A spectral confocal displacement measuring device

Abstract

This invention discloses a spectral confocal displacement measurement device, which includes a light source, a dispersion module, and a spectral analysis unit. The dispersion module faces the surface of the object being measured. After light is emitted from the light source, it passes through the dispersion module and exits, with the light spot illuminating the surface of the object. The reflected light returns to the dispersion module and is received by the spectral analysis unit. The spectral analysis unit processes the spectral information of the object to obtain the displacement value of the object. The dispersion module includes a collimation unit that collimates the light source into parallel light, and a dispersion and imaging unit that images the various colors of the parallel light at different positions. The spot size and dispersion range of the dispersion module can be changed by changing the collimation unit.

Description

Technical Field

[0001] This invention belongs to the field of measurement technology, and specifically relates to a displacement measuring device based on the principle of spectral confocality. Background Technology

[0002] Spectral confocal displacement measurement technology is a high-precision detection technique capable of detecting displacement changes. It can be used for displacement measurement of transparent targets such as glass and liquids, and has wide applications in industrial precision inspection. The key to this technology is to use a dispersive objective lens to focus a multispectral light source near the target object. The dispersive objective lens focuses light of different wavelengths at different positions. When the target object enters the measurement range, the light reflected from the target object is collected by the dispersive objective lens. After spectral analysis, the actual position of the object can be obtained.

[0003] Existing spectral confocal displacement sensing systems include at least one broadband light source, an optical dispersion unit, and a spectral analysis unit. The optical dispersion unit consists of at least one element, and the spectral analysis unit includes a linear array photosensitive element. Typically, a low Abbe number lens or lens group is used as the optical dispersion unit to generate the desired dispersion; a prism or grating is typically used as the spectral dispersive device, working in conjunction with the linear array photosensitive element to image the separated spectrum at different positions on the photosensitive element. The spectral analysis unit utilizes the correspondence between illuminance and photocurrent, allowing the photosensitive element to convert the spectral energy distribution on it into an electrical level signal.

[0004] For spectral confocal displacement measurement devices, the waveforms within the measurement range correspond one-to-one with the positions. However, the waveforms are affected by the roughness and color of the measured surface. Therefore, it is necessary to process the waveforms and reduce waveform variations caused by different materials to ensure the accuracy of the detection. Summary of the Invention

[0005] This invention provides a spectral confocal displacement measurement device that reduces the influence of material properties. The device consists of three parts: a composite color light source, a dispersive objective lens, and a spectral analysis unit. The composite color light source can be an LED or a laser fluorescent light source. The dispersive objective lens images the light source, with different wavelengths imaged at different positions in front of it. The spectral analysis unit consists of a spectrometer capable of resolving the spectrum of reflected light from the measured surface and a back-end processing circuit. The three components work together to achieve a one-to-one correspondence between waveform and position within the measurement range, unaffected by the surface roughness of the measured object. Furthermore, the waveform processing method of this spectral confocal displacement measurement device can restore the actual waveform of the measured object's surface, filter out interfering spectra, and effectively overcome measurement deviations introduced by the color of the measured object's surface. Attached Figure Description

[0006] The above and other objects, features, and advantages of exemplary embodiments of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings. Several embodiments of the invention are illustrated in the drawings by way of example and not limitation, wherein:

[0007] Figure 1 A schematic diagram of a spectral confocal displacement measuring device according to one embodiment of the present invention.

[0008] Figure 2 A schematic diagram of a dispersion module according to one embodiment of the present invention.

[0009] Figure 3 A schematic diagram of measurement error caused by the surface color of the object being measured according to one embodiment of the present invention.

[0010] Figure 4 A flowchart of the calibration process of a calibrator according to one embodiment of the present invention.

[0011] Figure 5 A flowchart of the measurement process for the test object according to one embodiment of the present invention.

[0012] Figure 6 A signal comparison diagram of a measurement example of a test object according to one embodiment of the present invention. Detailed Implementation

[0013] Current confocal spectral measurements typically use visible light in the 400–700 nm range, and calibration targets generally employ mirrors with a reflectivity exceeding 99% in the visible light band. The reflected light at this range is ideal, unaffected by roughness or color. However, in actual measurements, different objects exhibit color and surface roughness characteristics. This introduces other light sources, independent of the measurement position, into the reflected light from the object's surface, distorting the waveform obtained by the spectral analysis unit and introducing measurement errors.

[0014] According to one or more embodiments, the invention introduces a spectral confocal displacement measurement device that can reduce the influence of material properties. The system consists of three parts: a composite color light source, a dispersive objective lens, and a spectral analysis unit.

