Ultra-low frequency active vibration isolation in high precision equipment with electromagnetic suspension: Analysis and experiment

High performance vibration isolation requirement in the ultra-low frequency band for high precision equipment becomes more and more stringent. An active vibration isolation method is investigated to broaden the frequency band of isolation and improve the performance, in which an electromagnetic suspension is adopted to provide a static lifting force and an active control force. Naturally, the suspension is accompanied by a controllable negative stiffness. Two factors that might cause instability of the suspension system, i.e., the bias current and the base disturbance, are analyzed to give the stable region. The matching relationship between the currents in the upper and lower coils, and the mapping rules among the current, relative displacement, force and stiffness are discussed, respectively. The maximum base disturbance that the system can withstand is analyzed. On the basis of the theoretical analysis, the active position stabilization and vibration control methods are built and the performance is evaluated. Furthermore, experiments were conducted to verify the effectiveness of the active vibration isolation method. Numerical and experimental results demonstrate that the electromagnetic suspension is able to improve the vibration isolation performance in the ultra-low frequency range, where the transmissibility is below 0 dB in the entire frequency band and the initial vibration isolation frequency is less than 1Hz.