Common-path optical laser ranging system and method for correcting absolute distance based on phase method

Abstract

The application provides a co-calibration optical path laser ranging system and method for correcting absolute distance based on a phase method, a frequency of a laser signal of a semiconductor laser is modulated by using an arbitrary waveform generator; object light and reference light reflected by a fiber collimation unit are co-calibrated and transmitted, transmitted to a processor through a loop unit, a photoelectric detector and a data collector; a beat signal of interference light is extracted by the processor, absolute distance is obtained according to a frequency modulation continuous wave ranging principle, after a target moves, updated absolute distance is obtained, a phase difference is calculated according to intensity signals of interference light corresponding to positions of the target before and after the target moves, a relative distance measurement value is obtained, the absolute distance measured twice is corrected by using the relative distance measurement value, and the corrected absolute distance measured twice is obtained. The application combines advantages of a frequency difference method and a phase difference method, can obtain a high-precision measurement result, can avoid a phase ambiguity problem, and can adapt to precision requirements and measurement requirements in different occasions.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser radar, more particularly to a common-path optical path laser ranging system and method for correcting absolute distance based on a phase method. BACKGROUND

[0002] Laser radar is a kind of active detection technology that uses laser as light source and realizes target distance detection by photoelectric detection technology. It has the advantages of high precision, fast speed, small size, etc. and is widely used in automatic driving, unmanned aerial vehicles, intelligent robots, three-dimensional modeling, geographic surveying and mapping, etc. For example, laser radar can help robots to accurately locate and navigate in complex environments, and update the environment map in real time. In the manufacturing process, laser radar can be used to detect the size and shape changes of mechanical parts, monitor the position and movement of objects on the production line in real time, and ensure the accurate control of the production process. How to ensure the ranging accuracy of laser radar has always been an important research topic in the field.

[0003] For relative ranging, Zhang Enyao et al. disclosed in the document "Improvement of Performance of Fiber Optic Heterodyne Interferometric Displacement Sensor" that a double-path fiber-optic Fizeau interferometer was composed of a semiconductor laser (LD), an optical isolator, a fiber-optic directional coupler, a self-focusing lens, etc. The method of using a triangular wave current modulation and a differential phase detection of the upper and lower edge beat signals was adopted to improve the measurement sensitivity and displacement response speed. An auxiliary interferometer was used to detect the phase drift of the system, and the LD emission wavelength was feedback controlled to improve the stability of the system. However, the optical path structure in this technology is complex, which affects the measurement efficiency, and a coupler is needed for light splitting. The reference light and the object light have a large difference in optical path, and external factors (such as air turbulence, mechanical vibration, etc.) will have different effects on the phase of the two beams, resulting in an increase in phase noise and thus reducing the stability and precision of the measurement.

[0004] For absolute ranging, frequency difference method is often used for ranging, and absolute distance can be directly obtained by measuring frequency difference. The accuracy of frequency measurement is usually lower than that of phase difference method, especially in the case of short distance and high precision requirement; but phase difference method can only measure phase information to obtain relative distance information, and cannot obtain absolute distance. In summary, when using frequency information for ranging, although absolute distance can be directly obtained, the accuracy of frequency ranging is relatively low; when using phase information for ranging, information is obtained through phase difference, and only relative position information can be obtained through two measurements, and there is a multi-value problem when the ranging distance is greater than the wavelength of the ranging wave, that is, the phase difference is periodic within a period, and there is a phase ambiguity problem. The absolute ranging accuracy of frequency-modulated continuous wave can reach millimeter level, and the measurement accuracy is low, while the relative ranging has higher measurement accuracy than the absolute ranging, and can reach sub-nanometer. To realize high-precision absolute ranging, optical frequency comb technology is currently mainly used, which has large device volume and high cost, and the cost is generally one order of magnitude higher than that of frequency-modulated continuous wave ranging. Therefore, how to combine the relative ranging and absolute ranging schemes, improve the measurement accuracy and reduce the cost has become an important research topic in the field. SUMMARY

[0005] The purpose of the present application is to provide a co-programmed optical path laser ranging system and method for correcting absolute distance based on phase method, which combines the advantages of frequency difference method and phase difference method, can obtain high-precision measurement results, avoid phase ambiguity problem, reduce measurement cost, and adapt to precision requirements and measurement requirements in different occasions.

[0006] To achieve the above purpose, the present application provides the following scheme:

[0007] A co-programmed optical path laser ranging system for correcting absolute distance based on phase method, comprising an arbitrary waveform generator, a semiconductor laser, a loop unit, a fiber collimation unit, a photodetector, a data collector and a processor.

[0008] The arbitrary waveform generator generates a frequency-modulated signal, and the driving current of the semiconductor laser is controlled to frequency-modulate the laser signal of the semiconductor laser.

[0009] The output end of the semiconductor laser is connected to the loop unit through an optical fiber.

[0010] The loop unit and the fiber collimation unit are connected through an optical fiber, and are used for transmitting the laser signal to the fiber collimation unit.

[0011] The fiber collimation unit is used for transmitting a part of the laser signal to the target, collecting the object light reflected by the target, and reflecting a part of the laser signal to form reference light, and the object light and the reference light received by the fiber collimation unit are co-programmed.

[0012] The loop unit receives the object light and the reference light returned by the optical fiber collimation unit and transmits to the photodetector;

[0013] The photodetector is used to convert the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmit to the data collector;

[0014] The data collector is used to receive the electrical signal converted by the photodetector and transmit to the processor;

[0015] The processor is used to receive the electrical signal and extract the beat frequency signal of the interference light based on the electrical signal, obtain the absolute distance according to the principle of frequency-modulated continuous wave ranging, obtain the updated absolute distance after the target moves, calculate the phase difference to obtain the relative distance measurement value based on the intensity signals of the interference light corresponding to the positions of the target before and after the target moves, and correct the absolute distances measured twice by using the relative distance measurement value to obtain the corrected absolute distance measured twice.

[0016] Further, the arbitrary waveform generator is also used to generate a synchronization electrical signal and transmit to the data collector, and the synchronization electrical signal has the same frequency as the frequency-modulated signal.

[0017] Further, the signal transmission mode between the loop unit and the photodetector includes one or more of fiber transmission, collimator conversion transmission and free space optical transmission.

