JPH046884B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention uses a reflective scale to measure the moving distance of the scale by utilizing the interference between the diffracted lights of coherent light, and the phase delay of the light due to reflection on a half mirror. This invention relates to an optical scale reading device that measures the direction of movement of a scale by using a scale and achieves ultra-high resolution.

Various types of optical scale reading devices that utilize light interference have been known. FIG. 1 is a diagram showing the configuration of a conventional device of this type. The laser beam l emitted from the laser is reflected by the mirror M
1 and lenses L1 and L2 to scale 1.
is irradiated. The +1st-order diffracted light diffracted by the scale 1 enters the mirror M2, and the 0th-order diffracted light enters the mirror M3.
The +1st-order diffracted light from mirror M2 goes directly to lens L3.
Also, the 0th order diffracted light from the mirror M3 is diffracted on the scale 1 and enters the lens L3 as -1st order diffracted light. At this time, the +1st-order and -1st-order diffracted lights become linearly polarized lights whose polarization planes are shifted by 90 degrees from each other by polarizers P1 and P2, respectively. The light focused by the lens L3 is divided into three directions by the spectroscope 2 and sent to the photoelectric conversion element D.
1 to D3. here. The output of D3 is used for automatic gain control to keep the laser light constant. In addition, the light entering D1 and D2 is given a phase difference of 90° by the quarter-wave plate 13, and the plane of polarization is changed.
The ±1st-order diffracted lights are mixed by analyzers P3 and P4, which are set to have an angle of 45°. The light that has produced interference fringes as a result is converted into electrical signals by photoelectric conversion elements D1 and D2. The outputs of these photoelectric conversion elements are converted into scale movement values through predetermined processing.

FIG. 2 is a block diagram showing an embodiment of an optical scale reading device already proposed by the applicant. In the figure, 11 is a coherent light source, L11 is a first lens that condenses the output light of the light source, 12 is a polarizing cube prism that receives light passing through the lens, and 13 is a polarizing cube prism that receives light that has passed through the prism. 1/4 wavelength plate, L1
2 is a second condenser lens that receives the light transmitted through the wavelength plate; 14 is a scale that receives the light that passes through the lens;
A is an imaging section where the diffracted light reflected from the scale 14 forms an image. Reference numeral 15 denotes a stopper placed in the imaging section A to remove the 0th order diffracted light, S denotes interference fringes generated at the rear of the imaging section A, and 16 denotes a light receiving element that receives the interference fringes S.

The light emitted from the light source 11 passes through the following lens L1.
1 and enters the polarizing cube prism 12. Of the light incident on the Cube prism, only the component whose polarization angle matches that of the prism passes through the prism. When a semiconductor laser is used as the light source 11, most of the light is linearly polarized and can pass through the prism 12. The light that has passed through the Cube prism enters the 1/4 wavelength plate 13. 1/4 wavelength plate 1
The light that has passed through the lens L12 becomes circularly polarized light, is focused by the lens L12, and is irradiated onto the scale 14. The light incident on the scale 14 generates multimode diffracted light when reflected. Here, the first imaging point of the 0th-order diffracted light is O1, and the imaging points of the +1st-order diffracted light and the -1st-order diffracted light are P1 and Q1, respectively. scale 1
The diffracted light reflected by 4 is directed to the first imaging point O
1, P1, advance as if it came out from Q1 Lens L12
The light is focused. The light that has passed through the lens L12 enters the 1/4 long plate 13 again. Here, the reflected light is converted back into linearly polarized light. Moreover, since its polarization angle is 90° different from the incident linearly polarized light, all of the reflected light that has entered the cube prism 12 is reflected. The reflected diffracted light is again imaged at the imaging section A. In the figure, O2 is 0th order, P2 is +1st order, and Q2 is -1
Each of the following is an imaging point of diffracted light. The zero-order diffracted light is removed by a stopper 15 provided in the imaging section A, resulting in interference fringes between the ±1st-order diffracted lights. The light-receiving element 16 that receives the interference fringes S is composed of multi-divided photodiodes, and each photodiode generates an electric signal depending on the brightness and darkness of the light.

