JPH0577967B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an optical displacement detector, and in particular, the relative position of two members can be detected using a main scale formed with an optical grating and a reference scale formed with a corresponding optical grating. This invention relates to an improvement in an optical displacement detector that detects changes in photoelectric conversion signals caused by relative displacement with a scale.

[Conventional technology]

A main scale on which a first grating is formed is fixed to one of the facing members, and a reference scale on which a second grating is formed, an illumination system including a light source, and a light receiving element are fixed to the other member, 2. Description of the Related Art Optical displacement detectors that generate detection signals that periodically change according to the relative movement of two members are widely used in the field of measuring the feed rate of tools in machine tools. Conventional optical displacement detectors generally use a parallel illumination system, and the pitch of the first grating and the second grating are the same. In contrast, the present applicant has proposed a detector in which the pitch of the second grating is 1/n (n is a natural number) of the pitch of the first grating in Japanese Patent Application No. 61-191532. The detector where n is an even number is constructed as shown in FIG. 7, for example. The optical displacement detector shown in FIG. 7 includes a collimated illumination system 10 with an effective wavelength λ, consisting of a light emitting diode (LED) 12 and a collimator lens 14;
A main scale 16 on which a first grating 18 of pitch P is formed, and a second grating 22 with a gap of v from the first grating 18 and pitch Q=P/(2n) (n is a natural number) are formed. reference scale 2
0, a light receiving element 24 that photoelectrically converts the light from the parallel illumination system 10 filtered by the first and second gratings 18 and 22, and a preamplifier 26 that amplifies its output signal to obtain a detection signal a. It mainly consists of

[Problem to be solved by the invention]

The S/N ratio of the detection signal a is usually equal to the amplitude PP
and the direct current component DC = PP/DC. An example of the experimental results obtained when the lattice spacing v is varied in the case of pitch Q=P/2 is shown by a solid line A in FIG. As is clear from FIG. 8, the S/
Since the N ratio (=PP/DC) varies depending on the grid spacing v, when assembling the detector,
If the reference scale 20 is fixed at the minimum PP/DC, the S/N ratio of the detection signal a will deteriorate, and the noise resistance will deteriorate. Therefore, there have been problems in that positioning accuracy becomes strict and the cost of the detector increases.

[Purpose of the invention]

The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide an optical displacement detector in which the S/N ratio of a detection signal has less gap dependence than the conventional one.

[Means to solve the problem]

The present invention provides a main scale with a coherent diffused light source having an effective wavelength λ, a main scale provided with a first grating of pitch P disposed at a distance u from the diffused light source, and a main scale formed with a first grating of pitch P; a reference scale on which a second grating is formed, disposed at a distance of v, and a light receiving element that photoelectrically converts light from the diffused light source filtered by the first and second gratings, In the optical displacement detector that generates a detection signal that periodically changes depending on the relative displacement between the main scale and the reference scale, the light receiving element detects a grating of a geometric image of the first grating in the detection signal. In order to remove the variation due to the interval, the following relationship is established: {u ₂ v ₂ / (u ₂ + v ₂ )} − {u ₁ v ₁ / (u ₁ + v ₁ )}≒mP ² /λ...(1) W ≒n {(u ₂ + v ₂ ) sin θ ₂ − (u ₁ + v ₁ ) sin θ ₁ } ...(2) (However, m and n are integers, and u ₁ and u ₂ are the respective values between the diffused light source and the first grating. The optical path lengths of the light rays, v ₁ and v ₂ are the optical path lengths of each ray between the first grating and the second grating, W is the center distance of each ray on the second grating, θ ₁ and θ ₂ are the diffused light source The above object is achieved by receiving two light rays that satisfy the following angles (angles each light ray makes with respect to the perpendicular to the grid). Further, in the present invention, in a similar optical displacement detector, two light receiving elements are provided, and each light receiving element has the following relationship (u ₂ v ₂ )/(u ₂ +v ₂ ) −(u ₁ v ₁ ) / (u ₁ + v ₁ )≒mP ² /2λ ...(3) L≒n {(u ₂ + v ₂ ) sin θ ₂ − (u ₁ + v ₁ ) sin θ ₁ } ... (4) (However, m, n are The integers, u ₁ and u ₂ are the optical path length of each ray between the diffused light source and the first grating, v ₁ and v ₂ are the optical path length of each ray between the first grating and the second grating, and L is the optical path length of each ray between the first grating and the second grating. The distance between the centers on the second grating, θ ₁ and θ ₂ , is the angle that each ray makes with respect to the perpendicular line drawn from the diffused light source to the grating. The above objective is achieved by using the sum of the outputs of the elements as the detection signal.

