1. A laser doppler velocimeter, wherein the device comprises: the device comprises a seed laser (1), a first beam splitter (2), a modulator module (3), an optical fiber amplifier module (4), a circulator module (5) and an optical antenna module (6) which are sequentially connected through optical fibers;

the first beam splitter (2) is sequentially connected with the optical fiber attenuator (7), the coupler module (8) and the balanced photoelectric detector module (9) through optical fibers; the circulator module (5) is connected with the coupler module (8) through optical fibers; the balanced photoelectric detector module (9) is electrically connected with the signal processing module (10);

the signal processing module (10) is electrically connected with the modulator module (3); the signal processing module (10) is used for resolving the signal sent by the balanced photoelectric detector module (9) to obtain Doppler frequency shift, and calculating a radio frequency signal loaded to the modulator module (3) according to the Doppler frequency shift so as to change the frequency shift amount of the modulator module (3).

2. Laser doppler velocimeter according to claim 1, wherein the modulator module (3) comprises at least one modulator, which is an acousto-optic modulator or an electro-optic modulator.

3. Laser doppler velocimetry device according to claim 1, characterized in that the modulator module (3) comprises at least one acousto-optic modulator; the acousto-optic modulator comprises a first lens (30), an acousto-optic crystal (31) and a second lens (32);

the first lens (30) and the optical fiber form a first optical fiber collimator; the second lens (32) and the optical fiber form a coupling system; the acousto-optic crystal (31) is respectively coupled with the first optical fiber collimator and the coupling system in space;

the second lens (32) is used for receiving the light beam modulated by the acousto-optic crystal (31) and coupling the light beam into the optical fiber.

4. The laser Doppler velocity measurement device according to claim 3, wherein the clear aperture and the numerical aperture of the second lens (32) are larger than those of the first lens (30), and the light spot output by the second lens (32) is not larger than 2 times of the fiber mode field diameter, and the numerical aperture of the second lens (32) is not larger than 2 times of the fiber numerical aperture.

5. Laser doppler velocimetry device according to claim 1, characterized in that the circulator module (5) comprises a circulator, the optical antenna module (6) comprises an optical antenna, the coupler module (8) comprises a coupler, and the balanced photo detector module (9) comprises a balanced photo detector.

6. Laser doppler velocimetry device according to claim 1, characterized in that the device further comprises a second beam splitter (11) and a third beam splitter (12);

a third beam splitter (12) is connected between the first beam splitter (2) and the coupler module (8);

the circulator module (5) comprises a plurality of circulators, the optical antenna module (6) comprises a plurality of optical antennas, the coupler module (8) comprises a plurality of couplers, and the balanced photodetector module (9) comprises a plurality of balanced photodetectors; the modulator module (3) comprises a plurality of modulators; the optical fiber amplifier module (4) comprises a plurality of optical fiber amplifiers;

a second beam splitter (11) is connected between the first beam splitter (2) and each modulator; the plurality of modulators are correspondingly connected with the plurality of optical fiber amplifiers; the optical fiber amplifiers, the circulators and the optical antennas are sequentially connected in a one-to-one correspondence manner;

the circulators are connected with the couplers and the balance photodetectors in sequence in a one-to-one correspondence manner.

7. Laser doppler velocimetry device according to claim 6, characterized in that if a third beam splitter is connected between the first beam splitter (2) and the fiber optic attenuator (7), the device comprises a plurality of fiber optic attenuators (7); the plurality of optical fiber attenuators (7) are connected with the plurality of couplers in a one-to-one correspondence manner.

8. A laser doppler velocimetry method using the measuring device of any one of claims 1 to 7, characterized in that the measuring method comprises:

the signal processing module (10) is used for resolving the signal sent by the balanced photoelectric detector module (9) to obtain a signal frequency f 1; setting the working frequency point of a modulator as f 0; the feedback quantity of the signal processing module (10) is (f1-f0) K;

superimposing the feedback quantity as a negative feedback signal on a modulation signal of the modulator to enable the frequency of the modulation signal actually loaded by the modulator to be f0+ [ Kx (f0-f1) ];

after multiple feedbacks, until the signal processing module (10) calculates the signal sent by the balanced photoelectric detector module (9) to obtain a signal frequency close to the working point f 0;

the final Doppler shift is an accumulated value of the feedback amounts Δ f ∑ [ K × (f0-f1)]Calculating the moving speed V of the target according to the formulaWhere is the lambda laser wavelength.

