Rotation control of a variable-capacitance electrostatic motor for space equivalence principle tests with rotating masses.

Variable-capacitance electrostatic motors are ideal for driving the test mass in ultra-low-noise electrostatic accelerometers. Such devices are essential for testing the new equivalence principle (NEP) with rotating extended masses. However, as the air-film damping is greatly reduced by placing the sensor core assembly in a high-vacuum housing, this synchronous motor may easily fall out of step and suffer spin-up failures with traditional open-loop excitation. In this study, a synchronous electronic phase commutation scheme is proposed by sensing the three-phase position change of the rotor poles and activating the stator electrodes in careful correlation with the instantaneous rotor position. Experiments on a ground-test NEP instrument prototype show that the proposed closed-loop excitation scheme can spin-up the rotor synchronously and maintain stable constant-speed operation of this macroscale variable capacitance motor operated in a high-vacuum environment. This rotation control method is also applicable to the synchronous operation of micromachined variable-capacitance electrostatic motors.

Modeling and compensation of cross-axis coupling in an electrostatic accelerometer for testing the equivalence principle.

Electrostatic accelerometers have extremely high sensitivity and are ideal scientific instruments for measuring very weak acceleration. In particular, a single-sensitive-axis electrostatic accelerometer can be used for testing the equivalence principle in space. Sensitive-axis capacitances formed by axial electrodes and a cylindrical proof mass vary with the axial motion of the mass and are also affected by radial motion, which results in cross-axis coupling disturbances. A quantitative model is built to analyze the cross-axis coupling effect on the sensitive axis from the radial suspension loop, including a nonlinear model for large radial motion and a linear model for small radial motion. Frequency response simulation shows that the cross-axis coupling effect for a small signal case arises mostly in the high-frequency range. Experiments are carried out with a ground-based electrostatic accelerometer made of a single, non-rotating test cylinder, and in this case, the experimental results are utilized to verify the mathematical model. Cross-axis coupling for small signal perturbations is virtually removed if the equilibrium position of the proof mass is calibrated to the null position of the sensor cage. In addition, data post-processing can further attenuate the cross-axis coupling disturbances when dealing with large radial motion. The cross-axis coupling disturbances on both the position and the acceleration measurement signals in the sensitive axis are mostly removed in ground-based experiments. The proposed model and compensation can be extended to space equivalence principle instruments and other electrostatic accelerometers with a cylindrical proof mass.

Microgravity Level Measurement of the Beijing Drop Tower Using a Sensitive Accelerometer.

Drop tower is the most common ground-based facility to provide microgravity environment and widely used in many science experiments. A differential space accelerometer has been proposed to test the spin-gravity interaction between rotating extended bodies onboard a drag-free satellite. In order to assist design and test of this inertial sensor in a series of ground- based pre-flight experiments, it is very important to know accurately the residual acceleration of drop towers. In this report, a sensitive instrument for this purpose was built with a high-performance servo quartz accelerometer, and the dedicated interface electronics design providing small full-scale range and high sensitivity, up to 136.8 V/g0. The residual acceleration at the Beijing drop tower was measured using two different drop capsules. The experimental result shows that the microgravity level of the free-falling double capsule is better than 2 × 10(-4)g0 (Earth's gravity). The measured data in this report provides critical microgravity information for design of the following ground experiments.

Seizure resistance is dependent upon age and calorie restriction in rats fed a ketogenic diet.

The present study was designed to evaluate the effects of age on the efficacy of the ketogenic diet in suppressing seizures evoked by tail-vein infusion of pentylenetetrazole (PTZ). Male rats of various ages were divided into three groups and fed one of three diets: (1) a calorie-restricted ketogenic diet, (2) a calorie-restricted normal (rodent chow) diet, or (3) a normal diet, ad libitum. After animals had been on experimental or control diets for more than 20 days, seizure threshold and blood levels of beta-hydroxybutyrate (beta-OHB) were determined. Animals fed a ketogenic diet exhibited significant elevations in levels of beta-OHB and seizure resistance compared to animals fed either a calorie-restricted normal diet or a normal diet, ad libitum. The levels of beta-OHB and seizure resistance were greatest for young pups. A surprising finding was that young animals fed a calorie-restricted rodent chow diet exhibited a significantly increased resistance to seizures compared to those fed the same diet, ad libitum. Results presented here demonstrate that the ketogenic diet produces the highest levels of ketonemia and seizure threshold in young animals. Collectively, these data suggest that age and caloric restriction are important considerations for implementing the ketogenic diet.