Hwa-Suk Oh
Korea Aerospace University
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Featured researches published by Hwa-Suk Oh.
Journal of Mechanical Science and Technology | 2005
Hwa-Suk Oh; Dong-Ik Cheon
The reaction wheel, an actuator for satellite attitude control, produces disturbance torque and force as well as axial control torque. The disturbances are so crucial to the pointing stability of this high precision satellite that a measurement of disturbances is necessary for such a satellite. The measurement table that is equipped with several load cells is one of the favorite types of measurement devices. The disturbance force and torque caused by the wheels rotation, however, stimulate the elasticity of the loadcells from the measurement table and induce the vibration of the table. This then causes the measurement error, which is especially large near the resonance frequencies of the table. In order to reduce this type of error, a calibration process with frequency compensation is suggested in this paper. The “filtered” disturbance spectrum is obtained from the raw data and the degradation of data accuracy caused by the table vibration is alleviated. Since the exact measurement is made possible by this compensation process even in the resonance area, the measurement range can be expanded up to the frequency area including the resonance frequencies. The compensation method has been adopted for the HAU measurement table where three uni-directional loadcells are used. The validity of the use of uni-directional loadcells is also tested theoretically.
international conference on control, automation and systems | 2008
Sungwook Yang; Dong-Ik Cheon; Sangchul Lee; Hwa-Suk Oh
Reliable and efficient attitude control requires the accurate dynamic properties. On orbit, the mass and inertia values are to be changed by some activities such as the propellant use, the solar panel deployment, etc. To obtain the accurate mass properties, the estimation should be performed. For the initial on-orbit estimation of the mass properties, the gyro-based attitude data is applied for the batch method. The gyro-based attitude data includes the noise, which affects the estimation accuracy. We suggested several filtering methods including the extended Kalman filtering algorithm to obtain the well-conditioned attitude data. Applying the proper filters, we can obtain the reliable estimation result of the mass properties of STSAT3.
Journal of Mechanical Science and Technology | 2005
Byung-Hoon Lee; Bong Un Lee; Hwa-Suk Oh; Seon-Ho Lee; Seung-Wu Rhee
Reaction wheels and thrusters are commonly used for the satellite attitude control. Since satellites frequently need fast maneuvers, the minimum time maneuvers have been extensively studied. When the speed of attitude maneuver is restricted due to the wheel torque capacity of low level, the combinational use of wheel and thruster is considered. In this paper, minimum time optimal control performances with reaction wheels and thrusters are studied. We first identify the features of the maneuvers of the satellite with reaction wheels only. It is shown that the time-optimal maneuver for the satellite with four reaction wheels in a pyramid configuration occurs on the fashion of single axis rotation. Pseudo control logic for reaction wheels is successfully adopted for smooth and chattering-free time-optimal maneuvers. Secondly, two different thrusting logics for satellite time-optimal attitude maneuver are compared with each other: constant time-sharing thrusting logic and varying time-sharing thrusting logic. The newly suggested varying time-sharing thrusting logic is found to reduce the maneuvering time dramatically. Finally, the hybrid control with reaction wheels and thrusters are considered. The simulation results show that the simultaneous actuation of reaction wheels and thrusters with varying time-sharing logic reduces the maneuvering time enormously. Spacecraft model is KOrea Multi-Purpose SATellite (KOMPSAT) -2 which is being developed in Korea as an agile maneuvering satellite.
