CN110186551A - Square wave transforming amplitudes measuring device and method based on self-mixed interference - Google Patents

Abstract

The invention discloses a square wave transform amplitude measurement device based on self-mixing interference, which includes a light source unit, a signal processing unit and a data processing unit. The signal processing unit introduces a hysteresis comparator to process the signal to output the required square wave signal. The data processing unit searches for the turning point of the output square wave signal, so as to calculate the vibration amplitude of the object according to the number of stripes in the same oblique direction between adjacent turning points. The invention also discloses a square wave transform amplitude measuring method based on self-mixing interference. The invention uses a hysteresis comparator circuit with adjustable upper and lower limits to perform square wave conversion on the obtained laser self-mixing signal, effectively reducing the influence of external noise on measurement stability and accuracy. The self-mixing interference principle has no specific requirements on the coherence of the laser light source, and the laser has a large degree of freedom in selection. At the same time, the magnification of the signal is controllable, and the minimum measurable amplitude value can reach the nanometer level, which is easy to realize real-time measurement in a wide range.

Description

Device and method for measuring square wave transform amplitude based on self-mixing interference

technical field

The invention relates to the technical field of optical nanometer measurement, in particular to a square wave transform amplitude measurement device and method based on self-mixing interference.

Background technique

Nano-vibration measurement is becoming more and more important in many applications such as high-precision engineering technology, precision machining, and aerospace. At present, researchers have proposed some meaningful methods for measuring the amplitude of weakly vibrating objects. Among them, the measurement method based on optical technology has the advantages of non-contact and high precision, and has attracted much attention.

Commonly used optical vibration measurement methods include geometric optics and optical interferometry. However, most of the reported geometric optics methods require complex and precise optical adjustment systems; in the field of laser interferometry, due to its simple optical structure and easy alignment, self-mixing interference methods have been extensively studied and applied. In the self-mixing interferometric vibration method, there are frequency domain analysis measurement method and time domain fringe counting method.

The frequency domain measurement method is sensitive to external noise and has high requirements on the signal-to-noise ratio of the signal. When the noise becomes larger, the spectrum will be unstable, and useful frequency components are easily covered by the noise spectrum, making the measurement results inaccurate. The traditional fringe counting method is divided into manual counting and automatic counting. The manual counting method is time-consuming when the vibration amplitude of the measured object is large, while the traditional automatic counting method tends to count the noise voltage fluctuation at the threshold as an effective fringe, resulting in relatively large Large count error. At the same time, the accuracy of the fringe counting method can usually only reach λ/2, so it is not suitable for applications that require simple and stable real-time measurement and require nanometer-level high-precision resolution.

Contents of the invention

The object of the present invention is to provide a self-mixing interference-based square-wave transform amplitude measurement device and method, which has a simple and compact structure, simple and efficient method, stable and accurate measurement results, high resolution, and a wide range of applications.

To achieve the above object, the present invention adopts the following technical solutions:

Square wave transform amplitude measurement device based on self-mixing interference, including:

A light source unit, which is used to monitor the vibrating object to be measured, to generate a self-mixed laser intensity signal and convert it into a corresponding current signal;

A signal processing unit, which includes a signal pre-processing module and a signal post-processing module; the signal pre-processing module converts the current signal into a voltage signal and amplifies it; the signal post-processing module includes a hysteresis comparator, and the hysteresis comparator The device is used to implement signal processing, so that the number of square waves in the output square wave signal is the largest and the glitch level is the least in unit time;

The data processing unit is used to find the turning points of the output square wave signal, so as to calculate the vibration amplitude of the object according to the number of stripes in the same oblique direction between adjacent turning points.

Further, the data processing unit also calculates the vibration information of the vibrating object to be measured according to the sampling sequence number of the flip point.

Further, the light source unit includes a semiconductor laser, a laser drive circuit, and a photodetector; the signal pre-processing module includes a transconductance operational amplifier circuit, a DC blocking circuit, and a proportional operational amplifier circuit; the data processing unit includes an ADC sampling circuit and DSP real-time processing module; the laser drive circuit and the photodetector are respectively connected with the semiconductor laser, the photodetector, the transconductance operational amplifier circuit, the DC blocking circuit, the proportional operational amplifier circuit, the hysteresis comparator, the ADC The sampling circuit and the DSP real-time processing module are connected in sequence.

