A method and system for laser ranging using single photon avalanche diodes

Abstract

This invention provides a lidar ranging method and system using a single-photon avalanche diode (SPAD). The lidar ranging method using a SPAD includes the following steps: generating and transmitting a laser pulse sequence, the laser pulse sequence containing *n* pulses, with *n* time intervals between the *n* pulses, wherein each time interval is an integer multiple of the resolution of the time-to-digital converter (TD-SCDMA), and the sum of all time intervals is a preset constant value to fix the duration of a single measurement cycle; receiving the echo signal reflected from the target using a SPAD detector, and quantizing the echo signal into a received vector using the TD-SCDMA; performing a cross-correlation operation between the received vector and the corresponding transmitted vector of the laser pulse sequence, and calculating the target distance based on the peak position of the cross-correlation result. This invention achieves high-efficiency ranging with a stable frame rate while reducing hardware costs.

Description

Technical Field

[0001] This invention relates to the field of lidar technology, and specifically to a lidar ranging method and system using a single-photon avalanche diode. Background Technology

[0002] LiDAR, as an active optical detection device, calculates the target distance by emitting laser pulses and receiving the echo signals reflected from the target, using the time-of-flight of the light waves. It has wide applications in fields such as autonomous driving, robot navigation, and 3D mapping. Among them, lidar based on single-photon avalanche diode detectors has become an important direction in the development of lidar technology due to its single-photon-level detection sensitivity, enabling high-precision measurement of long-distance, weak-signal targets.

[0003] In existing technologies, lidar based on single-photon avalanche diodes (SPADs) mainly employs two technical approaches. The first approach combines SPAD array chips with traditional time-correlated single-photon counting technology, using parallel detection to improve the point cloud generation rate. However, the complex manufacturing process and low yield of SPAD array chips result in high hardware costs. Furthermore, traditional time-correlated single-photon counting technology requires constructing massive photon arrival time histograms to complete a single ranging operation, leading to huge data processing demands and limiting measurement speed.

[0004] The second technical approach employs a ranging scheme that combines pseudo-random encoded pulse trains with cross-correlation calculations. For example, Chinese invention patent application number 2017113471872 discloses a photon counting lidar based on composite pseudo-random encoding. This scheme obtains the target distance in one go by transmitting a pseudo-random encoded pulse train and using cross-correlation calculations, eliminating the need for multiple accumulations and thus shortening the imaging time. However, due to the nanosecond or even microsecond-level dead time of single-photon avalanche diodes, this scheme requires composite modulation of the traditional pseudo-random sequence, that is, forcibly inserting a zero code corresponding to the dead time after each transmitted pulse to avoid the decrease in detection efficiency caused by the dead time effect. This forced insertion of the dead time design causes the actual duration of each measurement to vary depending on the pulse distribution in the code pattern, making it impossible to keep the measurement cycle constant. This results in jitter and instability in the point cloud data refresh rate, which cannot meet the strict requirements of deterministic frame rates for applications such as autonomous driving. Summary of the Invention

[0005] Based on the above background, the purpose of this invention is to provide a lidar ranging method and system using single-photon avalanche diodes. By combining deterministic coded pulse sequences with pre-accumulated cross-correlation algorithms, this invention achieves high-efficiency ranging with stable frame rates while reducing hardware costs. It solves the problems of high cost of single-photon avalanche diode array chips, slow measurement speed of traditional time-correlated single-photon counting, and frame rate jitter caused by unstable measurement cycles in pseudo-random coding schemes in the prior art.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] A lidar ranging method using a single-photon avalanche diode, the method comprising the following steps:

[0008] Generate and emit a laser pulse sequence, the laser pulse sequence comprising One pulse, the There is a pulse interval time interval Each of the time intervals is an integer multiple of the resolution of the time-to-digital converter, and the sum of all time intervals is a preset constant value, so that the duration of a single measurement cycle is fixed.

[0009] The echo signal reflected from the target is received using a single-photon avalanche diode detector, and the echo signal is quantized into a receiving vector using the time-to-digital converter.

[0010] The receiving vector and the emission vector corresponding to the laser pulse sequence are cross-correlated, and the target distance is calculated based on the peak position of the cross-correlation result.

[0011] Preferably, the generation and emission of the laser pulse sequence specifically includes: continuous repetitive emission. The laser pulse sequence described below, wherein The specific steps of receiving the echo signal reflected from the target using a single-photon avalanche diode detector include: acquiring the corresponding... The second launch Group Original Received Vector ,right The original received vectors are summed in the time domain to obtain the synthesized received vector. The cross-correlation operation between the received vector and the emission vector corresponding to the laser pulse sequence specifically includes: calculating the synthesized received vector. The cross-correlation function with the emission vector.

