Laser self-mixing interference rotating speed measuring device based on current modulation and measuring method thereof

Abstract

The invention discloses a laser self-mixing interference rotating speed measurement method based on current modulation, and the method comprises the following steps: S1, building a light path, irradiating the laser emitted by a fixed-wavelength solid-state laser to a fixed measurement point of a measured rotating workpiece, and feeding back the light reflected by the workpiece in a diffuse manner into a laser cavity to form self-mixing interference; s2, laser current modulation: inputting a high-frequency sinusoidal modulation current with direct-current bias to the fixed-wavelength solid-state laser, and superposing a high-frequency modulation component in a self-mixing interference signal; s3, interference signal acquisition: a self-mixing interference signal containing a high-frequency modulation component is acquired and converted into a digital electric signal; and S4, signal demodulation and rotation speed calculation: processing the digital electric signal to extract stripe subdivision information, and calculating the instantaneous rotation speed of the workpiece. Compared with the prior art, the laser self-mixing interference rotating speed measuring device based on current modulation and the measuring method thereof have the advantages that high resolution and high signal-to-noise ratio are achieved.

Description

Technical Field

[0001] This invention relates to the field of high-precision rotational speed measurement technology, specifically to a laser self-mixing interferometry rotational speed measurement device and its measurement method based on current modulation. Background Technology

[0002] Rotational speed is a core parameter of the dynamic characteristics of a rotating system, and its high-precision measurement is of great significance in fields such as automation control, dynamic monitoring of microelectromechanical systems, and precision manufacturing.

[0003] Currently, commonly used speed measurement technologies mainly include photoelectric encoders, Doppler effect, and laser interferometry.

[0004] Among them, photoelectric encoders have the advantages of fast response and simple structure, but their resolution is limited by the number of grating lines, making it difficult to achieve nanometer-level displacement recognition.

[0005] Doppler velocimetry is suitable for measuring high-speed, large-scale targets, but the system is complex and has limited sensitivity to minute angle changes. In contrast, laser interferometry, with its advantages of being non-contact, highly sensitive, and highly accurate, has gradually become an important means of precision velocimetry and minute motion analysis.

[0006] Among them, laser self-mixing interferometry, as a self-feedback interferometric measurement technology, is widely used for high-precision measurements of displacement detection, vibration analysis, and acceleration analysis due to its simple optical path, easy collimation, compact system, and low cost.

[0007] Currently, some researchers have combined SMI with speckle effect or Doppler effect for rotational speed measurement, but this method has drawbacks such as poor signal-to-noise ratio and complex optical path. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the above-mentioned technical defects and provide a laser self-mixing interferometry rotation speed measuring device and its measuring method based on current modulation to achieve high resolution and high signal-to-noise ratio.

[0009] To solve the above-mentioned technical problems, the technical solution provided by the present invention is: a laser self-mixing interferometry rotation speed measurement method based on current modulation, comprising the following steps:

[0010] S1: Set up the optical path and irradiate the fixed measurement point of the rotating workpiece with the laser emitted by the fixed wavelength solid-state laser. The light reflected back from the workpiece is fed back into the laser cavity to form self-mixing interference.

[0011] S2: Laser current modulation, inputting a high-frequency sinusoidal modulation current with DC bias into a fixed-wavelength solid-state laser, superimposing a high-frequency modulation component into the self-mixing interference signal;

[0012] S3: Interference signal acquisition, including self-mixed interference signals with high-frequency modulation components, and converting them into digital electrical signals;

[0013] S4: Signal demodulation and rotational speed calculation. Extract stripe subdivision information from digital electrical signal processing and calculate the instantaneous rotational speed of the workpiece.

[0014] Preferably, the expression for the high-frequency sinusoidal modulation current in S2 is:

[0015] I = I b +I m sin(ω s t);

[0016] Where I b I is the DC bias value. m ω is the amplitude of the sinusoidal modulation. s Angular frequency of the sinusoidal modulation signal;

[0017] The I m The intensity of the modulation fringes used to control the interference fringes is adjusted for subdivision.

[0018] Preferably, the output power of the self-mixing interference signal in S3 is:

[0019] P=P0[1+mcos(ω F τ L )];

[0020] Where P and P0 represent the optical power with and without optical feedback, respectively, m represents the modulation coefficient of the self-mixing interference, and τ L =2L / c represents the round-trip time of the laser in the external cavity, ω F This indicates the angular frequency when there is optical feedback;

[0021] The optical power modulation coefficient is:

[0022] P total =P+P mod ·sin(ω s t);

[0023] Where P mod P is the optical power modulation coefficient of a laser under isothermal conditions and current modulation. total This represents the total power.

