Hyunjae Lee

Star Pattern Identification Technique by Modified Grid Algorithm

A star pattern identification algorithm, called modified grid algorithm, for attitude determination of spacecraft is addressed. The proposed algorithm is closely connected to a general pattern recognition technique. Each star is characterized by a well-defined pattern that can be determined by the surrounding stars. The so-called grid algorithm is one of the star identification algorithms based upon the pattern recognition approach. The proposed algorithm, is motivated by the conventional grid algorithm. To enhance the performance of the conventional grid algorithm, a modified method using polar grid, virtual grid, and multi-references is proposed. Simulation study is conducted for the demonstration of the new algorithm. The proposed modified grid approach turns out to make the grid algorithm more robust and reliable.

Airship control using neural network augmented model inversion

Airship is a highly nonlinear plant which requires gain scheduled controller for various flight conditions. Model inversion control is augmented with neural network to control the airship. The input-output feedback linearization allows one to design a linear controller from a nonlinear system. Neural network is used to compensate the nonlinearities resulting from this linearization. The control technique is applied to two different airship flight conditions. The results shows that control technique is successful in controlling both conditions.

Modified grid algorithm for star pattern identification by using star trackers

In this paper a star pattern identification algorithm is addressed. Proposed algorithm is associated in terms of pattern recognition. Each star has related with well-defined pattern that can be determined by the surrounding stars. One of the well-known types related the class is the grid algorithm. The proposed algorithm is based on the algorithm. To enhance the algorithm the modifying methods so called virtual grids and selection multi-reference stars are proposed. This method makes the grid algorithm more accurate and reliable.

Nonlinear Momentum Transfer Control of a Gyrostat with a Discrete Damper by Using Neural Networks

An adaptive feedback linearization technique combined with neural networks is addressed for the momentum transfer control of a torque-free gyrostat with an attached spring-mass-dashpot damper. The input normalization neural network is used to adaptively compensate for the model error uncertainties of a nutation damper as an internal dynamics and avoid the unnecessary assumptions for stability analysis. The whole spacecraft angular momentum component of the wheel spin axis is selected as the output function of the feedback linearization. Thus, a desired output function is predefined that the total angular momentum of the spacecraft is absorbed into the wheel spin direction at the steady state so that the nutation angle converges to zero. The ultimate boundedness of the tracking error is simply proved by the Lyapunov stability theory. We also investigate the effect of rotor misalignment on the steady spin of the spacecraft. The effectiveness of the proposed control law is verified through simulation results.

Study on the glyoxylate cycle in germinating sesame seed embryos

Abstract The glyoxylate-cycle system in germinated sesame seed embryos was examined from several viewpoints. This glyoxylate-cycle system was found in the mitochondrial fraction of germinated sesame seed embryos, and it also was found that this cycle should predominate over the TCA-cycle system in seed mitochondria during the germination, although the mitochondrial fraction of sesame seed also contained other TCA-cycle enzymes, condensing enzyme, aconitase, fumarase, etc., in addition to isocitritase and malate synthetase. The activities of both glyoxylate-cycle enzymes, isocitritase and malate synthetase, were increased remarkably by the period of seed germination after radicle emergency, and reached maximum activities at near the 8th day of seed germination. From these data and other information in the literature, the glyoxylate-cycle system in sesame seed embryos should act as a plausible route for the conversion of acetate into nucleic acid by means of some modified metabolic pathway combined with reversal of glycolysis.

Role of chloride ion as an allosteric activator of angiotensin-converting enzyme

The nature of chloride ion as an activator of angiotensin-converting enzyme was studied by a series of kinetic experiments with hog plasma enzyme preparation. The enzyme required the presence of chloride ion for its full catalytic activity, but its requirement of monovalent anion was not absolute. The KA value for the enzyme-chloride binding was estimated to be about 150 mM in all cases regardless of the peptide substrates employed. In the presence of chloride ion, the activity of the enzyme was increased, but its optimum pH was shifted gradually to the alkaline region up to pH 8.2 depending on the concentration of chloride ion. In addition, in the presence of chloride ion, the apparent Km values were reduced markedly while the Vmax values were not much altered; for example, for the hydrolysis of angiotensin I decapeptide, the Km value decreased by a factor of 50 while only an 18% increase in Vmax was observed when the enzyme was saturated with chloride ion. The result suggests that chloride ion acts as a conformational modifier inducing the affinity of synergistic binding of substrate.

Singularity avoidance of variable speed control moment gyros by optimization approach

Singularity avoidance steering laws for control momentum gyros (CMGs) and variable speed control moment gyros (VSCMGs) are addressed. The proposed methodology is based on a constrained optimization by minimizing a cost function which consists of a singularity index and a kinetic energy term. The new singularity avoidance law prevents CMG/VSCMG clusters from encountering a singularity condition. It turns out that the proposed optimal CMG steering law is in a general form including the conventional null motion solution. Also, an optimal steering law is developed to overcome the wheel speed saturation problem during the operation of VSCMG clusters. The performance of the new steering law is demonstrated through a simulation study with a three-parallel single-gimbal CMG/VSCMG cluster.

SINGULARITY AVOIDANCE LAW FOR CMG USING VIRTUAL ACTUATOR

Abstract CMGs are actuators for the attitude control of spacecraft. The simplest steering logic of CMGs is the well-known pseudo-inverse method, which is derived from the minimization problem. A drawback of this logic is that the CMG can be singular during the operation. In this paper, the singularity problem is investigated, and a singularity avoidance steering law for CMGs using virtual actuator is proposed. The virtual vector is determined by cross product with 4 column vectors of Jacobian matrix. Using this virtual vector, the system can escape the singular state effectively when CMGs approaches to singular configuration.

An experimental study on attitude control of spacecraft using neural networks

This primary objective of this study is to demonstrate ground-based experiment for the attitude control of spacecraft. A two-axis rotational simulator with a flexible arm is constructed with on-off air thrusters and reaction wheel as actuation. The simulator is also equipped with payload pointing capability by simultaneous thruster and DC servo motor actuation. The experiment of spacecraft attitude control is performed using the reaction wheel. Using the reaction wheel the performance of the PID and the three proposed neural network controllers is illustrated by simulation and experimental results for a two-axis rotational simulator.

The previewable switch: a light switch with feedforward

A light switch and its spatial mapping constitute an unsolved, user interface problem introduced to the realm of Human Computer Interaction by Norman. In this paper, we introduce the Previewable Switch which is a light switch enhanced with a touch sensor with an ability to provide a user a glimpse of which lights will turn on/off prior to compressing the switch. Such concept of communicating a result prior to taking an action is known as feedforward [6]. Feedforward is an important element to be considered in the user interface design as it provides clear and instant affordance of what will occur before the next action. We present the findings from the study in regard to behavior and spatial mapping. We will discuss the effect of feedforward as a design element in a user interface.