JPS6128923B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical scale reading device that utilizes optical interference.

Various types of optical scale reading devices that utilize light interference have been known. FIG. 1 is a diagram showing a conventional example of such a scale reading device. In the figure, 1 is a light source, and 2 is a condenser lens that receives the light output from the light source. 3 is a glass scale. 4 is a graduation grating engraved on the scale. Reference numeral 5 denotes a scanning plate provided with a grating scale having the same pitch as that of the scale 3. 6 is a photoelectric conversion element that receives the light that has passed through the scanning plate 5. The operation of the device configured as described above is summarized as follows.

If the scale 3 moves while being irradiated with light by the light source 1 and the condensing lens L, periodic brightness and darkness will occur in the amount of light entering the photoelectric conversion element 6. Due to this brightness and darkness, the photoelectric conversion element 6 generates a periodic electric signal matching the period. This periodic signal is amplified by a subsequent amplifier, passes through a dividing circuit, a Schmitt trigger circuit, and a direction discrimination circuit, and finally reaches a signal of 1 to 10 μm.
This becomes a rectangular wave pulse corresponding to the movement of the scale of m, enters the counter circuit, and displays the scale value on the display board. Here, the scale 3 is usually attached to a moving object. The device shown in FIG.
Although the purpose can be achieved with a relatively simple configuration, the tolerance of the distance between the scale 3 and the scanning plate 5
The drawback is that it must satisfy extremely strict conditions of within 0.03±0.003 mm. Therefore,
A method is used to hold down the scale 3 with a ball bearing, but for this reason the maximum scanning speed is, for example, 25
The speed is kept as low as cm/second.

FIG. 2 is a diagram showing another conventional example and shows the configuration of a holographic laser interferometer. The operation of this device is outlined below.

Laser light 1 emitted from a semiconductor laser, a He--Ne laser, or the like passes through a mirror M ₁ and lenses L ₁ and L ₂ and is irradiated onto the scale 10 . scale 10
The +-order diffracted light enters the mirror _M2 , and the 0-order transmitted light enters the mirror _M3 . The +1st order diffracted light from the mirror _M2 directly enters the lens _L3 , and the 0th order transmitted light from the mirror _M3 is diffracted on the scale 10 and enters the lens _L3 as -1st order diffracted light. At this time, the +1st-order and -1st-order diffracted lights are phase-rotated by the polarizers P ₁ ' and P ₂ , respectively, and become polarized lights with a phase shift of 90° from each other. The light focused by the lens _L3 is divided into three directions by the spectroscope 11 and enters the photoelectric conversion elements _D1 to _D3 . Here, the output of _D3 is used for automatic gain control to keep the laser light constant. Furthermore, the light entering D ₁ ′ and D ₂ is each converted into circularly polarized light by a quarter-wave plate P ₃ . The light that has generated interference fringes as a result is converted into electrical signals by photoelectric conversion elements D ₁ and D ₂ . The outputs of these photoelectric conversion elements D ₁ to _{D 3} are converted into scale values through the same operation as described in connection with FIG. 1. As explained above, the optical scale reading device shown in FIG. 2 does not have a problem with positioning accuracy like the device shown in FIG. 1, but the structure is extremely complicated.

The present invention has been made in view of these points, and provides an optical scale reading device with a simple configuration in which positioning accuracy is not a particular problem. 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, 30 is a coherent light source such as a laser. For example, a semiconductor laser, a He--Ne laser, or the like is used. 31 is a half mirror that receives the light output of the light source. _L10 is a lens that receives the light passing through the half mirror. 33 is a scale having a graduation grating in which reflective surfaces and transmitting surfaces are arranged at equal intervals. When the light passing through the lens _L10 is reflected by the scale, it generates multimode diffracted light. 32 is 0 among these multi-mode diffracted lights.
This is a stopper that blocks the next mode of light.

