Compact linear actuator

A fluid-driven linear actuator comprising:

• a piston configured for reciprocating motion in a bore defined by a piston chamber;
• a spool valve in a valve chamber, the valve chamber fluidly connected to a fluid input and to a fluid output, the spool valve configured to be hydraulically moved within the valve chamber between a plurality of spool valve configurations, the plurality of spool valve configurations comprising a first spool valve configuration wherein the valve chamber is fluidly connected to the piston chamber to thereby create a first fluid pressure differential which tends to force the piston in a first axial direction in the piston chamber and a second spool valve configuration wherein the valve chamber is fluidly connected to the piston chamber to thereby create a second fluid pressure differential which tends to force the piston in a second axial direction in the piston chamber;
• and
• a first switch valve located on a first axial side of the piston and a second switch valve located on a second axial side of the piston and wherein each of the first and second switch valves is configured to be switchable between a plurality of switch valve configurations by the reciprocating motion of the piston and to thereby create one or more differential pressure configurations which hydraulically move the spool valve;
• wherein the spool valve is held, between extremes of the reciprocating motion of the piston, in at least one of the plurality of spool valve configurations by fluid pressure, and
• wherein the first and second switch valves comprise first and second intersections of first and second fluid conduit channels with a piston rod, the piston rod coupled to the piston or integrally formed with the piston for axial movement therewith, the piston rod comprising a first groove at a first axial location and a second groove at a second axial location;
• wherein alignment of the first groove with the first fluid conduit channel creates a relatively high pressure of at least one of the one or more differential pressure configurations which hydraulically move the spool valve.