JP2017161294A - Displacement measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a displacement measurement device with which it is possible to maintain desired measurement accuracy even when a tilt around the optical axis of a diffraction grating occurs.SOLUTION: A displacement measurement device 100 comprises a light source 12, a diffraction grating 20 on which a light from the light source 12 is incident, a photodetector 30 for receiving an interference light obtained as the light passes through the diffraction grating, and an output unit 40 for outputting a relative displacement of the diffraction grating 20 and the photodetector 30 on the basis of a signal obtained by the photodetector 30. Since only one of the diffraction grating 20 is provided, there is no reduction in the accuracy of displacement measurement even when a tilt around the optical axis occurs in relation to the diffraction grating 20 and the photodetector 30. This means that desired measurement accuracy can be maintained.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a displacement measuring apparatus using optical interference.

  Patent Document 1 discloses a displacement measuring device including a diffraction grating pair composed of a first diffraction grating close to the light source side and a second diffraction grating facing the first diffraction grating. In the diffraction grating pair, + n-order diffracted light generated from the second diffraction grating when + n-order diffracted light from the first diffraction grating is incident on the second diffraction grating and -n-order diffracted light from the first diffraction grating are applied to the second diffraction grating. Interference light with + n-order diffracted light that is incident and generated from the second diffraction grating is generated. The displacement measuring device receives the interference light by a PD that is a light receiving unit, and outputs a relative displacement between the first diffraction grating and the second diffraction grating based on the received light signal (see, for example, Patent Document 1).

JP-A-2015-21890

  The apparatus of Patent Document 1 has a problem that the measurement accuracy decreases when a twist (tilt) around the optical axis occurs in the first diffraction grating and the second diffraction grating.

  An object of the present invention is to provide a displacement measuring apparatus that can maintain a desired measurement accuracy even when a tilt around the optical axis of a diffraction grating occurs.

In order to achieve the above object, a displacement measuring apparatus according to the present invention includes a light source, a diffraction grating, a light receiving unit, and an output unit.
The light from the light source is incident on the diffraction grating. The light receiving unit receives interference light obtained by transmitting the light through the diffraction grating.
The output unit outputs a relative displacement between the diffraction grating and the light receiving unit based on a signal obtained by the light receiving unit.

  In this displacement measuring apparatus, since only one diffraction grating is provided, even if a tilt around the optical axis occurs relative to the diffraction grating and the light receiving portion, there is little reduction in displacement measurement accuracy. That is, according to the present invention, desired measurement accuracy can be maintained.

The light receiving unit may be configured to receive interference light by + n-order diffracted light and -n-order diffracted light, where n is a natural number among diffracted light emitted from the diffraction grating.
This facilitates optical design of the displacement measuring device.

The output unit may acquire a sine wave signal directly output from the light receiving unit.
Thereby, it is not necessary to intentionally form a sine wave signal, and it is not necessary to use an expensive IC as the arithmetic unit.

  The diffraction grating has a width of one bright part in the arrangement direction of the bright part and the dark part among the plurality of bright parts and the plurality of dark parts constituting the interference fringes of the interference light. You may have the grating | lattice pitch set so that it might become more than a width | variety.

  The light receiving unit is composed of a plurality of light receiving units, and the grating pitch is set such that the width of the one bright portion in the arrangement direction is equal to or larger than the entire width of the plurality of light receiving units in the arrangement direction. May be.

The light receiving unit may be arranged such that a light receiving surface is located at a position where optical axes of two diffracted lights forming the interference light intersect.
Thereby, since a light-receiving part can obtain a high intensity | strength signal, it can raise SN ratio and can improve a measurement precision.

  As described above, according to the present invention, a desired measurement accuracy can be maintained even when a tilt around the optical axis of the diffraction grating occurs.