Further, the proportional operational amplifier circuit uses a rheostat as a negative feedback resistor to realize a variable magnification; or, the proportional operational amplifier circuit uses a fixed resistor as a negative feedback resistor to realize a fixed magnification.

Further, the hysteresis comparator uses a variable resistor as a feedback resistor to realize variable upper and lower threshold voltages.

The invention also discloses a method for measuring the amplitude of square wave transformation based on self-mixing interference, which includes the following steps:

S1. Signal generation and output: The outgoing light of the semiconductor laser hits the surface of the vibrating object to be measured, and part of the 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 it into a corresponding current signal output;

S2. Signal processing: convert the current signal into a voltage signal and amplify the signal; input the amplified voltage signal into the hysteresis comparator, and adjust the upper and lower threshold voltages of the hysteresis comparator within the range of the signal amplitude to make the output square The number of square waves in the wave signal is the largest and the glitches are the least in unit time;

S3, data processing:

S31. Perform AD sampling on the square wave signal output by S2, and calculate the width of each level of the sampled signal;

S32. Find the flipping point according to the width of the level: if the ratio of a certain level to two levels of the same polarity before and after both exceeds the set threshold, then the level is the flipping point;

S33. Calculate the vibration amplitude of the object: A=λ*N/4, where A is the amplitude of the object, λ is the wavelength of the laser, and N is the number of fringes in the same oblique direction between two adjacent flipping points.

Further, S3 also includes steps:

S34. Calculate the vibration frequency of the object: F=2*fs/(X ₂ -X ₁ ), where F is the vibration frequency of the object, X ₁ is the sampling sequence number of the previous flip point, and X ₂ is the sampling sequence number of the next flip point , fs is the sampling rate.

Further, in S31, the point where the high and low levels jump is detected to obtain the time coordinates of each edge of the square wave, and the time width of each level is obtained by calculating the difference of the time coordinates of adjacent edges, and the level width is obtained .

Further, in S32, the set threshold ≥ 2.

Further, in S33, if the sum of incomplete stripes in the same oblique direction between two adjacent flipping points is less than half a stripe, it is treated as 0.5 stripes, and if it is more than 0.5 stripes and less than 1 stripe, it is treated as 1 stripe.

After adopting the technical solution, the present invention has the following advantages compared with the background technology:

1. The present invention does not require a reference arm of a traditional laser interferometer, and its single optical path is easy to collimate and reduces the system's sensitivity to external noise. The principle of self-mixing interference has no specific requirements on the coherence of the laser light source, and the degree of freedom in laser selection is large.

2. Since the hysteresis comparator is used to process the self-mixing signal, the high and low square waves with different widths are output. The square wave level width corresponding to the square wave signal at the inversion point is obviously larger than the non-inversion point prescription wave level width, so that The requirement for the signal-to-noise ratio of the obtained self-mixing waveform is reduced, and the impact of the burr on the self-mixing signal on the measurement result is negligible, so there is no need for a low-pass filter circuit, and it is not necessary to use a high-precision, ultra-low-noise integrated operational amplifier circuit to form There is a low-pass filter, which reduces the cost; and when the self-mixing signal stripe shape is not good or has an envelope, the measurement accuracy is still good.

3. The overall design of the measurement system is easy to integrate, the measurement method is simple and reliable, and the resolution reaches λ/8, which is suitable for industrial applications.

Description of drawings

Figure 1 is a schematic diagram of laser self-mixing interference technology.

Fig. 2 is a structural schematic diagram of the amplitude measuring device of the present invention.

Fig. 3 is a schematic flow chart of the amplitude measurement method of the present invention.

Fig. 4 is a schematic diagram of a square wave obtained by the present invention.

FIG. 5 is a schematic diagram of a measurement result of the present invention.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

Before describing the embodiments, the basic principle of the present invention will be explained first. The invention is designed based on the principle of laser self-mixing interference. When the measured object is displaced, the reflected light carrying the optical path change signal of the external cavity will affect the internal cavity, which will disturb the laser power, that is, the displacement information of the measured object It is reflected in the fluctuation of optical power.