[0012] By performing time-domain accumulation on the K sets of original received vectors before cross-correlation operations, the cross-correlation operation that originally required K operations is reduced to a single operation, significantly reducing the algorithm complexity. This reduces the number of costly cross-correlation operations by a factor of K, making it compatible with low-cost edge computing hardware and greatly improving computational efficiency while ensuring ranging accuracy.

[0013] Preferably, the time interval The distribution is non-uniform, and the value of each of the time intervals is greater than the dead time of the single-photon avalanche diode detector.

[0014] By setting the time interval to a non-uniform distribution, with each interval being greater than the dead time of a single-photon avalanche diode, the dead time effect can be avoided without forcibly inserting additional zero codes. This retains the signal-to-noise ratio improvement brought by the coding gain while avoiding the interference of the dead time on the measurement cycle.

[0015] Preferably, the number of pulses in the laser pulse sequence The range of values ​​is ,in, and The value is a preset positive integer; provided that the sum of the time intervals is a constant, the time intervals are dynamically adjusted according to preset encoding rules. The specific numerical distribution is used to form an orthogonal emission sequence.

[0016] By limiting the number of pulses to a preset range and dynamically adjusting the numerical distribution of the time interval according to the encoding rules, it is possible to flexibly generate a variety of orthogonal transmission sequences while ensuring a fixed measurement period, which is suitable for multi-channel parallel measurement or anti-interference application scenarios.

[0017] Preferably, the result of the cross-correlation operation The mathematical expression is:

[0018] ;

[0019] In the formula, The emission vector, For the received vector, This refers to the time delay.

[0020] The target distance The mathematical expression is:

[0021] ;

[0022] In the formula, Represents the speed of light. Represents the result of cross-correlation operation The delay amount corresponding to the global maximum value.

[0023] A lidar ranging system using a single-photon avalanche diode, the lidar ranging system comprising:

[0024] The emission module is used to drive the laser to emit a sequence of laser pulses;

[0025] The receiving module, including a single-photon avalanche diode detector and a time-to-digital converter, is used to convert the received photon signal into a digitized receiving vector.

[0026] The control and processing module is configured to execute the lidar ranging method described above.

[0027] Preferably, the control and processing module includes:

[0028] Accumulator units are configured to store consecutive data in memory. The received vector for each measurement cycle is accumulated bit by bit;

[0029] The correlation calculation unit is configured to perform cross-correlation calculations on the accumulation result and the pre-stored transmission sequence after the accumulation is completed.

[0030] Preferably, the single-photon avalanche diode detector is a single-point single-photon avalanche diode detector or a linear array single-photon avalanche diode detector; the emission module further includes a scanning device, which is configured to change the spatial projection direction of the laser pulse sequence.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] The present invention provides a lidar ranging method and system using a single-photon avalanche diode. By using a global constraint that the sum of time intervals is a constant value, the time window for each point cloud measurement is strictly consistent, thereby achieving a deterministic data refresh rate. This effectively solves the technical problem of measurement cycle jitter as the code pattern changes in existing pseudo-random coding schemes, improves the frame rate stability of the lidar system, and achieves high-efficiency ranging with a stable frame rate.

[0033] This invention reduces the number of highly complex cross-correlation operations by a factor of K through algorithm optimization of first accumulation and then correlation, significantly reducing the consumption of computing resources and enabling the system to be adapted to low-cost edge computing hardware.

[0034] This invention avoids the dead zone effect of single-photon avalanche diodes through non-equal interval pulse design, while retaining the coding gain;

[0035] This invention significantly reduces system hardware costs by using single-point or a small number of single-photon avalanche diodes in conjunction with a scanning device, replacing expensive array chips. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0037] Figure 1 This is a schematic diagram of the architecture of a lidar ranging system using a single-photon avalanche diode according to the present invention.

[0038] Figure 2 This is a schematic diagram of the emission timing of the laser pulse sequence in this invention;

[0039] Figure 3 This is a schematic diagram of the timing of repeated transmissions in the transmission sequence of this invention;

[0040] Figure 4 This is a schematic flowchart of a lidar ranging method using a single-photon avalanche diode according to the present invention.

[0041] In the diagram: 10. Transmitting module; 11. Laser; 12. Scanning device; 20. Receiving module; 21. Single-photon avalanche diode detector; 22. Time-to-digital converter; 30. Control and processing module; 31. Accumulator unit; 32. Correlation calculation unit; 40. Target. Detailed Implementation

[0042] The technical solution of the present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the implementation of the present invention is not limited to the following embodiments, and any modifications and / or alterations made to the present invention will fall within the protection scope of the present invention.

