JPH0462002B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a rotary encoder, and more particularly, a radiation grating in which a plurality of grid patterns of transparent parts and reflective parts are periodically carved on the circumference is attached to a rotating object, and the radiation grating is attached to the radiation grating. For example, the present invention relates to a rotary encoder that irradiates a beam of light from a laser and uses diffracted light from the radiation grating to photoelectrically detect the rotation angle of a radiation grating or a rotating object.

Conventionally, computer equipment such as driving floppy desks, office equipment such as printers, or
Rotary encoders have been used as a means to detect the rotation angle of rotating mechanisms such as NC machine tools and VTR capsten motors and rotating drums.

The method using a photoelectric rotary encoder is to create a so-called main scale in which transparent parts and light-shielding parts are provided at equal intervals around a disc connected to the rotation axis, and corresponding transparent parts are provided at equal intervals to the main scale. A so-called fixed index scale having a light-shielding section and a light-shielding section are arranged in a so-called index scale system in which both scales are placed facing each other with a light emitting means and a light receiving means sandwiched between the two scales. In this method, as the main scale rotates, a signal is obtained that is synchronized with the interval between the light-transmitting part and the light-blocking part of both scales, and the rotation angle is detected by integrating this signal after waveform shaping.

In a rotary encoder, the finer the scale interval between the light-transmitting part and the light-blocking part of both scales, the higher the detection accuracy can be. However, when the scale interval is made finer, 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, it is conceivable to fix the total number of gratings in the light-transmitting part and the light-blocking part of the main scale, and widening the interval between the light-transmitting part and the light-blocking part to the extent that it is not affected by the diffracted light.
However, this has the disadvantage that the diameter and thickness of the main scale disc increases, making the entire device larger, and as a result, the load on the rotating object to be tested increases.

SUMMARY OF THE INVENTION An object of the present invention is to provide a small rotary encoder that reduces the load on a rotating object to be tested, reduces the influence of eccentricity of attachment to the rotating object, and is capable of detecting a rotation angle with high precision.

The main features of the rotary encoder for achieving the object of the present invention are a radiation grating in which a plurality of grid patterns are arranged at equal angles around the circumference of a disk, a rotating object connected to the radiation grating, and a luminous flux incident on the radiation grating. of the diffracted light from the luminous flux incident on the radiation grating, and
A second diffracted light beam is re-injected into a position approximately symmetrical to the rotation center of the rotating object with respect to the incident position on the radiation grating of the light beam by the first illumination means, via each quarter-wave plate. After superimposing the two diffracted lights of a specific order diffracted again by the illumination means and the radiation grating, the superimposed light beam is split by a light splitting means and the light splitting means for splitting the superimposed light beam into two light beams. and two light-receiving means for receiving the two light beams, respectively, through polarizing plates, and the rotation angle of the rotating object is determined using output signals from the two light-receiving means. .

Next, one embodiment of the present invention will be described with reference to each drawing.

FIG. 1 is a schematic diagram of an embodiment of the present invention. In the figure, 1 is a light source such as a laser, 2 is a collimator lens, 3 ₁ to 3 ₃ , 3 ₁ ′ to 3 ₃ ′ are cylindrical lenses, 4 ₁ , 4 ₂ , 4 ₁ ′, 4 ₂ ′ are reflecting mirrors,
5 is a radiation grating with a grid pattern of transparent parts and reflective parts arranged at equal angles on a disk, 6 is the rotation axis of the rotating object to be tested, and 7 and 7' are 1/4 wavelength plates, which are used to measure the radiation from the laser 1. They are arranged so that their axes are at 45 degrees and -45 degrees with respect to the linearly polarized light beam. 8 is a beam splitter, and 9 and 9' are polarizing plates, which are arranged so that the polarization direction of the polarizing plates 9 and 9' is 45 degrees with respect to each other. 1
0 and 10' are light receiving elements.

