Eye position automatic tracking device of optical axial length measuring instrument

Abstract

This invention discloses an automatic eye position tracking device for an optical axial length measuring instrument, relating to the field of optical measurement. It includes an optical path unit comprising a collimating lens, a two-dimensional MEMS micromirror, a focusing lens, a dichroic beam splitter, and an objective lens arranged sequentially along the measurement optical path. The deflection surface of the two-dimensional MEMS micromirror is optically conjugate to the exit pupil plane of the objective lens through the focusing lens; this exit pupil plane is used to accommodate the pupil of the subject's eye. A control unit includes a disturbance signal generation module, an error demodulation module, and a tracking control module. This automatic eye position tracking device for an optical axial length measuring instrument, by multiplexing the measurement beam itself as a probe for eye position sensing, eliminates the need for a separate pupil imaging camera and illumination source in the optical path, thus simplifying the system structure and eliminating the need for additional optical path registration.

Description

Technical Field

[0001] This invention relates to optical measurement technology, and more specifically to an automatic eye position tracking device for an optical axial length measuring instrument. Background Technology

[0002] Optical axial length measuring instruments (such as biomeasuring devices based on the principles of low-coherence reflection or partial-coherence interference) obtain key parameters such as axial length by emitting a near-infrared measuring beam into the eye and analyzing the interference signals of the reflected light at various interfaces. During the measurement process, the measuring beam must be precisely aligned with the center of the pupil. If the beam is partially blocked by the iris, the signal-to-noise ratio of the interference signal will drop sharply or even be completely lost, leading to measurement failure.

[0003] Currently in clinical practice, eye alignment mainly relies on two methods:

[0004] Manual alignment: The operator uses a joystick or shift stage, with the aid of an observation screen or eyepiece, to align the measuring beam with the patient's pupil. This method is highly dependent on the operator's experience and the patient's fixation cooperation. For children, the elderly, or patients with unstable fixation, repeated attempts are often required, which is time-consuming and has a low success rate.

[0005] Automatic tracking based on an external camera: An independent pupil imaging camera and illumination source are added outside the measurement optical path. The pupil center is identified through image processing, and then a displacement platform or scanning mechanism is driven to compensate for eye movements. This type of solution requires the introduction of additional optical components and detectors, and precise spatial calibration and registration with the main measurement optical path. The system structure becomes significantly more complex, increasing cost and size, and there is tracking lag due to camera frame rate and processing delay.

[0006] Neither manual nor external camera solutions have resolved a core contradiction: the measuring beam itself carries rich information about eye position deviation, yet current technology only uses it for distance measurement, not for sensing whether it is aligned with the pupil center. If the eccentricity error could be extracted in real time from the changes in the measuring signal and used to drive the beam deflection element for compensation, fully automatic eye tracking could be achieved without adding any additional imaging camera, illumination source, or complex calibration mechanism. Summary of the Invention

[0007] The purpose of this invention is to provide an automatic eye position tracking device for an optical axial length measuring instrument, in order to solve the problem that the measuring beam itself carries rich information about eye position deviation, but in the prior art it is only used for distance measurement and not for sensing whether it is aligned with the center of the pupil.

[0008] To achieve the above objectives, the present invention provides the following technical solution: an automatic eye position tracking device for an optical axial length measuring instrument, comprising an optical path unit, including a collimating lens, a two-dimensional MEMS micromirror, a focusing lens, a dichroic beam splitter, and an objective lens arranged sequentially along the measurement optical path; wherein, the deflection surface of the two-dimensional MEMS micromirror is optically conjugate with the exit pupil plane of the objective lens through the focusing lens, and the exit pupil plane is used to accommodate the pupil of the human eye being measured;

[0009] The control unit includes a disturbance signal generation module, an error demodulation module, and a tracking control module;

[0010] The disturbance signal generation module is used to generate at least one high-frequency sinusoidal disturbance signal and superimpose it onto the driving signal of the two-dimensional MEMS micromirror so that the measurement beam generates high-frequency micro-vibration in the pupil plane.

