JPS62200224A - Rotary encoder - Google Patents

Abstract

PURPOSE:To facilitate the detection of the number of interference fringes and enable the rotational condition of a rotating object to be inspected to be accurately measured by obtaining high contrast interference fringes by superposing two diffracted light beams of +n-th and -n-th orders on each other in a plurality of the diffracted light beams from a grating. CONSTITUTION:The light beam irradiated from a laser 1 is changed to a parallel light beam by a collimator lens 2 to be projected upon a beam splitter 3 and then upon a position M1 on one 4 of the radial diffraction gratings on a disk connected to a rotating object to be measured. Two diffracted light beams of +n-th and -n-th orders in the transmitted and diffracted light beams incident upon and diffracted by the radial grating 4 are reflected by optical means 5 and 5', respectively, and are returned in the same light path as in advance to be projected again upon approximately the same position as the one M1 on the radial grating 4. The diffracted light beams of the +n-th and -n-th orders which are diffracted again by the radial grating 4 are projected upon the beam splitter 3 and the two diffracted light beams wherein an n-th diffraction is conducted two times are superposed on each other to be led to light receiving means 6 and the intensity of interference fringes is detected.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a rotary encoder, in particular a rotary encoder having a plurality of diffraction gratings in a lattice pattern, for example, a transparent part and a reflective part, on the circumference.
A periodic radiation grating is attached to a rotating object, and the radiation grating is irradiated with a beam of light from a laser, for example, and the diffracted light from the radiation grating is used to determine the rotational speed or speed of the radiation grating or the rotating object. This invention relates to a rotary encoder that photoelectrically detects the rotational state such as the amount of variation in .

(Prior art) Conventionally, it has been used to measure the rotational speed and the amount of variation in rotational speed of computer equipment such as floppy desk drives, office equipment such as printers, NC machine tools, and rotating mechanisms such as VTR capsten motors and rotating drums. A photoelectric rotary encoder has been used as a means for detection.

For example, as shown in FIG. 6, a photoelectric rotary encoder has a so-called main scale 31 in which light-transmitting parts and light-shielding parts are provided at equal intervals around a disk 35 connected to a rotating shaft 30, and a corresponding main scale 31. A so-called fixed index scale 32 is provided with a light-transmitting part and a light-shielding part at equal intervals to the scale, and a so-called index scale system configuration is adopted in which both scales are placed facing each other with the light emitting means 33 and the light receiving means 34 sandwiching them. ing.

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-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 particle size can be. However, when the scale interval is made smaller, 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 the detected particle size. 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. As the thickness increases, the entire device becomes larger.
As a result, there are drawbacks such as an increase in the load on the rotating object to be tested.

(Problems to be Solved by the Invention) The present invention is characterized by providing a rotary encoder that has a small load on a rotating object to be inspected, can easily downsize the entire device, and can detect the rotational state with high precision. In particular, we aim to provide a rotary encoder that enables high-precision measurement by superimposing two diffracted lights, +n-order and -n-order out of multiple diffracted lights from a radiation grating, to improve the contrast of interference fringes. purpose.

(Means for solving the problem) A coherent light beam is made incident on a radiation grating on a disk connected to a rotating object, and the +nth and -nth order diffracted lights from the surface radiation grating are superimposed. Then, the light was guided to a light receiving means, and the rotational state of the rotating object was determined using the output signal from the light receiving means.

Other features of the invention are described in the Examples.

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

In this embodiment, the light beam emitted from the laser 1 is converted into a parallel light beam by the collimator lens 2, and the beam splitter
3 and is made incident on a position M1 of a radiation grating 4, which is provided with a radial diffraction grating on a disk connected to a rotating object to be measured. Then, of the transmitted diffracted light that is incident on the radiation grating 4 and diffracted, two diffracted lights of the +n order and -n order are reflected by the optical means 5° 5', and are caused to travel backward along the same optical path to approximately the same position on the radiation grating 4. It is re-injected into M1. The +n-order and -n-order diffracted lights re-diffracted by the radiation grating 4 are made incident on the beam splitter 3. Then, the two diffracted lights of the +nth order and the -nth order reflected by the beam splitter 3, that is, the two diffracted light beams that have undergone nth order diffraction twice, are superimposed and guided to the light receiving means 6. The intensity of interference fringes formed on the surface of the light receiving means 6 is detected by the light receiving means 6.

FIG. 2 is an explanatory diagram of one embodiment of the optical means 5, 5' shown in FIG.

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. As a reflection means, this Ryuteki 2
Any configuration, such as a cat's eye optical system or a plane mirror, may be used as long as it has the same function as shown in the figure. 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.

