CN110186551B - Square wave transformation amplitude measuring device and method based on self-mixing interference - Google Patents
Square wave transformation amplitude measuring device and method based on self-mixing interference Download PDFInfo
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Abstract
The invention discloses a square wave transformation amplitude measuring device based on self-mixing interference, which comprises a light source unit, a signal processing unit and a data processing unit, wherein a hysteresis comparator is introduced into the signal processing unit to process signals so as to output required square wave signals, and the data processing unit searches turning points for the output square wave signals so as to calculate the vibration amplitude of an object according to the number of stripes in the same inclination direction between adjacent turning points. The invention also discloses a square wave transformation amplitude measuring method based on self-mixing interference. The invention adopts the hysteresis comparator circuit with adjustable upper and lower limits to carry out square wave transformation on the obtained laser self-mixing signal, thereby effectively reducing the influence of external noise on the measurement stability and accuracy. The self-mixing interference principle is adopted, no specific requirement is made on the coherence of a laser source, and the freedom degree of laser selection is large. Meanwhile, the amplification factor of the signal is controllable, the measurable minimum amplitude value can reach the nanometer level, and the wide-range real-time measurement is easy to realize.
Description
Technical Field
The invention relates to the technical field of optical nano measurement, in particular to a square wave transformation amplitude measuring device and method based on self-mixing interference.
Background
Nano-vibration measurement is of increasing importance in many applications such as high-precision engineering, precision machining, aerospace, and the like. Researchers have proposed some meaningful methods for measuring the amplitude of a weakly vibrating object. Among them, the measurement means based on the optical technique has the advantage of non-contact high precision and is emphasized.
Common optical vibration measurement methods are geometrical optics and optical interferometry. However, most of the reported geometrical optics methods require complicated and precise optical adjustment systems; in the field of laser interferometry, due to the characteristics of simple optical structure and easy collimation, self-mixing interferometry is widely researched and applied. The self-mixing interference vibration measuring method includes a frequency domain analysis measuring method and a time domain fringe counting method.
The frequency domain measurement method is sensitive to external noise, has high requirements on the signal-to-noise ratio of signals, and when the noise becomes large, the frequency spectrum is unstable, useful frequency components are easily covered by the noise frequency spectrum, and the measurement result becomes inaccurate. The conventional fringe counting method is divided into manual counting and automatic counting, the manual counting method consumes time when the vibration amplitude of a measured object is large, and the conventional automatic counting method easily counts noise voltage fluctuation at a threshold as effective fringes to generate a large counting error. Meanwhile, the accuracy of the fringe counting method can only reach lambda/2, so that the method is not suitable for the application occasions of simple and stable measurement in real time and high accuracy resolution of nanometer level.
Disclosure of Invention
The invention aims to provide a square wave transformation amplitude measuring device and method based on self-mixing interference, which have the advantages of simple and compact structure, simple and efficient method, stable and accurate measuring result, high resolution and wide application range.
