JPH05340767A - Optical displacement detector - Google Patents

Abstract

PURPOSE:To reduce the gap dependency of the S/N of detecting signals. CONSTITUTION:The detector is provided with a coherent diffusion lighting system having an effective wavelength, main scale 16 on which a first grating 18 is formed at a pitch P, reference scale 20 on which a second grating 22 is formed at a pitch Q corresponding to the 2n-th order (n is a natural number) harmonics of the grating 18, and photodetectors 24 which perform photoelectric conversion of the light from the lighting system filtrated through the gratings 18 and 22 and generates detecting signals which periodically change in accordance with the relative displacement between the scales 16 and 20. When the magnification of the optical system of the detector is M, the pitch Q is set at MP/2n and the reference scale 20 is inclined by MmQ<2>/lambda (m is a natural number) against the main scale 16 so as to remove the fluctuation of a geometric image caused by the rating interval of the first grating 18 from the detecting signals.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical displacement detector, and more particularly, the relative position of two members is formed by a main scale having an optical grating and a corresponding optical grating. The present invention relates to an improvement of an optical displacement detector which detects from a change in a photoelectric conversion signal caused by a relative displacement with respect to a reference scale.

[0002]

2. Description of the Related Art A main scale having a first grating is fixed to one of opposing members, and a reference scale having a second grating, an illumination system, and a light receiving element are fixed to the other member. However, an optical displacement detector that generates a detection signal that periodically changes according to the relative movement of both members has become widespread in the field of measuring the feed amount of a tool of a machine tool.

The conventional optical displacement detector generally uses a parallel illumination system, and the pitches of the first and second gratings are the same.

On the other hand, the present applicant has filed Japanese Patent Application No. 61-1
91532 proposes 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. Among them, a detector in which n is an even number is shown in FIG. It is configured as shown.

The optical displacement detector shown in FIG. 4 includes a light emitting diode (LED) 12 and a collimator lens 14.
A parallel illumination system 10 having an effective wavelength λ, a main scale 16 on which a first grating 18 of pitch P is formed, a distance (gear) from the first grating 18 is v, and pitch Q =
A reference scale 20 on which a second lattice 22 of P / (2n) (n is a natural number) is formed, and the first and second lattices 18,
A light receiving element 24 for photoelectrically converting the light of the parallel illumination system 10 filtered by 22, and an output signal thereof is amplified to detect a signal a.
And a preamplifier 26 for obtaining

[0006]

Problems to be Solved by the Invention S of the detection signal a
The / N ratio is usually the ratio of the amplitude PP to the DC component DC = PP /
It is represented by DC. In the case of pitch Q = P / 2, an example of the experimental result when the lattice spacing v is changed is shown by the solid line A in FIG.

As is clear from FIG. 5, S of the detection signal a
Since the / N ratio (= PP / DC) varies depending on the lattice spacing v, when the detector is assembled, if the reference scale 20 is fixed at the position where PP / DC is the minimum,
The S / N ratio of the detection signal a deteriorates, and the noise resistance deteriorates. Therefore, there is a problem that the positioning accuracy becomes strict and the cost of the detector becomes high.

The present invention has been made to solve the above 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 is less dependent on a gear than in the prior art. .

[0009]

The present invention provides an effective wavelength λ
Of the coherent diffuse illumination system, the main scale on which the first grating of the pitch P is formed, and the second grating of the pitch Q corresponding to the 2n-th order (n is a natural number) harmonic of the first grating are formed. Detection that includes a reference scale and a light receiving element that photoelectrically converts light from the illumination system that is filtered by the first and second gratings, and that periodically changes according to relative displacement between the main scale and the reference scale. In an optical displacement detector that generates a signal, when the magnification of the optical system is M,
The pitch Q is set to MP / (2n), and the reference scale is set to Mm Q with respect to the main scale so as to remove a variation due to the lattice spacing of the geometric image of the first lattice in the detection signal. ^The object was achieved by inclining by ² / λ (m is a natural number).

[0010]

According to Fresnel's theory of diffraction, when the optical grating of the pitch P is illuminated with coherent parallel light rays, that is, the pitch is the original grating at a distance (gap) v from the optical grating. The same geometrical image of P (geom
etric image) and Pitch is 1/2 of the original lattice, that is, P / 2
Is known to form a diffractive image of the. Of these, the S / N ratio of the geometrical image is
It changes greatly periodically with changes in the lattice spacing v.

Further, the optical grating is generally formed as a bright and dark vertical stripe-shaped scale, and when Fourier analysis is performed, it contains a lot of harmonic components. The fact that these harmonic components also have a geometrical image and a diffraction effect image, respectively, is disclosed in Japanese Patent Application No. 61-
As disclosed in 208554, FIG. 3 shows a detector that also utilizes this harmonic component.

From this, the experimental result of FIG. 5 (solid line A)
Considering this, this PP / DC curve is the S / N of the diffraction effect image (pitched P / 2) of the original lattice (pitched P) of the first grating.
The ratio (broken line B in FIG. 5) and the S / N ratio (dashed line C in FIG. 5) of the geometrical image (pit P / 2) of the second harmonic (pit P / 2) of the first grating are combined. It is understood that it was done.