[0018] Further, the loop unit is provided with three interfaces, namely a port, a b port and a c port, wherein the a port is used to receive the laser signal from the DFB laser, the b port is used to output the laser signal to the optical fiber collimation unit and receive the object light and the reference light, and the c port is used to transmit the object light and the reference light to the photodetector.

[0019] Further, the optical fiber collimation unit adopts an optical fiber collimator with a pc type interface, adopts a lens with transmission and reflection functions or adopts a combined structure of an optical fiber and a collimation lens.

[0020] Further, the frequency-modulated signal is a symmetrical triangular wave or a sawtooth wave, and the synchronization electrical signal is a square wave with a duty cycle of 50%.

[0021] The application also provides a common-path optical laser ranging method for correcting absolute distance based on a phase method, which is applied to the common-path optical laser ranging system for correcting absolute distance based on a phase method and includes the following steps:

[0022] The arbitrary waveform generator generates a frequency-modulated signal and transmits to the semiconductor laser to frequency-modulate the laser signal of the semiconductor laser;

[0023] The frequency-modulated laser signal is transmitted to the optical fiber collimation unit through the loop unit.

[0024] The optical fiber collimation unit transmits a part of the laser signal to the target and collects the object light reflected by the target, and reflects a part of the laser signal to form reference light;

[0025] The object light received by the optical fiber collimation unit is co-transmitted with the reference light reflected at the end face of the optical fiber collimation unit, and is transmitted to the photodetector through the loop unit;

[0026] The photodetector converts the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmits it to the data collector;

[0027] The data collector receives the electrical signal converted by the photodetector and transmits it to the processor;

[0028] The processor receives the electrical signal and extracts the beat frequency signal of the interference light based on the electrical signal, obtains the absolute distance according to the principle of frequency-modulated continuous wave ranging, obtains the updated absolute distance after the target moves, calculates the relative distance measurement value based on the phase difference corresponding to the intensity signals of the interference light at the positions before and after the target moves, and corrects the absolute distances measured twice by using the relative distance measurement value to obtain the corrected absolute distances measured twice, specifically including:

[0029] According to the principle of frequency-modulated ranging, the absolute distances before and after the target moves are l1 and l2 respectively, and the relative distance theoretical value Δl' is obtained according to the absolute distances l1 and l2:

[0030] Δl' = l2 - l1

[0031] The calculation formula of the relative distance measurement value is as follows:

[0032]

[0033] In the formula, Δl represents the relative distance measurement value of the positions before and after the target moves; τ1 is the time difference corresponding to the optical path difference of the object light and the reference light when the position before the target moves is measured; τ2 is the time difference corresponding to the optical path difference of the object light and the reference light when the position after the target moves is measured; is the phase difference actually measured based on the intensity signals of the interference light at the positions before and after the target moves; f0 is the initial frequency of the laser signal; c is the speed of light;

[0034] Based on the error analysis method, the absolute error or relative error between the relative distance measurement value Δl and the relative distance theoretical value Δl' is obtained, and the absolute distances l1 and l2 are respectively corrected based on the absolute error or relative error.

[0035] Further, the error analysis method is used to obtain the absolute error or relative error between the relative distance measurement value Δl and the relative distance theoretical value Δl', and the absolute distance l1 and l2 are respectively corrected based on the absolute error or relative error, and the correction specifically includes:

[0036] The relative distance measurement value Δl is compared with the relative distance theoretical value Δl', and the error distribution rule is analyzed.

[0037] According to the error distribution rule, an error model is established by a data fitting method, and the absolute error correction parameter or the relative error correction coefficient is obtained.

[0038] The absolute distance l1 and l2 are respectively corrected based on the absolute error correction parameter or the relative error correction coefficient.

[0039] Further, the absolute distances of the target before and after moving are respectively l1 and l2 according to the frequency modulation ranging principle, and the specific formula is as follows:

[0040] l1=n1λ+r1n1=0,1,2,3,4……

[0041] l2=n2λ+r2n2=0,1,2,3,4……

[0042] Wherein, λ is the wavelength of the laser signal, n1 means the wavelength integer multiple part in the absolute distance l1, which corresponds to the phase periodic integer multiple in the interference light intensity I 上升 before the target moves, r1 means the decimal part in the absolute distance l1 except the wavelength integer multiple, 0 < r1 < λ, which corresponds to the phase part in the interference light intensity I 上升 after the target moves n2 means the wavelength integer multiple part in the absolute distance l2, which corresponds to the phase periodic integer multiple in the interference light intensity I' 上升 after the target moves, r2 means the decimal part in the absolute distance l2 except the wavelength integer multiple, 0 < r2 < λ, which corresponds to the phase part in the interference light intensity I' 上升 after the target moves

[0043] When only the phase ranging is used, Δl < λ, which means that even if the relative moving distance is greater than λ, the relative ranging method can only measure the decimal part except the wavelength integer multiple, which can be expressed as

[0044]

[0045] When the relative moving distance is greater than λ, the relative distance cannot be directly obtained from because The relative distance theoretical value Δl' is as follows:

[0046] Δl' = l2 - l1 = (n2 - n1) λ - (r2 - r1).

[0047] Further, the phase difference corresponding to the absolute distance l1 is obtained The distance corresponding to the phase part of the beat frequency interference signal The precision of r'1 is higher than that of r1, and the corrected absolute distance l'1 = n1 λ + r'1 is obtained.

[0048] The phase difference corresponding to the absolute distance l2 is obtained The distance corresponding to the phase part of the beat frequency interference signal The precision of r'2 is higher than that of r2, and the corrected absolute distance l'2 = n2 λ + r'2 is obtained.

[0049] According to the specific embodiments provided by the present application, the following technical effects are disclosed: the common-path optical laser ranging system and method for correcting absolute distance based on phase method provided by the present application, which obtains relative distance measurement value by phase method, obtains absolute distance by frequency-modulated continuous wave (FMCW) ranging, and uses phase method for ranging to correct frequency-modulated continuous wave (FMCW) ranging, especially when the frequency-modulated ranging accuracy is poor and system error exists, the high-precision characteristics of phase method for ranging can be used for correction.

