WO2023155657A1

WO2023155657A1 - Absolute six-degrees-of-freedom grating encoder

Info

Publication number: WO2023155657A1
Application number: PCT/CN2023/073097
Authority: WO
Inventors: 李星辉; 汪盛通
Original assignee: 清华大学深圳国际研究生院
Priority date: 2022-02-21
Filing date: 2023-01-19
Publication date: 2023-08-24
Also published as: CN114459516B; CN114459516A

Abstract

An absolute six-degrees-of-freedom grating encoder. A collimated beam which is generated by a light source module (10) passes through a PBS to generate a beam L1 and a beam L2. The beam L2 is first split into a beam L2-1, a beam L2-2 and a beam L2-3 by means of a composite grating light-splitting module (4). The beam L2-1 generates diffracted light by using a two-dimensional grating region (5-1) of a composite grating (5), and the diffracted light passes through an absolute four-degrees-of-freedom measurement module (20) to measure an absolute pose of the composite grating (5) in θ_x, θ_y, θ_z and z directions. The beam L2-2 and the beam L2-3 respectively enter reference code channels via an X-direction mask and a Y-direction mask. The masks have the same code as the reference code channels, and when being aligned with the code channels, the masks generate a negative pulse, so as to mark a zero position. Beams which are generated by the diffraction of the beam L2-1 and beams which are generated by the beam L1 at a two-dimensional reference grating (3) generate an interference phenomenon, and are received by an absolute two-degrees-of-freedom measurement module (30), phase changes in an x direction and a y direction are analyzed to obtain displacement increment information, and an absolute pose thereof is obtained by means of coupling a mark of the negative pulse.

Description

An Absolute Grating Encoder with Six Degrees of Freedom

technical field

The invention relates to the technical field of pose measurement, in particular to an absolute six-degree-of-freedom grating encoder.

Background technique

The synthetic aperture optical system is an expansion of the aperture, which can be mainly used in large telescopes and high-energy laser physical focusing mirrors. Scientific instrument telescope is one of the important means for human beings to explore the universe at present. No matter what kind of telescope, such as radio telescope or optical telescope, one of its main performance parameters is resolution, which is related to its aperture size, but a single large-area telescope The mirror surface is difficult to manufacture. Currently, the sub-mirror assembly method is mainly used. The observation performance is limited by the splicing accuracy of the sub-mirror. At the same time, due to the influence of environmental factors such as gravity load, temperature change, and humidity change, the relative position between the installed sub-mirror There will be small changes in the attitude, which will directly lead to a large surface shape error of the primary mirror. The other is the focusing mirror of the high-energy laser. This kind of focusing also requires the installation and cooperation of the sub-mirror to reduce the surface error as much as possible to ensure a high concentration of energy. The current pose measurement scheme has high requirements for environmental stability, and surface errors still occur after long-term use and correction. Therefore, a more stable and high-precision device that can measure the pose absolutely is urgently needed.

With the use of the telescope, the accumulation of errors will lead to a significant decline in its performance, for example, the observation performance of the South African Large Telescope due to the influence of humidity, and the observation performance of the Hobby Eberly Telescope due to the influence of temperature. In the soon-to-be-built Large Magellan Telescope, great attention is also paid to the measurement of the sub-mirror's pose and orientation. Participated by China, the 30m telescope, which is jointly researched by many countries, has a primary mirror composed of 492 hexagonal sub-mirrors. It is expected to become the world's first extremely large telescope after it is completed in 2027. On December 25, 2021, the James Webb Space Telescope, which cost about US$10 billion, was launched from the Kourou Space Center in French Guiana. One of the limitations of its lifespan is also the attitude of the mirror. The focusing process of high-energy laser physics research often requires a long working time, during which the pose standard of the mirror surface of the sub-mirror must be maintained. Therefore, the demand for spatial pose detection of the sub-mirror is very urgent, not only the absolute pose needs to be detected during installation, but also needs to be measured and fed back during subsequent use, so as to adjust the pose of the sub-mirror, and then actively control the sub-mirror to make the main mirror form to meet the needs. How to measure the position and orientation of the sub-mirror with high precision and provide a basis for realizing the adjustment of the sub-mirror's confocal and common phase is the core technology of large-aperture optical system research for astronomical observation.