[0011] The error demodulation module is connected to the detector output terminal of the axial length measuring instrument. It is used to acquire the intensity envelope of the measurement signal in real time and to demodulate the eccentric error signal that is in phase and frequency with the high-frequency sinusoidal disturbance signal from the intensity envelope using the lock-in amplification principle.

[0012] The tracking control module is used to generate a driving signal based on the eccentricity error signal and control the deflection of the two-dimensional MEMS micromirror to drive the eccentricity error signal to zero, thereby locking the center of the measurement beam at the center of the pupil.

[0013] Furthermore, the disturbance signal generation module generates a first high-frequency sine wave signal and a second high-frequency sine wave signal with different frequencies and orthogonal to each other, which are superimposed on the X-axis and Y-axis driving signals of the two-dimensional MEMS micromirror, respectively, to detect the eccentricity error in the two dimensions.

[0014] Furthermore, the error demodulation module includes:

[0015] The signal sampling unit performs analog-to-digital conversion on the intensity envelope;

[0016] The first mixer and the second mixer multiply the sampled signal by the first high-frequency sine wave signal and the second high-frequency sine wave signal, respectively.

[0017] The first low-pass filter and the second low-pass filter respectively filter out the high-frequency components after mixing and output the X-axis eccentricity error signal and the Y-axis eccentricity error signal.

[0018] Furthermore, the tracking control module is also configured to: drive the two-dimensional MEMS micromirror to perform a spiral scan before entering the locked tracking state; when the intensity envelope is detected to exceed a preset threshold, control the two-dimensional MEMS micromirror to jump the measurement beam to the position with the maximum current signal intensity, and then start the disturbance signal generation module and the error demodulation module to enter the closed-loop locked state.

[0019] Furthermore, when the amplitude of the eccentricity error signal remains below a preset threshold, the tracking control module outputs a tracking ready signal to allow the axial length measuring instrument to acquire data.

[0020] Furthermore, the tracking control module temporarily freezes the superposition of the high-frequency sinusoidal disturbance signal the instant it receives the trigger signal from the axial length measuring instrument, and automatically resumes the process after the acquisition is completed.

[0021] An automatic tracking method for the device includes the following steps:

[0022] S1. Control the two-dimensional MEMS micromirror to perform spiral scanning until the eye axis measuring instrument detector detects a valid measurement signal, and position the beam to the area with the maximum signal intensity.

[0023] S2. High-frequency perturbation signals of different frequencies and orthogonal are superimposed on the X and Y axis driving signals of the two-dimensional MEMS micromirror respectively.

[0024] S3. From the measurement signal intensity envelope output by the detector, the eccentricity error signals in the X and Y directions are demodulated in real time through lock-in amplification processing.

[0025] S4. Using the eccentricity error signal as feedback, a driving signal is generated through a closed-loop control algorithm to drive the two-dimensional MEMS micromirror to deflect, thereby continuously bringing the eccentricity error signal to zero and forming a closed-loop lock.

[0026] S5. Once the lock is determined to be stable, a signal to allow data acquisition is sent to the main unit of the axial length measuring instrument, and the superposition of the high-frequency perturbation signal is paused the instant the main unit performs data acquisition.

[0027] Compared with existing technologies, the automatic eye-tracking device for an optical axial length measuring instrument provided by this invention reuses the measurement beam itself as a probe for eye position sensing, eliminating the need for a separate pupil imaging camera and illumination source in the optical path. This simplifies the system structure and eliminates the need for additional optical path registration. The high-frequency micro-vibrations of the measurement beam in the pupil plane are directly correlated with the signal intensity envelope output by the detector. The error demodulation module demodulates the eccentricity error signal from this envelope in real time. Sensing and measurement share the same optical path, eliminating parallax, and tracking accuracy is directly guaranteed by the quality of the measurement signal. The tracking control module drives the deflection of a two-dimensional MEMS micromirror based on the eccentricity error signal to zero the error. The closed-loop response speed is faster than traditional camera solutions, enabling real-time compensation for eye movements and maintaining the measurement beam locked at the center of the pupil, thus improving measurement success rate and data reliability. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0029] Figure 1 This is a schematic diagram of the overall structural layout of the device of the present invention;

[0030] Figure 2 This is a block diagram illustrating the signal processing principle of the error demodulation module in this invention.

