Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1989-03-07
1990-03-06
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250237G, H01J 314
Patent
active
049068385
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an optical encoder and, more particularly, to an optical encoder capable of raising the noise tolerability of a light-receiving element.
BACKGROUND ART
An optical encoder has a rotary coding plate secured to a shaft, a fixed coding plate arranged to oppose the rotary coding plate with a predetermined spacing therebetween and secured to a housing, a light source, and a light-receiving element which receives the light from the light source via the rotary coding plate and fixed coding plate, and is adapted so as to be able to detect the rotating direction, rotational speed or rotational angle of the shaft using the output of the light-receiving element produced when the shaft is rotated.
When the noise tolerability of the optical encoder is considered, a case in which the noise source is external and a case in which it is internal must be taken into account.
With regard to noise from an external source, noise tolerability can be enhanced with comparative ease by physically surrounding the encoder in a space of 0[V]. However, enhancing noise tolerability with respect to noise generated internally is considerably more difficult. In particular, since the rotary shaft is a source of noise in the optical encoder, a pattern portion (a portion on which chrome is formed by vapor deposition) on the rotary coding plate secured to the shaft and a pattern portion on the fixed coding plate secured to the housing form secondary noise sources due to electromagnetic induction and, hence, it is difficult to enhance noise tolerability. More specifically, since noise tolerability is proportional to the square of distance from the noise source, it is necessary to keep at a distance from the noise source. In an optical encoder, however, in order to assure an output signal level from the light-receiving element, a small gap of less than 1 mm is required between the light-receiving element and the fixed coding plate, and a very small gap of 70-90 .mu.m is required between the fixed coding plate and the rotary coding plate. For this reason, the distance between the fixed coding plate, which is a source of noise, and the light-receiving element cannot be lengthened. Furthermore, in order to surround (shield) the light-receiving element by 0[V], a dielectric capable of being reduced to 0[V] is required to be inserted between the light-receiving element and the fixed coding plate. However, this also is very difficult to achieve in terms of space.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an optical encoder capable of enhancing noise tolerability with respect to noise generated internally, as from a rotary shaft.
To achieve the above object, chip-mounting portion of a flexible cable having a light-receiving element chip mounted thereon is place on a shielding plate, and a pattern portion on a fixed coding plate and the shielding plate are grounded, thereby shielding the periphery of the light-receiving element chip to improve noise tolerability with respect to noise generated internally.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view illustrating the principal portion of an optical encoder according to the present invention;
FIGS. 2(a) and 2(b) are views illustrating the mounted state of a light-receiving element chip; and
FIG. 3 is a view for illustrating a specific example of a method of grounding a shielding plate and a pattern portion.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a sectional view illustrating the principal portion of an optical encoder according to the present invention, FIGS. 2(a) and 2(b) illustrate the mounted state of a light-receiving element chip, and FIG. 3 is a view illustrating a specific example of a method of grounding a shielding plate and a pattern portion. In FIG. 2(a) is a sectional view taken along line A--A of FIG. 2(b). In FIG. 1, the dimensions of the chip and the like are drawn larger than in actuality for the sake of clarity.
With reference to FIG. 1, numeral 11 denotes a shaft
REFERENCES:
patent: 3770971 (1973-11-01), Somerset
patent: 4072201 (1978-02-01), Wiesler
Okuda Kanemasa
Sogabe Masatoyo
Yamamoto Tomonaga
Fanuc Ltd.
Hsia Sherrie
Nelms David C.
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