• Proceedings of the KIEE Conference
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• 1999.11c
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• pp.500-502
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• 1999

This thesis deals with study and implementation of a cross-coupling controller which can enhance the path-tracking performance of optically guided AGV(Automated Guided Vehicle). The AGV in this thesis is differential drive type and has front-side and rear-side optical sensors, which can identify the guiding path. When AGV from the path due to the inevitable error and the deviation must be corrected. It has been shown that compensation only the first term can lead to undesirable oscillatory results and even instability but compensating only the second term leads to a steady state offset error. Cross-coupling control directly minimizes the error by coordinating the motion of the two drive wheels. The cross-coupling controller is analyzed to evaluate its performance. The cross-coupling controller enhances transient performance of the controller is demonstrated by simulation and is compared with that of individual loop controller.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.04a
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• pp.842-846
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• 1997

Micro-hole drilling (holes less than 0.5 mm in diameter with aspect ratios larger than 10) is gaining increased attention in a wide spectrum of precision production industries. Alternative methods such as EDM, laser drilling, etc. can sometimes replace mechanical micro-hole drilling but are not acceptable in PCB manufacture because they yield inferior hole quality and accuracy. The major difficulties in micro-hold drilling are related to wandering motions during the inlet stage, high aspect ratios, high temperature,etc. However, of all the difficulties, the most undesirable one is the increase of drilling force as the drill penetrates deeper into hold. This is caused mainly by chip related effects. Peck-drilling is thus widely used for deep hole drilling despite the fact that it leads to low productivity. Therefore, in this paper, a method of cutting force regulation is proposed to achieve continuous drilling. A proportional plus derivative (PD) and a sliding modecontrol algorithm will be implemented for controlling the spinle rotational frequeency. Experimental results will show that sliding mode control reduces the nominal cutting force and its variation better than the PD control, resulting in a number of advantages such as an increase in drill life, fast stabilization of the wandering motion, and precise positioning of the hole.

• Journal of KSNVE
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• v.8 no.1
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• pp.157-170
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• 1998

This paper proposes a new technique to formulate the finite element model of a sandwich beam by using GHM (Golla-Hughes-McTavish) internal auxiliary coordinates to account for frequency dependence. Through the use of auxiliary coordinates, the equation of motion of undamped mass and stiffness matrix form is extended to encompass viscoelastic damping matrix. However, this methods all suffer from an increase in order of the final finite element model which is undesirable in many applications. Here we propose to combine the GHM method with model reduction techniques to remove the objection of increased model order.

• Journal of the Korean Society for Precision Engineering
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• v.15 no.1
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• pp.117-129
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• 1998

Micro-hole drilling (holes less than 0.5 mm in diameter with aspect ratio larger than 10) is recently having more attention in a wide spectrum of precision production industries. Alternative methods such as EDM. laser drilling, etc. can sometimes replace the mechanical micro-hole drilling but are not acceptable in PCB manufacture because of the inferior hole quality and accuracy. The major difficulties in micro-hole drilling are related to small signal to noise ratios, wandering motions of the inlet stage, high aspect ratios, high temperatures and so forth. Of all the difficulties. the most undesirable one is the increase of drilling force as the drill proceeds deeper into the hole. This is caused mainly from the chip effects. Peck-drilling is thus widely used for deep hole drilling despite that it suffers from low productivity. In the paper, a method of cutting force regulation is proposed to achieve continuous drilling. A PD and a sliding mode control algorithms were implemented through controlling the spindle rotating frequency. Experimental results show that the sliding mode control reduces the nominal cutting force and the variation of the cutting force better than the PD control. The advantages of the regulation, such as increase of drill life, fast stabilization of a wandering motion, and the precise positioning of the hole are verified in experiment.

• International Journal of Aeronautical and Space Sciences
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• v.6 no.2
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• pp.23-32
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• 2005

An experimental control system is proposed for the attitude control of a simplified 2-DOF helicopter model. The main rotor is a rigid one, and the fuselage is simply supported by a fixed hinge point where the longitudinal motion is decoupled from the lateral one since the translations and the rolling rotation are completely removed. The yaw trim of the helicopter is performed with a tail rotor, by which the azimuthal attitude can be adjusted on the rotatable post in the yaw direction. The robust sliding mode control tracking a given attitude angle is proposed based on the flight dynamics. A pitch damper is inserted for the control of pitching angle while the compensator to reaction torque is used for the control of azimuth angle. Several parameters of the system are selected through experiments. The results shows that the proposed control method effectively counteracts nonlinear perturbations such as main rotor disturbance, undesirable chattering, and high frequency dynamics.

