Mechanisms for controlling access to storage volumes on the secondary storage system is provided. The rotation angles of a first rotation body rotating at a rotation angle frequency in correspondence with rotation of the rotation object whose rotation is to be detected and the rotation angle of a second rotation body rotating at a rotation frequency that is different from the rotation angle frequency of the first rotation body are individually detected. A mouthpiece has a gas passageway and a retainer passageway and a first end of the gas passageway communicates with the internal chamber while a second free end of the mouthpiece has a gas opening therein. A connecting element has a shaft passing through a central aperture in the intermediate element, thus also extending through the aperture in the arm. The metal particles is covered by the compound. In this embodiment, the first and second stages are further for subtracting the leakage signal from the sense signal prior to the comparisons.