The absolute gravimeter plays an important role in metrology, geophysics, and geological exploration. Seismic and environmental vibration has been one of the most serious factors limiting its performance. Consequently, an ultra-low-frequency vertical vibration isolator is required to significantly improve its measurement precision. A novel active vertical vibration isolator employing geometric anti-spring (GAS) structure is proposed in this paper. The payload is supported by a GAS structure fixed on an inner frame, and the inner frame is hung by coil springs from the base. The relative movement of the payload with respect to the inner frame is detected, and the inner frame is driven by a voice coil actuator controlled by a feedback circuit to track the payload’s motion. The new isolator has a compact size, and it can be used for different load ranges by tuning the GAS structure. The practical closed-loop system has a resonance period of 19.2 s, compared with the period of 0.74 s in an open-loop system. Experiments showed that the new isolator has great performance in a homemade T-1 absolute gravimeter, reducing the measurement deviation by a factor of 32. It is expected to be used in both free-falling and atomic-interference absolute gravimeters. Future improvements may include optimizing the mechanical structure and integrating a temperature control subsystem.

Many efforts have been taken in recent years to push atomic gravimeters toward practical applications. We demonstrate an atomic gravimeter named NIM-AGRb2 that is transportable and suitable for high-precision gravity measurements. Constraint-structured active vibration isolation (CS-AVI) is used to reduce the ground vibration noise. The constraint structure in CS-AVI ensures that the isolation platform only has vertical translation, with all other degrees of freedom (DoFs) being constrained. Therefore, the stability of active vibration isolation is enhanced. With the implementation of CS-AVI, the sensitivity of NIM-AGRb2 reached as low as 20.5 μGal/Hz^1/2. The short-term sensitivity could be further reduced to 10.8 μGal/Hz^1/2 in a seismologic observation station. Moreover, we evaluated the system noise of the gravimeter, and the results were consistent with our observations.

Classical absolute gravimeters are widely used in many areas, including metrology, seismology, geophysics, and exploration. These gravimeters apply a free-fall method to measure absolute gravitational acceleration using a laser interferometer. In practice, the measurement precision is affected by ground vibrations. Vibration correction is an important way to deal with these ground vibrations for absolute gravity measurement. Most vibration correction methods use a commercial seismometer to measure the motion of a reference retroreflector indirectly. A vertical seismometer with a built-in retroreflector is designed to directly measure the motion of the retroreflector, which is fixed on the proof mass of the seismometer. The motion of the retroreflector relative to the ground vibration is measured through capacitance detection. A voice coil actuator is employed to drive the retroreflector to track the ground vibration. The sensitivity of the homemade vertical seismometer is approximately 15751 V/g. The bandwidth of the system is 80 Hz. The homemade vertical seismometer is integrated into the homemade T-1 absolute gravimeter to test its performance. The standard deviation of the measurement results is reduced by a factor of 16, when compared with the results obtained without any vibration treatment. Absolute gravimeters with the homemade vertical seismometer are expected to be used in field measurements.

For absolute gravimeters, which play important roles in geophysics and geological exploration, an ultra-low-frequency vertical vibration isolator is necessary to achieve the required measurement precision. A novel active vibration isolator that uses a geometric anti-spring (GAS) structure has been proposed by our team at Tsinghua University previously, but its performance is mainly limited by the large-scale drift in the detection signal of the system. In this paper, after a brief theoretical introduction to the overall system, recent improvements in this novel vibration isolator are presented. The main improvements to the isolator are the use of new blades in the GAS structure and the addition of an extra compensation circuit to eliminate the drift. The improved prototype has a resonance period of 29.2 s and a continuous working time of several days, as compared with the resonance period of 19.2 s and a working time lasting only several minutes of the previous prototype. Experiments show that the improved prototype performs well in the homemade T-1 laser-interferometry absolute gravimeter. The standard error of the mean (SEM) of a 50-drop measurement performed in Tsinghua University is reduced significantly from 404 μGal (1 μGal = 1 × 10⁻⁸ m s⁻²) without the vibration isolator to 10.8 μGal with the improved prototype at its best level. Additionally, the SEM of a 50-set measurement (including 800 drops) lasting for 25 h achieves 5.9 μGal with the improved prototype.

Measuring low-frequency and ultra-low-frequency vibration signals is of critical importance in fields such as structural mechanics, geological exploration, aerospace, precision machining, and biomedicine. Existing methods face limitations in achieving both ultra-low-frequency range and high precision. We present an ultra-low-frequency vibration measurement method based on the atom interferometer, capable of measuring vibration signals from 0.01 Hz to DC. The performance of measurement was experimentally demonstrated for vibrations between 0.007 Hz and 0.01 Hz, achieving a sensitivity of $1.1 \mathsf{\mu }\mathrm{m}/{\mathrm{s}}^2/\sqrt{\mathrm{Hz}}$. Incorporating active vibration isolation can further enhance the measurement range, increasing the optimal sensitivity to $0.2 \mathsf{\mu }\mathrm{m}/{\mathrm{s}}^2/\sqrt{\mathrm{Hz}}$.

The stability of absolute gravimeters during carriage-falling body separation is crucial for improving gravitational acceleration measurement accuracy. Transmission speed accuracy of the transmission system and system vibration are core factors determining this stability, while steel belt pre-tightening force, free-fall segment acceleration, and start-up segment displacement are key parameters influencing both. In-depth analysis of their coupling clarified their roles, and two objective function models (for speed accuracy and vibration) were established, with fitting accuracies R² = 0.8976 and R² = 0.8395, respectively. Since traditional single-objective optimization fails to balance “improving speed accuracy” and “suppressing vibration”, this study proposes a multi-objective optimization method: two Nondominated Sorting Genetic Algorithm II (NSGA-II) parameter sets were designed, Hypervolume (HV) index quantified solution set quality, and Wilcoxon signed-rank test was combined to determine the optimal parameter set; comparing the Global Criterion Method and Weighted Sum Method, the former was superior (no dimensional bias) and more suitable for this study, finally screening out the optimal parameter combination. Experimental results showed that the measured transmission speed accuracy was 0.09132 m/s (16.94% lower than the orthogonal experiment’s optimal level); the measured system vibration was 0.022 m/s², falling within the orthogonal experiment’s optimal range. Consequently, separation moment stability was significantly enhanced, with its standard deviation reduced by 45% pre-optimization. This method achieves global balance in transmission system dynamic performance, providing an effective parameter optimization strategy for improving absolute gravimeter measurement accuracy.