JPH08261729A - Scale inspecting apparatus - Google Patents

Abstract

PURPOSE: To provide a scale inspecting apparatus which can simply inspect the open area ratio of an optical grid formed on a scale used in an optical encoder in a short time. CONSTITUTION: The scale inspecting apparatus comprises photodetectors PD0, PD1, PD2 for photodetecting a plurality of diffraction lights transmitted or reflected at a scale 2 to be inspected in which a luminous flux from a coherence light source 1 such as an HeNe laser is incident, and a signal processor for obtaining an open area ratio based on the diffraction light photoelectrically converted by the photodetectors. The processor mounts an open area ratio calculator including a divider for obtaining primary and secondary diffraction light intensities for a zero-order diffraction light intensity and a relative intensity-open area ratio converter for calculating the open area ratio from the relative strengths of the primary and secondary diffraction lights in order to obtain the open area ratio by utilizing a phenomenon in which the intensity ratios of a plurality of the diffraction lights are different.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scale inspecting apparatus which is a constituent element of an optical encoder for detecting the position of a table in a machine tool or the like.

[0002]

2. Description of the Related Art FIG. 11 is a diagram showing a general optical encoder. The light emitted from the light source 201 is converted into parallel light by the collimator lens 202, and the main scale 203
Incident on. The light passes through the main scale 203 and then the index scale 204, and the photoelectric conversion element 2
It is incident on 05 and converted into an electric signal. Here, on the main scale 203 and the index scale 204,
An optical grating including a transparent portion and a non-transparent portion is provided. FIG. 12 is an example of an optical grating provided on the main scale 203.

When the main scale 203 provided with such an optical grating 206 is relatively displaced in the longitudinal direction of the scale, the intensity of light incident on the photoelectric conversion element changes. Therefore, the electric signal also changes according to the relative displacement. The optical encoder detects the position from this electric signal.

An electric signal that changes with respect to the relative displacement of the main scale 203 (hereinafter referred to as a displacement signal) also changes depending on the pitch of the optical grating 206 and the gap between the main scale 203 and the index scale 204, but normally, A sine wave signal as shown in FIG. 13 is generally used.

Here, in order to obtain a signal having a desired waveform as the displacement signal and a desired period, the ratio (aperture ratio) of the transparent portion and the non-transparent portion of the optical grating on the main scale and the index scale 204 is obtained. , And the pitch of the optical grating needs to be a predetermined design value. FIG. 14 is an enlarged view of the optical grating on the scale, in which the aperture ratio α is 0.4.
Thus, the optical grating is designed with a pitch P of 8 μm. Here, the aperture ratio represents the ratio of the transparent portion T and the non-transparent portion O of the lattice, and the aperture ratio α = transparent portion T / (transparent portion T +
It is defined by the non-transparent part O). Further, the pitch P is a cycle of repeating the transparent portion and the non-transparent portion. Although the aperture ratio is defined here for the transmission type diffraction grating, in the case of a reflection type diffraction grating consisting of repeating reflective and non-reflective parts, the transparent part corresponds to the reflective part, so the aperture ratio is The ratio α can be expressed as: reflection portion R / (reflection portion R + non-reflection portion B).

If the aperture ratio of the optical grating on the main scale is different from the designed value or if the pitch of the optical grating has an error compared with the designed value, the waveform of the displacement signal is disturbed or the displacement signal Accurate position detection cannot be performed because the pitch changes.

Therefore, it is necessary to inspect the aperture ratio and the pitch of the optical gratings on the main scale and the index scale.

Conventionally, when inspecting the aperture ratio and pitch of an optical grating, the pitch of the optical grating is as small as several μm to several tens of μm, and therefore, a part of the grating on the scale is measured by a magnifying projector or a measuring microscope. It was enlarged and the widths of the transparent part and the non-transparent part were measured and calculated.

[0009]

However, there have been the following problems in the prior art.

(1) With a microscope or the like, it is extremely troublesome to measure a long scale because the range that can be simultaneously observed is narrow.

In particular, in the case of inspecting a grating having a small pitch, it is necessary to increase the enlargement magnification for accurate measurement. However, conversely, the range that can be observed simultaneously becomes very narrow, and it takes a very long time to measure the required length. For example, in the case of an optical grating with a pitch of 10 μm,
Since there are 100,000 grids on a scale of 1 m length, it is difficult to actually perform the above-described measurement over the entire length.

(2) Accurate inspection cannot be performed if the width of the lattice varies.

As described above, it takes a lot of time to measure each grating by using a microscope or the like, and therefore some points on the scale must be inspected in a spotty manner. Accurate inspection cannot be performed if there is a variation in each line.

