JPH0588768B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rotary encoder, and particularly relates to a rotary encoder, in which a radiation grating in which a plurality of grid patterns of, for example, transparent parts and reflective parts are periodically carved on the circumference of a rotating object is used. The radiation grating is irradiated with a beam of light from, for example, a laser, and the diffracted light from the radiation grating is used to photoelectrically measure the rotational state of the radiation grating or rotating object, such as the rotational speed or the amount of variation in rotational speed. This can be well applied to a rotary encoder for detection.

(Prior art) Computer equipment such as floppy desk drives, office equipment such as printers, or
Photoelectric rotary encoders have been used as a means to detect the rotational speed and variation in rotational speed of rotating mechanisms such as NC machine tools and VTR capsten motors and rotating drums.

A photoelectric rotary encoder has a so-called main scale in which transparent parts and light-shielding parts are provided at equal intervals around a disc connected to the rotation axis, and correspondingly transparent parts and light-shielding parts are provided at equal intervals to the main scale. A so-called fixed index scale is provided, and a so-called index scale system is adopted in which both scales are placed facing each other with a light emitting means and a light receiving means sandwiched between them. In this method, as the main scale rotates, a signal is obtained that is synchronized with the interval between the light-transmitting part and the light-blocking part of both scales, and this signal is frequency-analyzed to detect fluctuations in the rotational speed of the rotating shaft. Therefore, the finer the scale interval between the light-transmitting part and the light-blocking part of both scales, the higher the detection accuracy can be. However, when the scale interval is made finer, the S/ of the output signal from the light receiving means is
There was a drawback that the N ratio decreased and the detection accuracy decreased. For this reason, if you fix the total number of gratings in the light-transmitting part and light-blocking part of the main scale, and try to increase the distance between the light-transmitting part and the light-blocking part to the extent that it is not affected by diffracted light, the diameter of the main scale disc will increase. This increases the size and thickness of the device, making the entire device larger, resulting in disadvantages such as an increase in the load on the rotating object to be tested.

(Problems to be Solved by the Invention) An object of the present invention is to provide a rotary encoder that has a small load on a rotating object to be inspected, is easy to miniaturize the entire device, and is capable of detecting a rotational state with high precision.

(Means for solving the problem) A light source that generates a coherent light beam and two light beams emitted from the light source are each reflected by a reflecting means to reverse the optical path, thereby achieving relative rotation. an optical system arranged to make the light incident twice on each disc-shaped radiation grating connected to the object to be measured; and a light receiving means for detecting interference light of the two diffracted lights from the radiation grating. A rotary encoder which obtains the relative rotational state of the disk based on the detection result of the light receiving means,
The incident position of each of the two light beams onto the radiation grating is made substantially point symmetrical with respect to the rotation center of the radiation grating, and the reflecting means directs the light direction in the direction of output of the diffracted light of a specific order from the radiation grating. The present invention includes lens means that are substantially coincident with each other, and a reflecting surface that reflects only the diffracted light of the specific order near the focal plane of the lens means.

Other features of the invention are described in the Examples.

(Embodiment) FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention.

In the figure, 1 is a coherent light source such as a laser, 2 is a collimator lens, and 3 ₁ and 3 ₂ are polarization beam splitters, whose polarization axis is 45 degrees with respect to the linearly polarized light from laser 1. It is arranged like this. 4 ₁ to 4 ₅ are each 1/4 wavelength plate, 5 ₁ to 5 ₄
are cylindrical lenses, 6 is a radiation grating in which, for example, a lattice pattern of transparent parts and reflective parts is provided at equal angles on a disk, and 7 is a rotation axis of a rotating object to be tested (not shown). 9 ₁ and 9 ₂ are reflecting mirrors, 10 is a 1/2 wavelength plate, 11
is a beam splitter, 12 ₁ and 12 ₂ are polarizing plates,
13 ₁ and 13 ₂ are light receiving elements. 14 ₁ and 14 ₂ are convex lenses with condensing properties, and 15 ₁ and 15 ₂ are convex lenses 14
₁ , 14 A reflecting mirror placed near the focal position of ₂ ,
In front of it are masters 16 ₁ , 16 ₂ that limit the luminous flux.
is provided.

In this embodiment, a convex lens 14 ₁ , a reflecting mirror 15 ₁ , and a mask 16 ₁ constitute an optical system that is part of the reflecting means.