In order to achieve the purpose, the invention adopts the following technical scheme:
the square wave transformation amplitude measuring device based on self-mixing interference comprises:
the light source unit is used for monitoring a vibrating object to be tested so as to generate a self-mixing laser intensity signal and convert the self-mixing laser intensity signal into a corresponding current signal;
the signal processing unit comprises a signal pre-processing module and a signal post-processing module; the signal preprocessing module converts the current signal into a voltage signal and amplifies the voltage signal; the signal post-processing module comprises a hysteresis comparator, the hysteresis comparator adopts a variable resistor as a feedback resistor to realize that the upper threshold voltage and the lower threshold voltage of the hysteresis comparator are variable, and the hysteresis comparator is used for realizing signal processing to ensure that the number of square waves in the output square wave signals in unit time is the most and the level of burrs is the least;
the data processing unit is used for searching the turning points of the output square wave signals so as to calculate the vibration amplitude of the object according to the number of the stripes in the same inclination direction between the adjacent turning points;
the light source unit comprises a semiconductor laser, a laser driving circuit and a photoelectric detector; the signal preprocessing module comprises a transconductance operational amplifier circuit, a blocking circuit and a proportional operational amplifier circuit; the data processing unit comprises an ADC sampling circuit and a DSP real-time processing module; the laser driving circuit and the photoelectric detector are respectively connected with the semiconductor laser, the photoelectric detector, the transconductance operational amplifier circuit, the blocking circuit, the proportional operational amplifier circuit, the hysteresis comparator, the ADC sampling circuit and the DSP real-time processing module are sequentially connected, the ADC sampling circuit samples a square wave signal output by the hysteresis comparator, the obtained sampling signal is input into the DSP real-time processing module, turning points are obtained by searching a method that the level width is obviously larger than that of adjacent stripes through a program, and then the number of the stripes in the same inclined direction between the turning points is calculated, so that the vibration amplitude of an object is calculated according to A = lambda N/4, wherein A is the amplitude of the object, lambda is the laser wavelength, and N is the number of the stripes in the same inclined direction between two adjacent turning points;
the data processing unit can also calculate the vibration frequency of the object according to the sampling ordinal number and the sampling rate of the turning point and according to F = 2X fs/(X2-X1), wherein F is the vibration frequency of the object, X1 is the sampling ordinal number of the previous turning point, X2 is the sampling ordinal number of the next turning point, and fs is the sampling rate, and the final calculation result is displayed and output through a display screen. Furthermore, the data processing unit also calculates the vibration information of the vibrating object to be measured according to the sampling ordinal number of the turning point.
Furthermore, the proportional operational amplifier circuit adopts a rheostat as a negative feedback resistor to realize variable amplification factor; or, the proportional operational amplifier circuit adopts a fixed resistor as a negative feedback resistor to realize fixed amplification factor.
Further, the hysteresis comparator adopts a variable resistor as a feedback resistor to realize that the upper threshold voltage and the lower threshold voltage of the hysteresis comparator are variable.
The invention also discloses a square wave transformation amplitude measuring method based on self-mixing interference, which comprises the following steps:
s1, generation and output of signals: emergent light of the semiconductor laser hits the surface of a vibrating object to be measured, and part of light returns to the laser cavity to generate a self-mixing laser intensity signal; the photoelectric detector detects the self-mixing laser intensity signal and converts the self-mixing laser intensity signal into a corresponding current signal for output;
s2, signal processing: converting the current signal into a voltage signal, and amplifying the signal; inputting the amplified voltage signal into a hysteresis comparator, and adjusting the upper threshold voltage and the lower threshold voltage of the hysteresis comparator within the signal amplitude range to ensure that square waves in the output square wave signal have the most number and the least burrs in unit time;
s3, data processing:
s31, AD sampling is carried out on the square wave signal output in the S2, and the width of each level is calculated on the sampled signal;
s32, finding a turning point according to the level width: if the ratio of a certain level to two levels with the same polarity before and after the certain level exceeds a set threshold value, the level is a turning point;
s33, calculating the vibration amplitude of the object: a = lambda N/4, wherein A is the amplitude of the object, lambda is the wavelength of the laser, and N is the number of stripes in the same oblique direction between two adjacent turning points;
s34, calculating the vibration frequency of the object: f =2 fs/(X2-X1), where F is the vibration frequency of the object, X1 is the sampling number of the previous flip point, X2 is the sampling number of the next flip point, and fs is the sampling rate.
Further, in S31, the point at which the high and low levels jump is detected to obtain the time coordinate of each edge of the square wave, and the time width of each level is obtained by subtracting the time coordinates of adjacent edges to obtain the level width.
Further, in S32, the set threshold value is ≧ 2.
Further, in S33, the incomplete stripe addition in the same oblique direction between two adjacent turning points is processed by 0.5 stripes for less than half stripes, and is processed by 1 stripe for more than 0.5 stripes and less than 1 stripe.
