JPH07119623B2 - Displacement measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a displacement measuring device, and more particularly, to rotating a radiation grating in which a plurality of grating patterns of a light transmitting portion and a reflecting portion are periodically engraved on a circumference. It is attached to an object, and the radiation grating is irradiated with a light flux from a laser, for example, and the diffracted light from the radiation grating is used to photoelectrically determine the rotation state of the radiation grating or the rotating object such as the rotation speed and the fluctuation amount of the rotation speed. It can be favorably applied to the displacement measuring device for detecting the above.

(Prior art) Conventionally, computer equipment such as drive of floppy desks, office equipment such as printers, NC machine tools, and rotation speed of rotation mechanism such as capstain motor of VTR and rotating drum, and fluctuation amount of rotation speed have been used. A photoelectric rotary encoder has been used as a means for detecting.

The photoelectric rotary encoder is, for example, as shown in FIG. 3, a so-called main scale 31 in which a light-transmitting portion and a light-shielding portion are provided at equal intervals around a disk 35 connected to a rotating shaft 30 and a main scale corresponding thereto. By adopting a so-called index scale system configuration in which both scales of a so-called fixed index scale 32 provided with a light-transmitting portion and a light-shielding portion at the same interval as the scale are sandwiched between the light projecting means 33 and the light receiving means 34 and are opposed to each other. There is.
With this method, a signal synchronized with the distance between the light-transmitting portion and the light-shielding portion of both scales is obtained with the rotation of the main scale, and this signal is subjected to frequency analysis to detect fluctuations in the rotation speed of the rotating shaft. . Therefore, the finer the scale interval between the light-transmitting portion and the light-shielding portion of both scales, the higher the detection accuracy can be. However, if the scale interval is made fine, there is a drawback that the S / N ratio of the output signal from the light receiving means is lowered due to the influence of the diffracted light, and the detection accuracy is lowered. For this reason, if the total number of gratings of the light-transmitting part and the light-shielding part of the main scale is fixed and the distance between the light-transmitting part and the light-shielding part is expanded to the extent that it is not affected by the diffracted light, the diameter of the disk of the main scale will change. There is a defect that the load is increased and the thickness is increased, the size of the entire apparatus is increased, and as a result, the load on the rotating object to be inspected is increased.

A laser is often used as a light source in the rotary encoder. When the speed of light from the laser is incident on the rotating object to be inspected, interference fringes are formed from the light flux from the rotating object to be inspected, and the rotation state of the rotating object to be inspected is measured using this interference fringe. It is necessary to strictly set the stability of the oscillation wavelength of the laser and the optical path length between the two light beams to be interfered with each other. A semiconductor laser is a small-sized laser light source and is advantageous for downsizing the entire apparatus, but the oscillation wavelength fluctuates depending on, for example, temperature. For this reason, if the semiconductor laser is used to reduce the size of the entire apparatus, the optical path lengths of the two light fluxes that interfere with each other change, which causes a decrease in measurement accuracy.

(Problems to be Solved by the Invention) In view of the above-mentioned drawbacks of the conventional example, the present invention enables high-accuracy measurement, the size of the object to be inspected is small, and the size of the entire apparatus is easily reduced. It is an object of the present invention to provide a displacement measuring device capable of easily adjusting the optical path length between light fluxes even when the oscillation wavelength of the light source is slightly changed when displacement measurement is performed by using interference fringes of a plurality of light fluxes using a light source such as.

(Means for Solving the Problems) A light source that generates coherent light, and a plurality of split light beams that are emitted from the light source and are formed at predetermined split positions are provided in a condenser lens and in the vicinity of the focal plane of the condenser lens. An optical system arranged to superimpose the plurality of divided luminous fluxes at substantially the dividing positions by causing reflection using a reflecting means having the arranged reflection plane to reverse the optical path, and superimposing by the optical system. And a light receiving means for detecting the separated luminous flux, and each of the divided luminous fluxes has a specific order by passing twice through substantially the same position on the diffraction grating to be the target of relative displacement measurement in the optical path in the optical system. In a displacement measuring device which is given a diffracting action and which uses the output signal from the light receiving means to measure relative displacement information with respect to the diffraction grating, it is necessary to match the optical path lengths of the plurality of divided light beams. Further, the adjusting means is provided for adjusting the optical path length of the optical paths until they are superposed by displacing the reflecting means in a direction that does not change the reciprocating optical path for at least one light beam of the plurality of divided light beams.

