CN113960032B - Online laser cleaning effect detection method and three-dimensional detection device - Google Patents
Online laser cleaning effect detection method and three-dimensional detection device Download PDFInfo
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Abstract
According to the online laser cleaning effect detection method and the three-dimensional detection device, the state of a cleaned sample is detected by adopting the infrared thermal wave nondestructive detection theory, a controllable thermal excitation source in the form of a function of pulse, step, period and the like is actively applied to an object, so that the internal structure (such as a defect or damage) of the object is represented in the form of a surface temperature field, the change of the temperature field before and after the surface of the object is continuously recorded by using an infrared thermal imager, and the time-series thermal wave signals are subjected to operation treatment, so that the qualitative and quantitative calculation and characterization of the internal anisotropic structure of the object can be realized, and the high-precision detection purpose is achieved; the three-dimensional detection device is simple and easy to realize.
Description
Technical Field
The invention belongs to the technical field of nondestructive testing, and particularly relates to an online laser cleaning effect detection method and a three-dimensional detection device.
Background
Industrial cleaning can be categorized according to the cleaning fineness requirements, cleaning media and cleaning techniques. The method can be divided into industrial cleaning, precise industrial cleaning and ultra-precise industrial cleaning according to the fine degree requirement. The cleaning medium may be classified into wet cleaning and dry cleaning. According to the cleaning technology principle, the method can be divided into physical cleaning, chemical cleaning and biological cleaning. The selection of the cleaning scheme is required to be selected according to the different objects to be cleaned, the different surface pollutants, the cleaning standard and the cost requirement.
The concept of laser cleaning was first proposed by s.m. bedapir in 1969. In 1973, J.Asmus and L.Lazzarini et al proposed protective cleaning of cultural relics and art with lasers. In 1975, the laser of the soviet union scientist eliminates the oil floating on the water surface, has higher eliminating efficiency and does not produce secondary pollution. In 1987, V.I. Beklemvshev et al utilized the photolysis of the laser to remove the surface metal particles. In 1988, e.y.assendel' ft et al studied wet laser cleaning for the first time. The montreal protocol in 1998 was in effect and limited the use of environmentally polluting organic solvents for chemical cleaning. In the same year, K.Liu and E.Garmie research laser rust removal effects of different wavelengths and different pulse widths, and found that using Q-switched Nd: YAG laser paint removal effect is best. The research of R.ultra, P.Meja et al aiming at rust removal of metal surfaces compares the relation of laser pulse width, wavelength energy, mass density, cleaning efficiency and cleaning quality. In 2007, dongsik Kim et al developed a laser shock wave cleaning technique of the nanoscale example. In 2010, g.x.chen et al have studied the theory and technique of laser paint removal for marine paint removal requirement systems.
Compared with the traditional cleaning means, the laser cleaning has the unique advantages of environmental protection, high precision, no contact, good repeatability, easy realization of micro-area and large-area cleaning, and the like. As an advanced cleaning mode, the fields of laser cleaning research and application currently include paint removal and rust removal of metal surfaces and laser dust removal of surfaces of circuit boards and optical elements. In addition, laser cleaning has application potential in various fields such as industrial molds, ultra-smooth optical surfaces, space garbage, nuclear radiation treatment and the like.
The mature laser cleaning system comprises a laser light source, a light beam adjustment transmission system, a mobile platform and an online detection system. The online laser cleaning detection device can detect the laser cleaning effect, so that the laser cleaning equipment can timely adjust the laser cleaning operation process. Real-time detection is important to the improvement of the technical level and safety of laser cleaning.
