• 제어로봇시스템학회:학술대회논문집
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• 2000.10a
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• pp.261-261
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• 2000

This paper analyzes the characteristics of pre-sliding friction of an X-Y table of CNC machining center at velocity reversal, and presents a simple and effective method of friction compensation based on this characteristics. At velocity reversal, a large position tracking error occurs because of the discontinuous change of friction. The relationship between the occurrence time of maximum position tracking error and the acceleration at zero velocity is analyzed by using the spring-like friction model. Furthermore, the experimental observation verifies this relation. From this, the state transition tine from pre-sliding regime into sliding regime can be predicted. Using the predicted transition time, the friction can be effectively compensated and table experimental results show its effectiveness.

• Journal of the Korea Society for Simulation
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• v.7 no.2
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• pp.105-114
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• 1998

Using finite elements, a simulation is prformed for the pipe systems to investigate free vibrational characteristics, that is natural frequencies, considering the intial tension due to the velocity and the pressure of the inside fluid flow. To confirm the program developed in this study, the results are compared with the results of commercial software ANSYS. When the initial tension is neglected in curved pipes, the natural frequencies are reduced as flow velocity increases, and the rapid decreases of the natural frequencies took place. However, when the initial tension is taken into account, the natural frequencies are not changed with the change of the flow velocity. In free vibrational simulation of pipe systems, it is necessary to calculate the initial state force due to the velocity and the pressure of the fluid flow from the equilibrium first, then the force should be included in the equation of motion of the systems to get more accurate natural frequencies.

• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.243-246
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• 2000

This paper addresses a new technique of measuring the mean flow velocity not only over the cross sectional area but also along the pipe by exploiting the acoustic plane waves in the pipe. When fluid flows in the pipe and two plane waves propagate oppositely through the medium in it, the flow velocity causes a change of the wave number of the plane waves. The wave number of the positive going plane wave decreases but oppositely that of negative going one increases in comparison to no flow of the medium in the pipe. Theoretical backgrounds of this method are in details discussed and measurement results of the mean flow velocity are illustrated to reveal the feasibility and effectiveness of the suggested technique.

• Journal of the Korean Society for Precision Engineering
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• v.17 no.1
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• pp.110-119
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• 2000

This paper presents simple and effective nonlinear friction compensation methods. When the direction of position command reverses, the integrator output of the PID controller does not change the sign of its output instantaneously, due to friction at zero velocity, i.e. stiction resulting tracking errors, that results in continuous push even though the command direction has been changed. To overcome this problem, we attempt to reverse the sign of the integrator output as the sign of velocity changes. The effectiveness of this approach is demonstrated by experiments on a 3-PRPS (Prismatic-Revolute-Prismatic-Shperical joints) in-parallel 6-D.O.F manipulator. The control strategy has been analyzed for stability. Also discussed are disturbance observer and velocity observer approaches for friction compensation.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.70.3-70.3
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• 2019

We present a method to estimate 3-component plasma velocity (Vx, Vy and Vz) at solar photosphere near solar disk center, using the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) called Space-weather HMI Active Region Patch (SHARP). In Heliocentric-Cartesian Coordinates, the component of Vz is obtained from Dopplergram while the components of Vx and Vy are derived from the relation of $B_z{\overrightarrow{u}}=B_z{\overrightarrow{{\nu}_t}}-{\nu}_z{\overrightarrow{B_t}}$ (Demoulin & Berger 2003) using a series of vector magnetograms by an optical flow technique NAVE (Nonlinear Affine Velocity Estimator). This velocity measurement method is applied to AR 12158 producing an X1.6 flare along with a coronal mass ejection. We find noticeable upflow motions at both ends of flux ropes which become a major eruption part, and strong transverse motions nearby them before the eruption. We will discuss the change of plasma motions and magnetic fields before and after the eruption.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.31 no.5
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• pp.314-319
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• 2019

