JPS62204127A - Encoder - Google Patents

Abstract

PURPOSE:To use luminous flux efficiently for the formation of interference firnges and to improve detection sensitivity by constituting a diffraction grating so that diffracted light beams of specific degree to be superposed among plural diffracted light beams are almost largest in intensity. CONSTITUTION:Luminous flux emitted by a laser 1 is made incident on a polari zation beam splitter 3 through a collimator lens 2 and split into reflected lumi nous flux and transmitted luminous flux which are almost equal in quantity. The reflected luminous flux and transmitted luminous flux are incident on positions M1 and M2 of a radial grating 7 and diffracted light beams of specific degree of the diffracted transmitted light are reflected by reflecting means 8 and 9 to travel through the same optical path in the opposite directions and impinge almost the same positions M1 and M2. When, for example, diffracted light of 1<st> order is utilized, the diffraction rate eta1approx.=10(%) and the diffracted light intensity is maximum at the time of phiw/phip=0.5. Here phip is angular pitch and phiw is the angle of a light transmission part. Consequently, the light beams are superposed by the polarization beam splitter 3 and the interference efficiency of interference fringes detected by photodetecting means 14 and 15 is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field> The present invention relates to an encoder, and particularly to an encoder, in which a coherent light beam is incident on a diffraction grating attached to a moving or rotating object, and the diffracted lights from the diffraction grating are made to interfere with each other to form interference fringes. This invention relates to an encoder that determines the movement state of a diffraction grating, that is, the movement or rotation state of a moving object, by Δ1 by counting bright and dark interference fringes.

<Prior Art> In recent years, precision machines such as NC machine tools and semiconductor printing equipment have required precise measuring instruments capable of measuring in lpm or less (submicron) units.

Conventionally, a linear encoder that uses coherent light beams such as a laser to form interference fringes from diffracted light from a moving object is well-known as a measuring instrument capable of measuring in submicron units. It is being

On the other hand, it has conventionally been used to detect the rotational speed and the amount of variation in rotational speed of computer equipment such as the drive of a floppy desk, office equipment such as a printer, or NC machine tools, as well as rotational mechanisms such as the gearbox motor and rotary drum of a VTR. A photoelectric rotary encoder has been used as a means for this purpose.

For example, a photoelectric rotary encoder is constructed by providing light-transmitting parts and light-shielding parts at equal intervals around a disc = 4 connected to the rotary axis ±*.
Both scales, the main scale 4 and a so-called fixed index scale in which a light-transmitting part and a light-blocking part are provided at equal intervals to the main scale, are sandwiched between a light emitting means and a light receiving means. It adopts a so-called index scale system configuration in which the sensors are arranged facing each other. In this method, as the main scale rotates, a signal synchronized with the interval between the light-transmitting part and the light-blocking part of both scales is obtained, and this signal is frequency diffracted to detect fluctuations in the rotational speed of the rotating shaft. For this reason, the finer the scale interval between the light-transmitting part and the light-blocking part of both scales, the higher the detection accuracy.

However, when the scale interval is narrowed, the S/N ratio of the output signal from the light receiving means decreases due to the influence of the diffracted light, resulting in a decrease in detection accuracy. For this reason, if we try to fix the total number of gratings in the light-transmitting part and the light-blocking part of the main scale and expand the interval between the light-transmitting part and the light-blocking part to the extent that it is not affected by the diffracted light, the diameter of the disc of the main scale This increases the thickness and increases the overall size of the housing, which has the disadvantage of increasing the load on the rotating object to be tested.

One possible method for solving the drawbacks of this type of conventional rotary encoder is to apply the interference fringe detection method of the linear encoder described above to the rotary encoder.

However, not only linear encoders and rotary encoders but also interference fringe detection systems using this type of diffracted light have the drawbacks of poor detection sensitivity due to the diffraction grating. Moreover, other unnecessary diffracted light may become flare or cause the generation of ghost light, resulting in a problem of lowering the S/N ratio when detecting interference fringes.

<Summary of the Invention> In view of the above-mentioned conventional drawbacks, an object of the present invention is to provide an encoder that efficiently uses the luminous flux emitted from a light source to form interference fringes and has improved detection sensitivity.

