JP4078809B2 - Optical encoder - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical encoder, and more particularly to an optical encoder using LEDs.
[0002]
[Prior art]
FIG. 9 is a perspective view showing a configuration of a conventional optical rotary encoder. In the figure, two members having slits are interposed in the optical path between the light source 101 and the light receiving element 102. Among them, the member on the light source 101 side is a rotating slit plate 103 that rotates together with the rotation amount to be measured, and slits 103a are formed radially and at equal intervals in the circumferential direction. The other member is a fixed slit plate 104, and slits 104a are formed at equal intervals with the rotary slit plate 103.
When the slits 103a and 104a are arranged in the Z direction in the figure, the light from the light source 101 passes through the slits 103a and 104a, and the passed light is detected by the light receiving element 102 disposed behind the fixed slit plate 104. Is done.
[0003]
[Problems to be solved by the invention]
In the conventional optical rotary encoder as described above, in order to obtain high resolution, it is necessary to make the slit pitch (interval in the circumferential direction) in each of the slit plates 103 and 104 as small as possible. However, if the slit pitch is reduced, the light that has passed through the slit 103a of the rotating slit plate 103 spreads due to the diffraction phenomenon, and the contrast of the light in the fixed slit plate 104 decreases. In order to avoid this, the slit pitch is reduced, the gap between the slit plates 103 and 104 (distance in the Z direction) is reduced, or a laser diode is used as the light source 101 to prevent diffraction. Conceivable. However, if the gap is reduced, there is no allowance for clearances such as machining errors and axial deflection of the rotating slit plate 103, and manufacturing becomes difficult. On the other hand, laser diodes are vulnerable to heat, generally have a short life, and have problems such as large output fluctuations due to temperature, and cannot be used in a rotary encoder for a servo motor that can be used at high temperatures.
[0004]
In view of the above-described conventional problems, an object of the present invention is to provide an optical encoder that has high resolution, is easy to manufacture, and can be used even in a high temperature environment.
[0005]
[Means for Solving the Problems]
The optical encoder of the present invention extracts an LED that emits light having a wavelength λ, and extracts only parallel light that is substantially parallel to the optical axis from the light emitted from the LED, and condenses it with a Fresnel zone plate. An optical unit integrated with a selective condensing unit and a collimator unit that collimates light that has spread from the focal point by the selective condensing unit, and maintains coherence at least up to a certain distance A light source device that emits light, and slits having a slit pitch P, each being a natural number n and being spaced apart from each other in the optical axis direction of the light source device by a gap G represented by G = nP ² / λ, Index slit plate and main slit plate constituting a slit shutter for light maintaining coherence, the index slit plate, And a light receiving element for receiving the light passing through each slit in-slit plate, the selection condensing unit, other of said Fresnel zone plate, the light not incident on the Fresnel zone plate of the emitted light from the LED A light-shielding portion that refracts or attenuates the optical axis outward with respect to the optical axis, and the Fresnel zone plate is disposed at a distance of at least 10 times the emission diameter from the LED. (Claim 1).
[0006]
In the optical encoder configured as described above, only the parallel light with respect to the optical axis is condensed by the selective condensing unit, and a secondary light source having a very small spot is formed at the focal point. Is done. The collimator unit radiates light that spreads again from the secondary light source. On the other hand, the gap G between the index slit plate and the main slit plate constituting the slit shutter is a natural number multiple of the distance P ² / λ where the Talbot image is formed. Accordingly, when the coherence-maintaining light is incident on the slit shutter, a Talbot image having an ideal light intensity distribution that is the same as or reversed from the periodic structure of one slit appears on the other slit. Thereby, the light receiving element detects an image having high contrast. Further, the value of the gap G that can be easily manufactured can be set by arbitrarily selecting the value of n. Note that LEDs are more resistant to heat than laser diodes and have longer lifetimes.