Next, the operation of the rotary encoder shown in FIG. 1 will be explained. The light beam emitted from the laser 1 is turned into a substantially parallel light beam by the collimator lens 2, enters the polarizing beam splitter ₃₁ , and is transmitted and reflected in substantially equal amounts. Of these, the transmitted luminous flux is 1/
It passes through a 4-wave plate, 4 ₁ , becomes circularly polarized light, and passes through a cylindrical lens 5 ₁ to a position on the radiation grating 6.
Linear irradiation of M ₁ .

Here, the cylindrical lens 5 ₁ is arranged as necessary so as to linearly irradiate the luminous flux in a direction perpendicular to the radiation direction of the radiation grating 6 . By performing the linear irradiation in this manner, it is possible to reduce the pitch error in the grid pattern between the light transmitting portion and the reflecting portion corresponding to the irradiated portion of the light beam on the radiation grating 6.

The diffracted light of a specific order, which is linearly irradiated onto the position M ₁ on the radiation grating 6 and diffracted, becomes a substantially parallel light beam by the cylindrical lens 5 ₂ and enters the convex lens 14 ₁ which is a part of the optical system. FIG. 2 is a schematic diagram of an optical system that is a part of the reflecting means in this embodiment, and as can be understood from FIG. 2, the convex lens 1
The optical axis of 4 ₁ substantially coincides with the traveling direction of the incident diffracted light of a specific order, and the reflecting mirror 15 ₁ is a convex lens 1
4 It is arranged approximately at the focal plane of ₁ . Therefore, the light flux that is parallel to the convex lens 14 ₁ is focused on the reflecting mirror 15 ₁ via the mask 16 ₁ . The focused luminous flux is
It is reflected and returns to the original optical path to a position on the radiation grating 6.
Re-irradiate _M1 . The light beam re-diffracted by _M1 passes through a quarter-wave plate ₄₁ , is reflected by a polarizing beam splitter ₃₁ , passes through a quarter-wave plate ₄₂ , is reflected by a reflecting mirror ₉₁ , and is again The light is transmitted through the polarizing beam splitter 3 ₁ via the 1/4 wavelength plate 4 ₂ . Then, the polarization direction is rotated by 90 degrees by the 1/2 wavelength plate 10, reflected by the polarizing beam splitter ₃₂ , and then reflected by the 1/4 wavelength plate _45.
The light is split by the beam splitter 11 and sent to the light receiving elements 13 ₁ , 13 ₂ via the polarizing plates 12 ₁ , 12 _{2 .}
The light is received by

On the other hand, the light beam emitted from the laser 1 and reflected by the polarizing beam splitter 3 ₁ passes through the 1/2 wavelength plate 10 , passes through the polarizing beam splitter 3 ₂ , and passes through the 1/4 wavelength plate 4 ₃ . It is reflected by the reflecting mirror 9 ₂ through the polarizing beam splitter 3 ₂ ,
A position M ₂ on the radiation grating 6 is linearly irradiated via a quarter-wave plate 4 ₄ and a cylindrical lens 5 ₃ .

Here, the positions M ₁ and M ₂ have a substantially symmetrical positional relationship with respect to the rotation center of the rotating object to be tested.

Among the light beams diffracted at the position _M2 , the diffracted light _L2 of a specific order becomes a substantially parallel light beam by the cylindrical lens ₅₄ in the same manner as the diffracted light _L1 , and enters the convex lens ₁₄₂ , and enters the light beam limiting mask 16. The light is focused onto a reflecting mirror _{152 through a mirror 152 .} The focused light beam is reflected and returns to the original optical path to a point on the radiation grating 6.
Reirradiate with _M2 . The light beam re-diffracted at point _M2 returns to the original optical path and enters the polarizing beam splitter 3.
₂ , the light is overlapped with the diffracted light L ₁ at the position M ₁ and received by the light receiving elements 13 ₁ and 13 ₂ . When the rotating object to be tested rotates, the diffracted light L ₁ at position M ₁ becomes
The frequency is shifted by Δ=rωsinθ _n /λ. Here, r is the distance from the center of rotation to M ₁ , ω is the angular velocity, θ _n is the diffraction angle of the m-th order diffracted light L ₁ , and λ is the wavelength of the laser 1 .