In order to achieve the above object, an encoder according to the present invention includes a light source means for obtaining a coherent light beam, and a first optical means for directing the coherent light beam to a diffraction grating formed on a movable or rotatable object. , comprising a second optical means for superimposing a plurality of diffracted lights emitted from the diffraction grating, and a light receiving means for receiving the light beam obtained by the second optical means and detecting interference fringes, and detecting interference fringes from the signal from the light receiving means. A device for detecting the movement or rotation state of the object, wherein the diffraction grating is configured so that the intensity of the diffracted light of a specific order to be superimposed is approximately the maximum among the plurality of diffracted lights emitted from the diffraction grating. It is characterized by what it did.

Further features of the present invention will become clear from the following examples.

<Embodiment> FIG. 1 is a schematic diagram showing an embodiment of an encoder according to the present invention, and here a rotary encoder is shown.

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 made incident on the polarizing beam splitter 3.
It is split into two linearly polarized beams. The reflected light flux passes through a quarter-wave plate 4, becomes circularly polarized light, and is made incident on a position M1 of a radiation grating 7, which is provided with a radial diffraction grating, on a disk 6 connected to the rotating object to be measured. There is. Of the transmitted diffracted light that is incident on the radiation grating 7 and diffracted, the diffracted light of a specific order is reflected by the reflecting means 8, travels the same optical path backwards, and is made to re-enter the radiation grating 7 at substantially the same position M1. Then, the diffracted light of a specific order re-diffracted by the radiation grating 7 is converted into linearly polarized light with a polarization direction that is 90 degrees different from that when it is incident through the quarter-wave plate 4, and is converted into polarized beam splitter 3.
It is input to.

In this embodiment, from the polarizing beam splitter 3 to the reflecting means 8
The round trip optical path of the diffracted light of a specific order is the same.

FIG. 2 is an explanatory diagram of one embodiment of the reflecting means shown in FIG. 1.

In the figure, a reflecting mirror 40 is arranged approximately on the focal plane of a condensing lens 41, and only the diffracted light of a specific order that is incident parallel to the condensing lens 41 passes through an opening 43 of a mask 42, and the reflecting mirror 40 After reflecting the light, the light travels back along its original path. Then, the diffracted light of other orders is masked 4.
2 to block light. In addition to this, there are other reflective means.
If the functionality is the same as shown in the diagram.

For example, a configuration such as a cat's eye optical system may be used. If such an optical system is used, for example, even if the oscillation wavelength of the laser changes and the diffraction angle changes somewhat, the light can be returned along substantially the same optical path.

Alternatively, a gradient index lens such as Selfoc Micro Lens (trade name) manufactured by Nippon Sheet Glass Co., Ltd. may be applied to the cat's eye optical system, and a reflective film may be provided on one side of the lens, with both ends being flat. Therefore, it can be effectively applied to the present invention as an optical element having a simple configuration and high productivity.

Returning to Figure 1, the 2 beams split by the polarizing beam splitter 3
Of the two light beams, the transmitted light beam is made into circularly polarized light through a quarter-wave plate 5, and is incident on a position M2 on a radiation grating 7 on a disk 6, which is approximately point symmetrical with respect to a position M1 on a radiation grating 7 and a rotation axis 50. . Then, the diffracted light of a specific order among the transmitted diffracted light that is incident on the radiation grating 7 and diffracted is caused to travel backward along the same optical path by a reflecting means 9 similar to the aforementioned reflecting means 8, so that the diffracted light of a specific order is caused to travel in the same optical path at approximately the same position M of the radiation grating 7.
2 is re-injected. When the diffracted light of a specific order re-diffracted by the radiation grating 7 is incident through the quarter-wave plate 5, it is converted into linearly polarized light with a 90 degree polarization direction and is made incident on the polarizing beam splitter 3.