[Effect]

According to Fresnel's theory of diffraction, when an optical grating with a pitch P is illuminated with coherent parallel light or a diffuse light source, the pitch becomes the original grating at a gap v from the optical grating. The geometric image of P, which is the same as
It is known that a diffractive image with a pitch of 1/2 of the original grating, ie, P/2, is formed. Among these, the S/N ratio of the geometric image changes greatly and periodically due to changes in the grating interval v. Furthermore, optical gratings are generally scaled in the form of bright and dark vertical stripes, and when analyzed by Fourier analysis, they contain many harmonic components. It has been clarified in Japanese Patent Application No. 61-208554 filed by the present applicant that these harmonic components also have a geometric image and a diffraction effect image, respectively. From this, considering the experimental results (solid line A) in Figure 8, this PP/DC curve is the diffraction effect image (pitch P/2) of the original lattice (pitch P) of the first grating.
(dashed line B in Figure 8) and the S/N ratio of 2 of the first grid.
It can be seen that the S/N ratio (dotted chain line C in FIG. 8) of the geometric image (pitch P/2) of the harmonic (pitch P/2) is synthesized. As is clear from FIG ^. 8, the S/N ratio of the geometric image (dotted chain line C) has a gap dependence, with peaks G ₁ ,
There are G ₂ , G ₃ , G ₄ , ..., and each peak G ₁ ,
The phases are reversed in G ₂ , G ₃ , G ₄ , etc. Therefore, in the case of a parallel illumination system, as the applicant proposed in Japanese Patent Application No. 62-231755, as shown in FIG. When mQ ² /λ (m is a natural number) is given, the geometric image is integrated by the period in which the S/N ratio changes, and the variation is removed. Therefore, the dependence of the S/N ratio of the detection signal on the grid spacing v is almost eliminated. However, when using a diffused light source such as a laser diode as is, the second grating 22 and the light receiving element 24 are provided in multiple stages in the width direction of the reference scale 20, especially for the purpose of direction discrimination or phase division. If the entire reference scale 20 is tilted, the difference in distance between the respective light receiving elements 24 will become too large, and the amount of light received will become unbalanced, which will not work. Further, there still remain problems such as the inability to apply this method to a reflection type detector in which only the second grating cannot be tilted. In contrast, in the present invention, the reference scale is not tilted, and the light receiving element 24 is
is offset from the position Z ₀ of the foot of the perpendicular line drawn from the diffused light source 30 such as a laser diode to the first grating 18 and the second grating 22, and so that the light receiving element 24 can simultaneously receive light corresponding to the period of fluctuation. To,
The size (W) in the scale width direction is set to a size that can receive both light beams B ₁ and B ₂ that satisfy the following equation. {u ₂ v ₂ / (u ₂ + v ₂ )} − {u ₁ v ₁ / (u ₁ + v ₁ )}≒mP ² /λ …(1) W≒n {(u ₂ +v ₂ ) sinθ ₂ −( u ₁ + v ₁ ) sinθ ₁ } ...(2) Here, m and n are integers (1, 2, 3,...), P
is the pitch of the first grating 18, and λ is the diffused light source 30.
