CN205981138U - Phase shift formula optical projection three -dimensional measurement system - Google Patents

Abstract

The utility model relates to a three-dimensional measurement system of phase-shifting light projection. The laser light generated by a single-mode laser is diffused into a beam through an optical beam expansion system. After the modulator generates Bragg diffraction, the angle is adjusted and projected to the beam combiner to form an interference light path. The interference light path is enlarged by the projection objective lens after the focusing lens, and the interference fringes are generated and projected onto the measured object, and the interference fringes projected on the surface of the measured object After being captured by the CCD camera, it is processed by a computer and finally restores the three-dimensional shape of the measured object. This system combines the acousto-optic modulation technology with the phase-shift fringe frequency conversion technology, and can quickly measure the three-dimensional topography of the surface of various objects with different structures on the machine. It is robust to the surface of complex objects.

Description

A phase-shifting light projection three-dimensional measurement system

technical field

The utility model relates to the field of three-dimensional measurement, in particular to a phase-shifting light projection three-dimensional measurement system.

Background technique

In the actual production process of modern industry and manufacturing, a lot of measurement work is required, so 3D shape measurement is playing an increasingly important role in modern industry and real life. Optical three-dimensional shape measurement has become one of the research hotspots because of its non-contact, high precision, high speed and other advantages. Optical three-dimensional shape measurement is a method and technology to obtain three-dimensional space information of objects by optical means. It has very important significance and Broad application prospects.

The traditional three-coordinate contact probe three-dimensional measurement method has inherent defects: 1. There is a certain contact pressure during measurement, and measurement errors will inevitably occur for some soft items; 2. Due to the fact that the probe radius is It is impossible to be zero, so it is impossible to measure the subtle features of some complex surfaces; 3. Because of the point-by-point contact measurement, the measurement speed is slow, and it is not suitable for measuring large parts. However, the current popular laser scanning method uses a charge-coupled device (CCD) or a position-sensitive device (PSD) for digital point laser image acquisition, which is limited in speed due to point-by-point or line-by-line scanning. At the same time, the scanning accuracy is greatly affected by the material and surface characteristics of the test piece. For this reason, the utility model adopts the laser interference fringe projection technology combined with the phase shift measurement method, and strives to realize the fast, efficient and high-precision measurement of the workpiece, which is different from the existing Compared with technology, the utility model has significant advantages: (1) the projected structured light is real interference fringes, has high brightness and high contrast, has infinite focal depth, and can reduce the difficulty of software processing; (2) the effective measurement area is large and the space The reproducibility is good, and the gap of the fringe image obtained by projection is very small, which makes the obtained 3D data more accurate. (3) The measurement process is not affected by the color, shape and motion state of the measured object; (4) The phase shift speed is fast and the accuracy is higher. At the same time, the time phase shift method can be used for measurement, and the measurement process is not affected by the measured surface. Influenced by unfavorable factors such as discontinuities and jumps, the measurement robustness is better.

One of the prior techniques (refer to "color combination coded stripe grating profilometry", Liu Weiyi, Wang Zhaoqi, Mu Guoguang, Fang Zhiliang, Acta Optics Sinica, 20(9), 1218-1234, 2000) studied a coded grating projection three-dimensional profile In this technique, the projected grating utilizes the independent characteristics of the three primary colors of red, green, and blue in the color space, and uses color stripes to encode the grating, starting with white stripes, followed by red, green, and blue (R, G, B) Stripes of three colors form a group. The coded grating increases the height measurement range under the premise of ensuring the same measurement accuracy. The accuracy of the measurement mainly depends on the resolution of the image. This method uses a projector for fringe projection, and its phase shift accuracy depends on the extraction phase accuracy of the color image. The color-coded fringe grating uses 4 fringes as a group, and at least 3 fringes are required to determine its location. If the space width of the measured object cannot accommodate 3 stripes, the measurement cannot be performed. This method has high requirements on the measurement conditions, and the repeatability of the measurement accuracy is low.