Besides, the features of the present invention are described in the embodiments.

(Example) FIG. 1A is a schematic view of an optical system of an example of the present invention. FIG. 1 (B) is a partial side view of FIG. 1 (A).

In this embodiment, the light beam emitted from the laser 1 is made into a parallel light beam by the collimator lens 2 and is incident on the polarization beam splitter 3 as a light splitting means, and is converted into two linearly polarized light beams, that is, a reflected light beam and a transmitted light beam having substantially equal light amounts. It is divided. The reflected light beam passes through the quarter-wave plate 4 to be circularly polarized light, passes through prisms 16 and 18, and is then provided with a radial diffraction grating on the circular plate 6 connected to the rotating object to be measured. It is incident on the position M _{1 of the} grating 7. At this time, the luminous flux is made incident so that the diffracted light of the specific order from the radiation grating 7 is emitted from the radiation grating 7 substantially vertically. Then, the diffracted light of a specific order out of the transmitted diffracted light that is incident on the radiation grating 7 and diffracted is reflected by the reflecting means 8 and the same optical path is reversed so that it is re-incident on the substantially same position M ₁ on the radiation grating 7. Then, the diffracted light of the specific order re-diffracted by the radiation grating 7 is made to be linearly polarized light having a 90 ° polarization direction different from that when the diffracted light is made incident through the 1/4 wavelength plate 4, and is made incident on the polarization beam splitter 3.

In this embodiment, the polarization beam splitter 3 to the reflection means 8 are used.
The same round-trip optical path of the diffracted light of a specific order leading to.
FIG. 2 is an illustration of an embodiment of the reflecting means shown in FIG.

In the figure, the reflecting mirror 40 is arranged on the substantially focal plane of the condensing lens 41, and only the diffracted light of a specific order incident in parallel to the condensing lens 41 is passed through the opening 43 of the mask 42 and the reflecting mirror 40 After being reflected at, the original optical path is reversed.
The diffracted light of other orders is blocked by the mask 42. As the reflecting means, any other structure such as a cat's eye optical system may be used as long as it has the same function as shown in FIG. If such an optical system is used, for example, the oscillation wavelength of the laser is changed, and even if the diffraction angle is changed a little, it is possible to return the light beam in substantially the same optical path.

Also, by applying a gradient index lens, for example, SELFOC micro lens (trade name) manufactured by Nippon Sheet Glass Co., Ltd. to the cat's eye optical system, and providing a reflective film on one side by paying attention to the flat points at both ends. The optical element having a simple structure and high productivity can be effectively applied to the present invention.

Returning to FIG. 1, 2 split by the polarization beam splitter 3
The transmitted light beam out of the two light beams is circularly polarized through the quarter-wave plate 5 and is approximately a point with respect to the position M ₁ on the radiation grating 7 on the rear disk 6 via the prisms 17 and 19 and the rotation axis 50. It is incident on the symmetrical position M ₂ . Then, the diffracted light of a specific order among the transmitted diffracted light that is incident on the radiation grating 7 and diffracted is made to go back to the same position M ₂ of the radiation grating 7 by reversing the same optical path by the reflecting means 9 similar to the above-mentioned reflecting means 8. It is incident. Then, the diffracted light of the specific order re-diffracted by the radiation grating 7 is made into linearly polarized light having a 90 ° polarization direction different from that when it is incident through the quarter-wave plate 5, and is made incident on the polarization beam splitter 3.