FIG. 1 is a diagram schematically showing a basic optical system configuration of a displacement measuring apparatus according to the first embodiment of the present invention. It is the figure which overlapped and showed the light-receiving part (light-receiving surface) of PD, and the interference light which injects into it. FIG. 3A shows a Lissajous waveform for this embodiment, and FIG. 3B shows a Lissajous waveform for apparatus A. FIG. 4 is a diagram showing an optical system for explaining the positional relationship between the diffraction grating and the PD. FIG. 5A shows a PD having a plurality of light receiving parts, for example, three light receiving parts, in the displacement measuring apparatus according to the second embodiment of the present invention. FIG. 5B shows a signal (signal input to the output unit) obtained by the PD shown in FIG. 5A. FIG. 6A shows a variation example of the grating pitch of the diffraction grating and the size of the light receiving portion of the PD. FIG. 6B shows signals obtained by these light receiving units. FIG. 7A shows a variation example of the grating pitch of the diffraction grating and the size of the light receiving portion of the PD. FIG. 7B shows signals obtained by these light receiving units. FIG. 8 is a diagram showing a displacement measuring apparatus according to the third embodiment of the present invention.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings.

  1. First embodiment

  1.1) Displacement measuring device

  FIG. 1 is a diagram schematically showing a basic optical system configuration of a displacement measuring apparatus according to the first embodiment of the present invention.

  The displacement measuring apparatus 100 includes a light source 12, a collimator lens 14, a diffraction grating 20, a PD (Photo Detector) 30 functioning as a light receiving unit, and an output unit 40.

  The light source 12 is an LD (Laser Diode) or an LED (Light Emitting Diode), and is driven by a driver (not shown).

  The collimator lens 14 turns the light emitted from the light source 12 into parallel light 15. At least the light source 12 and the collimator lens 14 constitute an optical system that generates parallel light. An aperture may be provided between the light source 12 and the collimator lens 14.

  The diffraction grating 20 is a transmission type. Light from the light source 12 and the collimator lens 14 enters the diffraction grating 20 and emits a plurality of diffracted lights. Among a plurality of diffracted lights (± nth order diffracted light, n is a natural number), for example, interference light is generated by + nth order diffracted light and −nth order diffracted light, and the PD 30 is arranged at a position for receiving the interference light. Typically, the PD 30 receives interference light caused by + 1st order diffracted light and −1st order diffracted light.

  For example, the interference light by the + 1st order and + 2nd order diffracted light, the interference light by the −1st order and −2nd order diffracted light, or the interference light by ± 2nd order diffracted light may be received by the PD 30. That is, it is only necessary to use diffracted light such that the optical axes of the diffracted light emitted from the diffraction grating 20 intersect at a position away from the diffraction grating 20. Actually, there are many diffracted lights other than those shown in FIG. 1, but they are not shown for easy explanation.

  The diffraction grating 20 has a plurality of grating lines which are grooves along the y direction in FIG. The pitch of the lattice lines (lattice pitch) P is not particularly limited, but is, for example, 50 μm to 500 μm. More preferably, it is set to 100 μm to 300 μm.

  The displacement measuring apparatus 100 according to the present embodiment uses the translational displacement of the diffraction grating 20 in the x direction, which is the arrangement direction of these grating lines, that is, the relative displacement between the diffraction grating 20 and the PD 30 in the x direction.

  In this specification, two axes orthogonal to the main optical axis direction (z direction) are defined as x and y axes. As described above, the direction along the grating lines of each diffraction grating 20 is the y direction, and the arrangement direction of the grating lines is the x direction.

  The light source 12, the collimator lens 14, and the diffraction grating 20 are integrally supported by a support structure 10 such as a housing or a frame, and can be moved integrally in the x direction with respect to the PD 30. The PD 30 is also supported by another support structure 13 such as a housing or a frame. These support structures 10 and 13 are connected by a guide and an elastic member (not shown), for example, and are configured to be relatively movable in the x direction.

  For example, the PD 30 may be supported integrally with an IC that constitutes the output unit 40.

