Dong-Ik Cheon

Precision Measurements of Reaction Wheel Disturbances with Frequency Compensation Process

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.

On-orbit estimation of dynamic properties for STSAT3

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.

Reaction Wheel Disturbance Reduction Method Using Disturbance Measurement Table

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

Disturbance reduction on the small satellite actuator

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.

Three Dimensional Measurements and Parameter Identification of Force and Torque Distmbances of High Speed Rotating Actuators

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.

An averaging algorithm for speed measurement of SCL/KAU reaction wheel

Reaction wheel, the satellite attitude actuator provides torque upon satellite body by changing its wheel speed. Accurate speed measurement is necessary for precision attitude control. Based on the conventional digital algorithm, a new speed detection method is suggested using averaging algorithm. The new algorithm is applied on a reaction wheel developed by Spacecraft Control Laboratory in Korea Aerospace University. Based on the mean errors and standard deviations of measurement speed data, the performance of the suggested algorithm is evaluated. The suggested method works well in the speed detection and shows the improved error performance better than the conventional algorithms.