At this time, the transmitted light beam also reaches the reflection means 9 from the polarizing beam splitter 3 in the same way as the reflected light beam described above. The round trip optical path of the diffracted light of a specific order is made the same. Then, after overlapping with the diffracted light incident through the reflection means 8,
The circularly polarized light is passed through a /4 wavelength plate 10, split into two beams by a light splitter 11, and each beam is split into two beams by polarizing plates 12 and 13 arranged with their polarization directions tilted by 45 degrees.
Each light receiving means 1 receives linearly polarized light with a phase difference of 0 degrees.
4.15. and light receiving means 14.15
The intensity of the interference fringes of the two light beams formed by this is detected.

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 n-th order diffracted light re-diffracted by the radiation grating 7 changes by 2nπ. As a result, (2m-2n) sine waveforms are obtained from the light receiving means as a whole. In this embodiment, the amount of rotation is measured by detecting the sine waveforms at this time.

For example, if the pitch of the diffraction grating is 3.2 pm, the diffracted light is 1
If we use the next and -1st orders, the rotating object will have a pitch of 3
．． When rotated by 2gm, four sine waveforms are obtained from the light receiving element. In other words, the resolution per sine waveform is 1/4 of 1 pitch of the diffraction grating = 3.274 = 0.8
p-m is obtained.

FIG. 3 is a diagram showing a radiation grating used in an encoder according to an unimplemented example. In the figure, as in Figure 1, 6 represents a disk, and 7
indicates a radiation grating, the shaded area is a light absorption part, the blank area is a light transmission part, ψp is the angular pitch of the radiation grating 7, and ψw is the angle of the light transmission part.

is set to ψw/ψp = 0.5. That is, by setting the duty ratio of the black and white chart to 50% in the so-called amplitude type diffraction grating as shown in FIG. As a result, the interference efficiency of the interference fringes that are superimposed by the polarizing beam splitter 11 and detected by the light receiving means 14 and 15 is improved. Further, although the details will be described later, if a radiation grating as shown in FIG. 3 is used, since no ±2nd order diffracted light is generated, it is possible to predict the occurrence of ghost light and the like.

Now, the diffraction efficiency of m-th order diffracted light in an amplitude type diffraction grating as shown in Fig. 3 can be expressed by the following equation (1), where ψp is the angular pitch and ψw is the angle of the transparent vortex part. I can do it.

5in2 (πmψw/ψp) ηm= −−−−(1)
π2゜2 FIG. 4 shows the relationship between ψw/ψp and diffraction efficiency ηm using equation (1). In Fig. 4, the horizontal axis represents the duty ratio ψw/ψp.

The diffraction effect *77 m is plotted on the vertical axis.

As can be seen from Figure 4, when using the first-order diffracted light, ψw
When /ψF = 0.5, the diffraction efficiency η1 is mainly 10 (%), and the intensity of the diffracted light is maximum.

Furthermore, since the diffraction efficiency η2 of the second-order diffracted light is η2=0 at this time, no ±second-order diffracted light is generated from the diffraction grating configured with ψm/ψp=0.5. for example,
In the encoder as shown in FIG. 1, in order to emit the diffracted light of a specific order approximately perpendicularly from the radiation grating 7, a coherent beam of light is emitted from the radiation grating 7 at a diffraction angle α of the diffracted light of this specific order.
In some cases, it may be input to At this time, reflected ghost light is generally generated which is reflected and diffracted at an angle β as shown in FIG. The condition for reflection diffraction is expressed as P (sina + sin β) = m in, whereas the condition for transmission diffraction when incident at the first-order diffraction angle is P sin α = in, so the above-mentioned condition for reflection diffraction is can be transformed as follows.

Psinβ=(m-1) Therefore, from the above equation, when m=2, α=β, and in this case, the second-order reflected diffraction light is emitted in the direction of the incident optical path and travels backward, so it becomes a 511 ghost light. This interferes with the detection of interference fringes. However, as mentioned above, the duty ratio is 50%,
By setting the rotation angle ψw/ψp = 0.5, it is possible to suppress the generation of second-order diffracted light, so that this type of ghost light does not degrade the detection accuracy of interference fringes.

In addition, when using second-order or third-order diffracted light to increase the resolution of measurement, as can be seen from Figure 4, the time for the second-order diffracted light is ψ
w/ψp=0.25.For 3rd order diffracted light, ψw/ψF =
It is sufficient to construct a diffraction grating (radiation grating) as 0.5. In addition, in this type of amplitude type diffraction grating,
) It is preferable to have a duty such that the diffraction efficiencies ψm and ψn are equal, and the brightness ratio (visibility) of the interference fringes is high and the detection accuracy is also good.

