CN108775878B - Grating heterodyne interference system and roll angle measuring method thereof - Google Patents
Grating heterodyne interference system and roll angle measuring method thereof Download PDFInfo
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Abstract
The invention discloses a grating heterodyne interference system, and belongs to the technical field of laser precision measurement. The system comprises a laser source, a polarization beam splitter prism, a reflector, a pyramid prism, a first wave plate and a second wave plate, wherein the laser source is arranged on a first side of an object to be measured; a first grating and a second grating disposed on the object to be measured; and a plane mirror group arranged on a second side of the object to be measured.
Description
Technical Field
The invention relates to the technical field of laser precision measurement, in particular to a grating heterodyne interference system and a roll angle measurement method thereof.
Background
The precision guide rail kinematic pair is a key common moving part of modern precision engineering, and is widely applied to the high-tech fields of high-grade equipment manufacturing, aerospace, advanced synchrotron radiation light sources and the like. The advanced roll angle measurement technology pre-method is one of bottleneck technologies to be solved and promoted in the field. Due to the structural characteristic that the arc (displacement) plane of the roll angle is orthogonal to the measuring axis of the instrument, the traditional methods such as a laser interferometer, an autocollimator and the like can not meet the measuring requirements.
Therefore, a plurality of new methods are provided for roll angle measurement, and the three methods mainly comprise a Wollaston prism laser interferometry, a structured light method based on auto-collimation, a light intensity/phase method based on polarization characteristics and the like.
The first method is based on geometrical optics principle, uses special optical elements as a sensing lens and a target lens of an interference system, and converts an angle into displacement to measure a roll angle. Therefore, it has high requirements for the precision and quality of the optical element, is difficult to realize in the process, and is difficult to realize high-precision measurement.
The second method is based on the auto-collimation principle, and converts the roll angle into the change of the spot Position through the geometric light path structure design, and the change is detected by a PSD (Position Sensitive Detector), thereby realizing the roll angle measurement. Although the method is simple to implement, the method is easily influenced by coupling of other factors such as straightness accuracy and the like, and particularly the requirements for measuring and compensating the single error of the high-precision roll angle are difficult to meet.
The third method is essentially to measure the sensitivity of the polarization device to the roll angle, mainly comprises a light intensity detection method and a phase detection method, wherein the former method is easy to realize and has lower precision, and belongs to the early proposal; the latter is a high-precision measurement method, which is the current development direction. The method can realize high resolution, but a nonlinear response curve needs to be calibrated, and the application of the corresponding perfect optical element is limited because the perfect optical element is difficult to obtain, such as Chinese patent publication No. CN103292744A, a roll angle measuring device and method based on diffraction grating displacement technology, and CN104535019A, a roll angle measuring device and method based on double diffraction grating heterodyne interference, and the like.
Therefore, a new grating heterodyne interferometry system and a roll angle measurement method thereof are needed.
The above information disclosed in this background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.
Disclosure of Invention
The invention aims to provide a grating heterodyne interference system with high resolution and easiness in processing and a roll angle measuring method thereof.
Additional features and advantages of the invention will be set forth in the detailed description which follows, or may be learned by practice of the invention.
According to an aspect of the present invention, there is provided a grating heterodyne interferometry system, including: a laser light source disposed at a first side of an object to be measured; the polarization beam splitter prism is arranged on the optical axis of the laser source and is positioned on the first side of the object to be measured; the reflector is arranged on a reflection optical axis of the polarization beam splitter prism and positioned on a first side of the object to be measured, and a preset horizontal distance is reserved between the reflector and the polarization beam splitter prism; the pyramid prism is arranged in the reverse direction of the reflection optical axis of the polarization beam splitter prism and is positioned on the first side of the object to be measured; the first wave plate is arranged on the transmission optical axis of the polarization beam splitter prism and positioned on the first side of the object to be measured, the first wave plate is positioned between the polarization beam splitter prism and the first grating, and the first wave plate is close to the polarization beam splitter prism; the second wave plate is arranged on the reflecting optical axis of the reflector and positioned on the first side of the object to be measured, the second wave plate is positioned between the reflector and the second grating, and the second wave plate is close to the reflector; the first grating is arranged on the transmission optical axis of the polarization splitting prism and is fixed on the object to be measured; the second grating is arranged on the reflection optical axis of the reflector and fixed on the object to be measured, wherein the first grating and the second grating are respectively positioned on two sides of the rolling center of the object to be measured, the first grating and the second grating are positioned on the same plane, and the first grating and the second grating have grating constants; the plane reflector group is arranged on the preset-order diffraction optical axes of the first grating and the second grating and is positioned on the second side of the object to be detected; wherein the first side and the second side are opposing sides.
In one embodiment, the plane mirror assembly comprises: the first plane mirror is arranged on the preset order diffraction optical axis of the first grating, and the normal line of the first plane mirror is parallel to the preset order diffraction light of the first grating; a first side edge and a second side edge of the first plane mirror, which are oppositely arranged, are parallel to a first incident plane, and the first incident plane is composed of preset-order diffracted light of the first grating and a normal of the first plane mirror; the second plane mirror is arranged on the preset order diffraction optical axis of the second grating, and the normal line of the second plane mirror is parallel to the preset order diffraction light of the second grating; the first side edge and the second side edge of the second plane reflector which are oppositely arranged are parallel to a second incident surface, and the second incident surface is composed of preset-order diffraction light of the second grating and a normal of the second plane reflector.
In an embodiment, the first plane mirror and the second plane mirror are the same strip-shaped plane mirror; wherein the first side edge and the second side edge have a predetermined length.
In one embodiment, the plane mirror assembly comprises: the whole plane reflector is arranged on the preset-order diffraction optical axes of the first grating and the second grating, and the normal line of the whole plane reflector is parallel to the preset-order diffraction light of the first grating and the second grating; the first side edge and the second side edge of the whole plane reflector which are oppositely arranged are parallel to a third incident surface, and the third incident surface consists of the first grating and the preset diffraction light of the second grating and the normal line of the whole plane reflector.
