Design and Fabrication of a Superconducting Relative Gravimeter With a Planar Spring

The relative gravimeter is in high demand to improve the performance of a navigation system. Therefore, we designed, fabricated, and tested a superconducting relative gravimeter. The proposed gravimeter features a superconducting planar spring, an electromagnetic levitation, and a superconducting quantum interference device (SQUID) sensor. The advantages of the superconductor include being able to enable the device with exceptional stability and having an extremely low noise. The coupling spring with electromagnetic levitation can also offer a low resonant frequency for better sensitivity. The device consists of a niobium mass-spring vibrometer having a planar spring, a levitation coil, and a SQUID sensor. The vibrometer was designed by conducting the mathematical analysis and the mechanical- electromagnetic finite element analysis simulation. Both the electrodischarge machining process and the high vacuum-thermal process were applied to fabricate superconducting structures. Experiments to store and discharge a persistent current in the levitation coil were carried out at 4.2 K cryogenic condition to confirm the numerical prediction. An experimental setup with a precision linear actuator was used to verify the relationship between induced inductance and displacement. The test result indicates that the gravimeter can measure the fine movement of the levitation mass by reading a SQUID output and the scale factor is ~28 mV/um.