Since the diffracted light L ₁ is reflected by the reflecting means and re-diffracted at the position M ₁ , the frequency is shifted by 2Δ when it enters the light receiving elements 13 ₁ and 13 ₂ . On the other hand, the diffracted light L ₂ at the position M ₂ is transmitted to the light receiving elements 13 ₁ , 1
3 ₂ , the frequency is shifted by -2Δ in the same way.

Therefore, the frequency of the output signals from the light receiving elements 13 ₁ and 13 ₂ is 4Δ. Also, if the pitch of the lattice pattern at positions M ₁ and M ₂ is P, then from the diffraction conditions,
Since sinθ _n =mλ/P, the frequency of the output signal of the light receiving element is F=4Δ=4mrω/P.

If the total number of grid patterns of the radiation grating 6 is N and the equiangular pitch is Δ, then from P=rΔ and Δ=2π/N, F=2mNω/π. Now, if the wave number of the output signal of the light receiving element during time Δt is n, and the rotation angle of the radiation grating 6 during Δt is θ, then n=FΔt,
Since θ=ωΔt, n=2mNθ/π (1), and by counting the wave number n of the output signal of the light-receiving element, the rotation angle θ of the radiation grating 6 can be determined by equation (1). In the embodiment shown in FIG. 1 configured in this way, since diffracted light is used, it is possible to use a small-diameter fine grating for the radiation grating 6, and therefore the entire device can have a small diameter. Therefore, the load on the rotating object to be tested is also reduced. In addition, in this example, the diffracted lights L ₁ and L ₂ are
By using convex lenses 14 ₁ and 14 ₂ and reflecting mirrors 15 ₁ and 15 ₂ as a reflecting means for re-irradiating M ₁ and M ₂ , the same function as when corner cube reflecting mirrors are used is achieved. .

In other words, the wavelength of the light beam incident on the radiation grating 6 may change due to changes in the surrounding temperature, or the rotation center of the rotating object to be tested and the rotation center of the radiation grating 6 may not coincide with each other. Light flux incidence position on 6
When the pitch of the lattice pattern in M ₁ and M ₂ changes as the radiation grating 6 rotates, the diffracted light L ₁ ,
The diffraction angle of L ₂ changes.

However, according to this embodiment, by configuring the optical system of the reflecting means as described above, the convex lens 14
After the light beams incident on the light beams ₁ and 14 ₂ are reflected by the reflecting mirrors 15 ₁ and 15 ₂ , the light beams can be outputted from the convex lenses 14 ₁ and 14 ₂ at the same angle as when they were incident, and the original optical path can be returned. Furthermore, the _convex lens ₁₄ is
It is possible to remove diffracted lights other _than the diffracted lights L ₁ and L 2 of specific orders, such as 0th-order diffracted lights that are incident on L 1 _and L ₂ . This allows the radiation grating 6 and the reflecting means 14 ₁ , 14 ₂ , 15 ₁ , 15 ₂ , 16 ₁ , 16
It becomes possible to shorten the distance between _{the two} . for example,
When the pitch of the grid pattern at the positions M ₁ and M ₂ of the radiation grating 6 is 10 μm and the wavelength of the incident light beam is 0.83 μm, plano-convex microlenses with a radius of 3 mm are used as the convex lenses 14 ₁ and 14 ₂ to When trying to reirradiate the radiation grating 6 with the diffracted light, the 0th order diffracted light enters the microlenses 14 ₁ and 14 ₂ at an angle of 4.8° with respect to the 1st order diffracted light, so the light flux limiting mask is used. If the apertures of 16 ₁ and 16 ₂ have a radius of (focal length of microlens = 6 mm) x (tan 4.8°) = 0.5 mm or less, the 0th order diffracted light can be removed. At this time, from the radiation grating 6 to the reflecting mirrors 15 ₁ , 1
_5. A distance of about 15 mm is sufficient.
In addition, convex lenses 14 ₁ , 14 ₂ and reflecting mirrors 15 ₁ , 1
5 ₂ and the light flux limiting masks 16 ₁ and 16 ₂ can both be easily manufactured, and have the advantage of being lower in cost than, for example, a case where a corner cube reflecting mirror is used.

In this embodiment, the reflecting mirrors 15 ₁ and 15 ₂ may be constructed of concave mirrors having the centers of curvature at the nodes of the convex lenses 14 ₁ and 14 ₂ instead of plane mirrors. Furthermore, it is preferable to construct the optical system of the reflecting means by integrating a convex lens, a mask, and a reflecting mirror as shown in FIG. 3, since this simplifies the entire apparatus.

