Patent Application
20070215797
The present invention relates to a photoelectric encoder, and more particularly to an improvement of a photoelectric encoder that has a telecentric optical system in which a lens and an aperture are interposed between a main scale and photoreceptors.
As described in Japanese Patent Laid-Open Publication No. 2004-264295, a photoelectric encoder has been devised in which, as shown in
In the photoelectric encoder using this telecentric optical system 40, an image on the main scale 20 is projected onto the photoreceptor array 34 via the lens optical system (42, 44). Here, since the aperture 44 is located in the focal position of the lens 42, the image formed on the photoreceptor array 34 can be prevented from varying in magnification even if the distance (gap) between the main scale 20 and the lens 42 changes as long as the physical relationship among the lens 42, the aperture 44, and the photoreceptor array 34 is unchanged.
Nevertheless, even with the photoelectric encoder using such a telecentric optical system 40, misalignment of the photoreceptors 35 in the gap direction can change the relationship between the distance a from the lens 42 to the main scale 20 and the distance b from the lens 42 to the photoreceptors 35 as shown in
Besides, the signal detection efficiency in the peripheral area also drops due to lens distortion and coma aberration.
Moreover, when miniaturization is intended, the optical system requires a lens that has a shorter focal length (a smaller diameter, in the case of a general-purpose lens). To maintain the aberrations smaller, however, it is necessary (1) to use an aspherical lens or (2) to use a plurality of lenses in combination (with adjustment). There have thus been such problems as a rise in cost and additional man-hours for adjustment.
The present invention has been achieved in view of the foregoing conventional problems, and it is thus an object thereof to improve the signal detection efficiency, increase assembly tolerance, and reduce the man-hours for adjustment.
The foregoing object of the present invention has been achieved by the provision of a photoelectric encoder having a telecentric optical system in which a first lens and an aperture located at a focal position of the first lens are interposed between a main scale and a photoreceptor, and wherein at least a second lens is interposed between the aperture and the photoreceptor with a focus of the second lens on the aperture, thereby constituting a bilateral telecentric optical system.
Moreover, the second lens is made identical to the first lens and is situated reversely. The second lens can thus inversely correct an aberration occurring from the first lens so that the aberration is cancelled out almost completely.
Moreover, at least either one of the first lens and the second lens is made of: a spherical ball lens, which has high distortion but is inexpensive; a GRIN lens of gradient refractive index type (also called SELFOC lens), which refracts light beams in a parabolic pattern inside its lens medium; or a drum lens. This allows compact configuration at low price.
Moreover, one or more second bilateral telecentric optical systems including a second aperture and third and fourth lenses arranged on both sides thereof is/are further interposed between the second lens and the photoreceptor.
Moreover, the aperture is formed as a slit oblong in a direction perpendicular to an axis of measurement, so that the amount of light to reach the photoreceptor is increased for the sake of power saving and improved reliability of the light source.
According to the present invention, the second lens can inversely correct the aberration occurring from the first lens. This makes it possible to reduce the aberration and improve the signal detection efficiency.
Moreover, even if the gap between the second lens and the photoreceptors varies, the optical magnification can be maintained constant. This can increase the assembly tolerance in the gap direction and reduce the man-hours for adjustment.
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
A first embodiment of the present invention is a photoelectric encoder having a telecentric optical system 40 like shown in
In the present embodiment, the first lens 42 and the second lens 52 are identical. The second lens 52 can thus inversely correct aberrations occurring from the first lens 42 almost completely. It is therefore possible to cancel out the aberrations almost completely with a significant improvement in the signal detection efficiency.
Moreover, since the second lens 52 is interposed so that its focus falls on the aperture 44, the outgoing light from the second lens 52 becomes parallel light as shown in
It should be appreciated that the first and second lenses 42 and 52 need not necessarily be plano-convex lenses which are used outward as shown in
Moreover, as in a seventh embodiment shown in
In this case, it might be impossible to eliminate aberrations completely, whereas the assembly tolerance in the gap direction increases.
Moreover, the aperture may have a non-circular shape. As in an eighth embodiment shown in
Furthermore, as in a ninth embodiment shown in
Moreover, the number of lenses to be added is not limited to one. As in a tenth embodiment shown in
The present invention is applicable to either of an encoder in which the index grating and the photoreceptors are formed separately and an encoder having a photoreceptor array having the two components formed integrally. Moreover, it is applicable not only to encoders of transmission type, but to encoders of reflection type as well.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/04924 | 3/18/2005 | WO | 9/19/2006 |