[0031] Figure 3 This is a schematic diagram illustrating the workflow of the automatic tracking method of the present invention.

[0032] Explanation of reference numerals in the attached figures:

[0033] 1. Light source; 2. Fiber optic coupler; 3. Collimating lens; 4. Two-dimensional MEMS micromirror; 5. Focusing lens; 6. Dichroic beam splitter; 7. Fixed target; 8. Objective lens; 9. Pupil; 10. Detector; 11. Control unit. Detailed Implementation

[0034] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0035] As attached Figure 1 To be continued Figure 3 As shown:

[0036] An automatic eye position tracking device for an optical axial length measuring instrument includes:

[0037] The optical path unit includes a collimating lens 3, a two-dimensional MEMS micromirror 4, a focusing lens 5, a dichroic beam splitter 6, and an objective lens 8 arranged sequentially along the measurement optical path; wherein, the deflection surface of the two-dimensional MEMS micromirror 4 is optically conjugate with the exit pupil plane of the objective lens 8 through the focusing lens 5, and the exit pupil plane is used to accommodate the pupil 9 of the human eye being tested.

[0038] Control unit 11 includes a disturbance signal generation module, an error demodulation module, and a tracking control module;

[0039] The disturbance signal generation module is used to generate at least one high-frequency sinusoidal disturbance signal and superimpose it onto the driving signal of the two-dimensional MEMS micromirror 4 so that the measurement beam generates high-frequency micro-vibration in the plane of the pupil 9.

[0040] The error demodulation module is connected to the output terminal of the detector 10 of the axial length measuring instrument. It is used to acquire the intensity envelope of the measurement signal in real time and to demodulate the eccentric error signal that is in phase and frequency with the high-frequency sinusoidal disturbance signal from the intensity envelope using the lock-in amplification principle.

[0041] The tracking control module is used to generate a drive signal based on the eccentricity error signal, and control the deflection of the two-dimensional MEMS micromirror 4 to drive the eccentricity error signal to zero, thereby locking the center of the measurement beam at the center of the pupil 9.

[0042] The measurement beam, emitted from the optical fiber, is collimated into parallel light by a collimating lens 3 made of N-BK7 optical glass. It then enters a two-dimensional electromagnetically driven MEMS micromirror 4 made of single-crystal silicon. Its deflection surface is optically conjugated by a focusing lens 5, also made of N-BK7 glass, and the exit pupil plane of an achromatic doublet mirror 8. This ensures that the angular deflection of the two-dimensional MEMS micromirror 4 is converted into a pure translation of the measurement beam on the plane accommodating the pupil 9 of the subject's eye. A dichroic beam splitter 6, with a dielectric film deposited on a fused silica substrate, transmits the measurement light to the objective lens 8, which then converges it at the pupil 9. The control unit 11, based on a Xilinx Spartan-7 FPGA, generates a 12kHz high-frequency sinusoidal perturbation signal through direct digital frequency synthesis. This signal is added to the digital compensation value output by the tracking control module and converted into an analog voltage by an AD5668 digital-to-analog converter. This voltage is then superimposed on the driving signal of the two-dimensional MEMS micromirror 4, causing the measurement spot to vibrate at a high frequency with an amplitude of 0.2mm on the plane of the pupil 9. Detector 10 uses an InGaAs PIN photodiode G12180-005A to convert the returned interference light into an electrical signal and output an intensity envelope. The error demodulation module uses an AD9240 analog-to-digital converter to sample this envelope. Within the FPGA, the sampled signal is multiplied by a reference signal of the same frequency and then filtered digitally to demodulate an eccentric error signal that is in phase and frequency with the high-frequency sinusoidal disturbance signal. The tracking control module implements a PID controller within the FPGA. After receiving the eccentric error signal, it generates a drive signal to control the deflection of the two-dimensional MEMS micromirror 4 until the eccentric error signal returns to zero, at which point the measurement beam center is locked at the center of the pupil 9.