The present invention has been made to solve the above problems, and an object thereof is to reduce the aperture ratio of an optical grating formed on a scale used in an optical encoder in a short time and easily. It is to provide a scale inspection device that can inspect.

[0015]

To achieve the above object, the present invention receives a coherent light source and a plurality of diffracted light transmitted or reflected by a test scale on which a light beam from the coherent light source is incident. A signal processing device that includes a light receiver and a signal processing device that obtains an aperture ratio based on a plurality of diffracted light beams obtained by the light receiver device, wherein the signal processing device compares the intensities of the plurality of diffracted light beams to perform the test. It is characterized by including aperture ratio calculating means for determining the aperture ratio of the optical grating on the scale.

Further, the signal processing apparatus is characterized in that it includes grating pitch calculating means for finding the pitch of the optical grating on the test scale by detecting the incident position of the diffracted light.

Further, the signal processing device includes an optical axis shift calculating means for obtaining an optical axis shift of the 0th order diffracted light by detecting an incident position of the 0th order diffracted light in the diffracted light obtained by the light receiver. It is characterized by

Further, a plurality of light receiving surfaces are provided on the light receiving device, and light incident on each of the light receiving surfaces is photoelectrically converted separately.

[0019]

According to the scale inspection apparatus of the present invention, when the coherent light incident on the optical grating is transmitted or reflected and is divided into a plurality of diffracted lights by the diffraction phenomenon, the intensities of the plurality of diffracted lights are increased by the aperture ratio of the optical grating. The ratio is determined, and the diffraction angle of multiple diffracted lights is determined by the pitch of the optical grating.By utilizing the physical phenomenon, coherent measurement light is incident on the test scale, and the aperture ratio of the optical grating is determined based on the diffracted light. The pitch can be easily calculated.

[0020]

【Example】

Example 1. FIG. 1 is a diagram showing a first embodiment of the present invention. The measurement light emitted from the coherent light source 1 such as a HeNe laser is a test scale 2 installed on a movable table (not shown).
On the test scale 2 and is diffracted by the optical grating on the test scale 2 to be separated into a plurality of diffracted lights. At this time, the light that travels in the same direction as the measurement light is called 0th-order diffracted light, and the other diffracted lights are called 1st-order and 2nd-order diffracted lights in order from the smaller diffraction angle θ. The diffracted light has higher order such as third order, fourth order, fifth order, etc., but it is ignored here. The respective diffracted lights are incident on the light receiving surfaces PS0, PS1 and PS2 on the photodetectors PD0, PD1 and PD2, and the electric signals S0 and S0.
1, converted to S2. Here, the position of the light receiver will be described. Diffraction angle θ of each diffracted light diffracted by the optical grating
If the wavelength λ of the incident light and the pitch P of the optical grating are determined, 0 (= 0), θ1 and θ2 can be calculated by the following formula 1 when the diffraction order is n.

[0021]

[Equation 1] Therefore, the position of the light receiver that receives each diffracted light is set to the following Equation 2 (where D is the distance between the optical grating and the light receiver) from the point where the optical axis of the incident light hits the light receiving plane. do it.

[0022]

[Equation 2] The light-receiving surface of the light-receiver may be set to be slightly larger than the size of the incident light flux in consideration of the error of the incident wavelength and the slight error of the optical grating. It is also effective to design so that the position of the light receiver can be adjusted.

Here, the aperture ratio and the intensity of diffracted light of each order will be described.

As shown in FIG. 14, the intensity ratio of the diffracted light of each order diffracted by the optical grating formed by repeating the transparent portion and the non-transparent portion is determined by the aperture ratio of the optical lattice. Let α be the relative intensity In of the n-th order diffracted light
Is represented by Equation 3. The relative intensity of diffracted light is a value obtained by dividing the intensity of diffracted light of each order by the intensity of diffracted light of order 0.

[0025]

(Equation 3) FIG. 2 shows the relative intensities of diffracted light from the 0th order to the 5th order when the aperture ratios are 0.4, 0.5, and 0.6. FIGS. 3 and 4 show the aperture ratios, respectively. Is a diagram showing the relative intensity of the primary light and the relative intensity of the secondary light in the range of 0.3 to 0.7. As can be seen from these figures, the relationship between the aperture ratio and the diffracted light of each order has the following characteristics.

When the aperture ratio is 0.5, the even-order diffracted light intensity is 0.

The second-order diffracted light becomes 0 at an aperture ratio of 0.5, but the intensity changes sensitively to the aperture ratio at aperture ratios other than 0.