[0024] Preferably, in step S4, the measurement resolution is obtained by counting the number of high-frequency modulation fringes within a single interference fringe, and the resolution S is:

[0025]

[0026] Where f f f is the frequency of the interference fringes. sNs is the frequency of the sinusoidal modulation, and Ns is the number of modulation fringes in a single interference fringe.

[0027] The f s Adjust the resolution according to the target measurement's expected resolution.

[0028] Preferably, calculating the instantaneous rotational speed of the workpiece in S4 includes constructing a correlation model between angular displacement and the change in the outer cavity length:

[0029]

[0030] Where ω is the instantaneous rotational speed, d is the distance between the laser spot and the workpiece rotation center, ΔL is the change in the outer cavity length, and Δt is the change in time.

[0031] Preferably, the fixed-wavelength solid-state laser in S1 is mounted on a TEC precision temperature-controlled base and operates under constant temperature conditions.

[0032] Preferably, signal demodulation in S4 includes:

[0033] S4.1: Perform variational mode decomposition on the digital electrical signal to extract the first eigenmode function containing high-frequency modulation information;

[0034] S4.2: Perform peak detection on the first eigenmode function and mark the periodic boundaries of the original interference fringes;

[0035] S4.3: Convert the first intrinsic mode function into a square wave signal, and count the number of modulation fringes in a single interference period by detecting the number of rising or falling edges of the square wave as the fringe subdivision value.

[0036] Another aspect of the present invention discloses a laser self-mixing interferometry rotation speed measurement device based on current modulation, comprising a solid-state laser, a photodetector, a current modulation module, and a signal processing unit;

[0037] The solid-state laser is used to emit fixed-wavelength laser light and receive external feedback light to form self-mixing interference.

[0038] The photodetector collects the mixed interference signal and converts it into a digital signal, which is then output to the signal processing unit.

[0039] The current modulation module injects a high-frequency modulation current into the solid-state laser, superimposing modulation fringes onto the self-mixing interference signal.

[0040] The signal processing unit demodulates the electrical signal to obtain the rotational speed data of the workpiece under test.

[0041] Preferably, the solid-state laser is a diode-pumped solid-state laser with an emission wavelength stabilized at 532nm.

[0042] Preferably, it also includes a polarizing beam splitter prism, which is placed in the optical path of the solid-state laser to form transmitted light and reflected light.

[0043] The advantages of this invention compared to the prior art are:

[0044] This invention is the first to utilize a current-modulated laser to subdivide self-mixing interference fringes, and then use the subdivision values ​​to measure rotational speed, achieving high-precision rotational speed measurement. Compared with traditional rotational speed measurement schemes, it improves accuracy, simplifies the device, increases sensitivity, has a more compact system structure, stronger robustness, and good integration and practicality.

[0045] The fringe subdivision value is obtained by demodulation using VMD, peak extraction algorithm and square wave edge extraction algorithm. The high-precision rotation speed is obtained by combining the subdivision time base with the fringe subdivision value.

[0046] Finally, the fringe subdivision method of self-mixed interference fringes was used to achieve high-precision measurement of minute rotation angles and rotational speeds. Attached Figure Description

[0047] Figure 1 This is a schematic diagram illustrating the principle of measuring rotation angle and rotation speed.

[0048] Figure 2 This is an example signal diagram of the interference fringe signal.

[0049] Figure 3 It is the signal processing flow.

[0050] Figure 4 It is a statistical analysis of the average resolution at different frequencies.

[0051] Figure 5 This is a schematic diagram showing the maximum and minimum instantaneous rotational speed measurements during the measurement process.

[0052] Figure 6 This is a schematic diagram showing the measured values ​​of different rotational speeds and their relative errors. Detailed Implementation

[0053] The present invention will now be described in further detail with reference to the accompanying drawings.

[0054] The technical solution adopted in this invention is as follows:

[0055] The device includes a solid-state laser, a beam splitter, a photodetector, and a laser driver. By adjusting the drive current input to the laser, the interference fringes are subdivided, thereby obtaining a high-resolution rotational speed by using the subdivision value and the subdivision time reference.

[0056] A rotational speed measurement device based on self-mixing interferometry includes:

[0057] Diode-pumped solid-state laser: Under isothermal conditions, the wavelength remains almost constant at 532nm for low-amplitude sinusoidal current modulation, with a maximum output power of 40mW.

[0058] Beam splitter (PBS): Used to split the light emitted by a laser into transmitted light and reflected light.

[0059] Photodetector (PD): Used to receive self-mixing interference signals and convert them into digital signals.

[0060] Laser driver: Provides sinusoidally modulated drive current for the laser.

[0061] The rotational speed measurement method based on self-mixing interferometry includes the following steps:

[0062] Step 1: Optical path collimation. The laser is split into two beams by a beam splitter. One beam is incident on a fixed measurement point of the standard part, and the other beam reaches the photodetector for collection.