The second prior technology (see "Novel 3-D free-form surface profilometry for reverse engineering", Liang-Chia Chen, Zhi-Xue Huang, 7th International Symposium on Measurement Technology and Intelligent Instruments , Journal ofPhysics: Conference Series 13, 174–177, 2005) adopted the concept of sensor integration, and developed an automatic reconstruction method to solve the key problems in the measurement process of free-form surfaces, specifically using laser scanning to scan, and then integrated edge detection algorithms and The image processing method processes in real time and performs three-dimensional reconstruction, but due to its own light shading problem, this method cannot detect concave objects, so its application is greatly limited.

The third prior technique (see "Profilometry by fringe projection", Luis Salas, Esteban Luna, Javier Salinas, Victor Garcia, Manuel Servin, Opt.Eng.42(11)3307–3314, 2003) uses high-intensity The light source projects the Ronchi grating onto the measured object through the projection lens, and resolves the phase through the high-density fringe pattern. The least square method is used to obtain the relevant three-dimensional reconstruction parameters, and the measurement accuracy of 1/80 equivalent wavelength is obtained. This method needs to make a precise Ronchi grating, and cannot realize the phase shift measurement, the measurement range is limited, the cost is high, the flexibility is not high, and the universal measurement cannot be realized.

The fourth advanced technology (see "Frequency Conversion 3D Digital Imaging Based on Dual Acousto-optical Deflectors", Zhang Peng, Peng Xiang, Qiu Wenjie, etc., Acta Photonica Sinica, 34(10), 1550-1553, 2005) proposes the use of dual acousto-optic deflectors Time-series frequency conversion three-dimensional digital imaging, using the diffraction order of two acousto-optic modulators to interfere to form spatially structured light fringes, the measurement process and phase can be controlled by computer programming, the measurement flexibility is good, and the universality is high, but it requires Two acousto-optic modulators, and to ensure the synchronization of the two acousto-optic modulators, the cost is high and the difficulty is relatively large.

The fifth of the prior art (refer to "A frequency conversion fringe projection system based on acoustic grating", Zhao Huijie, Zeng Junyu, Lei Yanzhang, Acta Optics Sinica, 28(2), 2008) proposed a frequency conversion fringe projection system based on an acoustic grating, the system The beat frequency signal is used to drive the acousto-optic deflector to form two overlapping gratings in the acousto-optic crystal. The laser light emitted by the light source is incident at the Bragg angle to form two beams of first-order diffracted light, which are focused by the lens to form a structure in which the light intensity is distributed according to the sinusoidal law. Light fringes, and a new phase solidification technology is proposed, so that the variation law of the fringe spatial frequency and phase has been well explained. The system measures the topography of plaster images and obtains phase maps for 3D imaging. The projection system has certain engineering application value for solving the three-dimensional measurement problem of objects with complex geometric shapes. This method uses frequency mixing technology combined with phase solidification method to realize fringe projection, which is very difficult in technical implementation, and at the same time, the error control of phase shift is not easy. , and its measurement accuracy is not mentioned.

Therefore, it is necessary to provide a three-dimensional measurement system for object surface measurement with low cost, high accuracy, fast measurement speed and strong robustness.

Utility model content

To sum up, in order to solve the existing technical problems, the technical problem to be solved by the utility model is to provide a three-dimensional measurement system of phase-shifting light projection.

