Two-stage servo valve

A two-stage servo valve comprising:

• a first drive stage;
• and
• a second fluid transfer stage, the fluid transfer stage comprising:
• a housing that defines an axial cavity within the housing and the housing having a plurality of ports;
• and
• a spool axially moveable within the axial cavity defined within the housing to control flow of fluid between the plurality of ports according to the axial position of the spool,
• wherein the first drive stage is configured to cause axial movement of the spool;
• wherein the housing comprises three ports, a first port being a supply port fluidly connected to a second port, being a control port, via a supply channel (sc) and a first control channel (cc1) via the spool cavity, and a third port being a return port fluidly connected to the control port via a return channel (rc) and a second control channel (cc2) via the spool cavity, and
• wherein the spool comprises a middle portion and first and second end portions, a first opening defined between a first edge (K1) of the middle portion and the first end portion and a second opening defined between a second edge (L1) of the middle portion and the second end portion, the first and second openings being sized such that in a neutral position, the first opening does not overlap both the supply channel and the first control channel and the first edge K1 is positioned to prevent fluid flow between the supply channel and the first control channel and the second opening does not overlap both the return channel and the second control channel and the second edge L1 is positioned to prevent fluid flow between the second control channel and the return channel;
• in a first axial position, the first edge K1 is positioned such that the first opening overlaps at least a portion of both the supply channel and the first control channel to allow fluid flow between the supply channel and the first control channel, and the second opening does not overlap both the return channel and the second control channel and the second edge L1 is positioned to prevent fluid flow between the second control channel and the return channel;
• and
• in a second axial position, the second edge L1 is positioned such that the second opening overlaps at least a portion of both the return channel and the second control channel to allow fluid flow between the return channel and the second control channel, and the first opening does not overlap both the supply channel and the first control channel and the first edge K1 is positioned to prevent fluid flow between the supply channel and the first control channel.