First-order diffracted light is inversely proportional to the aperture ratio.
(The value of the relative intensity of the primary light when the aperture ratio is 0.5 is about 0.
405) Based on the above-mentioned principle, the aperture ratio calculating unit 13 as the aperture ratio calculating means for calculating the aperture ratio from the electric signal obtained by the optical system having the configuration of FIG. 1 will be described. FIG. 5 shows a block diagram of the signal processing device 10 including the aperture ratio calculation unit 13.

The electrical signals S0, S1 and S2 photoelectrically converted by the light receiver of FIG. 1 are input to the aperture ratio calculation unit 13. In the aperture ratio calculator 13, first, the divider 11 calculates the 1st-order diffracted light intensity for the 0th-order diffracted light intensity (hereinafter referred to as the 1st-order light relative intensity) R1 = S1 / S0 and the 2nd-order diffracted light intensity for the 0th-order diffracted light intensity (hereinafter , Called secondary light relative intensity)
R2 = S2 / S0 is calculated.

Next, in the relative intensity-aperture ratio converter 12,
The aperture ratio is calculated from the relative intensity of the first-order diffracted light by the above-described relational expression of Equation 3 or the table showing the relation between the aperture ratio and the relative intensity. As can be seen from the relationship between the relative intensity of the primary light and the aperture ratio shown in FIG. 3, if the relative intensity of the primary light is known, the corresponding aperture ratio is specified, so that the aperture ratio can be easily determined.

As another method, the aperture ratio can be calculated by using the secondary light relative intensity which is more sensitive to the aperture ratio. However, when using the relative intensity of the secondary light, there are two aperture ratios corresponding to a certain relative intensity of the secondary light around the aperture ratio of 0.5 as shown in FIG. Depending on the strength, it is determined whether the aperture ratio is 0.5 or more and below, and then the accurate aperture ratio is determined.

The aperture ratio thus determined by the aperture ratio calculator 13 is output from the signal processing device 10.

By observing the output from the signal processing device 10 while moving the test scale 2 in the longitudinal direction by the scale inspection device constructed as described above, the aperture ratio according to the location of the test scale 2 can be continuously measured. Can be inspected for changes.

Embodiment 2 FIG. Next, a second embodiment according to the present invention will be described with reference to FIG. 6 differs from FIG. 1 showing the first embodiment in that the basic configuration is similar to that of FIG. 1, and therefore only the differences will be described. In FIG. 6, instead of the photodetector that photoelectrically converts the intensity of each diffracted light, a photodetector that can detect the intensity of the light incident on the light receiving surface and the position of the incident light is provided. This light receiver may be, for example, an image sensor such as a CCD or a light position detecting element (PSD).
It may be a unit using. In this embodiment, 0th order, 1st order,
Corresponding to the secondary light, the photodetectors PPI0, PP as described above
I1 and PPI2 are provided. From each of the light receivers, signals SI0, SI1, SI2 representing the intensity of light,
Signals SP0, SP1, and SP2 indicating the light incident position are output.

FIG. 7 shows a signal processing device 2 according to this embodiment.
It is a block diagram showing 0. In addition to the aperture ratio calculator 13 shown in the first embodiment, the signal processing device 20 according to the present embodiment further detects the incident position of the diffracted light to obtain the pitch of the optical grating on the test scale 2. The grating pitch calculation unit 14 as a calculation unit, and the optical axis as an optical axis shift calculation unit that obtains the optical axis shift of the 0th order diffracted light by detecting the incident position of the 0th order diffracted light among the diffracted light obtained by the light receiver It is characterized by having a shift calculation unit 15. The functions of this embodiment will be described below.

The signal SP1 representing the incident position of the first-order diffracted light output from the photodetector PPI1 that receives the first-order diffracted light is
It is input to the grating pitch calculator 14 to calculate and output the pitch corresponding to the incident position. If the incident position of the first-order diffracted light deviates from the design value with respect to the second-order diffracted light, it indicates that the first-order diffracted light has a larger diffraction angle than the designed value. That is,
It can be seen that the pitch of the test scale 2 is smaller than the design value.

On the other hand, a photodetector PPI for receiving the 0th order diffracted light
Signal SP representing the incident position of 0th-order diffracted light output from 0
0 is input to the optical axis shift calculation unit 15. If the incident position of the 0th-order diffracted light is deviated from the design value, it is considered that the optical axis of the measurement light is deviated or the test scale 2 is not installed correctly. By checking, it is possible to evaluate the reliability of the grating pitch detection itself.

Signals SI0, SI1, SI2 representing the intensities of the diffracted lights of the respective orders output from the respective light receivers.
Is input to the aperture ratio calculator 13, and the aperture ratio of the test scale 2 can be calculated as in the first embodiment.