[0043] In one embodiment of the present invention, the disturbance signal generation module generates a first high-frequency sine wave signal and a second high-frequency sine wave signal with different frequencies and orthogonal to each other, which are superimposed on the X-axis and Y-axis driving signals of the two-dimensional MEMS micromirror 4, respectively, to detect the eccentricity error in the two dimensions.

[0044] The disturbance signal generation module in the control unit 11 generates a first high-frequency sine wave with a frequency of 12kHz and a second high-frequency sine wave with a frequency of 15kHz through two direct digital frequency synthesis cores of the FPGA. The two signals are orthogonal to each other. The first high-frequency sine wave is superimposed on the X-axis drive signal of the two-dimensional MEMS micromirror 4, and the second high-frequency sine wave is superimposed on the Y-axis drive signal, so that the measurement spot performs independent high-frequency micro-vibration in the X and Y directions simultaneously in the plane of the pupil 9. When the pupil deflection causes the iris to partially block the spot, the intensity envelope output by the detector 10 will show fluctuation components with the same frequency as 12kHz and 15kHz, respectively, thus providing independent modulation information for subsequent detection of the eccentricity error of the X-axis and Y-axis.

[0045] In one embodiment of the present invention, the error demodulation module includes:

[0046] The signal sampling unit performs analog-to-digital conversion on the intensity envelope;

[0047] The first mixer and the second mixer multiply the sampled signal by the first high-frequency sine wave signal and the second high-frequency sine wave signal, respectively.

[0048] The first low-pass filter and the second low-pass filter respectively filter out the high-frequency components after mixing and output the X-axis eccentricity error signal and the Y-axis eccentricity error signal.

[0049] The signal sampling unit in the error demodulation module uses an AD9240 analog-to-digital converter to perform analog-to-digital conversion on the intensity envelope output by the detector 10 at a sampling rate of 1MSPS. The converted digital signal is fed into the first mixer and the second mixer in parallel. The first mixer multiplies the sampled signal with a 12kHz first high-frequency sine wave generated by the disturbance signal generation module. The second mixer multiplies the sampled signal with a 15kHz second high-frequency sine wave. The multiplication results are respectively input into the first low-pass filter and the second low-pass filter with a cutoff frequency of 100Hz implemented inside the FPGA. The first low-pass filter outputs an X-axis eccentricity error signal, and the second low-pass filter outputs a Y-axis eccentricity error signal. The two signals indicate the distance and direction of the light spot deviating from the center of the pupil 9 in the X and Y directions, respectively.

[0050] In one embodiment of the present invention, the tracking control module is further configured to: drive the two-dimensional MEMS micromirror 4 to perform a spiral scan before entering the locked tracking state; when the intensity envelope is detected to exceed a preset threshold, control the two-dimensional MEMS micromirror 4 to jump the measurement beam to the position with the maximum current signal intensity, and then start the disturbance signal generation module and the error demodulation module to enter the closed-loop locked state.

[0051] The tracking control module is configured with a spiral scanning state machine in the FPGA. Before closed-loop locking, it drives the two-dimensional MEMS micromirror 4 to perform an Archimedes spiral scan, covering the ±5mm area that the pupil 9 may appear in. During this process, the control unit 11 continuously monitors the intensity envelope sampled by AD9240. When the intensity envelope exceeds the preset threshold, the current deflection coordinates of the two-dimensional MEMS micromirror 4 are recorded. After one scan is completed, the signal intensity of each sampling point is compared. The two-dimensional MEMS micromirror 4 is immediately switched to the coordinate position with the maximum signal intensity. Then, the disturbance signal generation module and the error demodulation module are started to enter the closed-loop locking state, ensuring that the locking starting point is the point with the strongest signal.

[0052] In one embodiment of the present invention, when the amplitude of the eccentricity error signal is continuously lower than a preset threshold, the tracking control module outputs a tracking ready signal to allow the axial length measuring instrument to acquire data.

[0053] The tracking control module sets up a lock detection logic in the FPGA. When the amplitudes of the demodulated X-axis eccentricity error signal and Y-axis eccentricity error signal are continuously lower than the preset threshold value corresponding to the beam deviating from the center of the pupil by 50μm within 50ms, a general-purpose input / output pin of the control unit 11 outputs a high level as a tracking ready signal to the main control circuit of the axial length measuring instrument, allowing the axial length measuring instrument to perform data acquisition under the condition that the beam is stably aligned with the center of the pupil 9.

