| 摘要 |
PURPOSE:To detect a position correctly for a long time and to simplify a device by providing a light emitting element and a light receiving element for one of a fixed member and a movable member, providing a reflecting plate for the other and adding beforehand change in the lapse of time light quantity to emission quantity. CONSTITUTION:Outgoing light from a LED 20 driven by a variable constant power source 30 is irradiated to a reflecting mirror 22 on a supporting body 15, its reflected light is received by a photodiode 21 and a photoelectrically converted output is amplified by an amplifier 31. Since the LED 20 and the diode 21 are fixed on a cylindrical magnetic substance 9, the position of the supporting body 15, that is, the position of an objective lens 2 can be obtained from the quantity of the light incident on the diode 21. The supporting body 15 is driven by the current of a coil 5 and when the central position of its movable range is a C point, the output of the amplifier 31 in the C point is a value that an offset quantity is added to the center of its output range. Therefore, the offset quantity of a value to which the LED 20 is expected to lower with the passage of time is added to a current flowing in the LED 20 by a current source 30. |