At this time, similarly to the above-described reflected light flux, the transmitted light flux has the same round-trip optical path of the diffracted light of a specific order from the polarization beam splitter 3 to the reflection means 9. Then, after overlapping with the diffracted light that has entered through the reflecting means 8, the 1/4 wavelength plate 1
Circularly polarized light through 0, split into two light beams by the light splitter 11, and each light beam is passed through the polarizing plates 12 and 13 which are arranged with their polarization directions inclined by 45 degrees. A linearly polarized light with a mark is incident on each of the light receiving means 14 and 15. Then, the intensity of the interference fringes of the two light fluxes formed by the light receiving means 14 and 15 is detected.

In this embodiment, the polarization beam splitter 3 is used to radiate two light beams onto the radiation grating 7 and the radiation grating 7 on a line connecting the incident points M ₁ and M ₂ to each other.
Are arranged on a substantially perpendicular bisector or in a direction orthogonal to the bisector to make the optical path lengths between the two light beams equal.

In this embodiment, when the rotating object to be measured rotates by one pitch of the radiation grating 7, the phase of the m-th order diffracted light changes by 2mπ. Similarly, the phase of the diffracted light of the nth order re-diffracted by the radiation grating 7 changes by 2nπ. As a result, (2m-2n) sinusoidal waveforms are obtained from the light receiving means as a whole. In this embodiment, the rotation amount is measured by detecting the sine waveform at this time.

For example, if the diffraction grating has a pitch of 3.2 μm and the first and −1st orders are used as diffracted light, the rotating object has a pitch of 3.2 μm.
When rotated by .mu.m, four sine waveforms are obtained from the light receiving element. That is, the resolution per sine waveform is 3.2 / 4 = 0.8 μm, which is 1/4 of one pitch of the diffraction grating.

However, for example, the oscillation wavelength λ of the laser that is the light source
Is changed by Δλ due to the influence of temperature and the like, the phase shift Δφ of the two light fluxes is given by the difference in optical path length between the two light fluxes. Becomes

If the periodic signal output from the light receiving means is numerically divided by, for example, electrical division, the phase shift Δφ between the two light beams due to fluctuations in the oscillation wavelength of the laser is If the inequality becomes

For example, λ = 780mn, Δλ = 0.3nm (depending on mode hop),
If x = 4 In this embodiment, the adjusting means is provided so as to satisfy the above condition, and the optical path lengths between the two light beams are adjusted to be substantially equal.

As the adjusting means and the adjusting method, for example, (a) one of the reflecting means 8 and 9 is moved in the optical axis direction as indicated by an arrow Y1 shown in FIG.

(B) Each element surrounded by the dotted line 100 in FIG. 1 is integrally tilted as indicated by an arrow Y2 in FIG.

(C) A wedge-shaped prism 18 is put in and taken out in the direction of arrow Y3 in the vicinity of the radiation grating 7 and the thickness thereof is changed.

(D) Two wedge-shaped prisms 19 and 20 are superposed in the vicinity of the radiation grating 7 so as to form a plane parallel plate, and at least one prism is slid in the direction of arrow Y4.

(E) One optical path length is changed by using two reflecting mirrors or a prism having two reflecting surfaces.

In the present invention, particularly by adopting the configuration of (a), it becomes possible to adjust the optical path length without changing the reciprocating optical path, and it becomes possible to provide a simple adjusting means that does not unnecessarily displace other optical elements. .

In the present embodiment, the light splitter 11 divides the light flux into two and gives a 90-degree phase difference between the light fluxes so that the rotation direction of the rotating object can also be determined.

If only the amount of rotation is measured, the light splitter 11, the polarizing plates 12 and 13, and one light receiving means are unnecessary.

In this embodiment, there are two positions that are substantially point-symmetric with respect to the center of rotation.
By utilizing the diffracted light from M ₁ and M ₂ , the measurement error due to the eccentricity between the center of rotation of the rotating object and the center of the radiation grating is reduced.