34 is a screen that receives the reflected light from the half mirror 31. S is an interference fringe generated on the screen.
S ₁ and S ₂ are interference fringes that occur adjacent to each other among these interference fringes. d ₁ to _{d 4} are light receiving elements arranged with a phase shift of 90° between these interference fringes. For example, a photodiode is used as the light receiving element. A to D are puff amplifiers that receive electrical signal outputs from the respective light receiving elements _d1 to _d4 . A controller 35 receives the outputs of the buffer amplifiers for each phase and outputs a pulse output signal corresponding to the moving distance of the scale 33 and a signal indicating the moving direction of the scale 33. In addition to the signals described above, the controller also outputs control signals corresponding to the signal outputs of the light receiving elements _d1 to _d4 . 3
Reference numeral 6 denotes a drive circuit that receives this control signal output and drives the coherent light source 30 so that the value of the control signal is constant. The operation of the device configured in this way will be explained below.

A part of the light emitted from the light source 30 is reflected by the succeeding half mirror 31, and the rest passes through the half mirror. This passed light is collected by the subsequent lens _L10 . The focused light continues on the scale 3
A part of the incident light is reflected. At this time, the scale 33 acts as a reflective diffraction grating, and when light is reflected, multimode diffraction from the 0th order to the ±nth order (n is an integer) occurs. Of these multi-mode diffracted lights, the zero-order mode light, that is, mere reflected light, is stopped by the stopper 32. The diffracted reflected light is again focused on the lens _L10 . At this time, if the aperture ratio of the lens is selected appropriately, ±
It is possible to prevent light of secondary mode light or higher from passing through.

Therefore, the light passing through the lens _L10 is only the ±1st mode light. The light indicated by the broken line in FIG. 3 indicates the +1st mode light, and the light indicated by the dashed dotted line indicates the -1st mode light. This ±1st order light is transmitted to the following half mirror 31
A part of the light is reflected by the screen 34, and the reflected light interferes with each other to produce interference fringes S on the screen 34. Light receiving elements d ₁ to _{d 4} arranged in interference fringes S ₁ and S ₂ adjacent to each other
generates electrical signals depending on the brightness of the light.

Suppose now that the scale 33 is moved in a certain direction while being irradiated with coherent light from the light source 30. At this time, the light input to the light-receiving elements d ₁ to _{d 4} periodically exhibits brightness and darkness at a pitch twice the grating pitch of the scale 33 . As described above, these light receiving elements are installed at positions with a phase difference of 90° from each other, so the outputs of these light receiving elements are sine waves with a phase shift of 90° from each other.
These outputs are connected to the respective subsequent buffer amplifiers A~
Enter in D. Buffer amplifiers A to D amplify the input signal to an appropriate signal level and perform impedance conversion. Let the respective outputs of these buffer amplifiers be P _A , P _B , P _C , and P _D .

The controller 35 receives these P _A to P _D outputs and outputs a pulse corresponding to (P _A -P _C ) to one output terminal OUT1. The number of pulses output from the output terminal OUT1 corresponds to the moving distance of the scale 33. Also, from the other output terminal OUT2 of the controller 35, (P _A - P _C ) and (P _B - P
_D ) A signal indicating the moving direction of the scale 33 is output using the phase difference. As an output format of the signal, it is conceivable that, for example, when the scale moves to the right, it corresponds to 0, and when it moves to the left, it corresponds to 1. Or it may be the other way around. By using both outputs from the output terminals OUT1 and OUT2, this device can be used as an optical scale reading device.

By the way, the pulse output signal output from the output terminal OUT1 corresponds to ( _PA - P _C ) as mentioned above, but since the P _C output is 180° out of phase with the P _A output, (P _A - P _C )=2P _A , resulting in twice the amplitude. Therefore, reliability is increased when pulsing (P _A - P _C ). On the other hand, in addition to the above-mentioned output signal, the controller 35 outputs (P _A +P _C )
There is output. In the ( _PA + P _C ) output, the signal component is 0, contrary to the above-mentioned (P _A -P _C ) output, and only the offset voltage is output. The drive circuit 36 drives the light source 30 so that the value of ( _PA + P _C ) remains constant. Therefore, the light receiving elements d ₁ to _{d 4} can always obtain a constant amount of light.

