JPH0462003B2 - - 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 such an 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.

A further object of the present invention is to provide a highly accurate rotary encoder that reduces measurement errors due to errors in the grid pattern of the radiation grating and fluctuations in the output level from the light source.

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 light flux incident on the radiation grating. of the diffracted light from the luminous flux incident on the first illumination means and the radiation grating, and
A second diffracted light beam is re-injected into a position approximately symmetrical to the center of rotation 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 four light beams. and four light receiving means for receiving the four light fluxes through four polarizing plates whose polarization directions are shifted by 45 degrees, and the rotation is performed using output signals from the four light receiving means. This is to find the rotation angle of an object.

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 ₁ to 8 ₃ are beam splitters, and 9, 10, 11, and 12 are polarizing plates, and the polarizing plates 9, 10, 11, and 12 are arranged so that their polarization directions are shifted from each other by 45 degrees. 13, 14,
15 and 16 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 linearly irradiated onto a position _M1 on the radiation grating 5 by the cylindrical lens ₃₁ .

By linear irradiation in this way, the radiation grating 5
It is possible to reduce the pitch error of the grid pattern between the transparent part and the reflective part corresponding to the part irradiated with the light beam above.

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=±sinθ _n /λ=±rωsinθ _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 diffraction light overlaps, passes through the cylindrical lens 3 ₁ ′, becomes a parallel beam of light again, and is transformed into a parallel beam by the three beam splitters 8 ₁ , 8 ₂ , 8 ₃ . The light is divided into four beams, each of which passes through polarizing plates 9-12 and enters light-receiving elements 13-16. reflected at the irradiation position _M2 ,
The overlapping ±m-order diffracted light undergoes the Doppler frequency shift Δ of equation (2) again as the radiation grating 5 rotates, so in addition to the frequency shift when reflected at the irradiation position _M1 , the result is , the frequency shift amount of the ±m-order diffracted light reflected at the irradiation position _M2 is ±
It becomes 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 13 to 16 becomes 2Δ−(−2Δ)=4Δ. In other words, the frequency of the output signals of the light receiving elements 13 to 16 becomes =4Δ=4rωsinθ _n /λ, and from the diffraction condition equation (1), it becomes =4mrω/p. If the total number of grid patterns of the radiation grating 5 is N and the equiangular pitch is Δ, then from p=rΔ and 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.
A method is used to extract a signal indicating the direction of rotation from a 90° phase difference signal that accompanies rotation.

Furthermore, the center level of the output signal of the light receiving element may vary due to an error in the line width between the light-transmitting portion and the reflecting portion of the radiation grating 5, or due to fluctuations in laser output. Therefore, in this embodiment, in order to suppress this fluctuation, keep the center level constant, and stabilize the subsequent signal processing, we take the differential of two output signals with a 180° phase difference and remove the DC component. It uses the so-called push-and-bull method.

In this way, in this embodiment, in order to detect the rotation direction and at the same time keep the center level of the signal constant,
Four phase difference signals of 0°, 90°, 180°, and 270° are used as output signals of the light receiving element. In the configuration of this embodiment, these four phase difference signals are generated by a combination of linearly polarized laser light, two quarter-wave plates 7 and 7', and four polarizing plates 9 to 12.
Generally, a laser is linearly polarized light, but with respect to this polarization direction, in each optical path of the ±m-order diffracted light,
As mentioned above, the quarter-wave plates 7 and 7' are arranged so that their axes are at 45 degrees and -45 degrees. Then, the speed of light transmitted through the quarter-wave plates 7 and 7' becomes circularly polarized light in opposite directions, and when they overlap again at the irradiation position _M2 , they become linearly polarized light. , its polarization direction changes as the radiation grating 5 rotates. This luminous flux is divided into four luminous fluxes by the three beam splitters 8 ₁ to 8 ₃ as described above,
The light enters light receiving elements 13 to 16 via polarizing plates 9 to 12 whose polarization directions are shifted by 45 degrees. From the light receiving elements 13 to 16, as the radiation grating 5 rotates,
This results in signals whose phases are shifted by 90 degrees.
For example, if the phase of the output signal of the light receiving element 13 is 0°, the phases of the output signals of the light receiving elements 14, 15, and 16 are 90°, 180°, and 270°, respectively. Let these output signals be P ₀ , P ₉₀ , P ₁₈₀ , and P ₂₇₀ , respectively. As shown in Figure 1, the output signals P ₀ , P ₁₈₀ , P ₉₀ and
When P ₂₇₀ is input to the differential amplifiers 20 and 21, there is a 90° phase difference between the output signals of the differential amplifiers 20 and 21, and each output signal has a constant center level with the DC component removed. It's becoming a signal. Then, after waveform-shaping these signals and detecting the rotation direction, the rotation angle can be determined by inputting the signals into a counter and integrating them.

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, when a scale with the same number of divisions is used, rotation angle detection accuracy 4 m times that of the conventional example can be obtained.

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 rotational angle detection accuracy as in the conventional example, the number of divisions of the radiation grating may be 1/4 m. ±1
In the case of m=1 using the following 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.

In this example, two points M ₁ and M ₂ on the radiation grating 5 that are symmetrical with respect to the rotation center are used as irradiation points, that is, measurement points, and the eccentricity between the center of the radiation grating 5 and the rotation center of the rotating body to be tested is This reduces the impact of 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 Fig. 2, when there is an eccentric amount a between the center O of the radiation grating 5 and the rotation center O', the measurement point _M1 located at a distance r from the rotation center The Doppler frequency shift of
Compared to when there is no eccentricity, it changes from r/(r+a) to r/(ra-a). On the other hand, at this time, the frequency shift at measurement point _M2 , which is symmetrical to position _M1 with respect to _the center of rotation, changes from r/(ra-a) to r /(r+
a) Therefore, by using two measurement points at the same time, _M1 and _M2 , it is possible to reduce the effect of eccentricity, and as a result, it is possible to detect the rotational speed 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.

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.

Furthermore, by introducing the push-pull method,
It is possible to remove DC fluctuations in the output of the light receiving element due to the undefined line width of the reference radiation grating, etc., and it is possible to achieve a rotary encoder capable of stable signal processing.

[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′ is a quarter wavelength plate, 8 ₁
~8 ₃ is a beam splitter, 9 to 12 are polarizing plates,
13 to 16 are light receiving elements.

Claims (1)

[Claims] 1. A radiation grating in which a plurality of grid patterns are arranged at equal angles around the periphery of a disk, a rotating object connected to the radiation grating, a first illumination means for making a light beam incident on the radiation grating, and a radiation grating that is incident on the radiation grating. Out of the diffracted lights from the light beam, two diffracted lights of specific orders are placed at positions approximately symmetrical to the center of rotation of the rotating object with respect to the incident position of the light beam by the first illuminating means 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-entering through the /4 wavelength plate, the superimposed light beam is divided into four light beams. It has a light splitting means for splitting the light and four light receiving means for receiving the four light beams split by the light splitting means through four polarizing plates each having a polarization direction shifted by 45 degrees, A rotary encoder characterized in that the rotation angle of the rotating object is determined using output signals from four light receiving means.