[0054] In one embodiment of the present invention, the tracking control module temporarily freezes the superposition of high-frequency sinusoidal disturbance signals the instant it receives the acquisition trigger signal from the axial length measuring instrument, and automatically resumes after the acquisition is completed.

[0055] The tracking control module contains a freeze control logic. When the rising edge of the trigger signal from the axial length measuring instrument is received, this logic controls a gating switch inside the FPGA to temporarily disconnect the high-frequency sinusoidal disturbance signal output by the disturbance signal generation module. This ensures that only the DC compensation component generated by the PID controller is retained in the drive signal of the two-dimensional MEMS micromirror 4, and the micro-vibration of the light spot immediately stops. After the acquisition process lasts for 100μs, the gating switch automatically restores the superposition of the high-frequency sinusoidal disturbance signal, and the system re-enters the normal closed-loop tracking state.

[0056] In one embodiment of the present invention, a device-based automatic tracking method includes the following steps:

[0057] S1. Control the two-dimensional MEMS micromirror 4 to perform spiral scanning until the eye axis measuring instrument detector 10 detects a valid measurement signal, and position the beam to the area with the maximum signal intensity.

[0058] S2. High-frequency perturbation signals of different frequencies and orthogonal are superimposed on the X and Y axis driving signals of the two-dimensional MEMS micromirror 4, respectively.

[0059] S3. The eccentricity error signals in the X and Y directions are demodulated in real time from the measurement signal intensity envelope output by the detector 10 through lock-in amplification processing.

[0060] S4. Using the eccentricity error signal as feedback, a driving signal is generated through a closed-loop control algorithm to drive the two-dimensional MEMS micromirror 4 to deflect, continuously bringing the eccentricity error signal to zero and forming a closed-loop lock.

[0061] S5. Once the lock is determined to be stable, send an enable signal to the main unit of the axial length measuring instrument, and pause the superposition of high-frequency perturbation signals at the moment the main unit performs data acquisition.

[0062] In S1, control unit 11 drives single-crystal silicon two-dimensional MEMS micromirror 4 to perform Archimedes spiral scanning, InGaAs The PIN photodiode detector 10 converts the returned light into an intensity envelope. After analog-to-digital conversion by the AD9240, the FPGA determines that the intensity exceeds the threshold and finds the location of the maximum signal, positioning the beam there. In S2, the perturbation signal generation module inside the FPGA generates 12kHz and 15kHz orthogonal high-frequency perturbation signals, which are superimposed on the X and Y axis drive signals of the two-dimensional MEMS micromirror 4 in the digital domain, respectively. In S3, the intensity envelope output by the detector 10 is sampled by the AD9240, mixed with the corresponding reference signal of the same frequency in the FPGA, and demodulated in real time through digital low-pass filtering to obtain the X and Y direction eccentricity error signals. In S4, using the eccentricity error signal as feedback, the PID controller in the FPGA generates a drive signal to drive the two-dimensional MEMS micromirror 4 to deflect, continuously bringing the eccentricity error to zero to form a closed-loop lock. In S5, when the FPGA determines that the eccentricity error is continuously lower than the preset threshold, it sends a tracking ready signal to the eye axis measurement instrument host through a general-purpose input / output pin, and freezes the high-frequency perturbation superposition through an internal gating switch at the moment of acquisition triggering. After the acquisition is completed, the perturbation is automatically restored.