Detection of laser cleaning can be divided into three categories: the first type is a photodetection method, because the nature of laser cleaning is that laser interacts with a substance, a sample is ionized to generate plasma under the action of the laser, at the moment, the intensity or spectrum change characteristic condition of a laser plasma signal in the cleaning process can be monitored by an energy meter or a spectrometer, so that a cleaning threshold value is analyzed, and a relation between the luminous intensity or spectrum line characteristic and the current cleaning effect is established. The specific method can be divided into a plasma luminous intensity detection method and a plasma luminous spectrum detection method; the second type is an acoustic detection method, in which ultra-short laser pulses act on the surface of an object during laser cleaning, and intense vibration shock waves are generated inside the object and attenuated in air to form acoustic waves. The intensity, frequency and other parameters of the acoustic wave signal are related to the laser cleaning degree. The acoustic signals emitted in the cleaning process are collected through acoustic instruments such as a microphone, so that the cleaning process can be monitored in real time, and the method is called laser-induced acoustic wave detection; the third category is an image detection method, that is, image acquisition is performed on the object to be cleaned, and the cleaning process is comprehensively judged by combining a visual method with a laser interference pattern, including a surface speckle image detection method and the like.
The monitoring system dynamically analyzes the laser cleaning effect and feeds back the laser cleaning effect to the control system, so that the laser cleaning parameters and the laser cleaning process can be adjusted in time. The three existing methods have respective defects: firstly, for the above three monitoring methods, different materials, structures and surface states have influence on the monitoring signals, so that for different batches of samples to be cleaned, the monitoring threshold value needs to be determined in advance through experiments, and theoretical prediction cannot be performed. Secondly, all the current real-time detection methods can only qualitatively evaluate the current cleaning state, and cannot predict the cleaning state and give out quantitative information, so that the method is difficult to predict the cleaning state and is extremely easy to cause excessive cleaning. Third, in the photodetection method and the acoustic detection method, the generated signal is a combined signal generated from the portion to be cleaned, and when cleaning is performed by using a line laser or a plane laser, the local cleaning state cannot be determined. In contrast, although two-dimensional detection of the region to be cleaned can be achieved for image detection, visual detection relies on the personal experience of the detector to determine that there is no clear index. The distribution of laser speckle patterns in the speckle image detection method is strongly dependent on the surface state of a material, no indication parameter exists for the cleaning degree, the speckle image change can be caused only after the excessive cleaning occurs, the laser interference image is easily affected by vibration generated in the cleaning process, and a high stability requirement is provided for the detection environment.
Disclosure of Invention
Accordingly, the present invention is directed to an online laser cleaning effect detection method and a three-dimensional detection device, which can detect the laser cleaning effect online in real time, and have higher detection accuracy.
A laser cleaning effect detection method and a three-dimensional detection device comprise the following steps:
irradiating the position to be cleaned on the surface of the sample by adopting a thermal excitation source;
obtaining a temperature field change signal T of a position to be cleaned on the surface of a sample within a set time from the start of a thermal excitation source;
taking logarithm of the temperature field change signal to obtain ln (T), and then calculating first-order reciprocal of logarithm ln (T) of time T to obtain' logarithmic temperature first-order reciprocal
-a time t "curve, the time corresponding to the peak on the curve being determined, noted as t a ；
Cleaning the position to be cleaned on the surface of the sample by adopting a laser beam to finish one-time cleaning;
obtaining a temperature field change signal of a position to be cleaned on the surface of the cleaned sample, and obtaining the logarithmic temperature first order reciprocal at the moment
-time t' curve, and determining the time corresponding to the peak position of the curve, and recording as t b ；
According to the current t a And t b And (3) judging:
if it is
The cleaning stopping condition is met, and the cleaning is stopped;
if it is
Then cleaning is continued, and a temperature field change signal of the position to be cleaned is obtained again after cleaning, and a logarithmic temperature first-order reciprocal +.>
-time t "curve, and determining the time at which the peak position of the curve is present, denoted t c The method comprises the steps of carrying out a first treatment on the surface of the According to t a 、t b And t c And (3) judging: if->
Then the cleaning is continued; by the method, after each cleaning, the time t is corresponding to the current peak value c Corresponding time t to peak value of two times before current cleaning a And t b And judging until the cleaning stopping condition is met.
