JP6641650B2 - Absolute grid scale - Google Patents

Description

  The present invention relates to the field of metrology, and more particularly to an absolute grid scale.

  The main grid of the absolute grid scale is provided with two code bars, one of which is an incremental code bar, grid lines for measuring incremental displacement are imprinted at equal intervals, and the other code bar has a grid scale. At the time of activation, a plurality of reference point marks for specifying the position of the reference point are imprinted. Combining the measurement of the incremental displacement with the localization of the reference point enables an absolute measurement on a grid scale. Currently, most grid scales on the market use read heads that measure by reflection, and the resulting reference point signal is a negative pulse signal, which generally results in lower signal strength at the peak, Even a slight interference from an external optical signal greatly reduces the contrast of the signal, so that it is difficult to accurately specify the position of the reference point.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an absolute grid scale in which the contrast of a reference signal is effectively improved in order to solve the disadvantages of the prior art.

  The absolute grating scale according to the present invention includes a main grating and a read head device. The read head device includes an incremental displacement measuring unit. The read head device further includes a first beam splitter, a mask plate, and a reference position photoelectric detector. The main lattice is provided with a plurality of reference code bars, and a distance between any two adjacent reference code bars is different from a distance between any remaining two adjacent reference code bars. The first beam splitter divides the light emitted from the light source into a beam emitted to the main grating and a beam emitted to the incremental displacement measurement unit, and the beam emitted to the main grating passes through the mask plate. After reaching the main grating and being reflected, the light passes through the mask plate again and is received by the reference position photoelectric detector. The same code bar as the reference code bar is provided on the mask plate, and the mask plate is configured such that a beam emitted to the mask plate is not received by the reference position photoelectric detector after the beam emitted from the mask plate is reflected by the mask plate. Provided.

  Preferably, a beam emitted to the mask plate and a normal line of the mask plate form an acute angle, that is, the mask plate is shifted from a state parallel to the main grating by a predetermined angle around the θx direction in FIG. In this configuration, the beam emitted to the mask plate and the normal line of the mask plate form an acute angle.

  To increase the contrast of the pulse signal, the read head mask plate is configured to rotate at a small angle about a direction perpendicular to the slit. Thereby, the distribution of the light field after the beam has transmitted through the mask plate is identical to that before transmission, and the light reflected by the mask plate itself is deflected without being received by the photoelectric detector. The contrast of the pulse signal can be effectively increased, and the recognition accuracy for the reference position (reference code bar) can be improved.

  Preferably, the acute angle is less than 5 °.

  Preferably, a light transmitting portion and a light reflecting portion are provided on the code bar of the mask plate.

  Preferably, a beam emitted to the mask plate is larger than a width of a code bar of the mask plate.

  Preferably, the code composed of the code bar is 01100001000000000010011000011010000000100000000000100100000001000000010100000110001000001001000001110, 1 represents a light transmitting portion, and 0 represents a light reflecting portion.

  Preferably, each width of the light transmitting portion and the light reflecting portion is 10 μm.

  It can be aligned with the reference position by the peak of the pulse signal. The peak width of the pulse signal is proportional to the line width of the code bar. If the line width of the code bar is set to 10 μm, the obtained peak width is 26 μm. Therefore, the position of the reference point can be specified with an accuracy of 0.6 μm. realizable.

  The code bar is designed in consideration of an actual diffraction effect. Although the minimum slit width is 10 μm, which is far beyond the wavelength of 660 nm, even a slight diffraction effect significantly weakens the pulse peak of the reference signal, so that it is necessary to consider the influence from the diffraction effect. With such an optical path structure, the first diffraction occurs after the beam has passed through the mask plate of the read head, and this diffraction is treated as Fresnel diffraction. When the beam reaches the reference point region of the main grating, the reflected light undergoes a second Fresnel diffraction and is reflected by the read head mask plate, and then associates the light field of the second Fresnel diffraction with the slit structure of the mask plate. This makes it possible to obtain the waveform of the pulse signal for specifying the position of the reference point.

The present invention has the following advantages by being configured as described above.
1. Since the mask plate provided at an angle can reflect light reflected by itself to a region other than the photoelectric detector, the contrast of the reference signal can be increased, and the accuracy of specifying the position of the reference point can be improved.
2. With a 100-bit random code designed in consideration of the actual diffraction effect, a pulse signal having a peak width of 26 μm can be obtained, and the position of the reference point can be specified with an accuracy of 0.6 μm.

FIG. 4 is a schematic diagram of the principle of an absolute grid scale according to some embodiments of the present invention. FIG. 2 is a partial schematic diagram of FIG. 1. FIG. 7 is an incremental code bar of a main grid and its reference code bar according to some embodiments of the present invention. 4 is a code bar in a mask plate of a read head according to some embodiments of the present invention. 9 is a reference pulse signal according to some embodiments of the present invention.

