JPH0235246B2 - KOGAKUSUKEERUSOCHI - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical scale device, and particularly to an improvement for eliminating distortion and improving accuracy by removing third harmonic components from a detection scale signal.

[Background technology and its problems] As is well known, digital position display systems include
There are various methods such as magnetic, optical, electromagnetic induction, and capacitive methods, but all of them use a reference scale with a periodic pattern and a pick-up head that is displaced relative to this. Detects changes in magnetic flux, amount of light, induced voltage, capacitance, etc. corresponding to each displacement and converts them into electrical signals.

In order to increase the resolution of the detection output of the above system, it is often electrically divided (interpolated) to make it much smaller than the period of the reference scale pattern, so the waveform of the detection output has little distortion.
It is desirable that it approximate a sine wave as much as possible. If the waveform of the detection output is distorted, the electrical division described above may be possible nominally, but it will not be accurate.

Further, in the case of the optical system as shown in FIG. 1, the area of the light transmitting portion when the main scale 1 and the index scale 2 are superimposed increases or decreases in proportion to the amount of relative displacement between the two scales. Therefore, the detection output of the light-receiving element 3 is proportional to the area of the light transmitting part, and if the light transmitting area in the length direction defined by each optical grating of both scales is the same, the detection output of the light receiving element 3 is proportional to the area of the light transmitting part. The output is a triangular wave, and if the light transmission area in the length direction of each scale is different, it becomes a trapezoidal wave.
The detection output contains harmonic components mainly including third harmonic components.

In other words, the trapezoidal wave is expressed by a Fourier series as shown in Fig. 11, f ₁ (x) = 4/π a/b (1/1 ² sinbsinx + 1/3 ² sin3
bsinx…) (1) For the triangular wave, set b = π/2 in equation (1) and f ₂ (x) = 8a/π ² (sinx/1 ² −sin3x/3 ² + sin5x/5 ²
…) is expressed by the formula (2), and the distortion rate is Therefore, it can be seen that the third harmonic component is a major cause of distortion.

[Object of the Invention] An object of the present invention is to remove the third harmonic component that causes distortion of the output waveform with respect to the displacement x of the optical scale device.

[Summary of the Invention] In order to achieve the above object, the present invention provides index scale optical grating patterns such that each index scale optical grating pattern has a phase wave of 1/6 wavelength spatially. The present invention is characterized in that by combining the outputs, at least the third harmonic component is removed from the combined output.

[Example] The present invention will be described below with reference to an example shown in the drawings. In the optical scale apparatus shown in FIG.
When the duty of the optical grating is 1:1, the scale output f(x) detected by the light receiving element 3 is
is a triangular wave as shown in Figure 2, and in equation (2), a=
A/2 corresponds to a phase shift in the left direction by 1/2π and a shift in the positive direction by A/2, so it is expressed by the following equation (3).

f(x)=A/2+4A/π ² {cosx/1 ² +cos3x/3 ² +co
s5x/5 ² ...} (3) In Fig. 1, 4 is the light source, 5 is the lens system, and the duty ratio of 1:1 means that the dimensions of the light transmitting part and the light blocking part are equal in the optical grating. The sum of the two is the lattice constant.

For the triangular wave scale output of this f(x), ε
A triangular wave f expressed by the following equation (4) with a phase difference of
Consider the scale output of a composite wave F(x) formed by adding (x+ε) scale outputs.

f(x+ε)=A/2+4A/π ² {cos(x+ε)/1 ² +
cos3(x+ε)/3 ² +cos5(x+ε)/5 ² }(4) F(x)=f(x)+f(x+ε)=A+8A/π ² {1
/1 ² cos(x+ε/2) cosε/2+1/3 ² cos(3x+3ε
/2) cos3ε/2 +1/5 ² cos (5x+5ε/2) cos5/2ε...}(5) From equation (5), the conditions for making the term of the third harmonic component of the composite output F(x) zero are 3ε/2=π/2, ε=π/3 In other words, the triangular wave f(x) and the triangular wave f(x+π/3) which has a phase difference of π/3 with respect to x
It can be seen that by combining these, the third harmonic component is canceled out from the combined output F(x), a scale output with less distortion is obtained, and the accuracy of the optical scale device can be improved.

3 and 4 respectively show embodiments of the present invention based on the above-described principles (in each of these embodiments, n to n ⁴ is an arbitrary integer).

In Figure 3, index scale 2'
is used in an optical scale device as shown in FIG. 5, and its four pattern parts a, b, c, and d are each vertically divided into two, and the spatial phase difference ε of each divided pattern is λ/ 6 (here, one wavelength λ=2π, so ε=π/3), the optical element 3 generates a detection output f( x) and f
(x+π/3) is obtained and synthesized, the third harmonic component is canceled out from the synthesized scale output F(x) for the reason described above.