Note that the configuration of this embodiment utilizes diffracted light from two points that are substantially point-symmetrical, but it is not limited to being substantially point-symmetrical. You can get the effect. For example, 3 forming an angle of 120 ° with each other
It is also effective to use diffracted light from a point or diffracted light from any two points that are not close to each other.

Further, the light beam element near the center of the rotation axis of one light beam and the light beam element near the center of the rotation axis of the other light beam incident at a position substantially point-symmetrical are overlapped with each other. The effect of the wavefront aberration caused by the difference in pitch between the outer side and the inner side of the radiation grating is eliminated by superimposing them on each other.

In this embodiment, the polarization beam splitter 3 to the reflecting means 8,
By making the round-trip optical paths of the diffracted light of the specific order up to 9 the same, the degree of overlap of the two diffracted light beams in the polarization beam splitter 3 is facilitated, and the assembly accuracy of the entire device is improved.

In the above embodiments, the quarter-wave plates 4 and 5 may be arranged anywhere between the polarizing beam splitter 3 or the polarizing prism 22 and the reflecting means.

Further, in the embodiment, reflected diffracted light may be used instead of transmitted diffracted light.

The diffraction grating used in the present invention is a so-called amplitude type diffraction grating including a light transmitting portion and a light shielding portion, and a phase type diffraction grating including portions having different refractive indexes. In particular, a phase type diffraction grating can be produced, for example, by forming an uneven relief pattern on the circumference of a transparent disc, and can be mass-produced by processes such as embossing and stamper.

(Effect of the invention) According to the present invention, the adjusting means is used to change the optical path length of at least one of the plurality of divided light beams so that the optical path length between the plurality of light beams can be adjusted. It is possible to prevent the measurement accuracy from deteriorating when a light source with a variable fluctuation is used, and it is possible to perform highly accurate measurement. Further, according to the present invention, it is possible to provide a displacement measuring apparatus capable of downsizing the entire apparatus.

Further, since the adjusting means is configured to adjust the optical path length of the optical paths until they are superposed by displacing the reflecting means in a direction that does not change the reciprocating optical path for at least one light beam of the plurality of divided light beams, other optical elements can be used. A simple adjusting means that does not unnecessarily displace becomes possible.

[Brief description of drawings]

FIG. 1 is a schematic view of an optical system according to an embodiment of the present invention, FIG. 2 is a partial explanatory view of FIG. 1, and FIG. 3 is an explanatory view of a conventional photoelectric rotary encoder. In the figure, 1 is a laser,
2 is a collimator lens, 3 is a polarization beam splitter, 4, 5 and 10 are quarter-wave plates, 6 is a disc, 7 is a radiation grating, and 8 and 9
Are reflecting means, 12 and 13 are polarizing plates, and 14 and 15 are light receiving means.

Front page continued (72) Inventor Osamu Kasahara 770 Shimonoge, Takatsu-ku, Kawasaki-shi, Kanagawa Canon Inc. Tamagawa Plant (56) References

Claims (1)

[Claims] 1. A light source for generating coherent light, a condenser lens for each of a plurality of divided light fluxes emitted from the light source and formed at predetermined division positions, and a reflection plane for arranging in the vicinity of the focal plane of the condenser lens. An optical system arranged to superimpose the plurality of divided luminous fluxes at approximately the dividing position by reflecting the light by using a reflecting means having, and detecting the luminous fluxes superposed by the optical system. Each of the divided luminous fluxes is given a diffracting action of a specific order by passing twice through substantially the same position on the diffraction grating to be the target of relative displacement measurement in the optical path in the optical system. In a displacement measuring device for measuring relative displacement information with respect to the diffraction grating by using an output signal from the light receiving means, the plurality of divided light beams so as to match optical path lengths between the plurality of divided light beams. Displacement measuring apparatus being characterized in that an adjusting means for adjusting the optical path length of the optical path to be superimposed by displacing the reflecting means in a direction that does not change a reciprocating optical path with respect to at least Ikko bundle of inner.