An apparatus for converting pressure into motion includes a housing having a plurality of ports formed therein. Movably disposed within the housing is a piston having a first plurality of apertures communicating with a first chamber and a second plurality of apertures communicating with a second chamber...http://www.google.fr/patents/US4434704?utm_source=gb-gplus-shareBrevet US4434704 - Hydraulic digital stepper actuator
