Adaptive or intelligent structures which have the capability for sensing and responding to their environment promise a novel approach to satisfying the stringent performance requirements of future space missions. This research effort focuses on the development and performance evaluation of an active composite strut (ACS) with precision positioning and active vibration suppression capabilities for use in space structures. Such ACS has potentials to be used as active strut members in Stewart platforms, modified Stewart platforms (MSPs), space truss structures, etc. The developed ACS utilizes piezoelectric actuators/sensors providing precision positioning and vibration suppression capabilities that would enhance mission performance of space structures by fine position-tuning, and potentially eliminating the nonoperational period of a satellite while minimizing the fuel consumption utilized for its position correction in a thrust vector control (TVC) application. Precision positioning of the ACS is achieved by extending or contracting its internal piezoelectric actuator. To magnify the positioning capabilities of a stack piezoelectric actuator, a miniature inchworm mechanism is designed and incorporated within the ACS. A precision positioning experimental setup is developed and employed to demonstrate the precision positioning capabilities of the developed ACS. The axial vibration suppression capability of the ACS can also be provided by its internal piezoelectric stack actuator, and is demonstrated employing a developed vibration suppression experimental setup. Analytical and finite element analysis numerical verification, simulation, and modeling of the vibration suppression capabilities of the ACS are presented. Active vibration suppression schemes, using finite element analyses, are employed to numerically demonstrate the vibration suppression capabilities of the developed ACS. The ACS housing is a composite tubing. The numerical and experimental results show that the proposed ACS offers a promising method for achieving fine tuning of positioning tolerances as well as minimizing the effects of disturbances generated during a thruster firing of a satellite for the TVC application using a MSP.
This case demonstrates the importance of including idiopathic ovarian vein thrombosis in the differential diagnosis of nonperipartum females with pelvic pain.
Career executives often occupy administrative positions that determine the pace and content of policy, such as those responsible for developing regulations. Yet, presidential administrations need control over these positions to achieve policy aims. This article considers the extent to which new presidential administrations marginalize career executives in key regulatory positions by transferring responsibilities to another individual and whether the mere expectation of political conflict with a new administration drives career regulators from their positions. Using unique new data on 866 career regulators that led major rulemaking efforts between 1995 and 2013, we demonstrate that turnover among career executives in key regulatory positions increases following a party change in the White House. Turnover also increases during a presidential election year, but this effect is conditioned by bureaucrats’ expectations of the election outcome. Finally, career executives are more likely to depart in response to favorable labor market conditions. Given our findings that turnover in regulatory responsibilities is driven both by presidential marginalization and strategic exit by bureaucrats, we conclude with implications for presidential efforts to control the administrative state.
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