Jo Mar 16, 2024
A charge sensitive amplifier (CSA) is widely used as the preamplifier of an X-ray or gamma-ray detector. It is based on a low-noise high frequency field-effect transistor and a current feedback operational amplifier allowing the readout of fast charge pulses.
The pile-up of output pulses in a CSA is attributable to its long discharge time constant. A CR differentiator which shortens pulse width is used to remove pile-up. Due to the exponential decay of a CSA feedback circuit in combination with a differentiator circuit, undershoot occurs at the output of a shaping amplifier with the same time constant as the CSA. PZC circuit is inserted between the CSA and the shaping amplifier to eliminate undershoot. But this makes it difficult to decide the real differential time constant of the CR differentiator after elimination of undershoot and to process the signals like peak detection, time analysis, etc. An analog PZC circuit always has some uncertainties that result in pulse tail pile-up. This drawback can be overcome with a digital pole-zero, zero-pole compensation unit.
A digital PZC method also has some uncertainties. Although digital real time processing of pulses could, in principle, eliminate almost all pulse pile-up distortion in spectroscopy, it is not yet widespread.
Pae Won Sik, a researcher at the General Assay Office, has performed an analysis to search for the cause of undershoot after differentiation of input signals and designed a new PZC circuit with unchanged differential time constant by using a pot resistor. Then, he has conducted some simulations and experiments to verify its successful performance.
The results show that the proposed circuit effectively eliminates undershoot and significantly reduces pulse width, which leads to the improvement of pile-up cancelling, peak detection and time analysis.
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Jo Mar 15, 2024
Welding deformation is one of the most complex problems arising in manufacturing of welded structures and it is impossible to remove or correct it. Welding deformation reduces accuracy in the size and shape of structures and increases the amount of correction work which leads to rise in manufacturing cost. Therefore, predicting and controlling welding deformation is of key importance in manufacturing welded structures. Predicting and controlling of welding deformation can reduce the amount of correction work, thus saving labor and cost and raising productivity.
At present, analysis of welding deformation is conducted by thermal-elastic-plastic (TEP) FEM and inherent strain method. In the past, there were a large number of experiments and numerical models to obtain inherent deformation. Although constraint is, in general, a simple practical method to reduce angle distortion, past researches were focused on numerical analysis of welding deformation, and the control of welding deformation considering the effect of constraint on angle distortion of large structures have not been sufficiently investigated.
Kim Chang Son, a researcher at the Faculty of Shipbuilding and Ocean Engineering, has predicted the welding distortion of a ship panel structure with butt and zigzag joints and determined the reasonable constraint position to reduce fabrication cost by considering the effect of constraint on angle distortion.
First, he investigated the inherent strains of the welded structure by TEP FEM analysis under both constraint and free conditions. Next, through the elastic FEM based on inherent strain, he estimated the welding deformation of ship panel structure under consideration. After that, in order to quantitatively estimate the effect of constraint on angle distortion of welded structure, he performed a numerical analysis in a non-constraint free condition and various constraint positions, and determined the reasonable constraint position. Finally, he compared the angle distortion of ship panel structure under the constraint condition with that under the non-constraint free condition.
The simulation results show that angular distortion can be effectively reduced under a constraint condition.
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Jo Mar 13, 2024
Laser cutting, which is one of the laser machining technologies, has been widely used in industrial production due to its good processing flexibility, high quality, high cutting speed, high precision and high production rate.
In order to ensure high quality in laser cutting, not only the appropriate process parameters (laser power, focal position, cutting speed, type and pressure of assist gas, etc.) must be set, but the stand-off distance between a work piece and a cutting head has to be kept at a fixed distance.
An Chol Min, a section head at the Faculty of Physical Engineering, has built a precise mathematical model of a truncated cone-shaped capacitive sensor based on the theory related to the electrostatic field and the properties of conductors and dielectrics in the field. He has also proposed limitations and suppositions to get a unique solution of the capacitance while analyzing the model by MATLAB, and verified the correctness of the model through experiments.
The theoretical analysis and the experiments show that the truncated cone sensor has much higher sensitivity than a parallel-plate sensor in laser cutting.
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Jo Mar 12, 2024
Electron density is a key parameter in designing and constructing a pumping system for high power transversely excited atmospheric (TEA) pressure CO2 laser. It is necessary to ensure uniform volumetric discharge in order to enhance the output power and the efficiency of gas laser. It is, however, very hard for TEA-CO2 laser to achieve electron density of more than 1014cm-3 necessary for volumetric discharge as the pressure of active gas of the laser is higher than atmospheric pressure.
Recent-developed external pre-ionization systems such as those based on high-energy field emission, ultraviolet or X-ray have enabled atmospheric pressure volumetric discharge in a short time of about 1㎲ and raised the output power of pulsed laser up to the level of GW. The main factor of getting higher power (over tens of GW) is to ensure that electron avalanche caused by inelastic collisions between CO2 (or N2 and He) molecule and cold electrons with energy of hundreds of keV is spatially uniform, and that non-self-sustained main discharge is conducted as a uniform volumetric one. It means that finding out the characteristics of change of electron density is of primary importance in developing high power TEA-CO2 laser.
The model of electron density for uniform volumetric discharge is a nonlinear model describing microphysical processes such as pre-ionization, avalanche ionization, recombination, attachment and so on. So far, there have been no general solutions of this model but only special solutions for some extreme cases.
Yun Tu Hon, a researcher at the Faculty of Physical Engineering, has proposed a method of general solutions for the nonlinear model and verified the accuracy and generality of his method by comparing the results with the data reported in the literature. He also reported some new special solutions unknown in the literature and the experimental results for them.
The proposed method will prove to be valuable for high power TEA-CO2 laser of hundreds of MW.
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Jo Mar 7, 2024
Submerged arc welding process with high productivity has been one of the widely-used surfacing methods for recycling worn components or fabricating bimetal. Surfacing with a lay-down plate electrode is a new variation of submerged arc surfacing, in which metal is deposited on the surface of a workpiece by arc heat between the workpiece and a plate laid down on it.
Both direct and alternate current can be used as welding power source. Surfacing with a lay-down plate shows the possibility of reducing the cost for restoring worn components like large-scale straight teeth gears with large module, and of improving the productivity and quality of surfacing.
Temperature distribution resulted by spontaneous movement of electric arc has a big effect on the stability and quality of surfacing with a lay-down plate. However, to the best of our knowledge, there have not been many discussions regarding the spontaneous movement of electric arc and the analysis of temperature field during surfacing with a lay-down plate electrode.
In order to make an optimal surfacing process condition, Pak Sang Yun, a researcher at the Faculty of Materials Science and Technology, has modeled the spontaneous movement orbit of arc during surfacing with a lay-down plate electrode and investigated its temperature distribution by ANSYS.
He has drawn the following conclusions.
First, the periodic and stable movement of electric arc, which is self-controlled in the channel during surfacing, results from periodic variation of forces acting on electric arc.
Second, the dimension and shape of the weld pool varies with time, but the boundary of solidus temperature is constant regardless of time. Thus, the temperature field remains in quasi-stationary state during surfacing with a lay-down plate electrode.
Third, the distribution of quasi-stationary temperature field depends on build-up technical conditions and the width of plate electrodes.
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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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