In one embodiment, the first side and the second side have a predetermined length.
In an embodiment, the first wave plate and the second wave plate are the same 1/4 wave plates configured to convert a normally incident linearly polarized light into a circularly polarized light or convert a normally incident circularly polarized light into a linearly polarized light.
In one embodiment, the first grating and the second grating are the same diffraction grating.
According to another aspect of the present invention, there is provided a roll angle measuring method based on any of the above-mentioned embodiments, including: emitting a laser beam by the laser source; dividing the laser beam into a first polarized light and a second polarized light through the polarization beam splitter prism, wherein the linear polarization directions of the first polarized light and the second polarized light are orthogonal and different in frequency; the first polarized light is incident to the first wave plate and is changed into first circularly polarized light, and the first circularly polarized light is incident to the first grating and is subjected to first diffraction to generate first preset-order diffracted light; the first preset-order diffraction light enters the plane mirror group, returns along an original path after being reflected, enters the first grating and is diffracted for the second time to generate second preset-order diffraction light; the second preset-order diffracted light enters the first wave plate, is converted into the first polarized light by the first circularly polarized light, is reflected to the pyramid prism by the polarization beam splitter prism, is reflected by the pyramid prism, returns in parallel, is reflected again by the polarization beam splitter prism, penetrates through the first wave plate, is converted into second circularly polarized light by the first polarized light, and enters the first grating to generate third diffraction to generate third preset-order diffracted light; the third preset-order diffraction light enters the plane mirror group and returns along the original path through reflection, and enters the first grating to generate fourth diffraction to generate fourth preset-order diffraction light; the fourth preset-order diffraction light passes through the first wave plate, is converted into the first polarized light from the second circularly polarized light, and is transmitted through the polarization beam splitter prism to form a first measuring beam; after the second polarized light is reflected by the reflector, the second polarized light is incident to the second wave plate and becomes third circularly polarized light, and the third circularly polarized light is incident to the second grating and generates first diffraction to generate fifth preset-order diffraction light; the fifth preset-order diffracted light enters the plane mirror group, returns along the original path after being reflected, enters the second grating and is subjected to secondary diffraction to generate sixth preset-order diffracted light; the sixth preset-order diffracted light enters the second wave plate, is changed into the second polarized light by the third circularly polarized light, is reflected to the pyramid prism by the reflector and the polarization beam splitter prism in sequence, is reflected by the pyramid prism, returns in parallel, is reflected again by the polarization beam splitter prism and the reflector in sequence, penetrates through the second wave plate, is changed into the fourth circularly polarized light by the second polarized light, and enters the second grating to generate third diffraction to generate seventh preset-order diffracted light; the seventh preset-order diffraction light enters the plane mirror group and returns along the original path after being reflected, and enters the second grating to generate fourth diffraction to generate eighth preset-order diffraction light; the eighth preset-order diffracted light is converted into second polarized light from the fourth circularly polarized light through the second wave plate, and the second polarized light is reflected by the reflector and the polarization beam splitter prism in sequence to form a second measuring beam; the first measuring beam and the second measuring beam are coincided to generate an interference signal; obtaining a difference value of Doppler frequency changes of the first measuring beam and the second measuring beam caused by the rolling motion of the object to be measured according to the interference signal and a reference signal, wherein the frequency of the reference signal corresponds to the frequency difference of the first polarized light and the second polarized light; and obtaining the roll angle of the object to be measured according to the horizontal spacing, the grating constant and the Doppler frequency change difference value.
In an embodiment, when the grating heterodyne interference system is in a measurement state, the laser source, the polarization splitting prism, the reflector, the corner cube prism, the first wave plate and the second wave plate, and the first plane mirror and the second plane mirror are all fixed and are relatively static; the first and second optical gratings move with the object to be measured.
In one embodiment, the roll angle α is calculated by the formula:
wherein T is a counting time period, s is a relative angular displacement of the first grating and the second grating, L is the horizontal pitch, d is a grating constant of the first grating and the second grating, and Δ f is a doppler frequency variation difference of the interference signal and the reference signal.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and together with the description, serve to explain the principles of the invention.
FIG. 1 schematically illustrates a three-dimensional structure of a grating heterodyne interferometry system, in accordance with an illustrative embodiment of the present invention;
FIG. 2 is a schematic view of the left side view of FIG. 1 and the linear motion of the roll plane;
FIG. 3 is a top view of FIG. 1;
FIG. 4 schematically illustrates a three-dimensional structural view of another grating-heterodyne interferometry system, in accordance with an illustrative embodiment of the present invention;
fig. 5 schematically shows a flowchart of a roll angle measurement method based on a grating heterodyne interferometry system according to an exemplary embodiment of the present invention.
Detailed Description
Example embodiments will now be described more fully with reference to the accompanying drawings. Example embodiments may, however, be embodied in many different forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of example embodiments to those skilled in the art. The drawings are merely schematic illustrations of the invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and thus their repetitive description will be omitted.
Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, devices, steps, and so forth. In other instances, well-known structures, methods, devices, implementations, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.
The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of apparatus and methods consistent with certain aspects of the invention, as detailed in the appended claims.
In the description and claims, the terms "a" and "an" can refer broadly to a single item or a plurality of items unless the context specifically states the item.
