Hyun-Ung Oh

An experimental study of a semiactive magneto-rheological fluid variable damper for vibration suppression of truss structures

The purpose of this paper is to demonstrate that vibrations of a truss structure can be suppressed nicely by a magneto-rheological (MR) fluid variable damper for semiactive vibration suppression. A variable MR fluid damper was designed and fabricated for this study. The principal characteristics of an MR damper were measured in dynamic tests, and a mathematical model of the damper was proposed. To investigate if the variable damper effectively suppresses the vibration of actual truss structures, semiactive vibration suppression experiments were performed using a cantilevered ten-bay truss beam. The experimental result has shown that the vibration was suppressed nicely by the variable MR damper, and that was compared with that of an electro-rheological (ER) damper investigated in previous research. The MR damper showed a higher performance than that of the ER damper.

Semiactive Vibration Suppression with Electrorheological-Fluid Dampers

Semiactive vibration suppression with electrorheological (ER)-fluid variable dampers is proposed and demonstrated to be a robust approach to suppressing the vibration of space structures. The principal characteristics of an ER-fluid variable damper are measured, and a simple mathematical model of the damper is proposed. Based on the mathematical model of the damper, several semiactive on-off control laws are proposed. Their performance is investigated through both numerical simulations and experiments of cantilevered truss beams with ER-fluid variable dampers. Simulation results indicate that most of the proposed laws result in much more effective vibration suppression than is obtained when using an optimally tuned passive system. Under an ill condition, they are also shown to be much more robust than linear quadratic Gaussian active vibration controls whose vibration suppression performance is almost the same. A semiactive vibration suppression experiment with a cantilevered 10-bay truss beam demonstrates that vibrations are suppressed effectively by the ER-fluid variable damper.

Semiactive Isolator With Liquid-Crystal Type ER Fluid for Momentum-Wheel Vibration Isolation

A semiactive isolator filled with liquid-crystal type electro-rheological (ER) fluid was developed to attenuate the disturbances generated by a momentum-wheel and to improve pointing performance. The principal characteristics of an ER isolator were measured in dynamic tests, and a mathematical model of the isolator was proposed. Two control laws for a semiactive approach were proposed. Numerical simulation results indicated that the proposed semiactive control system produced much better isolation performance than a passive system.

Flywheel vibration isolation test using a variable-damping isolator

This study demonstrates the isolation performance of a variable-damping isolator using a bio-metal fiber (BMF) valve to enhance the pointing performance of observation satellites by isolating disturbances induced by reaction wheel assemblies. Vibration isolation tests of the variable-damping isolator were performed using an air-floating wheel disturbance detector to investigate whether the isolator can actually isolate flywheel vibration. In this paper, we first present a recently developed variable-damping isolator with low power consumption, and a reaction wheel disturbance detector, fabricated in a previous study, which detects low-frequency disturbances. Next, we describe the effectiveness of the variable-damping isolator based on flywheel vibration isolation test results.

Development of variable-damping isolator using bio-metal fiber for reaction wheel vibration isolation

This study focuses on the basic characteristics of a variable-damping isolator using a bio-metal fiber (BMF) valve to enhance the pointing performance of optical equipment on-board satellites. A variable-damping BMF valve isolator for isolating disturbances induced by reaction-wheel operation has been designed and fabricated. The opening and closing of the valve to change the damping of the isolator is implemented by using characteristic variation of the bio-metal fiber to the input electric current. The BMF isolator fabricated in this study has the advantages of being a simple variable-damping device with low power consumption, unlike conventional variable-damping devices, such as solenoid-valve-type oil dampers and ER or MR dampers. This paper presents the basic characteristics of the BMF isolator obtained from dynamic tests of the isolator.

Experimental demonstration of an improved magneto-rheological fluid damper for suppression of vibration of a space flexible structure

A low-power-consumption magneto-rheological (MR) fluid damper has been designed and fabricated to suppress the vibration of a space flexible structure. The MR damper fabricated in this study has advantages of both semi-active and optimal passive dampers, unlike the conventional MR and electro-rheological (ER) fluid semi-active damper. The vibration is thus damped quickly when the damper is controlled semi-actively, and the vibration energy is dissipated by an optimal passive damper in the absence of control. To implement the aforementioned damper, we fabricated an electromagnet combined with a permanent magnet. Its performance and the characteristics of the damper with the newly developed electromagnet were subsequently measured. The performance of the damper has been confirmed by vibration suppression experiments using 10-bay truss structures.