[0050] In the present application, the characteristics of the optical path are combined, the object light received by the optical fiber collimation unit is common-path transmitted with the reference light reflected at the end face of the optical fiber collimation unit and forms interference light, which is transmitted to the photoelectric detector through the loop unit. In the ranging, the phase difference is calculated based on the intensity signals of the interference light corresponding to the positions before and after the target moves. Since the object light and the reference light are common-path transmitted, the influence of external environment (such as temperature, pressure, vibration, etc.) on the object light and the reference light is consistent. When calculating the phase difference, the external interference can be offset by phase subtraction, which maximizes the measurement accuracy. The present application calculates the phase difference based on the intensity signals of the interference light corresponding to the positions before and after the target moves to obtain the relative distance measurement value and obtain the ranging result. Compared with the traditional frequency difference method for ranging, higher measurement accuracy can be achieved. The key of combining frequency-modulated continuous wave ranging and phase ranging lies in utilizing their respective advantages: frequency-modulated continuous wave is suitable for long-distance and large-range measurement, while phase ranging provides higher accuracy and can achieve sub-nanometer.

[0051] The phase method ranging can effectively correct the error of the frequency-modulated continuous wave (FMCW) ranging by providing high-precision short-distance measurement data. By establishing an error model, a correction factor, or a real-time correction strategy, accurate correction of the FMCW ranging result can be achieved, so as to obtain higher accuracy in the entire ranging range. This combined method can utilize the long-range range of the FMCW ranging and the high-precision characteristics of the phase ranging to achieve a more ideal ranging effect, without the need for large equipment, low cost, and high universal applicability. BRIEF DESCRIPTION OF DRAWINGS

[0052] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0053] Figure 1 A structure diagram of a common-path optical laser ranging system for correcting absolute distance based on a phase method according to the present application;

[0054] Figure 2 A triangular wave frequency modulation signal diagram according to an embodiment of the present application;

[0055] Figure 3 A structure diagram of a typical FMCW laser radar ranging system according to the prior art;

[0056] Figure 4 A beat signal diagram obtained by a typical FMCW laser radar ranging system according to the prior art;

[0057] Figure 5 A beat signal diagram obtained by a common-path optical laser ranging system for correcting absolute distance based on a phase method according to the present application;

[0058] Figure 6 A structure diagram of a fiber collimation unit formed by the combination of an optical fiber and a collimation lens;

[0059] BRIEF DESCRIPTION OF DRAWINGS

[0060] Figure 1 In the drawings, 1 is an arbitrary waveform generator; 2 is a semiconductor laser; 3 is a loop unit (divided into a, b, and c three ports, a port in and b port out, b port in and c port out); 4 is a fiber collimation unit; 5 is a photodetector; 6 is a data collector (two-way input); 7 is a processor; 8 is the position of the target before moving; 8' is the position of the target after moving;

[0061] Figure 3Among them, 3-1, tunable laser; 3-2, isolator; 3-3, coupler 1; 3-4, circulator; 3-5, lens; 3-6, target; 3-7, coupler 2; 3-8, balanced detector;

[0062] Figure 6 In the figure, 6-1 is optical fiber; 6-2 is collimating lens. DETAILED DESCRIPTION

[0063] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0064] The purpose of the present invention is to provide a common optical path laser ranging system and method for correcting absolute distance based on the phase method. By combining the advantages of the frequency difference method and the phase difference method, high-precision measurement results can be obtained while avoiding the phase ambiguity problem, and the system can adapt to the accuracy and measurement requirements of different occasions.

[0065] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0066] like Figure 1 As shown, the common optical path laser ranging system for correcting absolute distance based on the phase method provided by the present invention includes an arbitrary waveform generator 1, a semiconductor laser 2, a loop unit 3, a fiber collimation unit 4, a photodetector 5, a data collector 6 and a processor 7;

[0067] The arbitrary waveform generator 1 generates a frequency modulation signal and performs frequency modulation on the laser signal of the semiconductor laser 2 by controlling the driving current of the semiconductor laser 2;

[0068] The output end of the semiconductor laser 2 is connected to the loop unit 3 via an optical fiber;

[0069] The loop unit 3 is connected to the optical fiber collimation unit 4 via an optical fiber, and is used to transmit the laser signal to the optical fiber collimation unit 4;

[0070] The optical fiber collimation unit 4 is used to transmit a portion of the laser signal to the target and collect the object light reflected by the target, while reflecting a portion of the laser signal to form a reference light. The object light received by the optical fiber collimation unit 4 and the reference light are transmitted together.

[0071] The loop unit 3 receives the object light and reference light transmitted back by the optical fiber collimation unit 4 and transmits them to the photodetector 5;

[0072] The photoelectric detector 5 is used for converting the intensity signal of the interference light of the object light and the reference light into an electric signal, and transmitting to the data collector 6;

[0073] The data collector 6 is used for receiving the electric signal converted by the photoelectric detector 5, and transmitting to the processor 7;

[0074] The processor 7 is used for receiving the electric signal, extracting the beat frequency signal of the interference light based on the electric signal, obtaining the absolute distance according to the principle of the frequency-modulated continuous wave ranging, obtaining the updated absolute distance after the target moves, calculating the phase difference based on the intensity signals of the interference light corresponding to the positions before and after the target moves to obtain the relative distance measurement value, and correcting the absolute distances measured twice by using the relative distance measurement value to obtain the corrected absolute distances measured twice.

[0075] The relative distance measurement value in the application refers to the difference between the positions before and after the target moves, that is, the distance of the target movement, that is, the relative displacement of the target can be measured under the condition that the fiber collimation unit 4 is fixed. Figure 1 In the specific embodiment, l1 and l2 respectively represent the distances from the positions 8 before and after the target moves to the fiber collimation unit 4, and Δl represents the distance of the target movement.

[0076] The arbitrary waveform generator 1 is also used for generating a synchronous electric signal, transmitting to the data collector 6, the synchronous electric signal is the same frequency as the frequency-modulated signal, for example, 1khz. For example, the frequency-modulated signal is a symmetrical triangular wave or a sawtooth wave, and other waveforms can also be used, the synchronous electric signal is a square wave, and the duty cycle is 50%. The embodiment of the application takes the triangular wave frequency-modulated signal as an example.

