CN1189725C - Laser vibration detestion method and its equipment - Google Patents

Abstract

The laser vibration detection device of the invention relates to a device for accurately measuring and analyzing the vibration of an object, in particular to a device for accurately measuring and analyzing vibration by using an optical measurement method. In order to provide a laser vibration detection method and its implementation device, the measurement accuracy of which is not affected by laser wavelength drift, the technical solution adopted in the present invention is that a laser vibration detection method and its implementation device include a laser, a laser power supply, a focusing lens , transmission grating, diaphragm, λ/4 wave plate, reflective prism, focusing lens, lens, Wollaston prism, two photoelectric receivers, amplifying circuit unit, signal processing unit, and frequency mixing components. The invention is suitable for measuring the vibration of an object and the occasion where the measured vibration is smaller than the counting equivalent value.

Description

Laser vibration detection method and its implementing device

Technical field

The invention relates to a device for accurately measuring and analyzing the vibration of an object, in particular to a device for accurately measuring and analyzing the vibration by using an optical measurement method.

Background technique

The currently disclosed sensors used in the field of vibration and acceleration detection include electromagnetic sensors, piezoelectric sensors, and optical sensors. The common defect of electromagnetic sensors and piezoelectric sensors is the contact measurement method, which needs to be connected with the measured object, which will affect the measured vibration and make the measurement inaccurate. In addition, the electromagnetic sensor has relatively serious nonlinearity, and the dynamic range of measurement is small, while the low-frequency response of the piezoelectric sensor is relatively poor.

Optical sensors are non-contact measurement methods, usually in two types: interferometry and Doppler measurement. Generally speaking, the use of interferometry requires high precision of the device and strict requirements for alignment. The laser used requires stable frequency and long coherence length. Atmospheric disturbance and vibration of the external environment will cause changes in the interference signal. , leading to measurement errors. Therefore, interferometric vibration sensors are mostly used in experimental measurements. The Doppler measurement does not require interferometer components, does not require precise assembly, the frequency of the Doppler signal has a linear relationship with the measured velocity vector, and is not affected by environmental conditions and temperature. It is suitable for studying any complex object motion.

In the traditional laser Doppler measurement system, the Doppler signal is obtained from the scattered light of the measured object, the signal-to-noise ratio of the signal is low, and it includes the angle factors between the light source, moving speed, and receiver , may introduce large measurement errors. The calculation method for vibration characteristics such as amplitude is usually that each beat wave in the signal corresponds to a displacement equivalent value, and the measured amplitude is obtained by counting the number of beat waves between two adjacent flipping points. This method cannot obtain displacements smaller than the equivalent value, and its measurement resolution is very low. The subdivision method based on the displacement measurement of the sinusoidal beat wave produces considerable errors when the beat wave is no longer a sinusoidal signal when measuring vibration. In addition, when the measured amplitude is less than 1/2 displacement equivalent value, the subdivision method cannot obtain the measured amplitude value.

Contents of Invention

The purpose of the present invention is to overcome the deficiencies of the prior art and provide a laser vibration detection device whose measurement accuracy is not affected by laser wavelength drift; the optical path structure is simple and practical, and the signal-to-noise ratio of the received signal can be improved, and assembly and debugging Simple; easy to use, and its measurement accuracy does not depend on the measurement conditions (the distance between the laser and the measured object, atmospheric disturbance, etc.); it can measure displacements smaller than the counting quantitative value. For this reason, the technical scheme that the present invention adopts is:

A laser vibration detection method, comprising the following steps:

The laser beam emitted by the laser is focused by the lens, so that the focused beam is projected onto the transmission grating for diffraction, and the generated diffracted light is filtered by the diaphragm, so that one of the beams passes through the λ/4 wave plate and passes through the diaphragm. The rest of the light beams converge on the frequency mixing element through the focusing lens to diffract. Adjust the position of the focusing lens so that the light beams overlap with the diffracted light generated by the rest of the light beams, so that the coincident diffracted light is reflected back to the lens, and then focused to the splitter through the reflective prism and the lens. The birefringence phenomenon occurs in the beam device, and the generated beams are respectively received by the photoelectric receiver and the photoelectric receiver and converted into current signals, and the current signals are sent to the signal processing unit through the amplifying circuit unit, and the formula is: f _d = 2V/ d obtains the vibration frequency, where V is the motion velocity of the measured grating 20, d is the grating constant of the measured grating 20, and f _d is the AC signal frequency output by the signal processing unit;