, u ₁ and u ₂ are the optical path lengths of the respective rays B ₁ and B ₂ between the diffused light source 30 and the first grating 18, and v ₁ and v ₂ are the effective wavelengths of the beams B 1 and B 2 between the first grating 18 and the second grating 22. The optical path length, W, of each light beam B ₁ , B ₂ is the center distance of each light beam B ₁ , B ₂ on the second grating 22 (=Z ₂ −Z ₁ ≒ size of the light receiving element 24 in the scale width direction), θ ₁ , θ ₂ are the angles that each ray B ₁ , B ₂ makes with respect to the normal from the diffused light source 30 to the gratings 18 , 22 . Especially in the case of a reflection type detector, since u ₂ = v ₂ and u ₁ = v ₁ , if we set these as u ₂ = v ₂ = d ₂ and u ₁ = v ₁ = d ₁ , we can obtain the above ( Equations 1) and (2) are as shown below. (d ₁ − d ₁ )/2=mP ² /λ ……(5) W≒2n(d ₂ sin θ ₂ − _{d 1} sin θ ₁ ) ……(6) The coefficient of the gap dependent term of the geometric image is cos
[πλuv/{P ² (u+v)}], so the optical distance u ₁ of the two light rays B ₁ and B ₂ from the diffused light source 30 is
If the relationship between u ₂ , v ₁ , and v ₂ is determined to satisfy the above equations (1) and (2) or (5) and (6), the period of fluctuation can be received simultaneously, Geometric images can be almost canceled out. This has also been confirmed through experiments. Therefore, there is no need to tilt the reference scale, and even when a diffused light source is used or a reflection type detector is used, the dependence of the S/N ratio of the detection signal on the gap can be reduced. What is important in the present invention is that the light-receiving element 24 receives the two light beams B ₁ and B 2 that satisfy the above-mentioned formulas (1) and ( ₂ ) together, and that this condition is not satisfied. In this case, the position of the light receiving element 24 does not necessarily need to be offset from the position Z ₀ of the leg of the perpendicular line, and it is also possible to arrange it at a position that includes the position Z ₀ of the leg of the perpendicular line. Also, one ray, e.g. B ₁
may coincide with the perpendicular line. If the position of the light receiving element 24 is completely offset from the foot position _Z0 of the perpendicular line as in the example of FIG.
As in the first embodiment described later, another set of second grating 22 and light receiving element 24 having substantially the same light receiving signal level can be provided at symmetrical positions, and for the purpose of direction discrimination, phase division, etc. A total of two sets of second gratings 22 having different phases can be easily provided. Furthermore, the size of the light receiving element 24 does not necessarily have to be such that the light beams B ₁ and B ₂ can be received simultaneously by a single light receiving element 24, and for example, as shown in FIG. 2, a single detection signal can be generated. To achieve this, two light receiving elements (24A, 24
B) are provided, and the period of the gap dependence is different from each other by half a period, and the relationship of the following equation is satisfied.
After receiving the book's light beams C ₁ and C ₂ , the sum can be calculated and used as a detection signal. (u ₂ v ₂ )/(u ₂ +v ₂ ) −(u ₁ v ₁ )/(u ₁ +v ₁ )≒mP ² /2λ…(3) L≒n{(u ₂ +v ₂ ) sinθ ₂ −( u ₁ + v ₁ ) sin θ ₁ } ...(4) Here, L is the second lattice 2 of each ray C ₁ and C ₂
The center spacing on 2 (≒center spacing of the light receiving elements 24A, 24B) and other symbols are almost the same as in the above equations (1) and (2). In particular, in the case of a reflection type detector, the above equations (3) and (4) become as shown in the following equation, as in the previous case. d ₂ - d ₁ = mP ² / λ ... (7) L≒2n (d ₂ sin θ ₂ - _{d 1} sin θ ₁ ) ... (8) Therefore, in these cases, each light receiving element 24, 24B with a different half period By using the sum of the outputs as the detection signal, the geometrical image can be almost canceled out.

