A spatially multiplexed two-dimensional moiré fringe linear grating ruler

Abstract

The invention discloses a spatially multiplexed two-dimensional moiré fringe linear grating ruler, comprising a straight grating line grating and a straight oblique grating line grating, wherein the vertical grating line grating and the vertical grating line area of ​​the straight oblique grating line grating form vertical moiré fringes, and the vertical grating line grating and the oblique grating line area of ​​the straight oblique grating line grating form horizontal moiré fringes; the two gratings move left and right relative to each other by a grating pitch, the vertical moiré fringes roll left and right by a vertical moiré fringing width, and the horizontal moiré fringes roll up and down by a horizontal moiré fringing width. Photoelectric detection is performed by a two-dimensional photodiode array, and the photodiode arrays in the horizontal and vertical moiré fringing areas are respectively connected to their own decoding circuits to output dual-channel IQ signals of the same frequency. The invention has the advantages of excellent anti-interference capability, and the output signal of the photodiode is amplified and accumulated to output a standard orthogonal signal, the circuit is mature, and the technical complexity of the invention is not increased.

Description

Technical Field

[0001] The present invention relates to the technical field of geometric measurement of automated equipment, and in particular to a spatially multiplexed two-dimensional moiré fringe linear grating ruler. Background Art

[0002] As a displacement, velocity and acceleration measurement device, the grating ruler is one of the standard configuration devices on high-precision and high-speed CNC machine tools, electronic bonding machines and semiconductor wafer production equipment. It is widely used in high-precision, high-speed automated CNC feedback closed-loop systems. The structure of the grating ruler of domestic manufacturers consists of two one-dimensional gratings overlapping at a small angle, forming a visually nearly parallel, black and white one-dimensional moiré fringes. Four photodetectors are installed in one moiré fringe cycle for detection, and a DC op amp circuit is used to perform simple addition, subtraction and amplification calculations, and finally form two standard orthogonal IQ signal outputs. The orthogonal IQ signal is a standard interface for industrial control position information input and is compatible with various motion control cards.

[0003] Under ideal conditions, the one-dimensional moiré fringe scale can accurately measure the position and can be used as a position feedback element for high-speed, high-precision control equipment. However, under non-ideal conditions, such as when there is external interference, pollution, and uneven photoelectric signals, distortion and displacement errors will occur, which will greatly reduce the reliability of automation equipment. In extreme cases, the automatic control system will become unstable, resulting in machine damage and fatal accidents. Summary of the invention

[0004] The purpose of the present invention is to provide a spatially multiplexed two-dimensional moiré fringe linear grating ruler, which is mainly used to solve the problems of existing grating rulers that are prone to distortion and displacement errors under non-ideal conditions.

[0005] In order to achieve the above tasks, the present invention adopts the following technical solutions:

[0006] A spatially multiplexed two-dimensional moiré fringe linear grating ruler comprises a straight grating line grating and a straight oblique grating line grating, wherein the grating line pitch of the straight grating line grating is Wm; the straight oblique grating line grating is composed of a vertical grating line grating and an oblique grating line grating arranged in an alternating manner, wherein the vertical grating line pitch is Wdz and the length is L; the oblique grating line pitch is Wdx and the length is L, and the angle with the vertical direction is θ; in the vertical direction, the vertical grating line grating and the oblique grating line grating are repeatedly arranged for multiple times; when one of the straight grating line grating and the straight oblique grating line grating is a main grating, the other is used as an indicator grating; the main grating and the indicator grating are arranged in parallel up and down with a gap, and the angle between the axes of the two is 0°; the main grating is fixed, and in the process of the indicator grating moving left and right, the vertical grating line area of ​​the straight grating line grating and the straight oblique grating line grating forms vertical moiré fringes, and the oblique grating line area of ​​the straight grating line grating and the straight oblique grating line grating forms horizontal moiré fringes;

[0007] Assume cs = Wdx / Wm, and Wm>Wdx. If cos(θ) = cs, the angle between the horizontal moiré fringe and the horizontal axis is 0°, and the moiré fringe width Ms is:

[0008]

[0009] Assume cz = Wdz / Wm, and Wm>Wdz, the vertical moiré fringe is at an angle of 90° to the horizontal direction, and the moiré fringe width Mc is:

[0010]

[0011] The indicator grating moves left and right by one grating pitch relative to the main grating, the horizontal moiré fringes roll up and down by one horizontal moiré fringes width, and the vertical moiré fringes roll left and right by one vertical moiré fringes width.

