Abstract: A pneumatic servo valve comprising a piston in a cylinder. Both controlling surfaces of the piston are exposed to controlling air. The piston is provided with an axial drilling extending through it, which communicates via a radial groove with an inlet of controlling air. Both openings of the drilling are normally closed by spring actuated mushroom valves. Controlling chambers in the cylinder communicate through calibrated conduits in the piston with the radial groove, and can be put into communication with the surrounding atmosphere via outlets. The outlets are controlled by electromagnet valves. A microcomputer receives signals from correct and actual value indicators respectively, and administers opening and closing of the valves. When one of these is opened the connected controlling chamber is decompressed and the piston moves in the direction of the empty chamber. One of the mushroom valves follows the movement of the piston, while the other mushroom valve is prevented from moving by a valve seat.