Further, before the sample is cleaned, the surface layer thickness L1 is obtained according to the surface layer and substrate material parameters by using the following formula:
wherein T (0, T) tableShowing the surface temperature; q is the heat applied per unit area, alpha 1 E is the thermal diffusivity of the surface material 1 And e 2 The heat storage coefficient of the first layer and the second layer material, L 1 Is the first layer material thickness.
Further, after the sample is cleaned once, the thickness of the surface layer is calculated according to the parameters of the surface layer and the substrate material, so that the thickness p of the surface layer removed by the single cleaning is calculated, and the total cleaning times are predicted.
A three-dimensional detection device for realizing a detection method of a laser cleaning effect comprises a pulse laser, a thermal imager and a three-dimensional translation stage; the pulse laser is used as a cleaning laser source and is used as a thermal excitation source; the thermal imager is used for obtaining the temperature field change signal; the translation stage is used to move the sample.
And the device further comprises a beam shaping system which is used for shaping the pulse laser generated by the pulse laser into square light spots with set size so as to be used as a thermal excitation source to irradiate the surface of the sample to be cleaned.
Further, the lens further comprises a reflecting mirror M1, a reflecting mirror M2 and a converging lens L1; the reflector M1 and the converging lens L1 are sequentially arranged in a laser light path emitted by the pulse laser; laser emitted by the pulse laser is reflected by M1 and M2 in sequence and is led into a beam shaping system; the converging lens L1 is used for converging laser emitted by the pulse laser and is used as a cleaning light source to irradiate the position to be cleaned on the surface of the sample.
A three-dimensional detection device for realizing a detection method of a laser cleaning effect comprises a pulse laser, a thermal imager, a flash lamp and a three-dimensional translation table; the pulse laser is used as a cleaning laser source, and the flash lamp is used as a thermal excitation source; the thermal imager is used for obtaining the temperature field change signal; the three-dimensional translation stage is used to move the sample.
The invention has the following beneficial effects:
according to the online laser cleaning effect detection method and the three-dimensional detection device, the state of a cleaned sample is detected by adopting the infrared thermal wave nondestructive detection theory, a controllable thermal excitation source in the form of a function of pulse, step, period and the like is actively applied to an object, so that the internal structure (such as a defect or damage) of the object is represented in the form of a surface temperature field, the change of the temperature field before and after the surface of the object is continuously recorded by using an infrared thermal imager, and the time-series thermal wave signals are subjected to operation treatment, so that the qualitative and quantitative calculation and characterization of the internal anisotropic structure of the object can be realized, and the high-precision detection purpose is achieved;
the three-dimensional detection device is simple and easy to realize.
Drawings
FIG. 1 is a schematic diagram of the principle of infrared thermal wave nondestructive testing of the present invention;
FIG. 2 is a schematic diagram of a pulse excited two-layer dielectric model according to the present invention;
FIG. 3 is a plot of logarithmic temperature first derivative vs. logarithmic time for two layers of material under pulsed excitation in accordance with the present invention;
FIG. 4 is a graph showing the temperature of different adhesive tape layers on a stainless steel plate according to the present invention over time;
FIG. 5 is a graph showing the temperature of different adhesive tape layers on a stainless steel plate according to the present invention with time;
FIG. 6 is a graph showing the temperature of different adhesive tape layers on an aluminum plate according to the present invention over time;
FIG. 7 (a) is a schematic diagram of an on-line laser cleaning effect detection device by laser excitation according to the present invention;
FIG. 7 (b) is a schematic diagram of a device for detecting the cleaning effect of the on-line laser excited by the flash lamp according to the present invention;
FIG. 8 is a shaped laser spot of the present invention;
FIG. 9 is a flow chart of a method for detecting an online laser cleaning effect according to the present invention.
Detailed Description
The invention will now be described in detail by way of example with reference to the accompanying drawings.