  Hereinafter, preferred embodiments of the present invention will be described in detail.

  As shown in FIGS. 1 and 2, the absolute grating scale according to one embodiment includes a main grating 15 and a read head device. The read head device includes an incremental displacement measuring unit 23 and a reference position measuring unit 22. The reference position measuring unit 22 includes a first beam splitter 12, a mask plate 13, and a reference position photoelectric detector 14. The main grid 15 is provided with a plurality of reference code bars 2.

  When the light source 11 (for example, a laser diode) emits red laser light L0 having a wavelength of 660 nm, passes through a collimating lens to become a parallel beam, and then passes through an aperture stop, the beam is shaped into a beam having a diameter of 1.2 mm. . The first beam splitter (energy beam splitter) 12 splits the light emitted from the light source 11 into two laser lights. One of them is refracted by 90 ° and exits to the main grating 15, and the other passes through the first beam splitter 12 and exits to the incremental displacement measuring unit 23, and detects the incremental displacement that the read head moves with respect to the main grating 15. Measure. The beam L1 emitted to the main grating passes through the mask plate 13 and reaches the main grating 15, and is reflected by the main grating 15 and then passes through the mask plate 13 again (that is, the beam L2), and the first beam is emitted. The light passes through the beam splitter 12 and is received by the reference position photoelectric detector 14.

  The incremental displacement measuring unit 23 may be configured as generally used in the related art, and measures an incremental displacement that moves with respect to the movement of the main grating 15 of the read head.

  The mask plate 13 includes a series of light transmitting portions 8 (for example, white slits shown in FIG. 4) and non-transmitting portions 7 (for example, black lines shown in FIG. 4). A corresponding code bar is provided.

  The incremental displacement measuring section 23 includes an interference optical path, and the laser beam incident on the incremental displacement measuring section 23 is further split into two beams by the second beam splitter 16. One of them is emitted to the reference grating 19 and the other is emitted to the main grating 15. The + N-order diffracted light of the reference grating and the + N-order diffracted light of the main grating form interference fringes. The second-order diffracted light is diffracted in the beam after passing through the mirror 20 and the second beam splitter 16), and the −Nth-order diffracted light of the reference grating and the Nth-order diffracted light of the main grating form interference fringes (for example, of the reference grating 19). After the -1st-order diffracted light is reflected by the mirror 21 and the second beam splitter 16, the -1st-order diffracted light of the main grating 15 is diffracted in the beam transmitted through the mirror 25 and the second beam splitter 16). When the read head is displaced in the longitudinal direction of the main grating, the Doppler effect causes a single light-dark change in the interference fringes. Each time the read head moves by one grating period, a light-dark change is observed in the interference fringes. The optical path is also provided with a plurality of photoelectric detectors 17 and 18 for detecting a change in the intensity of light in the interference fringes. By reading the number of periodic light-dark changes in the interference fringes, the read head is read. Can be calculated.

  As shown in FIG. 3, the main grid 15 is provided with a plurality of reference code bars 2 and the rest are grid lines, that is, the incremental code bars 1. In the incremental code bar 1, grating lines having a period of 1 μm are provided at equal intervals, and a reflection type holographic diffraction grating is used as the grating. In the main grid 15, the distance between any two adjacent reference code bars 2 is different from the distance between the remaining two adjacent reference code bars 2, that is, the reference code bar 2 The incremental code bar 1 is set so as to code with the distance so that the distance between the two reference code bars becomes a value uniquely determined. For example, the distance between two adjacent reference code bars is D0 + kδ, and the distance between the remaining two adjacent reference code bars is D0 + (k + 1) δ. Since the distance between two reference code bars is uniquely determined, the absolute position where the read head is first located can be calculated each time the read head passes over two adjacent reference code bars.

  Assuming that the initial position of the read head is point a, when the read head moves and passes through two adjacent reference code bars 3 and 4, the incremental displacement measuring section 13 can calculate the incremental displacement x1 and read. When the head reaches the reference code bar 3, the mask plate 13 is aligned with the reference code bar 3. At this time, since the beam transmitted through the mask plate 13 is an optical pulse, the reference position photoelectric detector 14 is set to the corresponding reference position. The pulse signal can be detected, and the incremental displacement measuring unit 13 calculates the distance x2. When the read head reaches the reference code bar 4, the mask plate 13 is aligned with the reference code bar 4. At this time, since the beam transmitted through the mask plate 13 is an optical pulse, the reference position photoelectric detector 14 corresponds. The reference pulse signal can be detected, and the incremental displacement measuring unit 13 can determine the value of the distance x2 with this. Since the reference code bar of the main lattice is set so as to code at a distance, the distance between any two reference code bars is determined. The calculated distance x2 determines the absolute position where the reference code bar is located. -X2 determines the absolute position of the first point a of the read head.