For the displacement less than the counting equivalent value, the measured voltage is obtained by the following formula:

For M waveform: ${\hat{A}}^{\&prime;}=\frac{{\cos }^{-1}(V_p/V_m)}{\mathrm{\&pi;}}\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="C0313039000081.png"\; state="\{\{state\}\}">d</figure-callout>}}{2};$

For W waveform: ${\hat{A}}^{\&prime;}=\frac{d}{2}(1-\frac{{\cos }^{-1}(V_p/V_m)}{\mathrm{\&pi;}});$

For S waveform: ${\hat{A}}^{\&prime;}=\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="C0313039000081.png"\; state="\{\{state\}\}">d</figure-callout>}}{2}\frac{{\cos }^{-1}(1-2V_p/V_m)}{\mathrm{\&pi;}};$

In the formula, d is the constant of the mixing element, V _p and Vm are the voltages at the flipping points, is the displacement less than the count equivalent value.

Wherein, the signal processing unit includes the following working steps: carry out A/D conversion on the input signal, send it to the processor, obtain the vibration spectrum through fast Fourier transform, and identify the integer multiple relationship among its multiple spectral peaks The frequency component of is effective, and the lowest frequency among them is determined to be the measured vibration frequency.

A laser vibration detection device, including a laser, a laser power supply, a focusing lens, a transmission grating, an aperture, a λ/4 wave plate, a reflecting prism, a focusing lens, a lens, a Wollaston prism, two photoelectric receivers, and an amplification circuit The laser beam emitted by the laser is focused by the lens, then reaches the transmission grating for diffraction, and the generated diffracted light passes through the diaphragm, and one of the beams passes through the λ/4 wave plate Diffraction with the rest of the light beam passing through the diaphragm and converged on the mixing element through the focusing lens, and the coincident diffracted light generated by the light beam and the rest of the light beam is reflected back to the lens, and then focused to the beam splitter through the reflective prism and lens to cause birefringence Phenomenon, the generated light beams are correspondingly received by the photoelectric receiver and the photoelectric receiver and converted into current signals, which are sent to the signal processing unit through the amplifying circuit unit.

Wherein, the laser is one of He-Ne laser, semiconductor laser or other types of lasers;

The beam splitter is a prism, a grating or a splitter;

The mixing element is a reflective grating;

The signal processing unit is a single chip microcomputer, a digital signal processor or a computer.

Since the present invention adds a frequency mixing element to the measurement device, the measurement accuracy is not affected by the laser wavelength drift; the optical path structure is simple and practical, can improve the signal-to-noise ratio of the received signal, and is easy to assemble and debug; it is easy to use and its The measurement accuracy does not depend on the characteristics of the measurement conditions (the distance between the laser and the measured object, the atmospheric disturbance, etc.); meanwhile, based on the relationship between the small displacement and the voltage value near the flip point, corresponding steps are used to obtain the small displacement value, so the present invention also has Characteristic for measuring displacements less than a count-equivalent value.

Description of drawings

Fig. 1 is a structural schematic diagram of the present invention.

FIG. 2 is a block diagram of the vibration information processing unit in FIG. 1 .

Figure 3 is a vibration M-shaped beat wave diagram.

Figure 4 is a vibration W-shaped beat wave diagram.

Figure 5 is a vibration S-shaped beat wave diagram.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and examples.

In Fig. 1, including laser 1, laser power supply 2, focusing lens 3, transmission grating 4, aperture 5, λ/4 wave plate 6, reflecting prism 7, focusing lens 8, frequency mixing element 20, reflecting prism 7, lens 9. Beam splitter 10, photoelectric receivers 11, 12, amplifier circuit unit 13, signal processing unit 14, etc. The laser beam emitted by the laser 1 is focused by the lens 3 to the transmission grating 4, and diffraction occurs, and 0-order, ±1-order, ±2-order, ... diffracted lights are emitted, and the aperture 5 filters out +1-order light 15, - Other diffracted light other than the first-order light 16. These two beams of light are transformed by the lens 8 into beams 21 and 24 that are converged to the mixing element 20 and diffracted. The ±1st order lights diffracted by the beam 21 are 22 and 23; Adjusting the position of the lens 8 can make the light beams 22 and 25 overlap, retroreflect to the lens 8, focus to the beam splitter 10 through the reflective prism 7 and lens 9, and birefringence occurs to generate e light and o light, respectively by the photoelectric receiver 11 and 12 receive and transform into current signals, that is, beat waves. The λ/4 wave plate 6 and the beam splitter 10 are used as polarization phase shifting elements to generate sin and cos signals with a 90° phase difference to distinguish the direction of motion. In this embodiment, the beam splitter 10 is a Wollaston prism , the frequency mixing element 20 is a reflective grating.