After adopting the technical scheme, compared with the background technology, the invention has the following advantages:
1. the invention does not need the reference arm of the traditional laser interferometer, and the single light path of the invention has easy collimation and reduces the noise sensitivity of the system to the outside. The self-mixing interference principle is adopted, no specific requirement is made on the coherence of a laser source, and the freedom degree of laser selection is large.
2. Because the self-mixing signal is processed by the hysteresis comparator, high and low square waves with different widths are output, the square wave level width corresponding to the square wave signal at the turning point is obviously larger than that of the square wave signal at the non-turning point, the signal-to-noise ratio requirement of the obtained self-mixing waveform is reduced, and the influence of burrs on the self-mixing signal on a measurement result can be ignored, a low-pass filter circuit is not needed, a high-precision and ultra-low-noise integrated operational amplifier circuit is not needed to form an active low-pass filter, and the cost is reduced; and when the self-mixing signal has poor fringe shape or envelope, the measurement accuracy still keeps good.
3. The whole design of the measuring system is easy to integrate, the measuring mode is simple and reliable, the resolution ratio reaches lambda/8, and the method is suitable for industrial application.
Drawings
FIG. 1 is a schematic diagram of a laser self-mixing interference technique.
Fig. 2 is a schematic structural diagram of the amplitude measuring device of the present invention.
Fig. 3 is a flow chart of the amplitude measuring method according to the present invention.
Fig. 4 is a schematic diagram of a square wave obtained by the present invention.
FIG. 5 is a graph illustrating a measurement result according to the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Before describing the embodiments, the basic principle of the present invention will be explained first. The invention is designed based on the self-mixing interference principle of laser, when the measured object is displaced, the reflected light carrying the external cavity optical path change signal will affect the internal cavity, and the laser power is disturbed, that is, the displacement information of the measured object is reflected in the light power fluctuation.
Example 1
Referring to fig. 2, the present invention discloses a square wave transformation amplitude measuring device based on self-mixing interference, which includes a light source unit, a signal processing unit and a data processing unit.
The light source unit is used for monitoring a vibrating object to be detected so as to generate a self-mixing laser intensity signal and convert the self-mixing laser intensity signal into a corresponding current signal; the signal processing unit processes the signal output by the light source unit to obtain a required square wave signal; and the data processing unit analyzes and calculates the square wave signal and solves the amplitude and the frequency of the vibrating object to be tested.
In this embodiment, the light source unit includes a semiconductor laser, and a photodetector and a laser driving circuit respectively connected to the semiconductor laser. The photoelectric detector encapsulates in semiconductor laser, semiconductor laser's laser hits on the vibration object surface that awaits measuring after the focusing lens gathering of laser self-band, irradiates the laser part on its surface and returns to the laser intracavity, takes place to interfere with the laser of intracavity, produces from mixing laser intensity signal, photoelectric detector will mix laser intensity signal conversion from the meeting and become corresponding current signal.
The surface of the vibrating object to be measured requires certain reflectivity, the laser part irradiated on the vibrating object to be measured can be returned to the laser cavity, and if the surface can be pasted with the reflector, no requirement is made on the surface of the object to be measured.
Of course, an external independent photodetector may be used to detect the output intensity of the laser light via the beam splitter. The light source 1 may also be other lasers with collimation meeting the requirement, and the present invention is not limited in particular.
In this embodiment, the signal processing unit includes a signal preprocessing module and a signal postprocessing module.
The signal preprocessing module sequentially comprises a transconductance operational amplifier circuit, a blocking circuit and a proportional operational amplifier circuit. The transconductance operational amplifier circuit converts the current signal into a voltage signal; the DC blocking circuit filters out DC components in the signal through a DC blocking capacitor; the proportional operational amplifier circuit amplifies weak signals. In this embodiment, the proportional operational amplifier circuit may use a rheostat as a negative feedback resistor to realize variable amplification factor; a fixed resistor can also be used as a negative feedback resistor to realize fixed amplification factor; the same proportion operational amplifier can be adopted, and the reverse proportion operational amplifier can also be adopted. The present invention is not particularly limited.