[0012] Furthermore, the photoelectric detection of the spatially multiplexed two-dimensional moiré fringe linear grating ruler is performed by a photodiode array; the spacing of the photodiode array in the horizontal and vertical directions is respectively:

[0013] W PDS =Ms / N

[0014] W PDC =Mc / N

[0015] Regardless of whether it is horizontal or vertical moiré fringes, the number of photodiodes in one moiré fringes period is N.

[0016] Furthermore, the decoding of horizontal and vertical moiré fringes relies on the analog amplification operation method of the photodiode array; the photodiode arrays in the horizontal and vertical moiré fringes areas are respectively connected to their own decoding circuits without interfering with each other. The decoding circuit principles of the vertical and horizontal moiré fringes are the same, using IQ decoding in the one-dimensional phase shift direction and an averaging circuit in the other direction.

[0017] Furthermore, in the one-dimensional phase shift direction, the phases of adjacent π / 2 moiré fringes of the photodiodes are numbered j=0, 1, 2, ..., 7, a total of J=7, spanning two complete moiré fringes; each photodiode signal g(j) has two independent output channels s, c, and amplification factors s(j) and c(j):

[0018] s(j)=Im[w(j)e -iΦ(j) ];

[0019] c(j)=Re[w(j)e -iΦ(j) ].

[0020] Wherein, e is the natural logarithm base, i is the imaginary unit, and Φ(j) and w(j) are given in Table 1 below.

[0021] 7 games j＝0 j＝1 j＝2 j＝3 j＝4 j＝5 j＝6 Φ(j) -3π / 2 -π -π / 2 0 π / 2 π 3π / 2 w(j) 0.1464466 0.5 0.8535533 1.0 0.8535533 0.5 0.1464466

[0022] Then they are sent to two adder circuits respectively, and two orthogonal signals I and Q with the same frequency and 90° phase difference are output:

[0023]

[0024]

[0025] Furthermore, in the photodiode array of horizontal moiré fringes, the vertical direction is the phase shift demodulation direction, with a total of 2 moiré fringes distributed, separated by vertical fringes in the middle, and the photodiode numbering can be considered continuous; in the horizontal direction, the output signals I and Q are averaged respectively, for a total of 7 channels; finally, the orthogonal signal (Is, Qs) is output.

[0026] Furthermore, in the vertical moiré fringe photodiode array, the horizontal direction is the phase shift demodulation direction, with a total of 2 continuous moiré fringes distributed, and the photodiode number j = 0, 1, ..., 7; in the vertical direction, the output signals I and Q are averaged, with a total of 7 channels; and finally, the orthogonal signal (Ic, Qc) is output.

[0027] Furthermore, the grating ruler outputs two IQ signals of the same frequency, which are sent to the motion control card for demodulation. The demodulation process records the number of cycles through an addition and subtraction counter, and then calculates the angle within a cycle through -atan(I / Q); the displacement of the relative movement of the grating to the left and right = the grating pitch of the grating * (number of cycles + -atan(I / Q) / 2π) - zero point.

[0028] Compared with the prior art, the present invention has the following technical features:

[0029] 1. Anti-IQ quadrature signal amplitude and phase change errors caused by uneven LED lighting;

[0030] 2. Anti-grating surface contamination caused by IQ orthogonal signal amplitude and phase change errors;

[0031] The uneven LED lighting and grating surface contamination cause low-frequency errors in the uneven output of the photosensitive detector. Since the signal of the present invention spans 2x2 moiré fringe cycles, the range is increased, which is equivalent to doubling the spatial low-frequency of the uneven output of the photosensitive detector; at the same time, 98 photosensitive diodes and an averaging circuit are used, which is equivalent to a high-order low-pass filter with a better low-pass effect; therefore, the ability to resist the uneven output error of the photosensitive detector is greatly improved.