[0015] The composite color light source can be an LED or laser-excited phosphor. The light source wavelength is 400–700 nm visible light, so that it is visible to the human eye during assembly.

[0016] After being emitted, light is focused onto the surface of the object being measured by a dispersive objective lens, with different wavelengths focused at different positions. The spot size and dispersion range of the dispersive objective lens can be changed by altering the collimating unit within the lens, thereby minimizing the influence of different surface roughnesses of the object being measured.

[0017] The light spot illuminates the surface of the object under test, and the reflected light returns to the dispersive objective lens, then enters the spectral analysis unit via a semi-reflective beam splitter or fiber optic beam splitter. The spectral analysis unit consists of a beam splitter, an imaging element, and a photosensitive chip array. It can interpret and image the spectral information of the object under test onto the photosensitive chip. The image is then processed by back-end processing circuitry and algorithms to finally output the measurement result. Waveform processing methods can restore the actual waveform of the measured surface, filter out interfering spectra, effectively overcome measurement deviations introduced by the surface color of the object under test, and reduce the influence of changes in the surface material of the object under test on displacement measurement.

[0018] According to one or more embodiments, such as Figure 1 As shown, a spectral confocal displacement measurement device is described. The device includes a broadband light source, a beam splitter module, a dispersion module, and a spectral analysis unit. The light from the broadband light source is coupled into the beam splitter module; the coupling method can be direct coupling or coupling after focusing with a lens. The beam splitter module can be a beam splitter or a visible light circulator. The light enters the dispersion module after passing through the output end of the beam splitter module. The dispersion module is generally a miniature projection lens that can image the light spot on the fiber optic end face within a preset measurement range. When the light spot is focused on the surface of the object being measured, the light reflected from the surface is collected again by the dispersion module, passes through the beam splitter module, and enters the spectral analysis unit. Further analysis The actual position of the measured surface can then be obtained from the waveform. Here, Figure 1 The diagram shows a spectral confocal displacement sensing and measurement device based on a fiber optic splitter scheme. The various modules and units within the device are connected by optical fibers. The broadband light source can be a composite color light source, and the dispersive module uses a dispersive objective lens.

[0019] The composite color light source can be an LED or laser-excited phosphor. The light source wavelength is 400–700 nm visible light, so that it is visible to the human eye during assembly.

[0020] A dispersion module is typically a miniature projection lens that images a spot of light from the fiber optic end face within a preset measurement range. Generally, the spot size of the dispersion unit ranges from a few micrometers (µm) to tens of µm. If the surface roughness of the object being measured is at the µm level, due to vibration, the measurement spot may appear at different locations on the surface, possibly at a peak or a valley, leading to measurement instability. In this case, it is necessary to change the imaging spot size of the dispersion unit to make it larger, covering multiple peaks and valleys on the surface of the object being measured. This is equivalent to averaging the surface area and achieving stable measurement.

[0021] like Figure 2As shown in the figure. This embodiment provides a dispersive module model that can quickly change the size of the imaging spot. The first part is a collimation unit, which collimates the light source into parallel light, and the second part is a dispersive and imaging unit, which images the various colors of the parallel light spectrum at different positions in front of the dispersive unit.

[0022] According to Lagrange invariants in optical imaging, nuy = n'u'y'.

[0023] n and n' are the refractive indices before and after imaging. In air, both are taken as 1. u is the angle of incidence, u' is the angle of exit, y is the size of the light source, and y' is the size of the imaging spot. From the above formula, it can be seen that to change the size of the imaging spot y', the size of the light source and the angle of incidence can be changed.

[0024] With the aperture unchanged, the angle of incidence can be altered by changing the focal length of the collimating element. If the focal length decreases, the angle of incidence will increase, and all other things being equal, the size of the image spot y' will increase proportionally.

[0025] After the measuring light shines on the surface of the object being measured, it is collected by the dispersive unit, carrying the object's position information. This light is then received by the spectral analysis unit and imaged onto the surface of the photosensitive chip. Different positions on the object's surface reflect different spectral information, resulting in different image positions. The imaging spot has a Gaussian distribution and a size exceeding 20 μm. The center of the spot can be easily calculated using the centroid method, thus establishing the relationship between the object's position and the spot center, achieving "spectral encoding" of the position.

[0026] However, if the object being measured has color and its reflected spectrum happens to overlap with the region of enhanced reflection, the color information will interfere with the signal light to some extent, disrupting the Gaussian distribution of the imaging spot and causing a deviation in the center of the spot calculated by the centroid method, thus leading to measurement errors. For example... Figure 3 As shown.