The light beam emitted from the laser 1 is turned into a substantially parallel light beam by the collimator lens 2, and is moved to a position _M1 on the radiation grating ₅ by the cylindrical lens 31.
is irradiated in a linear manner. By irradiating in a linear manner in this manner, it is possible to reduce the pitch error in the grid pattern between the transparent portion and the reflective portion corresponding to the irradiated portion of the light beam on the radiation grating 5.

Incidentally, instead of the cylindrical lens, a slit or a lens and a slit may be used for linear irradiation.

The light beam from the laser 1 is reflected and diffracted by the grating pattern of the radiation grating 5. The current irradiation position of the luminous flux
If the pitch of the checkered pattern in M ₁ is p, ±m
The diffraction angle θ _n of the next reflected diffraction lights L ₁ and L ₂ is expressed as sin θ _n =±mλ/p (1). Here, λ is the wavelength of the luminous flux.

Assume now that the radiation grating 5 is rotating at an angular velocity ω. From the rotation center of the radiation grating, the irradiation position M ₁
If the distance to the irradiation point M1 is r, the circumferential velocity at the irradiation point _M1 is v=rω. At this time, the frequency of the ±m-order reflected diffraction light undergoes a so-called Doppler shift by an amount expressed by the following equation.

Δ=±vsinθ _n /λ=±ωsinθ _n /λ...(2) Then, the reflected diffracted light of order ±m is reflected by the reflecting mirrors 4 ₁ and 4 ₂ via the cylindrical lenses 3 ₂ and 3 ₃ ,
At a position M ₂ approximately symmetrical about the center of rotation, a reflecting mirror 4 ₁ ′,
4 ₂ ′, quarter wavelength plates 7 and 7 ′, and cylindrical lenses 3 ₂ ′ and 3 ₃ ′, the light is irradiated linearly again. Here, the quarter-wave plates 7 and 7' are arranged so that their respective axes are at 45 degrees and -45 degrees with respect to the linear polarization direction of the incident laser. In addition, the angle of incidence to the irradiation position _M2 is as follows for each diffracted light:
equal to the reflection diffraction angle θ _n at the irradiation position M ₁ ,
Moreover, the reflecting mirrors 4 ₁ ′ and 4 ₂ ′ are arranged so that the angles with respect to the circumferential velocity direction of the radiation grating 5 are equal.
Then, at the irradiation position M ₂ , the ±m-order reflected and diffracted lights overlap, pass through the cylindrical lens 3 ₁ ', become a parallel beam of light again, are split into two beams by the beam splitter 8, and pass through the polarizing plates 9 and 9'. The light passes through and enters the light receiving elements 10, 10'. Irradiation position
The overlapping ±m-order diffracted light reflected by M ₂ is
As the radiation grating 5 rotates, the Doppler frequency shift Δ of equation (2) is again applied, so in addition to the frequency shift when reflected at the irradiation position M ₁ , eventually,
The frequency shift amount of the ±m-order diffracted light reflected at the irradiation position M ₂ is ±2Δ. In this way, since the light that has undergone ±m-order diffraction twice overlaps, the frequency of the output signal from the light receiving elements 10 and 10' is 2Δ-(-
2Δ) = 4Δ. In other words, the light receiving elements 10,1
The frequency of the output signal at 0' is =4Δ=
4rωsinθ _n /λ, and from the diffraction condition equation (1),
=4mrω/p. If the total number of grid patterns of the radiation grid 5 is N, and the equiangular pitch is Δ, then p=
From rΔ, Δ=2π/N, =2mNω/π...(3). Now, if the wave number of the output signal of the light receiving element during time Δt is n, and the rotation angle of the radiation grating 5 during Δt is θ, then from n=Δt and θ=ωΔt, n=2mNθ/π... (4) By counting the wave number of the output signal waveform of the light receiving element, the rotation angle θ of the radiation grating 5 can be calculated as
It can be obtained using equation (4). By the way, it is more preferable if the direction of rotation can be detected when detecting the rotation angle. Therefore, in this embodiment, as is well known in conventional photoelectric rotary encoders, a plurality of light receiving elements are prepared and arranged so that the phases of their signals are shifted by 90 degrees, and the 90 degrees caused by rotation are A method is used to extract a signal indicating the rotation direction from the phase difference signal.