FIG. 4 is a diagram showing the relationship between the aforementioned screen and the light receiving element. In the same figure a, 34 is the screen mentioned above. As shown in the figure, the screen has interference fringes S.
Strip-shaped slits are provided with the same spacing. These slits are divided into four groups with a phase shift of 90 degrees. The aforementioned light receiving elements d ₁ to d ₄ are arranged behind the screen. FIG. 1B is a diagram showing the positional relationship between the slit and the interference fringes S. The light including the interference fringes that has passed through the slit is incident on the light receiving element. The light-receiving element generates electrical signals whose phases are shifted by 90 degrees for each of the four groups of slits.

FIG. 5 is a diagram showing an example in which light including interference fringes is incident on continuously arranged light receiving elements. The strip-shaped light-receiving elements shown in the figure are arranged at intervals of 1/4 of the interference fringes, as shown in figure b, and the phases of the respective light-receiving elements are shifted by 90 degrees. Therefore, as shown in FIG. 3B, if the outputs of the light receiving elements having the same phase are added together by the adders _A1 to _A4 , outputs whose phases are shifted by 90 degrees from each other can be obtained. Furthermore, since outputs of the same phase are added together, sensitivity can be increased.

In the explanation of Fig. 4, the case where there are four light-receiving elements has been described, but the number of these light-receiving elements does not need to be limited to four, and if there are at least two, the pulse output corresponding to the moving distance of the scale can be output. Obtainable.

In the apparatus shown in Fig. 3, the focal length of the lens _L10 is f, the scale pitch is d, the distance between the lens and photodiode is Z, the distance between the lens and the scale is h, and the distance between the lens and the imaging point of the light source is If H, the interference fringe pitch P is expressed by the following equation.

P=d{2(z-f)h-(z-f)H+
zf}/{f(h-H)}...(1) In other words, the interference fringe pitch P is the scale pitch d
{2(z-f)h-(z-f)H+zf}/{f
(h-H)} times larger. In equation (1), if the conditional expression H=fz/(f-z)...(2) is configured to be satisfied, the interference fringe pitch P in equation (1) is P=2d(z-f)/f...( 3), which is unrelated to the value of h.

The advantages of the device described above are listed below.

(1) The position of the interference fringes S does not change even if the working distance (distance between the lens L ₁₀ and the scale 33) h changes. Therefore, a large tolerance for h can be achieved.

(2) Since the tolerance of h can be large, it is easy to attach the scale to a moving object.

(3) Since the tolerance of h is large, there is no need to hold down the scale with a ball bearing or the like as in the conventional example shown in FIG. Therefore, high-speed operation is possible.

(4) Since the interference fringes have redundancy, even if there is some dirt, dust, or scratches on the scale, it will not cause measurement errors.

(5) One lens L ₁₀ can perform both irradiation and interference, and by using a half mirror, the number of parts can be reduced and the configuration can be simplified. Therefore, a small and simple device can be realized.

(6) Sensitivity is good because resolution is twice the scale pitch.

(7) The pitch of the graduation grating on the scale (e.g. 20μ
Since the interval between the interference fringes is enlarged (for example, 0.1 to 0.2 mm) compared to m), it is easy to manufacture the slits for the screen. Further, since the magnification is large, a large pitch P can be obtained even if the lens-photodiode distance Z is relatively small, and miniaturization is easy.

(8) Since the stopper (light-shielding plate) blocks the light by focusing the light on a spot, the light-shielding plate can be small and light-shielding is easy.

(9) Since the configuration is simple, adjustment is easy.

The features of the device shown in FIG. 3 have been described above. Although this device has the above-mentioned features, it has the drawback that positioning of the zero-order mode removal stopper 32 is extremely difficult. In order to eliminate such a problem, instead of attaching the stopper 32, it is conceivable to scratch the lens _L10 itself to prevent the passage of the zero-order mode light. However, it is difficult to select the location of damage.

FIG. 6 is a diagram showing an embodiment in which a diaphragm is used in place of a stopper such as the stopper 32 described above to facilitate positioning. Components in FIG. 6 that are the same as those in FIG. 3 are designated by the same numbers. In FIG. 6, 40 is the aperture mentioned above. The aperture is configured to block −1st mode light. Since this diaphragm has a flat plate configuration, it is easier to position the diaphragm than the stopper 32 described above.
In the case of the apparatus shown in FIG. 4, interference fringes are created by 0th-order mode light and +1st-order mode light. In the figure, a solid line indicates 0th-order mode light, and a broken line indicates +1st-order mode light. In this device, interference fringes are created by interference between 0th mode light and +1st mode light, so when the scale is moved, the pitch of brightness and darkness of the light receiving element is equal to the grating pitch of the scale, so the resolution is as shown in Figure 3. lower than the device shown in . The other operations and features are the same as those of the embodiment shown in FIG. 3, so their explanation will be omitted. In order to remove the -1st mode light, instead of using the aperture 40 shown in FIG. 6, the lens _L10 may be partially cut to remove the -1st mode light. Furthermore, instead of removing the −1st mode light, the +1st mode light may be removed.