  The output unit 40 outputs a displacement by processing a signal obtained by the PD 30. The output unit 40 is typically composed of an analog IC or a digital IC.

  When the output unit 40 includes a digital circuit, the output unit 40 mainly includes hardware such as a CPU (Central Processing Unit), a RAM (Random Access Memory), and a ROM (Read Only Memory). The output unit 40 may include a PLD (Programmable Logic Device) in addition to or instead of the CPU, or may include a DSP (Digital Signal Processor). The output unit 40 may be composed of a plurality of physically separated chip packages, elements, and the like.

  FIG. 2 is a diagram in which the light receiving portion (light receiving surface) of the PD 30 and the interference light incident thereon are overlapped. The light receiving surface of the PD 30 has a rectangular shape, for example. The interference light is configured, for example, by arranging bright portions and dark portions alternately in the x direction. The bright part pitch a and the bright part width b of the interference light are values determined by the grating pitch of the diffraction grating 20. The diffraction grating 20 according to the present embodiment has a grating pitch (for example, 50 μm to 500 μm described above) set so that the width b of the bright portion is equal to or larger than the width c of the PD 30 in the x direction.

  The intensity signal of the interference light obtained by the PD 30 according to the movement of the diffraction grating 20 becomes a sine wave signal. Therefore, the output unit 40 acquires a sine wave signal output directly from the PD 30.

  In the present embodiment, a one-plate method is adopted. That is, the displacement measuring apparatus 100 according to this embodiment uses one diffraction grating 20 to measure the relative displacement between the diffraction grating 20 and the PD 30.

  On the other hand, the general one-plate system uses one plate having a plurality of slits like a linear encoder or a rotary encoder. Then, the optical sensor receives transmitted light (not interference light) from the slit, and the relative displacement between the plate and the optical sensor is measured. In this case, the signal obtained by the optical sensor is a triangular wave signal.

  However, when processing a triangular wave signal, a high frequency band is required for processing the apex portion of the triangular wave, so that circuit design becomes difficult or an expensive IC is required. Therefore, in general, it is necessary to add an optical element so that a sine wave signal can be obtained by an optical sensor, or to generate a sine wave signal by a circuit (calculation) from a triangular wave signal from the optical sensor.

  On the other hand, since the present embodiment uses the interference light obtained by the diffraction grating 20, it is possible to output a sine wave intensity signal from the PD 30. And the output part 40 can acquire the sine wave signal, can convert it into a displacement, and can output it.

  Note that the output unit 40 can convert the displacement into displacement based on the acquired intensity signal, for example, by an increment method or a lookup table method.

  As described above, since the displacement measuring apparatus 100 has a structure in which only one diffraction grating 20 is provided, even if a tilt around the optical axis occurs relative to the diffraction grating 20 and the PD 30, the displacement measurement apparatus 100 has a displacement. There is little decrease in measurement accuracy. That is, according to this embodiment, desired measurement accuracy can be maintained.

  In the present embodiment, since the structure of the optical system is simple, the manufacture of the optical system is facilitated.

  1.2) Repeated measurement accuracy when tilt occurs

  The inventors tilt the displacement measuring apparatus 100 according to the present embodiment and the displacement measuring apparatus (hereinafter referred to as apparatus X) shown in the first embodiment (FIG. 1) of Patent Document 1 above. The accuracy of repeated measurement in the state where the error occurred was compared by simulation. The repeated measurement is to acquire a sine wave signal for a plurality of periods corresponding to the relative displacement within the maximum range of the relative displacement between the diffraction grating 20 and the PD 30, for example. The sine wave signal acquired for each tilt angle is converted into a Lissajous waveform as shown in FIGS.

  FIG. 3A shows a Lissajous waveform for this embodiment, and FIG. 3B shows a Lissajous waveform for device X. Here, in the present embodiment, the tilt angle is set to 0 °, 1 °, and 5 °, and in the apparatus X, the tilt angle is set to 0 ° and 1 °.