Regarding pose measurement, there are electromagnetic displacement measurement and optical displacement measurement, among which the electromagnetic measurement method Among them, the accuracy of the capacitive sensor can reach the nanometer level, and it is a multi-degree-of-freedom measurement system composed of multiple capacitive sensors. At present, it has been used for real-time detection of the sub-mirror pose of the Keck and Canary large telescopes, which has high accuracy and stability, but the measurement system composed of capacitive sensors is not only complicated, but also sensitive to temperature and humidity. There is a problem of large errors in accumulation.

Contents of the invention

The object of the present invention is to provide an absolute six-degree-of-freedom grating encoder that can reduce errors during use, so as to realize accurate monitoring of the pose of a synthetic aperture optical system.

The invention provides an absolute six-degree-of-freedom grating encoder, which includes a light source module, an absolute four-degree-of-freedom measurement module, and an absolute two-degree-of-freedom measurement module; the collimated light beam generated by the light source module passes through a polarization beam splitter Generate the first collimated beam L1 and the second collimated beam L2, the second collimated beam L2 is first divided into the beam L2-1, the beam L2-2 and the beam L2-3 through the composite grating beam splitting module, the beam L2-1 utilizes The two-dimensional grating area of the composite grating generates diffracted light, and the diffracted light is measured by the absolute four-degree-of-freedom measurement module to measure the absolute pose of the composite grating in the θ _x , θ _y , θ _z and z directions, the beam L2-2 and the beam L2- 3 Enter the X reference code channel through the X-direction mask, and enter the Y reference code channel through the Y-direction mask. The X-direction mask and the X reference code have the same code, and the Y-direction mask and the Y The reference code track has the same code, and when the X reference code track is aligned with the X-direction mask, and the Y reference code track is aligned with the Y-direction mask, a negative pulse is generated to mark the zero position, and the beam L2-1 passes through the compound grating The light beam produced by the diffraction of the grating area, and the beam produced by the first collimated light beam L1 in the two-dimensional reference grating produce interference phenomenon, and is received by the absolute two-degree-of-freedom measurement module, and the phase change in the x and y directions is analyzed to obtain the displacement Incremental information and the pulse signal generated at the same time can be zero-calibrated for incremental displacement to obtain its absolute displacement.

Preferably, the absolute four-degree-of-freedom measurement module includes three four-quadrant photodetectors.

Preferably, the diffracted light includes ±1st-order diffracted light and zero-order light in the x direction, the positions of the three beam spots on the four-quadrant photodetector change, and the θ _x , θ _y , θ _z and z directions are calculated absolute pose.

Preferably, the absolute two-degree-of-freedom measurement module includes a two-degree-of-freedom incremental signal measurement module and a two-degree-of-freedom zero pulse signal measurement module.

Preferably, the two-degree-of-freedom incremental signal measurement module includes a polarization beam splitter prism, a second quarter glass slide, a second beam splitter prism, a first prism collimation unit, a first quarter wave plate, a first Dichroic prism, second prism collimation unit, two-dimensional reference grating, and incremental signal de-DC module.

Preferably, the DC removing module includes a depolarizing beam splitter, a first polarizing plate, a third quarter glass plate, a second polarizing plate and a photodetector.

Preferably, the two-degree-of-freedom zero pulse signal measurement module includes a composite grating spectroscopic module and a photodetector.

Preferably, the two-degree-of-freedom zero-position pulse signal measurement module enters the two beams of light L2-2 and L2-3 into the X reference code track through the X-direction mask plate, and enters the Y reference code track through the Y-direction mask plate .