The beat signal output by the photoelectric receiver is sent to the amplifying circuit unit 13, amplified by the amplifier and converted into a voltage signal, filtered to remove circuit noise, and sent to the signal processing unit 14 to obtain vibration information.

When the frequency mixing element 20 is stationary, the frequencies of the light beams 22 and 25 are the same, and the photoelectric receiver outputs a DC signal. If the frequency mixing element 20 moves along the direction perpendicular to the optical axis, as shown in Figure 1, due to the Doppler effect, the frequencies of the beams 22 and 25 will change, and the frequency of the beam 22 is f ₂₂ =f ₀ +V/d , the frequency of the light beam 25 is f ₂₅ =f ₀ -V/d. Where f ₀ is the laser frequency emitted by the laser, V is the moving speed of the grating 20 under test, and d is the grating constant of the grating 20 under test. The photoelectric receiver will output an AC signal with a frequency of f _d =2V/d.

When the frequency mixing element 20 vibrates, the moving speed is the largest near the origin, and the corresponding beat wave frequency is the highest; at the maximum displacement, the moving speed is the smallest, and the beat wave frequency is the lowest, corresponding to the turning point of the beat wave. The actual waveform will have different shapes due to the measured amplitude and initial phase, M-shaped, W-shaped or S-shaped. Figure 3, Figure 4, and Figure 5 show various beat waveforms obtained through actual measurement.

In the signal processing unit shown in Figure 2, the amplified and filtered beat signal is first A/D converted, sent to the processor, and the vibration spectrum is obtained by fast Fourier transform. Among its multiple peaks, only The frequency components that are integer multiples of each other are valid, the lowest frequency is the measured vibration frequency, and the others can be identified as interference. This gives the frequency of the measured vibration.

In the beat wave, each beat wave corresponds to the measured displacement of d/2. The total displacement can be obtained by counting the total number of beat waves. The incomplete beat wave near the flip point corresponds to a displacement less than d/2. The measured amplitude A ₀ <d/4, and its beat waveform is S-shaped. The measured amplitude value can be obtained by measuring the voltage at the flip point:

$\stackrel{<span\; class="notranslate">\; ^</span>}{\mathrm{<span\; class="notranslate">\; A</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; d</span>}}{\mathrm{<span\; class="notranslate">\; 4</span>}}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>{\stackrel{<span\; class="notranslate">\; ^</span>}{\mathrm{<span\; class="notranslate">\; A</span>}}}^{<span\; class="notranslate">\; \&prime;</span>}$

For M waveform: ${\hat{A}}^{\&prime;}=\frac{{\cos }^{-1}(V_p/V_m)}{\mathrm{\&pi;}}\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="C0313039000081.png"\; state="\{\{state\}\}">d</figure-callout>}}{2}$

For W waveform: ${\hat{A}}^{\&prime;}=\frac{d}{2}(1-\frac{{\cos }^{-1}(V_p/V_m)}{\mathrm{\&pi;}})$

For S waveform: ${\hat{A}}^{\&prime;}=\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="C0313039000081.png"\; state="\{\{state\}\}">d</figure-callout>}}{2}\frac{{\cos }^{-1}(1-2V_p/V_m)}{\mathrm{\&pi;}}$

为小于计数当量值的位移。In this way, the amplitude value of the measured vibration is obtained. In the formula: d is the constant of the frequency mixing element, V _p and Vm are the voltages at the flipping point,

is the displacement less than the count equivalent value.

As described above, the laser Doppler vibration measurement device and method using the reflective grating are simple, reliable, and improve the signal-to-noise ratio of the measurement signal. Through the frequency analysis of the beat signal, the method of peak frequency ratio can eliminate the interference and obtain the measured vibration frequency; based on the relationship between the small displacement near the flip point and the voltage value, the displacement smaller than the counting equivalent value can be obtained by measuring the voltage. The measurement accuracy of tiny vibration displacement is improved, and the minimum resolution and dynamic range of system measurement are improved. This makes it possible to obtain various characteristics of vibration with a simple structure. The measurement system can be applied to the measurement and analysis of automobiles, machine tools, buildings, seismic waves, semiconductor industries, etc.