An infrared thermal wave nondestructive testing schematic diagram is shown in fig. 1. When a thermal stimulus is applied to an isotropic homogeneous material (i.e., k x ＝k y ＝k z ) In the absence of an external and internal heat source, the partial differential equation for heat conduction at the surface of the material can be expressed as:
where α=k/ρc is the thermal diffusivity of the material, k is the thermal conductivity of the material, ρ is the material density, and C is the material specific heat capacity. In a one-dimensional thermal conduction theoretical model of pulse thermal excitation infrared thermal imaging, the influence of three-dimensional thermal diffusion is ignored, and only the heat transfer in the x direction is considered. At this time, the thermal conduction differential equation can be written as:
where T (x, T) is the temperature at time x, T, and f (x, T) represents a pulsed heat source function. According to the formula (2), temperature analysis solutions generated by different structural materials can be obtained by utilizing Laplace transformation and a separation variable rule according to a specific physical model and combining corresponding initial conditions and boundary conditions.
(1) For semi-infinite materials, under excitation of an ideal pulse function (f (t) =q delta (t), where Q is the amount of heat applied per unit area and delta (t) is the dirac delta function), the material surface temperature expression is:
wherein,,
is the heat storage coefficient of the material.
(2) For a single layer of material with a limited thickness L, the surface temperature expression is:
(3) For a two layer construction material, as shown in fig. 2. If the thickness of the first layer medium is L 1 The second medium is semi-infinite (L 2 → infinity), pulse excitation heat conduction formulaCan be expressed as:
wherein e 1 And e 2 Is the heat storage coefficient of the first layer and the second layer material, alpha 1 Is the thermal diffusivity of the material of the first layer. After taking the logarithm of the two sides of the formula (5), we can obtain the curve shown in fig. 3. The value was-0.5 first, followed by a single peak. The positive and negative directions of the peaks and the peak size depend on the thermal storage coefficients of the first layer material and the second layer material. The position of the peak is then dependent on the thickness and thermal diffusivity of the first layer material, i.e. the thinner the first layer material, the more left the peak position is moved and the width is simultaneously widened as the horizontal axis is logarithmic.
For laser cleaning, whether paint or rust, the material structure can be approximated as the model of fig. 2 when the substrate thickness is much greater than the cleaning layer thickness. Thus, according to the principle, we can obtain the peak curve as shown in fig. 3 at any point by processing the temperature signals at each position of the measured material. As the laser cleaning process proceeds, the peak position in fig. 3 moves to the left. If the laser power density is unchanged and the cleaned layer is made of homogeneous material, the peak value will move leftwards at a uniform speed. When the cleaning is finished, the material is changed into a single-layer structure from a two-layer structure, the peak value disappears, and the temperature expression is described by a formula (3). The curve in fig. 3 becomes a straight line with a value of-0.5. By determining the peak movement speed, a prediction of the cleaning state, i.e. how many laser pulses are needed to completely remove the first layer of material, can be achieved. In addition, if we know the thermal parameters of the first layer and the matrix material, real-time measurement of the first layer material thickness can be achieved. Therefore, the real-time monitoring of the three-dimensional online laser cleaning effect can be realized.
Fig. 4 is experimental data of a stainless steel plate excited by a flash lamp, on which 1, 2, and 3 layers of black electrical tapes are respectively attached, wherein the horizontal axis is the natural logarithm of time. The horizontal axis of fig. 5 is a linear coordinate. The peak position of the curve shifts left as the number of layers of tape decreases. In addition, since we know the thermal parameters of the base plate and the tape, we can calculate the thickness of the tape layer by finding the peak position. In order to obtain a more accurate result, we can also help us find the accurate peak position by performing a nonlinear fit to the experimental curve.
Based on FIG. 5, we can assume a time t a 、t b 、t c Peak times corresponding to temperature change curves of 3 layers, 2 layers and 1 layer of adhesive tapes respectively. Assuming that p is the single layer tape thickness, there are:
according to formula (6) has
In the limit case, the scanning cleaning can be continued for one time; if appropriate->
When the cleaning process is needed to be stopped, or the laser energy density is reduced; when->
At least one cleaning scan may be continued.