  In this embodiment, as shown in FIG. 3A, it can be seen that there is no change in the Lissajous waveform at three tilt angles (all Lissajous waveforms are almost overlapped and coincide), and there is no degradation in the repeated measurement accuracy. On the other hand, in the apparatus X, as shown in FIG. 3B, it can be seen that when the tilt occurs, the repeated measurement accuracy is lowered. The closer the Lissajous waveform is to a perfect circle, the higher the repeat measurement accuracy.

  The tilt tolerance of the device X is −1 ° to + 2 °, while the tilt tolerance of the displacement measuring device 100 is, for example, ± 5 ° or more, and the tilt tolerance is high.

  1.3) Measurement scale accuracy (resolution)

  However, the accuracy of the measurement scale of the displacement measuring apparatus 100 according to the present embodiment, that is, the resolution, is lower than that of the apparatus X. The resolution is determined by the grating pitch. Specifically, from the diffraction equation sin θ = mλ / d, the larger the grating pitch d, the smaller the incident angle θ and the width of the interference light (for example, the bright portion width b shown in FIG. 2). Becomes lower. That is, the resolution has a trade-off relationship with the grating pitch.

  1.4) Arrangement relationship between diffraction grating and PD

  FIG. 4 shows an optical system for explaining the positional relationship between the diffraction grating 20 and the PD 30. It is optimal that the light receiving surface of the PD 30 is arranged at a position (b) where the optical axes of the two diffracted lights emitted from the diffraction grating 20 intersect.

  The degree of overlap of the diffracted light beams at positions (a) to (c) is shown below FIG. Usually, since the light beam shows the intensity distribution of the Gaussian beam, the light intensity at the center of the light beam, that is, the optical axis is the highest. At positions (a) and (c), the two diffracted light beams intersect, but the intensity is lower than at position (b).

  By arranging the PD 30 at the position (b), a signal with as high an intensity as possible can be obtained, the SN ratio can be increased, and the measurement accuracy can be increased.

  2. Second embodiment

  Next, a displacement measuring apparatus according to the second embodiment of the present invention will be described. In the following description, elements that are substantially the same with respect to members, functions, and the like included in the displacement measuring apparatus 100 according to the first embodiment are denoted by the same reference numerals, and the description thereof is simplified or omitted, and is different. The explanation will be focused on.

  2.1) Configuration of displacement measuring device

  FIG. 5A shows a PD 35 having a plurality of light receiving parts, for example, three light receiving parts 35A, 35B, and 35C. The arrangement direction of these light receiving portions 35A, 35B, and 35C is the x direction that is the direction in which the grating lines of the diffraction grating 20 (see FIG. 1) are arranged. The diffraction grating 20 according to the present embodiment has a grating pitch set so that the width b of the bright part is equal to or larger than the entire width e of the light receiving parts 35A, 35B, and 35C. The grating pitch was set to 1600 μm.

  FIG. 5B shows a signal (signal input to the output unit 40) obtained by the PD 35 of the displacement measuring apparatus configured as described above. The horizontal axis represents displacement (relative displacement between the diffraction grating 20 and the PD 35), and the vertical axis represents brightness (light intensity). Signals A, B, and C are obtained by the three light receiving portions 35A, 35B, and 35C, respectively. Signals A and C are 180 ° out of phase. Signal B is 90 ° out of phase with respect to A and C.

  In FIG. 5B, an A + C signal is also included as a DC signal. For example, when the output unit 40 calculates the sum of the signals A and C having phases different by 180 °, the DC signal is obtained. For example, this DC value includes the intensity of the 0th-order light that travels straight out from the diffraction grating 20. The output unit 40 can monitor the state of the light source, for example, power fluctuation, deterioration with time, and the like by monitoring the DC signal.

