| 摘要 |
An aperture closure member control arrangement includes a disc (20) which rotates as the aperture closure member moves. The disc (20) has a ring of teeth (24). A sensor (22), such as a Hall effect sensor (28) and permanent magnet (30), detect the passage of teeth (24) as a cyclic waveform of amplitude and frequency determined by the spacing of the teeth (24), and the speed of rotation of the disc (20). An irregularity (26), in the form of a missing tooth (24), creates an irregularity in the output of the sensor (22), in the form of a pulse of greater amplitude and lower frequency. Accordingly, pulses from the irregularity can be discriminated and counted to provide a coarse indication of position, or pulses from the teeth (24) can be counted, to provide a fine indication. |