Example 1

Please refer to FIG. 2 , the present invention discloses a square wave transform amplitude measurement 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 to monitor the vibrating object to be measured to generate a self-mixed laser intensity signal and convert it into a corresponding current signal; the signal processing unit processes the signal output by the light source unit to obtain the required square wave signal; the data processing unit analyzes and calculates the square wave signal to obtain the amplitude and frequency of the vibrating object to be measured.

In this embodiment, the light source unit includes a semiconductor laser, a photodetector and a laser driving circuit respectively connected to the semiconductor laser. The photodetector is packaged in a semiconductor laser, and the laser light of the semiconductor laser is focused by the focusing lens of the laser and hits the surface of the vibrating object to be measured, and the part of the laser light irradiated on the surface returns to the laser cavity, and the The laser light inside interferes to generate a self-mixed laser intensity signal, and the photodetector converts the self-mixed laser intensity signal into a corresponding current signal.

Among them, the surface of the vibrating object to be measured requires a certain reflectivity, which can return the laser part irradiated on it to the laser cavity. If the surface can be pasted with a reflector, the surface of the object to be measured is not required.

Of course, an external independent photodetector may also be used to detect the output light intensity of the laser through a spectroscope. The light source 1 can also use other lasers with satisfactory collimation, which is not specifically limited in the present invention.

In this embodiment, the signal processing unit includes a signal pre-processing module and a signal post-processing module.

The signal pre-processing module sequentially includes a transconductance operational amplifier circuit, a DC 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 the DC component in the signal through a DC blocking capacitor; the proportional operational amplifier circuit amplifies the weak signal. In this embodiment, the proportional op-amp circuit can use a rheostat as a negative feedback resistor to realize a variable magnification; it can also use a fixed resistor as a negative feedback resistor to realize a fixed magnification; it can use a proportional op-amp in the same direction, or Using inverse proportional op amp. The present invention is not specifically limited.

The signal post-processing module includes a hysteresis comparator, which processes the signal output by the proportional operational amplifier circuit to output a square wave signal. Wherein, the hysteresis comparator can cope with self-mixing signals with different amplitudes by adjusting the upper and lower threshold voltages, so that the number of square waves output per unit time is the largest and the level glitches are the least.

The data processing unit includes 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, and inputs the obtained sampling signal into the DSP real-time processing module, and searches for a method whose level width is significantly larger than adjacent stripes through a program. Get the flip point, and then calculate the number of stripes in the same direction between the flip points), so as to calculate the amplitude of the vibration of the object according to A=λ*N/4, where A is the amplitude of the object, λ is the wavelength of the laser, and N is the two-phase The number of stripes in the same tilt direction between adjacent flip points.

At the same time, the data processing unit can also calculate the vibration frequency of the object according to the sampling sequence number and sampling rate of the flip point according to F=2*fs/(X2-X1), wherein F is the vibration frequency of the object, and X1 is the previous flip The sampling number of the point, X2 is the sampling number of the next flip point, and fs is the sampling rate.

The final calculation result is output through the display screen.

Example 2

Please refer to FIG. 3 , the present invention also provides a method for measuring the amplitude of square wave transformation based on self-mixing interference, which includes three core steps.

S1. Signal generation and output: The outgoing light of the semiconductor laser hits the surface of the vibrating object to be measured, and part of the 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 it into a corresponding current signal output.

S2. Signal processing: convert the current signal into a voltage signal and amplify the signal; input the amplified voltage signal into the hysteresis comparator, and adjust the upper and lower threshold voltages of the hysteresis comparator within the range of the signal amplitude to make the output square In the wave signal, the number of square waves per unit time is the largest and the glitches are the least.

In this embodiment, the conversion of the current signal into a voltage signal is realized through a transconductance operational amplifier circuit; the signal amplification is realized through a proportional operational amplifier circuit.