The embodiment of the invention firstly provides a grating heterodyne interference system, which can comprise: a laser light source that may be disposed at a first side of an object to be measured; a polarization splitting prism that may be disposed on an optical axis of the laser light source and may be located at a first side of the object to be measured; a reflector which may be disposed on a reflection optical axis of the polarization splitting prism and may be located at a first side of the object to be measured, and which may have a predetermined horizontal interval therebetween; a pyramid prism which may be disposed in a reverse direction of a reflection optical axis of the polarization splitting prism and may be located on a first side of the object to be measured; a first wave plate that may be disposed on a transmission optical axis of the polarization beam splitter prism and may be located on a first side of the object to be measured, the first wave plate may be located between the polarization beam splitter prism and the first grating, and the first wave plate may be close to the polarization beam splitter prism; a second wave plate which may be disposed on a reflection optical axis of the reflector and may be located at a first side of the object to be measured, the second wave plate may be located between the reflector and the second grating, and the second wave plate may be close to the reflector; a first grating which may be disposed on a transmission optical axis of the polarization splitting prism and which may be fixed on the object to be measured; a second grating which may be disposed on a reflection optical axis of the reflector and which may be fixed on the object to be measured, wherein the first grating and the second grating may be respectively located at both sides of a roll center of the object to be measured, and the first grating and the second grating may be located in the same plane, and the first grating and the second grating may have grating constants; a plane mirror group, which may be disposed on a diffraction optical axis of a preset level of the first grating and the second grating (in the following embodiments, the +1 level or the-1 level is taken as an example for description, but the present invention is not limited thereto, and may be selected according to actual requirements), and may be located on a second side of the object to be measured; wherein the first side and the second side may be opposing sides.
In an exemplary embodiment, the planar mirror group may include: the first plane mirror can be arranged on the preset order diffraction optical axis of the first grating, and the normal line of the first plane mirror can be parallel to the preset order diffraction light of the first grating; a first side edge and a second side edge of the first plane mirror, which are oppositely arranged, can be parallel to a first incident plane, and the first incident plane can be composed of the preset-order diffracted light of the first grating and the normal of the first plane mirror; the second plane mirror can be arranged on the preset order diffraction optical axis of the second grating, and the normal line of the second plane mirror can be parallel to the preset order diffraction light of the second grating; the first side and the second side of the second plane mirror, which are oppositely disposed, may be parallel to a second incident plane, and the second incident plane may be composed of the preset-order diffracted light of the second grating and a normal of the second plane mirror.
In an exemplary embodiment, the first and second planar mirrors may be the same strip-shaped planar mirror; wherein the first side edge and the second side edge may have a predetermined length.
In an exemplary embodiment, the planar mirror group may include: the whole plane mirror can be arranged on the preset order diffraction optical axes of the first grating and the second grating, and the normal line of the whole plane mirror can be parallel to the preset order diffraction light of the first grating and the second grating; the first side edge and the second side edge of the whole plane mirror which are oppositely arranged can be parallel to a third incident surface, and the third incident surface can be formed by the preset-order diffraction light of the first grating and the second grating and the normal line of the whole plane mirror.
In an exemplary embodiment, the first side edge and the second side edge may have a predetermined length.
In an exemplary embodiment, the first and second wave plates may be the same 1/4 wave plate, and may be configured to convert a linearly polarized light at normal incidence into a circularly polarized light or to convert a circularly polarized light at normal incidence into a linearly polarized light. In the following embodiments, the first wave plate and the second wave plate are illustrated as 1/4 wave plates.
Among them, 1/4 wave plate (quartz-wave plate) is a birefringent single crystal sheet with a certain thickness. When the light is transmitted through the normal incidence, the phase difference between the ordinary light (o light) and the extraordinary light (e light) is equal to pi/2 or an odd multiple thereof, and such a wafer is called a quarter-wave plate or 1/4-wave plate. When linearly polarized light vertically enters 1/4 wave plate, the light polarization and the optical axis plane (vertical natural cleavage plane) of mica form angle theta, and then form elliptical polarized light after exiting. In particular, when θ is 45 °, the emitted light is circularly polarized light.
It should be noted that, because a certain error may exist in actual operation, in the embodiment of the present invention, the first wave plate and the second wave plate may not strictly convert the normally incident linearly polarized light into the circularly polarized light, or convert the normally incident circularly polarized light into the linearly polarized light, and may allow a certain error.
In an exemplary embodiment, the first grating and the second grating may be the same diffraction grating.
In an exemplary embodiment, the reflector may include any one of a right-angle reflecting prism, a plane mirror, a penta prism, and the like. In the following embodiments, the reflector is exemplified as a right-angle reflecting prism.
The grating heterodyne interference system is described below with reference to the accompanying drawings.
Fig. 1 schematically illustrates a three-dimensional structure diagram of a grating heterodyne interferometry system in accordance with an exemplary embodiment of the present invention. Fig. 2 is a left side view of fig. 1 and a schematic view of the linear motion of the roll plane. Fig. 3 is a top view of fig. 1.
With reference to fig. 1 to 3, a grating heterodyne interference system provided in an embodiment of the present invention may include: the device comprises a laser source 1, an interference mirror group, a double 1/4 wave plate 2, a double grating 3, a double plane mirror 4 and an object to be measured 5. Wherein the laser source 1 and the set of interference mirrors and the double 1/4 wave plate 2 are located on a first side of the object to be measured 5 and the double plane mirror 4 is located on a second side of the object to be measured 5, said first and said second sides being opposite sides.
Wherein the laser source 1 may be configured to provide a laser beam and a reference signal, the laser beam may include two polarization components with different frequencies and orthogonal linear polarization directions; the frequency of the reference signal corresponds to the difference in frequency of the two polarization components.
In the exemplary embodiment, a dual-frequency laser measuring head may be selected as the laser source 1, but the present invention is not limited thereto.
In some embodiments, the laser source 1 may also be used to implement functions such as optical reception, signal processing and circuit subdivision, communication with a PC (personal computer).
In the embodiment of the present invention, the interference mirror group and the double 1/4 wave plate 2 may further include an interference mirror group and a double 1/4 wave plate. Further, the interference mirror group may include a polarization splitting prism 201, a reflector 202 (here, a right-angle reflecting prism is taken as an example, but the invention is not limited thereto), and a corner cube 205. The dual 1/4 wave plates may include a first wave plate 203 and a second wave plate 204.
In the embodiment of the present invention, the first wave plate 203 and the second wave plate 204 are two identical 1/4 wave plates, and the double 1/4 wave plate is located between the object to be measured 5 and the interference mirror group and close to the interference mirror group.
In the embodiment of the present invention, the double grating 3 as a sensing element may further include two identical diffraction gratings, i.e., a first grating 301 and a second grating 302, and the double grating 3 may move together with the object to be measured 5.