[0032] 3. Anti-photodiode distribution position error caused by IQ quadrature signal amplitude and phase change error;

[0033] 4. Anti-Moire fringe spacing variation error caused by grating position and angle installation error;

[0034] Since the invention uses w(j) amplification coefficient distribution, it actually uses Hanning window function, which has two advantages: 1) the width of demodulation main frequency area is increased by 1 times, which can resist the change of demodulation frequency caused by the change of moiré fringe spacing; 2) the 4-field scanning grating signal sampling rectangular window function coefficient has the disadvantage of large spectrum leakage, the first and second side peak values ​​are 21% and 12.6% of the main peak respectively, and the attenuation is slow. The side peak attenuation of Hanning window is 18dB / otc, which is large, so the spectrum accuracy is greatly improved. Hanning window function resists spectrum cross contamination caused by the position error of photodiode distribution.

[0035] 5. Anti-IQ quadrature signal amplitude and phase change errors caused by low-frequency vibration; 1) Since the grating scale signal of the present invention is first phase-shift demodulated, that is, band-pass filtering at the demodulation frequency; the next step is averaging, that is, low-pass filtering; at the same time, the Hanning window is used to prevent spectrum leakage; 2) Horizontal and vertical demodulation, respectively curbing vibrations in the horizontal and vertical directions. Therefore, the anti-vibration performance is outstanding, far exceeding the existing grating technology.

[0036] 6. Due to the above advantages 1-5, the accuracy retention and reliability of the grating ruler of the present invention are much better than the prior art in high-speed measurement. The present invention is specifically used for full closed-loop position feedback of automation equipment.

[0037] 7. Although the number of photodetector channels increases from 4 to 98, each channel is based on a DC amplifier and adder analog circuit of a DC operational amplifier. Technically, the present invention is the same as the conventional technology, except that 4 discrete channels are integrated into 98 channels. Considering that the photodetector and its amplifier circuit use customized integrated circuit chips, the multi-channel photodetector does not increase complexity, and the space size and cost do not change much. Therefore, the present invention is highly practical and can be promoted as a large-scale replacement product for the currently large-scale 4-field scanning grating ruler. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 (a) and (b) are the transmission and reflection grating scale structures.

[0039] Figure 2 It is a spatially multiplexed two-dimensional moiré fringe grating ruler; (a) straight grating line grating; (b) straight oblique grating line grating; (c) grating ruler size and coordinate system.

[0040] Figure 3 Two-dimensional moiré fringes; (a) overlapping grating patterns; (b) two-dimensional moiré fringes.

[0041] Figure 4Photodiode array decoding circuit for two-dimensional moiré fringes.

[0042] Figure 5 Schematic diagram of horizontal moiré fringe photodiode array decoding.

[0043] Figure 6 Schematic diagram of vertical moiré fringe photodiode array decoding.

[0044] Figure 7 This is the grating scale structure of an embodiment of the present invention.

[0045] Figure 8 is the photodiode array size, unit: μm. DETAILED DESCRIPTION

[0046] Referring to the accompanying drawings, the present invention provides a spatially multiplexed two-dimensional moiré fringe linear grating ruler, including a straight grating line grating and a straight oblique grating line grating, such as Figure 2 As shown in (a) and (b), the grating line pitch of the straight grating line grating is Wm; the straight oblique grating line grating is composed of the vertical grating line grating and the oblique grating line grating arranged alternately; the vertical grating line pitch is Wdz, the length is L; the oblique grating line pitch is Wdx, the length is L, and the angle with the vertical direction is θ; in the vertical direction, the vertical grating line grating and the oblique grating line grating are repeated H times. In the straight grating line grating and the straight oblique grating line grating, when one is the main grating, the other is used as the indicator grating. In this scheme, the substrates of the straight grating line grating and the straight oblique grating line grating are both transparent materials, such as glass.

[0047] The grating ruler structure of the present invention can be used as a transmission grating ruler or a reflection grating ruler; when used as a transmission grating ruler, Figure 1 As shown in the figure, the parallel light emitted by the light source is transmitted through the main grating and the indicator grating, and finally imaged on the photodiode array; the indicator grating and the main grating are placed in parallel, and the angle between the axes is 0°. There is a gap between the two; the main grating is fixed, and the moiré fringes are formed in the process of the indicator grating moving left and right.