Preferably, the two-degree-of-freedom incremental signal measurement module generates x _s+1 , x _s-1 , y _s+1 and y _s-1 beams through the two-dimensional grating area of the composite grating, and generates the two-dimensional reference grating The x _r+1 , x _r-1 , y _r+1 and y _r-1 light beams produce interference phenomenon, and are received by the photodetector, and the phase changes in the x and y directions are respectively analyzed to obtain the displacement incremental information, and at the same time The generated pulse signal can be used for zero calibration of the incremental displacement to obtain its absolute displacement.

Preferably, the light source module includes a collimating lens and a diaphragm.

The present invention can more stably and accurately measure the absolute pose of the sub-mirror in six degrees of freedom, and has the advantage of being able to measure the absolute pose compared with the known solutions, with simple structure, more stability and stronger robustness. Because the measurement accuracy of the grating encoder mainly depends on the pitch of the grating, the physical structure is not easy to change, so it is more robust. And compared with the capacitive displacement sensor currently used, it has better robustness, is hardly affected by large changes in ambient temperature, humidity, etc., and has better integration, which can realize the measurement of absolute pose.

Description of drawings

FIG. 1 is a schematic plan view of an absolute six-degree-of-freedom grating encoder provided by an embodiment of the present invention;

Fig. 2 is a three-dimensional structural schematic diagram of an absolute six-degree-of-freedom grating encoder provided by an embodiment of the present invention;

Fig. 3 is the main diagram of the composite grating provided by the embodiment of the present invention;

FIG. 4 is a schematic diagram of an X-direction mask plate provided by an embodiment of the present invention;

FIG. 5 is a schematic diagram of a Y-direction mask plate provided by an embodiment of the present invention;

FIG. 6 is a schematic structural diagram of a composite grating light splitting module provided by an embodiment of the present invention;

FIG. 7 is a schematic diagram of assembly site measurement points provided by an embodiment of the present invention;

Fig. 8 is a schematic diagram of spot measurement points for pose adjustment provided by an embodiment of the present invention;

Fig. 9 is a schematic diagram of the change rule of the spot position with the displacement provided by the embodiment of the present invention;

Fig. 10 is a schematic diagram of a DC optical path module;

Fig. 11 is a schematic diagram of the generation principle of the pulse signal;

Fig. 12 is a schematic diagram of the generation form of the pulse signal;

Figure 13 is a schematic diagram of the detection of the absolute position;

Fig. 14 is a schematic diagram of the zero point marked by the zero position signal on the incremental signal;

Fig. 15 is a schematic diagram of phase cooperative positioning improving pulse positioning accuracy;

Fig. 16 is a schematic diagram of the relationship between the phase and the displacement of the reading head within the incremental signal period;

Fig. 17 is a schematic diagram of a three-dimensional structure of a single-point measurement absolute six-degree-of-freedom grating encoder;

Fig. 18 is a schematic diagram of single-point measurement composite integrated grating;

FIG. 19 is a schematic diagram of a composite integrated mask for single-point measurement.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, but not to limit the present invention. In addition, it should be noted that, for the convenience of description, only some structures related to the present invention are shown in the drawings but not all structures.