The embodiment of FIG. 4 has an index scale of 2'.
Each pattern section is horizontally divided into two parts, and the same effect can be obtained with this division method.

As mentioned above, each pattern part of the index scale is divided into at least two parts vertically and/or horizontally, and the spatial phase difference of each part is λ/6 (λ is the wavelength) to correspond to each divided pattern part. By combining the detection outputs, the third harmonic component can be canceled out from the combined scale output.
A similar effect can also be obtained by dividing the main scale 1 horizontally.

Note that even if the pattern portion of the index scale 2' has a structure as shown in FIGS. 6 and 7, distortion can be canceled in the same manner as in each of the above embodiments.

FIGS. 8a and 8b show still another embodiment in which the present invention is applied to the optical scale apparatus shown in FIG. In the figure, reference numeral 6 denotes a holder for the index scale 2', which is stored in a fixed frame 8 via fine adjustment screws 7.

The main scale 1 moves horizontally and horizontally in parallel with the index scale 2' while maintaining a constant interval. Further, light receiving elements 3 are arranged parallel to the main scale 1 and divided into approximately the same sections as each pattern of the index scale.

As shown in FIG. 9, the index scale 2' has its respective pattern portions a to d divided into upper and lower halves by a first light-shielding plate 9. As shown in FIG. First light shielding plate 9
As shown by diagonal lines, the width of the central portion is widened, so that the amount of light passing through pattern portions b and c can be balanced by slightly adjusting the light shielding plate left and right. Further, a second light shielding plate 10 is arranged to overlap the first light shielding plate 9, dividing the pattern portions a and d into upper and lower portions, and balancing the amount of light passing through each.

The index scale 2' is arranged slightly inclined with respect to the main scale 1, and the relationship between the index scale 2' and the main scale 1 is as shown in FIG.
Fine adjustment is made so that there is a phase difference of λ/6 above and below 0. For this reason, for example, if a part of the holder 6 of the index scale 2' is connected to the fixed frame 8 by a large circle R, and the structure is such that it can be moved slightly horizontally along this large circle and then fixed, the fine adjustment screw 7 can be fixed. With a relatively large rotation, the above-mentioned fine adjustment of the direction (angle) of the index scale can be easily performed.

With the above configuration, each pattern part of the index scale 2' can be divided vertically into two parts by the light shielding plate.
Therefore, the transmitted light flux is also divided into two, and the respective light fluxes are divided into the f(x) and f(x+π/3)
Since both light beams are incident on one light receiving element, an output of F(x), which is a combination of f(x) and f(x+π/3), is obtained.

In addition to the method of combining two outputs as in each of the embodiments described above, it is also possible to obtain the same effect as the present invention by making the scale pattern have a specific structure.

That is, assuming that the waveform of _the scale output is a trapezoidal wave as shown in FIG.
)(6) fo(x)=4a/πb(sinbsinx/1 ² +sin3bsi
n3x/3 ² + sin5bsin5x/5 ² ...) (7).

In equation (5), if b=π/3, As is clear from equation (6), the waveform of the scale output is b
It can be seen that the third harmonic component can be removed by configuring the scale to form an optical grating pattern that forms a trapezoidal wave with =π/3. To do this, for example, when the duty of one of the optical grating patterns of the main scale and the index scale is 1:1, the duty of the other may be set to 1:2 (the portion through which light passes is 1).

[Effects of the Invention] As is clear from the above explanation, according to the present invention, the third harmonic component can be removed from the composite scale output using a relatively simple method, and the accuracy of the scale is greatly improved. improves.

[Brief explanation of drawings]

1 and 5 are schematic diagrams showing the basic configuration of the optical scale device, FIG. 2 is a waveform diagram for explaining the operating principle of the present invention, and FIGS. 3 and 4 are respective diagrams showing the basic configuration of the optical scale device. A schematic diagram showing the structure of the main part of the embodiment,
6 and 7 are schematic diagrams for explaining other embodiments of the present invention, FIGS. 8a and b, 9 and 1.
0 is a schematic diagram showing still another embodiment of the present invention, and FIG. 11 is a waveform diagram for explaining the operation of still another embodiment of the present invention. 1... Main scale, 2, 2'... Index scale, 3... Light receiving element, 4... Light source, 5
...Lens system.

Claims (1)

[Claims] 1 A scale comprising a main scale, an index scale held parallel to this, and a light receiving means for detecting transmitted light from both scales, and which corresponds to the optical grating pattern of both scales according to the relative displacement between the scales. In the optical scale device in which the output is obtained by the light receiving means, the index scale has a plurality of optical grating pattern parts, and each pattern part is provided so as to have a spatial phase difference of 1/6 wavelength. An optical scale device characterized in that it is configured to remove a third harmonic component from the synthesized scale output by synthesizing the scale outputs corresponding to the respective pattern portions.