[0048] In the embodiment of the present invention, the straight oblique grating line grating is used as the main grating, and the straight grating line grating is used as the indicator grating. When the grating moves under the condition of overlapping the main grating space axis at an angle of 0°, a two-dimensional periodic distribution of black and white moiré fringes is formed visually, such as Figure 3 As shown in (a), the vertical grating line area of ​​the straight grating line grating and the straight oblique grating line grating forms vertical moiré fringes, and the oblique grating line area of ​​the straight grating line grating and the straight oblique grating line grating forms horizontal moiré fringes, and at this time, the angle between the moiré fringes and the horizontal axis is 0°.

[0049] In use, the grating ruler is a macroscopic observation and detection, which is equivalent to Figure 3 (a) is filtered by a large window average value, and the resulting two-dimensional moiré fringes are as follows Figure 3as shown in (b).

[0050] Assume cs = Wdx / Wm, and Wm>Wdx. If cos(θ) = cs, the angle between the moiré fringe and the horizontal axis is 0°, which is a horizontal moiré fringe. Its moiré fringe width Ms is:

[0051]

[0052] Here, the heights of the horizontal and vertical moiré fringe regions are both: L = Ms. When the indicator grating moves left and right by one grating pitch relative to the main grating, the horizontal moiré fringe rolls up and down by one horizontal moiré fringe width.

[0053] Assume cz = Wdz / Wm, and Wm>Wdz, the moiré fringe width Mc:

[0054]

[0055] At this time, the moiré fringes are at an angle of 90° to the horizontal direction and are vertical fringes; the indicator grating moves left and right by one grating pitch relative to the main grating, and the vertical moiré fringes roll left and right by one vertical moiré fringes width.

[0056] The above design is the grating scale measurement area, and there is also an alignment grating area on the grating, which is the same as the traditional grating design.

[0057] like Figure 1 As shown in the figure, the photoelectric detection of the grating ruler is completed by a photodiode array fixed on the indicating grating. The specific structure of the photodiode array is as follows Figure 4 As shown, the spacing of the photodiode array in the horizontal and vertical directions are:

[0058] W PDS =Ms / N

[0059] W PDC =Mc / N

[0060] Regardless of whether it is horizontal or vertical moiré fringes, the number of photodiodes in one moiré fringes period is N.

[0061] The two-dimensional moiré fringe decoding mainly relies on the analog amplification operation method of the photodiode array. The distribution of the photodiode array on the two-dimensional grating ruler is shown in Figure 4 , which is a matrix array. The photodiode arrays in the horizontal and vertical moiré fringe areas are connected to their own decoding circuits respectively, without interfering with each other. The decoding circuit principles of the vertical and horizontal stripes of the two-dimensional moiré fringe are the same, using IQ decoding in the one-dimensional phase shift direction and an averaging circuit in the other direction.

[0062] In the one-dimensional phase shift direction, N = 4, the phase of adjacent π / 2 moiré fringes of photodiodes are used, numbered j = 0, 1, 2, ..., 7, a total of J = 7, spanning 2 complete moiré fringes. Each photodiode signal g(j) has two independent output channels s, c, and amplification factors s(j) and c(j):

[0063] s(j)=Im[w(j)e -iΦ(j) ];

[0064] c(j)=Re[w(j)e -iΦ(j) ].

[0065] Wherein, e is the natural logarithm base, i is the imaginary unit, and Φ(j) and w(j) are given in Table 1 below.

[0066] Table 1 7-field π / 2 scanning method

[0067] 7 games j＝0 j＝1 j＝2 j＝3 j＝4 j＝5 j＝6 Φ(j) -3π / 2 -π -π / 2 0 π / 2 π 3π / 2 w(j) 0.1464466 0.5 0.8535533 1.0 0.8535533 0.5 0.1464466

[0068] Afterwards, they are sent to two adder circuits respectively, which output two orthogonal signals with the same frequency and a phase difference of 90°: I and Q.

