• Korean Journal of Applied Biomechanics
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• v.25 no.3
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• pp.283-291
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• 2015

Objective : The purpose of this study was to determine the differences in the head and tibial acceleration signal magnitudes, and their powers and shock attenuations between flat-footed and normal-footed running. Methods : Ten flat-footed and ten normal-footed subjects ran barefoot on a treadmill with a force plate at 3.22m/s averaged from their preferred running speed using heel-toe running pattern while the head and tibial acceleration in the vertical axis data was collected. The accelerometers were sampled at 2000 Hz and voltage was set at 100 mv, respectively. The peak magnitudes of the head and tibial acceleration signals in time domain were calculated. The power spectral density(PSD) of each signal in the frequency domain was also calculated. In addition to that, shock attenuation was calculated by a transfer function of the head PSD relative to the tibia PSD. A one-way analysis of variance was used to determine the difference in time and frequency domain acceleration variables between the flat-footed and normal-footed groups running. Results : Peaks of the head and tibial acceleration signals were significantly greater during flat-footed group running than normal-footed group running(p<.05). PSDs of the tibial acceleration signal in the lower and higher frequency range were significantly greater during flat-footed running(p<.05), but PSDs of the head acceleration signal were not statistically different between the two groups. Flat-footed group running resulted in significantly greater shock attenuation for the higher frequency ranges compared with normal-footed group running(p<.05). Conclusion : The difference in impact shock magnitude and frequency content between flat-footed and normal-footed group during running suggested that the body had different ability to control impact shock from acceleration. It might be conjectured that flat-footed running was more vulnerable to potential injury than normal-footed running from an impact shock point of view.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• v.1
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• pp.327-332
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• 2006

In airborne gravimetry, there are two data streams. One is the specific force measured by an air/sea gravimeter or accelerometers, the other is kinematic acceleration measured by DGPS. And the difference of them provides the gravity disturbance information. To satisfy the requirement of most applications, an accuracy of 1mGal $(1mCal=10^{-5}m/s^{2})$ with a spatial resolution of 1km is the aim of current airborne gravimetry. There are two different methods to derive the kinematic acceleration. The generally used method is to differentiate the position twice, and the position can be calculated by commercial DGPS software. The main defect of this method is that integer ambiguities need to be fixed to get the precise position solution, but it's not a trivial thing for long base line. And to fix integer ambiguities, the noisier iono-free measurement is used. When differentiation is applied, noise is amplified and will influence the accuracy of acceleration. The other method is to get carrier phase acceleration by differentiate the carrier phase first, and then using the acceleration of GPS satellite to derive the vehicle acceleration. The main advantages include that fixing integer ambiguities is not needed anymore, position can be relaxed to about 10 meters, and smoother acceleration can be got since iono-free measurement is not needed. In some literatures, it's considered that the dynamic performance of the second method is inferior to that of the first. Through analysis, it is found that the performance degradation in dynamic environment results from the simplification of the GPS carrier phase observable model. And an iterative algorithm is presented to compensate the model error. Using a dynamic GPS data from an aeromagnetic survey, the importance of this compensation is showed at last.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.65 no.2
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• pp.349-353
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• 2016

The purpose of this study was to investigate possible age differences in the attenuation of acceleration in the upper body (from pelvis through shoulder to head) during fast walking. Thirty young and 29 elderly subjects participated in this study. Wireless acceleration sensors were attached on head, shoulder, and pelvis. Subjects performed two trials of fast walking on a treadmill, where the fast speed was defined as 1.5 times of the comfortable speed. Root-mean-squared (RMS) accelerations of each axis were compared with age group and sensor position as independent factors. In the AP direction, the pelvis acceleration was greater in the young and the shoulder-to-head attenuation was also greater in the young (p<0.001), so that the head acceleration was comparable between age groups (p=0.581). In the ML direction, the pelvis acceleration was greater in the young and also the pelvis-to-shoulder attenuation was greater in the young (p<0.001), so that the head acceleration was greater in the elderly group (p<0.001). Insufficient attenuation ML acceleration in the elderly resulting in the greater acceleration in the head may deteriorate the balance control which utilize feedback signals from the sensory organs in head, e.g., vestibular and visual systems.