When the upper and lower threshold voltages of the hysteresis comparator are adjusted too large, the square wave corresponding to the stripes will not be inverted. Therefore, it is necessary to adjust the upper and lower threshold voltages of the hysteresis comparator to the signal amplitude range. At this time, The number of stripes is the largest, but if the upper and lower threshold voltages are adjusted too small, the burrs will increase. Therefore, here, the upper and lower threshold voltages of the hysteresis comparator should be adjusted within the signal amplitude range, so that the number of output square waves is the largest (that is, the stripes between the flip points will not be lost) and the glitch level is the least (the glitch level Refers to a level signal with a very small level width, usually less than one-sixth of the stripe width, which is caused by the noise fluctuation on the signal around 0V breaking through the upper and lower threshold voltages when the upper and lower threshold voltages are adjusted to be close to 0V). As shown in FIG. 4 , it is a schematic diagram of generating a square wave by processing the upper and lower threshold voltages of the hysteresis comparator.

S3, data processing:

S31. Perform AD sampling on the square wave signal output by S3, and calculate the width of each level of the sampled signal;

S32, looking for the flip point: according to the width of the square wave level corresponding to the flip stripe place is obviously greater than (usually more than 2 times, this embodiment is set to 2) its first two square wave level widths and the last two square wave levels Flat width, to determine the flip point;

S33, calculating the vibration amplitude of the object: A=λ*N/4, where A is the amplitude of the object, λ is the wavelength of the laser, and N is the number of stripes in the same oblique direction between two adjacent flipping points;

S34. Calculate the vibration frequency of the object: F=2*fs/(X2-X1), wherein F is the vibration frequency of the object, X1 is the sampling sequence number of the previous flip point, X2 is the sampling sequence number of the next flip point, and fs is the sample Rate.

In S31, the level width is obtained by the following method: detect the point where the high and low levels jump, and obtain the time coordinates of each edge of the square wave, and obtain the time width of each level by calculating the difference between the time coordinates of adjacent edges, that is Get the level width.

In this embodiment, in S33, the smallest unit of N is set to 0.5. Specifically, it is calculated as follows: if the sum of incomplete stripes in the same oblique direction between two adjacent flip points is less than half a stripe, it will be treated as 0.5 stripes, and if it is greater than 0.5 stripes and less than 1 stripe, it will be treated as 1 stripe. In this way, the measurement resolution can reach λ/8.

Please refer to Figure 5, which is an example of applying this method for measurement. 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 flipping points are located at the upper part, that is, the gray ellipse area. Among them, the laser wavelength λ is equal to 650nm (the measurement resolution is 81.25nm), N is calculated as 2.5, and the calculation result A=406.3nm is substituted into the formula in S43, and the actual vibration amplitude is estimated to be 422.5nm by directly reading the number of self-mixing fringes , with an error of 3.8%. In the experiment, the vibration frequency calculated by DSP is 211Hz, and the actual vibration frequency is 200Hz obtained through the signal generator, with an error of 5.5%, which can meet the requirements of industrial measurement accuracy.

Those skilled in the art can clearly understand that for the convenience and brevity of description, only the division of the above-mentioned functional units and modules is used as an example for illustration. The above-mentioned function allocation is completed by different functional units and modules, that is, the internal structure of the device is divided into different functional units or modules, so as to complete all or part of the functions described above. Each functional unit and module in the embodiment may be integrated into one processing unit, or each unit may exist separately physically, or two or more units may be integrated into one unit, and the above-mentioned integrated units may adopt hardware It can also be implemented in the form of software functional units. In addition, the specific names of the functional units and modules are only for the convenience of distinguishing each other, and are not used to limit the protection scope of the present application.

In the above-mentioned embodiments, the descriptions of each embodiment have their own emphases, and for parts that are not detailed or recorded in a certain embodiment, refer to the relevant descriptions of other embodiments.

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed in the present invention can easily think of changes or Replacement should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.