[0069]

[0070]

[0071] Horizontal Moiré fringe photodiode array decoding Figure 5 As shown, the vertical direction is the phase shift demodulation direction, with a total of 2 moiré fringes, separated by vertical fringes. Since the photodiode numbers j = 0, 1, ..., 7 are continuous, they can be considered continuous and demodulated according to the above formula; in the horizontal direction, the output signals I and Q are averaged and calculated respectively, with a total of 7 channels. Finally, the orthogonal signal IQ: (Is, Qs) is output, and a total of 49 photodiodes are used.

[0072] Vertical Moiré fringe photodiode array decoding Figure 6 As shown, the horizontal direction is the phase shift demodulation direction, with a total of 2 continuous moiré fringes, and the photosensitive diode number j = 0, 1, ..., 7, which is demodulated according to the above formula; in the vertical direction, the output signals I and Q are averaged, with a total of 7 channels. Finally, the orthogonal signal IQ is output: (Ic, Qc), using a total of 49 photosensitive diodes.

[0073] According to the grating principle, the indicator grating moves left and right relative to one grating pitch, and the vertical and horizontal moiré fringes roll one cycle, so the present invention outputs two IQ signals with the same frequency. Generally, the IQ signals are sent to the motion control card for demodulation. The present invention outputs two IQ signals with the same frequency, calculates the displacement of the two gratings moving left and right relative to each other, and obtains the final displacement after the two-way average calculation.

[0074] The demodulation process is to record the number of cycles through the addition and subtraction counter, and then calculate the angle within one cycle through -atan(I / Q); the displacement of the relative movement of the grating to the left and right = the grating pitch of the grating * (number of cycles + -atan(I / Q) / 2π)-zero point, the theoretical resolution: the grating pitch of the grating / 100000. IQ demodulation is a general method and is not within the scope of this invention. For details, please refer to "Sensors" (5th edition) edited by Professor Tang Wenyan of Harbin Institute of Technology.

[0075] Example:

[0076] In the embodiment of the present invention, when the grating ruler structure is used as a transmission grating ruler, Figure 7 As shown, the light emitted by the LED light source 1 is converted into parallel light by the beam expander collimator lens 2, and is transmitted through the indicator grating-straight line grating 3 and the main grating-straight line grating 4, and finally imaged on the photosensitive diode array 5. The indicator grating and the main grating are placed in parallel, with an angle of 0°. There is a gap between the two, the main grating is fixed, and the indicator grating moves left and right.

[0077] The two-dimensional moiré fringe ruler of this embodiment is as follows Figure 2 As shown. The grating pitch of the straight grating line is: Wm = 20.1μm; the grating pitch of the straight oblique grating line is: Wdz = 19.2μm, the length is L = 200.7485μm; the grating pitch of the oblique grating line is: Wdx = 20μm, the length is L = 200.7485μm, and the angle with the vertical direction is θ = 5.7177°.

[0078] The oblique lines of the straight grating and the straight oblique grating form horizontal moiré fringes with α=0° and a moiré fringing width of Ms=200.7485μm. The straight grating and the straight oblique grating form vertical moiré fringes with a moiré fringing width of Mc=428.8μm. The moiré fringes formed are as follows: Figure 3 (b) as shown.

[0079] The photodiode array distribution on the moiré fringe is as follows Figure 4 , the photodiode array size is Figure 8 shown.

[0080] The two-dimensional moiré fringe decoding mainly relies on the analog amplification operation method of the photodiode array. The distribution of the photodiode array on the two-dimensional grating ruler is shown in Figure 4, which is a 14x7 matrix array. The photodiode arrays in the horizontal and vertical moiré fringe areas are connected to their own decoding circuits respectively, without interfering with each other. The decoding circuit principles of the vertical and horizontal stripes of the two-dimensional moiré fringe are the same, using IQ decoding in the one-dimensional phase shift direction and an averaging circuit in the other direction.

