ABSTRACT
The self-leveling feature is intended entirely for steady-state or static leveling, it does not attempt to fight against the high-frequency forces and motions that might be present. We can thus consider the self-leveling feature separately since it is much too slow to affect the dynamic behavior of the air spring isolators. While some manufacturers use electromechanical leveling systems, simple all-mechanical systems are adequate for many applications. These devices are simple feedback systems which sense mechanically when the table top moves vertically away from a desired location and then change the spring air pressure to return the table to the desired location. This action has two important effects. First, the table, which is usually supported by four air springs, one at each corner, is kept at the same height and level, even when equipment is added or taken away from the table top. Second, because the air pressure force of the "piston" must exactly equal the payload weight when the table is at the desired height, the spring constant of the air spring automatically changes when the payload mass is changed. We will shortly show that this spring constant change is exactly what is needed to keep the system natural frequency, and thus its isolation performance, at the chosen design value. Ordinary metal springs do not have this feature; the system natural frequency would change whenever the mass changed.