Claims (10)

1. The square wave transform amplitude measuring device based on self-mixing interference, is characterized in that, comprises: A light source unit, which is used to monitor the vibrating object to be measured, to generate a self-mixed laser intensity signal and convert it into a corresponding current signal; A signal processing unit, which includes a signal pre-processing module and a signal post-processing module; the signal pre-processing module converts the current signal into a voltage signal and amplifies it; the signal post-processing module includes a hysteresis comparator, and the hysteresis comparator The device is used to implement signal processing, so that the number of square waves in the output square wave signal is the largest and the glitch level is the least in unit time; The data processing unit is used to find the turning points of the output square wave signal, so as to calculate the vibration amplitude of the object according to the number of stripes in the same oblique direction between adjacent turning points. 2. A self-mixing interference based square wave transform amplitude measurement device according to claim 1, characterized in that: the data processing unit also calculates the vibration information of the vibrating object to be measured according to the sampling sequence number of the flip point. 3. A kind of square-wave transform amplitude measurement device based on self-mixing interference as claimed in claim 1, characterized in that: said light source unit comprises a semiconductor laser, a laser drive circuit and a photodetector; said signal pre-processing module comprises A transconductance operational amplifier circuit, a DC blocking circuit and a proportional operational amplifier circuit; the data processing unit includes an ADC sampling circuit and a DSP real-time processing module; the laser drive circuit and the photodetector are respectively connected to the semiconductor laser, and the photoelectric A detector, a transconductance operational amplifier circuit, a DC blocking circuit, a proportional operational amplifier circuit, a hysteresis comparator, an ADC sampling circuit and a DSP real-time processing module are connected in sequence. 4. a kind of square-wave transformation amplitude measurement device based on self-mixing interference as claimed in claim 3, is characterized in that: described proportional op-amp circuit adopts rheostat as negative feedback resistance, realizes variable magnification; Or, described The proportional operational amplifier circuit uses a fixed resistor as a negative feedback resistor to achieve a fixed magnification. 5. A self-mixing interference-based square-wave transform amplitude measuring device as claimed in claim 1, characterized in that: said hysteresis comparator uses a variable resistor as a feedback resistor to realize variable upper and lower threshold voltages. 6. based on the square wave transformation amplitude measuring method of self-mixing interference, it is characterized in that, comprising the following steps: S1. Signal generation and output: The outgoing light of the semiconductor laser hits the surface of the vibrating object to be measured, and part of the 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 it into a corresponding current signal output; S2. Signal processing: convert the current signal into a voltage signal and amplify the signal; input the amplified voltage signal into the hysteresis comparator, and adjust the upper and lower threshold voltages of the hysteresis comparator within the range of the signal amplitude to make the output square The number of square waves in the wave signal is the largest and the glitches are the least in unit time; S3, data processing: S31. Perform AD sampling on the square wave signal output by S2, and calculate the width of each level of the sampled signal; S32. Find the flipping point according to the width of the level: if the ratio of a certain level to two levels of the same polarity before and after both exceeds the set threshold, then the level is the flipping point; S33. Calculate the vibration amplitude of the object: A=λ*N/4, where A is the amplitude of the object, λ is the wavelength of the laser, and N is the number of fringes in the same oblique direction between two adjacent flipping points. 7. the square wave transformation amplitude measurement method based on self-mixing interference as claimed in claim 6, is characterized in that, S3 also comprises the step: S34. Calculate the vibration frequency of the object: F=2*fs/(X ₂ -X ₁ ), where F is the vibration frequency of the object, X ₁ is the sampling sequence number of the previous flip point, and X ₂ is the sampling sequence number of the next flip point , fs is the sampling rate. 8. the square wave transformation amplitude measurement method based on self-mixing interference as claimed in claim 6, is characterized in that: in S31, the point that high and low level jumps is detected, obtains the time coordinate of each edge of square wave, by The time coordinate difference of adjacent edges is calculated to obtain the time width of each level, and the level width is obtained. 9. The self-mixing interference-based square wave transform amplitude measurement method according to claim 6, characterized in that: in S32, the set threshold value is ≥2. 10. The square wave transformation amplitude measurement method based on self-mixing interference as claimed in claim 6, characterized in that: in S33, the non-complete fringes in the same oblique direction between two adjacent flip points add up to less than half a fringe Treat as 0.5 stripes, and treat as 1 stripe if more than 0.5 stripes and less than 1 stripe.