9. The laser doppler velocity measurement method according to claim 8, wherein the feedback coefficient K is 0.01-0.5.

10. The laser Doppler velocity measurement method according to claim 8, wherein the working frequency f0 is 20-100 MHz.

Background

The laser Doppler velocity measurement technology is based on Doppler effect, calculates the motion velocity value of an object by utilizing the linear relation between the motion velocity of the measured object and Doppler frequency shift quantity, has the advantages of high measurement precision, high spatial resolution, fast dynamic response, wide velocity measurement range, non-contact measurement and the like, is widely applied to various fields, such as measurement of fluid velocity and solid motion velocity in industrial fields and scientific researches, monitoring of blood flow velocity of blood vessels in the medical field, measurement and positioning of wind velocity, turbulence, wind shear, automobile and the like in the safety monitoring field, measurement of aircraft velocity, measurement of engine tail flame airflow field, measurement of airspeed and the like in the aerospace field, and has wide application market and prospect.

Various laser speed measuring instruments have been developed at present, for example, a laser doppler blood flow meter developed by Perimed corporation in sweden is used for measuring dynamic change of single-point blood perfusion, a windrowcer laser wind measuring radar developed by Lockheed Martin usa is used for detecting airport turbulence, wind shear and the like, a 1.5 μm laser wind measuring radar developed by mitsubishi corporation in japan is used for detecting an atmospheric three-dimensional wind field and the like, but reported laser doppler speed measuring schemes are all open-loop working modes, namely, a speed is obtained by resolving a doppler frequency shift quantity related to the speed, and no corresponding feedback control is provided for change of the frequency shift quantity.

According to the principle of laser speed measurement, the measured speed value is in direct proportion to the Doppler frequency shift amount, when the measured speed value is large, such as the flight speed, the vacuum speed or the tail flame flow rate of an aircraft, the Doppler frequency shift of an optical signal is large (up to several hundred MHz magnitude), the frequency range of an intermediate frequency signal is wide, the requirements on a photoelectric detector, circuit bandwidth, AD sampling frequency and the like are high, the large data amount causes large pressure to signal processing, and even signal acquisition and processing are difficult to realize.

Disclosure of Invention

The purpose of the invention is as follows: the invention provides a closed-loop control scheme for laser speed measurement, wherein a part of Doppler frequency shift quantity is used for feedback control of a modulator, the frequency shift quantity of the modulator is changed, closed-loop control of signals is realized, and the problems of wide signal spectrum range, difficult data processing and the like in the open-loop control scheme are solved.

The technical scheme of the invention is as follows: in one aspect, a laser doppler velocity measurement device is provided, the device includes: the device comprises a seed laser 1, a first beam splitter 2, a modulator module 3, an optical fiber amplifier module 4, a circulator module 5 and an optical antenna module 6 which are sequentially connected through optical fibers;

the first beam splitter 2 is sequentially connected with an optical fiber attenuator 7, a coupler module 8 and a balanced photoelectric detector module 9 through optical fibers; the circulator module 5 is connected with the coupler module 8 through optical fibers; the balanced photoelectric detector module (9) is electrically connected with the signal processing module (10);

the signal processing module 10 is electrically connected with the modulator module 3; the signal processing module 10 is configured to calculate a signal sent by the balanced photodetector module 9 to obtain a doppler shift, and calculate a radio frequency signal loaded to the modulator module 3 according to the doppler shift, so as to change a frequency shift amount of the modulator module 3.

Optionally, the modulator module 3 comprises at least one modulator; the adjuster is an acousto-optic modulator or an electro-optic modulator.

When the modulator is an electro-optical modulator, the modulator can be a phase modulator or an intensity modulator, the intensity modulator can be of a Mach-Zehnder type or a cascade, parallel, orthogonal or other deformation structure based on the Mach-Zehnder type, and the purpose of the phase or intensity modulator is to enable the frequency of the seed laser to generate a certain variation.

Optionally, the modulator module 3 comprises at least one acousto-optic modulator; the acousto-optic modulator comprises a first lens 30, an acousto-optic crystal 31 and a second lens 32;

the first lens 30 and the optical fiber form a first optical fiber collimator; the second lens 32 and the optical fiber form a coupling system; the acousto-optic crystal 31 is spatially coupled with the first optical fiber collimator and the coupling system respectively;

the second lens 32 is used for receiving the light beam modulated by the acousto-optic crystal 31 and coupling the light beam into the optical fiber.