Claims (7)

1. A laser vibration detection method is characterized in that, comprising the following steps: The laser beam emitted by the laser (1) is focused by the lens (3), so that the focused beam is projected onto the transmission grating (4) for diffraction, and the generated diffracted light is filtered by the diaphragm (5), so that one of the beams The light beam (15) passes through the λ/4 wave plate (6) and the rest of the light beam (16) passing through the diaphragm (5) passes through the focusing lens (8) and converges on the frequency mixing element (20) for diffraction, and the focusing lens (8) is adjusted The position of the first beam (15) and the diffracted light produced by the remaining beams (16) overlap, so that the overlapped diffracted light returns to the lens (8), through the reflective prism (7), lens (9) Focusing on the beam splitter (10) produces birefringence, and the generated light beams are respectively received by the photoelectric receiver (11) and the photoelectric receiver (12) and converted into current signals, and the current signals are passed through the amplifying circuit unit (13 ) into the signal processing unit (14), so that the frequency mixing element (20) vibrates with the measured object; By formula: f _d =2V/d obtains the vibration frequency, wherein V is the motion velocity of the frequency mixing element (20), d is the constant of the frequency mixing element (20), and f _d is the AC signal output by the signal processing unit (14) frequency; For the displacement less than the counting equivalent value, the measured voltage is obtained by the following formula: For M waveform: ${\hat{A}}^{\&prime;}=\frac{{\cos }^{-1}(V_p/V_m)}{\mathrm{\&pi;}}\frac{d}{2};$ For W waveform: ${\hat{A}}^{\&prime;}=\frac{d}{2}(1-\frac{{\cos }^{-1}(V_p/V_m)}{\mathrm{\&pi;}});$ For S waveform: ${\hat{A}}^{\&prime;}=\frac{d}{2}\frac{{\cos }^{-1}(1-2V_p/V_m)}{\mathrm{\&pi;}};$ In the formula, d is the constant of the frequency mixing element, V _P and Vm are the voltages at the flipping point,

is the displacement less than the count equivalent value. 2. A kind of laser vibration detection method according to claim 1, is characterized in that, described signal processing unit (14) comprises the following working steps: carry out A/D conversion to input signal, send in the processor, pass The vibration spectrum is obtained by fast Fourier transform. Among its multiple spectrum peaks, the frequency components with an integer multiple relationship between each other are considered to be effective, and the lowest frequency among them is determined to be the measured vibration frequency. 3. A laser vibration detection device, comprising a laser (1), a laser power supply (2), a focusing lens (3), a transmission grating (4), an aperture (5), a λ/4 wave plate (6), and a reflective prism (7), focusing lens (8), lens (9), beam splitter (10), two photoelectric receivers (11,12), amplifying circuit unit (13), signal processing unit (14), it is characterized in that , also includes a frequency mixing element (20), after the laser beam emitted by the laser (1) is focused by the lens (3), it reaches the transmission grating (4) and undergoes diffraction, and the generated diffracted light passes through the diaphragm (5), and the A beam of light (15) passes through the λ/4 wave plate (6) and the rest of the light beams (16) passing through the diaphragm (5) converge on the frequency mixing element (20) through the focusing lens (8) to undergo diffraction. The coincident diffracted light produced by the light beam (15) and the remaining light beams (16) returns to the lens (8), and is focused to the beam splitter (10) through the reflective prism (7) and lens (9) for birefringence Phenomena, the generated light beams are received by the photoelectric receiver (11) and photoelectric receiver (12) respectively and converted into current signals, which are sent to the signal processing unit (14) through the amplifying circuit unit (13). 4. A laser vibration detection device according to claim 3, characterized in that the laser (1) is one of a He-Ne laser and a semiconductor laser. 5. A laser vibration detection device according to claim 3, characterized in that the beam splitter (10) is a prism, a grating or a light splitter. 6. A laser vibration detection device according to claim 3, characterized in that the frequency mixing element (20) is a reflective grating. 7. A laser vibration detection device according to claim 3, characterized in that the signal processing unit (14) is a single-chip microcomputer, a digital signal processor or a computer.

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