Further, in the present embodiment, if the measurement is carried out by a calibration scale in which the grating pitch is accurately measured in advance at the time of inspection, an error occurs in the pitch measurement due to a change in the diffraction angle due to an error in the wavelength of the measuring light, for example. To prevent
It is possible to maintain a more reliable inspection function.

By observing the output from the signal processing device 20 while moving the test scale 2 in the longitudinal direction by the scale inspection device configured as described above, the aperture ratio according to the location of the test scale 2 can be continuously measured. It is possible to inspect changes in the grid pitch.

In the present embodiment, a light receiver for receiving diffracted light of each order is provided for each diffracted light. For example, one image sensor is used to detect the intensity and incident position of each order. It is also possible to do so.

Example 3. Next, a third embodiment according to the present invention will be described with reference to FIG. 8 differs from FIG. 1 showing the first embodiment in that the basic configuration is similar to that of FIG. 1, and therefore only the differences will be described. In FIG. 8, the light receiving surface of the light receiver that photoelectrically converts each diffracted light is divided into two, and the electric signals photoelectrically converted by the two light receiving surfaces can be separately taken out. For example, two light receiving surfaces PS1a and PS1b are provided on a light receiver PC1 that receives and photoelectrically converts the first-order diffracted light, and electric signals S1a and S1b are output from each light receiving surface. The photodetectors PC0 and PC2 for receiving the 0th-order diffracted light and the 2nd-order diffracted light are also provided with a light-receiving surface divided into two.

FIG. 9 shows an example of the light receiver used in FIG. 8 and having a light receiving surface divided into two parts. Two light receiving surfaces P on the light receiver PC
Sa and PSb are arranged so that the light beam LF of the diffracted light enters the two light receiving surfaces. If the pitch of the optical grating is as designed, the two light-receiving surfaces PS
a and PSb are incident on the two light receiving surfaces PS by the same amount of light.
The position of the photodetector PC is set so that the electric signals from a and PSb are equal. When the measurement light is made incident on the test scale 2 having an error with respect to the design value by the scale inspection device having such a configuration, the two light receiving surfaces PS1
a, the amount of light incident on PS1b becomes unbalanced,
The resulting electrical signal will also be unbalanced. The light-receiving surfaces PS1a and PS1b preferably have the same area in consideration of the manufacture of the light-receiving device PC, but as described above, the electric signals from the light-receiving surfaces PSa and PSb are equal when the design values are met. Since it suffices to arrange the photodetector PC so that
The areas do not necessarily have to be the same.

FIG. 10 is a block diagram showing the signal processing device 30 in this embodiment. The signal processing device 30 according to the present embodiment is the aperture ratio calculating unit 13, the grating pitch calculating unit 14, and the optical axis deviation calculating unit 1 shown in the second embodiment.
In addition to 5, the adder 16 is further provided on the input side of the aperture ratio calculator 13. The adder 16 sums the signals input from the respective light receiving surfaces for each light receiving device and outputs the summed signals to the aperture ratio calculation unit 13. The functions of this embodiment will be described below.

2 on the photodetector PC1 for receiving the first-order diffracted light
The electric signals S1a and S1b from one light receiving surface are input to the grating pitch calculation unit 14 to calculate and output a pitch corresponding to the imbalance of the two electric signals. If the electric signal S1b is larger than the electric signal S1a, it means that the first-order diffracted light has a larger diffraction angle than the design value. That is,
It can be seen that the pitch of the test scale 2 is smaller than the design value. The relationship between the unbalance amount and the pitch may be measured in advance.

On the other hand, a photodetector PC0 for receiving the 0th order diffracted light
The electrical signals S0a and S0b from the upper two light receiving surfaces are input to the optical axis shift calculation unit 15. If an imbalance occurs between the electrical signals S0a and S0b, it is possible that the optical axis of the measurement light is misaligned or the test scale 2 is not installed correctly. The reliability of the grating pitch detection itself can be evaluated by checking the unbalanced amount.

Further, each of the light receivers PC0, PC1,
The signals input from the respective light-receiving surfaces of the PC 2 are summed for each light-receiver by the adder 16 and input to the aperture ratio calculation unit 13, and the aperture ratio of the test scale 2 is calculated as in the first embodiment. can do.

Further, in the present embodiment, if the measurement is carried out by a calibration scale in which the grating pitch is accurately measured in advance at the time of inspection, an error occurs in the pitch measurement due to a change in the diffraction angle due to an error in the wavelength of the measuring light, for example. To prevent
It is possible to maintain a more reliable inspection function.

