Jae Hyuk Lim

An experimental study of the two-way shape memory effect in a NiTi tubular actuator

In this paper, the two-way shape memory effect (TWSME) in a Ti?54.5 Ni(wt%) alloy was investigated experimentally to develop a NiTi linear actuator. The two-way shape memory effect was induced through a compressive shape memory cycle comprising four steps: (1)?loading to maximum deformation; (2)?unloading; (3)?heating; and (4)?cooling. Six types of specimen (one solid cylindrical and five tubular) were used to obtain the two-way shape memory strain and two-way recovery stress and to evaluate the actuating capacity. The two-way actuating strain showed a saturated tendency after several training cycles for the same maximum deformation. A maximum value of the two-way strain was obtained for 7% of maximum deformation, independently of the geometry of the tubular specimens. The two-way strains obtained by the shape memory cycles and two-way recovery stress linearly increase as a function of the maximum deformation and the two-way strain, respectively, and the geometry of specimen affects the two-way recovery stress. Although the results show that sufficient recovery stress can be generated by either the two-way shape memory process or by the one-way shape memory process, the two-way shape memory process can be applied more conveniently to actuating applications.

Finite Element Model Updating and Validation of Satellites for Coupled Load Analysis

When developing medium satellites or large satellites, coupled load analysis(CLA) is performed in order to verify satellite design as a final assessment under launch environment. Maximum acceleration, gap between adjacent parts, internal loads obtained from CLA are used to assess the safety of satellite design by comparing them with the allowable loads of every component. To achieve reliable CLA results, satellite FE model have to be properly updated to match with the sine vibration test results. In this paper, the validation procedure of satellite FE model and its results are discussed.

Design of a Structural Model for Korean Lunar Explorer

Korean lunar explorer will be launched by korean launcher KSLV-2 in the 2020s in accordance with national space development strategy. Korean lunar explorer is composed of two unmanned orbiter and lander and should be developed as small size and light weight within 550kg of launch mass due to launcher`s loading capability. A structure of lunar explorer is required to have sufficient stiffness and strength under launch and operational environment as well as to accommodate mission equipment. This paper describes the result of a preliminary study on structural model design for korean lunar explorer.

Feasibility of PBGA packaging for high random vibration applications

In this study, the solder joint reliability of plastic ball grid array under harsh random vibration and thermal loadings was investigated. The chips were assembled on the daisy chained circuit boards for the test samples preparation, and part of the samples was processed for underfill to investigate the underfill effects on the solder failures. Two consequential steps of the random vibrations, named as the qualification and acceptance levels, were applied by pneumatic shaker. Overall controlled RMS of the power spectrum densities of the steps were 22.7 Grms and 32 Grms, respectively. Then the samples were undergone to thermal cycling tests. It was found that the samples survived without any solder failure during the test requirements, demonstrating the robustness of the packaging structure for potential avionics and space applications.

Thermal pointing error analysis of the observation satellites with interpolated temperature based on PAT method

In this work, we conduct a thermal pointing error analysis of the observation satellites considering seasonal and daily temperature variation with interpolated temperature based on prescribed average temperature (PAT) method. Maximum 200 degree temperature excursion is applied to the observation satellites during on-orbit operation, which cause the line of sight (LOS) to deviate from the designated pointing direction due to thermo-elastic deformation. To predict and adjust such deviation, the thermo-elastic deformation analysis with a fine structural finite element model is accomplished with interpolated thermal maps calculated from the results of on-station thermal analysis with a coarse thermal model. After verifying the interpolated temperatures by PAT with two benchmark problems, we evaluate the thermal pointing error.

Satellite finite element model updating for the prediction of the effect of micro-vibration

In this work, satellite FE (finite element) model updating for the prediction of the effect of micro-vibration is described. In the case of satellites launched in low earth orbit, high agility and more mission accomplishments are required by the customer in order to procure many images from satellites. To achieve the goal, many mechanisms, including high capacity wheels and antennas with multi-axis gimbals have been widely adopted, but they become a source of micro-vibration which could significantly deteriorate the quality of images. To investigate the effect due to the micro-vibration in orbit on the ground, a prediction is conducted through an integrated model coupling the measured jitter sources with FE (finite element) model. Before prediction, the FE model is updated to match simulation results with the modal survey test. Subsequently, the quality of FE model is evaluated in terms of frequency deviation error, the resemblance of mode shapes and FRFs (frequency response functions) between test and analysis.

SMA Damped Tape Spring Hinge for Quasi-Static Deployment of a Satellite Solar Array

A tape spring hinge is one of the most typical deployment devices and is frequently used in miniaturized satellite due to its simplicity, lightweight, low cost and high deployment reliability. But a tape spring hinge mechanism has limited performance due to the performance trade-off between deployed stiffness and latch-up shock.In this study, to improve a conventional tape spring hinge mechanism, SMA damped tape spring hinge is proposed so that a satellite solar array can be quasi-statically deployed. The test result shows the feasibility of the proposed concept.

Development of a Novel Deployment Hinge Mechanism for a Spacecraft Using Axiomatic Design

This paper presents a novel hinge mechanism for deployment of spacecraft subsystems such as antennas, solar arrays. By using Axiomatic design theory, the conceptual design of the hinge mechanism is suggested, which has not only high deployed stiffness and low deployment shock but also does not require lubrication and accurate fabrication. That is, optimization of deployment torque and maximization of the deployed stiffness can be possible since this suggested hinge mechanism is decoupled design. And the suggested hinge mechanism is fabricated and tested to evaluate the feasibility. Quasi-static analysis is performed to optimize deployment torque for low deployment shock by using FEM. Also, the bending stiffness is measured by 4 point bending test.