Jo Sep 30, 2024
Trajectory planning is one of the most important problems in robotics. It refers to generation of position commands, velocity, and acceleration of all degrees of freedom of a robot.
Pick-and-place trajectory planning is finding a smooth and continuous trajectory from the starting position to the desired terminal position within the workspace of a robot.
Many studies have been conducted for effective trajectory planning of a robot. However, previous studies revealed some drawbacks in the aspect of minimization of workspace, especially the equations of velocity and acceleration/deceleration with little information of the path, and therefore, the equations were not directly related to the geometry of the generated path.
To overcome such shortcomings, Choe Myong Song, a researcher at the Faculty of Mechanical Science and Technology, has developed an asymmetric bisect elliptical trajectory planning with minimum workspace suited to the geometry of the real-time path for pick-and-place operation in which both the picking points and placing points are changed randomly in the workspace.
First, he proposed an optimal asymmetric bisect elliptical path with two vertical straight line segments and a method of generating it. Then, he presented a method of generating the optimal motion profile suited to geometry of the elliptical path based on the radius of curvature. Finally, he conducted a simulation to demonstrate the effectiveness of the proposed method.
He drew the following conclusions.
The proposed motion profile based on the radius of curvature of the path is pretty suited to the geometry of the optimal elliptical path, and therefore, its velocity, acceleration and jerk are continuous and very smooth, which enables smoother pick-and-place robot operation. In addition, the proposed trajectory planning approach reduces a cycle period and ensures smoothness of working of robot actuators and real-time transhipping.
For further details, please refer to his paper “An Approach for Elliptical Trajectory Planning with Vertical Straight Line Segments of Pick-and-Place Robot Operation with Height Clearance” in “Mathematical Problems in Engineering” (SCI).
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Jo Sep 27, 2024
Nowadays, global warming is intensifying more and more and fossil fuels are running out all over the world, which leads to increasing numbers of efforts for protecting the environments against green house effects and for using natural energy.
In the production of electric power using natural energy, electrical power fluctuation occurs due to the natural conditions such as luminous intensity, wind speed, tidal current, etc. Therefore, it is necessary to make up for electrical power fluctuation for stabilizing the power supply. Compensation for electrical power fluctuation can be carried out by secondary batteries or a flywheel energy storage system (FESS). Since the FESS using the SMB has longer life time than secondary batteries, it has already been applied in several countries.
The important problems of the FESS are to heighten load capacity, to decrease loss and to raise stability. These problems are ascribable to the improvement of the performance of SMB.
A number of basic experiments were performed to design the SMB and to improve its performance, and the FESSs with SMBs of different load capacities and structures were developed. On the other hand, numerical simulations on SMB were conducted by MATLAB, FORTRAN and COMSOL Multiphysics.
Some researchers presented a way to increase the levitation force using opposite magnetization effect through experiments and simulations, and obtained 170.14N in maximum levitation force. However, they failed to intend optimization on it.
Jo Ju Hak, a researcher at the Faculty of Physical Engineering, has simulated a three surface levitation-superconducting magnetic bearing (TSL-SMB) in two-dimensional axisymmetric system using H-formulation and Taguchi method.
The simulation was conducted by field cooling (FC) method. The effects of geometric parameters on levitation force in TSL-SMB were considered using Analysis of Variance (ANOVA). He concluded that in the optimized TSL-SMB structure the effect of concentric ring PMs with opposite polarization did not play a decisive role. Therefore, he presented four-surface levitation-superconducting magnetic bearing (FSL-SMB) as advanced multi-surface levitation system to improve the performance of SMB.
The simulation results in the FSL-SMB system showed that the gap between the stator and the rotor and levitation force per permanent magnet volume might be increased more than those of TSL-SMB system in the condition of same load capacity.
For more information, you can refer to his paper “Simulation on modified multi-surface levitation structure of superconducting magnetic bearing for flywheel energy storage system by H-formulation and Taguchi method” in “Physica C: Superconductivity and its applications” (SCI).
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Jo Sep 25, 2024
Graphite has chemical inertness and corrosion resistance as well as good thermal and electrical conductivity. At present, flake graphite, the most common graphite form, has a high world market share. Crystalline flake graphite is a plate-like particle of hexagonal structure with irregular fragment edges. Graphite has a layered structure, in which carbon atoms in the monoatomic layers share a strong covalent bond, but the relatively weak π-π bonds connect the layers.
In the graphite structure, gangue minerals have the characteristics of accumulation between layers, stacks or clusters. These impurities can coexist with the graphite flake, either adhering to the surface of the graphite flake or being trapped by adjacent graphite flakes. Therefore, it is essential to study the release of impurities from graphite.
Ri Ho Chol, a section head at the Faculty of Mining Engineering, has succeeded in converting graphite flotation concentrate GFC into high purity graphite from laboratory scale to pilot scale through flotation, alkali roasting and acid leaching processes.
In GFC, carbon content was 85.6%, and quartz and hematite were the main gangue minerals. The high-grade flotation concentrate with 95% carbon content was produced by low pulp concentration flotation. And the silicate mineral was converted to the soluble salt at 500℃ by alkali roasting. The soluble roasting products and other impurities such as hematite were dissolved by acid, and the carbon content of the produced high purity graphite was higher than 99.8%.