Characteristics of spaceborne cooler passive vibration isolator by using a compressed shape memory alloy mesh washer

Cryogenic coolers produce undesirable micro-vibrations during on-orbit operation, which may seriously affect the image quality of high-resolution observation satellites. Micro-vibrations can be easily isolated by mounting the cooler on a vibration isolator with low stiffness to attenuate the vibration transmitted to the satellite structure. However, the structural safety of a cooler supported by an isolator with low stiffness cannot be guaranteed under the much more severe vibration condition of a launch environment. In this study, to guarantee vibration isolation performance in a launch environment while effectively isolating the micro-vibrations from the cooler on-orbit, a new type of passive vibration isolation system by using a compressed shape memory alloy mesh washer was proposed and investigated. The basic characteristics of the isolator were measured in static and free vibration tests of the isolator, and a simple equivalent model of the isolator was proposed. The effectiveness of the isolator design in a launch environment was demonstrated through sine vibration, random vibration, and shock tests.

Improved Electrorheological-Fluid Variable Damper Designed for Semiactive Vibration Suppression

The studied damper can be modeled as a variable coulomb-friction damper. The friction in the damper is not zero, even when no voltage is applied to the electrode of the damper, and is an increasing function of the absolute value of the voltage. Semiactive vibration suppression exploits this variation of friction. To suppress large-amplitude vibrations to small-amplitude ones effectively, we need to vary the friction over a large range. In other words, the ratio of the maximum and minimum values of the frictional force needs to be large. An investigation of decreasing the minimum frictional force by vibrating the electrode of the damper to increase this ratio is reported. The effectiveness of the electrode vibration was demonstrated in static experiments, as well as in semiactive vibration suppression experiments, indicating that electrode vibration reduces the minimum frictional force by 1/3-1/2 without changing the maximum frictional force, thereby greatly reducing the vibration amplitude.

Experimental feasibility study for micro-jitter attenuation of stepper-actuated X-band antenna-pointing mechanism by using pseudoelastic SMA mesh washer

A stepper motor mechanism activates the azimuth and elevation direction of an X-band downlink antenna-pointing mechanism for effective data transfer from a satellite to a ground station. The stepper motor activation, and an imperfect intermeshed teeth harmonic-drive-gear configuration of the antenna mechanism may induce micro-jitter in the system, which can seriously affect the image quality of high-resolution observation satellites. In this study, to confirm the feasibility of on-orbit micro-jitter isolation of a whole-antenna mechanism assembly, a new type of passive vibration isolation system was proposed and investigated. This proposed system uses a pseudoelastic shape memory alloy compressed mesh washer isolator. The basic characteristics of the isolator were measured by performing static test on the isolator. The effectiveness of the design was also demonstrated via a micro-jitter measurement test, with and without the isolation system.

Characteristics of a Liquid-Crystal Type ER-Fluid Variable Damper for Semiactive Vibration Suppression

A liquid-crystal type ER-fluid variable damper was fabricated to semiactively suppress the vibration of space truss structure. The principal characteristics of a liquid-crystal variable damper were measured in quasi-static tests, and a simple mathematical model of the damper was proposed. Semiactive control laws were derived from this model and the LQR theory. Numerical simulations were performed to compare the effectiveness of the semiactive vibration suppression using liquid-crystal type and particle-dispersion type ER-fluid variable dampers. The results indicated that the proposed control laws resulted in much better damping performances than is obtained with an optimally tuned passive system. In some cases such as large amplitude vibration, a variable damper using liquid-crystal type ER-fluid showed higher performance than one using particle-dispersion type ER-fluid. The results of semiactive vibration suppression experiments with a ten-bay truss beam also demonstrated that the liquid-crystal variable damper effectively suppresses vibration.