[0077] For example, the semiconductor laser 2 can select a narrow line width semiconductor laser, for example, a DFB (Distributed Feedback Laser) laser; the loop unit (3) can use a fiber loop.

[0078] For example, the signal transmission mode between the loop unit 3 and the photoelectric detector 5 includes one or more of fiber transmission, collimator conversion transmission, and free space optical transmission.

[0079] Specifically, the loop unit 3 is provided with three interfaces, which are a port, a b port and a c port, wherein the a port is used for receiving the laser signal from the DFB laser 2, the b port is used for outputting the laser signal to the fiber collimation unit 4 and receiving the object light and the reference light, and the c port is used for transmitting the object light and the reference light to the photoelectric detector 5.

[0080] For example, the fiber collimation unit 4 can adopt a fiber collimator with a pc type interface, a lens with transmission and reflection functions, or a combination of a fiber and a collimating lens, etc. The combination of a fiber and a collimating lens is shown in Figure 6 The present application does not limit the selection of the fiber collimation unit 4, as long as the fiber collimation unit 4 can realize the transmission and reflection functions of the laser signal, and the laser signal is reflected at the end face of the fiber collimation unit 4 to form the reference light. After the object light passes through the fiber collimation unit 4, it is co-optical-path with the reference light and transmitted to the photodetector 5 through the loop unit 3, so that the optical path difference between the measurement reference light and the object light is closer to the distance to be measured, and the measurement error is reduced.

[0081] In FMCW laser ranging, a triangular wave is generally used as the modulation waveform, and the frequency of the laser is linearly modulated, as shown in Figure 2 The variation of the laser frequency with time under the modulation of the periodic triangular wave is shown by the solid trace, and the optical frequency of the reference light is represented by the dashed trace. Due to the time delay between them, there is a difference in frequency when they are mixed, and the frequency difference jumps within a frequency modulation period. However, since the change time is extremely short, the influence can be ignored, so the difference frequency signal obtained by mixing can be regarded as a sinusoidal wave with a fixed frequency.

[0082] After the modulation of the laser signal by the frequency modulation signal, the photodetector 5 receives a sinusoidal wave with a phase jump within a T / 2 period, which is an alternating current signal, and can be filtered and signal amplified by the processing method of the alternating current signal. If there is no modulation of the frequency modulation signal, the photodetector 5 obtains a direct current signal at a measurement position.

[0083] The instantaneous frequency of the reference light under the modulation of the rising edge of the triangular wave is represented as:

[0084]

[0085] where f0 represents the initial frequency of the laser, and a = 2B / T is the frequency modulation rate, i.e. the slope of the rising edge of the triangular wave, and B is the frequency modulation bandwidth.

[0086] Within a frequency modulation period T, the expression of the reference light signal is:

[0087]

[0088] The expression of the object light signal is:

[0089] where is the initial phase of the reference light, E1 is the amplitude of the reference light, and E2 is the amplitude of the object light.

[0090] The time domain expression of the difference frequency signal light intensity after mixing interference is:

[0091]

[0092] Since the quadratic term is much smaller than the phase term, the quadratic term can be ignored:

[0093]

[0094] And when the falling edge, the instantaneous frequency of the reference signal is expressed as Similarly, we can get:

[0095]

[0096] According to the odd function property of the cos function, we can get:

[0097]

[0098] The synchronous square wave generated at the arbitrary waveform generator can be used to separate the rising edge and falling edge signals of the triangular wave at the data acquisition device, and it can be known from the expression that the phase difference between the rising edge and the falling edge is 4πf0τ.

[0099] Where, τ represents the E r (t) relative to the time delay of E e (t).

[0100] The ranging result of the application can only be used for relative distance measurement, because the phase difference will be wrapped when it is greater than one 2π, and the specific phase value cannot be determined. In addition, when performing relative distance measurement, the phase corresponding to the change value of the distance between two measurements also needs to be less than 2π.

[0101] The FMCW (Frequency Modulated Continuous Wave) radar realizes ranging by transmitting a continuous wave signal whose frequency changes linearly with time. There is a frequency difference between the signal received by the receiving end and the transmitted signal, and the frequency difference is proportional to the target distance. Therefore, according to the frequency modulated continuous wave ranging principle, the calculation formula of the absolute distance is as follows:

[0102]

[0103] In the formula, l represents the absolute distance of the distance to be measured in a single measurement;

[0104] τ is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance to be measured;

[0105] α is the frequency modulation rate, α = 2B / T, where B is the frequency modulation bandwidth and T is the frequency modulation period;

[0106] f is the beat frequency signal of the interference light obtained based on a single measurement, and a frequency difference is obtained.

[0107] Based on the above principle, the application provides a common-path optical path laser ranging method for correcting absolute distance based on a phase method, which is applied to the common-path optical path laser ranging system for correcting absolute distance based on a phase method, and includes the following steps:

[0108] An arbitrary waveform generator 1 generates a frequency modulation signal and transmits the frequency modulation signal to a semiconductor laser 2 to perform frequency modulation on a laser signal of the semiconductor laser 2.

[0109] The frequency-modulated laser signal is transmitted to a fiber collimation unit 4 through a loop unit 3.

[0110] The fiber collimation unit 4 transmits a part of the laser signal to a target and collects object light reflected by the target, and reflects a part of the laser signal to form reference light.

[0111] The object light and the reference light received by the fiber collimation unit 4 are transmitted in common, and are transmitted to a photodetector 5 through the loop unit 3.

[0112] The photodetector 5 converts an intensity signal of interference light of the object light and the reference light into an electrical signal and transmits the electrical signal to a data collector 6.

[0113] The data collector 6 receives the electrical signal converted by the photodetector 5 and transmits the electrical signal to a processor 7.

[0114] The processor 7 receives the electrical signal and extracts a beat frequency signal of the interference light based on the electrical signal, obtains an absolute distance according to a frequency-modulated continuous wave ranging principle, obtains an updated absolute distance after the target moves, calculates a relative distance measurement value based on a phase difference corresponding to intensity signals of the interference light before and after the target moves, and corrects the absolute distances measured twice by using the relative distance measurement value to obtain corrected absolute distances, and specifically includes the following steps.