The technical solution of the utility model to solve the above-mentioned technical problems is as follows: a phase-shifting light projection three-dimensional measurement system includes a single-mode laser CCD camera and a computer, the computer is connected with the CCD camera, and is arranged sequentially along the advancing direction of the single-mode laser to generate laser light Optical beam expander system composed of focusing lens, one-dimensional beam splitting grating, acousto-optic modulator, beam combiner, focusing lens and projection objective lens;

The laser light generated by the single-mode laser is diffused into a beam by the optical beam expander system and then decomposed into two laser beams by the one-dimensional beam splitting grating. A first optical wedge and a second optical wedge are also provided, and the first optical wedge and the second optical wedge respectively adjust the deflection angle of the two laser beams decomposed by the one-dimensional beam splitting grating to be equal to the incident angle entering the acousto-optic modulator , causing the two laser beams to generate Bragg diffraction after passing through the acousto-optic modulator;

A first spatial filter and a second spatial filter are provided between the acousto-optic modulator and the beam combiner, and the first spatial filter and the second spatial filter will respectively pass through the acousto-optic modulator The last two laser beams form first-order diffracted light; a third optical wedge is provided between the first spatial filter and the beam combiner, and a third optical wedge is provided between the second spatial filter and the beam combiner. The fourth optical wedge, the third optical wedge and the fourth optical wedge respectively project the first-order diffracted light on the beam combiner at an adjusted angle, and form an interference optical path after passing through the beam combiner; the interference The optical path passes through the focusing lens and is amplified by the projection objective lens to form interference fringes and project them onto the measured object. After the interference fringes are acquired by the CCD camera, the computer processes them and finally restores the three-dimensional shape of the measured object.

The beneficial effects of the utility model are as follows: a single acousto-optic modulator is used as a phase-shifting and frequency-converting device, and an equal optical path structure is used as a main body to reduce vibration and other noise disturbances, and the actual interference fringes are generated by using the interference method for projection, and the fringe clarity is improved. It is not affected by defocusing, etc., and at a lower cost, it can achieve accurate three-dimensional measurement and increase the speed of workpiece measurement.

On the basis of the above scheme, the utility model can also be further improved as follows:

Further, the acousto-optic modulator is connected to a computer through a radio frequency power amplifier and a direct digital frequency synthesizer, and the computer changes the frequency control word to change the exit angle of light passing through the acousto-optic modulator, thereby changing the fringe period.

The beneficial effect of adopting the above-mentioned further scheme is: changing the frequency control word N through the computer to change the exit angle of the light passing through the acousto-optic modulator Thus changing the fringe period, since the frequency change of the direct digital frequency synthesizer only takes tens of nanoseconds, the period and phase transformation of the interference fringe can be completed in a very short time, and the structured light of the phase shift fringe can be completed in a short time Projection is more convenient to use.

Further, the one-dimensional beam-splitting grating is a one-dimensional diffraction grating that splits the two laser beams in two with the same light intensity.

The beneficial effect of adopting the above further solution is to improve the clarity of interference fringe projection.

Further, a mirror for changing the direction of illumination to reduce the space occupied by the measurement system is provided between the optical beam expander system and the one-dimensional beam splitting grating and/or between the beam combiner and the focusing lens.

The beneficial effect of adopting the above further scheme is that the volume of the phase-shifting light projection three-dimensional measurement system is reduced, and the flexibility of use is improved.

Further, the acousto-optic modulator is a high-bandwidth modulator capable of modulating multiple wavelengths of laser light, the bandwidth of which is at least 40 MHz, and the diffraction efficiency of the acousto-optic modulator is greater than 60%.

The beneficial effect of adopting the above further scheme is that the interference fringes can be converted into multiple frequencies to adapt to different measurement objects; and there is sufficient light intensity to form clear interference fringes.

Further, the frequency adjustment range of the direct digital frequency synthesizer is greater than 40MHz.

The beneficial effect of adopting the above further scheme is that the digital frequency synthesizer is compatible with the frequency bandwidth of the acousto-optic modulator 8 .

Further, the beam combiner is an optical flat plate.

The beneficial effect of adopting the above further solution is that the optical flat panel is easy to manufacture and easy to obtain.

Further, the projection objective lens is a measurement projector objective lens with replaceable magnification, and the magnification error of the objective lens is ≤0.08%.

The beneficial effect of adopting the above further solution is to ensure that the interference fringes projected onto the measured object are sufficiently clear.

Further, the acquisition frame rate of the CCD camera is not less than 30fps.