Fig. 1 is a schematic structural diagram of an absolute six-degree-of-freedom grating encoder provided according to an embodiment of the present invention, including a light source module 10, an absolute four-degree-of-freedom measurement module 20, and an absolute two-degree-of-freedom measurement module 30; The collimated light beam generated by the light source module 10 is passed through the polarization beam splitter PBS to generate the first collimated light beam L1 and the second collimated light beam L2, and the second collimated light beam L2 first enters the composite grating light splitting module 4 to generate the light beam L2-1 , light beam L2-2 and light beam L2-3, the schematic structural diagram of the compound grating light splitting module 4 is shown in Figure 6, the light beam L2-1 utilizes the two-dimensional grating area of the compound grating 5 to generate diffracted light, and the diffracted light is passed through an absolute four-degree-of-freedom The measurement module 20 measures the absolute pose of the composite grating 5 in the θ _x , θ _y , θ _z and z directions. The light beam L2-2 and the light beam L2-3 enter the X reference code track through the X-direction mask respectively, and pass through the Y-direction The mask plate enters the Y reference code track, the X-direction mask plate and the X reference code track have the same code, the Y-direction mask plate and the Y reference code track have the same code, and the X-direction mask plate and the X-direction mask When the stencil, the Y reference code track and the Y-direction mask are aligned, a negative pulse is generated to mark the zero position, and the beam generated by the composite grating 5 and the beam generated by the first collimated beam L1 on the two-dimensional reference grating 3 The interference phenomenon is generated, and is received by the absolute two-degree-of-freedom measurement module 30, and the phase changes in the x and y directions are analyzed to obtain displacement incremental information, and the pulse signal generated at the same time can perform incremental displacement Zero calibration, get its absolute displacement. The composite grating beam splitting module 4 includes a group of light prisms, which can split the output of the second collimated light beam L2 into the light beam L2-1, the light beam L2-2 and the light beam L2-3. The two-dimensional grating area 5-1, the X-direction reference code track 5-2, and the Y-direction reference code track of the composite grating 5 are shown in Figure 3, the X-direction mask plate is shown in Figure 4, and the Y-direction mask plate is shown in Figure 4. 5.

The light source module 10 is used to generate collimated light beams, and includes a light source LD, a collimating lens CL1 and an aperture 6 . The collimated light beam generated by the light source module 10 first passes through the polarization beam splitter PBS to obtain the light beam L1 and the light beam L2. The light beam L1 is irradiated on the two-dimensional reference grating 3 and diffracted to generate four beams of diffracted light. The four beams of diffracted light are respectively beam x _r+1 , The beam x _r-1 , the beam y _r+1 and the beam y _r-1 , and the beam L2 pass through the composite grating beam splitting module 4 etc. to generate three beams of light, which are beam L2-1, beam L2-2, and beam L2-3. The absolute four-degree-of-freedom measurement module includes three four-quadrant photodetectors QPDA/B/C, two mirrors M1, M2, and three focusing lenses CL1/2/3. The light beam L2-1 is irradiated on the two-dimensional grating area of the composite grating to generate five beams of diffracted light, namely x _s+1 , x _s-1 , y _s+1 , y _s-1 and zero-order light. Before entering the absolute two-degree-of-freedom measurement module, the second beam splitting prism BS2 splits x _s+1 , x _s-1 and zero-order light, and the generated beam is set to x` _s+1 , x` _s-1 and zero Level light enters the absolute four-degree-of-freedom measurement module, and the absolute pose in the θ _x , θ _y , θ _z and z directions can be obtained by decoupling the position changes of the three light spots on the four-quadrant photodetector. The absolute two-degree-of-freedom measurement module 30 includes a two-degree-of-freedom zero pulse signal measurement module and a two-degree-of-freedom incremental signal measurement module. The two-degree-of-freedom zero pulse signal measurement module includes a polarization beamsplitter prism PBS, a second quarter-wave plate QWP2, a second beam-splitting prism BS2, a first prism collimation unit 1, a first quarter-wave plate QWP1, and a first quarter-wave plate QWP1. A beam splitting prism BS1, a second prism collimation unit 2, a two-dimensional reference grating 3, and a DC removal module for incremental signals, the DC removal module includes a depolarization beam splitter NPBS, a first polarizer P1, and a third quarter Slide QWP3, second polarizer P2 and photodetectors PD3, PD4. As shown in Figure 2, the second collimated light beam L2 passes through the composite grating beam splitting module 4 to generate two other beams of light L2-2 and L2-3, respectively enter the X reference code track through the X-direction mask plate, and pass through the Y-direction mask plate Enter the Y reference code channel. The two-degree-of-freedom zero-position pulse signal measurement module includes composite grating light-splitting modules BS3, BS4, BS5, BS6, and photodetectors PD1 and PD2. The beam x _s+1 , the beam x _s-1 , the beam y _s+1 and the beam y _s-1 produced by the two-dimensional grating area 5-1 of the compound grating 5, and the beam x _r+ 1 produced by the two-dimensional reference grating 3 , the light beam x _r-1 , the light beam y _r+1 and the light beam y _r-1 produce an interference phenomenon at the polarization beam splitter PBS, and are received by the photodetector PD3 and photodetector PD4, respectively analyzing the phase changes in the x and y directions Then get the displacement increment information. At the same time, the pulse signal detected by the photodetector PD1 and the photodetector PD2 can be zero-calibrated for the incremental displacement to obtain its absolute position.