You can find the details in his paper “Effective purification of graphite via low pulp concentration flotation-low temperature alkali roasting-acid leaching route: From laboratory-scale to pilot-scale” in “Minerals Engineering” (SCI).
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Jo Sep 23, 2024
An important problem arising in underground excavation is stabilizing the country rock that surrounds openings at depth. An increase in the in situ rock stresses is the essential difference between rocks at depth and near the surface. As a consequence of such an increase in the ground stress, rock burst may be of common occurrence in hard rocks, or large squeezing deformations may appear in soft and weak rocks. It has been detected in lots of mines that such phenomena begin to occur at the depth of about 600–800m below the surface level and become more significant below 1 000m. At these depths, conventional support devices may not be adapted for severe rock conditions.
Over the years, many researchers have tried to develop various ground support techniques and products for support and retention of the newly exposed faces and internal reinforcement of the soil and rock masses surrounding the excavations. One of them is a D bolt.
A D bolt is a smooth steel bar with a number of anchors along its length. The anchors, which can be spaced evenly or unevenly along its length, are firmly fixed within a borehole using either cement grout or resin, while the smooth sections of the bolt between the anchors may freely deform in response to rock dilation.
Pyon Kwang Nam, a section head at the Faculty of Mining Engineering, has carried out a numerical simulation of D bolt, a type of energy-absorbing rock bolt which is not fully encapsulated but multipoint anchored in a fully grouted borehole. Then, he considered the influence of the spacing arrangement of its anchors on its axial stress distribution and determined reasonable structural parameters.
The numerical simulation results show that for the D bolt with its whole length of 2.4m, the length of its exposed section of 0.1m, 4 anchors with the length of 0.1m and the ratio of the spacing between anchors (RSA) of 30:40:50:70, the maximum tensile stress of 3.25GPa is generated, which is about 1.13–1.31 times lower than other D bolts with different ratio of spacing, and the changing range of stress is also the smallest. Here, the ratio of 30:40:50:70 indicates a ratio of lengths of deformable sections which is determined by turns from the innermost section of rock mass around roadway to the outermost section of roadway space.
For more information, please refer to his paper “Numerical Simulation Study on Influence of a Structural Parameter of D Bolt, an Energy-Absorbing Rock Bolt, on its Stress Distribution” Advances in Civil Engineering” in “Advances in Civil Engineering” (SCI).
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Jo Sep 22, 2024
The mechanical parameters and initial stress of coal have predominant influences on the stability of the surrounding coal of an entry in the coal seam. In practice, however, it is very difficult to determine the values of time-dependent soft coal mass parameters. To obtain these values of the time-dependent soft coal mass parameters based on very limited amounts of measured data, back-analysis is the most commonly used approach worldwide. Field measurement data may include displacements, strains, and stresses.
Because the displacements of a rock mass can be measured easily and reliably, back-analysis based on displacements has long been an active topic of research. The basic methods used in most of the studies have been numerical simulation methods and optimization algorithms. In these methods, numerical simulations were used to compute the displacements of the surrounding rock based on certain assumed values of the mechanical parameters and the initial stress. Then, by optimizing the error between the computed displacements and the measured displacements via an optimization algorithm, the initially assumed values were corrected into real values.
However, there are few papers that deal with back-analysis method based on timber load and displacement measured in the entry when the rock pressure and displacement of the entry increase continuously owing to the coal softening with time in very soft coal such as anthracite coal.
In order to ensure the accuracy in determining the mechanical parameters of time-dependent anthracite coal and to ease consideration and calculation in a set of algorithm, Sin Myong Nam, a researcher at the Faculty of Mining Engineering, has conducted a FLAC3D numerical simulation based on measured load and displacement, established a three-layer BPNN from it and found a methodology for predicting the mechanical parameters by SSA.
The proposed method can provide a useful basis for predicting time-dependent behavior of media surrounding the drift or chamber driven in such soft rock as the anthracite coal.
You can find the details in his paper “Determination of Mechanical Parameters of Anthracite Coal using Flying Squirrel Search Algorithm with Timber Load and Displacement Data” in “Journal of Mining and Environment” (EI).
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Jo Sep 20, 2024
GFRP, the polymer matrix composite, has been widely used in many industrial domains including the development of natural energy by wind, construction, ship-building, automobile, power and chemical industries, and it occupies the majority of the total production of composite materials all around the world.
VARTM, one of the Liquid Composite Molding (LCM) processes for producing large composite parts such as wind turbine blades or hulls at room temperature, is a useful manufacturing method for high fiber content, low cost of equipment, high material quality and low porosity. VARTM process usually uses room temperature cured resins such as unsaturated polyesters and room temperature-curable VARTM epoxy resin.
There are various kinds of methods of manufacturing GFRP pipes, for example, hand lay-up, filament winding, pultrusion, pre-deposited fabric winding, centrifugal casting, RTM, etc., whereby long or short pipes with different dimensions and properties for various fields are produced.
However, there are very few studies on the process for GFRP pipes by VARTM.
Sin Song Bom, a researcher at the Faculty of Materials Science and Technology, has investigated the effect of several parameters on the resin filling behavior when high temperature cured epoxy GFRP pipes undergo VARTM in the heating conditions, in association with ANSYS simulations.
He found that resin flow gets faster when it is at 60℃ for infusion and semi-curing, and room temperature VARTM materials such as PE vacuum films and other auxiliary material can be used.
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