Alternatively, the first grating 301 and the second grating 302 in the double grating 3 are respectively located on both sides of the roll center of the object to be measured 5 and located on the same plane. However, the invention is not limited thereto, for example, the first grating 301 and the second grating 302 may also be arranged on the same side of the object to be measured, i.e. the roll center of the roll plane 5. The first grating 301 and the second grating 302 are disposed on both sides of the roll center in this embodiment, which can improve the accuracy of roll angle measurement.
In an exemplary embodiment, first grating 301 and second grating 302 are the same diffraction grating.
In an exemplary embodiment, the first and second gratings 301 and 302 are two identical blazed gratings, which have both grating constants d.
In the embodiment of the present invention, the dual plane mirror 4 may further include a first plane mirror 401 and a second plane mirror 402 in a bar shape, which are the same.
In the embodiment of the present invention, the object to be measured 5 may be a circular plate, but the shape of the object to be measured 5 is not limited thereto, and may be any other shape such as a rectangle. Further, other arbitrary means that can separately support the first grating 301 and the second grating 302 and move the first grating 301 and the second grating 302 together therewith are used as the object to be measured 5 instead of a circular plate.
Specifically, the polarization splitting prism 201 is disposed on the optical axis of the laser source 1, and a right-angle reflecting prism 202 is disposed on the reflection optical axis of the polarization splitting prism 201; a corner cube 205 is arranged in the opposite direction of the reflection light axis of the polarization beam splitter 201; a first wave plate 203 and a first grating 301 are sequentially arranged on a transmission optical axis of the polarization beam splitter prism 201; a second wave plate 204 and a second grating 302 are sequentially arranged on the reflection optical axis of the right-angle reflection prism 202; a first plane mirror 401 is arranged on the +1 or-1 order diffraction optical axis of the first grating 301; a second plane mirror 402 is disposed on the +1 or-1 order diffraction optical axis of the second grating 302.
In the embodiment of the invention, the laser source 1, the interference mirror group, the double 1/4 wave plates and the double plane mirror 4 are all fixed and are relatively static; only the bigrating 3 can move with the object 5 to be measured.
In the embodiment of the present invention, the normal of the first plane mirror 401 may be parallel to the +1 or-1 st order diffracted light of the first grating 301, that is, it can be ensured that the +1 or-1 st order diffracted light emitted from the first grating 301 is normally incident on the first plane mirror 401, and the long sides (two sides with the length H in fig. 2 are referred to as long sides) of the first plane mirror 401 are parallel to the incident surface of the +1 or-1 st order diffracted light emitted from the first grating 301. The incidence plane of the +1 or-1 st order diffracted light emitted from the first grating 301 is composed of the +1 or-1 st order diffracted light emitted from the first grating 301 and the normal of the first plane mirror 401.
Further, the normal of the second plane mirror 402 can be parallel to the +1 or-1 st order diffracted light of the second grating 302, i.e. it can be ensured that the +1 or-1 st order diffracted light emitted from the second grating 302 is normally incident on the second plane mirror 402, and the long sides (two sides with length H in fig. 2 are referred to as long sides) of the second plane mirror 402 are parallel to the incident surface of the +1 or-1 st order diffracted light emitted from the second grating 302. The incidence plane of the +1 or-1 st order diffracted light emitted from the second grating 302 is composed of the +1 or-1 st order diffracted light emitted from the second grating 302 and the normal of the second plane mirror 402.
FIG. 4 schematically illustrates a three-dimensional structural view of another grating-heterodyne interferometry system, in accordance with an exemplary embodiment of the present invention.
As shown in fig. 4, this embodiment provides a system that is essentially identical in principle to the previous embodiment (see fig. 1-3), except that the dual plane mirror is structurally modified to a single complete full plane mirror 4, which is more suitable for roll angle measurement applications in the case of a small differential pitch of the dual grating 3 (here equal to the horizontal pitch L between the polarizing beam splitter prism 201 and the right angle mirror prism 202).
In the embodiment shown in fig. 4, the entire plane mirror 4 may be a rectangle, and the rectangle may include a first side edge and a second side edge which are oppositely arranged, wherein the first side edge and the second side edge have a predetermined length H; the rectangle may further include a third side and a fourth side that are oppositely disposed, and the third side and the fourth side are perpendicular to the first side and the second side. Optionally, the lengths of the third side and the fourth side are approximately equal to the horizontal distance L between the polarization splitting prism 201 and the right-angle reflecting prism 202.
The principle and the preferred embodiment of the present invention have been explained above, and the explanation is made by taking a dual-frequency laser interferometer as an example, but the principle and the apparatus of the present invention are also applicable to a single-frequency laser interferometer system, and only the dual-frequency laser measuring head and the corresponding function are replaced by a single-frequency laser measuring head and the corresponding function. The preferred embodiments described above should be considered in all respects as illustrative and not restrictive.
Fig. 5 schematically shows a flowchart of a roll angle measurement method of a grating heterodyne interferometry system based on any of the above embodiments according to an exemplary embodiment of the present invention.
In step S510, a laser beam is emitted by the laser source.
In step S520, the laser beam is split into first polarized light and second polarized light through the polarization splitting prism, wherein the linear polarization directions of the first polarized light and the second polarized light are orthogonal and have different frequencies.
In step S530, the first polarized light is incident to the first wave plate and becomes a first circularly polarized light, and the first circularly polarized light is incident to the first grating and is diffracted for the first time to generate a first preset-order diffracted light; the first preset-order diffraction light enters the plane mirror group and returns along the original path through reflection, and enters the first grating to generate second diffraction to generate second preset-order diffraction light; the second preset-order diffracted light enters the first wave plate, is converted into the first polarized light by the first circularly polarized light, is reflected to the pyramid prism by the polarization beam splitter prism, is reflected by the pyramid prism, returns in parallel, is reflected again by the polarization beam splitter prism, penetrates through the first wave plate, is converted into the second circularly polarized light by the first polarized light, and enters the first grating to generate third diffraction to generate third preset-order diffracted light; the third preset-order diffraction light enters the plane mirror group and returns along the original path through reflection, and enters the first grating to generate fourth diffraction to generate fourth preset-order diffraction light; the fourth preset-order diffracted light passes through the first wave plate, is changed into the first polarized light from the second circularly polarized light, and is transmitted through the polarization beam splitter prism to form a first measuring beam.