Optionally, the clear aperture and the numerical aperture of the second lens 32 are larger than those of the first lens 30, and the light spot output by the second lens 32 is not larger than 2 times of the fiber mode field diameter, and the numerical aperture of the second lens 32 is not larger than 2 times of the fiber numerical aperture.

Optionally, the laser power of the seed laser 1 is 10-100 mW.

Optionally, the circulator module 5 comprises a circulator, the optical antenna module 6 comprises an optical antenna, the coupler module 8 comprises a coupler, and the balanced photodetector module 9 comprises a balanced photodetector.

Optionally, the apparatus further comprises a second beam splitter 11 and a third beam splitter 12;

a third beam splitter is connected between the first beam splitter 2 and the coupler module 8;

the circulator module 5 comprises a plurality of circulators, the optical antenna module 6 comprises a plurality of optical antennas, the coupler module 8 comprises a plurality of couplers, and the balanced photodetector module 9 comprises a plurality of balanced photodetectors; the modulator block 3 comprises a plurality of modulators; the optical fiber amplifier module 4 includes a plurality of optical fiber amplifiers;

a second beam splitter is connected between the first beam splitter 2 and each modulator; the plurality of modulators are correspondingly connected with the plurality of optical fiber amplifiers 4; the plurality of optical fiber amplifiers 4, the plurality of circulators and the plurality of optical antennas are sequentially connected in a one-to-one correspondence manner;

the circulators are connected with the couplers and the balance photodetectors in sequence in a one-to-one correspondence manner.

Alternatively, if the third splitter is connected between the optical fibre attenuators 7 of the first splitter 2, the apparatus comprises a plurality of optical fibre attenuators 7; the plurality of optical fiber attenuators 7 are connected to the plurality of couplers in a one-to-one correspondence.

In another aspect, there is provided a laser doppler velocity measurement method using the measurement apparatus as described above, the measurement method including:

the signal processing module 10 calculates the signal sent by the balanced photoelectric detector module 9 to obtain a signal frequency f 1; setting the working frequency point of a modulator as f 0; the feedback quantity of the signal processing module (10) is (f1-f0) K;

superimposing the feedback quantity as a negative feedback signal on a modulation signal of the modulator to enable the frequency of the modulation signal actually loaded by the modulator to be f0+ [ Kx (f0-f1) ];

after multiple feedbacks, until the signal processing module (10) calculates the signal sent by the balanced photoelectric detector module (9) to obtain a signal frequency close to the working point f 0;

the final Doppler shift is an accumulated value of the feedback amounts Δ f ∑ [ K × (f0-f1)]Calculating the moving speed V of the target according to the formulaWhere is the lambda laser wavelength.

Optionally, the feedback coefficient K is 0.01 to 0.5.

Optionally, the working frequency point f0 takes a value of 20-100 MHz.

The invention has the advantages that: the invention provides a speed measuring device and a closed-loop control method based on the laser Doppler speed measuring principle based on the laser Doppler speed measuring and coherent detection principle, and the invention has the following advantages:

1. after closed-loop control, the frequency of the heterodyne signal is always changed in a small range, so that the frequency spectrum estimation range is greatly reduced, and the frequency spectrum estimation precision and the speed measurement precision can be improved;

2. the signal frequency range is greatly reduced, the requirements on photoelectric detectors, circuit bandwidth and AD sampling frequency are low, and the design difficulty of circuits such as filters and amplifiers is reduced;

3. a fixed band-pass filter can be arranged to filter useless frequency spectrum information, so that the signal-to-noise ratio of the system is improved;

4. the nonlinearity of measurement is reduced, and the measurement precision is improved;

5. the AD acquisition data volume is greatly reduced, the data processing pressure of the rear end is reduced, the hardware calculation power consumption is reduced, and the calculation speed is increased.