The beneficial effect of adopting the above further solution is to ensure that the CCD camera can clearly collect the interference fringes on the measured object.

Description of drawings

Fig. 1 is a structural schematic diagram of the system described in the present invention. In the picture:

1—single-mode laser, 2—laser beam expander lens group, 3—mirror,

4—beam splitting grating, 5—first optical wedge, 6—second optical wedge,

7—Direct digital frequency synthesizer and RF amplifier, 8—Acousto-optic modulator,

9—the first spatial filter, 10—the second spatial filter, 11—the third optical wedge,

12—the fourth optical wedge, 13—beam combiner 14—computer, 15—focusing lens,

16—projection objective lens, 17—measured object, 18—CCD camera.

detailed description

The principles and features of the present utility model are described below in conjunction with the accompanying drawings, and the examples given are only used to explain the utility model, and are not used to limit the scope of the utility model.

As shown in Figure 1, a phase-shifting light projection three-dimensional measurement system includes a single-mode laser 1, an optical beam expander system 2 composed of a focusing lens and a one-dimensional beam splitting grating 4 are sequentially arranged along the advancing direction of the laser light generated by the laser 1 , an acousto-optic modulator 8, a beam combiner 13, a focusing lens 15 and a projection objective 16.

The laser light generated by the single-mode laser 1 is diffused into a beam by the optical beam expander system 2 and then decomposed into two laser beams by the one-dimensional beam splitting grating 4. The one-dimensional beam splitting grating 4 is divided into two One-dimensional diffraction grating, and the light intensity of the two laser beams is the same. A first optical wedge 11 and a second optical wedge 12 are also provided between the one-dimensional beam splitting grating 4 and the acousto-optic modulator 8, and the first optical wedge 11 and the second optical wedge 12 adjust The deflection angle θ _i of the two laser beams decomposed by the one-dimensional beam splitting grating 4 is equal to the incident angle entering the AOM 8 , so that the two laser beams will generate Bragg diffraction after passing through the AOM 8 . A first spatial filter 9 and a second spatial filter 10 are arranged between the acousto-optic modulator 8 and the beam combiner 13, and the first spatial filter 9 and the second spatial filter 10 are respectively The two laser beams passing through the acousto-optic modulator 8 form first-order diffracted light, and the first-order diffracted light is θ _B .

The acousto-optic modulator 8 is a high-bandwidth modulator capable of modulating lasers of various wavelengths, with a bandwidth of at least 40 MHz, capable of modulating lasers of various wavelengths so that the interference fringes can be converted into multiple frequencies, and its diffraction efficiency is greater than or equal to 60%, so that there is enough light intensity to form interference fringes. When the exit light θ _o passing through the optical wedge 5 and the optical wedge 6 enters the acousto-optic deflector 8 at the Bragg angle θ _B , the energy of the diffracted light is concentrated in the 0th order and the first order (±1 order) diffracted light, and the spatial filter is used 9 and the spatial filter 10 filter out 0-order light and one of the first-order lights to obtain a single light beam. After passing through the acousto-optic modulator, the relationship between the light intensity of the 0th-order diffracted light I ₀ and I _1st -order diffracted light and the incident light intensity I _i is:

Where L is the length of the transducer, and ν=2π/λΔnL is the phase delay generated by the light wave passing through the ultrasonic field. Δn is the acoustically induced refractive index change of the ultrasonic crystal, and the first-order diffraction efficiency is:

Where _M2 is the quality factor of the acousto-optic material, _Ps is the ultrasonic power, and H is the width of the transducer.

A third optical wedge 11 is provided between the first spatial filter 9 and the beam combiner 13, a fourth optical wedge 12 is provided between the second spatial filter 10 and the beam combiner 13, The third optical wedge 11 and the fourth optical wedge 12 respectively project the first-order diffracted light onto the beam combiner 13 at an adjusted angle.

