A method for condition monitoring a rotor of a wind energy plant includes the steps of measuring activity signals of the blades in the rotor and establishing one or more differential values between the activity signals of different blades. Data is sent from the sender to the recipient by divided sending of the data over the plural connections. This valve has a moveable member such as a diaphragm sealing off a pressure chamber. A spring ring is set in the annular space from the distal end, moved with a telescoping motion, and locked under exertion of its spring effect. The passive member is fixed to the hub easily.