JP4488065B2 - Optical encoder - Google Patents

Description

  The present invention relates to an optical encoder, and more particularly to an optical encoder using LEDs.

FIG. 9 is a perspective view showing a configuration of a conventional optical rotary encoder (see, for example, Patent Document 1). 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.

JP-A-10-325742 (FIG. 5)

  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.

  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.

The optical encoder of the present invention includes a light source device that emits light that maintains coherence up to a distance Lc, and an LED that emits light of wavelength λ and a collimator lens that collimates the light, and the distance Lc from the light source device. Each of which 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 the coherence An index slit plate and a main slit plate that constitute a slit shutter for the light, and a light receiving element that receives light that has passed through each slit of the index slit plate and the main slit plate, and the light source device has a light emission diameter of the LED The coherence is improved by making the diameter smaller than 6 times the slit pitch P. Is.

In the optical encoder configured as described above, the collimator lens radiates the light emitted from the LED in parallel. 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. Therefore, when the light maintaining the coherence 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.
Further, by setting the light emission diameter of the LED to 6 times or less of the slit pitch P, the coherence of light is appropriately ensured in relation to the slit pitch.
Further, since the coherence of light is appropriately secured in relation to the slit pitch, it is possible to always ensure the coherence of light with certainty.

In the optical encoder, the distance between the index slit plate and the main slit plate immediately before the light receiving element and the light receiving element may be a natural number multiple of P ² / λ.
Moreover, the light source device may be integrally formed continuously from the support portion that supports the LED to the collimator lens.

According to the optical encoder of the present invention, a Talbot image having an ideal light intensity distribution that is the same as or inverted from the periodic structure of one slit 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.
Further, by setting the light emission diameter of the LED to 6 times or less of the slit pitch P, the light coherence is appropriately ensured in relation to the slit pitch.
Further, since the coherence of light is appropriately secured in relation to the slit pitch, it is possible to always ensure the coherence of light with certainty.
And such an effect can be easily acquired with the light source device only of LED and a collimator lens.

  Hereinafter, an optical rotary encoder according to a reference example 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 reference example. 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.

  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 orthogonal 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.

  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 part 13b attached to the center of the right end surface of the mounting base 13a, and a lead part electrically connected to the light emitting part 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.

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.

  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.

  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.

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.

  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 light 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.

Next, a reference example of the present invention using the light source device 1 configured as described above and an optical rotary encoder according to an embodiment described later 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.

  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.

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.

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. Further, 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 reference example, 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.

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.

  In the above reference example, the light source device 1 that forms the secondary light source by the FZP 11c is used. However, as shown in FIG. 8, the light emitting unit 13b having a relatively small light emitting diameter (for example, 50 μm) is used as an embodiment of the present invention. In the case of the LED 13 having, it is possible to emit light having parallelism and coherence only by using the collimator lens 21 that has been subjected to sufficient aberration correction. 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.

In the reference example and the 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 reference example and the 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, similar to the gap G.
Moreover, although the said reference example and embodiment demonstrated the rotary encoder, the same structure can also be applied to a linear encoder.

It is sectional drawing which shows the structure of the light source device used for the optical rotary encoder by one reference example of this invention. It is an enlarged view of the B section in FIG. FIG. 1 is a cross-sectional view in which a part of FIG. 1 is further enlarged to describe a light path. It is sectional drawing which shows the light source device of a structure different from FIG. It is a perspective view which shows the basic composition of the optical rotary encoder by the reference example and one Embodiment of this invention. (A) is the front view which expanded the index slit board in FIG. 5, (b) is the enlarged view further. It is a figure explaining the Talbot effect. It is sectional drawing which shows the light source device which consists of LED and collimator lens by one Embodiment of this invention. It is a perspective view which shows the conventional optical rotary encoder.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,22 Light source device 2 Light receiving element 3 Index slit plate 4 Main slit plate 3s, 4s Slit 11 Lens body 11a Collimator part 11b Light-shielding part 11c FZP (Condensing part)
13 LED
13b Light emitting part 21 Collimator lens A Optical axis d2 Light emitting diameter G Gap L Distance P Slit pitch

Claims (3)

A light source device that emits light that maintains coherence up to a distance Lc, comprising an LED that emits light of wavelength λ and a collimator lens that collimates the light;
Located within the distance Lc from the light source device, each having slits with a slit pitch P, separated 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. An index slit plate and a main slit plate that constitute a slit shutter for the light that is arranged and maintains the coherence,
A light receiving element that receives light that has passed through each slit of the index slit plate and the main slit plate,
The optical light source device is characterized in that the coherence is improved by making the light emission diameter of the LED smaller than 6 times the slit pitch P. The optical encoder according to claim 1, wherein a distance between the index slit plate and the main slit plate immediately before the light receiving element and the light receiving element is a natural number multiple of P ² / λ.   2. The optical encoder according to claim 1, wherein the light source device is integrally formed continuously from a support portion that supports the LED to the collimator lens.

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