[0013] As is clear from FIG. 5, S / N ratio of the geometrical image (dashed line C), there is Giyatsupu dependent, peak G ₁ at lattice spacing v is an integer multiple of Q ² / lambda, G ₂ ,
G ₃ , G ₄ , ... And each peak G ₁ ,
The phases are inverted in G ₂ , G ₃ , G ₄ ,.
Therefore, as shown in FIG. 2, when the reference scale 20 is provided with an inclination amount δ = m Q ² / λ (m is a natural number) that is an integral multiple of the fluctuation period, the geometric image changes in S / N ratio. Then, the change is removed by integrating for the period. Therefore, the dependency of the S / N ratio of the detection signal on the lattice spacing v is almost eliminated.

In particular, as shown in FIG. 3, in the case of an optical displacement detector using a diffused light source as proposed by the applicant in Japanese Patent Application No. 61-208554, the diffused light source and the first grating are used. If the distance is u, the magnification M of the optical system is (u + v) /
However, it is clear that the pitch of the second grating and the tilt amount of the reference scale may be M times the value in the parallel illumination system.

[0015]

Embodiments of the present invention will now be described in detail with reference to the drawings.

As a comparative example of the parallel illumination system for explaining the principle of the present invention, as shown in FIGS. 1 and 2, a coherent parallel illumination system 10 having an LED 12 and a collimator ring 14 having an effective wavelength λ and a pitch. The main scale 16 on which the P first grating 18 is formed and the first scale 18
Second pitch Q of pitch Q corresponding to the nth harmonic (n is a natural number)
A reference scale 20 having two gratings 22 formed out of phase with each other by 90 degrees, and the first and second gratings 18,
A light receiving element 24 for photoelectrically converting the light from the parallel illumination system 10 filtered by 22, and the detection signals a and b whose phases are shifted by 90 degrees by amplifying the output of the light receiving element 24, respectively.
In the optical displacement detector including the preamplifier 26 for obtaining the above, as shown in detail in FIG. 2, the reference scale 20 is connected to the first grating 18 in the detection signals a and b.
The inclination amount δ = m Q ^{2 with} respect to the main scale 16 so as to remove the variation due to the lattice spacing v of the geometrical image of
/ Λ (m is a natural number).

A laser diode is ideal as a light source of the parallel illumination system 10, but a tungsten lamp or the LED 12 used in the embodiment may be used.

Now, assuming that the pitch P of the first grating 18 is 20 μm, the pitch Q of the second grating 22 is, for example, 10 μm.
(M = 1), 5 μm (m = 2), ...

In this comparative example, among the fluctuations due to the lattice spacing of the detection signal as shown by the solid line A in FIG.
Since the variation due to the geometrical image as indicated by the alternate long and short dash line C is removed, the S / N ratio by the diffraction effect image shown by the broken line B in FIG. 5 is almost constant regardless of the lattice spacing. S /
A detection signal with a high N ratio can be obtained.

The comparative example uses a parallel illumination system as the illumination system, but the present invention has a diffused light source using the laser diode 32 as it is, as in the example shown in FIG. In this case, the above principle is applied.

The other structure is the same as that of the comparative example, and the description thereof is omitted.

Regarding the operation, the part relating to the principle of the present invention is the same as that of the comparative example, and the other parts are in Japanese Patent Application No. 61-2.
Since it is described in detail in No. 08554, the description will be omitted.

In the above embodiment, the present invention is applied to the transmission type linear displacement measuring device, but the applicable range of the present invention is not limited to this, and the rotational displacement detector (rotary encoder) and the reflection type It is obvious that it can be applied to the optical displacement detector of the type as well.

[0024]

As described above, according to the present invention, the signal due to the geometrical image in the detection signal can be removed, and the dependency of the S / N ratio of the detection signal on the lattice spacing is almost eliminated. To be done. Therefore, the positioning accuracy becomes less severe and the cost of the detector can be reduced, which is an excellent effect.

[Brief description of drawings]

FIG. 1 is a perspective view showing a configuration of a comparative example of an optical displacement detector for explaining the principle of the present invention.

FIG. 2 is a sectional view as seen from the direction of arrow II in FIG.

FIG. 3 is a plan view showing a configuration of a main part of an embodiment of the present invention.

FIG. 4 is a plan view showing the configuration of an applied comparative example of the optical displacement detector proposed by the applicant in Japanese Patent Application No. 61-191532.

FIG. 5 is a diagram showing the dependence of the S / N ratio of the detection signal on the lattice spacing in the comparative example.

[Explanation of symbols]

 10 ... Parallel illumination system 16 ... Main scale 18 ... First grating 20 ... Reference scale 22 ... Second grating P, Q ... Pitch v ... Lattice spacing 24 ... Photodetector a, b ... Detection signal λ ... Inclination amount 32 ... Laser diode (Diffuse light source)

Claims (1)

[Claims] Claims 1. A coherent diffuse illumination system having an effective wavelength λ, a main scale on which a first grating of a pitch P is formed, and a pitch Q corresponding to a 2n-th order (n is a natural number) harmonic of the first grating. And a light receiving element that photoelectrically converts the light from the illumination system filtered by the first and second gratings into a relative scale between the main scale and the reference scale. In an optical displacement detector that generates a detection signal that changes periodically according to the above, when the magnification of the optical system is M, the pitch Q is MP /
(2n), and the reference scale is Mm Q ² / λ (m with respect to the main scale so as to remove a variation due to the lattice spacing of the geometric image of the first lattice in the detection signal. Is a natural number), which is an optical displacement detector.