First, a laser with a wavelength of 660nm is emitted from a laser diode, collimated into a parallel laser by the collimator lens CL1, and passes through a diaphragm 6 with an aperture of 2mm, and the parallel laser beam is shaped into a laser with a diameter of 2mm. After passing through a polarization beam-splitting prism PBS, it is divided into two beams of light. One beam of light is split by the composite grating beam-splitting module 4 and turned, and finally passes through the X-direction mask and the Y-direction mask, and is received by photodetectors PD1 and PD2. Zero pulse signal, another beam of light first enters the incremental displacement measurement module, and at the same time enters the angle measurement module.

1. The actual use is the splicing of multiple sub-mirrors. Here we take the splicing of two sub-mirrors as an example, which can cover all monitoring situations during installation and use.

(1) All the sub-mirrors are spliced into a complete telescope main mirror at the assembly site, and the wavefront sensor is used to adjust the pose of each sub-mirror until the wavefront sensor obtains a complete interference image without distortion.

(2) At this time, the reading head installed at the telescope bracket reads and records the position and posture of each sub-mirror, for example, as shown in Figure 7, the absolute position information of the reading head recording sub-mirror A and B is: X _A ,Y _A ,Z _A ,Rx _A ,Ry _A ,Rz _A ,X _B ,Y _B ,Z _B ,Rx _B ,Ry _B ,Rz _B . By analogy, the absolute pose of each sub-mirror is calculated.

2. On-site use

After adjusting the rough pose of each sub-mirror, the absolute six-degree-of-freedom grating encoder is used to monitor the pose, as shown in Figure 8, so that

After the adjustment, fix the position of the reading head, and then adjust the pose of another sub-mirror that needs to be spliced, so that the display of the reading head is as follows:

At this time, the relative pose of the two sub-mirrors can be guaranteed to return to the pose state of the assembly stage, and then the installation of the rest of the sub-mirrors can be completed. The posture is monitored during use for the actuator to adjust the posture.

During the working stage of the telescope, if the pose of the sub-mirror changes due to factors such as gravity, wind load, and temperature, the absolute six-degree-of-freedom readhead can obtain the pose data of the sub-mirror in real time, and the active controller can adjust the sub-mirror according to the feedback information. Mirror pose to restore to the pose state of each sub-mirror of the telescope in the assembly stage. In this way, a closed-loop control system is formed, which can adjust the position and orientation of the sub-mirror in real time when the telescope is working.

Working principle of the absolute four-degree-of-freedom measuring module

The specific positions x _A , y _A , x _B , y _B , x _C , y _C of the light spot are calculated according to the back-end photocurrent information I, and the specific calculation formula can be expressed as:

Where k ₁ and k ₂ are proportional coefficients, α=A, B, C.

Firstly, the coordinates of the spot position are transformed into the coordinates of the origin, that is, when it is not measured, its coordinate system is artificially defined, so that all diffracted light can be located at the origin of the coordinates. Figure 9 shows the position change of the light spot when the specified displacement of the measurement grating occurs.