[0007]
Further, since the Fresnel zone plate is arranged at a distance of at least 10 times the emission diameter from the LED , the light incident on the Fresnel zone plate can be regarded as being substantially parallel to the optical axis. . Furthermore, the light that is not incident on the Fresnel zone plate can be refracted or attenuated outward with respect to the optical axis due to the presence of the light shielding portion.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an optical rotary encoder according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing a configuration of a light source device 1 used in the optical rotary encoder of the embodiment. In the figure, a lens body 11 formed by molding a transparent resin is a compound optical element having an optical axis A as a center line of a hatched cross-sectional shape portion, and a collimator constituting an aspherical lens on the right end side thereof. A portion 11a is formed, and a mortar-shaped light shielding portion 11b is formed inside the left side. Moreover, the right end bottom part (B part) of the light-shielding part 11b is a condensing part by the Fresnel zone plate (henceforth FZP) described below.
[0012]
FIG. 2 is an enlarged view of a portion B in FIG. 1, and the light-shielding portion 11b forms an inclined surface around the optical axis A with an angle θ (90 ° or less is as small as possible). In addition, an FZP 11c having a diameter d1 perpendicular to the optical axis A is formed on the bottom portion formed at the center of the mortar-shaped light-shielding portion 11b by integral molding with the lens body 11 or a semiconductor microfabrication technique. The FZP 11c can be easily manufactured with a minute size compared to the lens, and an arbitrary focal length can be obtained by designing the annular zone. In this way, an integrated optical unit including the collimator unit 11a, the light shielding unit 11b, and the FZP 11c as the light collecting unit is formed.
[0013]
Returning to FIG. 1, a cylindrical support portion 11 d that supports the LED 13 is formed on the left end side of the lens body 11. The LED 13 includes a disk-shaped mounting base 13a having a hem that spreads in a bowl shape, a light-emitting portion 13b that is attached to the center of the right end surface of the mounting base 13a, and a lead portion that is electrically connected to the light-emitting portion 13b. 13c. The LED 13 is fixed to the lens body 11 by fitting the mounting base 13 a into the support portion 11 d of the lens body 11. In a state where the LED 13 is fixed to the lens body 11, the optical axis of the light emitting portion 13b coincides with the optical axis A of the optical unit. The diameter d2 of the light emitting portion 13b, that is, the light emitting diameter is substantially the same as the diameter d1 (FIG. 2) of the FZP 11c.
[0014]
FIG. 3 is a sectional view further enlarging a part of FIG. With the above configuration, the light emitting unit 13b, the FZP 11c, the light shielding unit 11b, and the collimator unit 11a share the optical axis A, and the FZP 11c is disposed at a position spaced apart by a distance L on the optical axis of the light emitting unit 13b. This distance L is sufficiently larger than the diameter d2 of the light emitting portion 13b and the diameter d1 of the FZP 11c. Here, “sufficiently” specifically means 10 times or more, preferably 20 times or more. The collimator unit 11a is disposed further forward on the optical axis than the focal position (focal point F) of the FZP 11c, and the focal position is set as the focal point of the collimator unit 11a itself. That is, in FIG. 3, the focal length of the FZP 11c is f1, the focal length of the collimator unit 11a is f2, the effective beam diameter of the collimator unit 11a is d3, and the distance from the principal point P of the collimator unit 11a to the outermost peripheral position of the effective beam. If h,
(F2 + h) / f1 = d3 / d2
It becomes.