Suppose now that the scale is moved in a certain direction while being irradiated with coherent light from the light source 11. At this time, the interference fringes S move on the light receiving element 16 in accordance with the movement of the scale 14. If you arrange the photodiodes so that their phases differ by 90 degrees,
Each of these photodiodes outputs a sine wave with a phase difference of 90 degrees. The displacement of the scale 14 can be determined by processing the outputs of these photodiodes using a control circuit (not shown). In either example, the output is two sine waves with a 90° phase difference.The amount of movement of the scale can be determined by counting the peak value of the sine waves, and the direction of movement can be determined by determining the phase relationship between the two sine waves. can be measured respectively. If you want to obtain even higher resolution here, it is conceivable to divide the sine wave voltage in an analog manner. Therefore, in that case the shape of the sine wave and the 90° phase difference need to be as accurate as possible.

All of the above-mentioned devices use a transmission type scale, so it is difficult to move the scale.
The configuration was complicated, requiring polarizers, analyzers, quarter-wave plates, beam splitters, etc. to create a 90° phase difference. In addition, in the case of Fig. 2, if the pitch of the scale 14 is small, the diffraction angle becomes large, so a high NA lens is required for L12, and the light receiving element 1
The disadvantage is that the area on the scale 14 on which the light reaching the top of the scale 6 is reflected becomes very small, making it susceptible to dust and dirt on the scale.

The present invention has been made in view of these points, and uses a reflective scale as the scale, increases the diameter of the light beam irradiated to the scale, reduces the influence of dust, and furthermore, uses a half mirror. By creating a 90° phase difference using the phase delay of light due to reflection, we have achieved an ultra-high resolution optical scale reading device that is simple in configuration and easy to operate.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 3 is a configuration diagram showing an embodiment of the present invention. In the figure, 21 is a coherent light source using a semiconductor laser or the like, 22 is a condenser lens that receives the emitted light from the light source, 23 is a reflective scale, and 24 and 25 are mirrors that each receive the diffracted light reflected by the scale 23. 26 is a first half mirror that receives reflected light from these mirrors, 27 is a second half mirror that receives transmitted light from the first half mirror and mixes and interferes with it, and 28 and 29 are phases of the second half mirror. 30 and 31 are amplifiers that amplify the outputs of these light receiving elements, and 32 receives the outputs of these amplifiers and performs arithmetic processing to calculate the moving distance of the scale 23. A signal processing circuit, 33 is a display section that displays the output of the signal processing circuit, and 34 is a light receiving element that receives reflected light from the first half mirror 26. The operation of the device configured as described above will be explained as follows.

The output light of the semiconductor laser 21 is converted into an angle (or parallel light) to be focused on the light receiving elements 28 and 29 by the lens 22. At this time, the plane of polarization should be oriented as shown in the figure. This light is scale 23
to project. As the scale, for example, a diffraction grating in which grooves are precisely carved at regular intervals or a diffraction grating made by holographic technology is used. The projected light is therefore diffracted. If the diffraction angle θ at this time is the scale pitch d and the wavelength of the semiconductor laser 21 is λ, then the following equation holds true.