• Korean Journal of Applied Biomechanics
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• v.30 no.2
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• pp.145-154
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• 2020

Objective: The purpose of this study was to determine the relationship between impact and shear peak force, and tibia-accelerometer variables during running. Method: Twenty-five male heel strike runners (mean age: 23.5±3.6 yrs, mean height: 176.3±3.3 m/s, mean mass: 71.8±9.7 kg) were recruited in this study. The peak impact and anteroposterior shear forces during treadmill running (Bertec, USA) were collected, and impact shock variables were computed by using a triaxial accelerometer (Noraxon, USA). One-way ANOVA was used to test the influence of the running speed on the parameters. Pearson's partial correlation was used to investigate the relationship between the peak impact and shear force, and accelerometer variables. Results: The running speed affected the peak impact and posterior shear force, time, slope, and peak vertical and resultant tibial acceleration, slope at heel contact. Significant correlations were noticed between the peak impact force and peak vertical and resultant tibia acceleration, and between peak impact average slope and peak vertical and resultant tibia acceleration average slope, and between posterior peak (FyP) and peak vertical tibia acceleration, and between posterior peak instantaneous slop and peak vertical tibial acceleration during running at 3 m/s. However, it was observed that correlations between peak impact average slope and peak vertical tibia acceleration average slope, between posterior peak time and peak vertical and resultant tibia acceleration time, between posterior peak instantaneous slope and peak vertical tibial acceleration instantaneous slope during running at 4 m/s. Conclusion: Careful analysis is required when investigating the linear relationship between the impact and shear force, and tibia accelerometer components during relatively fast running speed.

• Earthquakes and Structures
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• v.21 no.5
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• pp.551-562
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• 2021

In this study, the generalized intensity measure (IM) named I_Npg is analyzed. The recently proposed proxy of the spectral shape named N_pg is the base of this intensity measure, which is similar to the traditional N_p based on the spectral shape in terms of pseudo-acceleration; however, in this case the new generalized intensity measure can be defined through other types of spectral shapes such as those obtained with velocity, displacement, input energy, inelastic parameters and so on. It is shown that this IM is able to increase the efficiency in the prediction of nonlinear behavior of structures subjected to earthquake ground motions. For this work, the efficiency of two particular cases (based on acceleration and velocity) of the generalized I_Npg to predict the peak floor acceleration demands on steel frames under 30 earthquake ground motions with respect to the traditional spectral acceleration at first mode of vibration Sa(T₁) is compared. Additionally, a 3D reinforced concrete building and an irregular steel frame is used as a basis for comparison. It is concluded that the use of velocity and acceleration spectral shape increase the efficiency to predict peak floor accelerations in comparison with the traditional and most used around the world spectral acceleration at first mode of vibration.

• Tribology and Lubricants
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• v.39 no.5
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• pp.203-211
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• 2023

This study experimentally investigates the effects of angular acceleration on the friction and wear performances of a gas foil thrust bearing (GFTB) using a typical GFTB with six pads. The outer radius of the bearing is 31.5 mm, the total bearing area is 2,041 mm² , and the bump foil and incline (ramp) height are both 500 ㎛. The newly developed GFTB test rig for measuring the friction torque and coefficient measures the axial load, drag torque, lift-off speed, and touch-down speed. The experiment is conducted for angular accelerations of 78.5, 314.2, and 328.3 rad/s² at axial loads of 5, 10, and 15 N, respectively. The test shows that the start-up friction coefficient increases with increasing axial load at the same angular acceleration, and the friction coefficient decreases with increasing angular acceleration under the same axial load. As the angular acceleration increases, the lift-off speed at the motor start-up increases, and the touch-down speed at the motor stop decreases. The wear distance of the GFTB for a single on/off cycle increases with increasing axial load at the same angular acceleration and decreases nonlinearly with increasing angular acceleration under the same axial load. The test results suggest that adjusting the rotational angular acceleration helps reduce bearing friction and wear.