When the thickness of the surface layer of the sample to be cleaned is extremely thin, single cleaning should be performed first. The curve peak time when not cleaned is taken as t a The time of the peak of the curve after a single wash is taken as t b . When (when)
When the time is the limit condition, the scanning cleaning can be continuously carried out for one time; if->
It is necessary to stop the cleaning process or reduce the laser energy density; when->
At least one cleaning scan may be continued. Whether this step needs to be performed or not, may be the case. But using more than two characteristic times (e.g. t a 、t b 、t c ) In this case, the judgment can be made more accurate.
Fig. 6 is experimental data for a flash-excited aluminum plate with 1, 3, 5 layers of black electrical tape applied thereto, respectively. The data in the graph are further processed, the temperature curve of each pixel point in a certain selected area is calculated according to the formula (5), and then the average value is obtained, so that the thicknesses of the surface layer materials are respectively 0.1439mm,0.5132mm and 0.7204mm. Each layer of tape was measured to a practical thickness of 0.143mm. The thickness of the 1, 3, 5 layers of tape should therefore theoretically be about 0.143mm,0.429mm and 0.715mm. The actual measured value being slightly greater than the theoretical value may be due to the presence of gaps between each layer of tape.
The schematic diagrams of the detection device according to the present invention are shown in fig. 7 (a) and 7 (b). The device comprises an Nd: YAG laser with wavelength of 1064nm, pulse width of 10ns, maximum single pulse laser output energy of 2J, and repetition frequency of 0.3-10Hz; the pixel of the detector of the refrigeration type long-wave thermal infrared imager is 320x256, and the highest frame frequency is 2000Hz; a pulse shaping system; a three-dimensional translation stage; a computer synchronized control thermal excitation system and infrared acquisition system and three-dimensional translation stage are shown in fig. 7 (a). Fig. 7 (b) further includes a flash lamp with a power of 2.4 KW. In the figure, M1 and M2 are reflectors, L1 and L2 are converging lenses, and H1 is a laser beam even wave plate.
As shown in fig. 9, the online laser cleaning effect detection of the present invention includes the steps of:
step 2 a), if laser excitation is adopted, the M1 reflecting mirror needs to be turned up, and laser light emitted by the pulse laser is sequentially reflected by M1 and M2 and is led into a shaping system (composed of a laser beam uniform wave plate H1 and a converging lens L2), as shown in fig. 7 (a). After beam shaping, the laser beam becomes a square uniform spot, and irradiates the surface of the sample to be cleaned at the position to be cleaned, as shown in fig. 8. At this time, the energy density can be reduced to be less than the cleaning threshold due to the increase of the laser spot area.
The pulse laser can also be used as a laser source for cleaning a sample, M1 is cut out at the moment, and laser is converged on the surface of the sample through a lens L1.
Step 2 b) when the area of the sample to be cleaned is excited by an additional light source (e.g. a flash lamp), the system does not need to be provided with a beam shaping system, and the device is shown in fig. 7 (b).
And step 3, starting the thermal imager before triggering thermal excitation, and recording a temperature field change signal of a position to be cleaned on the surface of the sample within a set time. The set time depends on the thickness of the layer to be cleaned and its thermal diffusivity, preferably greater than 2 times the peak time of the coating.
And step 5, cleaning the position to be cleaned on the surface of the sample by adopting a laser beam, and moving the translation stage to complete one-time cleaning.
And step 6, repeating the step 3 and the step 4 once, carrying out thermal wave nondestructive testing on the sample again to obtain a curve at the moment, and determining the time of the peak position of the curve at the moment, and marking as tb. If the skin and substrate material parameters are known, the skin thickness at this time can be calculated according to equation (5), whereby the skin thickness p removed by a single cleaning can be calculated, at which time the total number of cleaning can be predicted.
Step 7, the results obtained in step 4 and step 6 can be judged according to ta and tb, if
And (3) cleaning can be continued, and the step (8) is executed; otherwise, stopping cleaning.