In step S540, after the second polarized light is reflected by the reflector, the second polarized light enters the second wave plate and is changed into third circularly polarized light, and the third circularly polarized light enters the second grating and is diffracted for the first time to generate fifth preset-order diffracted light; the fifth preset-order diffraction light enters the plane mirror group and returns along the original path through reflection, and enters the second grating to generate second diffraction so as to generate sixth preset-order diffraction light; the sixth preset-order diffracted light enters the second wave plate, is changed into the second polarized light by the third circularly polarized light, is reflected to the pyramid prism by the reflector and the polarization beam splitter prism in sequence, is reflected by the pyramid prism, returns in parallel, is reflected again by the polarization beam splitter prism and the reflector in sequence, penetrates through the second wave plate, is changed into the fourth circularly polarized light by the second polarized light, and enters the second grating to generate third diffraction to generate seventh preset-order diffracted light; the seventh preset-order diffraction light enters the plane mirror group and returns along the original path after being reflected, and enters the second grating to generate fourth diffraction to generate eighth preset-order diffraction light; and the eighth preset-order diffracted light passes through the second wave plate, is converted into the second polarized light from the fourth circularly polarized light, and is reflected by the reflector and the polarization beam splitter prism in sequence to form a second measuring beam.
In step S550, the first measuring beam and the second measuring beam are coincident to generate an interference signal.
In step S560, a difference in doppler frequency changes of the first measuring beam and the second measuring beam caused by the rolling motion of the object to be measured is obtained from the interference signal and a reference signal, wherein the frequency of the reference signal corresponds to a frequency difference of the first polarized light and the second polarized light.
In step S570, a roll angle of the object to be measured is obtained from the horizontal pitch, the grating constant, and the doppler frequency variation difference value.
With reference to fig. 1 to 3, a roll angle measuring method based on the grating heterodyne interference system is described as an example, and the method according to the embodiment of the present invention may include the following steps:
in the first step, the laser beam emitted from the laser source 1 is split into P light and S light by the polarization splitting prism 201 in the interference mirror group.
Secondly, after the P light passes through the polarization beam splitter 201, the P light passes through the first wave plate 203 of the double 1/4 wave plates for the first time, the P light is changed into circular polarized light, after the P light is incident on the first grating 301 of the double grating 3, the P light is diffracted for the first time, the +1 or-1 order diffracted light is incident on the first plane reflector 401 of the double plane reflector 4, the reflected P light returns along the original path, after the reflected P light passes through the first grating 301 of the double grating 3, the P light is diffracted for the second time, the +1 or-1 order diffracted light passes through the first wave plate 203 for the second time, the circular polarized light is changed into P light, after the P light is incident on the polarization beam splitter 201, the P light is reflected to the pyramid prism 205 again, the P light returns in parallel after being reflected by the pyramid prism 205, after the P light is reflected by the polarization beam splitter 201, the P light passes through the first wave plate 203 for the third time, and, the positive 1 or-1 order diffracted light is reflected after being normally incident on the first grating 301 for the third time, and then is normally incident on the first plane mirror 401 again, and then is reflected by the first plane mirror 401, and then returns along the original path, and is diffracted for the fourth time after passing through the first grating 301, and the +1 or-1 order diffracted light thereof is transmitted through the first wave plate 203 for the fourth time, and then is changed from circularly polarized light to P light again, and then is transmitted through the polarization beam splitter prism 201 again to be used as the first measuring beam.
Thirdly, the S light is transmitted through the polarization beam splitter 201, then reflected by the right angle reflection prism 202 in the interference mirror group, then first passes through the second wave plate 204 in the double 1/4 wave plate, changes the S light into circularly polarized light, is first diffracted after being incident on the second grating 302 in the double grating 3, is first reflected after being incident on the second plane mirror 402 in the double plane mirror 4, is reflected and then returns along the original path, is second diffracted after being transmitted through the second grating 302 in the double grating 3, is second diffracted after being second transmitted through the second wave plate 204, is changed into the S light from circularly polarized light, is reflected by the right angle reflection prism 202 in the interference mirror group, is reflected to the pyramid prism 205 after being incident on the polarization beam splitter 201, is reflected again by the pyramid prism 205 and then returns in parallel, the light passes through the polarization beam splitter 201, is reflected by the right-angle reflection prism 202, passes through the second wave plate 204 for the third time, is changed into circularly polarized light again from S light, is diffracted for the third time after being normally incident on the second grating 302, is reflected after being normally incident on the second plane mirror 402 again, is returned along the original path after being reflected by the second plane mirror 402, is diffracted for the fourth time after passing through the second grating 302, is changed into S light from circularly polarized light again after being transmitted through the second wave plate 204 for the fourth time, and is then reflected again by the right-angle reflection prism 202 and the polarization beam splitter 201 as a second measurement beam.
It should be noted that the second step and the third step may be performed simultaneously, and the execution sequence is not divided.
And fourthly, the first measuring beam and the second measuring beam are coincided and interfered, and are reflected back to the receiving hole of the laser source 1 to be received. The received interference signal is a heterodyne interference measurement signal, and is subjected to difference with a reference signal provided by the laser source 1, so that a doppler frequency variation difference Δ f between the first measurement beam and the second measurement beam caused by the rolling motion of the object to be measured 5 can be obtained. By the heterodyne interferometry data processing method using the grating constant d as the length measurement reference, the tiny relative linear displacement of the double grating 3 can be obtained, the tiny linear displacement is approximated to an angular displacement, and the value of the roll angle α to be measured can be obtained according to the geometric relationship α of the relative angular displacement s and the relative radius, i.e., the distance L between the polarization splitting prism 201 and the right-angle reflecting prism 202, which is s/L. Wherein a frequency of the reference signal corresponds to a frequency difference of a first polarization component and a second polarization component included in the laser beam, which are different in frequency and orthogonal in linear polarization direction.
