JPH0151926B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photoelectric encoder for measuring relative movement, particularly to an encoder using a diffraction grating. Figure 1 shows the basic optical system of an encoder that combines a transmission type diffraction grating and a reflection type diffraction grating.The light emitted from the light source 1 is turned into almost parallel light by the collimator lens 2, and the semi-transparent mirror After passing through the diffraction grating 5, it enters the transmission type diffraction grating 3. The diffracted light diffracted into different orders by the diffraction grating 3 is further diffracted by the reflective diffraction grating 4. The diffracted light diffracted by the diffraction grating 4 is again diffracted by the transmission type diffraction grating 3, reflected by the anti-transparent mirror 5, and focused onto the photoelectric conversion element 7 by the condensing lens 6. As shown in FIG. 2, the orders of two arbitrarily selected diffracted lights among the diffracted lights by the transmission type diffraction grating 3 are m ₁₀ and m ₁₁ , and the orders of the above-mentioned diffraction lights m ₁₀ , After the diffracted light of m ₁₁ is diffracted, among the lights that are further diffracted by the transmission type diffraction grating 3, the orders of the diffracted lights that are diffracted in the same direction and used for measurement are m ₂₀ and m ₂₁ . , the diffracted light of order _m20 and the diffracted light of order _m21 interfere on the photoelectric conversion element 7. The signal I(x) obtained by the photoelectric conversion element 7 by the movement of the diffraction grating 3 with respect to the diffraction grating 4 is I(x)=A+Bsin[2πx/d{(m ₁₁ +m
₂₁ ) − (m ₁₀ + m ₂₀ )}+].

However, in the above equation, A and B are constants, d is a grating constant, x is the amount of movement of the transmission type diffraction grating 3, and is a phase term.

Figure 2 shows the case where m ₁₁ = m ₂₁ = 1, m ₁₀ = m ₂₀ = 0, in which case the above equation becomes I(x) = A + Bsin (2π・2x/d+). .

Here, if d=2 μm, I(x)=A+Bsin(2π·x+). That is, according to this equation, it can be seen that a sinusoidal signal that changes periodically every time the transmission type diffraction grating 3 moves by 1 μm is obtained from the photoelectric conversion element 7.

Generally, in order to measure the amount of movement of the transmission type diffraction grating 3 with high precision, including the direction, the phase term of the signal obtained from two or four reflection type diffraction gratings 4 is π/2. The photoelectric conversion elements are arranged so as to be shifted from each other, and are arranged to correspond to each of the divided diffraction gratings 4. As a result, the signals obtained from the photoelectric conversion elements arranged corresponding to each diffraction grating are: I ₁ =A+Bsin (2π・2x/d+) I ₂ =A+Bcos (2π・2x/d+) I ₃ =A −Bsin(2π・2x/d+) I ₄ =A−Bcos(2π・2x/d+) By passing these signals through an appropriate circuit, the signals I ₁ −I ₃ , I ₂ −I ₄ can be obtained. The calculation results in I ₁ −I ₃ =2Bsin (2π・2x/d+) I ₂ −I ₄ =2Bcos (2π・2x/d+).

That is, if the signals I ₁ - I ₃ and I ₂ - I ₄ are obtained, the constant A can be removed and signals with a phase difference of π/2 can be obtained, so these two signals I ₁
By passing −I ₃ and I ₂ −I ₄ through an appropriate circuit, the amount of movement of the diffraction grating 3 can be measured with high precision. These signal processing processes are common to photoelectric encoders other than diffraction gratings. The above is an explanation that extracts only the diffracted light of the order necessary for measuring the amount of movement out of many diffracted lights, but in reality, diffracted light of orders other than the desired order enters the photoelectric conversion element, so The output signal of the photoelectric conversion element is the value {(m ₁₁ +
m ₂₁ )−(m ₁₀ +m ₂₀ )} different I(x) are superimposed, and as a result, a clean sine waveform is not obtained. That is, in Fig. 3, Fig. 3a shows the output signal of the photoelectric conversion element obtained only by the diffracted light of a desired order, and Fig. 3b shows the output signal of I(x) obtained by the diffracted light of the desired order. The output signal of the photoelectric conversion element when a small I(x) of value {(m ₁₁ +m ₂₁ )−(m ₁₀ +m ₂₀ )} is superimposed, and FIG. 3c shows the I( x) has a value {(m ₁₁ + m ₂₁ ) − (m ₁₀ + m ₂₀ )}
This is the output signal of the photoelectric conversion element when a large I(x) is superimposed. When trying to measure more detailed movement distance by dividing the signal, Fig. 3 b and c
A signal that deviates from a sine waveform, such as the one shown in FIG.