【Example】

Hereinafter, embodiments of the present invention applied to a reflection type detector will be described in detail with reference to the drawings. As shown in FIGS. 3 to 5, the first embodiment of the present invention includes a diffused light source 30 including an LD chip 34 (see FIG. 4) housed in a storage container 32, and a first grating of pitch P. 18 and four corresponding second gratings 22A to 22D.
(See FIG. 5) Photoelectrically converts the light from the diffused light source 30 that has been reflected by the first grating 18 and passed through each of the second gratings 22A to 22D. 4 light receiving elements 2
4 (see FIG. 4), the main scale 1
In the reflective linear displacement detector that generates periodic detection signals a and b according to the relative displacement between the reference scale 20 and the reference scale 20, the size of each light receiving element 24 in the scale width direction is determined by the size of each second grating 22A to 22D. Two light rays B ₁ and B ₂ with a center spacing W that satisfies the relationships of equations (5) and (6) above in terms of position (they are sized to be able to receive light taking into account both figures in Fig. 1). The above-mentioned diffused light source 30, as shown in detail in FIG. 4, the LD chip 34 as a primary point light source;
It is configured to include a cylindrical distributed index lens 40 as a condenser lens that converges the diverging light from the LD chip 34 to generate a secondary point light source, and the secondary point light source is connected to the reference scale. 20 of the second grating forming surfaces (chromium-deposited surfaces) 42. The main scale 16 is made of a glass plate, and as shown in FIG.
The first grating 18 is formed of a periodic scale with a pitch P in the form of vertical stripes. As shown in detail in FIG. 5, the reference scale 20 has four divisions on the chrome-deposited surface 42 corresponding to phases of 0°, 180°, 90°, and 270° at the same pitch. The second lattice 2 arranged in a field shape
2A, 22B, 22C, 22D and a central opening 52 through which the secondary point light source is focused and passes. The central opening 52 has a height of 0.4 mm and a width of 0.1 mm, for example.
mm, into which the diverging light of the LD chip 34 is focused by the distributed index lens 40 (for example, self-occurring lens manufactured by Nippon Sheet Glass Co., Ltd.), and the secondary point light source 54 is formed (see Figure 4). A total of four light receiving elements 24 respectively corresponding to the second gratings 22A to 22D are as shown in FIG.
They are arranged on the light receiving board 56, and have a positional relationship as shown by broken lines in FIG. 5, and form a pair of two, and the detection signals a and b are generated by the differential amplifiers 60 and 62, respectively. The gradient refractive index lens 40 is also inserted into the center of the light receiving substrate 56. In this first embodiment, for example, the grid pitch P
=8μm, light source wavelength λ≒0.8μm, m=n=1, u=
When v=d=5mm and θ ₁ =8°, the equations (5) and (6) above are
d ₁ , d ₂ , and θ ₂ are respectively as follows. d ₁ = d/cosθ ₁ ≒5.049mm d ₂ = 2P ² /λ+d ₁ ≒5.209mm cosθ ₂ ≒5/5.209, so θ ₂ ≒16.3゜ Substituting these into equation (6), W≒1.518≒1.5 mm, the size of each light receiving element 24 in the scale width direction is large enough to receive two light beams B ₁ and B ₂ with a center spacing W≒1.5 mm at the positions of each second grating 22A to 22D. The geometrical image can be almost canceled out by setting the value to 1 or an integer multiple thereof. In this embodiment, there is no need to increase the number of light receiving elements, and the configuration is simple. Further, in this embodiment, since the distributed index lens 40 is used to form the secondary point light source 54, an almost ideal diffused light source can be obtained. Note that the method for forming the diffused light source 30 is not limited to this, and it is also possible to directly use a laser diode as the diffused light source, or to use a tungsten lamp or a light emitting diode other than the laser diode. Next, a second embodiment of the present invention will be described in detail. In the second embodiment, in a reflection type displacement detector similar to the first embodiment, two light receiving elements 24, 24A and 24B are provided for each of the second gratings 22A to 22D, as shown in FIG. Provided, each light receiving element 2
4A and 24B in the scale width direction at the positions of the corresponding second gratings 22A to 22D (7),
In order to receive the two light beams C ₁ and C ₂ (Fig. 2) that satisfy the equation (8), the size is set to correspond to the interval L in the equation (8), and adders 64A to 64D are used to each After finding the sum of the outputs of the light receiving elements 24A and 24B,
Differential amplification is performed by differential amplifiers 60 and 62 similar to those in the first embodiment, and the detection signals a and b are used for direction discrimination, phase division, etc. In this second embodiment, for example, the grating pitch P=8 μm, the light source wavelength λ≒0.8 μm, m=n=1, u
= v = d = 5 mm, θ ₁ = 8°, the above equations (7) and (8)
d ₁ , d ₂ , and θ ₂ are respectively as follows. d ₁ ≒5.049mm d ₂ ≒5.129mm θ ₂ ≒12.9゜ Substituting these into equation (8), L≒0.884≒0.9mm
Therefore, the center spacing in the scale width direction of each light receiving element 24A, 24B is equal to the second grating 22A to 22D.
If it is approximately 0.9mm at the position or an integral multiple thereof,
By summing the outputs, the geometric images are approximately canceled. In each of the above embodiments, the present invention was applied to a reflective linear displacement detector including a reflective scale made of glass, but the scope of application of the present invention is not limited to this, and The present invention can be similarly applied to a transmissive detector, a transmissive detector, and a rotational displacement detector.

