Natural-frequency-adjustment characteristics of symmetric solenoids in a superconducting gravimeter

Natural frequency tuning is a critical aspect of superconducting gravimeter adjustment. The coupling of the natural frequency tuning with the levitation position of the test mass in our previous design complicates the natural-frequency-adjustment process. We introduce a new magnetic spring oscillator structure, along with novel superconducting circuits. By employing symmetric solenoids in this structure, the positional shift of the test mass is indicated by the excitation response, enabling the optimal levitation position to be identified. The levitation position remains nearly unchanged when the natural frequency is adjusted at this position. This approach decouples natural frequency tuning from position regulation. The experimental results confirm the feasibility and efficiency of this method, with the natural frequency adjusted from 11.9 to 1.7 Hz. Over ten days of solid tide observations demonstrated the reliable performance of the superconducting gravimeter.