In addition, by applying a refractive index gradient lens, such as 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, focusing on the fact that both ends of the lens are flat. , it can be effectively applied to the present invention as an optical element having a simple structure and high productivity.

In this embodiment, when the rotating object to be measured rotates by one pitch of the radiation grating 4, the phase of the n-th order diffracted light changes by 2nπ. In this embodiment, the radiation grating 4 re-diffracts the light, so the phase changes by a total of 4ni. As a result, 4n 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 waveform at this □ time.

For example, if the pitch of the diffraction grating is 3.2 μm, the diffracted light is 1
If we use the next and -1st orders, the rotating object will be
．． When rotated by 2 μm, four sine waveforms are obtained from the light receiving element. That is, the resolution per sine waveform is 0.8 μm, which is 3.28% of 1 pitch of the diffraction grating.

In this embodiment, the contrast (visibility) of the interference fringes is improved by superimposing the diffracted lights of the same order among the plurality of diffracted lights from the radiation grating 4, for example, two diffracted lights of +9th order and -0th order, The aim is to improve the detection accuracy when detecting the number of interference fringes by the light receiving means.

That is, when the intensities of the two light beams to be interfered with are each I++, the contrast V of the interference fringes is V=2E dingding completed/(I
I+I2). Therefore 1. When V=I2, V=1 and the contrast ■ becomes the best.

In this embodiment, the detection accuracy of interference fringes is improved by using two diffracted light beams that satisfy I and -I2.

FIG. 3 is an explanatory diagram regarding the grating pitch when the radiation grating 4 shown in FIG. 1 is constituted by, for example, an amplitude division type diffraction grating. In the figure, d represents the light beam passage area, and P represents the grating pitch.

Figure 4 shows the intensity distribution of diffracted light when the ratio d/P between the length d of the passage area of the diffraction grating shown in Figure 3 and the grating pitch P is varied, with the horizontal axis representing the order and the vertical axis representing the intensity. It is an explanatory diagram when taken. In the figure, for example, when '/P=0.5 and first-order and -1st-order diffracted lights are used, the intensity of the 0th-order light is at point 41,
The figure shows how the intensity of the +1st-order light becomes the intensity at a point on the curve represented by a point 42, '/P=0.5.

In this example, as shown in FIG. 4, d/ is appropriately determined,
By using the two diffracted lights of +9th order and -nth order obtained at this time, for example, 1st order and -1st order, or 2nd order and 12th order, interference fringes with good contrast are obtained.

FIG. 5 is a schematic diagram of an optical system according to another embodiment of the present invention.

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 polarizing beam splitter 51, and the reflected light beam and the transmitted light beam are divided into substantially equal amounts of reflected light beam and transmitted light beam.
It is split into two linearly polarized beams. The reflected light beam passes through a wavelength plate 52, becomes circularly polarized light, passes through a prism 53 having two reflecting surfaces, and enters an optical member 54 made of a prism. The light beam is then made incident on a position Ml of the radiation grating 4, which is provided with a radial diffraction grating on a disc connected to the rotating object to be measured via the optical member 5.

At this time, the light flux emitted perpendicularly to the radiation grating 4 from the prism 53 is transmitted to the optical member 54 as shown in FIG. 5(B).
By specifying the shape of the radiation grating 4, the +9th order diffracted light by the radiation grating 4 is made to enter the radiation grating 4 so as to be emitted substantially perpendicularly to the radiation grating 4. Of the transmitted diffracted light incident on the radiation grating 4 and diffracted, +9th order diffracted light is transmitted to the optical means 5.
The light beam is reflected by the light beam, travels the same optical path backwards, and enters the radiation grating 4 at substantially the same position M1. Then, the +9th-order diffracted light re-diffracted by the radiation grating 4 is converted into linearly polarized light with a polarization direction 90" different from that when it is incident through the wavelength plate 52, and is made incident on the polarizing beam splitter 51. In this embodiment, The round trip optical path of the +9th order diffracted light from the polarizing beam splitter 51 to the optical means 5 is made the same.

On the other hand, the transmitted light beam out of the two light beams split by the polarizing beam splitter 51 becomes circularly polarized light through the % wavelength plate 55, passes through a prism 56 having two reflective surfaces, and enters an optical member 57 made of a prism. It is incident. The light is then made incident through the optical member 57 at a position M1 on the radiation grating 4 on the disc and a position M2 which is approximately symmetrical with respect to the rotation axis 50. At this time, by specifying the shape of the optical member 57 for the light beam emitted perpendicularly to the radiation grating 4 from the prism 17 in the same way as in the case of the reflected light beam described above, the = nth order diffracted light by the radiation grating 4 is reflected by the radiation grating 4. The radiation is made incident on the radiation grating 4 so as to be emitted perpendicularly to the radiation grating 4. Of the transmitted diffracted light incident on the radiation grating 4 and diffracted, the −n-order diffracted light is caused to travel backward along the same optical path by an optical means 5' similar to the optical means 5 described above, so that the radiation grating 4 is at approximately the same position M2. It is re-injected into the Then, the −0th order diffracted light re-diffracted by the radiation grating 4 is made into a linear scratched light whose polarization direction is different by 90 degrees from that when it is incident through the daylight wavelength plate 55, and is made incident on the polarizing beam splitter 51.