[0115] According to the frequency-modulated ranging principle, the absolute distances before and after the target moves are l1 and l2 respectively, and a relative distance theoretical value Δl' is obtained according to the absolute distances l1 and l2.

[0116] Δl' = l2 - l1

[0117] The calculation formula of the relative distance measurement value is as follows:

[0118]

[0119] Wherein, Δl represents the relative distance measurement value of the positions before and after the target moves; τ1 is the time difference corresponding to the optical path difference of the object light and the reference light when the position before the target moves is measured; τ2 is the time difference corresponding to the optical path difference of the object light and the reference light when the position after the target moves is measured; is the phase difference actually measured based on the intensity signals of the interference light corresponding to the positions before and after the target moves; f0 is the initial frequency of the laser signal; and c is the light speed.

[0120] Based on the error analysis method, the absolute error or the relative error between the relative distance measurement value Δl and the relative distance theoretical value Δl' is obtained, and the absolute distances l1 and l2 are respectively corrected based on the absolute error or the relative error.

[0121] Specifically, the common-path laser ranging method for correcting the absolute distance based on the phase method of the embodiment of the present application is introduced as follows:

[0122] 1. Relative distance measurement by using the phase ranging:

[0123] Specifically, the phase difference is calculated based on the intensity signals of the interference light corresponding to the positions before and after the target moves, and the relative distance measurement value is obtained, which specifically includes:

[0124] When the position before the target moves is measured, the time domain expression of the light intensity of the beat frequency signal obtained after the object light and the reference light interfere is:

[0125]

[0126] Wherein, I 上升 , I 下降 are the light intensities of the rising edge and the falling edge of the beat frequency signal obtained after the object light and the reference light interfere when the position before the target moves is measured; I1 and I2 are the light intensities of the reference light and the object light; τ1 is the time difference corresponding to the optical path difference of the object light and the reference light when the position before the target moves is measured; and α = 2B / T is the frequency modulation rate, that is, the slope of the rising edge of the triangular wave, B is the frequency modulation bandwidth, and T is the frequency modulation period.

[0127] Therefore, the phase difference of the light intensities of the rising edge and the falling edge when the position before the target moves is measured is 4πf0τ1.

[0128] Similarly, when the position after the target moves is measured, the time difference corresponding to the optical path difference of the object light and the reference light is τ2, and the time domain expression of the light intensity of the beat frequency signal obtained after the object light and the reference light interfere is:

[0129]

[0130] Wherein, I' 上升 , I' 下降When measuring the distance to the target's position after it moves, the intensity of the rising and falling edges of the difference frequency signal are obtained after the interference of the object light and the reference light;

[0131] When measuring the distance to the target after it moves, the phase difference between the rising and falling light intensities is 4πf0τ2;

[0132] The combined phase difference is calculated based on the intensity signal of the interference light at the position before and after the target moves:

[0133] 4πf0τ1-4πf0τ2=4πf0(τ1-τ2);

[0134] Depend on Conclusion

[0135] Among them, because the phase difference of the light intensity at the rising edge and the falling edge is doubled, the corresponding measured phase difference also needs to be doubled, which is

[0136] but,

[0137] The range is 0-2π, when hour,

[0138]

[0139] The value range of Δl is 0-λ, where λ is the wavelength of the laser signal.

[0140] For phase difference ranging, the maximum ranging distance depends on the wavelength of the laser signal, and the ranging accuracy depends on the resolution of the phase difference.

[0141] The present invention further calculates the combined phase difference of the intensity signals of the interference light at the position before and after the target moves based on the phase difference of the rising and falling edges. The scheme of obtaining the distance difference value by using the combined phase difference of the rising and falling edges doubles the distance accuracy at the same phase difference resolution compared with measuring the distance by comparing the phase difference of the two rising edges.

[0142] In the above, the phase difference actually measured is based on the intensity signal of the interference light at the position before and after the target moves. Specifically, the existing technologies in the art can be used to calculate, such as the cross-correlation method, the fast Fourier transform method, the Hilbert transform method, the zero-crossing method (interpolation method), etc. The cross-correlation method: the correlation peaks are found by correlating two discrete signals, the time offset is calculated, and then the sampling frequency is converted into a phase difference, which is suitable for processing periodic signals and non-periodic signals. The fast Fourier transform (FFT) method: the FFT of the signal is obtained, the phase information in the frequency spectrum is obtained, and then the phase difference of the same frequency points of the two signals is calculated, which is suitable for stable frequency sinusoidal signals. The Hilbert transform method: the Hilbert transform of the signal is obtained, the analytic signal (including amplitude and phase information) is obtained, and then the phase difference is calculated. The zero-crossing method (interpolation method): the zero-crossing points are found in the sampling data, the time difference of the zero-crossing points is accurately positioned by the interpolation method, and the phase difference is calculated, which is suitable for high sampling rate data.

[0143] In addition, the following embodiments can also be used for the calculation of the phase difference:

[0144] When performing signal processing, the rising edge part can also be directly referenced to obtain the phase difference when measuring the positions of the target before and after movement, that is:

[0145] 2πf0τ1-2πf0τ2=2πf0(τ1-τ2)

[0146] From

[0147] Similarly, the above

[0148] As Figure 3 is a typical FMCW laser radar ranging system in the prior art, two couplers are provided, and the reference light and the object light are not co-transmitted in the fiber collimation unit. The beat frequency signal obtained when the system performs ranging is as shown in Figure 4 It can be seen that before the improvement of the optical path of the present application, the beat frequency signal does not have obvious sinusoidal characteristics, it is difficult to observe obvious sinusoidal signals from the signal, and the phase cannot be judged.

[0149] Figure 5 The beat frequency signal obtained after the improvement of the optical path of the high-precision laser phase ranging system of the present application is shown, which is compared with Figure 4 It can be seen that the present application can observe a part with obvious sinusoidal signal characteristics, which can be used to obtain phase information after fitting, and the waveform integrity is significantly improved compared with the signal before the improvement of the optical path. It can be seen that the present application has better technical effects compared with the traditional FMCW laser radar ranging system, and realizes technical improvement.