An object of the present invention is to obtain a sinusoidal signal in a photoelectric encoder using a diffraction grating.

The present invention will be described below based on embodiments shown in the drawings.

First, for simplicity, the above formula I(x)=A+Bsin[2πx/d{(m ₁₁ +m
₂₁ )−(m ₁₀ +m ₂₀ )}+], let {(m ₁₁ +m ₂₁ )−(m ₁₀ +m ₂₀ )}=l,
For example, consider the case where l=2 by selecting m ₁₁ =m ₂₁ =1 and m ₁₀ =m ₂₀ =0.

If l=2, I(x) in the above equation becomes I(x)=A+Bsin [2x/d·2π+]. Therefore, the signal I(x) takes the same value every d/2 of the movement amount x of the transmission type diffraction grating 3. On the other hand, if l=1, I(x) becomes I(x)=A+Bsin [x/d・2π+], so I(x) is the value where the sign of the movement amount x is reversed every d/2. will be taken.

Therefore, if a split grating is constructed by making up a pair of reflection-type diffraction gratings 4 shifted by d/2 in advance, on the photoelectric conversion element 7, a signal of l=2 is generated by the diffracted light from the two diffraction gratings 4. The signal is added, but l
The signal of =1 almost disappears because the diffracted lights from the two diffraction gratings 4 cancel each other out.

FIG. 4 shows an example of a reflective diffraction grating 4 formed with such a split grating, and FIG. 5 shows an optical system according to an embodiment of the present invention using this diffraction grating.

As shown in FIG. 4, the diffraction grating 4 is divided into eight gratings A, B, C, D, E, F, G, and H.
With respect to grid A, grid B is 4/8 d, grid C is 1/8 d, grid D is 5/8 d, grid E is 2/8 d, grid F is 6/8 d, grid G is 3/8 d, grid H is out of phase by 7/8d. That is, gratings A and B, gratings C and D, gratings E and F, and gratings G and H form a set, and each set constitutes a divided grating.

As shown in FIG. 5, such a reflection type diffraction grating 4 is arranged opposite to the transmission type diffraction grating 3. The photoelectric converter 70 includes a total of four photoelectric conversion elements 7a, 7b, 7c, and 7d corresponding to each grating set of the diffraction grating 4. That is, the diffracted light from gratings A and B is transmitted to the photoelectric conversion element 7a, the diffracted light from gratings C and D is transmitted to the photoelectric conversion element 7b, the diffracted light from gratings E and F is transmitted to the photoelectric conversion element 7c, and the diffracted light from gratings G is transmitted to the photoelectric conversion element 7c. The diffracted light from and H enters the photoelectric conversion element 7d. A pinhole plate 8 as a spatial filter is disposed at the rear focal point of the condenser lens 6 between the condenser lens 6 and the photoelectric converter 70, and only light on the optical axis and its vicinity is transmitted. ,
This prevents other unnecessary light from entering the photoelectric converter 70.

According to such an optical system, most of the diffracted light having a large angle with respect to the desired diffracted light is blocked by the pinhole plate 8. Photoelectric conversion element 7a,
l= generated from the diffracted light reaching 7b, 7c, 7d
1 signal is removed as described above, and l=1
Other odd-numbered signals are also removed in the same way. l
When is an even number, since l=-2 is the same signal as l=2, there is no adverse effect, and therefore, the case where l=4 has the greatest effect. Since the signal with l=4 has a frequency twice as high as the signal with l=2, the transmission type diffraction grating 3 and the reflection type diffraction grating 4 should be formed so that the directions of the gratings are slightly twisted. Most of them can be removed by As a result, the higher frequency l=
Magnitude 6 can also be removed. Of course, it can also be removed using an electric filter. However, when the frequency is about twice as high, it is difficult to completely eliminate the distortion due to the above-mentioned twist structure, so in order to completely eliminate the distortion, it is sufficient to further divide the reflection type diffraction grating 4.
That is, as shown in FIG. 6, the lattice A in FIG.
By dividing the grid into two gratings A ₁ and A ₂ that are shifted by 1/8d, and also dividing the grating B into two gratings B ₁ and B ₂ that are shifted by 1/8d from each other, the grids A ₁ and A ₂ The signals caused by the lattice B ₁ and B ₂ have a phase shift of π with respect to the signal with l=4, so they can be made to cancel each other out. For the signal with l=2, the signals from gratings A ₁ and A ₂ and the signals from gratings B ₁ and B ₂ have a phase shift of π/2, so the contrast decreases and the phase also shifts, but the effect of l = 4 Since it is a clean sine wave, it can be used for measurements without any problems. Of course, lattices C other than lattices A and B,
The same applies to D, E, F, G, and H.