Description of the drawings:

FIG. 1 is a schematic diagram of a closed loop control scheme for laser Doppler velocity measurement;

FIG. 2 is a schematic diagram of a power spectrum of an acquired signal;

FIG. 3 is a closed loop control flow diagram;

FIG. 4 shows Bragg conditions satisfied by the acousto-optic modulator, where k1 is an incident light wave vector, k2 is a sound wave vector, and k3 is an emergent light wave vector;

FIG. 5 is a structural diagram of an acousto-optic modulator, (a) is a structural diagram of a traditional acousto-optic modulator, and (b) is a structural diagram of a modified acousto-optic modulator;

FIG. 6 is a schematic diagram of a closed loop control scheme for multi-path laser Doppler velocity measurement

Description of reference numerals: 1-seed laser, 2-first beam splitter, 3-modulator module, 4-optical fiber amplifier module, 5-circulator module, 6-optical antenna module, 7-optical fiber attenuator, 8-coupler module, 9-balanced photoelectric detector module and 10-signal processing module; 5-1-incident optical fiber, 5-3-emergent end coupling optical fiber, 5-4-self-focusing lens and 5-5-self-focusing lens; 31-acousto-optic crystal, 11-second beam splitter and 12-third beam splitter.

The specific implementation mode is as follows:

example 1

In this embodiment, referring to fig. 1, a laser doppler velocity measurement device is provided, where the device includes: the device comprises a seed laser 1, a first beam splitter 2, a modulator module 3, an optical fiber amplifier module 4, a circulator module 5 and an optical antenna module 6 which are sequentially connected through optical fibers; the first beam splitter 2 is sequentially connected with an optical fiber attenuator 7, a coupler module 8 and a balanced photoelectric detector module 9 through optical fibers; the circulator module 5 is fiber-connected with the coupler module 8. The balanced photodetector module 9 and the signal processing module 10 are electrically connected.

Wherein, the signal processing module 10 is electrically connected with the modulator module 3; the signal processing module 10 is configured to calculate a signal sent by the balanced photodetector module 9 to obtain a doppler shift, and calculate a radio frequency signal loaded to the modulator module 3 according to the doppler shift, so as to obtain a frequency shift amount of the acousto-optic modulator module 3.

As one of the preferred embodiments of this embodiment, the modulator module 3 includes at least one modulator, which may be an acoustic-optical modulator or an electro-optical modulator.

In particular, in the present embodiment, the modulator module 3 comprises at least one acousto-optic modulator. As shown in fig. 4 and 5, the acousto-optic modulator includes an incident end optical fiber, a first lens 30, an acousto-optic crystal 31, a second lens 32, and an exit end coupling optical fiber;

the first lens 30 and the incident end fiber form a first fiber collimator; the second lens 32 and the exit end optical fiber form a coupling system; the acousto-optic crystal 31 is spatially coupled with the first optical fiber collimator and the coupling system respectively;

the second lens 32 is used for receiving the light beam modulated by the acousto-optic crystal 31 and coupling the light beam into the optical fiber.

In the present embodiment, the acousto-optic modulator is based on the acousto-optic bragg diffraction principle, and as shown in fig. 4, the incident light vector k1, the acoustic wave vector k2 and the diffracted light vector k3 satisfy the bragg condition, so if the acoustic wave frequency is changed to change the frequency shift amount of the acousto-optic modulator, the magnitude and direction of the diffracted light vector k3 will change. The structure of the traditional acousto-optic modulator is shown in fig. 5(a), wherein 5-1 is an incident end optical fiber, 31 is an acousto-optic crystal, and 5-3 is an emergent end coupling optical fiber; 5-4 and 5-5 are both self-focusing lenses or spherical lenses. 5-1 and 5-4, 5-3 and 5-5 respectively form an optical fiber collimator, and the optical fiber collimator is used for collimating the optical signal and then transmitting the optical signal to the acousto-optic crystal, and simultaneously coupling the diffracted light beam in the acousto-optic crystal to the optical fiber. The traditional acousto-optic modulator works under a specific sound wave frequency, the k3 direction is fixed, the direction and the position of the corresponding diffracted light are fixed, and the coupling mode of the optical fiber collimator can obtain ideal coupling efficiency, but if the sound wave frequency is changed, the direction and the position of the diffracted light are changed, the optical fiber coupling efficiency is suddenly reduced, even no light is output, and the modulation frequency range corresponding to 90% reduction of the power of the acousto-optic modulator of a certain model is actually measured to be 22 MHz. Therefore, the modulation frequency of the acousto-optic modulator can only be changed within a small range, and the application of the closed-loop scheme is limited.

