the control electronics 4 drive the motor 6 via the signal line 5 the motor 6 then rotates the spool 8 about its longitudinal axis within the cavity 9 via the drive shaft 7 the rotation of the spool 8 causes the grooves 11 a 11 b to move into or out of alignment with the internal ports located on the inner surface of cavity 9 when a groove 11 is aligned with a fluid inlet and a fluid outlet fluid may flow from the pressurised supply port 12 to either of the first or second control ports 16 18 depending on the alignment of the groove 11 alternatively or as well depending on the form of the groove see below fluid may flow from either of the first or second control ports 16 18 to the tank outlet port 14 the interaction of the groove the spool and the internal ports is discussed in more detail below in relation to fig2 if the fluid flows from the servo valve 1 via the first control port 16 then the pressure in the first cylinder chamber 22 increases the increase in pressure in the first cylinder chamber 22 acts on piston 26 to move the actuator arm 20 in a first direction if the fluid flows from the servo valve 1 via the second control port 18 then the pressure in the second cylinder chamber 24 increases the increase in pressure in the second cylinder chamber 24 acts on piston 26 to move.