Further, here, the light receiving surface on the light receiving device PC is 2
The example has been described that is divided into two. In this case, the luminous flux LF
The position of the light flux LF can be accurately detected when the light flux is straddling two light-receiving surfaces, but when the light flux LF is greatly deviated from the ideal position and is completely included in either light-receiving surface, The position of LF is not known exactly (dead zone). If it is desired to detect the accurate position of the light flux even if the light flux LF deviates largely from the ideal position, the dead band can be obtained by increasing the number of divisions and making the width of one light receiving surface smaller than the size of the light flux. It is possible to detect the exact position of the light flux.

By observing the output from the signal processing device 30 while moving the test scale 2 in the longitudinal direction with the scale inspecting device configured as described above, the aperture ratio according to the location of the test scale 2 can be continuously measured. It is possible to inspect changes in the grid pitch.

In the above embodiment, the optical grating formed by repeating the transparent portion and the non-transparent portion, that is, the optical grating having only two values of transmittance depending on the location is described. The present invention can be applied to an optical grating that continuously changes and a reflection type optical grating. It can also be applied to a scale having a radial optical grating such as used in a rotary encoder.

[0052]

According to the scale inspection apparatus of the present invention, by moving the test scale installed in the scale inspection apparatus, the aperture ratio of the optical grating is determined based on the intensity or incident position of the diffracted light obtained by the light receiver. Since the pitch and pitch can be continuously inspected, the inspection time can be remarkably shortened as compared with the case of measuring the lattice dimensions one by one with a microscope or the like.

Further, unlike measurement by a microscope or the like, since the size of the luminous flux of the measurement light can be matched with the size of the luminous flux used by the actual encoder, the optical characteristics similar to those obtained by the actual encoder can be obtained. Can be inspected.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of a first embodiment of a scale inspection device according to the present invention.

FIG. 2 is a diagram showing the relative intensity of diffracted light of each order.

FIG. 3 is a diagram showing a relative intensity of primary light.

FIG. 4 is a diagram showing a secondary light relative intensity.

FIG. 5 is a block diagram illustrating a signal processing device according to a first embodiment.

FIG. 6 is a schematic diagram showing the configuration of a second embodiment of the scale inspection device according to the present invention.

FIG. 7 is a block diagram illustrating a signal processing device according to a second embodiment.

FIG. 8 is a schematic diagram showing the configuration of a third embodiment of the scale inspection apparatus according to the present invention.

FIG. 9 is a schematic view showing a light receiver used in a third embodiment.

FIG. 10 is a block diagram illustrating a signal processing device according to a third embodiment.

FIG. 11 is a schematic diagram showing a configuration of a general optical encoder.

FIG. 12 is a schematic view of an optical grating on the main scale.

FIG. 13 is a diagram showing a displacement signal obtained from a general optical encoder.

FIG. 14 is an enlarged view of the optical grating on the scale.

[Explanation of symbols]

1 coherent light source, 2 test scale, 10, 20, 30
Signal processing device, 11 divider, 12 relative intensity-aperture ratio converter, 13 aperture ratio calculation unit, 14 grating pitch calculation unit, 15 optical axis deviation calculation unit, 16 adder, 201 light source, 202 collimator lens, 203 main scale , 204 index scale, 205 photoelectric conversion element, 206 optical grating, PD0, PD1, PD2, P
C0, PC1, PC2, PPI0, PPI1, PPI2
Light receiver.

Claims (4)

[Claims] 1. A coherent light source, a photoreceiver for receiving a plurality of diffracted lights transmitted or reflected by a test scale on which a light flux from the coherent light source is incident, and a plurality of diffracted lights obtained by the photoreceiver. A signal processing device for obtaining an aperture ratio based on the above, wherein the signal processing device includes an aperture ratio calculating means for obtaining an aperture ratio of the optical grating on the test scale by comparing intensities of a plurality of diffracted lights. A scale inspection device characterized in that 2. The scale inspecting apparatus according to claim 1, wherein the signal processing device includes a grating pitch calculating unit that detects a pitch of an optical grating on the test scale by detecting an incident position of diffracted light. Scale inspection device characterized by. 3. The scale inspection device according to claim 1, wherein the signal processing device detects the 0th-order diffracted light by detecting the incident position of the 0th-order diffracted light among the diffracted light obtained by the light receiver. A scale inspecting device, comprising: an optical axis deviation calculating means for obtaining an optical axis deviation. 4. The scale inspection apparatus according to claim 1, wherein a plurality of light receiving surfaces are provided on the light receiver, and light incident on each of the light receiving surfaces is photoelectrically converted separately. A scale inspection device characterized in that