Session: 01-08-01: Vibration and Acoustic Measurements, Signal Processing, and Test Facilities

Paper Number: 68659

Start Time: Wednesday, 01:20 PM

68659 - Analysis and Optimization of the Recoil-Compensated Absolute Gravimeter 

The free-fall absolute gravimeter is the instrument for precise gravitational acceleration measurement and has been used in the study of metrology, geology, seismology, etc. During the measurement process of gravimeter, a test mass falls freely in the vacuum dropping chamber and its trajectory is tracked by one arm of an interferometer. The g value is then calculated through the quadratic fitting of the acquired position-time data pairs. However, this procedure is practically complicated by disturbances of both the instrument and its support. Vibrations during the measurement process will introduce systematic changes in the optical path length and eventually cause errors in the computed gravitational acceleration results. There are two types of disturbances that must be dealt with, the first one is seismic noise which is from external conditions, and the other is generated by the gravimeter itself. Previous study is mainly concerned with seismic vibrations, while the self-vibration of the instrument needs further exploration. The recoil vibrations are generated by the release of the test mass due to the movement of the center of mass of the chamber. These vibrations are transmitted to the dropping chamber and reference mirror, thus introducing noises to the measurement of the trajectory. Past research has made efforts to deal with the recoil vibrations by using the recoil-compensated structure to optimize the test procedure. Niebauer et al added counterweights in the dropping chamber for FG5 gravimeter, aiming to achieve a basically stationary center of mass during the drop. Our group developed a recoil-compensated structure R-1 absolute gravimeter to reduce the recoil vibration amplitude, which also showed its superiority in simulation.

Despite these efforts, further exploration of our homemade gravimeter is needed to confirm the simulation and collect its dynamic characteristics for future improvement. In this paper, various mass of counterweight are tested on R-1 gravimeter and the recoil vibrations are recorded and analyzed. Simulation results show that proper compensated counterbalance can largely reduce the recoil vibration amplitude, thus making it promising to obtain a more precise measurement. In the experiment part, accelerometers and seismometers are used respectively to measure the recoil vibrations of the dropping chamber and reference reflector simultaneously. These vibration signals of different sets are analyzed and compared in both time and frequency domain, and the outcomes confirm the conclusion drawn by simulation. The present study determines an optimized mass of the counterbalance based on our existing gravimeter and measures the perturbing vibration. Results and discussions of this research indicate that with a proper design of compensated mass, the recoil vibration amplitude can be significantly eliminated during the test procedure, which has a positive meaning to the improvement of high precision measurement.

Presenting Author: Yicong Chen Tsinghua University

Authors:

Yicong Chen Tsinghua University
Kang Wu Tsinghua University
Yi Wen Tsinghua University
Lijun Wang Tsinghua University