In this way, each photoelectric conversion element 7a, 7
The signals obtained from b, 7c, and 7d can be formed into a substantially sinusoidal waveform. Therefore, it is also possible to perform accurate divided reading by appropriately combining the signals of the respective photoelectric conversion elements 7a, 7b, 7c, and 7d.

Note that when obtaining the l=2 signal as explained above, this is usually the easiest configuration to adopt, but to generalize the discussion, consider the case where the l=m signal is erased in order to obtain the l=n signal. As a result, it can be seen that in this case, one of the two divided reflective diffraction gratings should be shifted from the other by p/n (1+2k)d. However, in this formula, p is an integer that satisfies n=2mp, and k is an arbitrary integer. If we determine the amount of deviation of the diffraction grating divided into two in this way, d/n = 2π, so the diffracted light from each diffraction grating will be shifted by p (1 + 2k) × 2π for l = n. , are in phase with each other, so they strengthen each other and produce a sine wave signal, but for light with l=m, d/m=2π, so the diffracted light from each diffraction grating is (1+2k)×
Since they are shifted by π and have opposite phases, they cancel each other out and no signal is generated. The above embodiment is for n=2, m=1, p=1, k=0.

In addition, in the above explanation, the transmission type diffraction grating 3 is a moving grating, but the transmission type diffraction grating 3 is fixed and the other members (members 1, 2, etc. in FIG. 5) are fixed.
4, 5, 6, 7, 8) may be moved.

Furthermore, the transmission type diffraction grating 3 is configured as a divided grating as shown in FIG. 4, and the reflection type diffraction grating (this grating is similar to the grating 4 in FIG. ) Only 4 may be moved.

As described above, according to the present invention, in a photoelectric encoder with two opposing diffraction gratings,
A spatial filter that transmits only the light on and around the optical axis is provided at the rear focal point of the condensing lens, and
Since one of the diffraction gratings (index grating) is configured as a split grating shifted by a predetermined amount in order to remove error components from the output signal of the photoelectric conversion element, a sinusoidal signal can be obtained. Therefore, by interpolating the obtained signal, accurate reading becomes possible even when a dividing circuit is used, such as reading a finer amount of movement.

[Brief explanation of drawings]

Figure 1 is a diagram showing the basic optical system of a photoelectric encoder using a diffraction grating, Figure 2 is an optical path diagram of diffracted light by a transmission type diffraction grating and a reflection type diffraction grating, and Figure 3 is a moving diagram. FIG. 4 is a diagram of the output waveform of the photoelectric conversion element due to the movement of the grating. FIG. 4 is a layout diagram of the reflection type diffraction grating according to the embodiment of the present invention. FIG. FIG. 6 is a diagram showing a layout diagram of another example of a reflection type diffraction grating. Explanation of symbols of main parts, 1... Light source, 2... Collimator lens, 3... Transmission type diffraction grating, 4...
...Reflection type diffraction grating, 6... Condensing lens, 7a,
7b, 7c, 7d...photoelectric conversion element, 8... spatial filter.

Claims (1)

[Scope of Claims] 1. A pair of relatively moving diffraction gratings are arranged facing each other in a parallel light flux as measurement light, and the light involved by the pair of diffraction gratings is collected by a condensing lens system. condensing light onto a photoelectric conversion element, one of the pair of diffraction gratings is integrally formed with respect to the photoelectric conversion element so that there is no positional change, and the amount of movement of the other diffraction grating with respect to the one diffraction grating; In a photoelectric encoder for reading a photoelectric conversion element, a spatial filter that transmits only light on and around the optical axis is disposed at a rear focal plane position of the condenser lens system, and one of the diffraction gratings is connected to the photoelectric conversion element. An encoder characterized in that the encoder is configured as a divided grating consisting of gratings whose phase is shifted by a predetermined amount in order to remove error components from an output signal.