From the above position change law, it can be deduced that when only a single degree of freedom pose change occurs, the change law of the absolute pose change and the spot position is:

In the formula, k _z , k _θx , k _θy and k _θz are undetermined parameters determined by experiments, f is the focal length of the front convex lens of the photodetector, and L is the equivalent between photodetectors A and B (or C and B). distance.

Assume that the positions of the existing spot in the three four-quadrant photodetectors QPDA/B/C are (x ₁ .y ₁ )(x ₀ ,y ₀ ),(x _-1 ,y _-1 ), due to the zero order The position of the light has changed, and the position change of the zero-order light is only related to θ _x , θ _y , so the values of the two can be directly deduced.

In the formula, k _θxz1 and k _θxz-1 are the asymmetric influence coefficients of θ _x on the y-direction position of the +1 and -1 order spots during the movement, k _θydz1 and k _θydz-1 are the θ _y to +1 and -1 during the movement The asymmetric influence coefficient of the x-direction position of the order spot.

At this time, the position of the spot on the four-quadrant photodetector QPDA and the four-quadrant photodetector QPDC will also change under the influence of the attitude changes of θ _x and θ _y , which can be calculated by the asymmetric influence factor and the influence can be excluded. In a small range, the θ _z attitude change only changes the y-direction spot position of QPDA and QPDB, and the z-direction attitude change only changes the x-direction spot position of QPDA and QPDB, so the summary is as follows:

In the theoretical process, a zero-order diffracted light and a set of ±1-order diffracted light are needed to complete the measurement.

The working principle of the absolute two-degree-of-freedom measurement module:

1) Two degrees of freedom incremental signal measurement module

After the light beam passes through the polarizing beam splitter PBS, two sets of 0° and 90° signals can be obtained through diffracted light interference, and then the signals can be processed.

Note that the z direction at this time is measured by the incremental displacement module, and the data value is the incremental displacement. With the assistance of the absolute four-degree-of-freedom measurement module, it can be converted into absolute position coordinates.

2) Two degrees of freedom zero pulse signal measurement module

The two-dimensional reference grating and the incremental grating have the same grating period, so when the reading head moves along the X direction, the photodetector PD3 and photodetector PD4 convert the interference signal into an electrical signal at the coincident beam for processing and calculation to obtain the incremental displacement information. The second collimated light beam L2 emitted by the second beam-splitting prism BS2 passes through the composite grating beam-splitting module 4, and then splits into beams L2-1, L2-2, and L2-3, respectively irradiating the two-dimensional grating areas 5-1, X-direction reference code track 5-2 and Y-direction reference code track 5-3, light beam L2-2 and L2-3 respectively pass through the X-direction mask and the Y-direction mask and reflect to the photodetector PD1 and photodetector PD2. The X-direction mask, the Y-direction mask and the corresponding reference code track are equipped with zero marks of the same code. When the reading head moves, the reflected light intensity changes accordingly, and the reading head passes one of the zero marks every time. The photodetector can detect a corresponding negative pulse zero position signal, and the generation principle and form of the pulse signal are shown in Fig. 11 and Fig. 12 .

Through the compound grating beam splitting modules BS3, BS4, BS5, BS6, the light beams L2-2 and L2-3 can pass through the X-direction mask and the Y-direction mask respectively to the X reference code track and the Y reference code track, and finally Received by photodetectors PD1 and PD2 respectively. Therefore, when the X-direction mask and the Y-direction mask are displaced relative to the composite grating, the reflected light signals received at PD1 and PD2 are a negative pulse signal.

