Fax: (818) 393-4440 spano s@ c s i Abstract The paper presents analytical and experimental results with a 6-axis actively controlled vibration isolator. The isolator was designed to attenuate vibrations from reaction wheels, cryogenic coolers and other noisy machines located on board precision optical-class spacecraft. It consists of six active struts each of which includes an electromagnetic voice coil actuator in parallel with a soft spring. Six decoupled analog controllers were used to close broad-band feedback loops around six force sensors and the isolator multi-axis transmissibility was measured with twelve accelerometers (six on the noisy side and six on the quite side). Experimental results show a ten-fold improvement in performance over passive isolation alone.
Future direct imaging missions such as HabEx and LUVOIR aim to catalog and characterize Earth-mass analogs around nearby stars. The exoplanet yield of these missions will be dependent on the frequency of Earth-like planets, and potentially the a priori knowledge of which stars specifically host suitable planetary systems. Ground or space based radial velocity surveys can potentially perform the pre-selection of targets and assist in the optimization of observation times, as opposed to an uninformed direct imaging survey. In this paper, we present our framework for simulating future radial velocity surveys of nearby stars in support of direct imaging missions. We generate lists of exposure times, observation time-series, and radial velocity time-series given a direct imaging target list. We generate simulated surveys for a proposed set of telescopes and precise radial velocity spectrographs spanning a set of plausible global-network architectures that may be considered for next generation extremely precise radial velocity surveys. We also develop figures of merit for observation frequency and planet detection sensitivity, and compare these across architectures. From these, we draw conclusions, given our stated assumptions and caveats, to optimize the yield of future radial velocity surveys in support of direct imaging missions. We find that all of our considered surveys obtain sufficient numbers of precise observations to meet the minimum theoretical white noise detection sensitivity for Earth-mass habitable zone planets, with margin to explore systematic effects due to stellar activity and correlated noise.
Lead magnesium niobate (Pb(Mg1/3Nb2/3)O3 or PMN) exhibits many attractive properties required by actuators for precision submicron control. At room temperature hysteresis is negligible, thermal expansion is less than 1 microstrain degrees C-1, and the longitudinal strain sensitivity is 375 microstrain at 600 V mm-1. There has been recent interest in using PMN actuators in applications near 0 degrees C, which is near the Curie temperature of the material. The purpose of this paper is to use the nonlinear constitutive relations describing the fundamental material behavior, along with supporting to test results, to establish a foundation for an engineering description of electrostrictive materials for the design of integrated actuator and structure systems. This paper presents an experimental characterization of PMN strain response, hysteresis and dielectric permittivity for temperatures between -50 degrees C and 100 degrees C. Measurements were made of the transverse strain of a thin PMN plate at bias fields of 0 and 400 V mm-1 and for frequencies between 1 and 1000 Hz. Field dependent effects are discussed in terms of the nonlinear electrostrictive equations derived from a parametric elastic Gibbs free energy function. The temperature dependence of the permittivity and induced piezoelectric coefficient are modelled by a modified Curie-Weiss law.
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