## 4 Current Progress of Magnetic Negative Stiffness in QZS Isolators
### QZS-Based Electromagnetic Negative Stiffness Elements  
Electromagnets function by harnessing the magnetic field generated by the electric current flowing through the wire to either attract or repel magnetic materials, thereby facilitating the adjustment of the mechanism's rigidity. Within the negative stiffness mechanism, employing electromagnets enables alterations in the strength or orientation of the magnetic field through adjustments in the current magnitude or direction, consequently modifying the stiffness properties of the mechanism. Through theoretical analysis, simulation, and experimental validation, the researchers undertook a comprehensive investigation into the utilization of electromagnets within negative stiffness mechanisms. They delved into  
Figure 26: Configuration of an MNSI based on Maxwell magnetic normal stress: (**a**) cross-section view of isolator; (**b**) configuration of excitation mechanism; (**c**) relationship between negative stiffness \(\text{k}_{\text{n}}\) and air gap \(\text{x}_{0,}\); and (**d**) relationship between negative stiffness \(\text{k}_{\text{n}}\) and ratio T.  
various configurations of electromagnets, strategies for current control, and designs for mechanism structure, all aimed at attaining the targeted negative stiffness properties. As shown in Figure 27, [PERSON] [73] and his fellow researchers directed their attention toward an active seismometer tailored for low-frequency vibration measurements, incorporating adjustable electromagnetic negative stiffness (AENS). The primary objective of this study is to devise a seismometer aimed at enhancing the efficacy of low-frequency measurements. Typically, seismometers manifest high-pass filter characteristics, featuring substantial phase advancement and amplitude attenuation below their natural frequencies. To surmount this issue, the authors advocated for an active seismometer employing electromagnetic forces to counterbalance the positive stiffness of a mechanical spring, thereby diminishing the overall stiffness while preserving the high-frequency stability. The primary contributions of the paper comprise: a compact AENS employing two pairs of magnetic rings and a coil winding to furnish adjustable linear negative stiffness spanning a few millimeters; an exploration of the ramifications of the initial mounting misalignment and nonlinearities inherent to the AENS; and an elucidation of the necessity to amalgamate the AENS and active control within active seismometers, with both endeavors serving to elongate the low-frequency performance whilst maintaining the constant sensitivity and bandwidth. Furthermore, the paper offers an exhaustive discourse on the design, principles, experimental findings, sensitivity computations, and noise analysis. The experimental outcomes indicate that the prototype boasts a measurable bandwidth ranging from 0.1 Hz to 25 Hz and a sensitivity of 275 V/(m/s). Nonetheless, owing to the utilization of commercial actuators and sensors, the noise floor surpasses the majority of the frequency spectrum of the NHNM. The authors suggested the possibility of enhancing the existing design by incorporating bridge-based capacitive displacement sensors, custom-made lightweight moving coil motors, and premium-grade actuators. Overall, this study holds significance in enhancing the precision and sensitivity of low-frequency vibration measurements, along with advancing the design and utilization of seismometers. The distinctive design of the Adjustable Electromagnetic Negative Stiffness (AENS) plays a pivotal role in advancing seismology. Conventional seismometers face limitations in the low-frequency range, often characterized by notable phase advancement and amplitude attenuation.