In this embodiment, a 90° phase shift between the output signals of the light receiving elements 10 and 10' is created by combining linearly polarized laser light, a quarter wavelength plate, and a polarizing plate. That is, although a laser is generally a linearly polarized light, the two quarter-wave plates 7 and 7' are arranged so that their axes are in the direction of ±45° with respect to this polarization direction. Then, the light beams transmitted through the quarter-wave plates 7 and 7' become circularly polarized light in opposite directions,
When the light is reflected and diffracted at the irradiation position _M2 and overlaps, it becomes linearly polarized light again, but the polarization direction is different from the radiation grating 5.
It changes with the rotation of. Then, the light receiving element 1
By shifting the polarization directions of the polarizing plates 9 and 9' provided on the front surfaces of the light receiving elements 1 and 10' by 45 degrees, the light receiving element 1
A phase difference of 90° can be provided between the 0 and 10' output signals. As shown in FIG. 1, for example, the output signals of the light-receiving elements 10, 10' are waveform-shaped, the rotation direction is detected, and then the rotation angle is determined by integrating the signals with a counter.

By the way, in the conventionally used index scale type photoelectric rotary encoder, the wave number n of the output signal from the light receiving element, the total number N of the main scale, and the rotation angle θ correspond to equation (4). The relationship is n=Nθ/2π...(5), so the rotation angle Δθ per wave number is Δθ=2π/N (radians)...(6). On the other hand, in this embodiment, from equation (4), Δθ=π/2mN (radian) (7).

Therefore, according to this embodiment, even if a scale with the same number of divisions is used, the rotation angle can be detected with an accuracy 4 m times higher than in the conventional example.

Further, in a conventional photoelectric rotary encoder, the distance between the light-transmitting part and the light-blocking part is limited to about 10 μm, considering the influence of light diffraction.

Now, in order to obtain a rotation angle detection accuracy of, for example, 30 seconds, in the conventional example, the number of divisions of the main scale is calculated from equation (6), N = 360 × 60 × 60 / 30 =
Only 43200 is needed. Therefore, if the interval between the light-transmitting part and the light-blocking part at the outermost circumference of the main scale is 10 μm, the diameter of the main scale is 0.01 mm x 43200/
π=137.5mm is required. However, according to this embodiment, in order to obtain the same rotation angle detection accuracy as in the conventional example, the number of divisions of the radiation grating may be 1/4 m. ±
In the case of m=1 using first-order diffracted light, the number of grating divisions of the radiation grating 5 to obtain rotation angle detection accuracy of 30 seconds may be 43200/4=10800. In this example, if laser diffraction light is used, the distance between the transparent part and the reflective part can be narrow, so for example, if this is 4 μm, the diameter of the radiation grating is 0.004 mm.
×10800/π=13.75mm is sufficient. In other words, according to this embodiment, the shape can be 1/10 or less in size to obtain rotational angle detection accuracy equivalent to that of a conventional index scale type photoelectric rotary encoder. Therefore, the load on the rotating object to be tested is much smaller than in the conventional example, and accurate measurements can be performed.

FIG. 2 is an explanatory diagram of the irradiation positions M ₁ and M ₂ of the light beam on the radiation grating 5 of a part of FIG. 1 and the eccentricity between the center of the radiation grating 5 and the rotation center of the rotating object to be detected.