  As shown in FIGS. 1 and 2, the incident light first passes through the mask plate and is modulated for the first time, and the code bar of the mask plate 13 modulates the incident beam to form a light-dark alternating striped beam. it can. The modulated striped beam then impinges on the reference code bar area of the main grating and is reflected back to the mask plate, at which time the light is modulated a second time. Finally, the light is transmitted through the mask plate 13 again and modulated for the third time, and then received by the photoelectric detector. In this embodiment, a reflective grating is used for the main grating 15 and the reference grating 19. When the mask plate and the main grating 15 are displaced relatively, the intensity of the light received by the photodetector also changes, which corresponds to the autocorrelation in the code structure of the mask plate itself. When the code bars of the mask plate are exactly aligned with the same reference code bar 2 of the main grating, the correlation is highest and the intensity of light received by the detector is minimal. When the code bar of the mask plate is offset by one bit from the reference code bar of the main grating, the light intensity is greatly increased, thereby forming a negative reference pulse signal to align with the reference position. As shown in FIG. 5, when the mask plate is precisely aligned with the reference point of the main grating, the intensity of light received by the detector is theoretically zero, ie, corresponding to the peak W2 of the pulse signal, The position of the reference code bar can be specified using this peak value.

  As shown in FIG. 3, since the light intensity at the peak of the reference pulse signal is extremely small, even if external light causes slight interference on the detector, the contrast of the reference signal is seriously affected. . In fact, since the mask plate is generally made of glass fiber, its reflection cannot be eliminated even if its surface is coated with an anti-reflection material, and the mask plate itself reflects some light to the detector. This is equivalent to adding a DC component to the reference pulse signal, which greatly reduces the contrast and affects the accuracy of the reference code bar location, but moves the mask plate in a direction perpendicular to the slit by less than 5 °. By rotating by an angle and reflecting the light reflected by itself to a region other than the detector, the contrast of the light intensity signal can be increased, and the accuracy of specifying the position of the reference code bar can be improved.

  In one embodiment, the number of bits for coding the code bar of the mask plate is 100 bits, the line width is 10 μm, the number of light transmitting portions 8 is 23, and the number of non-transmitting portions 7 is 77. As shown in FIG. 2, a beam having a total width of 1 mm and a diameter of 1.2 mm can cover an area of 1 mm. In the present embodiment, the slit width is 10 μm, which is far beyond the wavelength of 660 nm of the laser light used, so that a slight diffraction effect occurs. However, since the signal at the peak of the reference pulse is extremely weak, a slight The signal may be shielded by noise even with a simple diffraction effect. Therefore, when setting the code, the influence from the diffraction effect is also taken into consideration. A 100-bit code structure is set by the enumeration method, and specifically, it is “011000010000000000001001100001101000000000000000000000001100100000001000000010100000110001000001001000001110”. It includes a pulse signal having a peak, has a peak width W1 of 26 μm, and is further divided by a circuit, whereby the position of the reference point can be specified with an accuracy of 0.6 μm.

  What has been described above merely describes the present invention in detail with preferred embodiments, and the embodiments of the present invention are not limited to the above description. It should be noted that those skilled in the art can make various changes or modifications without departing from the spirit of the present invention, and these are all considered to be protected by the appended claims.

Claims (7)

  In an absolute grating scale including a main grating and a read head device including an incremental displacement measurement unit, the read head device further includes a first beam splitter, a mask plate, and a reference position photoelectric detector, The main grid is provided with a plurality of reference code bars, and a distance between any two adjacent reference code bars is different from a distance between any remaining two adjacent reference code bars, and The beam splitter splits the light emitted from the light source into a beam emitted to the main grating and a beam emitted to the incremental displacement measurement unit, and the beam emitted to the main grating passes through the mask plate and passes through the mask. After arriving at the grating and being reflected, it passes through the mask plate again and is received by the reference position photoelectric detector. Code bar is provided in the mask plate, after said beam exiting the mask plate is reflected by the mask plate, it is provided so as not to be received by said reference position photoelectric detector, an absolute expression grid scale.   The absolute grating scale according to claim 1, wherein a beam emitted to the mask plate and a normal to the mask plate form an acute angle.   3. The absolute grid scale of claim 2, wherein the acute angle is less than 5 [deg.].   The absolute-type grating scale according to any one of claims 1 to 3, wherein a light transmitting portion and a light reflecting portion are provided on a code bar of the mask plate.   The absolute grating scale according to any one of claims 1 to 4, wherein a beam emitted to the mask plate is larger than a width of a code bar of the mask plate.   The absolute scale grid according to any one of claims 1 to 5, wherein the code composed of the code bar is 011000010000000000001001100001101000000010000000000000011000000000000000000010100000110001000001001000001110, 1 is a light transmitting portion, and 0 is a light reflecting portion.   The absolute grating scale according to claim 6, wherein each width of the light transmitting portion and the light reflecting portion is 10 µm.

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