sinθ=mλ/(m; integer) However, -90°≦θ≦90°, -1≦mλ/d≦+1Here, for example, if λ=0.78μm and d=0.83μm, m=0, ±1 and θ =0° (0th order diffracted light when m=0) θ=±70.0° (±1st order diffracted light when m=±1). The ±1st-order diffracted lights are reflected by mirrors 24 and 25, respectively, and after passing through a half mirror 26, they are mixed and interfered by a second half mirror 27. This interfered light is transmitted to light receiving elements 28 and 29, respectively.
is converted into an electrical signal. At this time, the interfering light must have a phase difference of 90°. The method is shown below. Figure 4 shows half mirror 27
FIG. In the figure, 40 is glass and 41 is a metal semi-transparent surface.
Generally, when light is reflected from a metal surface, the phase is delayed, but when reflected or transmitted from a glass surface, the phase is not delayed. In the figure, the phase delay due to reflection of the -1st order diffracted light by the half mirror 27 is shown as δ _r1 , the phase delay due to the reflection of the +1st order diffracted light is shown as δ _r2 , and the phase delay of the half mirror in the glass medium is shown, respectively. so that δ _t1
~δ _t3 . The +1st order light is reflected and transmitted by the half mirror and goes towards the light receiving elements 28 and 29 as P ₊₁ ,
Let P _-1 and Q _-1 be the lights in which the Q ₊₁ and −1st order lights similarly go in the direction of the light receiving element. The phase delays of these four beams are as follows.

P ₊₁ ; δ _t1 + δ _r2 + δ _t2 , P _-1 ; δ _t3 Q ₊₁ ; δ _t1 , Q _-1 ; δ _r1 Therefore, the phase difference Δ ₁ between P ₊₁ and P _-1 , Q ₊₁ The phase difference Δ ₂ of Q _-1 is expressed by the following equations.

Δ ₁ = δ _t1 + δ _r2 + δ _t2 − δ _t3 Δ ₂ = δ _t1 − δ _r1Here , if the optical paths of P ₊₁ and P _-1 are matched, δ _t2 =
δ _t3 . Therefore, the following equation holds.

Δ ₁ = δ _t1 + δ _r2 Now, P ₊₁ and P _-1 and Q ₊₁ and Q _-1 interfere with each other and enter the light receiving elements 28 and 29. At this time, if the phase difference between the outputs of the light receiving elements 28 and 29 is α, then α=Δ ₁ −Δ ₂ =δ _t1 +δ _r2 −δ _t1 +δ _r1 =δ _r1 +δ _r2 . Therefore, the phase difference between the light receiving elements 28 and 29 is determined only by δ _r1 and δ _r2 and is unrelated to the thickness of the glass of the half mirror 27. δ _r1 on the metal surface,
The value of δ _r2 is determined by the incident angle φ and the angle of the polarization plane of the incident light. δ _r1 and δ _r2 become maximum when the plane of polarization is oriented as shown in FIG. 4, and in this case, the following equation holds true from Fresnel's formula and the law of refraction.

Rp=tan(φ−χ)/tan(φ+χ)Ap sinχ=sinφ/n(1+ik) However, Rp: Complex amplitude of reflected light Ap: Complex amplitude of incident light χ: Complex refraction angle n: Refractive index of metal k: Attenuation If the constant χ is eliminated from the above equation, the phase delay δ of the reflected light is expressed by the following equation. .

δ＝tan ^-1 Rp／Ap＝tan ^-1 [2nktanφsin(tan ^2φ +
1) /tan ² φ{n ² +(nk) ² }−sin ² φ(tan ² φ+1) ²
] However, in the case of a half mirror, it is thought that there is reflection from the glass surface in addition to the metal surface.
The phase of reflection on a glass surface is reversed by 180° at the Bryutas angle.

