An Improved Vertical Seismometer with Build-in Retroreflector

A classical absolute gravimeter is widely used to measure the gravitational acceleration, normally known as g. It applies a Mach-Zehnder interferometer to track a free-falling retroreflector in a vacuum chamber. Theoretically, it needs a static reference retroreflector in an inertial frame. Practically, the reference retroreflector is always disturbed by the ground vibration. Vibration correction methods apply the seismometer output to calculate the motion of the reference retroreflector. Transfer function between them is hypothesized to approach the real transfer function. The error between them limits the measurement effect. A direct measurement method, which puts the reference retroreflector on the sensitive beam, is proposed. A differential parallel plate capacitance detection is used to detect the movement of the reference retroreflector relative to the ground. A closed-loop feedback controller is applied to drive the reference retroreflector tracking the ground vibration. The feedback voltage represents the ground vibration acceleration, which is the motion acceleration of the reference retroreflector. A spring, instead of another voice coil actuator in previous version, is used to balance the sensitive beam gravity. It greatly reduces the thermal noise of the seismometer. The sensitivity and acceleration resolution of the capacitance detection are respectively 3.1 V/pF and 25 μGal (1 μGal = 10−8 m/s2). In the closed-loop system, the sensitivity and resolution of the instrument are respectively 12274 V/g and 644 μGal. The bandwidth of the system is 43 Hz. The damping ratio of the system needs to be increased. In the future, it will be used for T-1 absolute gravimeter.