• Structural Engineering and Mechanics
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• v.55 no.2
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• pp.365-377
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• 2015

Large-amplitude vibration of overhead sign structures can cause unfavorable psychological responses in motorists, interfere with readability of the signs, and lead to fatigue cracking in the sign structures. Field experience in Texas suggests that an overhead sign structure can vibrate excessively when supported within the span of a highway bridge instead of at a bent. This study used finite element modeling to analyze the dynamic displacement response of three hypothetical sign structures subjected to truck-passage-induced vertical oscillations recorded for the girders from four actual bridges. The modeled sign bridge structures included several span lengths based on standard design practices in Texas and were mounted on precast concrete I-girder bridges. Results revealed that resonance with bridge girder vertical vibrations can amplify the dynamic displacement of sign structures, and a specific range of frequency ratios subject to undesirable amplification was identified. Based on these findings, it is suggested that this type of sign structure be located at a bridge bent if its vertical motion frequency is within the identified range of bridge structure excitation frequencies. Several alternatives are investigated for cases where this is not possible, including increasing sign structure stiffness, reducing sign mass, and installing mechanical dampers.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.24 no.10
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• pp.803-806
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• 2011

1 mol% $Li_2O$ excess $0.9(Na_{0.52}K_{0.48})NbO_3-0.1LiTaO_3$ lead-free piezoelectric multilayer ceramic actuators were investigated to determine their aging properties. To reduce the thermal aging behavior, we applied a rectified unipolar electric field of 5 kV/mm to the specimen to accelerate the electric aging behavior. By employing a rectified unipolar electric field for the piezoelectric actuators, we could remove undesirable heating from the relaxation current in the motion of the ferroelectric domain. To accelerate the aging test, the applied electric fields had a frequency of 900 Hz. To have enough time for charging and discharging, we employed an accurate time constant to design the equivalent circuit model for the aging tester. To extract exact aging behavior, we measured the pseudo-piezoelectric coefficient before and after the aging process. We also measured the electro-mechanical coupling coefficient, the frequency-dependent dielectric permittivity, and the impedance to compare with fresh and aged specimen.

• The Transactions of the Korea Information Processing Society
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• v.6 no.11
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• pp.3165-3177
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• 1999

With the recent outstanding advance in computer software and hardware, a number of researches to enhance the manufacturing speed and the process accuracy has been undertaken in many fields of textile industry. Frequently issued problems of automatic recognition of textile wrinkles in a grey scale image are as follows. First, changes in grey level intensity of wrinkles are so minute. Second, as both colors and patterns in a grey scale image appear in grey level intensity, it is difficult to sort out the wrinkle information only. Third, it is also difficult to distinguish grey level intensity changed by wrinkles from those by uneven illumination. This paper suggests a method of automatic recognition of textile wrinkles that can solve above problems concerned with wrinkles, which can be raised in a manufacturing process as one of errors. In this paper, we first make the outline of wrinkles distinctly, apply the block matching algorithm used in motion estimation, and then estimate block locations of target images corresponding to blocks of standard images with the assumption that wrinkles are kind of textile distortions caused by directional forces. We plot a "wrinkle map" considering distances between wrinkles as depths of wrinkles. But because mismatch can occur by different illumination intensity and changes in tensions and directions of the force, there are also undesirable patterns in the map. Post processing is needed to filter them out and get wrinkles information only. We use average grey level intensity of wrinkle map to recognize wrinkles. When it comes to textile with colors and patterns, previous researches on wrinkles in grey scale image hasn't been successful. But we make it possible by considering wrinkles as distortion.istortion.

• Journal of the Ergonomics Society of Korea
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• v.32 no.2
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• pp.207-215
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• 2013

Objective: This study aimed to investigate important human factors that should be considered when developing ultra-high definition TVs by reviewing measurement methods and main characteristics of ultra-high definition displays. Background: Although much attention has been paid to high-definition displays, there have been few studies for systematically evaluating human factors. Method: In order to determine human factors to be considered in developing human-friendly displays, we reviewed subjective and objective measurement methods to figure out the current limitations and establish a guideline for developing human-centered ultra-high definition TVs. In doing so, pros and cons of both subjective and objective measurement methods for assessing humans factors were discussed and specific aspects of ultra-high definition displays were also investigated in the literature. Results: Hazardous effects such as visually-induced motion sickness, visual fatigue, and mental fatigue in the brain caused by undesirable TV viewing are induced by not only temporal decay of visual function but also cognitive load in processing sophisticated external information. There has been a growing evidence that individual differences in visual and cognitive ability to process external information can make contrary responses after exposing to the same viewing situation. A wide vision, ultra-high definition TVs provide, can has positive and negative influences on viewers depending on their individual characteristics. Conclusion: Integrated measurement methods capable of considering individual differences in human visual system are required to clearly determine potential effects of super-high vision displays with a wide view on humans. All of brainwaves, autonomic responses, eye functions, and psychological responses should be simultaneously examined and correlated. Application: The results obtained in this review are expected to be a guideline for determining optimized viewing factors of ultra-high definition displays and accelerating successful penetration of the next-generation displays into our daily life.

• International Journal of Fluid Machinery and Systems
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• v.4 no.1
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• pp.179-190
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• 2011

The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The influence of outlet bubble cavitation and air injection is also investigated for low cavitation number. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.