[0015]
The light shielding part 11b and the FZP 11c constitute a selective condensing part that extracts only the parallel light with respect to the optical axis A out of the light emitted from the light emitting part 13b of the LED 13 and condenses it. That is, the light shielding unit 11b refracts light that is not incident on the FZP3 out of the light emitted from the light emitting unit 13b, with respect to the optical axis A (optical path P1). The refracted light is excluded by exiting from the outer peripheral surface of the lens body 11. On the other hand, among the light emitted from the light emitting unit 13b, the light incident on the FZP 11c includes parallel light having ideal parallelism with respect to the optical axis A and a slight angle with respect to the optical axis A. It is included. However, as described above, since the distance L between the two is sufficiently larger than the diameter d2 of the light emitting portion 13b and the diameter d1 of the FZP 11c, the angle is very small and can be ignored. Accordingly, substantially all of the light incident on the FZP 11c is parallel light. These parallel lights converge on the focal point F by the lens action of the FZP 11c. By converging only the parallel light, the light at this focal point F forms a secondary light source with a minimal spot.
[0016]
The light that has spread again from the secondary light source is collimated by the collimator unit 11 a and is emitted from the right end side of the lens body 11 to the outside. In this way, light with high precision parallelism can be emitted. Further, since the parallel light corresponding to the size of the light emitting portion 13b is collected by the FZP 11c, the light intensity at the focal point F is higher than in the case where the parallel light is extracted by a pinhole. Therefore, the finally emitted light also has a high light intensity.
[0017]
Moreover, it was confirmed by experiment that the light emitted from the light source device 1 as described above has coherence within a certain distance Lc. Therefore, in the range of the distance Lc, light having parallelism and coherence can be emitted. The distance Lc varies depending on the light emission diameter d2 of the light emitting portion 13b, and becomes longer as the light emission diameter is smaller.
As described above, the light source device 1 can emit light having high precision parallelism and coherence. Further, the LED 13 is more resistant to heat than a laser diode and has a longer lifetime. Therefore, use in a high temperature environment is also possible. The LED 13 is also highly practical in that it is less expensive than a laser diode.
[0018]
Instead of the mortar-shaped light shielding part 11b as described above, a mask-shaped light shielding part may be provided. FIG. 4 is a cross-sectional view showing an example of such a light shielding portion. In this case, the left half of the lens body 11 is formed in a cylindrical shape, and a black mask-shaped light-shielding portion 14 that hardly reflects light is mounted on a central circular bottom portion 11e. The hole 14a at the center of the light shielding part 14 is on the optical axis A, and the diameter thereof is substantially equal to the emission diameter d2 of the light emitting part 13b. The FZP 11c is provided in the lens body 11 at the position of the hole 14a as in the previous example. The light (optical path P <b> 2) that has entered the light shielding unit 14 without entering the FZP 11 c hits the light shielding unit 14 and is attenuated. On the other hand, the light incident on the FZP 11c is condensed and collimated as in the previous example.
[0019]
Next, an optical rotary encoder according to an embodiment of the present invention using the light source device 1 configured as described above will be described.
FIG. 5 is a perspective view showing the basic configuration of the optical rotary encoder (note that the dimensions shown are not necessarily proportional to the actual dimensions). In the figure, the light source device 1 emits coherent light collimated as described above. If the direction of coherent light is the Z direction, the light receiving element 2 is disposed at the end in the Z direction. Between the light source device 1 and the light receiving element 2, a rectangular index slit plate 3 and a circular main slit plate 4 are arranged in parallel to an XY plane orthogonal to the Z direction. The index slit plate 3 is fixed, but the main slit plate 4 is rotatably supported and rotates together with the rotation amount to be measured. In the main slit plate 4, slits 4 s are formed radially around the position of the radius R from the rotation center O and at equal intervals (slit pitch P) in the circumferential direction.
[0020]
FIG. 6A is an enlarged front view of the index slit plate 3. The index slit plate 3 has a slit portion 3a composed of four slit windows 3b, and (b) is an enlarged view thereof. In each slit window 3b, slits 3s are formed radially around the position of the radius of curvature R (same as the radius R described above) and at equal intervals (slit pitch P) in the circumferential direction.
