Jo Mar 2, 2024
CMM, one of precision measuring instruments, is an important component of advanced manufacturing technology. Nowadays, CMMs are widely used in the fields of machine-building, automobile manufacturing, aerospace, flexible manufacturing system and computer integrated manufacturing system due to the advantages of strong versatility, high degree of automation and high precision.
However, CMMs have some weaknesses. They are large, expensive and not portable.
In comparison with traditional CMMs, AACMM offers many advantages such as large workspace and ease of use, high degree of freedom, portability and low cost. However, it also reveals some drawbacks due to its many articulated arms like bigger accumulated errors and lower measuring precision than CMMs.
In an attempt to improve the measurement accuracy of AACMM and reduce its manufacturing cost, Rim Chang Hyon, a researcher at the Faculty of Mechanical Science and Technology, has performed an error analysis and an optimum tolerance allocation.
Firstly, based on the analysis of error source of AACMM, he constructed a systematic error model.
Next, he calculated by the analytical and numerical methods the position error of measuring probe due to the errors of each structural parameter and that due to the systematic error of individual rotary encoders.
Lastly, from the perspective of processing and assembly, he established a mathematical model that converts the perpendicularity error of the AACMM into the arm’s length error, and performed a calculation about the optimal tolerance allocation of each structural parameter and then verified its accuracy.
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Jo Feb 29, 2024
The geometry of TICS has an influence on the output signal of sensors. This relation is determined by the cell constant given by the size of a sensor.
Several researchers demonstrated three methods to determine the cell constant of TICS― experimental method using standard solutions with known conductivity, single-loop calibration method and analytical method by rough approximation. They pointed out that in order to measure the lower ranges of conductivity which require a smaller cell constant, the diameter of sensor and measuring container must be enlarged.
But until now it has not been suggested to deal with the influence of the geometric parameter of a sensor on the cell constant and the regulation band of the cell constant with the size of a sensor. So, it is worthwhile to do some research on the relation between the cell constant and the size of a sensor.
Jang Hyon, a researcher at the Faculty of Electronics, has analyzed the relation between the geometry of a TICS (inner radius r1, outer radius r2 and height t of the sensor toroid) and its cell constant through simulations by the finite element method and experiments.
Taking account of actual conditions, he considered the sizes (r1, r2 and t) of a sensor within several tens of millimeters.
A key factor which influences the cell constant is the hole diameter (2r1) of the sensor toroid. The effective superior limit of inner radius of the sensor toroid that can give the smallest cell constant is about 30 mm.
The theoretical results on the cell constants of sensors in different sizes coincided with the experiment results with less than about 5% relative error.
The conductivity below limit value that can be adjusted by cell constant is about 0.36 times the lowest bound value of conductance measurement range.
You can find the details in his paper “Optimization of the cell constant for application of the transformer type inductive conductivity sensor (TICS) in low-conductive solution” in “IEEE Sensors Letters” (SCI).
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Jo Feb 28, 2024
Satellite-based free space optical quantum key distribution system is being considered as a very promising technique which is capable of overcoming the transmission limits of fiber-based QKD system and constructing a global quantum communication network.
It was demonstrated that laser beam has orbital angular momentum generated from spiral topological structure of itself in addition to spin angular momentum polarization property. In this case, unlike polarized quantum state, a photon may have a number of quantum states. Using optical orbital angular momentum as information carriers, information can be modulated in the infinite dimensional space. Furthermore, there is a possibility of increasing data throughput efficiently and enhancing the stability of system. Consequently, applying the orbital angular momentum to quantum key distribution, it is possible to increase the quantum key generation rate remarkably and this mode is suitable for free space optical quantum communication.
Some researchers considered polarization coding in FSO QKD system which is one of the good ways to realize quantum communication in free space. For convenience of analysis, they assumed that the air is in a steady state when QKD is performed between two parties in free space. However, the air is in a dynamic state of continuous flow rather than in a fixed state. It is known that there is a possibility of upgrading the performance of FSO QKD system when orbital angular momentum is applied to QKD.
O Ju Hyok, a researcher at the Faculty of Communications, has investigated more suitable QKD protocols for FSO quantum communication and analyzed the impact of atmospheric turbulence on quantum bit error rate of QKD system. He has also developed two QKD protocols suitable for FSO QKD system (one formed by combining random phase modulation with BB84 protocol and the other by combining entanglement modulation using photon orbital angular momentum with BB84 protocol) and analyzed the effects of atmospheric turbulence on the performance of FSO QKD system.