The signal post-processing module comprises a hysteresis comparator which processes the signal output by the proportional operational amplifier circuit to output a square wave signal. The hysteresis comparator can deal with self-mixing signals with different amplitudes by adjusting the upper threshold voltage and the lower threshold voltage, so that the number of square waves output in unit time is the largest and level burrs are the least.
The data processing unit comprises an ADC sampling circuit and a DSP real-time processing module. The ADC sampling circuit samples the square wave signal output by the hysteresis comparator, the obtained sampling signal is input into the DSP real-time processing module, turning points are obtained by searching a method that the level width is obviously larger than that of adjacent stripes through a program, and then the number of the stripes in the same inclination direction between the turning points is calculated), so that the vibration amplitude of the object is calculated according to A = lambda N/4, wherein A is the amplitude of the object, lambda is the laser wavelength, and N is the number of the stripes in the same inclination direction between the two adjacent turning points.
Meanwhile, the data processing unit can also calculate the vibration frequency of the object according to the sampling ordinal number and the sampling rate of the turning point and according to F = 2X fs/(X2-X1), wherein F is the vibration frequency of the object, X1 is the sampling ordinal number of the previous turning point, X2 is the sampling ordinal number of the next turning point, and fs is the sampling rate.
And finally, displaying and outputting the calculation result through a display screen.
Example 2
Referring to fig. 3, the present invention further provides a square wave transformation amplitude measurement method based on self-mixing interference, which includes 3 core steps.
S1, generation and output of signals: emergent light of the semiconductor laser hits the surface of a vibrating object to be measured, and part of light returns to the laser cavity to generate a self-mixing laser intensity signal; the photodetector detects the self-mixing laser intensity signal and converts the signal into a corresponding current signal for output.
S2, signal processing: converting the current signal into a voltage signal, and amplifying the signal; and inputting the amplified voltage signal into a hysteresis comparator, and adjusting the upper threshold voltage and the lower threshold voltage of the hysteresis comparator within the signal amplitude range to ensure that square waves in the output square wave signal have the most number and the least burrs in unit time.
In the embodiment, the conversion of the current signal into the voltage signal is realized through the transconductance operational amplifier circuit; the signal amplification is realized by a proportional operational amplifier circuit.
When the upper and lower threshold voltages of the hysteresis comparator are adjusted to be too large, the square waves corresponding to the stripes cannot be turned over, so that the upper and lower threshold voltages of the hysteresis comparator need to be adjusted to be within a signal amplitude range, the number of the stripes is the largest at the moment, and if the upper and lower threshold voltages are adjusted to be too small, burrs are increased. Therefore, the upper and lower threshold voltages of the hysteretic comparator should be adjusted within the signal amplitude range so that the number of output square waves is the largest (i.e. the stripes between the flip points are not lost) and the glitch level is the smallest (the glitch level is a level signal with a very small level width, usually less than one sixth of the stripe width, and is caused by the fact that when the upper and lower threshold voltages are adjusted to be close to 0V, the noise fluctuation on the signal around 0V breaks through the upper and lower threshold voltages). Fig. 4 is a schematic diagram of the square wave generated by the upper and lower threshold voltage processing of the hysteresis comparator.
S3, data processing:
s31, AD sampling is carried out on the square wave signal output in the S3, and the width of each level is calculated on the sampled signal;
s32, finding a turning point: judging a turning point according to the fact that the width of the square wave level corresponding to the turning stripe is obviously larger than (usually more than 2 times, the embodiment is set as 2) the width of the front two square wave levels and the width of the back two square wave levels;
s33, calculating the vibration amplitude of the object: a = lambda N/4, wherein A is the amplitude of the object, lambda is the wavelength of the laser, and N is the number of stripes in the same oblique direction between two adjacent turning points;
s34, calculating the vibration frequency of the object: f =2 fs/(X2-X1), where F is the vibration frequency of the object, X1 is the sampling number of the previous flip point, X2 is the sampling number of the next flip point, and fs is the sampling rate.