[0043] In this invention, unless otherwise specified, all parts and percentages are by weight, and the equipment and raw materials used are commercially available or commonly used in the art. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art. Unless otherwise specified, the components or equipment in the following embodiments are general standard parts or components known to those skilled in the art, and their structures and principles can be learned by those skilled in the art through technical manuals or conventional experimental methods.

[0044] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In this detailed description, numerous specific details are set forth to facilitate explanation and provide a thorough understanding of the embodiments of the present invention. However, one or more embodiments may be practiced by those skilled in the art without these specific details.

[0045] This invention discloses a lidar ranging method using a single-photon avalanche diode and a corresponding lidar ranging system. For example... Figure 1 As shown, the lidar ranging system includes a transmitting module 10, a receiving module 20, and a control and processing module 30. These three functional modules work together to complete the transmission of laser pulses, the reception of echo signals, and the calculation of distance information.

[0046] The transmitting module 10 is configured to drive the laser 11 to emit a laser pulse sequence and control the scanning device 12 to change the spatial projection direction of the laser pulse sequence according to the requirements of the ranging scenario, thereby achieving scanning coverage of the spatial area where the target 40 is located. The receiving module 20 includes a single-photon avalanche diode detector 21 and a time-to-digital converter 22. The single-photon avalanche diode detector 21 is configured to receive the echo photon signal reflected from the target 40 with single-photon sensitivity, and the time-to-digital converter 22 is configured to quantize the photon arrival time into a digital timestamp to form a digitized receiving vector. The control and processing module 30 is electrically connected to the transmitting module 10 and the receiving module 20 and is configured to control the transmission timing, receive data, and execute the ranging algorithm. The control and processing module 30 includes an accumulator unit 31 and a correlation operation unit 32, which are used to perform time-domain accumulation and cross-correlation operations on the received vector, respectively.

[0047] like Figure 2 As shown, the laser pulse sequence emitted by laser 11 consists of x pulses, with x-1 time intervals between these x pulses, denoted as . Each time interval y is an integer multiple of the resolution of the time-to-digital converter 22. For example, when the resolution of the time-to-digital converter 22 is 50 picoseconds, the time interval y can take discrete values ​​such as 100 picoseconds, 150 picoseconds, and 200 picoseconds. Crucially, the sum S of all x-1 time intervals is a preset constant. This global constraint ensures that regardless of the specific distribution of the values ​​of each time interval, the total duration of a single measurement cycle remains constant, guaranteeing a deterministic data refresh rate.

[0048] The time intervals are distributed non-uniformly, meaning that the values ​​of each interval can be different, and each time interval value is greater than the dead time of the single-photon avalanche diode detector 21. The single-photon avalanche diode detector 21 requires a dead time after detecting a photon to recover its detection capability; a typical dead time is on the order of 10 nanoseconds to 100 nanoseconds. By setting the time interval between adjacent pulses to be greater than this dead time, it is ensured that the single-photon avalanche diode detector 21 has sufficient time to recover after the echo signal of the previous pulse arrives, thus avoiding the decrease in detection efficiency caused by the dead time effect, and eliminating the need to forcibly insert a fixed zero-code interval after each pulse as in existing technologies.

[0049] The range of values ​​for the number of pulses x in the laser pulse sequence is set to... Where X1 and X2 are preset positive integers. Under the premise that the sum of time intervals S is a constant, the specific numerical distribution of each time interval can be dynamically adjusted according to preset encoding rules to form a transmission sequence with good autocorrelation characteristics. This adjustable encoding design enables the system to generate multiple sets of mutually orthogonal transmission sequences, suitable for multi-channel parallel measurement or time-division multiplexing scenarios, improving the system's flexibility and anti-interference capability.

[0050] like Figure 3 As shown, to improve ranging accuracy and signal-to-noise ratio, a repetitive emission and pre-accumulation processing strategy is adopted. Specifically, the same laser pulse sequence is continuously emitted K times, where K is a positive integer greater than or equal to 2. During each emission, the single-photon avalanche diode detector 21 receives the echo signal reflected by the target 40, and the time-to-digital converter 22 converts the photon arrival event into a digitized raw received vector, denoted as . The accumulator unit 31 performs bit-by-bit accumulation on the K sets of original received vectors in the memory, that is, adds the values ​​at corresponding times to obtain the synthesized received vector Ts. This time-domain accumulation operation can effectively suppress random noise and improve the signal-to-noise ratio of the signal.