[0150] ​In addition, compared with the typical FMCW lidar ranging system in the prior art, the present invention forms a reference light by reflection of the fiber collimation unit, which can reduce the use of couplers, reduce the energy loss generated by the couplers, and improve the overall efficiency of the system. At the same time, various errors introduced by the couplers are reduced. Meanwhile, the object light and the reference light received by the fiber collimation unit are transmitted in a common path, reducing the non-common path error, further improving the ranging accuracy of the relative distance measurement, and providing a reliable data basis and technical support for correcting the absolute distance.

[0151] 2. Regarding the absolute distance measurement using FMCW ranging:

[0152] The length l of the absolute distance can be expressed as

[0153] l = 2nλ + r n = 0, 1, 2, 3, 4……

[0154] where r > 0. Therefore, for a determined l, the displacement-determined n and r can be obtained. Numerically, n is equal to the integer part of l divided by 2λ, and r is equal to the remainder.

[0155] Then there is

[0156] l1 = 2n1λ + r1 n1 = 0, 1, 2, 3, 4……

[0157] l2 = 2n2λ + r2 n2 = 0, 1, 2, 3, 4……

[0158] where λ is the wavelength of the laser signal. The meaning of n1 is the integer multiple part of the wavelength in the absolute distance l1, corresponding to the integer multiple of the phase periodicity in the interference light intensity I 上升 (i.e., corresponding to n12π), and the meaning of r1 is the decimal part of the absolute distance l1 excluding the integer multiple of the wavelength, 0 < r1 < λ, corresponding to the phase part in the interference light intensity I 上升 in That is, 2πf0τ1); similarly, the meaning of is the integer multiple part of the wavelength in the absolute distance l2, corresponding to the integer multiple of the phase periodicity in the interference light intensity I' 上升 in, and the meaning of r2 is the decimal part of the absolute distance l2 excluding the integer multiple of the wavelength, 0 < r2 < λ, corresponding to the phase part in the interference light intensity I' 上升 in

[0159] When only phase ranging is used, Δl < λ, which means that even if the relative moving distance is greater than λ, the relative ranging method can only measure the decimal part excluding the integer multiple of the wavelength, which can be expressed as

[0160] [[ID=4E]] [[ID=4F]]

[0161] When the relative moving distance is greater than λ, at this time, it is impossible to obtain from The relative distance is directly obtained because The formula of the relative distance theoretical value Δl' is as follows:

[0162] Δl' = l2 - l1 = (n2 - n1) λ - (r2 - r1).

[0163] For the absolute distance, the phase of the beat frequency interference signal corresponding to l1 is obtained The distance corresponding to the phase part is Since the phase has higher precision than the frequency, the precision of r'1 is higher than that of r1, and the corrected l'1 = 2n1 λ + r'1 can be obtained, and the corrected l'2 = 2n2 λ + r'2 can be obtained in the same way. This part is the correction of the phase method for the absolute distance, and the starting point is that the phase part has higher precision.

[0164] As mentioned above, if only the relative distance measurement data is obtained, the condition This is actually because of the periodicity of the cos function, when When combined with absolute distance measurement, there is

[0165]

[0166] Therefore, the phase period ambiguity problem of the relative distance can be demodulated using the difference of the absolute distance; this part is a correction of the relative distance by the absolute distance, which solves the problem of the phase period of the relative distance that cannot be measured.

[0167] 3. Correction of FMCW ranging by phase method

[0168] 3.1 Correction principle

[0169] The frequency modulated continuous wave (FMCW) ranging method usually relies on frequency modulation and signal time delay to calculate the target distance, and can achieve a larger ranging range, but due to the influence of signal propagation conditions, nonlinear errors, system deviations and other factors, there may be certain errors. In contrast, the phase method of distance measurement has higher precision and is suitable for shorter distance measurement range, but is also affected by problems such as multipath effect and phase ambiguity.

[0170] By using the high precision characteristics of the phase method of distance measurement, we can correct the FMCW ranging result. The basic idea of correction is: use the phase ranging to accurately measure the short distance, and then correct the system error of the FMCW ranging according to this accurate result.

[0171] 3.2 Correction steps

[0172] (1). Measure and compare the results of the two methods

[0173] Using FMCW ranging method to measure the distance of the target, get a rough distance value (may have some error).

[0174] At the same target distance, use phase ranging method for accurate measurement, get an accurate distance value.

[0175] By comparing the FMCW ranging value and the phase ranging value, the difference between the two can be found. These differences mainly come from the error in FMCW ranging method (such as nonlinear error, system bias, etc.).

[0176] (2). Establish error model

[0177] Compare the measurement value of FMCW ranging with the measurement value of phase ranging, analyze the error distribution rule.

[0178] Through data fitting method (such as linear regression, nonlinear fitting, etc.), establish error model, find out the system bias in FMCW ranging (including sensor calibration error, frequency response error, etc.). For example, if the difference between FMCW ranging and phase ranging in a certain distance range is a linear relationship, then a error correction coefficient can be obtained by linear fitting.

[0179] (3). Correct FMCW ranging result

[0180] According to the error model or correction coefficient, correct the FMCW ranging result. For example, if FMCW ranging and phase ranging show a linear relationship, the FMCW result can be directly corrected by a correction factor (such as error compensation coefficient): the phase ranging result multiplied by the correction factor, which can get the FMCW ranging result; if the difference between FMCW ranging and phase ranging can be calculated by error model, then the error value calculated by error model can be used for correction: the phase ranging result minus the error value, which can get the FMCW ranging result.

[0181] (4). Consider correction in different ranging ranges

[0182] In practical application, the error model of FMCW and phase ranging may not be consistent globally, that is, in different ranging ranges, the error characteristics of the two may be different. Therefore, the error model can be subdivided into multiple different ranging intervals, and different correction factors are used for different intervals.

[0183] For example, in short distance (effective range of phase ranging), the phase ranging result can be used to correct the FMCW ranging; in long distance range (advantageous interval of FMCW ranging), more complex nonlinear error correction needs to be considered.

[0184] (5). Real-time correction and dynamic correction

[0185] In practical applications, the measurement data of FMCW ranging and phase ranging can be collected in real time, and the FMCW ranging result is adjusted according to the error of real-time comparison.

[0186] Through dynamic error compensation algorithm (such as Kalman filter or other adaptive filtering method), the FMCW ranging result is continuously corrected.