Therefore, in the case of a half mirror, when the relationship between the incident angle φ and the phase difference α between the light receiving elements 28 and 29 is actually measured, it is as shown in Fig. 5, which combines the characteristics of metal surface reflection and glass surface reflection. In the case of a mochi Inconel half mirror, φ = approximately 75° and α = 90°.
In the figure, the horizontal axis represents the incident angle φ, and the vertical axis represents the phase difference α between the light receiving elements 28 and 29, respectively.
Therefore, the direction of movement of the scale can be determined from this output, and the amount of movement can be determined by counting the number of waves of the sine wave. Since the output is a sine wave with an accurate 90° phase difference, further analog interpolation is performed to achieve a resolution of 1/100 ~
Ultra-high resolution of 1/1000 μm can be obtained, which can be displayed or used for position control. Various types of general configurations can be considered as the configuration for processing this 90° phase difference signal depending on the purpose. With this configuration, the diameter of the light beam projected onto the scale is approximately 4 to 5 mm, and if the pitch d of the scale is 0.8 μm, there are approximately 5000 gratings within this beam diameter, and all of these gratings This will create one interference fringe. Therefore, the influence of lattice defects of the scale, small pitch irregularities, and dust and dirt attached to the scale can be greatly reduced. By the way, the first half mirror 26 and the light-receiving element 34 in FIG. It is something that is created. In this way, even if the diffraction efficiency fluctuates at each location on the scale, or the intensity of the ±1st-order diffracted light changes due to dust or dirt, and the outputs of the light receiving elements 28 and 29 change, an accurate sine wave can be obtained. It can be accurately converted into pulses. However, if the scale is uniform and the change in position or angle during movement is small, it may not be necessary. The features of the device of the present invention described above are listed below.

(1) Ultra-high resolution of 1/100 to 1/1000 μm over 100 mm.

(2) Utilizing the phase delay caused by the reflection of the half mirror,
The configuration is simple because no polarizing plate or quarter-wave plate is used.

(3) Since the light beam projected onto the scale is thick and no spatial filter is used on the light-receiving element, there is little influence from dust or dirt on the scale or from defective pitch irregularities on the scale.

(4) Large tolerance between scale and head spacing.

(5) Since a reflective scale is used, scale installation is easy.

FIG. 6 is a diagram showing another embodiment of the present invention. In both figures, when light is obliquely incident from a coherent light source, a third half mirror 50 is provided to separate the incident light into two, and the diffracted light at two points on the scale is used. It is something. In this case, since the interference fringes change depending on the rotation of the scale, it is also possible to measure extremely small angles.

As explained in detail above, according to the present invention, a reflective type scale is used, the diameter of the light beam irradiated to the scale is increased to reduce the influence of dust, and the light beam reflected by the half mirror is By creating a 90° phase difference by using the phase delay of

[Brief explanation of drawings]

FIGS. 1 and 2 are diagrams showing the configuration of a conventional device, FIG. 3 is a configuration diagram showing an embodiment of the present invention, FIG. 4 is a diagram showing the phase relationship when interference occurs with a half mirror, and FIG. This figure is a diagram showing the relationship between the incident angle and the light receiving element, and FIG. 6 is a diagram showing another embodiment of the present invention. 1, 14... Transmission type scale, 2... Spectrometer,
L ₁ to _{L 3} , L ₁₁ , L ₁₂ , 22...Lens, M ₁ to _{M 3} ,
24, 25...Mirror, _P1 , _P2 ...Polarizer, _P3 ,
P ₄ ... Analyzer, 13 ... 1/4 wavelength plate, D ₁ ~ _{D 3} , 1
6, 28, 29, 34...light receiving element, 11, 21
...Coherent light source, 12...Polarizing cube prism, 15...Stopper, 23...Reflection scale, 26, 27, 50...Half mirror, 30,
31...Amplifier, 32...Signal processing circuit, 33...
...Display section. 40...Glass, 41...Metal anti-transparent surface.

Claims (1)

[Scope of Claims] 1. A coherent light source is irradiated onto a reflective scale, and two reflected diffraction lights of different orders from the reflective scale are incident on a half mirror having a metal thin film on the base from both sides at a specific angle of incidence. The phase difference between the interference lights emitted on both sides of the half mirror is set to 90° by mixing and interfering with each other, and each of the interference lights is converted into an electric signal by a light receiving element, and then arithmetic processing is performed to determine the direction of movement of the scale and An optical scale reader that can measure the amount of movement. 2. The optical scale reading device according to claim 1, wherein an Inconel thin film is used as the half mirror and the incident angle is 75°. 3. The optical scale reading device according to claim 1, further comprising a half mirror for separating the obliquely incident light from the coherent light source into two parts and irradiating the two parts to the reflective scale. .