[0051] After completing the accumulation operation, the related operation unit 32 performs a cross-correlation operation between the synthesized received vector Ts and the pre-stored transmitted vector. The transmitted vector G(n) is generated based on the sequence of emitted laser pulses, taking a value of 1 when there is an emitted pulse and a value of 0 when there is no emitted pulse, with the time interval being the resolution of the time-to-digital converter 22.

[0052] The result of cross-correlation operation The mathematical expression is:

[0053] ;

[0054] In the formula, For the emission vector, For the receive vector, This refers to the time delay.

[0055] By iterating through different time delays, the cross-correlation function curve is calculated, and the determination is made. The latency corresponding to the global maximum value .

[0056] Based on the principle of time-of-flight ranging, the target distance... The mathematical expression is:

[0057] ;

[0058] In the formula, Represents the speed of light. Represents the result of cross-correlation operation The delay amount corresponding to the global maximum value.

[0059] like Figure 4 As shown, the lidar ranging method using a single-photon avalanche diode includes the following steps:

[0060] First, a laser pulse sequence is generated and emitted, the laser pulse sequence containing One pulse, There is a pulse interval time interval Each time interval is an integer multiple of the resolution of the time-to-digital converter, and the sum of all time intervals is a preset constant to keep the duration of a single measurement cycle fixed.

[0061] Secondly, a single-photon avalanche diode (SPAD) detector is used to receive the echo signal reflected from the target, and a time-to-digital converter (TDC) is used to quantize the echo signal into a receiving vector.

[0062] Then, determine whether repeated accumulation is needed. If so, repeat the transmission K times and obtain K sets of original received vectors. Then, perform time-domain accumulation on the K sets of original received vectors to obtain the synthetic received vector.

[0063] Finally, the received vector or synthesized received vector is cross-correlated with the emission vector corresponding to the laser pulse sequence, and the target distance is calculated based on the peak position of the cross-correlation result.

[0064] The following analysis explains the working principle and advantages of the lidar ranging system and method using single-photon avalanche diodes, based on the operating principle.

[0065] After the system starts, the control and processing module 30 generates a timing control signal for the laser pulse sequence according to preset parameters. This signal specifies the emission time of x pulses, the time interval between adjacent pulses is an integer multiple of the resolution of the time-to-digital converter 22, and the sum of all intervals is constant at S. The laser 11 in the emission module 10 emits laser pulses according to the timing control signal, while the scanning device 12 changes the beam direction according to a preset scanning trajectory to achieve coverage of the target area.

[0066] After the laser pulse strikes the surface of target 40, it is reflected. Some of the reflected photons return along their original path and are captured by the receiving module 20. The single-photon avalanche diode detector 21 operates in Geiger mode, possessing single-photon-level detection sensitivity. When the echo photon is incident on the photosensitive surface of the detector, it triggers avalanche breakdown, generating an electrical pulse signal. The time-to-digital converter 22 records the time delay of the electrical pulse signal relative to the emission time, discretizing the continuous time information into a digital receiving vector. Due to the quantum efficiency of the single-photon avalanche diode detector 21 and the influence of ambient background light, the receiving vector contains noise components in addition to the target reflection signal.

[0067] Within a single measurement cycle, a definite time correspondence exists between the emitted laser pulse sequence and the received echo signal. This time correspondence is determined by the flight time of the light wave, which is proportional to the target distance. By performing a cross-correlation operation on the received and emitted vectors, the autocorrelation characteristics of the emitted sequence can be utilized to extract the echo signal component matching the emitted sequence from the noise background. The peak position of the cross-correlation function corresponds to the optimal time alignment point between the emitted and received sequences, which is the flight time of the light wave.

[0068] When employing a repetitive transmission strategy, accumulator unit 31 accumulates bit-by-bit the received vectors from K consecutive measurement cycles in memory. Since the target reflected signal has a definite time position in each measurement, while noise is random, the accumulation operation coherently enhances the signal components and incoherently superimposes the noise components, thus significantly improving the signal-to-noise ratio. By using an accumulation-then-correlation processing method, the high-complexity operation that originally required cross-correlation calculations on K sets of received vectors is reduced to a single cross-correlation operation on the accumulated composite vector, reducing computational complexity by a factor of K. This enables the system to achieve real-time ranging on resource-constrained edge computing platforms.

