1. Field of the Invention
The present invention relates to a displacement sensing apparatus for a movable member, and more particularly, to a displacement sensing apparatus for use in an optical encoder and a mouse.
2. Description of the Prior Art
FIGS. 1 and 2 show a conventional displacement sensing apparatus for a movable member for use in an encoder and a mouse. FIG. 1 is a perspective view. FIG. 2 is a view schematically showing the displacement sensing apparatus including its circuit portion. This structure is disclosed by Japanese Laid-open Utility Model Application H2-89314.
The displacement sensing apparatus includes a light emitting portion 61 having a light emitting diode (LED) 60, a rotating disc 63 in which a plurality of slits 62 are formed along its circumference, a light receiving portion 66 having two photodiodes 64 and 65, and comparators 67 and 68 for comparing outputs of the two photodiodes 64 and 65 of the light receiving portion 66 with a reference voltage Vref.
Light emitted from the light emitting portion 61 passes through the slits 62 of the rotating disc 63 which is rotating, and reaches the light receiving portion 66. Outputs of the photodiodes 64 and 65, which are current values, are converted into voltage values by current-to-voltage converters 69 and 70, and compared with the reference voltage Vref by the comparators 67 and 68, respectively.
FIG. 3 shows outputs (voltage values) 64a and 65a of the photodiodes 64 and 65. FIG. 4 shows outputs 67a and 68a of the comparators 67 and 68. In FIG. 3, the reference voltage Vref is represented by an alternate long and short dash line. When it is required that a phase difference between the outputs 64a and 65a of the two photodiodes 64 and 65 of the light receiving device is 90.degree., the positions of the photodiodes 64 and 65 are adjusted in advance so that the phase difference is 90.degree. as shown in FIGS. 3 and 4.
However, there is sometimes nonuniformity in the light quantity of the LED 60 of the light emitting portion 61. This nonuniformity is generated at the time of manufacturing or when the light quantity gradually decreases according to the time of use. For example, when the light quantity of the LED 60 decreases, the output waveforms 64a and 65b of the photodiodes 64 and 65 decrease as shown in FIG. 5, so that if the reference voltage Vref is constant, the phase difference deviates from the desired 90.degree. as shown in FIG. 6.
The deviation of the phase difference due to the non-uniformity of the light quantity of the LED 60 can be corrected by varying the reference voltage Vref in accordance with the light quantity. However, it requires much labor since it is necessary to individually make an adjustment with respect to all of the light receiving device, and it is impossible to completely cope with the decrease of the light quantity while the device is being used.