• Smart Structures and Systems
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• v.4 no.5
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• pp.583-603
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• 2008

In this study, a real-time damage detection method using output-only acceleration signals and artificial neural networks (ANN) is developed to monitor the occurrence of damage and the location of damage in structures. A theoretical approach of an ANN algorithm that uses acceleration signals to detect changes in structural parameters in real-time is newly designed. Cross-covariance functions of two acceleration responses measured before and after damage at two different sensor locations are selected as the features representing the structural conditions. By means of the acceleration features, multiple neural networks are trained for a series of potential loading patterns and damage scenarios of the target structure for which its actual loading history and structural conditions are unknown. The feasibility of the proposed method is evaluated using a numerical beam model under the effect of model uncertainty due to the variability of impulse excitation patterns used for training neural networks. The practicality of the method is also evaluated from laboratory-model tests on free-free beams for which acceleration responses were measured for several damage cases.

• Journal of ILASS-Korea
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• v.18 no.3
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• pp.132-139
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• 2013

Smoke emissions from light duty diesel vehicles were measured using light extinction method with the free acceleration test mode. The smoke emissions for each measurement cycle of the free acceleration method showed large variations according to driver's pedal pushing pattern. The smoke values for each measurement cycle initially increased and reach a peak value. Integration of the smoke emissions with time for each measurement cycle was performed to get a representative smoke value which was obtained by averaging the integrated results. Two kinds of integration time range were used. One is range over the whole measurement cycle of the free acceleration method. The other is only the acceleration range in the measurement cycle. Overall, variation of the representative smoke values obtained by the integration method was reduced comparing to the traditional representative smoke value which was obtained from a peak smoke value over the measurement cycle. Ten vehicles of the same model with 2.5 liter diesel engines, and seven vehicles of the same model with 2.7 liter diesel engines, were tested using the free acceleration test method.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.25 no.11
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• pp.739-746
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• 2015

Several damage counting methods can be applied for the fatigue issues of a ground vehicle system using strain data and acceleration data is partially used for a high cyclic loading case. For a vibration test, acceleration data is, however, more useful than strain one owing to the good nature of signal-to-random ratio at acceleration response. The test severity can be judged by the fatigue damage and the pseudo-damage from the acceleration response stated in ISO-16750-3 is one of sound solutions for the vibration test. The comparison of fatigue damages, derived from both acceleration and strain, are analyzed in this study to determine the best choice of fatigue damage over multi-spectral input pattern. Uniaxial excitation test was conducted for a notched simple specimen and response data, both acceleration and strain, are used for the comparison of fatigue damages.

• Journal of Institute of Control, Robotics and Systems
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• v.15 no.1
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• pp.99-104
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• 2009

This article describes the analysis of stable grasping for multi-fingered robot. An analysis method of stable grasping, which is based on the three-dimensional acceleration convex polytope, is proposed. This method is derived from combining dynamic equations governing object motion and robot motion, force relationship and acceleration relationship between robot fingers and object's gravity center through contact condition, and constraint equations for satisfying no-slip conditions at every contact points. After mapping no-slip condition to torque space, we derived intersected region of given torque bounds and the mapped region in torque space so that the intersected region in torque space guarantees no excessive torque as well as no-slip at the contact points. The intersected region in torque space is mapped to an acceleration convex polytope corresponding to the maximum acceleration boundaries which can be exerted by the robot fingers under the given individual bounds of each joints torque and without causing slip at the contacts. As will be shown through the analysis and examples, the stable grasping depends on the joint driving torque limits, the posture and the mass of robot fingers, the configuration and the mass of an object, the grasp position, the friction coefficients between the object surface and finger end-effectors.