Active isolation systems using nozzle-flapper valves and pneumatic springs are capable of much improved isolation characteristics when compared with conventional isolation systems, but they suffer from the fact that large quiescent leakage flows are required for satisfactory dynamic performance of the isolation system (1)‡. This quiescent power loss due to quiescent leakage flow is a very serious problem, because high pressure air is consumed when no useful work is done. In this paper it is shown how quiescent leakage flow is eliminated by replacing the nozzle-flapper valve with a critically centred servo-valve. The damping produced by the quiescent leakage flow in a nozzle-flapper controlled isolation system is produced by ‘transient pressure feedback’ obtained by connecting the pneumatic spring to a surge tank through capillary resistances, in a servo-valve controlled isolation system.