In order to expand the modulation frequency variation range of the acousto-optic modulator, this embodiment proposes an improved scheme as shown in fig. 5(b), in which a fiber collimator formed by 5-3 and 5-5 is changed into a coupling system formed by the exit-end coupling fiber 5-3 and the second lens 32, so as to improve the fiber coupling efficiency when the modulation frequency varies. The first lens 30 is a self-focusing lens or a spherical lens, and 5-1 and 30 constitute a fiber collimator.

Specifically, in this embodiment, the second lens 32 may be a combined lens system or a single lens, the input of which has a larger clear aperture and numerical aperture, and the output of which matches the numerical aperture and mode field of the optical fiber.

Further, the clear aperture and the numerical aperture of the second lens 32 are larger than those of the first lens 30, and the light spot output by the second lens 32 is not larger than 2 times of the fiber mode field diameter, and the numerical aperture of the second lens 32 is not larger than 2 times of the fiber numerical aperture.

In a preferred embodiment of the present embodiment, the seed laser 1 has a laser power of 10 to 100 mW.

As one of the preferred embodiments of this embodiment, referring to fig. 1, the circulator module 5 includes a circulator, the optical antenna module 6 includes an optical antenna, the coupler module 8 includes a coupler, the balanced photodetector module 9 includes a balanced photodetector, and the fiber amplifier module 4 includes a fiber amplifier.

The seed laser 1, the first beam splitter 2, the acousto-optic modulator, the optical fiber amplifier, the circulator and the optical antenna are sequentially connected; the input end 1 of the coupler is connected with three ports of the circulator, the input end of the optical fiber attenuator 7 is connected with the other output end of the first beam splitter 2, and the output end of the optical fiber attenuator is connected with the input end 2 of the coupler; two output ends of the coupler are connected with a balanced photoelectric detector, and the balanced photoelectric detector is connected with the signal processing module 10. The signal processing module 10 is connected to the seed laser 1, the acousto-optic modulator, and the optical fiber amplifier via electrical interfaces for supplying power, but may also supply power in other manners. The optical devices are all connected with each other through polarization maintaining fiber welding or through flanges by using connectors such as FC/APC.

As a preferred embodiment of this embodiment, referring to fig. 6, the apparatus further includes a second beam splitter and a third beam splitter; a third beam splitter 12 is connected between the first beam splitter 2 and the coupler module 8; the circulator module 5 comprises a plurality of circulators, the optical antenna module 6 comprises a plurality of optical antennas, the coupler module 8 comprises a plurality of couplers, and the balanced photodetector module 9 comprises a plurality of balanced photodetectors; the modulator block 3 comprises a plurality of acousto-optic modulators. The fiber amplifier module 4 includes a plurality of fiber amplifiers.

A second beam splitter is connected between the first beam splitter 2 and each acousto-optic modulator; each acousto-optic modulator is correspondingly connected with a plurality of optical fiber amplifiers; the plurality of optical fiber amplifiers, the plurality of circulators and the plurality of optical antennas are connected in sequence in a one-to-one correspondence manner. The circulators are connected with the couplers and the balance photodetectors in sequence in a one-to-one correspondence manner.

Furthermore, if the third splitter is connected between the optical fibre attenuators 7 of the first splitter 2, the apparatus comprises a plurality of optical fibre attenuators 7; the plurality of optical fiber attenuators 7 are connected to the plurality of couplers in a one-to-one correspondence.

As one of the preferred embodiments of this embodiment, the optical fiber attenuator 7 may be an optical power attenuator of any principle or an optical fiber splitter of any fractional ratio. The optical antenna can be in a focusing working mode or a collimating working mode.

As one of the preferred embodiments of the present embodiment, the optical antenna module 6 includes an optical antenna that can simultaneously transmit and receive a plurality of laser signals with different field angles; the optical antenna is connected to a plurality of circulators.

The working principle of the embodiment is as follows: laser emitted by the seed laser 1 is divided into two beams by the first beam splitter 2, one beam enters the acousto-optic modulator as signal light, and the other beam enters the optical fiber attenuator as local oscillation light. The signal light is modulated into a pulse signal by the acousto-optic modulator, enters the optical fiber amplifier, enters the circulator after being amplified, enters the optical antenna through two ports of the circulator and is then emitted into the air or a target object. The signal scattered by air or reflected by the target enters the circulator through the optical antenna, enters the coupler from the three ports of the circulator, and enters the coupler together with the local oscillator light output from the optical fiber attenuator to realize optical frequency mixing. Then the mixed signal is divided into two beams of light with equal power by the coupler and respectively enters two input ends of the balanced detector to realize heterodyne detection, and heterodyne signals output by the balanced photoelectric detector enter the signal processing module. In this embodiment, the signal processing module provides a modulation signal and a feedback control signal for the acousto-optic modulator.