[0069] This invention solves the technical problem of unstable measurement period in existing pseudo-random coding schemes by using a global constraint that the sum of time intervals is a constant. In existing technologies, because a zero code corresponding to the dead time needs to be forcibly inserted after each transmitted pulse, the code pattern containing different numbers of pulses has different actual lengths, resulting in jitter in the measurement period. This invention, through a non-uniform interval design, satisfies the dead time requirement without inserting additional zero codes, while ensuring the determinism of the measurement period through a sum constraint, thus achieving a balance between frame rate stability and dead-time resistance.

[0070] Furthermore, this invention employs a single-point or linear array of single-photon avalanche diode detectors in conjunction with a scanning device, replacing the expensive single-photon avalanche diode array chip. Single-point detectors or linear arrays composed of a small number of detectors offer advantages such as simple structure, low cost, and low complexity of driving circuitry. Combined with a mechanical scanning device, they can achieve a large field of view coverage, significantly reducing hardware costs while meeting the basic performance requirements of lidar, thus providing an economical and feasible solution for the large-scale application of lidar.

[0071] In summary, this invention achieves technological breakthroughs in frame rate stability, computational efficiency, and hardware cost through the synergistic combination of deterministic encoded pulse sequences, pre-accumulated cross-correlation algorithms, and low-cost detector configurations, providing a high-performance, low-cost technical option for LiDAR applications in fields such as autonomous driving and robot navigation.

[0072] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A lidar ranging method using a single-photon avalanche diode, characterized in that: The method includes the following steps: Generate and emit a laser pulse sequence, the laser pulse sequence comprising One pulse, the There is a pulse interval time interval Each of the time intervals is an integer multiple of the resolution of the time-to-digital converter, and the sum of all time intervals is a preset constant value, so that the duration of a single measurement cycle is fixed. The echo signal reflected from the target is received using a single-photon avalanche diode detector, and the echo signal is quantized into a receiving vector using the time-to-digital converter. The receiving vector and the emission vector corresponding to the laser pulse sequence are cross-correlated, and the target distance is calculated based on the peak position of the cross-correlation result.

2. The lidar ranging method using a single-photon avalanche diode according to claim 1, characterized in that: The generation and emission of the laser pulse sequence specifically includes: continuous repetitive emission. The laser pulse sequence described below, wherein The specific steps of receiving the echo signal reflected from the target using a single-photon avalanche diode detector include: acquiring the corresponding... The second launch Group Original Received Vector ,right The original received vectors are summed in the time domain to obtain the synthesized received vector. The cross-correlation operation between the received vector and the emission vector corresponding to the laser pulse sequence specifically includes: calculating the synthesized received vector. The cross-correlation function with the emission vector.

3. The lidar ranging method using a single-photon avalanche diode according to claim 1, characterized in that: The time interval The distribution is non-uniform, and the value of each of the time intervals is greater than the dead time of the single-photon avalanche diode detector.

4. A lidar ranging method using a single-photon avalanche diode according to claim 1, characterized in that: The number of pulses in the laser pulse sequence The range of values ​​is ,in, and The value is a preset positive integer; provided that the sum of the time intervals is a constant, the time intervals are dynamically adjusted according to preset encoding rules. The specific numerical distribution is used to form an orthogonal emission sequence.

5. A lidar ranging method using a single-photon avalanche diode according to claim 1, characterized in that: The result of the cross-correlation operation The mathematical expression is: ； In the formula, The emission vector, For the received vector, This is the amount of time delay; The target distance The mathematical expression is: ； In the formula, Represents the speed of light. Represents the result of cross-correlation operation The delay amount corresponding to the global maximum value.

6. A lidar ranging system using a single-photon avalanche diode, characterized in that: The lidar ranging system includes: The transmitting module is used to drive the laser to emit a sequence of laser pulses; The receiving module, including a single-photon avalanche diode detector and a time-to-digital converter, is used to convert the received photon signal into a digitized receiving vector. The control and processing module is configured to perform the lidar ranging method as described in any one of claims 1-5.

7. A lidar ranging system using a single-photon avalanche diode according to claim 6, characterized in that: The control and processing module includes: Accumulator units are configured to store consecutive data in memory. The received vector for each measurement cycle is accumulated bit by bit; The correlation calculation unit is configured to perform cross-correlation calculations on the accumulation result and the pre-stored transmission sequence after the accumulation is completed.

8. A lidar ranging system using a single-photon avalanche diode according to claim 6, characterized in that: The single-photon avalanche diode detector is a single-point single-photon avalanche diode detector or a linear array single-photon avalanche diode detector; the emission module also includes a scanning device, which is configured to change the spatial projection direction of the laser pulse sequence.

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