[0027] This embodiment uses a Gaussian fitting data table to record the Gaussian distribution coefficients at typical locations as a basis for waveform inspection. Furthermore, the Gaussian fitting coefficients are used to identify waveform data with stray light, and the waveform is corrected using an assignment method or a difference method as the processed waveform for the centroid method, reducing the influence of stray light on the measurement.

[0028] Gaussian fitting is a fitting method that uses a Gaussian function of the form Gi(x) = Ai*exp((x-Bi)^2 / Ci^2) to approximate a data point set. Here, Ai, Bi, and Ci represent the constant coefficients of the Gaussian function at point i, A is the peak value of the curve, B is the x-coordinate of the peak value, and C is the standard deviation. During system calibration, Table 1 is used to record the Gaussian coefficients at typical locations and write them into the processor of the spectral processing and analysis unit.

[0029] Table 1

[0030] 1 …… N

[0031] The calibration process flowchart is as follows: Figure 4 As shown, the testing process is as follows: Figure 5 As shown.

[0032] like Figure 4 The calibration process shown includes the following steps:

[0033] 101. The waveform of the calibration object is acquired, and the centroid of the calibration object is calculated. Here, the object used for calibration is a standard part, generally a white card with a reflectivity of 90%. As for the acquired waveform, the centroid method is used to calculate the centroid. There are various methods, which will not be elaborated here.

[0034] 102, waveform Gaussian fitting;

[0035] 103, record the peak position, centroid, and Gaussian fitting coefficients;

[0036] 104. Has the displacement platform reached the measurement endpoint? If not, return to step 101.

[0037] 105. If the endpoint is reached, the calibration data will be saved and written to the memory within the spectral analysis unit.

[0038] like Figure 5 As shown, the testing process includes the following steps:

[0039] 201, Acquire waveform;

[0040] 202, Calculation of the centroid of the measured object;

[0041] 203. Look up the table and estimate the coefficients of the current Gaussian fitting equation;

[0042] 204, waveform Gaussian fitting;

[0043] 205. Determine if the Gaussian fitting coefficients at the same peak value have a large deviation. If so, determine that it is an abnormal waveform and use an elimination algorithm to remove the outlier.

[0044] 206. If the deviation of the Gaussian fitting coefficient is within the expected range, the calculated centroid is output as the displacement value of the measured object.

[0045] Here, the centroid is the centroid of the waveform, defined by CMOS image pixels. For example, a centroid of 100 means the centroid is located at pixel 100. The displacement is the actual movement of the measured object, such as +5mm. There is a calibration conversion process between the centroid and the actual displacement. After the centroid calculation is processed with or without correction, the centroid-to-displacement conversion, as described in the "spectral encoding" process, is performed, and the final displacement value is output.

[0046] After calculating the Gaussian fitting coefficients of the waveform, the calibration data at the same peak position is obtained mainly by looking up a table. If B in the test data is equal to B in the calibration data, or the deviation is within a certain range (generally 0.2), the waveform is considered to be free of stray light. Otherwise, stray light is present. The position of the stray light is determined by the size of the centroid, and then the stray light is filtered out. The waveform is then corrected and recalculated.

[0047] like Figure 6 The table shows a comparison of normal signals and abnormal signals affected by color in a measurement example of an object being measured. After preliminary calculations, the data in Table 2 can be obtained.

[0048] Table 2

[0049] Calibration value 106 106.002 287.7149 105.8729 5.5395 Measurements before correction 106 105.275 211.5432 105.0081 6.9352

[0050] As shown in Table 2, the peak values ​​of the measured values ​​and the calibrated values ​​are consistent, but the coefficient B differs by more than 0.8, indicating stray light interference in the measured waveform. The centroid of the measured value is smaller than the calibrated value, indicating stray light on the side of the measured waveform with smaller pixels. The amplitude of the measured values ​​was recalculated using the assignment method, and the data in Table 3 was obtained.

[0051] Table 3

[0052] Calibration value 106 106.002 287.7149 105.8729 5.5395 Measurements before correction 106 105.275 211.5432 105.0081 6.9352 Corrected positive measurement value 106 105.87 240.7935 106.0355 5.5104

[0053] After correction, the centroid deviation is less than 0.2, which is 18% of the original value, indicating that the waveform processing algorithm can effectively reduce the measurement deviation caused by the surface color of the measured object.

