Coal is an essential fossil fuel in China; however, coal mining and its utilization are being under the increasing pressure from ecological and environmental protection. Therefore, the consulting ...project “Technical Revolution in Ecological and Efficient Coal Mining and Utilization & Intelligence and Diverse Coordination of Coal-based Energy System,” initiated by Chinese Academy of Engineering, puts forward three stages (3.0, 4.0 and 5.0) of China’s coal industry development strategy. Aimed at “reduced staff, ultra-low ecological damage, and emission level near to natural gas,” breakthroughs should be achieved in the following three key technologies during the China Coal Industry 3.0 stage (2016–2025): including intelligent coal mining, ecological mining, ultra-low emission and environmental protection. This paper focuses on the development trends of the China Coal Industry 3.0 and its support for China Coal Industry 4.0 and 5.0 is analyzed and prospected as well, which may offer technical assistance and strategy orientation for realizing the transformation from traditional coal energy to clean energy.
This paper aims to clarify global and local force harmonics and their effects on vibration in permanent magnet (PM) synchronous machines. The local force includes both the tangential and radial ...forces distributing on the stator bore while global force refers to cogging torque and torque ripple. First, the expressions of local tangential and radial forces are derived based on the periodicity of magnetic field. The relationship between local tangential force and global force is also established. Then, the work continues with the effects of global and local forces on vibration. It is found global force may induce remarkable lateral motion when the mounting stiffness is low, while the main combined effect of local radial and tangential forces is inducing radial vibration. At last, influence of reducing global force on the vibration is investigated. It is shown even the vibration induced by global force itself does not always decrease, because this sort of vibration depends on both the harmonic components of optimized global force and the specific mounting conditions. The influence on vibration induced by local force is determined by the overall variation of flux density harmonics. Moreover, it is shown that PM arc shaping is a promising candidate for vibration mitigation.
For the purpose of reducing cost and volume, techniques of reconstructing three-phase currents through a single current sensor have been reported for permanent magnet synchronous motor vector control ...system. In existing studies, the reconstruction precision is largely affected by the dead zones in space vector PWM plane, which requires additional efforts to compensate the dead zones either by modifying pulse width modulation (PWM) modulation strategy or by phase-shifting of PWM signal. In this paper, a novel zero voltage vector sampling method (ZVVSM) is proposed, which can move the current reconstruction dead zones in low modulation region and sector boundary regions toward the outline of the space vector hexagon without modifying PWM signal. By arranging the single current sensor at a novel position, the proposed method is able to sample current in two zero voltage vectors (ZVV). ZVVSM avoids the complicated algorithms as well as the increase of the switching times, so that it is beneficial to the PMSM drive performance. The proposed method is validated by both simulation and experiments.
•Self-crosslinking chitosan hydrogel was fabricated using PBS and NaCl.•Chitosan hydrogel formed only at particular temperature and concentration.•The properties of the hydrogel depended on the ...preparation conditions.
Self-crosslinking chitosan hydrogels are a highly suitable material for biomedical applications owing to their biodegradability and biocompatibility. However, strong bases, such as sodium hydroxide, which are often used in the preparation of such hydrogels, are known to affect biocompatibility and even destroy the bioactive factors or drug payload of the hydrogel. In the present study, strong bases were replaced by sodium chloride (NaCl) and phosphate buffer saline (PBS, pH=7.4), which were used as gelling solutions for hydrogel fabrication via the freeze-melting-neutralization method. Non-cytotoxicity was showed in MTT assay for hydrogel. Our findings suggest that hydrogel microstructure and physical properties may be adjusted by modifying parameters, such as concentration, temperature, and pH, during the gelling process. Furthermore, the present hydrogel was found to exhibit pH-and ionic strength-responsive properties and may be utilized as a stimulus-responsive material for biomedical applications such as controlled drug release.
Additive manufacturing (AM) zirconia shows excellent prospects for use in clinical applications. In this work, AM zirconia samples were fabricated in horizontal (H) and upright (U) fashion using a ...stereolithography appearance (SLA) system. The dimensional accuracy, density, translucency, surface quality, flexural strength and fracture toughness (KIc) of the samples were then assessed. AM zirconia fabricated in a H fashion shows excellent dimensional accuracy. Samples fabricated in a U fashion exhibit a higher density (relative density 95.4%) and translucency (4.393), but H group samples exhibit a higher KIc value (12.635 ± 1.372 MPa m1/2). The flexural strengths of the samples were measured and the values were compared according to their different build orientations, surface quality and fracture modes. Manual defects that arose in the samples as a result of their separation from the build platform in which they were made were found to lead to samples with irregular surface morphologies and increased surface roughness. However, this type of defect does not affect the flexural strength of samples fabricated in a H fashion. Polished-H samples that fracture from the stress concentration area exhibit the highest flexural strength (1151.08 ± 166.41 MPa) amongst all the samples prepared in this work. However, the flexural strength of the samples prepared in a U fashion is obviously low, even after polishing (225.44 ± 46.10 MPa). The Weibull characteristic strengths and Weibull moduli of the as-sintered samples are 920.22 MPa and 6.50 for H and 219.59 MPa and 7.99 for U, respectively. Overall, it was found that the dimensional accuracy, density, translucency, surface quality and mechanical properties of materials vary according to their different build orientations.
