Optical encoder

Details Optical encoder Optical encoder

1998-11-20

2001-03-13

Lee, John R. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C356S370000

Reexamination Certificate

active

06201239

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical encoder for use in detecting an amount of movement of a movable portion of an apparatus.
It has been difficult to reduce the size and cost of a conventional optical encoder, since various optical components need to be assembled and adjusted. The applicant of the present invention, on the other hand, has developed an optical encoder which has a surface emitting semiconductor laser (hereinafter referred to as “surface emitting laser”) as a light source and which permits reduction in size and cost.
FIG. 32
shows the structure of an optical encoder having the surface emitting laser developed by the applicant as a light source. In this optical encoder, when a beam has been radiated onto a scale
20
from a surface emitting laser
10
, the beam reflected from or passing through the scale
20
is detected by a photodetector
30
,
30
′.
In general, the surface emitting laser has a smaller widening angle of an emission beam, which is determined by an emission window width (opening length)
a
, than a conventional edge-emitting semiconductor laser. Accordingly, the optical encoder developed by the applicant does not require a collimator lens or an index scale, which is needed in the conventional optical encoder. Thus, the size and cost of the optical encoder developed by the applicant can be reduced.
In the optical encoder using the conventional surface emitting laser, it is supposed that when a distance L between a light source and a scale is less than a predetermined distance (a
2
/&lgr;), the size of the optical spot formed on the scale by the surface emitting laser is substantially equal to an opening width
a
in the scale pitch direction of the emission window of the surface emitting laser. Thus, there is no choice but to limit the value of the scale pitch p to p≧a. In addition, since it is supposed in the prior art that the size of the optical spot formed on the scale is substantially equal to L&lgr;/a when L>a
2
/&lgr;, the scale pitch p is limited to p≧L&lgr;/a. It has not been considered to more decrease the value p.
On the other hand, an increase in resolution of the encoder has still been demanded in order to achieve a higher performance of a machine tool, a measuring device, etc. High resolution is indispensable for small-sized, low-cost optical encoders.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide an optical encoder using a surface emitting laser, wherein when a light source and a scale are situated relatively close to each other, the scale pitch can be made less than that in a conventional optical encoder.
In order to achieve this object, in the optical encoder according to the present invention, in a case where a distance between the movable scale and a light beam emission plane of the surface emitting laser light source along a center axis of the light beam is L, a wavelength of the light beam is &lgr;, a scale pitch of the movable scale is p, an angle between a pitch direction of the movable scale and a plane perpendicular to the center axis of the light beam is &thgr;, and an opening width of the surface emitting laser light source along a direction in which the pitch direction of the movable scale is projected on the plane perpendicular to the center axis of the light beam is
a
, a relationship, p.cos &thgr;<a, is satisfied when L≦1.0a
2
/&lgr;, or a relationship, p.cos &thgr;<L&lgr;/a, when L≧a
2
/&lgr;.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.


REFERENCES:
patent: 3881113 (1975-04-01), Rideout et al.
patent: 5129725 (1992-07-01), Ishizuka et al.
patent: 5671052 (1997-09-01), Kawakubo et al.
patent: 5991249 (1999-11-01), Lee
patent: 6020585 (2000-02-01), Shimonaka et al.
patent: 7-306058 (1995-11-01), None

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