However, the positional relationship of the above elements is not limited thereto, and any optical path condition that can realize the above embodiments falls within the scope of the present invention.
The following examples illustrate the principles and mathematical models for roll angle measurement using the system shown in FIGS. 1-3 above.
In fig. 1-3, the laser beam is diffracted for the first time from the normal incidence (incidence angle is 0 degree) in the Z direction to the first grating 301 and blazed at the +1 or-1 order, so as to ensure the high diffraction efficiency of the order (up to 70% -85%), and the ± 1 st order diffracted light thereof satisfies the grating equation:
d sinθ＝±λ (1)
in formula (1): d is the grating constant or pitch. When the ± 1 st order diffracted light of the first diffraction enters the first grating 301 at an angle θ and is diffracted for the second time, the +1 or-1 st order diffracted light exits orthogonally to the grating plane of the first grating 301, which also satisfies equation (1), and similarly, the third diffraction and the fourth diffraction of the first grating 301 similar to the first and second diffraction also satisfy equation (1).
When the first grating 301 performs a minute motion due to the object to be measured 5, i.e., the roll plane, rolling (roll angle is α), the motion can be approximated to an X-direction linear motion (assuming that the grating linear velocity is V) in a small range. According to the Doppler principle, the + -1 st order diffracted light will produce a frequency change, i.e., a frequency shift, and the +1 or-1 st order diffracted light (exiting orthogonally to the plane of the grating, i.e., parallel to the original incident light) of the first, second, third and fourth diffractions will be shifted by the same amount, and the fourth frequency shift will have a superposition effect. Thus, the amount of frequency shift of the first measuring beam can be expressed as:
in the formula,. DELTA.f1Expressed as an amount of frequency shift of the first measuring beam; λ is the wavelength of the laser beam, and 633nm is preferable if the He-Ne laser is used; theta is the diffraction angle of the 1 st order of the first diffraction.
The following formulae (1) and (2) give:
similarly, the second measuring beam will also generate Doppler shift, but since the first grating 301 and the second grating 302 are located at two opposite sides of the roll center to form a differential structure, so that the frequency shift directions of the first measuring beam and the second measuring beam are opposite, then the first measuring beam and the second measuring beam will generate Doppler shiftFrequency shift quantity delta f of two measuring beams2Can be expressed as:
according to the heterodyne interference principle, two beams corresponding to the reference arm and the measurement arm, namely the first measurement beam and the second measurement beam interfere, and the doppler frequency change difference Δ f of the interference signal relative to the reference signal can be obtained by the difference of the equations (3) and (4):
from formula (5):
let the relative angular displacement of the first grating 301 and the second grating 302 be s, which is the time integral of the linear velocity V of the gratings. The integral of the time t is obtained from two sides of the equation (6):
in the formula: t is the integration time, i.e. the time period for counting Δ f.
The counting operation of the doppler frequency variation difference Δ f, usually performed by an electronic circuit (or counter), is represented by the number of frequencies within the counting period T, i.e. the number of pulses.
According to the formula (7), each original pulse equivalent in displacement measurement is d/8, which is eight times of optical subdivision and 2 times of that of the traditional laser interferometer. Obviously, the original pulse equivalent d/8, i.e. the resolution, is difficult to meet the requirement of high-precision measurement, and the resolution needs to be further improved by means of higher electronic circuits, software subdivision and the like. At present, after 2 times of optical subdivision, namely electronic subdivision with the original pulse equivalent of lambda/2 and 512 times or more, the displacement measurement resolution of the traditional commercial laser interferometer can reach 1nm or less. Based on the same processing circuit and electronic subdivision multiple, the displacement measurement resolution of the invention is (d/8)/(lambda/2) ═ d/(4 lambda) nanometers.
In an exemplary embodiment, L is the relative roll radius (which may also be referred to as differential spacing) according to the geometric relationship α -s/L, where the formula for calculating the value of the roll angle α to be measured may be:
equation (8) is a mathematical model of the roll angle measurement of the present invention.
Where α is a roll angle, T is a counting time period, s is a relative angular displacement of the first grating 301 and the second grating 302, L is a relative roll radius, d is a grating constant of the first grating and the second grating, and Δ f is a doppler frequency variation difference between the interference signal and the reference signal.
In an exemplary embodiment, the relative roll radius L is the horizontal separation of the polarization splitting prism 201 and the right angle reflecting prism 202. The roll angle α is calculated from the reference signal provided by the laser source 1 and the received interference signal, wherein the grating constant d and the relative roll radius L, i.e. the horizontal distance between the polarization beam splitter prism 201 and the right-angle reflection prism 202, are preset before measurement.
From the above analysis, assuming that the displacement measurement resolution s _ res of the present invention is d/(4 λ) nm and L is m in units, the angular measurement resolution α _ res can be expressed as:
as shown in fig. 2, from the triangular geometry we can derive:
H＝D sinθ (10)
in the formula, D is the linear motion range of the object 5 to be measured, i.e. the rolling plane, in the Z direction; h is the length of the long sides of the first plane mirror 401 and the second plane mirror 402.
The following equations (1) and (10) can be obtained:
thus, formula (11) can be substituted for formula (9):
two conclusions can be drawn from equation (12):
first, by increasing the horizontal interval L between the polarization splitting prism 201 and the right-angle reflecting prism 202, the angular measurement resolution can be improved.
Second, angular measurement resolution can also be improved by decreasing the grating constant d, i.e., increasing the ± 1 st order diffraction angle θ. However, for a determined value of wavelength λ (633nm), this will result in a decrease in the value of D/H as well, which will tend to result in a decrease in the range of linear motion D in the Z direction or an increase in the length H of the plane mirror. For measurements, D is expected to be as large as possible, which obviously degrades the angular resolution; h is generally not limited by factors such as the existing manufacturing level. Thus, the angular measurement resolution, although determined by the grating constant D, is determined more by the Z-direction range of motion D and the combined mirror length H.