[0021]
Both the slits 3s and 4s constitute a diffraction grating having a slit pitch P and a slit width P / 2. The XY coordinate (see FIG. 5) of the center of curvature of the slit window 3b in the index slit plate 3 coincides with the rotation center O of the main slit plate 4, and therefore each slit 3s and 4s is on the optical axis (see FIG. 5). 5 on the Z axis). That is, the index slit plate 3 and the main slit plate 4 constitute a slit shutter for the light from the light source device 1. The gap G in the optical axis direction between the index slit plate 3 and the main slit plate 4 is such that the wavelength of light is λ and n is a natural number (1, 2, 3,...)
G = nP ² / λ
It is set to be the relationship. The index slit 3 and the main slit 4 are within the above-described distance Lc, and the light reaching both the slits 3 and 4 has parallelism and coherence. In addition, stricter coherence is required as the slit pitch P is smaller. In order to improve coherence, it is effective to secure a distance of at least 10 times the light emission diameter d2 of the LED 13 in the light source device 1 between the light emitting unit 13b and the FZP 11c.
[0022]
FIG. 7 is a diagram for explaining the Talbot effect. When the diffraction grating having the slit pitch P as described above is irradiated with coherent light having the wavelength λ, the light intensity distribution is the same as the periodic structure of the diffraction grating every Talbot distance Z _t = 2P ² / λ in the light propagation direction. The Talbot statue appears. In addition, a Talbot image having a light intensity distribution obtained by inverting the periodic structure of the diffraction grating appears at every odd number of times Z _t / 2 (= P ² / λ). Accordingly, a Talbot image having a light intensity distribution that is the same as or inverted from the periodic structure of the diffraction grating always appears at the position of the distance nP ² / λ corresponding to a natural number multiple of Z _t / 2. Here, since the slit width is ½ of the slit pitch P, the light intensity distribution identical to the periodic structure of the diffraction grating and the light intensity distribution obtained by inverting it are substantially the same regardless of which is adopted. It is. This distance nP ² / λ is equal to the gap G. That is, on the main slit plate 4 in the above embodiment, an image having an ideal light intensity distribution that is the same as or inverted from the periodic structure of the slits 3a of the index slit plate 3 appears. Moreover, as described above, the light emitted from the light source device 1 has a high light intensity. Therefore, the image detected by the light receiving element 2 also has high contrast and high resolution can be obtained.
[0023]
By designing the gap G using the Talbot effect as described above, the value of the gap G can be selected as a value that is easy to manufacture. For example, when the slit pitch P is 10 μm (high resolution level) and the wavelength λ is 860 nm, n = 1.
G = (10 × 10 ⁻⁶ ) ² / (860 × 10 ⁻⁹ ) ≈116 × 10 ⁻⁶
Thus, a gap of 116 μm can be secured. Usually, the standard that is difficult to manufacture is about 40 μm. Therefore, manufacturing is facilitated by securing such a sufficient gap. A preferable range of the gap G is 80 to 200 μm from the viewpoint of ease of manufacture and the contrast of the Talbot image.
Also, when trying to obtain a higher resolution (slit pitch P is 10 μm or less), for example, if P = 6 μm under the above conditions, the value of G is about 42 μm, which is below the preferred range. By setting n to 2 or 3, G can be set to about 84 μm or 126 μm and fall within a suitable range.
[0024]
In the above embodiment, the light source device 1 that forms the secondary light source by the FZP 11c is used. However, in the case of the LED 13 having the light emitting portion 13b having a relatively small light emitting diameter (for example, 50 μm) as shown in FIG. Even using only the collimator lens 21 subjected to aberration correction, light having parallelism and coherence can be emitted. In this case, the distance Lc at which coherence is ensured is, for example, about 300 μm when the emission diameter d2 is 50 μm. Therefore, by using such a light source device 22 in place of the light source device 1 described above, and arranging the index slit 3 and the main slit 4 within the distance Lc, it is possible to obtain the same effect as the above case. it can. In this case as well, the smaller the slit pitch P, the more severe the coherence is required. In order to improve the coherence, it is necessary to reduce the emission diameter d2 of the LED 13. Specifically, it is considered that the relationship between the emission diameter d2 and the slit pitch P is preferably d2 ≦ 6 × P.