Although the explanation has been made regarding the amplitude type diffraction grating, a phase type diffraction grating can also be used in the encoder according to the present invention. In particular, a single-phase diffraction grating is effective because it has a higher diffraction efficiency than an amplitude diffraction grating and can greatly increase the luminous flux utilization efficiency.

FIGS. 6(A) and 6(B) are schematic cross-sectional views showing a phase-type diffraction grating, in which a relief pattern of various concavities and convexities forms a diffraction grating that produces a phase difference in a light beam. In addition, it is possible to obtain various phase-type diffraction gratings of transmission or reflection type by forming gratings with alternating refractive indexes in a transparent member or by applying a reflective film on top leaf patterns. I can do it.

In FIG. 6, (A) shows a rectangular phase grating, and (B) shows a triangular wave-like phase grating. In addition, there are various grating shapes, such as a sinusoidal phase grating and an asymmetrical phase grating called a so-called blazed diffraction grating. Therefore, the behavior of diffracted light in this type of phase-type diffraction grating is determined by many parameters such as the grating shape, the refractive index of the material forming the grating, the grating height, and the grating pitch. I will refrain from deriving it.

However, a feature of the phase type diffraction grating is that by reducing the pitch of the grating, the order of the emitted diffracted light can be controlled; for example, it is possible to emit only the 0th order and +1st order.

Furthermore, the separation angle between adjacent orders can also be increased. In addition, by controlling the height T of the grating and the refractive index 〇 of the material forming the grating, all the energy of the incident light can be absorbed without emitting a new 0-order beam, with respect to the wavelength 0 of the idle interference beam used. For example, only ±1st order diffracted light, +1st order and +
It is possible to concentrate only the second-order diffracted light, and the diffraction efficiency is naturally high.

For example, in the rectangular diffraction grating shown in FIG. 6(A),
If the surrounding refraction=V is nQ.

(m=o, L, 2.3 ---) If the diffraction grating is formed so as to satisfy It is preferable to detect interference fringes using +1st-order diffracted light in terms of measurement accuracy and device configuration.

Furthermore, the aforementioned blazed diffraction grating greatly increases the intensity of diffracted light of a specific order. For example, the energy of the incident light is concentrated in the +1st order, +3rd order, etc. diffracted light, and the intensity of other diffracted lights becomes weak, which is extremely effective. Therefore, by using diffracted light of the +3rd order, etc., not only the resolution of the measurement is improved by several steps, but also sufficient intensity of the diffracted light can be obtained, resulting in good measurement accuracy.

As described above, even if a phase-type diffraction grating is used, it is possible to maximize the intensity of the diffracted light of a specific order to be superimposed, and it is also possible to improve the luminous flux utilization efficiency, the interference efficiency, and the Al11 constant accuracy. be.

Returning to FIG. 1, in this embodiment, the light beam is divided into two by the light splitter 11, and a phase difference of 90 degrees is created between the two light beams, so that the direction of rotation of the rotating object can also be determined.

, If only the amount of rotation is to be measured, the light splitter 1
1. A polarizing plate, 12.13, and one light receiving means are unnecessary.

In this embodiment, there are two positions M that are approximately symmetrical about the center of rotation.
1. By using the diffracted light from M2, measurement errors due to eccentricity between the center of rotation of the rotating object and the center of the radiation grating are reduced.

Note that although the configuration in this example uses diffracted light from two points that are approximately point symmetrical, substantially the same effect can be achieved by using diffracted light from multiple positions, not limited to approximately point symmetrical. can be obtained. For example, 3 with a 120° angle to the tail.
It is also effective to use diffracted light from a point or to use diffracted light from arbitrary two points that are not close to each other.

Furthermore, the luminous flux elements of one luminous flux near the center of the rotation axis and the luminous flux elements of the other luminous flux incident at a substantially point-symmetrical position near the center of the rotation axis are overlapped with each other, and similarly the luminous flux elements near the outside of the rotation center are overlapped with each other. The shadow of wavefront aberration caused by the difference in pitch between the outside and inside of the radiation grating!
is being removed.