[0063] In one embodiment of the present invention: the automatic eye position tracking device of the optical axial length measuring instrument of this embodiment includes an optical path unit and a control unit 11. The optical path unit is composed of a collimating lens 3, a two-dimensional MEMS micromirror 4, a focusing lens 5, a dichroic beam splitter 6, and an objective lens 8 arranged sequentially along the measurement optical path. The collimating lens 3 is made of N-BK7 optical glass, which collimates the diverging measurement beam emitted from the optical fiber into a parallel beam with a diameter of 2 mm. This parallel beam is incident on the two-dimensional MEMS micromirror 4 at a 45° angle. The two-dimensional MEMS micromirror 4 is made of single-crystal silicon and achieves two-dimensional deflection through electromagnetic drive. Its deflection surface is imaged onto the exit pupil plane of the objective lens 8 by the focusing lens 5, which is also made of N-BK7 optical glass. The objective lens 8 is an achromatic cemented doublet lens. The exit pupil plane is used to accommodate the pupil 9 of the subject's eye. The deflection surface of the two-dimensional MEMS micromirror 4 is strictly optically conjugate with the pupil 9 plane, so that the X-axis or Y-axis angular deflection of the two-dimensional MEMS micromirror 4 is only converted into a pure translation of the measurement beam in the pupil 9 plane without changing the incident angle. The dichroic beam splitter 6 uses fused silica as a substrate and is coated with a dielectric film. It efficiently transmits the 840nm measurement beam to the objective lens 8, while simultaneously reflecting the fixation light in the visible light band to the pupil 9, ensuring that the patient's fixation point is coaxial with the measurement optical axis. The control unit 11 outputs a tracking ready signal to the main unit of the axial length measuring instrument through general-purpose input / output pins to allow data acquisition. Upon receiving the acquisition trigger signal, the FPGA's internal gating switch temporarily disconnects the superposition of high-frequency sinusoidal disturbance signals. After the acquisition process lasts for 100μs, the gating switch automatically restores the disturbance signal superposition, and the system returns to the normal closed-loop tracking state.

[0064] Working Principle: After the operator places the pupil 9 of the subject's eye in front of the objective lens 8, the measurement is initiated. The tracking control module in the control unit 11 first drives the two-dimensional MEMS micromirror 4 to perform an Archimedean spiral scan. The detector 10 converts the returned light into an intensity envelope in real time and sends it to the signal sampling unit of the error demodulation module for analog-to-digital conversion. The tracking control module continuously judges whether the intensity envelope exceeds a preset threshold. When a signal exceeding the threshold is detected, the intensity of each sampling point is recorded and compared to determine the position of the maximum signal intensity. Then, the two-dimensional MEMS micromirror 4 is controlled to jump the measurement beam to that position. Immediately afterwards, the disturbance signal generation module generates a first high-frequency sine wave signal and a second high-frequency sine wave signal. The two signals have different frequencies and are orthogonal to each other. They are superimposed on the X-axis and Y-axis drive signals of the two-dimensional MEMS micromirror 4, respectively, causing the measurement beam to generate high-frequency micro-vibrations in the plane of the pupil 9. The signal sampling unit of the error demodulation module performs analog-to-digital conversion on the intensity envelope output by the detector 10. The converted signal... The sampled signal is fed into the first mixer and the second mixer in parallel. The first mixer multiplies the sampled signal with the first high-frequency sine wave signal, and the second mixer multiplies the sampled signal with the second high-frequency sine wave signal. The multiplication results are filtered by the first low-pass filter and the second low-pass filter to remove high-frequency components, and output X-axis eccentricity error signals and Y-axis eccentricity error signals. After receiving the eccentricity error signals, the tracking control module generates a drive signal through a closed-loop control algorithm. After being converted into an analog voltage by the digital-to-analog converter, the drive signal drives the two-dimensional MEMS micromirror 4 to deflect, continuously driving the eccentricity error signal to zero to form a closed-loop lock. When the tracking control module determines that the amplitude of the eccentricity error signal is continuously lower than the preset threshold, it outputs a tracking ready signal to the eye axis measuring instrument host to allow data acquisition. At the moment the eye axis measuring instrument acquisition trigger signal is received, the superposition of high-frequency sine wave disturbance signals is temporarily frozen. After the acquisition is completed, the superposition of high-frequency sine wave disturbance signals is automatically restored, and the system re-enters the closed-loop tracking state until all measurements are completed.