In the figure, 18 ₁ is a condenser lens, 18 ₂ is a reflective surface, and 18 ₃ is a mask.

Furthermore, as shown in Fig. 4, the lens shown in Fig. 3 is made into a gradient index lens, for example, a self-occurring microlens (trade name: manufactured by Nippon Sheet Glass Co., Ltd.), and a reflecting mirror is deposited only at the center of its flat end face. By doing so, manufacturing becomes easy, and the device becomes small and simple.

In Fig. 4, 19 ₁ is a gradient index lens;
19 ₂ is a reflecting part (back mirror).

The diffraction grating used in the present invention may be a so-called amplitude type diffraction grating consisting of a light shielding part and a light transmitting part, a phase type diffraction grating consisting of parts having mutually different refractive indexes, or the like. In particular, phase-type diffraction gratings (phase gratings) can be obtained, for example, by forming a relief-type uneven pattern on the circumference of a transparent disk, and are effective because they can be mass-produced using stampers, embossing, etc. . Further, a reflective phase grating can be easily created by forming a reflective film such as vapor deposition on a concavo-convex pattern.

In this embodiment, a case has been described in which transmitted diffracted light is used, but the object of the present invention can be similarly achieved by using reflected diffracted light.

(Effects of the Invention) As described above, according to the present invention, by using a reflecting means having an optical system in which a reflecting surface is arranged at the focal plane, the load on the rotating object to be inspected is small, and the entire apparatus can be miniaturized. The precision of rotary encoder can be achieved.

Furthermore, by making the incident positions of the two light beams on the radiation grating approximately point symmetrical with respect to the rotation center of the radiation grating, for example, disturbances can cause a "shake" in the relative rotation, and the radiation grating can be rotated. Even if the eccentricity occurs with respect to the center, the eccentricity gives opposite error components to each of the two light beams that are incident on positions that are approximately symmetrical with respect to the rotation center (as viewed from the rotation direction). a lens means capable of canceling out error components by causing two diffracted lights emitted from the radiation grating to interfere with each other, and whose optical axis direction substantially coincides with the emission direction of the diffracted light of a specific order from the radiation grating; By means of a reflecting means having a reflecting surface that reflects only the diffracted light of the specific order near the focal plane of the means, unnecessary diffracted light can be effectively removed. Even if the position of the reflecting means changes, fluctuations in the output optical path of the luminous flux can be minimized, resulting in a rotary encoder that is more resistant to external disturbances.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention;
Figure 2 is an explanatory diagram of a part of Figure 1, Figures 3 and 4.
The figure is a partial explanatory diagram of another embodiment of the present invention. In the figure, 1 is the light source, 2 is the collimator lens, and 3
₁ and 3 ₂ are polarizing beam splitters, 4 ₁ to 4 ₅ are 1/
4 wavelength plate, 5 ₁ to 5 ₄ are cylindrical lenses, 6
is a radiation grating, 7 is a rotation axis, 9 ₁ and 9 ₂ are reflecting mirrors, 1
2 ₁ and 12 ₂ are polarizing plates, 13 ₁ and 13 ₂ are light receiving elements,
14 ₁ and 14 ₂ are convex lenses, 15 ₁ and 15 ₂ are reflecting mirrors, and 16 ₁ and 16 ₂ are light flux limiting masks.

Claims (1)

[Claims] 1. A light source that generates a coherent light beam, and two light beams emitted from the light source are each reflected by a reflecting means to reverse the optical path to the object to be measured whose relative rotation is to be measured. An optical system is arranged so that the light is incident twice on each connected disc-shaped radiation grating, and two beams from the radiation grating are arranged.
and a light receiving means for detecting interference light of two diffracted lights, and the rotary encoder determines the relative rotational state of the disk based on the detection result of the light receiving means, wherein each of the two light beams is incident on the radiation grating. The reflecting means includes a lens means whose position is approximately symmetrical with respect to the center of rotation of the radiation grating, and whose optical axis direction substantially coincides with the direction in which diffracted light of a specific order is emitted from the radiation grating; A rotary encoder comprising: a reflecting surface that reflects only the diffracted light of the specific order near the focal plane.