The first optical wedge 5, the second optical wedge 6, the third optical wedge 11 and the fourth optical wedge 12 are circular optical wedges with the same size and material, and a cylindrical glass is cut multiple times during processing to obtain Ensure that the surface shape and size errors are consistent; the first optical wedge 5 and the second optical wedge 6 are symmetrically placed in the two optical paths; the optical wedge 11 and the optical wedge 12 are placed in the two optical paths behind the acousto-optic modulator in the same way. The function of the first optical wedge 5 and the second optical wedge 6 is to adjust the Bragg diffraction angle position of the light beam to the acousto-optic deflector 8 . The propagation direction of light after passing through the wedge is changed according to the following formula:

Among them, θi is the incident angle of the incident light wedge, _{n is the refractive index of the light wedge, θ o is} the exit angle of the light wedge, and α is the wedge angle of the light wedge.

The beam combiner 13 is an optical element capable of interfering laser beams, preferably: the beam combiner 13 is an optical flat plate. After the first-order diffracted light is projected onto the beam combiner 13 at an adjusted angle, it passes through the beam combiner 13 to form an interference optical path; the interference optical path passes through the focusing lens 15 and is enlarged by the projection objective lens 16 to form an interference The fringes are projected onto the measured object 17, and the projection objective lens 16 is a measurement projector objective lens with replaceable magnification, and the magnification error of the objective lens is ≤0.08%. The CCD camera 18 acquires the interference fringes (sinusoidal grating) and processes them through the computer 14 to finally restore the three-dimensional shape of the measured object 17. The acquisition frame rate of the CCD camera 18 is not less than 30 fps.

In addition, the acousto-optic modulator 8 is connected to the computer 14 through the radio frequency power amplifier and the direct digital frequency synthesizer 7. The output angle of the light modulator 8, thereby changing the period of the interference fringes to achieve the purpose of phase shifting fringes, as follows:

The frequency adjustment range of the radio frequency power amplifier and direct digital frequency synthesizer 7 is greater than 40MHz, to be compatible with the frequency bandwidth of the acousto-optic modulator, the frequency F of its output sinusoidal analog signal is F=N f/2 ^t (N is frequency Control word, f is the clock frequency, t is the number of bits of the phase accumulator). The acoustic wave frequency f _s in the AOM 8 is equal to the output frequency F of the RF power amplifier and the direct digital frequency synthesizer 7, so the deflection angle of the first-order diffracted light after the light passes through the AOM 8 is equal to the Bragg angle θ _B , as:

Where n is the refractive index of the acousto-optic medium, v _s is the speed of sound, f _s is the frequency of the sound wave, and λ is the wavelength of the light wave.

Therefore, by changing the frequency control word N through the computer, the angle of the outgoing light can be changed, thereby changing the frequency of the interference fringes. The fringe frequency fr and the exit angle The relationship between can be expressed as (Outgoing angle very small ).

The acousto-optic modulator 8 is connected with the radio-frequency power amplifier and the direct digital frequency synthesizer 7, and the radio-frequency power amplifier amplifies the signal of the direct digital frequency synthesizer, and changes the frequency control word N through the computer 14 to change the light passing through the acousto-optic modulator 8. exit angle Thus changing the fringe period, since the frequency change of the direct digital frequency synthesizer only takes tens of nanoseconds, the period and phase transformation of the interference fringe can be completed in a very short time, and the structured light of the phase shift fringe can be completed in a short time projection.

The three-dimensional measurement system is provided with a reflector 3 between the optical beam expander system 2 and the one-dimensional beam splitting grating 4 and/or between the beam combiner 13 and the focusing lens 15 to change the illumination direction to reduce the space occupied by the measurement system , to improve the flexibility of the three-dimensional measurement system.

The above descriptions are only preferred embodiments of the present utility model, and are not intended to limit the present utility model. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present utility model shall be included in this utility model. within the scope of protection of utility models.

