How does hydraulic actuator servo valve feedback work? 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):

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Is the first figure simply a poor explanation and the second figure is more representative of how these hydraulic linear actuators work?
The actuator in the first picture is a position follower system and only works as intended if the servo valve is closed as a consequence of the resulting piston displacement. The system in the picture works if the cylinder end is connected to the flight control surface (the left eyelet with the cross in it) and the piston end is connected to the structure frame. The key for understanding is the shift in the centre of rotation:




The servo valve is closed when the summing bar is vertical.
At first, the cylinder is stationary and the pilot deflects the stick, rotating the summing bar about the lower hinge and opening the servo valve. (Black dotted line).
While the pilot keeps the stick stationary, the cylinder moves in response of the servo valve opening. If the initial deflection was to the right, the piston moves to the right. The centre of rotation is now the top hinge (mechanical input).
When the summing bar becomes vertical again at the point where the pilot now holds the stick, the servo valve closes. (Red dotted line).


The second diagram does not show how the cylinder is attached and where the mechanical input is connceted to at the botttom. It could be a position follower system or a simple velocity output where input deflects the servo valve, oil starts to flow as long as the input is open, and the output end moves with a speed proportional to the input deflection. If the lower "First Mechanical Connection" is attached to the moving surface and the cylinder is free to move, we get a mechanical position follower again.

The mechanical feedback provided by the summing link was how the first generations of hydraulically operated systems worked. Nowadays controllers are electronic, not mechanical: a position transducer measures cylinder output, and adjusts servo valve input in response to the control laws programmed in the controller.