And 8, repeating the step 3 and the step 4 to obtain the peak position tc of the temperature curve. According to t a 、t b And t c And (3) judging: if it is
Then the cleaning is continued; by the method, after each cleaning, the time t is corresponding to the current peak value c Corresponding time t to peak value of two times before current cleaning a And t b And judging until the cleaning stopping condition is met.
In summary, the above embodiments are only preferred embodiments of the present invention, and are not intended to limit the scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. The method for detecting the laser cleaning effect is characterized by comprising the following steps of:
irradiating the position to be cleaned on the surface of the sample by adopting a thermal excitation source;
obtaining a temperature field change signal T of a position to be cleaned on the surface of a sample within a set time from the start of a thermal excitation source;
taking logarithm of the temperature field change signal to obtain ln (T), and calculating first derivative of logarithm ln (T) of time T to obtain logarithmic temperature first derivative
-a time t "curve, the time corresponding to the peak on the curve being determined, noted as t a ；
Cleaning the position to be cleaned on the surface of the sample by adopting a laser beam to finish one-time cleaning;
obtaining a temperature field change signal of a position to be cleaned on the surface of the cleaned sample, and obtaining a logarithmic temperature first derivative at the moment
-time t' curve, and determining the time corresponding to the peak position of the curve, and recording as t b ；
According to the current t a And t b And (3) judging:
if it is
The cleaning stopping condition is met, and the cleaning is stopped;
if it is
Then cleaning is continued, and a temperature field change signal of the position to be cleaned is obtained again after cleaning, and a logarithmic temperature derivative reciprocal +.>
-time t "curve, and determining the time at which the peak position of the curve is present, denoted t c The method comprises the steps of carrying out a first treatment on the surface of the According to t a 、t b And t c And (3) judging: if->
Then the cleaning is continued; by the method, after each cleaning, the time t is corresponding to the current peak value c Corresponding time t to peak value of two times before current cleaning a And t b And judging until the cleaning stopping condition is met.
2. The method for detecting the cleaning effect of laser light according to claim 1, wherein the thickness L1 of the surface layer is obtained by using the following formula according to the parameters of the surface layer and the base material before cleaning the sample:
wherein T (0, T) represents the surface temperature; q is the heat applied per unit area, alpha 1 E is the thermal diffusivity of the surface material 1 And e 2 The heat storage coefficient of the first layer and the second layer material, L 1 Is the first layer material thickness.
3. The method for detecting the cleaning effect of laser light according to claim 2, wherein after the sample is cleaned once, the thickness of the surface layer at this time is calculated again based on the parameters of the surface layer and the base material, thereby calculating the thickness p of the surface layer removed by the single cleaning, and predicting the total number of cleaning times.
4. A device for realizing the method for detecting the cleaning effect of the laser according to claim 1, 2 or 3, which is characterized by comprising a pulse laser, a thermal imager and a three-dimensional translation stage; the pulse laser is used as a cleaning laser source and is used as a thermal excitation source; the thermal imager is used for obtaining the temperature field change signal; the translation stage is used to move the sample.
5. The apparatus of claim 4, further comprising a beam shaping system for shaping the pulsed laser light generated by the pulsed laser into square spots of a set size for irradiation as a thermal excitation source to the sample surface at the locations to be cleaned.
6. The apparatus of claim 4, further comprising a mirror M1, a mirror M2, a converging lens L1; the reflector M1 and the converging lens L1 are sequentially arranged in a laser light path emitted by the pulse laser; laser emitted by the pulse laser is reflected by M1 and M2 in sequence and is led into a beam shaping system; the converging lens L1 is used for converging laser emitted by the pulse laser and is used as a cleaning light source to irradiate the position to be cleaned on the surface of the sample.
7. A device for realizing the method for detecting the cleaning effect of the laser according to claim 1, 2 or 3, which is characterized by comprising a pulse laser, a thermal imager, a flash lamp and a three-dimensional translation stage; the pulse laser is used as a cleaning laser source, and the flash lamp is used as a thermal excitation source; the thermal imager is used for obtaining the temperature field change signal; the three-dimensional translation stage is used to move the sample.
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