  2.2) Variations in grating pitch and light receiving area size of diffraction grating

  6A shows an example in which the grating pitch of the diffraction grating 20 is made smaller than that in FIG. 5A, and the size of each light receiving portion of the PD 36 is made smaller than that in FIG. 5A. The grating pitch was set to 1000 μm.

  FIG. 6B shows a signal obtained by the PD 36 shown in FIG. 6A. As shown in FIG. 6B, the resolution increases as the grating pitch is reduced.

  As described above, even if the size of the light receiving unit is sufficiently reduced, each light receiving unit can obtain an appropriate signal although there is some signal disturbance.

  FIG. 7A shows an example in which the grating pitch of the diffraction grating 20 is made larger than that in FIG. 5A, and the size of each light receiving portion of the PD 37 is made larger than that in FIG. 5A. The grating pitch was set to 2000 μm.

  FIG. 7B shows a signal obtained by the PD 37 shown in FIG. 7A. As shown in FIG. 7B, the resolution is lowered as the grating pitch is increased. As for the signals C and A + C, only signals having a displacement in the range of about 0 to 600 μm can be used, and measurement is impossible at 600 μm or more.

  3. Third embodiment

  FIG. 8 is a diagram showing a displacement measuring apparatus according to the third embodiment of the present invention. In this displacement measuring apparatus, for example, the light source 12, the collimator lens 14, and the PD 30 are integrally supported by a support structure 11 such as a housing or a frame. The diffraction grating 20 is supported by another support structure 21. These support structures 11 and 21 are relatively movable.

  The displacement measuring device having such a configuration acts in the same manner as the displacement measuring device 100 according to the above embodiment, and can obtain the same effect.

  4). Various other forms

  The present invention is not limited to the embodiment described above, and other various embodiments can be realized.

  For example, the diffraction grating 20 described above is a transmissive type, but a reflective type may be used. In that case, interference light may be guided to the PD by using a mirror or another optical element. As shown in FIG. 1, a transmissive diffraction grating 20 may be used. By using a mirror or another optical element, the optical path may be bent halfway.

  For example, the diffraction grating 20 has grating lines arranged only in the x direction, but in addition to this, the diffraction grating 20 may have grating lines arranged in the y direction. In this case, at least the PD has two or more light receiving portions that respectively receive displacements in the x and y directions, and the output portion outputs signals corresponding to the displacements in the two directions, respectively.

  It is also possible to combine at least two feature portions among the feature portions of each embodiment described above.

DESCRIPTION OF SYMBOLS 12 ... Light source 20 ... Diffraction grating 30,35,36,37 ... PD (light-receiving part)
40 ... Output unit 100 ... Displacement measuring device

Claims (6)

A light source;
A diffraction grating on which light from the light source is incident;
A light receiving unit that receives interference light obtained by transmitting the light through the diffraction grating, and an output unit that outputs a relative displacement between the diffraction grating and the light receiving unit based on a signal obtained by the light receiving unit. Displacement measuring device provided. The displacement measuring apparatus according to claim 1, wherein the light receiving unit receives interference light by + n-order diffracted light and −n-order diffracted light, where n is a natural number among diffracted light emitted from the diffraction grating. The displacement measuring apparatus according to claim 1, wherein the output unit acquires a sine wave signal directly output from the light receiving unit. The diffraction grating has a width of one bright part in the arrangement direction of the bright part and the dark part among the plurality of bright parts and the plurality of dark parts constituting the interference fringes of the interference light. The displacement measuring device according to any one of claims 1 to 3, wherein the displacement pitch is set to be equal to or greater than the width. The light receiving unit is composed of a plurality of light receiving units,
The displacement measuring apparatus according to claim 4, wherein the lattice pitch is set such that a width of the one bright portion in the arrangement direction is equal to or greater than an entire width of the plurality of light receiving portions in the arrangement direction. The displacement measuring device according to any one of claims 1 to 5, wherein the light receiving unit is disposed such that a light receiving surface is positioned at a position where optical axes of two diffracted lights forming the interference light intersect. .