[0025]
In the above embodiment, as shown in FIG. 5, the index slit plate 3 is arranged on the light source device 1 side and the main slit plate 4 is arranged on the light receiving element 2 side. However, these arrangements may be reversed.
In the above embodiment, the distance between the main slit plate 4 and the light receiving element 2 may be a value corresponding to a natural number multiple of Z _t / 2, similarly to the gap G.
Moreover, although the rotary encoder was demonstrated in the said embodiment, the same structure can also be applied to a linear encoder.
[0026]
【The invention's effect】
The present invention configured as described above has the following effects.
According to the optical encoder of claim 1, light that is collimated using an LED as a light source and has coherence is incident on a slit shutter having a gap G = nP ² / λ between both slits. A Talbot image having an ideal light intensity distribution that is the same as or reversed from the periodic structure of the above appears on the other slit. Thereby, since the light receiving element detects an image having a high contrast, a high resolution can be obtained. Moreover, since the value of the gap G that can be easily manufactured can be set by arbitrarily selecting the value of n, the optical encoder is easy to manufacture. Further, since the LED is more resistant to heat than a laser diode and has a longer lifetime, it can be used in a high temperature environment.
[0027]
Further, since the light incident on the Fresnel zone plate can be regarded as substantially parallel to the optical axis, the parallelism of the light can be easily ensured. Furthermore, the light that is not incident on the Fresnel zone plate can be refracted or attenuated outward with respect to the optical axis due to the presence of the light shielding portion.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration of a light source device used in an optical rotary encoder according to an embodiment of the present invention.
FIG. 2 is an enlarged view of a portion B in FIG.
FIG. 3 is a cross-sectional view in which a part of FIG. 1 is further enlarged, showing a light path.
4 is a cross-sectional view showing a light source device having a configuration different from that of FIG.
FIG. 5 is a perspective view showing a basic configuration of an optical rotary encoder according to an embodiment of the present invention.
6A is an enlarged front view of the index slit plate in FIG. 5, and FIG. 6B is an enlarged view of the index slit plate.
FIG. 7 is a diagram illustrating a Talbot effect.
FIG. 8 is a cross-sectional view showing a light source device including an LED and a collimator lens.
FIG. 9 is a perspective view showing a conventional optical rotary encoder.
[Explanation of symbols]
1, 22 Light source device 2 Light receiving element 3 Index slit plate 4 Main slit plates 3s, 4s Slit 11 Lens body 11a Collimator portion 11b Light shielding portion 11c FZP (light collecting portion)
13 LED
13b Light emitting part 21 Collimator lens A Optical axis d2 Light emitting diameter G Gap L Distance P Slit pitch

Claims (1)

LED that emits light of wavelength λ, and a selective condensing unit that extracts only parallel light with respect to the optical axis out of the light emitted from the LED and condenses it with a Fresnel zone plate, and the selective collection An optical unit integrated with a collimator unit that collimates the light that has converged on the focal point by the light unit, and that emits light that maintains coherence at least to a certain distance; and
A slit shutter for light that has slits with a slit pitch P and is spaced apart from each other in the optical axis direction of the light source device by a gap G represented by G = nP ² / λ, where n is a natural number, and maintains coherence An index slit plate and a main slit plate constituting
A light receiving element that receives light that has passed through each slit of the index slit plate and the main slit plate ,
In addition to the Fresnel zone plate, the selective condensing unit includes a light shielding unit for refracting or attenuating light that is not incident on the Fresnel zone plate out of the light emitted from the LED with respect to the optical axis. And the Fresnel zone plate is disposed at a distance of at least 10 times the emission diameter from the LED .

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