[0081] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A spatially multiplexed two-dimensional moiré fringe linear grating ruler, characterized in that: The invention comprises a straight grating line grating and a straight oblique grating line grating, wherein the grating line pitch of the straight grating line grating is Wm; the straight oblique grating line grating is composed of a vertical grating line grating and an oblique grating line grating arranged in an alternating manner, wherein the vertical grating line pitch is Wdz and the length is L; the oblique grating line pitch is Wdx and the length is L, and the angle with the vertical direction is θ; in the vertical direction, the vertical grating line grating and the oblique grating line grating are repeatedly arranged for many times; when one of the straight grating line grating and the straight oblique grating line grating is a main grating, the other is used as an indicator grating; the main grating and the indicator grating are arranged in parallel up and down with a gap, and the angle between the axes of the two is 0°; the main grating is fixed, and in the process of the indicator grating moving left and right, the vertical grating line area of ​​the straight grating line grating and the straight oblique grating line grating forms vertical moiré fringes, and the oblique grating line area of ​​the straight grating line grating and the straight oblique grating line grating forms horizontal moiré fringes; Assume cs = Wdx / Wm, and Wm>Wdx. If cos(θ) = cs, the angle between the horizontal moiré fringe and the horizontal axis is 0°, and the horizontal moiré fringe width Ms is: Assume cz = Wdz / Wm, and Wm>Wdz, the vertical moiré fringe is at an angle of 90° to the horizontal direction, and the vertical moiré fringe width Mc is: The indicator grating moves left and right by one grating pitch relative to the main grating, the horizontal moiré fringes roll up and down by one horizontal moiré fringes width, and the vertical moiré fringes roll left and right by one vertical moiré fringes width; The photoelectric detection of the spatially multiplexed two-dimensional moiré fringe linear grating ruler is completed by a photodiode array; the spacing of the photodiode array in the horizontal and vertical directions is respectively: W PDS =Ms / N W PDC =Mc / N Regardless of whether it is horizontal or vertical moiré fringes, the number of photodiodes in one moiré fringes period is N.

2. The spatially multiplexed two-dimensional moiré fringe linear grating ruler according to claim 1, characterized in that: The decoding of horizontal and vertical moiré fringes relies on the analog amplification operation method of the photodiode array. The photodiode arrays in the horizontal and vertical moiré fringes areas are respectively connected to their own decoding circuits without interfering with each other. The decoding circuit principles of vertical and horizontal moiré fringes are the same, using IQ decoding in the one-dimensional phase shift direction and an averaging circuit in the other direction.

3. The spatially multiplexed two-dimensional moiré fringe linear grating ruler according to claim 1, characterized in that: In the one-dimensional phase shift direction, the phase of adjacent π / 2 moiré fringes of the photodiodes is numbered j = 0, 1, 2, ..., 6, the field number J is 7 fields, spanning 2 complete moiré fringes; each photodiode signal g(j) has two independent output channels s, c, with amplification factors s(j) and c(j): s(j)=Im[w(j)e -iφ(j) ]: c(j)=Re[w(j)e -iφ(j) ]; Where e is the natural logarithm base, i is the imaginary unit, Φ(j) and w(j) are given in the following 7-field parameter table; Then they are sent to two adder circuits respectively, and two orthogonal signals I and Q with the same frequency and 90° phase difference are output:

4. The spatially multiplexed two-dimensional moiré fringe linear grating ruler according to claim 3, characterized in that: In the photodiode array of horizontal moiré fringes, the vertical direction is the phase shift demodulation direction, with a total of 2 moiré fringes distributed, separated by vertical fringes in the middle, and the photodiode numbering can be considered continuous; in the horizontal direction, the output signals I and Q are averaged respectively, for a total of 7 fields; finally, the orthogonal signal (Is, Qs) is output.

5. The spatially multiplexed two-dimensional moiré fringe linear grating ruler according to claim 4, characterized in that: In the vertical moiré fringe photodiode array, the horizontal direction is the phase shift demodulation direction, with a total of 2 continuous moiré fringes distributed, and the photodiode number j = 0, 1, ..., 6; in the vertical direction, the output signals I and Q are averaged, with a total of 7 fields; finally, the orthogonal signal (Ic, Qc) is output.

6. The spatially multiplexed two-dimensional moiré fringe linear grating ruler according to claim 4, characterized in that: The grating ruler outputs two I and Q signals of the same frequency. The I and Q signals are sent to the motion control card for demodulation. The demodulation process records the number of cycles through an addition and subtraction counter, and then calculates the angle within a cycle through -atan(I / Q); the displacement of the relative movement of the grating to the left and right = the grating pitch of the grating * (number of cycles + -atan(I / Q) / 2π) - zero point.

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