[0054] The spectral confocal displacement measurement device of this invention proposes a method for controlling the spot size of the dispersive objective lens through an improved dispersive module, thereby reducing the influence of the roughness of the measured surface. Simultaneously, the spectral analysis unit in the device employs a new waveform processing algorithm to reduce measurement deviations caused by the surface color of the measured object.

[0055] Therefore, the beneficial effects of the technical solution of this invention, besides the new design method for dispersive objectives, which allows for flexible adjustment of the detection spot size, also include the ability to reduce waveform variations caused by different materials and ensure detection accuracy through the aforementioned data processing method using Gaussian fitting coefficients as the criterion.

[0056] It is worth noting that although the spirit and principles of this invention have been described with reference to several specific embodiments, it should be understood that this invention is not limited to the disclosed specific embodiments, and the division of aspects does not imply that the features in these aspects cannot be combined; such division is merely for the convenience of description. This invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A spectral confocal displacement measurement device, comprising a light source, a dispersion module, and a spectral analysis unit, wherein the dispersion module faces the surface of the object being measured; light emitted from the light source passes through the dispersion module and exits, with the light spot illuminating the surface of the object; reflected light returns to the dispersion module and is received by the spectral analysis unit; the spectral analysis unit processes the spectral information of the object to obtain the displacement value of the object, characterized in that… The dispersion module includes a collimation unit that collimates the light source into parallel light, and a dispersion and imaging unit that images the various colors of the parallel light spectrum at different positions. The spot size and dispersion range of the dispersion module can be changed by modifying the collimation unit. The spot size can cover multiple peaks and valleys on the surface of the object being measured, and the surface of the object being measured can be averaged. The spectral analysis unit consists of a spectroscopic element, an imaging element, and a photosensitive chip array. It analyzes and images the spectral information of the analyte onto the photosensitive chip array, and then processes the data through back-end processing circuitry and algorithms to output the measurement results. The spectral analysis unit processes waveforms using a Gaussian fitting data table, recording the Gaussian distribution coefficients at typical locations as the basis for waveform inspection. Gaussian fitting coefficients are used to identify waveforms with stray light, and the waveforms are corrected using either assignment or difference methods as the processed waveforms for the centroid method. The Gaussian fitting data table is established by acquiring waveforms of the calibration object during the calibration process, calculating the centroid, performing Gaussian fitting on the waveform, and recording the peak position, centroid, and Gaussian fitting coefficients. The Gaussian fitting coefficients include the constant coefficients A, B, and C of the Gaussian function, where A is the peak value of the curve, B is the abscissa of the peak value, and C is the standard deviation. During the test, the spectral analysis unit calculates the centroid of the acquired waveform of the object under test, obtains calibration data for the same peak position using a lookup table, estimates the current Gaussian fitting equation coefficients, performs Gaussian fitting on the waveform of the object under test, and determines whether the Gaussian fitting coefficients at the same peak position have a large deviation. If the deviation is large, it is judged as an abnormal waveform. The position of stray light is determined by the size of the centroid. After correcting the waveform using an assignment method or a difference method, the centroid is recalculated, and the displacement value of the object under test is output. The determination of whether the Gaussian fitting coefficient at the same peak has a large deviation is achieved by judging whether B in the test data is equal to or the deviation is within a preset range, where the preset range is 0.

2.

2. The spectral confocal displacement measuring device according to claim 1, characterized in that, The dispersion module consists of a dispersion objective lens, which is a miniature projection objective lens that can image the light spot on the end face of the optical fiber within a preset range.

3. The spectral confocal displacement measuring device according to claim 1, characterized in that, The device further includes a spectrophotometer module, the output of which is connected to a first connection terminal of the spectrophotometer module, a second connection terminal of which is connected to a light source, and a third connection terminal of which is connected to a spectral analysis unit. The beam splitting module is a semi-reflective, semi-transparent beam splitter, beam splitter, or visible light circulator.

4. The spectral confocal displacement measuring device according to claim 1, characterized in that, The light source is a composite color light source, which uses LEDs or lasers to excite the phosphor sheet. The light emitted by the light source is coupled into the beam splitter module. The coupling method is direct coupling or coupling after focusing by a lens. The light source uses visible light in the wavelength range of 400~700nm.

5. The spectral confocal displacement measuring device according to claim 1, characterized in that, The light spot imaged by the dispersion and imaging unit has a Gaussian distribution and an imaging size exceeding 20 μm. The center of the light spot is calculated using the centroid method, thereby establishing the relationship between the position of the object under test and the center of the light spot, and realizing the spectral encoding of the position.

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