A Compound Recoil-Compensated Chamber Design for Free-Fall Absolute Gravimeters

High-precision measurements of the absolute value of Earth’s gravity acceleration (g) have seen increasingly importance in many fields, such as basic scientific research, resource survey, geophysics and so on. The high-precision absolute gravimeter has become a necessity for such applications. One classical free-fall method determines the gravity value by tracking the free-fall trajectory of a test mass in a vacuum with laser interferometer. In this procedure, the ground vibration causes the light path length variation and thus disturbs the measured positions of the test mass. And the instrumental recoil vibration generated by the release of the test mass is one of the most troublesome problems. It is highly reproducible from drop to drop with coherent phase so that it can cause systematic bias. In order to achieve high accuracy of μGal magnitude (1 μGal = 10^-8 m/s²), the instrumental recoil vibration must be reduced or eliminated.

Two recoil-compensated dropping chamber designs have been proposed by Faller et al and Niebauer et al so far. These designs both employ an auxiliary cart moving opposite in direction to the main cart to counterbalance the dropping chamber and eliminate the recoil. Faller uses an asymmetric double cam for drive mechanism. The off-centre double cam accelerates a compensating cart upwards while it also accelerates a main cart containing the test mass downwards. The motions of the main cart and the compensating cart can be artificially defined by the shapes of two cams respectively, and the two cams together are balanced around their common axis of rotation. The net effect is that the centre of mass of the dropping chamber and free-falling test mass remains stationary during the measurements, thus in principal eliminating recoil effects. But the cam radius limits its drop length, which reduces its measurement precision. Niebauer designs a belt-driven mechanism to counterbalance the dropping chamber. A belt pulls the main cart in one direction and two counterweights in the opposite direction to reduce recoil. The movement of them are 180 degrees out of phase. This design allows for a drop up to 33cm, much longer than the 3 cm available with the cam system. However, its overall centre of mass is not kept (theoretically) completely fixed during measurements because motions of the counterweights and main cart cannot be defined respectively.

A new counterbalanced design is proposed in this paper in order to achieve excellent recoil compensation as well as long freefall length for high precision applications. Based on the belt-driven mechanism proposed by Niebauer, our design adds a small cam-driven structure to drive an auxiliary mass for additional recoil-compensation. In this way, the drop length is ensured by the belt-driven mechanism and the centre of mass of the chamber and test mass can be completely fixed with cam-based structure at the same time. We first give out the theoretical scheme of this new design. Then, the modelling and simulation of the new compound mechanism is carried out by Simulink. The simulation result shows that the system with new counterbalanced design reduces the instrument vibration caused by the recoil effect by a factor of 6 compared with the previous structure. And this confirms the feasibility and superiority of this new design. Furthermore, the absolute gravimeter based on this compound chamber design is expected to obtain more precise gravity measurement results than those with aforementioned chamber structures.