Sun Q., He X.

In recent years, there has been an increasing demand for a high performance and large bandwidth vibration isolation system that could house ultra-high precision measurement equipment. Existing commercially available vibration isolators commonly have cross-over frequencies at or above 1 to 2 Hz, thereby effectively isolating the ground disturbance at frequencies higher than 2 Hz. Due to the low level of vibration magnitude at low frequencies, it is practical and technically valid to leave vibration below 1 Hz un-attenuated in most industrial environments and scientific laboratories. However, for ultra-high precision measurement such as the scanning tunneling microscope, it is highly desirable that the cross-over frequency of a vibration isolation system is at least one or two orders of magnitude lower. This is an extremely difficult task for several reasons. A passive isolation stage through the mass-spring set-up with a resonance frequency of 0.1Hz requires a prohibitively long spring extension of more than 25 meters. Sensors measuring the floor movement are limited by their insensitivity to small magnitudes at low frequencies. Noise due to the electronics devices including sensors, actuators, and power amplifiers is within the order of magnitude of the signal, which poses a serious challenge to the active control approach. In this paper, we propose a solution of cascaded multiple stages of active suspension. In particular, we propose to design a second stage that will be used on top of a commercial stage to target at low frequency isolation. As such, the bandwidth of the combined two stages will be enlarged. However, this is true only if the two stages do not interfere with each other. Through analysis, we conclude with design specifications for optimal performance. Some experimental results will be presented as well.