In this embodiment, from the polarizing beam splitter 3 to the reflecting means 8
．． By making the reciprocating optical path of the diffracted light of the specific order up to 9 the same, the degree of overlapping of the two diffracted light beams in the polarizing beam splitter 3 is facilitated, and the assembly precision of the entire device is improved.

In each of the above embodiments, the quarter-wave plate 4.5 may be placed anywhere between the polarizing beam splitter 3 and the reflecting means 8.9.

Further, in each embodiment, reflected diffraction light may be used instead of transmitted diffraction light.

As described above, the diffraction grating used in this embodiment is a so-called amplitude type diffraction grating consisting of a light-transmitting part and a light-blocking part, and a phase-type diffraction grating consisting of parts having mutually different refractive indexes. In particular, phase-type diffraction gratings can be created, for example, by forming an uneven relief pattern on the circumference of a transparent disk, and can be mass-produced by processes such as embossing and stamping.

So, here we will focus on the rotary encoder.
Although the present invention has been described in detail, it can of course also be applied to linear encoders. That is, the present invention is applicable to all encoders that detect interference fringes by superimposing diffracted light of a specific order among diffracted lights emitted from a diffraction grating based on the movement or rotation of the diffraction grating.

It goes without saying that there are various modifications and applications based on the idea of the present invention.

<Effects of the Invention> Hereinafter, the encoder according to the invention has excellent luminous flux utilization efficiency and high precision l! by increasing the interference efficiency of interference fringes.
This is a device that can be used for Furthermore, by controlling the configuration of the diffraction grating, harmful light such as ghost light is prevented from entering the light receiving means, and stable measurements can be performed.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of an encoder according to the present invention. FIG. 2 is a diagram showing an example of the reflecting means in FIG. 1. Figure 3 is a diagram showing an example of the radial grating shown in Figure 1. FIG. 4 is a diagram showing the relationship between duty ψw/ψp and diffraction efficiency η of an amplitude type diffraction grating. FIG. 5 is an explanatory diagram regarding reflected ghost light. FIGS. 6(A) and 6(B) are schematic diagrams showing examples of diffraction gratings. i ----------11---Light source 2------
−−−−−−−Collimator lens 3-11−−−−−
-----Polarizing beam splitter 4, 5, l
0------1/4 wavelength plate 6------------
One circular plate 7 --- One radiation grating 8 , 9 ---
----------One reflection means 11-----
1 light splitter 12, l 3----- 1 polarizing plate 14, 15----- 1 light receiving means 40----- 1 reflecting mirror 41-------- ----- One converging lens 42 -----
-------------Mask 43-------11-------Opening 50-----One rotation axis ψp--"'--=-Angle pitch ψw−−−−−−−−−−−Light transmission angle M 1 ,
M 2-------Position of light east incidence on one radiation grating 1δth M 9th

Claims (5)

[Claims] (1) A light source means for obtaining a coherent light beam, a first optical means for directing the coherent light beam toward a diffraction grating formed on a movable or rotatable object, and a plurality of diffracted lights emitted from the diffraction grating are superimposed. An apparatus comprising a second optical means and a light receiving means for receiving a light beam obtained by the second optical means and detecting interference fringes, and detecting a movement or rotation state of the object based on a signal from the light receiving means. and an encoder in which the diffraction grating is configured such that the intensity of the diffraction light of a specific order to be superimposed among the plurality of diffraction lights emitted from the diffraction grating becomes approximately maximum. (2) The encoder according to claim (1), wherein the diffraction grating is an amplitude type diffraction grating. (3) The encoder according to claim (1), wherein the diffraction grating is a phase type diffraction grating. (4) The ratio ψw/ψp of the angular pitch ψp of the amplitude type diffraction grating to the angle ψw of the light transmitting part or the light reflecting part is (ψw)/(ψp)≒0.5. The encoder described in range item (2). (5) Claim No. 1 (1) characterized in that the specific order diffraction light to be superimposed is ±1st order diffraction light.
) Encoder described in section.