In the context of distributed generation and renewable energy penetration toward smart grid, grid-connected inverter with LCL filter has drawn many attentions, whose current control conventionally ...requires several sensors to realize active damping and grid synchronization. In this paper, a novel full status observation strategy based on extended state observer (ESO) is proposed using inverter current feedback only and without grid voltage sensor. The proposed observation strategy contains observation and transformation process. Unlike conventional Luenberger observer, by using ESO, the system parameters do not appear in observation process, thus the observer dynamics and parameter mismatch error can be separately handled, providing more robust observation dynamics during parameter variation. Parameter mismatch study was carried out, and it is found that purposely choosing the observer parameters smaller than the real value can achieve relatively low estimation error and a large adaption range for parameter variation. The proposed observation strategy is simple to implement, without the need of expert knowledge-based parameter tuning such as pole placement. Experimental tests validated that the proposed observation-based control is able to give satisfactory performance in both dynamic and steady states, as well as adaption for system parameter variation.
Commutations existing in a brushless dc motor (BLDCM) drive system not only aggravate the torque ripple and reduce the power factor but also cause the catastrophic failure of a sensorless drive based ...on the zero-crossing detection of back electromotive force when the commutation duration approaches or exceeds 30 electrical degrees. For a sensorless BLDCM drive system with a long commutation time, it is necessary to shorten the commutation time. In this paper, a technique making the commutation time significantly reduced by means of a novel inverter topology is presented. The proposed topology is based on the idea of supplying higher dc-link voltage for the freewheeling phase. First, the dc link of the backward freewheeling path is decoupled from that of the forward energy flow path. The superiority of dc-link decoupling for commutation control is analyzed. Then, in order to provide adjustable voltage higher than the dc source voltage for the dc link of the freewheeling path, a double-layer C-dump converter is proposed. Compared with the traditional unipolar C-dump converter, it not only upraises the potential of the positive rail but also lowers the potential of the negative rail. Finally, the proposed inverter topology is verified by simulation and experimental results.
This paper deals with equivalent 2-D finite-element (FE) models of permanent magnet lead screw (PMLS), to accurately predict the thrust force. Due to the helical-shape permanent magnets (PMs), the ...magnetic fields of PMLS are typically 3-D and non-symmetric. However, the PMLS is approximately symmetric in some cases and an equivalent 2-D axis-symmetric FE model is developed instead of 3-D models. The problem lies in that no evaluation about the accuracy and applicability of the 2-D axis-symmetric FE model was offered in previous literature. In this paper, the 2-D axis-symmetric FE model is derived based on the concept of equivalent current sheets of PMs. The limitations of the existing 2-D axis-symmetric FE model are clarified, and a novel equivalent 2-D torque FE model is then proposed to predict thrust force with the gear ratio when the 2-D axis-symmetric FE model is not appropriate. It is shown that the choice of equivalent 2-D FE model depends on the lead angle of PMLS, 2-D axis-symmetric FE model is applicable in the range of (0°, 15°), and 2-D torque FE model is preferred in the range of (65°, 90°).
Although conventional slotted limited-angle torque motors (LATMs) have a high torque density, its fatal shortcoming of non-uniform torque profile makes it difficult to apply for precise positioning ...systems. This paper presents a novel radial-flux slotted LATM with asymmetrical teeth for the torque performance improvement including average output torque and angular operation range. A magnetic equivalent circuit (MEC) for radial-flux slotted LATMs is first derived and implemented in the design process. Then, the finite-element analysis is utilized to validate the related analytical equations and to make the comparison of the torque performance between the radial-flux conventional and proposed LATMs. Finally, a prototype machine of the proposed LATM is accomplished, and its torque characteristic experiment is carried out to confirm the torque performance improvement. The related experimental results are presented and discussed.
Abstract
Background
When evaluating the efficacy and safety of various desensitizing products in vitro, their mechanism of action and clinical utility should be considered during test model ...selection. This study aimed to evaluate the effects of two desensitizers, an in-office use material and an at-home use material, on dentin specimen permeability, and their dentin barrier cytotoxicity with appropriate test models.
Methods
Two materials, GLUMA desensitizer (GLU) containing glutaraldehyde and remineralizing and desensitizing gel (RD) containing sodium fluoride and fumed silica, were selected. Human dentin specimens were divided into three groups (n = 6): in groups 1 and 2, GLU was applied, and in group 3, RD was applied and immersed in artificial saliva (AS) for 24 h. Dentin specimen permeability before and after each treatment/post-treatment was measured using a hydraulic device under a pressure of 20 cm H
2
O. The perfusion fluid was deionized water, except in group 2 where 2% bovine serum albumin (BSA) was used. The representative specimens before and after treatment from each group were investigated using scanning electron microscopy. To measure cytotoxicity, test materials were applied to the occlusal surfaces of human dentin disks under which three-dimensional cell scaffolds were placed. After 24-h contact within the test device, cell viability was measured via 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays.
Results
GLU significantly reduced the dentin permeability and occluded the dentinal tubules when 2% BSA was used as perfusion fluid. RD significantly reduced dentin permeability and occluded the tubules, but permeability rebounded after AS immersion. GLU significantly decreased cell viability, but RD was non-cytotoxic.
Conclusions
In vitro GLU application induced effective dentinal tubule occlusion only following the introduction of simulated dentinal fluid. RD provided effective tubule occlusion, but its full remineralization potential was not realized after a short period of immersion in AS. GLU may harm the pulp, whereas RD is sufficiently biocompatible.
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