Claims (9)

1. a kind of phase-shift type optical projection three-dimension measuring system it is characterised in that include single-mode laser (1) CCD camera (18) and Computer (14), computer (14) is connected with CCD camera (18), along single-mode laser (1) produce laser direction of advance according to The optical beam-expanding system (2) of secondary setting condenser lenses composition, one-dimensional beam-splitting optical grating (4), acousto-optic modulator (8), bundling device (13), Condenser lenses (15) and projection objective (16)；

The laser that described single-mode laser (1) produces diffuses into after light beam again by described one-dimensional through described optical beam-expanding system (2) Beam-splitting optical grating (4) resolves into two bundle laser, is additionally provided between described one-dimensional beam-splitting optical grating (4) and described acousto-optic modulator (8) One wedge (5) and the second wedge (6), described first wedge (5) and described second wedge (6) adjust one-dimensional beam-splitting optical grating respectively (4) deflection angle of the two bundle laser decomposing is equal to the angle of incidence entering acousto-optic modulator (8), makes two bundle laser through described acousto-optic Manipulator (8) produces Bragg diffraction afterwards；

It is provided with the first spatial filter (9) and second space filtering between described acousto-optic modulator (8) and described bundling device (13) Device (10), described first spatial filter (9) and described second space wave filter (10) respectively will be after acousto-optic modulators (8) Two bundle laser formed first-order diffraction light；It is provided with the 3rd light between described first spatial filter (9) and described bundling device (13) Wedge (11), is provided with the 4th wedge (12), described 3rd light between described second space wave filter (10) and described bundling device (13) Wedge (11) and described 4th wedge (12) project first-order diffraction light adjustment angle on described bundling device (13) respectively, and pass through Described bundling device (13) forms optical interference circuit afterwards；Described optical interference circuit passes through described condenser lenses (15) again by described projection objective (16) form interference fringe after amplifying and project on testee (17), described CCD camera (18) is led to after obtaining this interference fringe Cross computer (14) to carry out processing the final three-dimensional appearance recovering testee (17).

2. phase-shift type optical projection three-dimension measuring system according to claim 1 is it is characterised in that described acousto-optic modulator (8) it is connected with computer (14) with Direct Digital Frequency Synthesizers (7) by radio-frequency power amplifier, described computer (14) is led to Cross the frequency of change radio-frequency power amplifier and Direct Digital Frequency Synthesizers (7) and pass through acousto-optic modulator (8) changing light The angle of emergence, thus changing fringe period.

3. phase-shift type optical projection three-dimension measuring system according to claim 1 is it is characterised in that described one-dimensional beam splitting light Grid (4) are the one-dimensional diffraction grating of one-to-two, and it is identical to separate the light intensity of two bundle laser.

4. phase-shift type optical projection three-dimension measuring system according to claim 1 is it is characterised in that in described optical beam-expanding system It is provided with change direction of illumination between system (2) and one-dimensional beam-splitting optical grating (4) and/or between bundling device (13) and condenser lenses (15) The illuminator (3) in the space being occupied with contract measurement system.

5. phase-shift type optical projection three-dimension measuring system according to claim 1 is it is characterised in that described acousto-optic modulator (8) it is high bandwidth manipulator, the laser of modulated multi-wavelength, its bandwidth is at least 40MHz, described acousto-optic modulator (8) Diffraction efficiency is more than 60%.

6. phase-shift type optical projection three-dimension measuring system according to claim 2 is it is characterised in that described Direct Digital frequency The frequency-tuning range of synthesizer (7) is more than 40MHz.

7. phase-shift type optical projection three-dimension measuring system according to claim 1 is it is characterised in that described bundling device (13) is Optical flat.

8. according to the arbitrary described phase-shift type optical projection three-dimension measuring system of claim 1 to 7 it is characterised in that described projection Object lens (16) are the measuring projector object lens of replaceable multiplying power, object lens enlargement ratio error≤0.08%.

9. according to the arbitrary described phase-shift type optical projection three-dimension measuring system of claim 1 to 7 it is characterised in that described CCD phase Machine (18) acquisition frame rate is no less than 30fps.