[0063] Step 2: Modulate the laser current by inputting a low-amplitude, high-frequency sinusoidal modulated drive current into the laser.

[0064] Step 3: Rotate the workpiece to generate interference signals.

[0065] Step 4: Analyze the specific subdivision values ​​and derive the instantaneous rotational speed according to the formula.

[0066] Theoretical Model

[0067] The distance between the laser spot A and the rotation center O is d. The rotation angle θ during the time interval Δt causes a change in the external cavity length ΔL. The rotational speed ω and ΔL satisfy the following relationship:

[0068]

[0069] When Δt is sufficiently small, ω approximates the instantaneous rotational speed.

[0070] In a voltage-controlled current source circuit, the output current is strictly controlled according to the input voltage. Based on this characteristic, it can be used as a laser driving circuit. When a biased sinusoidal voltage is input, its output current I is:

[0071] I = I b +I m sin(ω s t) (2

[0072] Among them, I b I is the DC bias value. m ω is the amplitude of the sinusoidal modulation. s ω is the angular frequency of the sinusoidal modulation signal.

[0073] In a self-mixing interferometric measurement system, according to the three-mirror Fabry-Perot theory, the formula for the output power of the self-mixing interferometry is as follows:

[0074] P=P0[1+mcos(ω F τ L )] (3

[0075] Where P and P0 represent the optical power with and without optical feedback, respectively, m represents the modulation coefficient of the self-mixing interference, and τ L =2L / c represents the round-trip time of the laser in the external cavity, ω F This represents the angular frequency when there is light feedback.

[0076] As can be seen from the characteristics of a fixed-wavelength laser, under constant temperature conditions, the center wavelength of the laser remains stable. At this time, small-amplitude current modulation mainly causes periodic changes in the laser output power, while its effect on the interference phase is negligible. Therefore, from equations (2) and (3), we can obtain:

[0077] P total =P+P mod ·sin(ω s t) (4)

[0078] Among them, P mod P is the optical power modulation coefficient of a laser under constant temperature conditions and current modulation, which is determined by the modulation amplitude. total This represents the total power.

[0079] In a self-mixed interferometric measurement system, when the external reflector is displaced by a length of λ / 2 relative to the laser, an interference fringe is generated. According to equation (4), when the sinusoidal modulation frequency is higher than the interference fringe frequency, multiple high-frequency modulation fringes will be superimposed within one interference cycle. Based on this theory, the interference fringes can be subdivided from the modulation fringes and possess adaptive subdivision capability. The resolution depends on the sinusoidal modulation frequency and the interference fringe frequency, expressed as:

[0080]

[0081] Among them, f f f is the frequency of the interference fringes. s Let Ns be the frequency of the sinusoidal modulation and Ns be the number of modulation fringes within a single interference fringe. According to signal processing principles, the frequency ratio can be converted into counting the number of modulation fringes within a single interference fringe period. Therefore, the resolution S can be obtained by counting the number of modulation fringes.

[0082] in:

[0083] To verify the effectiveness of the rotational speed measurement, the following experiment was designed. A Thorlabs DJ532-40 diode-pumped solid-state laser, composed of Nd:YVO4 and KTP crystals, was used and mounted on a TEC precision temperature-controlled base. Device characteristics show that under isothermal conditions, for low-amplitude sinusoidal current modulation, the wavelength remains stable at 532 nm.

[0084] The standard part is fixed on a precision rotary table. A laser beam is incident on the diffuse reflector of the standard part via a beam splitter. The distance between the measurement point and the rotation axis is 54 mm. The initial rotation speed of the rotary table is set to 0.5 rad / min, and the rotation angle is 0.5°. Signal acquisition uses an external PD to acquire signals with a bandwidth of 70 MHz. The current modulation circuit uses a 30 kHz sinusoidal voltage output from a signal generator to drive a voltage-controlled current source to output a 30 kHz frequency and 0.5 mA amplitude driving current, thus achieving current modulation.

[0085] In specific implementations of this invention, the contents not described in detail in this specification are existing technologies known to those skilled in the art.

[0086] The working principle of this invention is as follows: Based on the speckle effect, rotating the workpiece generates an interference signal to the laser, and then high-frequency modulation is applied to introduce high-frequency small fringes on the original interference fringes, thereby achieving adaptive subdivision of the interference fringes. The resolution increases with the increase of the modulation frequency, thereby obtaining high-precision rotation speed. This invention has the advantages of simple structure, strong anti-interference ability, and high resolution, and is suitable for the precise measurement of minute rotation angles and rotation speeds.

[0087] Table of meanings for proper nouns and English abbreviations:

[0088]

[0089]

[0090] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0091] The present invention and its embodiments have been described above. This description is not restrictive, and the accompanying drawings are only one embodiment of the present invention; the actual structure is not limited thereto. In conclusion, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the invention, such designs should fall within the protection scope of the present invention.

