The Design of a Sub-Hz Magnetic-Suspended Geophone

A geophone usually behaves like a high-pass filter. The lower bound of its measuring bandwidth is often dependent on the dynamics of the interior mass-spring mechanical structure. Reducing the stiffness of the interior mechanical spring could be an effective way to improve the low-frequency performance of a geophone. Unfortunately, the resultant system may produce a nonlinear response and suffer a stability problem. In this article, a novel magnetic-suspended (MS) geophone is developed. It employs four well-designed concentric magnetic cylinders as the magnetic spring to suspend the proof-mass, therein, the two middle magnetic cylinders produce a uniform magnetic flux around the induction coil. Moreover, two compensating coils are introduced to be independently driven by adjustable direct currents. They provide a low linear stiffness and can also compensate for the magnetic force errors of the magnetic spring caused by the manufacturing and assembly. Therefore, the sensitivity of the MS geophone is kept constant, and a sub-Hz natural frequency is obtained. A physical prototype has been built to evaluate the dynamic behaviors. The results have shown that the natural frequency and the sensitivity of the developed MS geophone are, respectively, 0.59 Hz and 412 V/(m/s). The proof-mass excursion is within the range of ±1.2 mm. The prototype may be used as a complementation of the commercial geophone for the applications of low-frequency active vibration isolation.