Now, in this embodiment, two points M ₁ and M ₂ on the radiation grating 5, which are symmetrical with respect to the center of rotation, are the irradiation points.
In other words, the measurement point is used to reduce the influence of eccentricity between the center of the radiation grating 5 and the rotation center of the rotating body to be tested.
That is, it is difficult to make the center of the radiation grating 5 and the center of rotation completely coincide with each other, and eccentricity between the two is unavoidable. For example, as shown in Figure 2,
Between the center O of the radiation grating 5 and the rotation center O',
When the amount of eccentricity is a, the distance r from the center of rotation
The Doppler frequency shift at the measurement point M ₁ located at the position is r/(r+
a) to r/(ra-a). on the other hand,
At this time, the frequency shift at position M ₁ and measurement point M ₂ , which is symmetrical with respect to the center of rotation, is
Contrary to the change in M ₁ , from r/(ra-a) to r/
(r+a), so by using two measurement points at the same time as positions _M1 and _M2 , the influence of eccentricity can be reduced, and as a result, the rotational speed can be detected with high accuracy. can.

FIG. 3 is a schematic diagram of a portion of another embodiment of the present invention, showing the vicinity where the light beam is incident on the radiation grating 5 of FIG. In the figure, the numbers assigned to each element indicate the same elements as shown in FIG. The ±m-order transmitted diffracted light of the light beam incident on the position M ₁ of the radiation grating 5 is transmitted through the cylindrical lenses 3 ₂ , 3 ₃ , 3 ₂ ',
3 ₃ ', and through the reflecting mirrors 4 ₁ , 4 ₂ , 4 ₁ ', and 4 ₂ ', the light is again incident on a position M ₂ that is approximately symmetrical to the center of the rotation axis 6, and is similar to the embodiment shown in FIG. It's getting an effect.

In each of the embodiments described above, two diffracted lights of order ±m were used, but two diffracted lights of different orders may be used instead of diffracted lights of order ±m. Alternatively, the lattice pattern on the radiation grating may be composed of only transmitting portions or only reflecting portions, and only transmitted diffracted light or reflected diffracted light may be used.

In the present invention, only the rotation angle may be measured without detecting the rotation direction of the rotation angle. In this case, the beam splitter 8, polarizing plates 9, 9', quarter wavelength plates 7, 7', light receiving element 10', etc. shown in FIG. 1 are not necessary.

Further, the light source in the present invention is not limited to a laser, but any light source that emits a single wavelength can be used.

As described above, according to the present invention, it is possible to achieve a small, high-precision rotary encoder that has a small load on the rotating object to be tested and reduces the eccentricity error between the center of the radiation grating and the rotation center of the rotating object.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is an explanatory diagram showing the eccentricity between the center of the radiation grating and the center of rotation, and Fig. 3 is a partial block diagram showing another embodiment of the present invention. It is. 1 is a laser, 2 is a collimator lens, 3
₁ to 3 ₃ , 3 ₁ ′ to _{3 3} ′ are cylindrical lenses, 4 ₁ ,
4 ₂ , 4 ₁ ', 4 ₂ ' are reflecting mirrors, 5 is a radiation grating, 6 is the rotation axis of the rotating object to be tested, 7, 7' are 1/4 wavelength plates, 8 is a beam splitter, 9, 9' is a polarizing plate, 10,
10' is a light receiving element.

Claims (1)

[Scope of Claims] 1. A radiation grating in which a plurality of grid patterns are arranged at equal angles on the periphery of a disk, a rotating object connected to the radiation grating, and a first illumination means for making a luminous flux incident on the radiation grating. and two diffracted lights of specific orders among the diffracted lights from the light flux that have entered the radiation grating, and the first illumination means generate a point approximately equal to the center of rotation of the rotating object with respect to the incident position of the light flux on the radiation grating. After superimposing the two diffracted lights of a specific order diffracted again by the second illumination means and the radiation grating for re-injecting them into symmetrical positions via the quarter-wave plate, the superimposed A light splitting means for splitting a luminous flux into two luminous fluxes, and two light receiving means for respectively receiving the two luminous fluxes split by the light splitting means via a polarizing plate, the two light receiving means A rotary encoder characterized in that the rotation angle of the rotating object is determined using an output signal from the rotary encoder. 2. The rotary encoder according to claim 1, wherein the first and second illumination means irradiate the luminous flux linearly in a direction perpendicular to the radiation direction of the grating of the radiation grating.