FIG. 7 is a configuration diagram showing a modification of the present invention. Components that are the same as those shown in FIG. 3 are designated by the same numbers. The apparatus shown in FIG.
Light is incident obliquely from zero, and the reflected and diffracted light from the scale is received by a mirror 50 instead of a half mirror. The diffracted light incident on the mirror 50 produces interference fringes S. The interference fringes are received by the light receiving element d and converted into an electrical signal. The following operations are the same as those of the apparatus described above, so the explanation will be omitted. This device uses a mirror instead of a half mirror, so the amount of light is large. Therefore, it has the advantage that the sensitivity of the light receiving element d can be increased. On the other hand,
Since the lens is not symmetrical about the optical axis, it may be affected by aberrations.

As described above in detail, according to the present invention, it is possible to realize an optical scale reading device with a simple configuration without the problem of positioning accuracy of scales and the like.

[Brief explanation of the drawing]

FIGS. 1 and 2 are diagrams showing conventional examples of optical scale reading devices. FIG. 3 is a block diagram showing one embodiment of the present invention, and FIG. 6 is a block diagram showing another embodiment. FIGS. 4 and 5 are diagrams showing the relationship between interference fringes and light receiving elements. FIG. 7 is a diagram showing a modification of the present invention. DESCRIPTION OF SYMBOLS 1...Coherence light source, L...Condensing lens, 3...Scale, 4...Graduation grating, 5...Scanning plate, 6...Photoelectric conversion element, _M1 - _M3 ...Mirror, _L1 - _L3 ...Lens, 10 ...Scale, 11...Spectroscope, _D1 - _D3 ...Photoelectric conversion element, 30...Coherent light source, 31...Half mirror,
32...stopper, 33...scale, 34...screen,
35...Controller, 36...Drive circuit, _L10 ...Lens,
S, S ₁ , S ₂ ... interference fringe, A to D ... buffer amplifier,
d ₁ - _{d 4} ... light receiving element, OUT1, OUT2 ... output terminal, A ₁ - _{A 4} ... adder, 40 ... aperture, 50 ... mirror.

Claims (1)

[Scope of Claims] 1. A coherent light source, a half mirror that receives the optical output of the light source, a lens that receives the transmitted light of the half mirror, a reflecting surface that receives the convergent spherical wave that is the transmitted light of the lens, and A scale having a graduation grating with transmitting surfaces lined up at equal intervals, a stopper that blocks O-order mode diffracted light of the reflected diffracted light from the scale, and interference fringes generated by the ±1st-order mode diffracted light reflected by the half mirror. at least two or more light-receiving elements arranged with a phase shift of 90 degrees from each other, and a signal indicating a pulse corresponding to the moving distance of the scale and a signal indicating the moving direction thereof upon receiving the output of each of the light-receiving elements. An optical scale reading device consisting of a controller that outputs . 2 A coherent light source, a half mirror that receives the light output of the light source, a lens that receives the transmitted light of the half mirror, and a reflective surface and a transmitting surface that receive the convergent spherical wave that is the transmitted light of the lens are arranged at equal intervals. A scale having a row of graduation gratings, an aperture that blocks one of the ±1st mode diffracted light of the reflected diffracted light of the scale, and an Oth mode diffracted light and ±1st mode diffracted light reflected by the half mirror. at least two light-receiving elements arranged with a phase shift of 90 degrees with respect to interference fringes generated by the first-order mode diffracted light of the light that is not blocked by the aperture, and the light-receiving elements; An optical scale reading device comprising: a controller that receives outputs from each element and outputs a pulse corresponding to the moving distance of the scale and a signal indicating the moving direction thereof.