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If fly-by-wire systems or digital flight-control systems analog electrohydraulic actuators are used for surface control. According to the actual state of technology the analog electro-hydraulic signal conversion is accomplished by servo valves, as so far only these can meet the very high requirements for dynamic response. Due to the complexity and the relatively poor reliability of the servo valves, an actuator using simpler and more sturdy fast switching ball valves will be presented. The unsteady control of the fluid flow is achieved by pulse-width modulation of an on-off switching valve. The displacement of the valve-controlled actuator is achieved by opening the valve for a predetermined time. By using discrete control in an electro-hydraulic actuator, it is achieved that the digital signal structure is maintained far into the actuator. The example of a modified production actuator, the set-up of the complete system, the unsteady valve control, the fast switching ball valve itself and special design features of the system are described and discussed. The performance of such a 'digital' servo actuator is demonstrated by means of static and dynamic characteristics which have been determined in laboratory tests. (Peters)