The book Aircraft Systems uses the following diagram for a mechanically controlled hydraulic linear actuator (for moving a control surface e.g. an aileron):
The following explanation is given about its operation:
As the pilot feeds a mechanical input to the flight control actuator, the summing link will rotate about the bottom pivot, thus applying an input to the servo valve. Hydraulic fluid will then flow into one side of the ram while exiting the opposite side resulting in movement of the ram in a direction dependent upon the direction of the pilot’s command. As the ram moves, the feedback link will rotate the summing link about the upper pivot returning the servo valve input to the null position as the commanded position is achieved.
However, I fail to understand this explanation given the diagram. If the mechanical signal is to the right, the summing link will slant to the right (i.e. /). I assume this is a command for the cylinder to move to the right. When the cylinder moves to the right, the feedback link causes the summing link to eventually return to the upright position (i.e. |). But when this position is achieved, the Servo Valve (SV) input is now more extended than it was before (the whole summing link is now upright but with a net shift to the right). How is this the null position that would cause the SV to close?
In search for an answer, I came across a better diagram which makes sense with a sliding sleeve of the SV (but this diagram is in my opinion not what the first figure illustrates):
Is the first figure simply a poor explanation and the second figure is more representative of how these hydraulic linear actuators work?