[0187] FMCW method is suitable for long distance measurement, and phase method is suitable for short distance, so it is necessary to reasonably select and correct the range to ensure the complementary advantages of the two in practical application.

[0188] Phase ranging can effectively correct the error of frequency-modulated continuous wave (FMCW) ranging by providing high-precision short-range measurement data. By establishing an error model, a correction factor, or a real-time correction strategy, accurate correction of the FMCW ranging result can be achieved, thereby achieving higher accuracy in the entire ranging range. This combined method can not only take advantage of the long-range range of FMCW ranging, but also take advantage of the high-precision characteristics of phase ranging, achieving a more ideal ranging effect.

[0189] In addition, to combine frequency-modulated continuous wave (FMCW) ranging and phase ranging to achieve both wide ranging range and high precision, the following strategies can be considered:

[0190] 1. Multi-stage ranging (multi-segment measurement)

[0191] Take advantage of the different advantages of frequency-modulated continuous wave (FMCW) and phase ranging to switch measurement methods in different ranging ranges. Specifically:

[0192] Short distance high precision stage: For closer targets, phase ranging can be used because it has higher accuracy and is suitable for accurate measurement of short distances.

[0193] Long distance wide range stage: For long distance targets, frequency-modulated continuous wave (FMCW) can be used because it has a larger dynamic range and is suitable for measuring long distance targets.

[0194] In this method, some algorithms and circuit designs are used to switch between different distance segments, so that the overall system can maintain good accuracy and dynamic range in the entire ranging range.

[0195] 2. Compound measurement (fusion ranging method)

[0196] Another method is to fuse the results of the two ranging methods. The phase ranging and FMCW ranging methods can be combined through multi-sensor fusion or data processing fusion:

[0197] Using the coarse distance information provided by FMCW, the approximate location of the target and the rough ranging result are obtained.

[0198] Then use phase ranging to fine measure the near distance and accurately calculate the distance of the target.

[0199] This method can use the wide ranging range of FMCW for rough measurement, and then accurately calculate through the method of phase ranging, so that the system can provide high-precision measurement in different distance ranges.

[0200] 3、Dynamic gain switching

[0201] In a system, by dynamically switching the gain, the performance of the two methods in different distance ranges is optimized:

[0202] When the target is far away, the FMCW method can handle the problem of large signal attenuation due to its large dynamic range, while maintaining low noise.

[0203] When the target is close, using phase ranging can improve accuracy and reduce measurement error.

[0204] By dynamically adjusting the gain of the signal, the best measurement method can be selected according to the distance of the target.

[0205] 4、Hybrid signal processing and calibration

[0206] By processing the measurement results of the two ranging methods, the advantages of the two can be complementary. For example:

[0207] For the error caused by multipath effect in phase ranging, the reliability of FMCW method at long distance can be used for calibration.

[0208] For the low precision problem of FMCW method at close distance, the high precision of phase ranging can be compensated.

[0209] In this way, through signal processing and data fusion, the ranging range and accuracy of the system are improved.

[0210] 5、Dual-frequency or multi-frequency technology

[0211] If the hardware allows, higher ranging accuracy and range can be achieved through dual-frequency or multi-frequency ranging. The frequency-modulated continuous wave method can use different frequencies for multiple measurements, and the accuracy can be improved through difference frequency technology; phase ranging can also measure the phase at multiple frequencies, and through multi-frequency information decoding, the error can be reduced.

[0212] In summary, the key to combining the frequency-modulated continuous wave ranging and phase ranging lies in utilizing their respective advantages: the frequency-modulated continuous wave is suitable for long-distance and large-range measurement, and the phase ranging provides higher accuracy. By dynamically switching the measurement methods on different distance segments, or by data fusion technology, a ranging system with both high accuracy and large ranging range can be realized.

[0213] The principles and implementation manners of the present application are described herein by using specific examples, and the above examples are only used to help understand the method of the present application and its core idea; meanwhile, for those skilled in the art, the specific implementation manners and application ranges will be changed according to the idea of the present application. In summary, the content of the present specification should not be understood as a limitation on the present application.

Claims

1. A co-designed optical path laser ranging system for correcting absolute distance based on phase method, characterized in that, The system comprises an arbitrary waveform generator (1), a semiconductor laser (2), a loop unit (3), a fiber collimation unit (4), a photodetector (5), a data collector (6) and a processor (7). The arbitrary waveform generator (1) generates a frequency modulation signal, and the frequency modulation signal is used to modulate the frequency of the laser signal of the semiconductor laser (2) by controlling the driving current of the semiconductor laser (2). The output end of the semiconductor laser (2) is connected to the loop unit (3) through an optical fiber. The loop unit (3) is connected to the fiber collimation unit (4) through an optical fiber, and is used to transmit the laser signal to the fiber collimation unit (4). The fiber collimation unit (4) is used to transmit a part of the laser signal to a target, collect the object light reflected by the target, and reflect a part of the laser signal to form reference light, and the object light and the reference light received by the fiber collimation unit (4) are co-transmitted. The loop unit (3) receives the object light and the reference light transmitted by the fiber collimation unit (4) and transmits them to the photodetector (5). The photodetector (5) is used to convert the intensity signal of the interference light of the object light and the reference light into an electric signal and transmit the electric signal to the data collector (6). The data collector (6) is used to receive the electric signal converted by the photodetector (5) and transmit the electric signal to the processor (7). The processor (7) is used to receive the electric signal, extract the beat frequency signal of the interference light based on the electric signal, obtain an absolute distance according to the principle of frequency modulation continuous wave ranging, and obtain an updated absolute distance after the target moves. Based on the intensity signals of the interference light corresponding to the positions of the target before and after the target moves, a phase difference is calculated to obtain a relative distance measurement value, and the relative distance measurement value is used to correct the absolute distances measured twice to obtain the corrected absolute distances measured twice.

2. The co-designed optical path laser ranging system for correcting absolute distance based on phase method according to claim 1, characterized in that, The arbitrary waveform generator (1) is also used to generate a synchronous electric signal, and the synchronous electric signal is transmitted to the data collector (6), and the synchronous electric signal has the same frequency as the frequency modulation signal.