For example, when the reading head moves along the compound grating X direction and passes two zero marks, the absolute position of the current reading head can be obtained according to the incremental signal and the zero position signal. When the reading head moves from point O, a negative pulse signal is formed after the first zero mark, and the reference point R1 is determined by the pulse signal. When the reading head passes the second zero mark, the zero negative pulse signal can determine the reference point Point R2, and finally the reading head continues to move to A and stops. The distance between the reference points R1 and R2 is D1, the distance between the reference point R2 and the reference point R3 is D2, and so on are respectively D3, D4, D5, . . . . At this time, D1, D2, D3, etc. are set as distance codes. During measurement, the incremental signal module can measure the distance between the two reference points as S12, and compare it with the above distance codes, then the two reference points can be known and calculated. absolute coordinates. For example, if S12=D1, it can be known that the two reference points passed at this time are R1 and R2 respectively, and the absolute positions P1 and P2 of the reference point R1 and the reference point R2 can be obtained. The incremental signal can calculate the incremental displacement between any adjacent zero reference points. The zero signal and the reference signal are sampled at the same time. According to the reference points R1 and R2 determined by the zero signal, the corresponding incremental signal from R1 to R2 can be calculated. Displacement, by which the absolute position is determined against the coded distance. When the absolute positions of R1 and R2 are determined, the current position PA of the reading head can be calculated according to the geometric position relationship, as shown in Figure 13.

P _A =P _II +S ₂₃ (8)

Continue to use phase cooperation for positioning within the range of zero pulse positioning accuracy, which is a combination of coarse and fine positioning, as shown in Figure 14 and Figure 15. That is, within the pulse positioning range, only the pulse peak can be fitted, so the position of the pulse tip point changes with the degree of fitting within this range, and the accuracy cannot be guaranteed. Therefore, with the help of the phase assistance of the incremental signal, within each different pulse positioning accuracy range, it corresponds to a certain phase in a 0.5 μm phase in the incremental signal, that is, the zero true value point. Finally, there is no need for fitting to find the pulse tip point, and the absolute positioning analysis is performed on this true value point, which avoids errors caused by noise and data fluctuations, and its positioning accuracy is directly related to the subdivision accuracy of the incremental signal of the grating ruler.

According to the incremental displacement calculation theory of the grating ruler, when the grating ruler is displaced along the X direction, the phase of the incremental signal and the displacement have the following one-to-one correspondence:

Among them, Ω _X is the phase of the incremental signal, Ω _X0 is the initial phase of the incremental signal, ΔX is the incremental displacement in the X direction, and g is the grating pitch of the measuring grating.

When the displacement is constant, the phase displacement of the incremental signal is shown in Figure 16. This position (phase) in the incremental signal period can be marked as the position of the zero pulse. When a zero pulse signal is obtained each time, It is only necessary to find the marked position in the corresponding incremental signal period, so that the positioning accuracy of the zero signal can be greatly improved, and its positioning accuracy is directly related to the subdivision accuracy of the incremental signal of the grating ruler.

The three-dimensional structural diagram of the single-point measurement absolute six-degree-of-freedom grating encoder is shown in Figure 17, the single-point measurement composite integrated grating 6 is shown in Figure 18, and the single-point measurement composite integrated mask 7 is shown in Figure 19. The X- and Y-direction reference code tracks of the composite grating are integrated with the two-dimensional grating area. The advantage is that a single measuring point can generate the required five beams of diffracted light, avoiding the Abbe error. While ensuring the interference effect of the grating diffracted light, the reference code in the grating area can be used for absolute positioning, forming an absolute six-degree-of-freedom grating encoder with a single measuring point.

In the process of measuring the movement of the grating, an image analysis method can be used to analyze the position of the measuring grating relative to the mask. The judgment process is to extract the image codes of the mask plate and the measurement grating respectively, and perform real-time position comparison. The degree of overlap and direction between the two changes with the position, so as to obtain the absolute coordinates in the x and y directions.

The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments, and it cannot be assumed that the specific implementation of the present invention is limited to these descriptions. For those of ordinary skill in the technical field of the present invention, without departing from the concept of the present invention, some simple deduction or replacement can be made, which should be regarded as belonging to the protection scope of the present invention.