Example 2

In this embodiment, as shown in fig. 3, a laser doppler velocity measurement method is provided, where the measurement device in embodiment 1 is used, and the measurement method includes the following steps:

the signal processing module 10 calculates the signal sent by the balanced photoelectric detector module 9 to obtain a signal frequency f 1; setting the working frequency point of the acousto-optic modulator as f 0; the feedback quantity of the signal processing module 10 is (f1-f0) × K;

superimposing the feedback quantity as a negative feedback signal on a modulation signal of the acousto-optic modulator to enable the frequency of the modulation signal actually loaded by the acousto-optic modulator to be f0+ [ Kx (f0-f1) ];

after multiple feedbacks, the signal frequency obtained by the signal processing module 10 resolving the signal sent by the balanced photoelectric detector module 9 approaches the working point f 0;

the final Doppler shift is an accumulated value of the feedback amounts Δ f ∑ [ K × (f0-f1)]Calculating the moving speed V of the target according to the formulaWhere is the lambda laser wavelength.

Specifically, in this embodiment, the closed-loop control method based on the laser doppler velocity measurement principle includes: the power of a seed laser is 10-100 mW, laser is emitted, the laser passes through a beam splitter with a beam splitting ratio of 90/10-50/50, one end with high power generally serves as signal light, enters an acousto-optic modulator module to modulate the signal, a certain frequency shift amount f0 is generated, the frequency shift amount is 20 MHz-100 MHz, the signal can be modulated into pulses, a continuous mode can be maintained, and then the signals enter an optical fiber amplifier; one end with low power as local oscillation light enters the optical fiber attenuator, is attenuated by a certain amplitude value and then enters the 50/50 coupler, and an optical signal enters the circulator through the optical fiber amplifier and then enters the optical antenna to be transmitted to the moving body. The signal light is scattered after encountering a moving body, and then a backward scattering signal enters the fiber antenna again and enters the 50/50 coupler after passing through the circulator. At the moment, the local oscillation light and the signal light jointly enter the 50/50 coupler, the local oscillation light and the signal light respectively enter the two optical input ports of the balanced photoelectric detector after being split by the coupler, heterodyne frequency mixing detection is finally completed in the balanced photoelectric detector, the intermediate frequency signal output by the balanced photoelectric detector enters the signal acquisition, processing and driving circuit for signal processing, and a feedback signal is output to the acousto-optic modulator for closed-loop control.

The specific closed-loop control method comprises the following steps: the working frequency point of the acousto-optic modulator is set to be f0, the signal processing module 10 calculates the signal sent by the balanced photodetector module 9 to obtain a signal frequency f1, as shown in fig. 2, calculates the Power Spectral Density (PSD) of the signal, and estimates the frequency f1 of the signal. The feedback quantity of the signal processing module 10 is (f1-f0) × K, where K is the feedback coefficient. Multiplying the Doppler frequency shift by a certain feedback coefficient K to be used as a negative feedback signal to be superposed on a modulation signal of the acousto-optic modulator, wherein the frequency of the modulation signal actually loaded by the acousto-optic modulator is f0+ [ K x (f0-f1)]. After multiple feedbacks, the signal processing module 10 calculates the signal sent by the balanced photoelectric detector module 9 to obtain a signal frequency close to the working point f 0; the final Doppler shift is an accumulated value of the feedback amounts Δ f ∑ [ K × (f0-f1)]Calculating the moving speed V of the target according to the formulaWhere is the lambda laser wavelength.

As one of the preferred embodiments of this embodiment, the feedback coefficient K takes a value of 0.01 to 0.5. The feedback coefficient is small in value and good in control stability, but the closed loop speed is low, the frequency band is narrow, the feedback coefficient is large, the closed loop speed is high, the frequency band is high, but the stability is poor, and sometimes the control process is easy to disperse.

The direction of the measured speed can be conveniently judged by setting the working frequency point, the influence of low-frequency noise on signal calculation is sometimes considered to be removed, the signal is limited within the measuring speed range, and the working frequency point f0 is 20-100 MHz.