• International Journal of Aeronautical and Space Sciences
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• v.7 no.2
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• pp.14-25
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• 2006

This paper is focused on designing an implementable control law to perform spacecraft various missions using momentum exchange devices such as reaction wheels(RWs) and control moment gyros(CMGs). A compact equation of motion of a spacecraft installed with various momentum exchange devices is derived in this paper. A hybrid control law is proposed for precision attitude control of agile spacecraft. The control law proposed in this paper allocates control torque to the CMGs and the RWs adequately to satisfy the precision attitude control and large angle maneuver simultaneously. The saturation problem of reaction wheels and the singularity problem of control moment gyros are considered. The problems are successfully resolved by using the proposed hybrid closed loop control law. Finally, the proposed hybrid control law is demonstrated by numerical simulations.

• Journal of Institute of Control, Robotics and Systems
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• v.18 no.9
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• pp.837-844
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• 2012

We introduce a pattern generation method for a hopping one-legged robot and verify it experimentally. The pattern is derived from the liner and angular momentum of a COM (Center of Mass), which are pre-scheduled. Because of the relation between angular velocities of joints and momemtums of the COM, joint angle trajectories are easily obtained. In addition, the landing impact force is reduced by only adjusting the landing timing. In the experiment, the one-legged robot hops in place with 0.06 s of flying time, and makes continuous hopping. Based on our experimental results, the proposed method can be applied to hopping and running of biped humanoid robots.

• KDI Journal of Economic Policy
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• v.30 no.1
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• pp.171-209
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• 2008

The Great Depression is one of the most important economic incidents in the twentieth century. A significant and long-lasting impact of this event is the rise of the government intervention to the economy. Under the catastrophic downturn of the economic condition worldwide, people required their government to play an active role for economic recovery, and this $mentalit{\acute{e}}$ prolonged even after the Second World War. Social science textbooks taught at Korean high schools mostly referred to the Great Depression for explaining the reason of government intervention in economy. However, the mainstream view commonly found in the textbooks provides a misleading theological interpretation. It argues that inherent flaws of the market economy causes over-production/under-consumption, and that this mismatch ends up with economic crisis. The chaotic situation was resolved by substitution of the governments for the market, and the New Deal was introduced as the monumental example ('laissez-faire economy ${\rightarrow}$over-production${\rightarrow}$the Great Depression${\rightarrow}$government intervention${\rightarrow}$economic recovery'). Based on economic historians' researches for past three decades, I argue that this mainstream view commits the fallacy of ex-post justification. Unlike what the mainstream view claims, the Great Depression was neither the result of the 'market failure', nor the recovery from the Great Depression but was due to successful government policies. For substantiating this claim, I suggest three points. First, blaming the weakness or instability of the market economy as the cause of the Great Depression is groundless. Unlike what the textbooks describe, the rise of the U.S. stock price during the 1920s cannot be said as a bubble, and there was no sign of under-consumption during the 1920s. On the contrary, a new consensus emerging from the 1980s among economic historians illustrates that the Great Depression was originated from 'the government failure' rather than from the 'market failure'. Policymakers of European countries tried to return to the gold standard regime before the First World War, but discrepancies between this policy and the reality made the world economy vulnerable. Second, the mainstream view identifies the New Deal as Keynesian interventionism and glorifies it for saving the U.S. economy from the crisis. However, this argument is not true. The New Deal was not Keynesian at all. What the U.S. government actually tried was not macroeconomic stabilization but price and quantity control. In addition, New Deal did not brought about economic recovery that people generally believe. Even after the New Deal, industrial production or employment level remained quite low until the late 1930s. Lastly, studies on individual New Deal policies show that they did not work as they were intended. For example, the National Industrial Recovery Act increased unemployment, and the Agricultural Adjustment Act expelled tenants from their land. Third, the mainstream view characterizes the economic order before the Great Depression as laissez-faire, and it tends to attribute all the vice during the Industrial Revolution era to the uncontrolled market economy. However, historical studies show that various economic and social problems of the Industrial Revolution period such as inequality problems, child labor, or environmental problems cannot be simply ascribed to the problems of the market economy. In conclusion, the remedy for all these problems in high school textbooks is not to use the Great Depression as an example showing the weakness of the market economy. The Great Depression should be introduced simply as a historical momentum that had initiated the growth of government intervention. This reform of high school textbooks is imperative for enhancing the right understanding of economy and history.