Self-Vibration Analysis of the Free-Fall Absolute Gravimeter

The ballistic free-fall absolute gravimeters are widely used in acquiring information of gravity field and the self-vibration of the absolute gravimeter is crucial for high precision gravitational measurement. The self-vibration of the T-1 absolute gravimeter has a multi-directional full-band excitation. The resulted horizontal swing has the major impact, since the T-1 absolute gravimeter is simplified as a cantilever beam. A laser vibrometer was applied to directly measure the mechanical self-vibration. The frequency of self-vibration has a nonlinear effect on the measurement error of g, and the peak frequencies should be avoided. The vibration signal was analyzed in time and frequency domain by continuous wavelet transform (CWT). The close frequency profiles were measured in the scalogram and the beat vibrations were observed in time domain as the results of the horizontal swing. The 38 Hz self-vibration had the dominating effect on the measurement error of g for T-1 absolute gravimeter. After optimizing the structure of the tripod, the dominating frequency increased from 38 Hz to 42 Hz. A 11% increase of the vibration frequency can reduce the measurement error of g.