Claims

An absolute six-degree-of-freedom grating encoder is characterized in that it includes a light source module, an absolute four-degree-of-freedom measurement module, and an absolute two-degree-of-freedom measurement module; the collimated light beam generated by the light source module passes through a polarizing beam splitter Generate the first collimated beam L1 and the second collimated beam L2, the second collimated beam L2 is first divided into the beam L2-1, the beam L2-2 and the beam L2-3 through the composite grating beam splitting module, the beam L2-1 utilizes The two-dimensional grating area of the composite grating generates diffracted light, and the diffracted light is measured by the absolute four-degree-of-freedom measurement module to measure the θ x , θ y , θ z and the absolute pose of the composite grating in the z direction. The beams L2-2 and L2- 3 Enter the X reference code channel through the X-direction mask, and enter the Y reference code channel through the Y-direction mask. The X-direction mask and the X reference code have the same code, and the Y-direction mask and the Y The reference code track has the same code, and when the X reference code track is aligned with the X-direction mask, and the Y reference code track is aligned with the Y-direction mask, a negative pulse is generated to mark the zero position, and the beam L2-1 passes through the compound grating The light beam produced by the diffraction of the grating area, and the beam produced by the first collimated light beam L1 in the two-dimensional reference grating produce interference phenomenon, and is received by the absolute two-degree-of-freedom measurement module, and the phase change in the x and y directions is analyzed to obtain the displacement Incremental information and the pulse signal generated at the same time can be zero-calibrated for incremental displacement to obtain its absolute displacement.
The absolute six-degree-of-freedom grating encoder according to claim 1, wherein the absolute four-degree-of-freedom measurement module includes three four-quadrant photodetectors.
The absolute six-degree-of-freedom grating encoder according to claim 2, wherein the diffracted light includes ±1st-order diffracted light and zero-order light in the x direction, and the positions of the three beam spots on the four-quadrant photodetector Change, and solve the absolute pose of θ x , θ y , θ z and z directions.
The absolute six-degree-of-freedom grating encoder according to claim 1, wherein the absolute two-degree-of-freedom measurement module includes a two-degree-of-freedom incremental signal measurement module and a two-degree-of-freedom zero pulse signal measurement module.
The absolute six-degree-of-freedom grating encoder according to claim 4, characterized in that Therefore, the two-degree-of-freedom incremental signal measurement module includes a polarization beamsplitter prism (PBS), a second quarter slide (QWP2), a second beamsplitter prism (BS2), and a first prism collimation unit (1), The first quarter-wave plate (QWP1), the first dichroic prism (BS1), the second prism collimation unit (2), the two-dimensional reference grating (3), and the DC removal module of the incremental signal.
The absolute six-degree-of-freedom grating encoder according to claim 5, wherein the direct current removal module includes a depolarization beam splitter prism (NPBS), a first polarizer (P1), and a third quarter glass (QWP3), second polarizer (P2) and photodetectors (PD3, PD4).
The absolute six-degree-of-freedom grating encoder according to claim 4, wherein the two-degree-of-freedom zero pulse signal measurement module includes a composite grating light splitting module (BS3, BS4, BS5, BS6), a photodetector ( PD1, PD2).
The absolute six-degree-of-freedom grating encoder according to claim 7, wherein the two-degree-of-freedom zero pulse signal measurement module passes two beams of light L2-2 and L2-3 through the X-direction mask respectively Enter the X reference code track, and enter the Y reference code track through the Y mask.
The absolute six-degree-of-freedom grating encoder according to claim 5, wherein the two-degree-of-freedom incremental signal measurement module generates x s+1 , x s-1 , The y s+1 and y s-1 beams interfere with the x r+1 , x r-1 , y r+1 and y r-1 beams generated by the two-dimensional reference grating, and receive them to analyze x and The phase change in the y direction can then obtain the incremental displacement information, and the pulse signal generated at the same time can perform zero calibration on the incremental displacement to obtain its absolute displacement.
The absolute six-degree-of-freedom grating encoder according to claim 1, wherein the light source module includes a light source, a collimating lens (CL1) and an aperture.