The simulation results demonstrate two facts. First, QBER of FSO QKD system increases with increase in air refraction index structural constant, that is, the intensity of atmospheric turbulence. Second, for FSO QKD, entanglement-based OAM QKD is more suitable than random phase modulation method.
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Jo Feb 26, 2024
Since the ocean covers over 70% of the earth’s surface and marine transportation has many advantages, it is continuously increasing and shipbuilding is still considered as an important industry. This is constantly increasing the demands for ships as well as for the efficiency improvement of shipbuilding. Therefore, shipbuilding field is widely introducing advanced techniques including computer programs for ship design and shipbuilding to raise its modernization level.
Some processes such as seam landing of hull surface are carried out on computers, but they are mainly based on the skills and experiences of designers.
Seam or butt landing (simply seam landing) of shell plates in the hull lofting process which is the first step of shipbuilding plays an important role in raising the speed and quality of shipbuilding and lowering its cost. However, this work has not been modernized until now because of its complexity and it has been done manually on paper and on mold lofts by the skills and experiences of designers.
Some computer programs for ship design and shipbuilding such as AVEVA, Ship Constructor, Maxsurf, etc. give help with seam landing of shell plate. These programs contain some graphics processing functions for seam landing in ship surfaces. However, seam landing of hull surface by computers still depends on the skills and experiences of designers. Therefore, the result of seam landing of hull surface is dependent on designer’s ability and this work lacks scientific accuracy. The main reason for low scientific accuracy of this work is its complexity.
Pae Ho Rim, a researcher at the Faculty of Shipbuilding and Ocean Engineering, has proposed an optimization model for seam landing and a solving method by GA. On this basis, he has confirmed the validity of the proposed method.
First, for the optimum seam landing, he built an optimization model with the sum of cutting, shearing, forming and welding cost as an objective function. As obvious in the objective function, costs of cutting, shearing, forming and welding depend on seam landing.
Second, he proved that the optimization problem for seam landing of hull surface is a topological optimization one when the number and form of seams are design variables and it is a form optimization one when only the form is a design variable.
Finally, he demonstrated that optimization method using GA gives a better solution than the seam landing of an existing ship and that by Taguchi’s method.
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Jo Feb 25, 2024
In magneto-telluric (MT) method, data processing for transfer functions (TF) estimation is routinely carried out based on the regression model of 4-element impedance under the assumption of plane electromagnetic wave. However, observed EM field contains not only MT signals subjected to the assumption of plane wave but also remarkable amounts of various noises with non-planar nature.
Jo Kyong Rae, a researcher at the Faculty of Earth Science and Technology, has introduced a regression model of impedance and admittance with 6 elements. He has also derived a mathematical formula for their estimation.
To decrease the influence of industrial noise as well as long-period noise, he employed 6-element regression models of impedance and admittance tensors in MT data processing to improve the predicted coherences of TFs estimation by about 0.02, both in single station and remote reference modes. He tested the proposed method through the processing of MT data, from four stations in the northern part of our country.
The results showed that the 6-element model makes it possible to improve the predicted coherences in TFs estimation and to decrease the scattering of the curves of apparent resistivity and phases, and that the difference between common 4-element and 6-element models gradually gets bigger as frequency decreases. It was also demonstrated that when the vertical magnetic component has amplitude as great as or greater than horizontal ones in auto-power curves, there is no doubt that the 6-element regression model is more appropriate for MT data processing than commonly used 4-element regression models.
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Jo Feb 23, 2024
Drawing up the most reasonable charging schedule is one of the most important challenges to be solved in wireless rechargeable sensor networks (WRSNs).
However, in existing on-demand charging schemes using multicriteria, it leaves space for efficient combination of multicriteria in the proactive charging process as well as in determination of charging location and charging time.
To solve this problem, Ri Man Gun, an institute head at the Faculty of Communications, has proposed a novel charging scheduling scheme called eIFVT (exploiting an Integrated FAHP-VWA-TOPSIS).
The eIFVT first calculates the exact weights of multicriteria characterizing the sensor nodes including charging request nodes using FAHP-VWA. These weights are then used to select the most suitable next-to-be-charged nodes and future potential-to-be-bottlenecked nodes with TOPSIS and to determine the partial charging time of the on-demand charging scheme adaptively.
The extensive simulation results show that the eIFVT greatly improves the charging and network performance for various metrics compared with existing schemes.
For more information, please refer to his paper “eIFVT: Exploiting an Integrated FAHP-VWA-TOPSIS in Whole-Process of On-Demand Charging Scheduling for WRSNs” in “IEEE SYSTEMS JOURNAL” (SCI).
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