For example, for a linear movement range D in the Z direction of 1000mm, if the pitch of the first measuring beam and the second measuring beam is 0.5m with respect to the roll radius L and the length H of the long side of the plane mirrors 401 and 402 is 500mm, the corresponding grating constants D and ± 1-order diffraction angles θ are 1.266 μm and 30 °, respectively, and if a blazed grating is used, the blazed angle γ thereof is 14.5 °, and the angular measurement theoretical resolution is 1nrad (about 0.0002 "). In terms of accuracy, if the measurement accuracy of the monograting displacement is 0.05 μm, the measurement accuracy of the bigrating displacement s is 0.05 μm, and it can be seen from equation (8) that the roll angle measurement accuracy using the method of the present invention will reach 0.05 μ rad (0.01 ").
In summary, compared with the long reflection prism adopted in the roll angle measuring device and method for double diffraction grating heterodyne interference of chinese patent publication No. CN104535019A, on one hand, four diffractions of the first polarized light and the second polarized light can be realized by the polarization beam splitter prism, the right-angle reflection prism, the pyramid prism, the double wave plate, the double grating, and the double plane mirror or the whole plane mirror, and finally the first measuring beam and the second measuring beam after the four diffractions are reflected back to the laser source, so that the long side length H of the plane mirror adopted in the present invention can be reduced by 1 time compared with the prior art under the condition that the resolution and the Z-direction linear movement range D are equal. For this reason, the plane mirror is not only structurally simpler than the long reflecting prism in the prior art method, but also smaller in size, which obviously provides for its high precision machining. Therefore, no matter the type or size of the reflector, the plane reflector adopted in the invention replaces a long reflecting prism, so that the method has the advantages of easier processing and higher precision, and simultaneously can overcome the defect that the light path is difficult to adjust due to the target reflecting prism in the existing method. On the other hand, the invention is a roll angle measuring scheme of heterodyne grating interference based on eight-time optical subdivision of the plane mirror, and under the condition that the length H of the long side of the plane mirror is the same, the plane mirror has higher optical subdivision, so that the resolution can be further improved, and the measuring resolution is improved by 1 time.
The technical scheme of the invention can be realized on the basis of the original system of the conventional commercial dual-frequency laser interferometer, and the device has the roll angle measuring function through corresponding accessories and optical paths. The device is particularly suitable for high-precision measurement of the small roll angle of linear motion positioning, such as the fields of precision engineering of a new generation of synchrotron radiation device, a numerical control machine, a three-coordinate measuring machine, a photoetching machine and the like.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
Exemplary embodiments of the present invention are specifically illustrated and described above. It is to be understood that the invention is not limited to the precise construction, arrangements, or instrumentalities described herein; on the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
Claims (8)
1. A grating heterodyne interferometry system, comprising:
a laser light source disposed at a first side of an object to be measured;
the polarization beam splitter prism is arranged on the optical axis of the laser source and is positioned on the first side of the object to be measured;
a reflector disposed on a reflection optical axis of the polarization splitting prism and located on a first side of the object to be measured, the reflector and the polarization splitting prism having a predetermined horizontal interval therebetween;
the pyramid prism is arranged in the reverse direction of the reflection optical axis of the polarization beam splitter prism and is positioned on the first side of the object to be measured;
the first wave plate is arranged on the transmission optical axis of the polarization beam splitter prism and positioned on the first side of the object to be measured, the first wave plate is positioned between the polarization beam splitter prism and the first grating, and the first wave plate is close to the polarization beam splitter prism;
the second wave plate is arranged on the reflecting optical axis of the reflector and positioned on the first side of the object to be measured, the second wave plate is positioned between the reflector and the second grating, and the second wave plate is close to the reflector;
the first grating is arranged on the transmission optical axis of the polarization beam splitter prism and is fixed on the object to be measured;
the second grating is arranged on the reflection optical axis of the reflector and fixed on the object to be measured, wherein the first grating and the second grating are respectively positioned on two sides of the rolling center of the object to be measured, the first grating and the second grating are positioned on the same plane, and the first grating and the second grating have grating constants;
the plane reflector group is arranged on the preset-order diffraction optical axes of the first grating and the second grating and is positioned on the second side of the object to be detected; wherein the first side and the second side are opposing sides;
the first wave plate and the second wave plate are the same 1/4 wave plates and are configured to convert the normally incident linearly polarized light into circularly polarized light or convert the normally incident circularly polarized light into linearly polarized light;
the plane mirror group includes:
the first plane reflector is arranged on the preset order diffraction optical axis of the first grating, and the normal line of the first plane reflector is parallel to the preset order diffraction light of the first grating; a first side edge and a second side edge of the first plane reflector, which are oppositely arranged, are parallel to a first incident plane, and the first incident plane is composed of preset-order diffracted light of the first grating and a normal of the first plane reflector;
the second plane mirror is arranged on the preset order diffraction optical axis of the second grating, and the normal line of the second plane mirror is parallel to the preset order diffraction light of the second grating; the first side edge and the second side edge of the second plane reflector which are oppositely arranged are parallel to a second incident surface, and the second incident surface is composed of preset-order diffraction light of the second grating and a normal of the second plane reflector.
2. The grating heterodyne interferometry system of claim 1, wherein the first planar mirror and the second planar mirror are identical strip-shaped planar mirrors; wherein the first side edge and the second side edge have a predetermined length.
3. The grating heterodyne interferometry system of claim 1, wherein the first grating and the second grating are the same diffraction grating.
