Optical encoder

Details Optical encoder Optical encoder

2001-12-05

2003-08-26

Le, Que T. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S23700G, C356S027000

Reexamination Certificate

active

06610975

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical encoder that can be compactly produced as a small device and can detect the position etc. of a moving body with high accuracy.
2. Prior Art Description
A detection apparatus for detecting the rotation or distance moved by a moving body is normally called a rotary encoder or a linear encoder. In most cases, a construction is used where a moving grating plate and a static grating plate are arranged between a light source and photodetectors. The amount of movement of a moving body, with which the moving grating plate is integrally provided, is detected based on changes in the amount of light from the light source that passes through the moving grating and static grating formed by these grating plates.
The resolving power of an optical encoder with the above construction is determined by the pitch of the gratings, so that the pitch of the gratings should be reduced in order to produce a encoder with high resolving power. However, in order to reduce the pitch of the gratings, the gap between the moving grating plate and the static grating plate needs to be reduced to prevent a drop in the signal to noise ratio (S/N ratio) due to the loss of light. Furthermore, the S/N ratio also falls if a reduction cannot be made in the fluctuation in the gap between the grating plates due to the movement of the moving grating.
Since there are limits on how narrow the gap between the grating plates can be made and on the extent to which fluctuation in the gap can be suppressed, one effective way of avoiding reductions in the S/N ratio due to the loss of light and fluctuation in the gap is to use a parallel light beam. A divergent light beam can be converted into a parallel light beam using a lens optical system, such as a collimating lens, and optical encoders that use such an optical system are conventionally known. However, the most commonly used light source for an optical encoder is a light-emitting diode (LED), and since LEDs are not point light sources, it is difficult to produce a high-quality parallel light beam. As a further problem, the addition of a lens optical system results in a corresponding increase in the dimensions of an optical encoder apparatus.
On the other hand, there is a conventional method for producing an optical encoder with high resolving power that uses the diffracting property of light. In an encoder that uses this method, light is produced by a point light source, such as a semiconductor laser, and is converted into a parallel light beam using lenses. This kind of encoder detects the movement of a moving body based on the changes in the amount of light received by photodetectors due to the diffraction and interference that occur when the parallel light beam passes through a grating with extremely fine pitch. When this method is used, the pitch of the gratings can be made finer than in an encoder of the construction described earlier, and the distribution of light due to interference approximates to a sine wave, so that the electrical signal can be precisely divided. However, both the gratings and the apparatus construction have to be produced with high precision, making such apparatuses expensive. A further problem is the poor reliability of the semiconductor laser used as the light source.
Another example of an optical encoder has been proposed by the applicant of the present specification in Japanese Laid-Open Patent Application H06-118088. This optical encoder is a spatial filter encoder where an image of the moving grating passes through lenses and is formed on an array of photodetectors that are arranged in a grid. With this method, the high-frequency components of the signal produced by the movement of the grating are removed due to the filter effect, so that a signal that approximates to a sine wave can be obtained. Accordingly, by using a signal divider, a high resolving power can be achieved. However, when the pitch of the gratings is reduced, it is difficult to raise the contrast of the image formed on the photodetectors. Also, since a lens optical system is used, there is the further problem of an increase in the dimensions of the apparatus.
“Analysis of Grating Imaging and its Application to Displacement Metrology” SPIE Vol. 136 1
st
European Congress on Optics Applied to Metrology (1977), pp. 325-332 describes a triple-grating theory and its application in the measurement of displacement. As disclosed in the above article, an index grating plate and a reflective grating plate are arranged in facing positions, with the light source and photodetectors being provided behind the index grating plate. Light from the light source is shone at the index grating plate, and the light that passes an index grating in the index grating plate is reflected back off a reflective grating on the reflective grating plate and passes back through the index grating in the index grating plate to the photodetectors, so that the movement of the reflective grating can be detected.
With the above construction, the gap between the index grating plate and the reflective grating plate can be large without affecting the contrast, with fluctuations in the gap between the gratings also having little effect on the contrast.
Consequently, by using the triple-grating theory where a reflective grating is used, it is possible to produce an optical encoder with high resolving power that is unaffected by the width of the gap between the static grating and the moving grating and by fluctuations in this gap.
However, the following problems remain to be solved when adopting this method in an optical encoder.
First, with an encoder of the construction described above, it is necessary to arrange the light source and photodetectors behind the index grating, which makes the construction complicated and leads to poor efficiency for the detection of light. Also, to use this kind of device as an encoder, it is necessary to use at least two photodetectors and to obtain signals that have a phase difference of one-quarter wavelength to detect the direction in which the moving body is moving. However, it is actually very difficult to produce a construction where the light source and at least two photodetectors are arranged behind the index grating and signals with a phase difference of one-quarter wavelength are obtained from the photodetectors.
Also, since the cost of an encoder with the above construction is directly linked to the size of the photodetectors, it is desirable to make these components extremely small. In order to raise the efficiency with which the emitted light is received by the photodetectors, it is also desirable to provide the photodetectors as close as possible to the light source.
In view of the above, by setting the emission point of an LED that is commonly used as a light source at a position that is close to a light transmitting grating and raising the divergence angle of the LED, an increase can be made in the area of the photodetectors that receives reflected light. However, due to the dimensions of the lens for the LED and other factors, there is no realistic way to raise the divergence angle of the LED. If the photodetectors are brought close to the light source without the divergence angle of the LED being increased, this leads to the undesirable result of a large decrease in the effective light-receiving area of the photodetectors for reflected light.
In view of the problems stated above, it is an object of the present invention to provide, based on the triple-grating theory where a reflective grating is used, a compact, small-scale optical encoder that can detect both the speed of movement and direction of movement (moved-to position) of a moving body.
SUMMARY OF THE INVENTION
In order to achieve the stated object, the present invention is an optical encoder, including a light source, a reflective grating of a predetermined form and a fixed pitch, a light transmitting grating of a predetermined form and a fixed pitch, and photodetectors with light receiving surfaces of predeter

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