Claims

1. A laser self-mixing interferometry rotation speed measurement method based on current modulation, characterized in that: Includes the following steps: S1: Set up the optical path and irradiate the fixed measurement point of the rotating workpiece with the laser emitted by the fixed wavelength solid-state laser. The light reflected back from the workpiece is fed back into the laser cavity to form self-mixing interference. S2: Laser current modulation, inputting a high-frequency sinusoidal modulation current with DC bias into a fixed-wavelength solid-state laser, superimposing a high-frequency modulation component into the self-mixing interference signal; S3: Interference signal acquisition, including self-mixed interference signals with high-frequency modulation components, and converting them into digital electrical signals; S4: Signal demodulation and rotational speed calculation. Extract stripe subdivision information from digital electrical signal processing and calculate the instantaneous rotational speed of the workpiece.

2. The laser self-mixing interferometry rotation speed measurement method based on current modulation according to claim 1, characterized in that: The expression for the high-frequency sinusoidal modulation current in S2 is: I=I b +I m sin(ω s t); Where I b I is the DC bias value. m ω is the amplitude of the sinusoidal modulation. s Angular frequency of the sinusoidal modulation signal; The I m The intensity of the modulation fringes used to control the interference fringes is adjusted for subdivision.

3. The laser self-mixing interferometry rotation speed measurement method based on current modulation according to claim 2, characterized in that: The output power of the self-mixing interference signal in S3 is: P=P0[1+mcos(ω F t L )]; Where P and P0 represent the optical power with and without optical feedback, respectively, m represents the modulation coefficient of the self-mixing interference, and τ L =2L / c represents the round-trip time of the laser in the external cavity, ω F This indicates the angular frequency when there is optical feedback; The optical power modulation coefficient is: P total =P+P mod ·sin(ω s t); Where P mod P is the optical power modulation coefficient of a laser under isothermal conditions and current modulation. total This represents the total power.

4. The laser self-mixing interferometry rotation speed measurement method based on current modulation according to claim 1, characterized in that: In step S4, the measurement resolution is obtained by counting the number of high-frequency modulation fringes within a single interference fringe. The resolution S is: Where f f f is the frequency of the interference fringes. s Ns is the frequency of the sinusoidal modulation, and Ns is the number of modulation fringes in a single interference fringe. The f s Adjust the resolution according to the target measurement's expected resolution.

5. The laser self-mixing interferometry rotation speed measurement method based on current modulation according to claim 4, characterized in that: The calculation of the instantaneous rotational speed of the workpiece in S4 includes constructing a correlation model between angular displacement and the change in the outer cavity length: Where ω is the instantaneous rotational speed, d is the distance between the laser spot and the workpiece rotation center, ΔL is the change in the outer cavity length, and Δt is the change in time.

6. The laser self-mixing interferometry rotation speed measurement method based on current modulation according to claim 1, characterized in that: The fixed-wavelength solid-state laser in S1 is mounted on a TEC precision temperature-controlled base and operates under constant temperature conditions.

7. The laser self-mixing interferometry rotation speed measurement method based on current modulation according to claim 5, characterized in that: The signal demodulation in S4 includes: S4.1: Perform variational mode decomposition on the digital electrical signal to extract the first eigenmode function containing high-frequency modulation information; S4.2: Perform peak detection on the first eigenmode function and mark the periodic boundaries of the original interference fringes; S4.3: Convert the first intrinsic mode function into a square wave signal, and count the number of modulation fringes in a single interference period by detecting the number of rising or falling edges of the square wave as the fringe subdivision value.

8. A current-modulated laser self-mixing interferometry rotational speed measuring device for use in any one of the methods of claims 1-7, characterized in that: Includes solid-state lasers, photodetectors, current modulation modules, and signal processing units; The solid-state laser is used to emit fixed-wavelength laser light and receive external feedback light to form self-mixing interference. The photodetector collects the mixed interference signal and converts it into a digital signal, which is then output to the signal processing unit. The current modulation module injects a high-frequency modulation current into the solid-state laser, superimposing modulation fringes onto the self-mixing interference signal. The signal processing unit demodulates the electrical signal to obtain the rotational speed data of the workpiece under test.

9. The laser self-mixing interferometry rotation speed measuring device based on current modulation according to claim 8, characterized in that: The solid-state laser is a diode-pumped solid-state laser with an emission wavelength stabilized at 532nm.

10. The laser self-mixing interferometry rotation speed measuring device based on current modulation according to claim 8, characterized in that: It also includes a polarizing beam splitter, which is placed in the optical path of the solid-state laser to form transmitted and reflected light.

Images