Quadruple redundant closed loop electro-hydraulic servo system

Claim 1

A servo system having four redundant channels coupled to a common output member for displacing the member, each of the channels comprising: means for supplying hydraulic fluid at a predetermined pressure; hydraulic ram means; a transfer valve intermediate the fluid supplying means and the ram means for transferring fluid to said ram means and said ram means thereupon exerting a force; friction means coupled to the ram means for providing a friction force at a predetermined level; the common output member being displaced by the net of the ram and friction forces; means for providing a command signal; follow-up means for providing a signal corresponding to the net of the ram and friction forces; a force sensor for sensing the net of the ram and friction forces and for providing a corresponding signal; means for applying the command, follow-up and force sensor signals to the transfer valve with the transfer valve being responsive to said signals for transferring the fluid to the ram means; and the predetermined friction level being slightly higher than the maximum expected force difference produced by the hydraulic fluid supply pressure of two remaining channels after two channels have failed, and following a third channel failure displacement of the common output member is prevented.