4. A grating heterodyne interferometry system, comprising:
a laser light source disposed at a first side of an object to be measured;
the polarization beam splitter prism is arranged on the optical axis of the laser source and is positioned on the first side of the object to be measured;
a reflector disposed on a reflection optical axis of the polarization splitting prism and located on a first side of the object to be measured, the reflector and the polarization splitting prism having a predetermined horizontal interval therebetween;
the pyramid prism is arranged in the reverse direction of the reflection optical axis of the polarization beam splitter prism and is positioned on the first side of the object to be measured;
the first wave plate is arranged on the transmission optical axis of the polarization beam splitter prism and positioned on the first side of the object to be measured, the first wave plate is positioned between the polarization beam splitter prism and the first grating, and the first wave plate is close to the polarization beam splitter prism;
the second wave plate is arranged on the reflecting optical axis of the reflector and positioned on the first side of the object to be measured, the second wave plate is positioned between the reflector and the second grating, and the second wave plate is close to the reflector;
the first grating is arranged on the transmission optical axis of the polarization beam splitter prism and is fixed on the object to be measured;
the second grating is arranged on the reflection optical axis of the reflector and fixed on the object to be measured, wherein the first grating and the second grating are respectively positioned on two sides of the rolling center of the object to be measured, the first grating and the second grating are positioned on the same plane, and the first grating and the second grating have grating constants;
the plane reflector group is arranged on the preset-order diffraction optical axes of the first grating and the second grating and is positioned on the second side of the object to be detected; wherein the first side and the second side are opposing sides;
the first wave plate and the second wave plate are the same 1/4 wave plates and are configured to convert the normally incident linearly polarized light into circularly polarized light or convert the normally incident circularly polarized light into linearly polarized light;
the plane mirror group includes:
the whole plane reflector is arranged on the preset-order diffraction optical axes of the first grating and the second grating, and the normal line of the whole plane reflector is parallel to the preset-order diffraction light of the first grating and the second grating; the first side edge and the second side edge of the whole plane reflector, which are oppositely arranged, are parallel to a third incident surface, and the third incident surface consists of the preset-order diffraction light of the first grating and the second grating and the normal of the whole plane reflector.
5. The grating heterodyne interferometry system of claim 4, wherein the first side and the second side have a predetermined length.
6. A roll angle measurement method based on the grating heterodyne interference system of any one of claims 1 to 5, comprising:
emitting a laser beam by the laser source;
dividing the laser beam into a first polarized light and a second polarized light through the polarization beam splitter prism, wherein the linear polarization directions of the first polarized light and the second polarized light are orthogonal and different in frequency;
the first polarized light is incident to the first wave plate and is changed into first circularly polarized light, and the first circularly polarized light is incident to the first grating and is subjected to first diffraction to generate first preset-order diffracted light; the first preset-order diffraction light enters the plane mirror group and returns along an original path through reflection, and enters the first grating to generate second diffraction to generate second preset-order diffraction light; the second preset-order diffracted light enters the first wave plate, is converted into the first polarized light by the first circularly polarized light, is reflected to the pyramid prism by the polarization beam splitter prism, is reflected by the pyramid prism, returns in parallel, is reflected again by the polarization beam splitter prism, penetrates through the first wave plate, is converted into the second circularly polarized light by the first polarized light, and enters the first grating to generate third diffraction to generate third preset-order diffracted light; the third preset-order diffraction light enters the plane mirror group and returns along the original path through reflection, and enters the first grating to generate fourth diffraction to generate fourth preset-order diffraction light; the fourth preset-order diffracted light passes through the first wave plate, is changed from the second circularly polarized light into the first polarized light, and is transmitted through the polarization beam splitter prism to form a first measuring beam;
after the second polarized light is reflected by the reflector, the second polarized light is incident to the second wave plate and becomes third circularly polarized light, and the third circularly polarized light is incident to the second grating and generates first diffraction to generate fifth preset-order diffracted light; the fifth preset-order diffraction light enters the plane mirror group and returns along the original path after being reflected, and enters the second grating to generate second diffraction so as to generate sixth preset-order diffraction light; the sixth preset-order diffracted light enters the second wave plate, is changed into the second polarized light by the third circularly polarized light, is reflected to the pyramid prism by the reflector and the polarization beam splitter prism in sequence, is reflected by the pyramid prism, returns in parallel, is reflected again by the polarization beam splitter prism and the reflector in sequence, penetrates through the second wave plate, is changed into the fourth circularly polarized light by the second polarized light, and enters the second grating to generate third diffraction to generate seventh preset-order diffracted light; the seventh preset-order diffraction light enters the plane mirror group and returns along the original path after being reflected, and enters the second grating to generate fourth diffraction to generate eighth preset-order diffraction light; the eighth preset-order diffracted light is converted into second polarized light from the fourth circularly polarized light through the second wave plate, and the second polarized light is reflected by the reflector and the polarization beam splitter prism in sequence to form a second measuring beam;
the first measuring beam and the second measuring beam are coincided to generate an interference signal;
obtaining a difference value of Doppler frequency changes of the first measuring beam and the second measuring beam caused by rolling motion of the object to be measured according to the interference signal and a reference signal, wherein the frequency of the reference signal corresponds to the frequency difference of the first polarized light and the second polarized light;
and obtaining the roll angle of the object to be measured according to the horizontal spacing, the grating constant and the Doppler frequency change difference value.
7. The roll angle measuring method according to claim 6, wherein when the grating heterodyne interferometry system is in a measuring state, the laser source, the polarization splitting prism, the reflector, the corner cube, the first and second wave plates, and the first and second plane mirrors are fixed and relatively stationary; the first and second optical gratings move with the object to be measured.
8. The method of roll angle measurement of claim 6 wherein the value of the roll angle α is calculated by the formula:
wherein T is a counting time period, s is a relative angular displacement of the first grating and the second grating, L is the horizontal pitch, d is a grating constant of the first grating and the second grating, and Δ f is a doppler frequency variation difference of the interference signal and the reference signal.
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| CN109443249A (en) * | 2018-12-05 | 2019-03-08 | 哈尔滨工业大学 | High precision roll angle measurement method and device based on transmission grating |
| CN109470177B (en) * | 2018-12-05 | 2020-07-24 | 哈尔滨工业大学 | Three-dimensional angle measuring method and device based on double gratings |
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