The numerical simulation method was used to analyze the flow velocity change and mechanical coefficient characteristics of the blade in the shroud system that changes with the initial flow velocity and the blade rpm. In the analysis condition, the initial flow velocity was varied from 0.35 m/s to 1.0 m/s, and the blade rpm varied from 50 rpm to 300 rpm. Through this, the mechanical coefficient was estimated. The flow velocity changes tended to increase more than 1.8 times at the middle point compared to the inlet. When the flow velocity ratio was 0.75 m/s compared to the initial flow velocity of 0.5 m/s, the flow velocity ratio decreased. The mechanical coefficient using the torque of the blade also showed the highest coefficient at 0.5 m/s, and the trends were similar. On the other hand, the maximum coefficient was estimated to be about 20.88% in TSR 4.77 when the initial flow velocity was 0.5 m/s. The mechanical coefficient analysis of blades in this study is expected to provide the basic data for hydraulic model experimental.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.3
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• pp.427-435
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• 1997

Conditional sampling techniques are utilized to investigate the relation between the wall skin-friction and stream wise velocity fluctuations in a turbulent boundary layer. Conditionally averaged results using a peak detection and the VITA (variable-interval time-averaging) technique show that a high skin friction is associated with high frequency components of the wall skin-friction fluctuations. The conditionally averaged wall skin-friction fluctuations obtained by using the VITA technique have a positively-skewed characteristics compared with the conditionally averaged stream wise velocity fluctuations. It is confirmed that there exists a phase shift between the wall skin-friction and stream wise velocity fluctuations, which was also found from the long-time averaged space-time correlations. The amount of phase shift between the wall skin-friction and stream wise velocity fluctuations is the same as that from the long-time averaged space-time correlations and does not change despite the variation of the detection threshold.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.10
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• pp.1378-1386
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• 2002

In the present study, velocity profile and wall shear stress distributions of developing turbulent oscillatory flows in an oscillator connected to straight duct located in exit region of a curved duct was investigated experimentally. The experimental study for air flows was conducted to measure axial velocity profiles, shear stress distributions by using the Laser Doppler Velocimetry(LDV) system with the data acquisition and processing system of Rotating Machinery Resolver(R.M.R) and PHASE software. The results obtained from experimental studies are summarized as follows. The critical Reynolds number for a change from transitional oscillatory flow to turbulent flow was about 7500, in the 60region of dimensionless axial position which was considered as a fully developed flow region. The turbulent oscillatory flow, velocity profiles of the inflow period in the entrance region were gradually developed, but those of the outflow period were not changed nearly. Velocity profiles of inflow and outflow were shown as a symmetric form in a fully developed flow region. The wall shear stress distributions of turbulent oscillatory flow increase rapidly as the flow proceeds to downstream and flow was in good agreement with the theoretically.

• Journal of Sensor Science and Technology
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• v.1 no.1
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• pp.35-41
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• 1992

A constant temperature type of flow sensor using a solid state micromachining technology was developed for measuring the velocity of gas or liquid. It was designed to detect only the heat convection related to flow velocity. Other heat transfer terms and common mode interferences are canceled by differentiating both reference and exposed flow sensor. It employs the principle that the change of current through the sensing element can be used to measure the flow velocity. An experimental study of the behavior on this flow sensor was performed in a narrow tube(diameter : 8mm) for city water. The relation between power consumption of the flow sensor and square-root of flow velocity is almost linear in the low velocity range(0-200 cm/sec).

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.10
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• pp.2678-2689
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• 1995

In this study, a new hot-wire anemometer which has greater sensitivity than that of a constant temperature anemometer (CTA) was proposed. In contrast to CTA, the wire working resistance of the new anemometer increases with flow velocity, that is, the operating mode of the wire becomes variable temperature. The variable temperature anemometer(VTA) was made by substituting a voltage controlled variable resistor such as photoconductive cell or transistor for one of the resistors in the bridge. By positively feeding back the bridge top signal to the input side of these electronic components, the wire overheat ratio could be increased with velocity automatically. Static response analyses of the VTA, constant voltage anemometer (CVA) and CTA were made in detail and calibration experiments were performed to validate the proposed operating principle. The wire operating resistance of the CVA decreases with velocity and this leads to lower sensitivity than that of a CTA. But the sensitivity of the newly proposed VTA is superior to that of a CTA, since the wire overheat ratio increases with velocity. Consequently, it is found that the major factor that is responsible for large sensitivity of a VTA is not the working resistance itself but the change of the wire working resistance with velocity.