A drive mechanism for driving a heald frame of a weaving machine, the drive mechanism having a sensor device, wherein the sensor device has at least three members having both a target set with one or more targets and a detector set with one or more detectors, wherein one of the detector set and the target set is arranged at the swivel lever and the other one is arranged stationary on the weaving machine, targets of the target set and/or detectors of the detector set have different characteristics for generating a first signal when approaching the measuring position from the upper position or when departing from the measuring position towards the upper position and for generating a second signal when approaching the measuring position from the lower position or when departing from the measuring position towards the lower position, and the second signal differs from the first signal.