[0065] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. An automatic eye position tracking device for an optical axial length measuring instrument, characterized in that, include: The optical path unit includes a collimating lens (3), a two-dimensional MEMS micromirror (4), a focusing lens (5), a dichroic beam splitter (6), and an objective lens (8) arranged sequentially along the measurement optical path. The deflection surface of the two-dimensional MEMS micromirror (4) is optically conjugate with the exit pupil plane of the objective lens (8) through the focusing lens (5), and the exit pupil plane is used to accommodate the pupil (9) of the human eye being tested. The control unit (11) includes a disturbance signal generation module, an error demodulation module, and a tracking control module; The disturbance signal generation module is used to generate at least one high-frequency sinusoidal disturbance signal and superimpose it onto the driving signal of the two-dimensional MEMS micromirror (4) so ​​that the measurement beam generates high-frequency micro-vibration in the pupil (9) plane; The error demodulation module is connected to the output terminal of the detector (10) of the axial length measuring instrument. It is used to acquire the intensity envelope of the measurement signal in real time and to demodulate the eccentric error signal that is in phase and frequency with the high-frequency sinusoidal disturbance signal from the intensity envelope using the lock-in amplification principle. The tracking control module is used to generate a driving signal based on the eccentricity error signal and control the deflection of the two-dimensional MEMS micromirror (4) to drive the eccentricity error signal to zero, thereby locking the center of the measurement beam at the center of the pupil (9).

2. The automatic eye position tracking device of an optical axial length measuring instrument according to claim 1, characterized in that, The disturbance signal generation module generates a first high-frequency sine wave and a second high-frequency sine wave with different frequencies and orthogonal to each other. These signals are superimposed on the X-axis and Y-axis drive signals of the two-dimensional MEMS micromirror (4) to detect the eccentricity error in the two dimensions respectively.

3. The automatic eye position tracking device for an optical axial length measuring instrument according to claim 2, characterized in that, The error demodulation module includes: The signal sampling unit performs analog-to-digital conversion on the intensity envelope; The first mixer and the second mixer multiply the sampled signal by the first high-frequency sine wave signal and the second high-frequency sine wave signal, respectively. The first low-pass filter and the second low-pass filter respectively filter out the high-frequency components after mixing and output the X-axis eccentricity error signal and the Y-axis eccentricity error signal.

4. The automatic eye position tracking device of an optical axial length measuring instrument according to claim 1, characterized in that, The tracking control module is also configured to: drive the two-dimensional MEMS micromirror (4) to perform a spiral scan before entering the locked tracking state; when the intensity envelope is detected to exceed the preset threshold, control the two-dimensional MEMS micromirror (4) to jump the measurement beam to the position with the maximum current signal intensity, and then start the disturbance signal generation module and the error demodulation module to enter the closed-loop locked state.

5. The automatic eye position tracking device for an optical axial length measuring instrument according to claim 1, characterized in that, When the amplitude of the eccentricity error signal remains below a preset threshold, the tracking control module outputs a tracking ready signal to allow the axial length measuring instrument to acquire data.

6. The automatic eye position tracking device for an optical axial length measuring instrument according to claim 5, characterized in that, The tracking control module temporarily freezes the superposition of the high-frequency sinusoidal disturbance signal the moment it receives the trigger signal from the axial length measuring instrument, and automatically resumes after the acquisition is completed.

7. An automatic tracking method based on the device according to any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Control the two-dimensional MEMS micromirror (4) to perform spiral scanning until the eye axis measuring instrument detector (10) detects a valid measurement signal and positions the beam to the area with the maximum signal intensity. S2. High-frequency perturbation signals of different frequencies and orthogonal are superimposed on the X and Y axis driving signals of the two-dimensional MEMS micromirror (4); S3. From the measurement signal intensity envelope output by the detector (10), the eccentricity error signals in the X and Y directions are demodulated in real time through lock-in amplification processing. S4. Using the eccentricity error signal as feedback, a driving signal is generated through a closed-loop control algorithm to drive the two-dimensional MEMS micromirror (4) to deflect, so that the eccentricity error signal is continuously returned to zero, forming a closed-loop lock. S5. Once the lock is determined to be stable, a signal to allow data acquisition is sent to the main unit of the axial length measuring instrument, and the superposition of the high-frequency perturbation signal is paused the instant the main unit performs data acquisition.

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