Journal of Astronomy and Space Sciences | 2010
Dong-Hoon Kim; Daegyun Choi; Hwa-Suk Oh
E-mail: [email protected] Tel: +82-2-3159-0147 Fax: +82-2-300-0147This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://cre-ativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Journal of Astronomy and Space Sciences | 2011
Dong-Ik Cheon; Eunjeong Jang; Hwa-Suk Oh
Momentum changing actuators like reaction wheels and control moment gyros are generally used for spacecraft attitude control. This type of actuators produces force and torque disturbances. These disturbances must be reduced since they degrade the quality of spacecraft attitude control. Major disturbances are mainly due to static and dynamic imbalances. This paper gives attention to the reduction of the static and dynamic imbalance. Force/torque measurement system is used to measure the disturbance of the test reaction wheel. An identification method for the location and magnitude of the imbalance is suggested, and the corrections of the imbalance are performed using balancing method. Through balancing, the static and dynamic imbalance is remarkably reduced
Journal of The Korean Society for Aeronautical & Space Sciences | 2009
Dong-Hoon Kim; Henzeh Lee; Hwa-Suk Oh
In general, accurate attitude information is essential to perform the mission. Two algorithms are well-known to determine the attitude through two or more vector observations. One is deterministic method such as TRIAD algorithm, the other is optimal method such as QUEST algorithm. This Paper suggests the idea to improve performance of the TRIAD algorithm and to determine the attitude by combination of different sensors. First, we change the attitude matrix to Euler angle instead of using orthogonalization method and also use covariance in place of variance, then apply an unbiased minimum variance formula for more accurate solutions. We also suggest the methodology to determine the attitude when more than two measurements are given. The performance of the Averaging TRIAD algorithm upon the combination of different sensors is analyzed by numerical simulation in terms of standard deviation and probability.
Journal of Astronomy and Space Sciences | 2005
Su-Jeoung Kim; Sun-Ok Kim; Byoung-Young Moon; Young-Keun Chang; Hwa-Suk Oh
This paper addresses development of the ultra-light analog sun sensors for small satellite applications. The sun sensor is suitable for attitude determination for small satellite because of its small, light, low-cost, and low power consumption characteristics. The sun sensor is designed, manufactured and characteristic-tested with the target requirements of FOV (Field of View) and pointing accuracy of . Since the sun sensor has nonlinear characteristics between output measurement voltage and incident angle of sunlight, a higher order calibration equation is required for error correction. The error was calculated by using a polynomial calibration equation that was computed by the least square method obtained from the measured voltages vs. angles characteristics. Finally, the accuracies of 1-axis and 2-axes sun sensors, which consist of 2 detectors, are compared.
2011 2nd International Conference on Instrumentation Control and Automation | 2011
Dong-Ik Cheon; Daegyun Choi; Eunjeong Jang; Hwa-Suk Oh
High speed rotating actuators like reaction wheels or control moment gyros are generally used for spacecraft attitude control. Torque and force disturbances are inherent in this type of actuators. Major disturbances are mainly due to static and dynamic imbalances. These imbalances must be reduced since they degrade the control quality of spacecraft attitude. This paper gives attention to the reduction of the static imbalance. A reaction wheel for small satellite is considered. Torque/force measurement table is used to measure the disturbance of the reaction wheel. An identification method for the location and magnitude of the imbalance is suggested, and the corrections of the imbalance are performed using balancing method. Through balancing, the static imbalance is remarkably reduced.
Transactions of The Korean Society of Mechanical Engineers A | 2007
Hyun-Ho Lee; Hwa-Suk Oh; Dong-Ik Cheon
A momentum type actuator produces force and torque disturbances as well as its designed control torque. These disturbances are ones of the largest disturbance sources inside the spacecraft, which deteriorate the pointing stability of the high precision spacecraft. The measurement and analysis of actuator disturbances are therefore imperative for such a spacecraft, and thus a three dimensional torque measurement table has been developed for that purpose. The data acquired from the measurement table are processed in the frequency domain and displayed in the power spectral density(PSD). Through this process, disturbance model parameters are obtained and used for the attitude stability simulation. The process has been adopted for the disturbance measurement of the reaction wheel, and the validity of measurements and parameter identification procedure is verified.
Journal of Mechanical Science and Technology | 2007
Bong-Un Lee; Hwa-Suk Oh; Ja-young Kang
The geometric singularity problem is one of principal difficulty when using single-gimbal control moment gyros as spacecraft attitude control devices. To overcome singularity, new steering logics are suggested in this paper which results in a reduction in the difficulty of generating gimbal rates around a singular state. One of the suggested steering laws presented is the reduced dimensional singular value decomposition steering law, which adopts the singular value decomposition in reduced dimensional forms. Two other steering laws make use of the least square method in reduced dimensional forms. All of the suggested steering laws have been generated for the compensation of the torque insufficiency. These logics are verified mathematically and simulations at a singular condition and non-singular condition are performed to see how well they work.