demonstrate that the natural specimen strength is greater than that of seven-day (7D) saturated specimens under both uniaxial compression and triaxial static compression loading; however, the dynamic strength of 7D saturated specimens is lower than that of natural specimens under one-dimensional static-dynamic loading. The particle size of the 7D saturated specimens is relatively small under uniaxial static compression and one-dimensional static-dynamic loading, and the specimen particle sizes before and after static triaxial loading tests and three-dimensional static-dynamic loading tests do not exhibit an obvious difference

Experimental Study on Strength, Acoustic Emission, and Energy Dissipation of Coal under Naturally and Forcedly Saturated Conditions

Coal seam water injection (CSWI) is an effective technology that is widely used for preventing rock burst in coal mines. To deepen the understanding of the mechanism of CSWI to prevent rock burst, new equipment was designed to prepare forcedly saturated coal samples in this study and a series of mechanical experiments was conducted to investigate the mechanical properties, acoustic emission (AE), and energy dissipation characteristics of the coal samples in natural, naturally saturated, and forcedly saturated states. The experimental results show that the forced saturation treatment can significantly improve P-wave velocity and water content of coal samples, as water can penetrate more into micropores and fractures. The forced saturation method also significantly promotes the deformation capacity of the coal sample and reduces the strength by 83.37%. The main reason of the bearing capacity decrease for the forcedly saturated coal samples is plastic yielding rather than brittle crack propagation and slip. The derivative of the volumetric dissipation energy was proposed to evaluate the outburst proneness. The forced saturation method significantly reduces the risk of sudden release of energy and is more effective in preventing rock burst in coal seams than the natural saturation method

LVRT capability enhancement of DFIG based wind turbine with coordination control of dynamic voltage restorer and inductive fault current limiter.

According to the coordination control of a dynamic voltage restorer (DVR) and an inductive fault current limiter (FCL), this paper proposes an efficient low-voltage ride-through (LVRT) scheme for a doubly fed induction generator (DFIG) based wind turbine. The DVR is located to the DFIG's stator circuit for stabilizing the terminal voltage and decreasing the generator current. The inductive FCL is connected to the DFIG's rotor circuit for suppressing the rotor overcurrent and protecting the converter. Theoretical discussions on structure, principle and scale criterion of the combined DVR-FCL are conducted, and simulation analyses of the proposed approach to handle symmetrical and asymmetrical faults are done in MATLAB/Simulink. In this study, the dynamic characteristics of the DFIG during the faults are analyzed from multiple aspects, and a detailed comparison of the proposed approach and the single action of DVR or FCL is carried out. From the simulation results, the effectiveness and superiority of the proposed approach are well demonstrated

Deep Q-Network for Optimal Decision for Top-Coal Caving

In top-coal caving, the window control of hydraulic support is a key issue to the perfect economic benefit. The window is driven by the electro-hydraulic control system whose command is produced by the control model and the corresponding algorithm. However, the model of the window’s control is hard to establish, and the optimal policy of window action is impossible to calculate. This paper studies the issue theoretically and, based on the 3D simulation platform, proposes a deep reinforcement learning method to regulate the window action for top-coal caving. Then, the window control of top-coal caving is considered as the Markov decision process, for which the deep Q-network method of reinforcement learning is employed to regulate the window’s action effectively. In the deep Q-network, the reward of each step is set as the control criterion of the window action, and a four-layer fully connected neural network is used to approximate the optimal Q-value to get the optimal action of the window. The 3D simulation experiments validated the effectiveness of the proposed method that the reward of top-coal caving could increase to get a better economic benefit

Study of SCA-Induced Rock Crack Propagation under Different Stress Conditions Using a Modified Cohesive Element Method

When inducing cracks, soundless cracking agents (SCAs) do not generate vibration, harmful gas, dust, nor flying rock fragment, making them suitable for hard rock roof breaking, rock burst prevention, oil or gas reservoir stimulation, and building demolition. In this study, SCA-induced crack initiation and propagation in different stress conditions were modelled using a modified cohesive element method. A new traction-separation law for describing rock compressive shear strength was proposed. The crack length and direction in bidirectional isobaric and unequal stress fields were analyzed in detail. The crack initiation pressure and the incremental ratio of crack length to SCA expansion pressure were proposed as two indicators to evaluate the difficulty in rock breaking in deep underground. Results indicate that (1) the modified cohesive element method used in this study is feasible to model crack propagation in deep rocks; (2) the maximum expansion pressure of SCAs depends on rock elastic modulus and geostress field and should be measured under a condition similar to deep underground prior to SCA borehole spacing design; when using the SCAs with a maximum expansion pressure of 100 MPa in 600 m underground, the suggested borehole spacing is less than 220 mm; and (3) σ3 dominates the crack initiation pressure while the principal stress ratio σ3/σ1 and notch direction control the direction of crack propagation