In S31, the level width is obtained by: and detecting the jumping points of the high and low levels to obtain the time coordinate of each edge of the square wave, and obtaining the time width of each level by the difference of the time coordinates of adjacent edges, namely obtaining the level width.
In S33, the minimum unit of N is set to 0.5. Specifically, the calculation is as follows: the incomplete fringe sum in the same inclination direction between two adjacent turning points is processed according to 0.5 fringe less than half of the fringe, and is processed according to 1 fringe more than 0.5 fringe and less than 1 fringe. Thus, the resolution of the measurement can reach lambda/8.
Fig. 5 shows an example of the measurement performed by the method. In the figure, the upper part is the amplified voltage signal, and the lower part is the square wave signal output after processing, in this example, the turning points are all located at the upper part, namely the gray oval area. Where the laser wavelength λ is equal to 650nm (and the measurement resolution is 81.25 nm), N is calculated to be 2.5, and the result a =406.3nm is calculated by substituting the formula in S43, the actual vibration amplitude is estimated to be 422.5nm by directly reading from the number of mixed fringes, and the error is 3.8%. In the experiment, the vibration frequency calculated by the DSP is 211Hz, the actual vibration frequency is 200Hz obtained by the signal generator, the error is 5.5 percent, and the requirement of industrial measurement precision can be met.
It is obvious to those skilled in the art that, for convenience and simplicity of description, the foregoing division of the functional units and modules is only used as an example, in practical applications, there may be another division manner in practical implementation, and the foregoing function distribution may be completed by different functional units and modules according to needs, that is, the internal structure of the apparatus is divided into different functional units or modules to complete all or part of the above-described functions. Each functional unit and module in the embodiments may be integrated in one processing unit, or each unit may exist alone physically, or two or more units are integrated in one unit, and the integrated unit may be implemented in a form of hardware, or in a form of software functional unit. In addition, specific names of the functional units and modules are only for convenience of distinguishing from each other, and are not used for limiting the protection scope of the present application.
In the above embodiments, the descriptions of the respective embodiments have respective emphasis, and reference may be made to the related descriptions of other embodiments for parts that are not described or illustrated in a certain embodiment.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (7)
1. Square wave transform amplitude measuring device based on self-mixing interference, characterized by, includes:
the light source unit is used for monitoring a vibrating object to be tested so as to generate a self-mixing laser intensity signal and convert the self-mixing laser intensity signal into a corresponding current signal;
the signal processing unit comprises a signal pre-processing module and a signal post-processing module; the signal preprocessing module converts the current signal into a voltage signal and amplifies the voltage signal; the signal post-processing module comprises a hysteresis comparator, the hysteresis comparator adopts a variable resistor as a feedback resistor to realize that the upper threshold voltage and the lower threshold voltage of the hysteresis comparator are variable, and the hysteresis comparator is used for realizing signal processing to ensure that the number of square waves in the output square wave signals in unit time is the most and the level of burrs is the least;
the data processing unit is used for searching the turning points of the output square wave signals so as to calculate the vibration amplitude of the object according to the number of the stripes in the same inclination direction between the adjacent turning points;
the light source unit comprises a semiconductor laser, a laser driving circuit and a photoelectric detector; the signal preprocessing module comprises a transconductance operational amplifier circuit, a blocking circuit and a proportional operational amplifier circuit; the data processing unit comprises an ADC sampling circuit and a DSP real-time processing module; the laser driving circuit and the photoelectric detector are respectively connected with the semiconductor laser, the photoelectric detector, the transconductance operational amplifier circuit, the blocking circuit, the proportional operational amplifier circuit, the hysteresis comparator, the ADC sampling circuit and the DSP real-time processing module are sequentially connected, the ADC sampling circuit samples a square wave signal output by the hysteresis comparator, the obtained sampling signal is input into the DSP real-time processing module, turning points are obtained by searching a method that the level width is obviously larger than that of adjacent stripes through a program, and then the number of the stripes in the same inclined direction between the turning points is calculated, so that the vibration amplitude of an object is calculated according to A = lambda N/4, wherein A is the amplitude of the object, lambda is the laser wavelength, and N is the number of the stripes in the same inclined direction between two adjacent turning points;
the data processing unit can also be used for processing the data according to the sampling ordinal number and the sampling rate of the turning point and according to F = 2X fs/(X)2-X1) Calculating the vibration frequency of the object, wherein F is the vibration frequency of the object, X1Sampling ordinal number, X, for the previous roll-over point2And f, the sampling ordinal number of the next turning point is obtained, fs is the sampling rate, and the final calculation result is displayed and output through a display screen.