【Effect of the invention】

As explained above, according to the present invention, even when a diffused light source is used or a reflective type, it is possible to remove the signal due to the geometric image in the detection signal, and the S/N ratio of the detection signal is The dependence on the lattice spacing is almost eliminated. Therefore, the positioning accuracy is not strict, and the cost of the detector can be reduced, which is an excellent effect.

[Brief explanation of the drawing]

1 and 2 are diagrams for explaining the principle of the present invention, FIG. 3 is a perspective view showing the overall configuration of the first embodiment of the optical displacement detector according to the present invention, and FIG. The figure is a cross-sectional view taken along the - line in Figure 3;
The figure is a cross-sectional view taken along the - line in FIG. 191532, FIG. 8 is a diagram showing the dependence of the S/N ratio of the detection signal on the grid spacing in the comparative example, and FIG. 9 is a plan view showing the configuration of the comparative example proposed by the applicant. FIG. 2 is a sectional view showing the configuration of a comparative example proposed in Japanese Patent Application No. 62-231755. 16...Main scale, 18...First grid, 20
...Reference scale, 22, 22A to 22D... Second grating, 24, 24A, 24B... Light receiving element, a, b...
Detection signal, 30... Diffuse light source, 34... Laser diode (LD) chip, 54... Secondary point light source, 64A
~64D...Adder.

Claims (1)

[Claims] 1. A coherent diffused light source with an effective wavelength λ; a main scale disposed at a distance u from the diffused light source and on which a first grating of pitch P is formed; a reference scale on which a second grating is formed, which is disposed at a distance v from the grating; and a light receiving element that photoelectrically converts light from the diffused light source filtered by the first and second gratings. In the optical displacement detector that generates a detection signal that periodically changes according to the relative displacement between the main scale and the reference scale, the light-receiving element detects the geometrical shape of the first grating in the detection signal. In order to remove the variation due to the image grid spacing, the following relationship is established: {u ₂ v ₂ / (u ₂ + v ₂ )} − {u ₁ v ₁ / (u ₁ + v ₁ )}≒mP ₂ /λ W≒ n {(u ₂ + v ₂ ) sin θ ₂ − (u ₁ + v ₁ ) sin θ ₁ } (where m and n are integers, u ₁ and u ₂ are the optical path lengths of each ray between the diffused light source and the first grating, v ₁ , _v2 are the optical path lengths of each ray between the first grating and the second grating, W is the center distance of each ray on the second grating, _θ1 , _θ2 are the perpendicular lines drawn from the diffused light source to the grating What is claimed is: 1. An optical displacement detector characterized in that the optical displacement detector is configured to receive two light beams that both satisfy the angle (the angle that each light beam makes with respect to the angle that each light beam makes). 2. A coherent diffused light source with an effective wavelength λ, a main scale on which a first grating of pitch P is formed, which is disposed at a distance u from the diffused light source, and a distance v from the first grating. a reference scale on which a second grating is formed, and a light receiving element that photoelectrically converts light from the diffused light source that has been filtered by the first and second gratings; In an optical displacement detector that generates a detection signal that periodically changes according to the relative displacement of a reference scale and a reference scale, two of the light receiving elements are provided, and each light receiving element has the following relationship (u ₂ v ₂ )/ (u ₂ +v ₂ ) −(u ₁ v ₁ )/(u ₁ +v ₁ )≒mP ² /2λ L≒n {(u ₂ +v ₂ ) sinθ ₂ − (u ₁ +v ₁ ) sinθ ₁ } (however, m , n are integers, u ₁ , u ₂ are the optical path lengths of each ray between the diffused light source and the first grating, v ₁ , v ₂ are the optical path lengths of each ray between the first grating and the second grating, and L is the optical path length of each ray between the first grating and the second grating. , the distance between the centers of each light ray on the second grid, θ ₁ and θ ₂ are the angles each light ray makes with respect to the perpendicular line drawn from the diffused light source to the grid), and two light rays are received respectively. , An optical displacement detector characterized in that the detection signal is the sum of outputs of each light receiving element.