At this time, the transmitted light beam also has the same round-trip optical path of the -0th order diffracted light from the polarizing beam splitter 51 to the optical means 5', similar to the above-mentioned reflected light beam.

After overlapping with the +n-order diffracted light incident through the optical means 5, it is made into circularly polarized light through the % wavelength plate 58, and split into two light beams by the light splitter 59. Both light beams are made incident on each light receiving means 62, 63 as linearly polarized light with a phase difference of 90 degrees via polarizing plates 60, 61 arranged at an angle of 45 degrees. Then, the intensity of the interference fringes of the two beams formed by the light receiving means 62 and 63 is detected.

In this embodiment, the light beam is divided into two by a light splitter 59, and a phase difference of 90° is created between each beam, so that the direction of rotation of the rotating object can also be determined.

Note that if only the amount of rotation is to be measured, the light splitter 59, the polarizing plate 60, 81, and one of the light receiving means are unnecessary.

In this embodiment, when the optical members 54 and 57 are used to make the light beam incident on the radiation grating 4, the diffracted light of a specific order from the radiation grating 4 is emitted approximately perpendicularly to the radiation grating 4. Efforts are being made to simplify and improve assembly accuracy.

Furthermore, in this embodiment, measurement errors due to eccentricity between the rotation center of the rotating object and the center of the radiation grating are reduced by using the diffracted lights from two positions Ml and M2 that are approximately symmetrical with respect to the rotation center. .

Although the configuration in this example uses diffracted light from two points that are approximately point symmetrical, it is possible to obtain approximately the same diffraction light by using diffracted light from multiple positions, not limited to approximately point symmetrical. You can get the effect. For example, it is also effective to use diffracted light from three points that are at an angle of 120 degrees to each other, or to use diffracted light from arbitrary two points that are not close to each other.

Note that by overlapping the luminous flux elements of one luminous flux closer to the rotation axis center and the luminous flux elements of the other luminous flux incident at a substantially point-symmetrical position closer to the rotation axis center, the luminous flux elements located outside the rotation center are similarly It is preferable to overlap these elements because it is possible to eliminate the influence of wavefront aberration caused by the difference in pitch between the outer and inner sides of the radiation grating.

In this embodiment, by making the round trip optical paths of the +n-th and -n-th order diffracted lights from the polarizing beam splitter 51 to the optical means 5, 5' the same, the two diffracted light beams in the polarizing beam splitter 51 overlap. This simplifies the process and improves the assembly accuracy of the entire device.

Although the present invention has been applied to a rotary encoder above, the technical idea of the present invention can also be successfully applied to a linear encoder.

The diffraction grating used in the present invention is a so-called amplitude type diffraction grating consisting of a light transmitting part and a light shielding 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.

(Effects of the Invention) According to the present invention, among the plurality of diffracted lights from the radiation grating, +n
By superimposing the two diffracted lights of order and −0 order,
Achieved a rotary encoder that can obtain interference fringes with good contrast, make it easy to detect the number of interference fringes, and measure the rotational state of a rotating object with high precision, while reducing the overall size of the device. can do.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention, FIG.
3 is an explanatory diagram of a part of FIG. 1, FIG. 4 is an explanatory diagram of the intensity distribution of diffracted light of each order when the diffraction grating shown in FIG. 3 is used, and FIG. 5 (8). (B) is a schematic diagram of an optical system according to another embodiment of the present invention, and FIG. 6 is an explanatory diagram of a conventional photoelectric rotary encoder. In the figure, 1 is the laser, 2 is the collimator lens, and 3
is a beam splitter - 151 is a polarizing beam splitter
152, 55, 58 are each % wave plates, 4 is a radiation grating, 5
, 5' are optical means, 60. fit is a polarizing plate,
6. [i2.63 are each light receiving means. Figure 1 Figure 2 Figure 5 (A) Figure 5 (8) Figure 6
wing

Claims (1)

[Claims] A coherent light beam is made incident on a radiation grating on a disk connected to a rotating object, the +n-th order and -n-th order diffracted light from the radiation grating are superimposed, and the light is guided to a light receiving means, and the light is received by the light receiving means. A rotary encoder characterized in that the rotational state of the rotating object is determined using an output signal from the means.