2. The square wave transformed amplitude measuring device based on self-mixing interference as claimed in claim 1, wherein: and the data processing unit also calculates the vibration information of the vibrating object to be detected according to the sampling ordinal number of the turning point.
3. The square wave transformed amplitude measuring device based on self-mixing interference as claimed in claim 1, wherein: the proportional operational amplifier circuit adopts a rheostat as a negative feedback resistor to realize variable amplification factor; or, the proportional operational amplifier circuit adopts a fixed resistor as a negative feedback resistor to realize fixed amplification factor.
4. The square wave transformation amplitude measuring method based on self-mixing interference is characterized by comprising the following steps of:
s1, generation and output of signals: emergent light of the semiconductor laser hits the surface of a vibrating object to be measured, and part of light returns to the laser cavity to generate a self-mixing laser intensity signal; the photoelectric detector detects the self-mixing laser intensity signal and converts the self-mixing laser intensity signal into a corresponding current signal for output;
s2, signal processing: converting the current signal into a voltage signal, and amplifying the signal; inputting the amplified voltage signal into a hysteresis comparator, wherein the hysteresis comparator adopts a variable resistor as a feedback resistor to realize that the upper threshold voltage and the lower threshold voltage of the hysteresis comparator are variable, and the upper threshold voltage and the lower threshold voltage of the hysteresis comparator are adjusted within a signal amplitude range, so that square waves in the output square wave signal have the largest number and the smallest burrs in unit time;
s3, data processing:
s31, AD sampling is carried out on the square wave signal output in the S2, and the width of each level is calculated on the sampled signal;
s32, finding a turning point according to the level width: if the ratio of a certain level to two levels with the same polarity before and after the certain level exceeds a set threshold value, the level is a turning point;
s33, calculating the vibration amplitude of the object: a = lambda N/4, wherein A is the amplitude of the object, lambda is the wavelength of the laser, and N is the number of stripes in the same oblique direction between two adjacent turning points;
s34, calculating the vibration frequency of the object: f =2 fs/(X)2-X1) Wherein F is the vibration frequency of the object, X1Sampling ordinal number, X, for the previous roll-over point2The sampling ordinal number of the latter turning point, fs is the sampling rate.
5. The square wave transform amplitude measurement method based on self-mixing interference of claim 4, wherein: in S31, the point at which the high and low levels jump is detected to obtain the time coordinate of each edge of the square wave, and the time width of each level is obtained by subtracting the time coordinates of adjacent edges to obtain the level width.
6. The square wave transform amplitude measurement method based on self-mixing interference of claim 4, wherein: in S32, the set threshold value is greater than or equal to 2.
7. The square wave transform amplitude measurement method based on self-mixing interference of claim 4, wherein: in S33, the incomplete stripes in the same oblique direction between two adjacent turning points are added to the less than half stripes and processed according to 0.5 stripes, and the more than 0.5 stripes and the less than 1 stripe and processed according to 1 stripe.
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