3. The co-designed optical path laser ranging system for correcting absolute distance based on phase method according to claim 1, characterized in that, The signal transmission mode between the loop unit (3) and the photodetector (5) comprises one or more of optical fiber transmission, collimator conversion transmission and free space optical transmission.

4. The co-designed optical path laser ranging system for correcting absolute distance based on phase method according to claim 1, characterized in that, The loop unit (3) is provided with three interfaces, namely a port, a b port and a c port, wherein the a port is used to receive the laser signal from the DFB laser (2), the b port is used to output the laser signal to the fiber collimation unit (4) and receive the object light and the reference light, and the c port is used to transmit the object light and the reference light to the photodetector (5).

5. The co-designed optical path laser ranging system for correcting absolute distance based on phase method according to claim 1, characterized in that, The fiber collimation unit (4) adopts a fiber collimator with a pc type interface, adopts a lens with transmission and reflection functions, or adopts a combination structure of an optical fiber and a collimating mirror.

6. The co-designed optical path laser ranging system for correcting absolute distance based on phase method according to claim 2, characterized in that, The frequency modulation signal is a symmetrical triangular wave or a sawtooth wave, and the synchronous electric signal is a square wave with a duty cycle of 50%.

7. A co-designed optical path laser ranging method for correcting absolute distance based on phase method, applied to the co-designed optical path laser ranging system for correcting absolute distance based on phase method in any one of claims 1-6, characterized in that, The system comprises the following steps: The arbitrary waveform generator (1) generates a frequency modulation signal, and the frequency modulation signal is transmitted to the semiconductor laser (2) to modulate the frequency of the laser signal of the semiconductor laser (2); The frequency modulated laser signal is transmitted to the fiber collimation unit (4) through the loop unit (3); The optical fiber collimation unit (4) transmits a part of the laser signal to the target and collects the object light reflected by the target, and reflects a part of the laser signal to form reference light; The object light received by the optical fiber collimation unit (4) is co-transmitted with the reference light reflected on the end face of the optical fiber collimation unit (4), and is transmitted to the photoelectric detector (5) through the loop unit (3); The photoelectric detector (5) converts the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmits it to the data collector (6); The data collector (6) receives the electrical signal converted by the photoelectric detector (5) and transmits it to the processor (7); The processor (7) receives the electrical signal and extracts the beat frequency signal of the interference light based on the electrical signal, obtains the absolute distance according to the frequency-modulated continuous wave ranging principle, and obtains the updated absolute distance after the target moves; Based on the intensity signals of the interference light corresponding to the positions of the target before and after moving, the phase difference is calculated to obtain the relative distance measurement value, and the relative distance measurement value is used to correct the absolute distances measured twice to obtain the corrected absolute distances measured twice, specifically including: According to the frequency-modulated ranging principle, the absolute distances before and after the target moves are l1 and l2 respectively, and the relative distance theoretical value Δl' is obtained according to the absolute distances l1 and l2: Δl' = l2 - l1 The calculation formula of the relative distance measurement value is as follows: In the formula, Δl represents the relative distance measurement value of the positions before and after the target moves; τ1 is the time difference corresponding to the optical path difference of the object light and the reference light when the position before the target moves is measured; τ2 is the time difference corresponding to the optical path difference of the object light and the reference light when the position after the target moves is measured; is the actual measured phase difference based on the intensity signals of the interference light corresponding to the positions before and after the target moves; f0 is the initial frequency of the laser signal; and c is the speed of light. Based on the error analysis method, the absolute error or relative error between the relative distance measurement value Δl and the relative distance theoretical value Δl' is obtained, and the absolute distances l1 and l2 are respectively corrected based on the absolute error or relative error.

8. The co-designed optical path laser ranging method for correcting absolute distance based on phase method according to claim 7, characterized in that, The error analysis method is used to obtain the absolute error or relative error between the relative distance measurement value Δl and the relative distance theoretical value Δl', and the absolute distances l1 and l2 are respectively corrected based on the absolute error or relative error, specifically including: Compare the relative distance measurement value Δl with the relative distance theoretical value Δl', and analyze the error distribution law; According to the error distribution law, an error model is established by a data fitting method to obtain an absolute error correction parameter or a relative error correction coefficient; Based on the absolute error correction parameter or the relative error correction coefficient, the absolute distances l1 and l2 are respectively corrected.

9. The co-designed optical path laser ranging method for correcting absolute distance based on phase method according to claim 7, characterized in that, According to the frequency-modulated ranging principle, the absolute distances before and after the target moves are l1 and l2 respectively, and the formula used is as follows: l1 = n1λ + r1 n1 = 0, 1, 2, 3, 4… l2 = n2λ + r2 n2 = 0, 1, 2, 3, 4… wherein λ is the wavelength of the laser signal, n1 is the integer part of the absolute distance l1, corresponding to the phase periodic integer multiple in the interference light intensity I 上升 before the target moves, and r1 is the decimal part of the absolute distance l1, 0 < r1 < λ, corresponding to the phase part in the interference light intensity I 上升 after the target moves n2 is the integer part of the absolute distance l2, corresponding to the phase periodic integer multiple in the interference light intensity I' 上升 after the target moves, and r2 is the decimal part of the absolute distance l2, 0 < r2 < λ, corresponding to the phase part in the interference light intensity I' 上升 after the target moves When only phase ranging is used, Δl < λ, which means that even if the relative moving distance is greater than λ, the relative ranging method can only measure the decimal part of the integer multiple of the wavelength, which can be represented as When the relative moving distance is greater than λ, the relative distance cannot be directly obtained from because The theoretical value of the relative distance Δl' is given by the following formula: Δl' = l2 - l1 = (n2 - n1)λ - (r2 - r1).

10. The co-transmission optical path laser ranging method for correcting absolute distance based on phase method according to claim 9, characterized in that, The phase of the beat frequency interference signal corresponding to the absolute distance l1 is obtained The distance corresponding to the phase part The precision of r'1 is higher than that of r1, and the corrected absolute distance l'1 = n1λ + r'1 is obtained. The phase of the beat frequency interference signal corresponding to the absolute distance l2 is obtained The distance corresponding to the phase part The precision of r'2 is higher than that of r2, and the corrected absolute distance l'2 = n2λ + r'2 is obtained.

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